Final Report Summary - SWIFF (Space Weather Integrated Forecasting Framework)
Executive Summary:
Space weather refers to a complex condition state resulting from the Sun activity, propagating in the interplanetary space and possibly affecting the Earth space environment, which all together can influence, not only the performance and reliability of space-borne and ground-based technological systems, but more significantly can endanger human life or health. Adverse conditions in the space environment can cause disruption of satellite operations, communications, navigation, and electric power distribution grids, leading to a variety of socioeconomic losses. In addition, the space weather influences the Earth climate. It is well known that the Sun activity affects the total amount of heat and light reaching the Earth and the amount of cosmic rays arriving in the atmosphere, directly contribute to a phenomenon responsible for the amount of cloud cover and precipitation. Given these crucial impacts on society, the space weather is attracting a growing worldwide attention and progressively increases presence within international research projects.
The actors of space weather are the Sun, the Earth and the vast space in between. Like any star, the sun is made of a highly energetic and highly conductive gas, called plasma. In plasmas, the atoms have been broken into their nuclei and their electrons that become free to move. The hot plasma of the Sun is confined by gravity and moves in complex patterns that produce large currents and large magnetic fields. The gravitational confinement is not perfect and a highly varying outflow of plasma, called solar wind, is emitted from the Sun to permeate the whole solar system, reaching the Earth. The Earth has itself a magnetic field. The Earth magnetic field makes compasses point towards the north pole and allows many species of animals to find their way during migrations. This very same field protects the Earth from the incoming solar wind and its disturbances. Only a small part of the particles can reach the Earth surface, the so-called cosmic rays. Most of the incoming plasma is stopped and deflected, reaching only high strata of the atmosphere at the polar regions and causing the aurora seen by the people of the northern latitudes as the northern lights.
To describe and predict such space weather processes, both electromagnetic fields and plasma particles need to be modelled and simulated. The nuclei (mostly protons, the nuclei of hydrogen) and the electrons of the plasma are loosely coupled, where each species has its own typical scales. The electromagnetic fields keep the species coupled forming a very non-linear and multi-scale system.
SWIFF (Space Weather Integrated Forecasting Framework, swiff.eu) aims at addressing the crucial challenges in the modelling and forecasting space weather: the presence of multiple scales and multiple physics.
Multiple physics is present because the conditions found in space are widely different in different regions spanning many orders in density range, intensity of the magnetic field and temperature. High density relatively cold plasmas (e.g. the photosphere and the ionosphere) are collisional and are best described by fluid models. Dilute and hot plasmas (e.g. the Earth magnetopshere) are collisionless and require a kinetic treatment. Additionally processes of radiative emission, chemical reactions and many others need to be included in certain regions, making the need to handle multiple physics models interacting with each other a key requirement for space weather.
Swiff wants to join in a integrated framework fluid and kinetic models under the same roof able to work together to provide a full description of all coupled scales.
Project Context and Objectives:
The Sun-Earth connection is a much more dynamic and eventful system than the common-life experience of looking at the sky can suggest. The Sun is far from a stationary ball emitting light. Instead it produces a highly diverse and varying flow of plasma, called solar wind, and a magnetic field that is ejected towards the rest of the solar system, interacting actively with the Earth. The term "Space weather" refers to conditions on the Sun and in the solar wind, magnetosphere, ionosphere, and thermosphere that can influence the performance and reliability of space-borne and ground-based technological systems and can endanger human life or health. Adverse conditions in the space environment can cause disruption of satellite operations, communications, navigation, and electric power distribution grids, leading to a variety of socioeconomic losses (Definition used by the U.S. National Space Weather Plan (http://www.ofcm.gov/nswp-sp/text/a-cover.htm)).
The study of space weather has become key in our technologically advanced society. Space weather disturbances can adversely affect the health of humans in space and in high altitude commercial flights (especially those on polar orbits flying over northern Canada and the north pole but also many of those over Europe as well). The astronauts on the space station are subject to a high risk even while partially shielded by the Earth 's magnetic field (so much so that the space station has a specially shielded area to seek refuge during space weather storms). But future missions to the Moon or Mars are critically affected by space weather effects, to a degree that it is currently expected that, without proper actions, the life of at least one astronaut would be lost due to radiation exposure generated by space weather events. Technology is also very susceptible. Satellites have been lost to space weather storms and critical land infrastructures (power lines, pipelines) have been disrupted during particularly severe space weather storms.
There is critical realisation at the political and scientific level that action should be taken to address space weather issues. If meteorological predictions are a good paradigm, to achieve the same level of predictive capability for space weather two directions need to be followed. Modern weather forecasts, shown for example on TV before/after the newscast, are based on two critical developments: the existence of a wide network of measuring stations and of meteosats, and on the existence of sophisticated computer models to use the observational information to project conditions in the future. This is a great advance over what was typical a generation ago when General Eisenhower decided the day of D-day based on the educated guess of expert meteorologists that could not have any meteosat or computer model to help. While great progress has been made in space science, we are comparatively, for space weather forecasting, still in the modus operandi of meteorology in the days of General Eisenhower.
However unlike those days, Europe is relatively affluent and at peace. A plan is in the making to understand and control space weather: space situational awareness (SSA) now well under way. Of the various goals of the SSA, the present project developed the cloest relation specifically with the virtual space weather modelling (VSWMC) effort. Numerical modelling is one of the two pillars that have advanced regular weather models. A number of advances numerical algorithms at centres around the world have been created to model the evolution of the atmosphere and oceans in order to predict the weather of the following few days. The eventual goal is to have the same capability for space weather.
Here, we are not trying to precede the SSA effort. ESA will lead the way for the creation of a space weather modelling effort under its virtual space weather modelling effort. Instead, we aimed at preparing the ground and show the way to follow in establishing a framework for that effort. This has been our goal:
1. Zero-in on the physics of all aspects of space weather and design mathematical models that can address them.
2. Develop specific computational models that are especially suited to handling the great complexity of space weather events where the range of time evolutions and of spatial variations are so much more challenging than in regular meteorological models.
3. Develop the software needed to implement such computational models on the modern supercomputers available now in Europe.
Other similar efforts are underway in the rest of the world. The US, even leaving aside the numerous and important military programs, funds several centres to model space weather (e.g. the NOAA space weather forecasting centre in Boulder, the CISM centred in Boston and funded by NSF, the CCMC funded by NASA and the SWMC in Michigan to name just perhaps some of the most important). Japan has recently initiated one similar centre following the example of the Americans.
Europe needed to act in this direction too. We implemented with Swiff a plan that, while taking full advantage of the American experience, intended to follow a different path, designed especially to combine synergistically the current strengths of Europe while avoiding any weakness to become a hindrance in reaching our goal. But before outlining our plan, we would like to stress our admiration and support to other approaches followed elsewhere. The coordinator has worked in the US system until 2 years ago and has learnt there to appreciate the great advances of the American research in this field. We fully intend to continue our collaborations in the US. While explicitly not part of this effort, we count on our many sources of input, support and collaboration with the US. However, as explained above, we will follow a different path.
The American way has been to leverage on their superior base of existing tools to mesh them together, by making different codes, developed for other related activities, talk to each other. This has of course the advantage of leading to a quick development. However the experience has been that the models and codes present serious challenges in their coupling and models can be so fundamentally incompatible as to make the progress difficult and uncertain. For this reason, our approach was different and followed the steps already mentioned above.
1. A strength of the European research is that not having had the tools and the shear financial size of the American system we have not crystallised into established widely used software tools and we have left more room to the investigation of different and more risky options. Instead of basing the software on the methods and knowledge of the 60's and 70's like some of the national labs in USA have tended to do, more research has gone into new innovative models and computational methods. Not that research has not gone in this direction in the US also, and indeed one of the key tools we will use was developed there (by the coordinator). But the early establishment of huge gorilla-codes has hindered the acceptance of new methods.
2. The existence of a wide range of new ideas and new methods in Europe was used as a springboard to design space weather models from the bottom up. By analysing
• at the physics present
• at the mathematical models needed to represent it
• at the algorithms best suited to simulate the models on computers
• at the best software to implement the models on modern supercomputers.
Instead of trying to piece together relatively incompatible software originally designed for other purposes, we designed models and tools from the beginning to be a seamless description of space weather. Nothing needed this more acutely than the coupling of large scale processes best described by macroscopic methods (fluid approach) with small scale description of microscopic processes (kinetic approach). These are the regions where the American software plug-in approach has found the most difficulties and needs a new fresh perspective. We provided such new perspective by redesigning the modelling with the focus immediately on these regions of coupling. The task of Swiff has been to develop methods to couple and analyse multiple methods and codes to treat compled multiscale systems.
The project has been structured around:
• developing of models and software for space weather (WP2)
• studying the coupling at the Sun (WP3)
• studying the coupling in space between the solar wind and the magnetosphere (WP4)
• studying the coupling between the Earth's atmosphere and the Earth-space environment (WP5)
A management work package (WP1) and a dissemination work package (WP6) complete the overview. The management and dissemination WP are described in Section B2 and B3, respectively. The other science WPs are described below.
Project Results:
The fundamental goal of the SWIFF project has been to progress beyond the state of the art in two ways.
First, in the development of mathematical models and computational methods and software especially designed to handle the multiple physics and the multiple scales characteristic of space weather phenomena. The project intended to produce an integrated forecasting framework able to model space weather events from their solar origin to the impact on Earth and its space environment, focusing on the effect on humans and technology in space and on the ground infrastructure. This is the goal of WP2.
Second, we intended to demonstrate for practical space weather events and processes the validity and usefulness of the SWIFF integrated space weather forecasting framework. The WP 3 to 5 include all phases of space weather events and have pushed forward the state of the art in modelling specific space weather events and processes.
Below the progress in both directions is described in detail.
1.2.1 Multiscale modelling for space weather (WP2)
Computationally, both fluid and kinetic approaches have been dominated by explicit methods. Explicit techniques are well-known for being conceptually simple and of straightforward implementation. In the context of multiple scale modelling where the kinetic scale has to be considered, the limitations arising with the use of explicit methods in the treatment of fluid models, become a minor consideration when compared with the limits imposed by the explicit discretisation of the kinetic level of description. For kinetic modelling, two constraints need to be satisfied. First, they need to resolve the fastest time scale (typically the electron plasma frequency for space plasma simulations) supported by the model, which may be orders of magnitude faster than the dynamical time scale of interest. Furthermore, explicit methods need to resolve also the smallest spatial scale, the Debye length that is typically several orders of magnitude smaller than the other scales of interest. Given these limitations, the fact is that 3D explicit fully kinetic simulations for realistic choice of parameters will remain out of reach in the foreseeable future. Therefore, we have taken a different approach which serves as the motivation for the proposed work.
A well known technique for handling multi-scale problems is Adaptive Mesh Refinement (AMR). However, an explicit full particle code with AMR capability will still keep the problems of space weather out of reach for realistic parameters. The reason is that the time step limitations suffered by explicit methods will still be present in any AMR explicit PIC code. This limits the advantage that AMR can offer. As a result, we propose to couple the use of implicit kinetic methods which removes the restriction of Debye length resolution and of time step resolution, so opening the possibility to take full advantage of AMR, resulting in performance advantage of several orders of magnitude over uniform-grid explicit schemes. In this context, instead, it is not crucial to rely on implicit modelling of the fluid description, as the time scales involved in that are already much longer than the kinetic scales and typically need to be resolved to achieve sufficient accuracy.
Implicit methods allow one to handle fast time scales and small length scales to resolve the dynamical scales of interest. Implicit PIC methods have been shown to be free from both spatial and temporal limitations, and as a consequence have been able to push the limits of kinetic plasma physics simulation to regimes where explicit methods have not been able to reach, even with the use of powerful supercomputers. Thus, implicit PIC approaches not only deal with multiple time scales, but are by design ideally suited to deal with multiple length scales in a reliable manner. Fluid simulations have also benefited from fully implicit approaches, as demonstrated in resistive and Hall MHD. Pioneering work in implicit MHD treatments show that it is possible to step over fast time scales without compromising accuracy or efficiency, reporting CPU speed-ups of implicit versus explicit methods of an order of magnitude.
Implicit kinetic plasma simulation has taken essentially two lines of investigation that over the years have converged to a similar framework and the distinction has become largely of historical interest. The two approaches are: the direct implicit method [Hewett, Langdon et al.] and the implicit moment method. Both led to well established codes with a wealth of applications. The first method has been developed primarily at Lawrence Livermore National Laboratory (LLNL) and the second has been developed primarily at the Los Alamos National Laboratory (LANL). The present effort takes full advantage of all the progress made to date on each of the methods, but relies primarily on the implicit moment method for two reasons. The first is practical, one of the proponents has been long involved in the development of the method and of the modern codes based on them. The second is theoretical: the implicit moment method is based on solving the kinetic equations by coupling them with the moment equations (i.e. with fluid equations): since it speaks both the kinetic and the fluid language, it is naturally suited to handle the task of using fluid equations in some regions and kinetic equations in other.
The major application for the hybrid multi-scale simulation tool is to address magnetic reconnection, going beyond the detailed inter-code comparisons in the context of the well documented GEM and Newton challenges. The challenges are to revisit the many recent insights obtained by the various teams involved in this project, realising both 2D and 3D multi-scale simulations in especially collisionless magnetic reconnection regimes. We intend to perform detailed simulations tailored to plasma configurations relevant for coronal mass ejections in the solar corona, heliospheric current sheet conditions, and magnetospheric reconnection sites. In all of these settings, the role of cospatial shear flow combined with magnetic shear is to be explored in detail. Using the hybrid simulation suite, we aim to quantitatively address the role of incorporating ideal to Hall-MHD, single to multi-fluid, as well as fluid to kinetic aspects under realistic and reproducible initial and boundary conditions.
A major challenge in modelling many space and laboratory reconnection events is to reproduce the observed (high) reconnection rates. For this it is crucial to know if, at the small scales, the process is laminar or turbulent. At these small scales, fluid modelling has to be complemented with kinetic modelling of the charged particle velocity distributions. While the plasma fluid turbulence exhibits vortices and shear flows in 2D or 3D, the energy dependent particle trajectories in the kinetic model effectively extend the turbulent fluctuations to a higher-dimensional phase space, which may lead to enhanced reconnection or to collisionless damping due to phase space mixing. Using the many (hybrid, PIC and grid-adaptive) codes that we jointly share at the start of this project, we could immediately perform a first detailed comparison of the different algorithmic approaches within the first half year of our efforts, and swiftly moved on to develop and integrate this knowledge in the targeted multi-scale approach.
1.2.2 Coupling at the solar surface: photosphere, chromosphere and corona (WP3)
Models of the source regions of solar energetic events can be developed along two paths; 1) they can be based on empirically deduced magnetic field arrangements and topologies, or 2) they can be based on realistic numerical models of solar active regions. We plan to use a combination of these two approaches; while gradually developing increasingly realistic models of solar active regions we indeed in any case need to be able to check if the magnetic flux emergence and magnetic stress build-up in the models is sufficiently similar to observed situations. In particular, we need to evaluate if the models generate similar statistics with respect to the frequency and severity of the generated energetic events, and if these events in turn are able to drive space weather with statistics similar to the observed space weather.
CMEs can be triggered by newly emerging magnetic fields that destabilise the overlying coronal fields. The state-of the art models for the initiation of CMEs use either an imposed shearing motion at the photosphere to create the conditions for an eruption or an imposed time evolution of the radial component of the magnetic field at the computational base to simulate the emergence of new magnetic fields. Simulations of flux emergence have shown that the shearing motions and high-density eruptions can occur in a self-consistent manner in response to the Lorentz force generated by emerging magnetic fields and the interaction with the over-lying coronal field. These simulations can provide the appropriate boundary conditions on magnetic field components and horizontal velocity as input to the solar wind codes.
Many models of the coronal magnetic field are constructed at a single instant in time through either potential or linear force-free extrapolations from observed photospheric magnetograms. However, these instantaneous descriptions of the coronal field do not take into account any previous history of magnetic interactions and the slow build up of non-potential effects. To take these into account over long periods of time (months to years), the evolution and injection of electric currents due to flux emergence, differential rotation and the meridional flow must be considered. Such processes have been developed in the quasi-static, non-linear, force free model of Mackay and van Ballegooijen (2006) that uses observed magnetograms. To date this model has successfully explained the large-scale transport of helicity across the Sun and the associated chirality of solar filaments. In addition, it has a 50% success rate in predicting the sites of onset of CMEs (far exceeding any other model). The aim is to use the magnetic field distributions produced by this code as input into full 3D MHD simulations, allowing us to follow the dynamical evolution of CMEs as they are ejected from the Sun.
In order to produce realistic simulations of the Sun, realistic boundary conditions must be applied to the numerical models. In many cases this involves applying the magnetic field and velocity on the model solar surface. These can be supplied through the use of magnetic feature tracking algorithms that can extract the important elements of observed photospheric magnetograms.
To identify and understand the fundamental physics processes that are behind specific plasma behaviours requires the use of specially designed focused analysis tools. Members of the St Andrews node have been focusing on developing such tools and have tested a state-of-the-art suite of algorithms designed to work on general numerical data to determine the magnetic skeleton (and quasi-skeleton) of the magnetic field. Knowing these sites is important as they enable many of the key locations of magnetic reconnection to be identified and also the locations at which waves may be focused and hence dissipate.
The possibility and means to develop realistic models of solar active regions have emerged over the last few years, as a consequence of the ever increasing computational capacity. This has led, on the one hand to the possibility of modelling the structure of individual sunspots and pairs of sunspots, as done by Rempel et al (2008), and on the other to the possibility of modelling the multi-scale structure of convective motions that probably are the cause of the near self-similarity of magnetic field structures emerging through the solar surface, as done by Stein, Lagerfjärd and Nordlund (2009). These models needed to be extended to larger scales, and studies needed to be made with respect to the amount of spatial resolution that is necessary to obtain quasi-realistic results.
The sub-surface and photospheric models then needed to be coupled to models that incorporate chromospheric and coronal physics. Such models have been developed in recent years by Gudiksen and Hansteen and collaborators in Oslo, and the methods needed to represent the chromospheric and coronal physics with reasonable accuracy are by now well developed. As part of WP3 we developed and carried these investigations forward, reaching the goal of modelling energetic, space weather driving events ab initio.
1.2.3 Coupling in space: solar wind-magnetosphere (WP4)
The solar wind-magnetosphere coupling strongly depends on the solar wind properties and their variability. The connection between the solar wind and magnetosphere is mediated through the magnetosheath and magnetopause boundaries. The properties of the magnetosheath shocked plasma and its electromagnetic field near the magnetopause determine the magnetopause location and properties including the position of the reconnection regions and its stability.
Satellite measurements have supplied clear evidence of rolled-up vortices at the flank of the Magnetopause. In this region, the Kelvin - Helmholtz instability (KHI) has been proved to play a crucial role in the interaction between the solar wind and the Earth's magnetosphere and to provide a mechanism by which the solar wind enters the Earth's magnetosphere. The understanding of the mechanisms responsible for the solar wind entering in the Earth's magnetosphere is of great importance in the context of the physics of the magnetosphere and, as a consequence, for space weather phenomena such as magnetic storms and aurorae.
The KHI has been invoked as a possible mechanism to account for the increase of the plasma transport since during northward magnetic field periods, when magnetic reconnection is considered as inefficient, a relevant mixing between the solar wind and the magnetospheric plasma is observed, even larger than during southward configurations. KHI instability driven by a velocity shear can grow at low latitude since the nearly perpendicular magnetic field does not inhibit instability developments. This provides an efficient mechanism for the formation of a mixing layer and for the entry of the solar plasma into the magnetosphere, explaining the efficient transport during northward solar wind periods. Several observations support this explanation and show that the physical quantities observed along the magnetopause flank at low latitude are compatible with a KH vortex. The vortex formation process drags the magnetic field component parallel to the solar wind direction into the flow. The magnetic field is thus more and more stretched inside the vortices until it reconnects, redistributing the initial kinetic energy into accelerated particles, heating, etc., but also playing a significant role in the transport properties of the plasma.
Magnetic Reconnection is one of the most important basic processes of magnetised plasmas and is the only process able to reorganize the large-scale topology of the magnetic field as well as to influence the energetics of the system. We underline that magnetic reconnection is considered to be the driver of the most important energetic phenomena observed in the solar atmosphere and in many regions of the (outer) Earth Magnetosphere.
The research effort in the understanding of the physics underlying magnetic reconnection is impressive and has reached remarkable progress in the last ten years. However, magnetic reconnection still remains a formidable challenge, in particular on the problem of the conditions for the system to spontaneously lead to reconnection. Theoretically, two different approaches are used to investigate magnetic reconnection: either as an initial value problem within which a specific magnetic configuration is considered such that the instability develops by itself or as a boundary value problem in a "stable" magnetic configuration where reconnection is forced to develop by an external driver. In both cases, the system is ad hoc prepared to develop reconnection.
In the low latitude magnetopause context, another important secondary vortex instability that can develop on the shoulder of KH vortices is the Rayleigh-Taylor (RT) instability driven by the initial density jump between the solar wind and the magnetosphere since the vortex centrifugal acceleration acts as an efficient gravity making the vortex arms RT unstable. The process stops the vortex pairing process and leads finally to the generation of a turbulent mixing layer. In a simplified 2D geometry, the linear and early nonlinear evolution of large-scale (as compared to the ion skin depth di) vortices is essentially a MHD process depending only on the initial velocity and magnetic field, regardless of the specific small-scale physics. If the value of the in-plane magnetic field parallel to the initial flow is “low” enough, the motion of the vortices advects, stretches and rolls-up the magnetic field lines leading in a natural way to magnetic inversion layers where the process of magnetic reconnection can develop. Thus, as already stated, magnetic reconnection acts as a secondary instability on the primary KH instability. Although large-scale vortices are essentially MHD structures, their motion is not only capable of creating favourable conditions for reconnection to act, but in particular to build up sub-di current sheets within which two-fluid dynamics develops. Transition to “fast” reconnection is observed. A very important result has been to demonstrate that reconnection, despite being a very localised process, is capable of changing the large-scale magnetic topology as established by the flow evolution. This global variation is due to the small-scale physics generated dynamically by the large-scale evolution. The possibility of changing the magnetic topology is of fundamental importance concerning the transport properties of the low latitude magnetosphere system.
The solar wind-magnetosphere coupling also manifests itself in the presence of strongly turbulent regions. In particular, in situ observations clearly demonstrated that the magnetosheath region is significantly turbulent. The plasma turbulence originates in the solar wind, is transformed (typically amplified) by the collisionless shock which itself is a source of turbulence/wave activity driven by large departures of the particle distribution functions from Maxwellian ones. Further fluctuating wave energy is injected to the system as the magnetosheath plasma flows around the magnetospheric cavity leading to plasma compression/expansion and stretching of magnetic field lines as they are draped over the magnetopause. Such processes typically lead to the ion perpendicular temperatures larger than the parallel ones. For lower betas this temperature anisotropy generates Alfven ion cyclotron waves whereas for higher beta it generates mirror mode waves. The mirror instability leads to formation of coherent structures with the form of pressure-balanced non propagating magnetic enhancements or depressions. The mirror mode structures are very robust and survive to regions which are linearly stable with respect to the mirror instability. Recent CLUSTER observations indicate that the mirror structures influence or even drive the magnetic reconnection at the magnetopause and consequently they influence the transport of the solar wind plasma to the magnetosphere. Similarly it is expected that important plasma turbulence/wave activity have an impact on the magnetopause properties and plasma transport. The nonlinear physics of plasma turbulence and kinetic instabilities is only poorly understood and even less is known about their interaction with the magnetopause (we note that a relevant phenomenon called impulsive penetration is considered as an alternative to magnetic reconnection). For the mirror mode instability in situ observations indicate that magnetic enhancements/humps are present close to the bow shock whereas the magnetic depressions/holes are observed further downstream, even close to the magnetopause in low beta plasmas. This behaviour may be related to theoretical work indicating a subcritical bifurcation (leading to bistability behaviour) of the mirror instability near threshold. However, in reality the magnetosheath plasma is typically far from the mirror threshold where the theoretical approach fails. Local hybrid simulations (standard ones and simulations using expanding box models to investigate an influence of plasma property changes as it flows around the magnetopause) exhibited properties similar to the observed ones. In these simulations the mirror instability saturated in the form of the magnetic humps which transformed to magnetic holes later on. The quasi-parallel portion of the bow shock and proton foreshock is a source of large-amplitude, almost isotropic magnetic fluctuations/turbulence essentially of the kinetic nature. Their properties and generating mechanism is not well understood and even less is known about their evolution further downstream and their interaction with the magnetopause. Large global (2D and 3D) kinetic simulations of the solar wind/magnetosheath/magnetopause were needed and carried out by Swiff (complemented by detailed local simulations of relevant phenomena) in order to investigate the role of these processes in the transport of particles to the magnetosphere and their consequences on the space whether.
1.2.4 Coupling at the Earth: magnetosphere-ionosphere-thermosphere coupling (WP5)
Kinetic models for space weather have been developed to understand the physical mechanisms responsible of their time variations during disturbed periods. For each region of the inner magnetosphere, forecasting models has been developed on the basis of these physical kinetic models. The models have been coupled to take into account the interactions between the magnetospheric regions.
Concerning the terrestrial plasmasphere, where the cold plasma is trapped by the magnetic field in the inner magnetosphere, a three dimensional kinetic dynamic model has been developed at BISA [Pierrard and Stegen, JGR, 113, A10209, 2008 and reference therein]. The core of the plasmasphere is obtained from the kinetic exospheric approach assuming a kappa velocity distribution function for the particles. The position of the plasmapause, the limit of the plasmasphere, is determined by the interchange instability mechanism and an empirical model of the convection electric field. The model that gives the position of the terrestrial plasmapause as a function of the geomagnetic activity level can be run on the space weather portal: http://www.spaceweather.eu. Plasmaspheric exploration got a major boost since 2000, when IMAGE and CLUSTER satellites were launched. These observations allow us to improve the physics-based model by using empirical data.
The plasmasphere is the high altitude extension of the ionosphere at low and mean latitudes, so that a coupling with this region would be highly useful, especially to determine the variability of the total electron content that influences the radio wave propagation and Global Positioning System (GPS). For the ionosphere, the IRI (International Reference Ionosphere) model has been used. This model describes the number density and temperature of the ionized particles as a function of the altitude (less than 700 km), the time and date. The transition region between 700 km (ionospheric model) and 2000 km (plasmaspheric model) requires specific investigations for coupling.
For radiation belts, we started with the database of satellite observations and empirical models available on SPENVIS (www.oma.spenvis.be) developed at BISA. Analyses of SAMPEX, CRRES, Equator-S, CLUSTER and SAC-C observations has been made to improve the understanding of radiation variability. The satellite observations available in the SPENVIS database has been used to develop a dynamic model of the space radiations. Swiff went beyond the pre-existing static radiation models by introducing dynamic features based on observations of transient flux events. This approach facilitates the investigation of the physical processes involved in the flux variations in the magnetosphere.
Moreover, radiation belt populations are very sensitive to the core plasmasphere distribution and specifically to the position of the plasmapause. There is a relationship between the position of the plasmapause and the internal border of the external radiation belt. This relationship has been investigated in more detail to couple the plasmaspheric model to the radiation belts variability.
The polar wind corresponds to the plasma outflowing along open magnetic field lines and escaping from the ionosphere at high latitudes. To obtain forecasting in this region, we started from the kinetic models developed at BISA [Lemaire and Pierrard, Astrophys. Space Sci., 277, 2, 169-180, 2001] that especially emphasise the effects of non-Maxwellian velocity distributions. The presence of suprathermal particles in the tail of the distribution, as often observed in space plasmas, leads to a significant increase of the escape flux. The relationship existing between the current and the voltage has also been used in the auroral regions.
The WP5 had the opportunity to validate the physical models and to constrain them by empirical observations to determine precisely the effects of the solar and geomagnetic activity. The models have been used as tools for forecasting and nowcasting of space weather. By coupling the different regions of the magnetosphere (and with solar wind disturbance), it has been possible to obtain a full description of the magnetospheric perturbations.
S&T Approach
The fundamental challenge in the modelling of space weather is the need to account for multiple physics processes developing simultaneously in the same region of space or progressing in a cascade in different regions, and characterised by widely different spatial and temporal scales. For each process and for each scale the correct and most efficient method is different. In the course of a storm or a substorm, the Earth magnetic tail (the region of the magnetotail on the night side of the Earth opposite to the Sun) becomes active with many waves and instabilities developing and ultimately affecting the Earth ionosphere and even (indirectly) the ground.
The agents are different: electrons, ions and electromagnetic fields (here gravity is not important but on the Sun it is adding one more worry for the method developer). Each agent evolves on very different spatial and temporal scales, spanning times from the tens of microseconds to the hours and from hundreds of meters to millions of kilometres. The fast small processes are best treated by the study of single particle trajectories or by kinetic models, where one studies the statistical probability of finding electrons or ions with a certain velocity in a certain position. But such models are not best suited for larger scales, as the cost of the simulation would be prohibitive. In that case one need to resort to fluid models where the information on the statistical distribution is lost and the model treats only average quantities (e.g. density, average flux or temperature).
Moreover different populations of particles are present. For example in space weather events, to the "usual" solar wind, population of high energy, even relativistic, particles are present travelling at much faster speeds, close to the speed of light. Such high energy particles are one of the main threats for humans and technology in space. Of course the physical model for such particles is often different from that of other type of particles. Another challenge is that the conditions in different regions can be so different that model that work in one region would not work in another: for example the lower strata of the Sun's surface and atmosphere (photosphere, chromosphere, corona) are much more collisional than higher strata, and similarly the upper Earth atmosphere and ionosphere is more collisional than the magnetosphere and the physical processes are largely different.
The summary of this short description left out on purpose some aspects for the interest of brevity, but not because such processes would not be considered (see the WP description to find a complete list of all aspects considered). Yet from this summary one can immediately understand that SWIFF had to achieve two goals that critical for the space weather community:
1. The ability to treat concurrently in a single integrated framework microphysics (treated kinetically) and macrophysics (treated with fluid methods). The need to couple fluid and kinetic models is without doubt the most burning and crucial challenge not just for space weather modelling but also for many other aspects of computational science.
2. The ability to couple different physics in the same region at the same time or in cascade in the same or different regions. This effort is explicitly mentioned as a key goal of the present call from the European Commission. The document European Commission C (2009) 5893 of 29 July 2009 explicitly asks for "space weather models to improve specification and prediction capabilities, with emphasis on the linkage of the different physical processes that occur simultaneously or sequentially in many domains.” (quoted directly from the document). This is exactly what the WP 3 to 5 in close connection with the modelling efforts of WP2 achieved. We focused solely on developing a software framework to simulate in the modern supercomputers the linkage (or coupling) of different physical processes. And we do so not in general but with the eye fixed on the specific key areas of linkage, as summarised by each WP:
• studying the coupling at the Sun (WP3)
• studying the coupling in space between the solar wind and the magnetosphere (WP4)
• studying the coupling between the Earth's atmosphere and the Earth-space environment (WP5)
In the next section, we describe for each science WP how this approach is implemented in a specific strategy. The WP1 and WP6 are described respectively in Section B2 and B3.
1.3.2 Multiscale modelling for space weather (WP2)
Within this work package, we started our activities with an immediate intra-comparison of all codes available to the teams of this project. All teams had state-of-the-art simulation tools developed in house, and we carefully selected a comparison on idealized problem setups that combine the main physics challenges that recur in the later work packages. In particular, reconnection is a recurring theme of interest to all later applications, and we wish to identify the main advantages and disadvantages of using particular algorithms and discretizations on identical problems on the one hand, while on the other hand better identifying the role of ever more complex physics in addressing reconnection effects in detail. For this purpose, GEM-type 2D and 3D setups need to be revisited with all (grid-adaptive) fluid, hybrid, PIC and multi-physics solvers already available at the start of the consortium. This helped us select the best strategy for consecutive coupling and integration efforts, where truly multi-physics modelling is pursued. In 3D reconnection, the identification of the magnetic skeleton is a useful aid to unravel the complex topological changes developing in global MHD models. We started with identifying the potential role of using different discretization schemes in global (grid-adaptive) MHD models on the skeleton evolution. This must give us the detailed insight needed for interpreting the large-scale effects in CME initiation studies from WP3. In a similar spirit, we repeated this comparison exercise for shear flow dynamics, as known to be important for local magnetopause conditions targeted in WP4. Again, running similar idealized configurations using both single to multi-fluid models confirm or identify the main physical ingredients, and help separate between physically versus algorithmically driven details of the ensuing dynamics.
We next pursued optimal coupling strategies, in particular in due preparation for the science goals of WP3: starting from detailed flux emergence simulations and/or observationally based extrapolations of the magnetic topology, we wish to make more realistic space weather event initiation models. That required to improve on current strategies, where e.g. in global MHD simulations the static potential background field is subtracted and the problem is reformulated in a (nonlinear) perturbative sense about this equilibrium, to one where the initial conditions from modern extrapolation codes are incorporated much more accurately (possibly designing more sophisticated splittings about force-free states). In the same spirit, we needed to couple flux emergence models to grid-adaptive CME initiation modelling, and that ultimately means that we must be able to handle differing discretizations in various spatially distinct regions of our simulation domain. Indeed, the most advanced flux emergence simulations typically employ high order finite difference treatments, ideally suited to also include more complex, radiation-related physics, while the CME models to date mostly build on modern shock-capturing techniques employing approximate Riemann solver methodologies in finite volume solvers. Having the capability to combine these in a scale-encompassing simulation brought the CME initiation models to the next level of realism. This has been compared to strategies where a much looser coupling is used (currently in effect in some of the US based frameworks), namely one where differing codes get coupled through mutual boundary condition prescriptions.
In a next level of complexity, we deployed differing physics solvers per grid level in AMR simulations. In the first subtask of the work package, we have identified the best hybrid approach, and it is in this stage that we like to bring these together in the same grid-adaptive, multi-physics simulation. This has been done separately for coupled fluid treatments on the one hand (from single fluid MHD, over Hall-MHD, to two-fluid descriptions), and in a PIC type framework exploiting the implicit moment method as pioneered in Parsek3D (initiated by the project coordinator) and related code efforts. In the latter PIC-AMR code, we demonstrated seamless multi-level simulations where the grid changes (in the computation of the electromagnetic fields) and the particle movers/splitters have no adverse effects on the simulated dynamics. Finally, the ultimate goal of this workpackage was to design and implement a novel approach that handles both fluid and kinetic treatments in multi-scale simulations. This combined the AMR strategies for PIC treatments alone on the one hand, and for coupled fluid treatments on the other hand. The implicit moment method is again very suited in this aim, since we readily were able to compute the needed moments and fluxes that connect both descriptions accurately. Ultimately, we then incorporated also the detailed insights gained from kinetic models that go down to thermospheric layers (from WP5) in global CME initiation to arrival modelling. A demonstration was given in Deliverable 2.6 on selected, well-documented space weather events, where multi-spacecraft (especially CLUSTER) info was used for validation.
The codes developed in this work package have been applied in the following application-oriented tasks, and has been delivered to the scientific community as opensource software, complete with detailed documentation on their usage and typical range of validity.
1.3.3 Coupling at the solar surface: photosphere, chromosphere and corona (WP3)
A key objective of global, long-term solar simulations is to determine the 3D coronal magnetic field configurations in which CMEs can occur. For example, one model for the initiation of CMEs is the flux cancellation model, which produces flux ropes that lose equilibrium and are ejected from the corona. To determine the location of formation and subsequent ejection of flux ropes, the global non-linear force-free field simulations has been used. These simulations used realistic photospheric magnetic field distributions (derived from observations) to model the slow evolution of the large-scale coronal field through a series of quasi-static, non-linear force free field equilibria over periods of weeks to months. New magnetic flux in the form of emerging bipoles has been added in accordance with observations to maintain the accuracy of both photospheric and coronal fields. Through this process, the gradual formation of magnetic flux ropes (and their subsequent ejection) has been studied. Once ejections are found, the 3D magnetic field configuration just prior to the onset of ejections has been taken and used as the initial condition for full 3D MHD simulations. The MHD simulations then consider for the first time the dynamics of the eruptions over short times-scales derived from observations. The resulting structures and properties then were compared with observations.
Efficient magnetic reconnection is required to eject structures as large as CMEs and it is extremely likely that the reconnection does not occur at a single site. Instead large-scale current sheets fragment and reconnection occurs at many localised sites. In order to properly understand the energetic consequences of flares and CMEs, it is important to be able to determine these local sites of energy release and to be able to understand the behaviour of the plasma and magnetic fields at these sites on the micro-scale. However, to see the overall global consequences of reconnection one needs to know how the local reconnection sites are linked. It has recently been shown that in a number of situations the magnetic field may reconnect not just once, but many times at neighbouring local sites enabling much more energy to be released than one might initially imagine. (This extra energy comes from a prolonged injection of Poynting flux that is maintained as the magnetic field does not take the quickest route to its lower energy state.) Moreover, this energy release has a longer time duration and a wider spatial area than initially anticipated. A key to understanding CMEs and, in particular, their consequences for space weather, has been to establish how significant a role this type of behaviour plays in the reconnection involved in CME initiation and whether the extra magnetic energy released is converted in to bulk plasma flows or heating. To do this MHD models of CMEs must be initiated from non-linear force free magnetic fields that are driven in order to create a CME, rather than simply starting from a marginally stable magnetic configuration with an initial imposed stressed configuration.
From the empirically based 3D MHD models of the solar (interior and) atmosphere that produce CMEs, we used the new suite of analysis tools in order to determine the key characteristics required for CME and flare initiation and to establish whether there are a range of magnetic configurations and velocity flow patterns that can produce CMEs or whether there is one preferred configuration.
We also studied initialization of energetic solar events based on ab initio models of solar active regions. Such models are just now becoming possible, based upon large scale MHD simulations stretching down at least 20 Mm below the solar surface. When sufficient amounts of magnetic flux are injected at the lower boundary of such models the interaction of this magnetic flux with multi-scale convection can yield realistic models of solar active regions. As in the case of real solar active regions an infinite number of dynamic arrangements is possible, and one of the main challenges of this approach was to learn how to seed the sub-surface magnetic field in such a way that energetic events result in the overlying solar corona when the magnetic fields emerge through the solar surface.
1.3.4 Coupling in space: solar wind-magnetosphere (WP4)
Our final goal was to model the solar wind plasma entry into the magnetosphere by a “global” simulation including large and small scale physics. Such a problem involves an impressive range of scales and frequencies, at least of the order of four orders of magnitude. Until now, the work concerning the low latitude magnetosphere system dynamics has been focused on the basic processes as magnetic reconnection effects, assuming a constant density profile, or as RT effects induced by the initial density jump, assuming a purely perpendicular initial magnetic field. In the “real” case these two effects and the pairing process compete and are strongly influenced by the specific vortex configuration process generated by the KH instability in a magnetised plasma where initially practically all the macroscopic quantities are inhomogeneous and varies not necessarily on the same spatial scale length. It was therefore crucial to establish the role of secondary instabilities on the dynamics of the "real" system, since they strongly influence the increase of the width of the mixing layer and its internal dynamics that are the most important factors for the evolution at the flank of the Earth's Magnetosphere.
From this point of view, the possibility to access CLUSTER and THEMIS satellite data is of crucial importance. This has been made by means of a well established collaboration with the colleagues of the LPP laboratory at « Ecole Polytechnique » in France. We note that at LPP, Anna Tenerani has just started a PhD thesis in partnership (supervisors F. Califano and O. Le Contel) and that Matteo Faganello that made most of the work on this subject in Pisa during his PhD thesis has now a post-doc position at LPP, thus giving to the Pisa group and all other teams here the possibility to access THEMIS and CLUSTER data from the magnetosphere. Furthermore, a realistic model should consider kinetic effects, as for example Finite Larmor Radius effects, particle resonances, wave - particle interaction. Due to the strong difficulties of studying numerically the full physics and geometry of this problem, we need first a description “simple”' enough to follow the inhomogeneous and geometrically complex large-scale motion. At the same time, any model must be capable of describing the small-scale microphysics including effects allowing for “fast” reconnection.
For these reasons, we continued to adopt in the first part of the project a collisionless, two-fluid description of the plasma that neglects completely kinetic effects but allows us to take into account a relatively large range of scales and frequencies, from MHD to electron-MHD. In the second part of this project, we started a kinetic study of the full problem by making use Eulerian, Vlasov low noise, codes for an "accurate" investigation of selected kinetic processes from the full system evolution, as for example the formation of coherent kinetic structures. A Hybrid approach (kinetic ions, fluid electrons) in Eulerian and Lagrangian formulations has been also used as a complementary tool to reduce the computational cost of kinetic simulations.
In summary, the main points that we developed are:
1. Nonlinear 2D evolution of the two-fluid model. We used "realistic" initial conditions of the macroscopic fields as extrapolated from satellite data. We investigated the role of the main parameters of the system and study the influence of the magnetic tension on the onset of the RT instability that, in turns, could force magnetic field lines to reconnect.
2. Nonlinear 3D evolution of the two-fluid model. The first relevant 3D effect studied is the effect of a small equilibrium variation along the perpendicular direction that can affect the pairing mechanism. Furthermore, the 3D geometry can change the large-scale flow plasma compression responsible for the formation of sub-di current sheets structures. Finally, a 3D configuration allowed us to study the role of the connection of magnetospheric field lines to the Earth.
3. Role of kinetic effects. A) We modelled some of the most relevant kinetic effects (ex.: FLR effects), still adopting a fluid approach. B) We used results of two-fluid simulations in order to localise the regions where, locally, kinetic effects could be relevant and to extrapolate the initial conditions for reproducing the same dynamics, but using now a full kinetic Vlasov approach. A typical example is the formation of magnetic islands by the secondary magnetic reconnection instability observed in two-fluids simulations. C) We made use of 2D and 3D PIC codes. This is probably the most important step concerning the possibility of considering a large-scale system but including all kinetic effects. Such simulations are obviously computationally heavy to be performed and the analysis of the corresponding data a complex task. Therefore, this approach has been performed after a first screening made by 2D and 3D two-fluids investigations.
4. We applied the mathematical - numerical global models developed in WP2 to obtain a self-consistent fluid to kinetic description of Solar Wind interaction with the Magnetosphere at low latitude focusing on transport problem. This allowed us to introduce our results in the context of the space whether forecasting problem.
1.3.5 Coupling at the Earth: magnetosphere-ionosphere-thermosphere coupling (WP5)
To investigate the chain of processes in the inner magnetosphere during quiet and disturbed periods, we started from physical kinetic models developed at BISA. These models were mainly developed to study the physical mechanisms implicated in the dynamics of the magnetospheric regions. In the present project, they has been improved by using constraints coming from empirical satellite observations. They has been used to develop nowcasting and forecasting models that have a direct application for space weather. The solar wind-magnetosphere coupling models developed in WP4 has been used as inputs to predict levels of geomagnetic activity.
Concerning the plasmasphere, the 3D kinetic model of the plasmasphere developed by Pierrard and Stegen (2008) has been improved by including the temperature and the ion composition, as well as the dynamic variability of the plasmaspheric density and plasmapause location as a function of day, season, solar-cycle, and storm-time variability.
The model has been constrained by empirical observations used mainly as boundary conditions. The model has been validated by comparisons with recent observations, mainly from CLUSTER and IMAGE satellites. EUV from IMAGE gives a global view of the Earth’s plasmasphere when the spacecraft flights above the North Pole. This is especially useful to determine the dynamics of the plasmaspheric region.
A forecasting and nowcasting model has been developed for quiet and disturbed periods: the user will introduce a date (the time), a point or an orbit in space. Deducing the geomagnetic activity index Kp, the model will predict the number density of electrons, protons and other particle species, the temperature and the position of the plasmapause as a function of the magnetic local time.
The model of plasmasphere has then been coupled with the ionosphere. The empirical International Reference Ionosphere (IRI) has been used for this task. This extend the model to lower altitudes. The IRI model gives values from 200 to 700 km. The plasmaspheric model gives values from 2000 km of altitude to 9 Re. The coupling in the transition region from 700 km to 2000 km requires specific investigations. The model has been used to determine the Total Electron Content (TEC) and its large scale variations that affect GPS and radio signals. The effects of plasmaspheric plumes formed during geomagnetic storms on the dynamics of the magnetosphere especially affects the TEC. The coupling with the thermosphere and especially the heating has been investigated with the MSIS empirical model that provides thermospheric temperature and density based on in-situ data from seven satellites and numerous rocket probes. The model coupling the plasmasphere, the ionosphere and the thermosphere has been implemented on a space weather website as a tool for the scientific community.
The energetic particles of the radiation belts are especially important for space weather due to their effects on spacecraft and the health of astronauts. We studied the dynamic of the radiation belts by using SAC-C, CLUSTER and other satellite observations provided on SPENVIS (www.spenvis.oma.be). This improved the steady state average empirical models generally developed to estimate the radiation flux encountered by spacecraft along their orbit.
The goal was to determine the flux enhancement and the decay time after a geomagnetic storm. The flux enhancement following the recovery phase of a geomagnetic storm depends on the driver by a Corotating Interaction Regions (CIR) or a Coronal Mass Ejection (CME). The solar wind parameters are thus important input for forecasting. We determined the most appropriate solar wind parameters to be used for prediction (solar wind velocity, number density, pressure, magnetic field direction…). The plasmasphere-radiation belt coupling has been provided by investigating the relationship between the position of the plasmapause and the internal border of the external radiation belt.
A forecasting model for space radiations has been developed: the user introduces a point, respectively an orbit, and the time. Regarding the solar parameters and/or geomagnetic indices, the model calculates the real-time fluxes or forecasts. Such predictions are useful for flying spacecraft since the energetic particles of space damage electronic circuits and solar panels of satellites. The radiation model has been implemented on a space weather website as a tool for the scientific community.
The description of the inner magnetosphere has been extended at high latitudes with a model for the terrestrial polar wind. Based on the kinetic models developed at BISA, a study of the photoelectrons has been provided to determine more precisely the flux of ions escaping at high latitudes. The model has been compared to empirical observations. Moreover, the polar wind escape is in some ways similar to the solar wind escape, so that their theoretical studies followed the same historical evolution. The comparisons with solar wind models and especially between models based on the kinetic and the MHD approaches improved our understanding of these phenomena.
The forecasting model of the polar wind uses time as an input. Determining the geomagnetic disturbance, it predicts the escape flux of light ions like hydrogen and helium. The polar wind model has been implemented on a space weather website as a tool for the scientific community.
Potential Impact:
Space weather refers to conditions on the Sun, in the interplanetary space and in the Earth space environment that can influence the performance and reliability of space-borne and ground-based technological systems and can endanger human life or health. Adverse conditions in the space environment can cause disruption of satellite operations, communications, navigation, and electric power distribution grids, leading to a variety of socioeconomic losses. The conditions in space are also linked to the Earth climate. The activity of the Sun affects the total amount of heat and light reaching the Earth and the amount of cosmic rays arriving in the atmosphere, a phenomenon linked with the amount of cloud cover and precipitation. Given these great impacts on society, space weather is attracting a growing attention and is the subject of international efforts worldwide.
SWIFF aimed at forming a wide collaboration between teams dealing with different fields of solar- space- and geophysics. The main goal is to create a framework for space weather modelling in the form of: a) mathematical models suitable to represent the physics processes behind space weather events, b) computational methods to solve such models, c) software to form a integrated framework to model space weather events. The outcome of the SWIFF research activities will led to an improvement of the methods that can be used to predict space weather events, and their near Earth space and terrestrial impact (geoeffectiveness). The achieved developments and outputs are expected to contribute to the future development of space weather science and space weather forecasting, with a legacy continuing long after the end of the project.
Our activity was developed with the goal of impacting two major aspects of the European research in space weather:
1. Addressing the needs expressed by the document European Commission C (2009) 5893 of 29 July 2009, under theme 9 – Space, with specific focus on the Area 9.2.3: Research into reducing the vulnerability of space assets as expressed in the call: SPA.2010.2.3-01 Security of space assets from space weather events.
2. Forming the fertile ground for the future activities of ESA in the area of Space Situational Awareness (SSA), as expressed in the plans of the First phase: “SSA preparatory Programme” of ESA presented at the Space Weather Working Team meeting in Brussel in September and in documents presented there (http://www.esa-spaceweather.net/index.html).
Additionally SWIFF addressed fundamental issues and developed tools of general interest to space research as described in the same EC document mentioned above under the other areas of the Activity: 9.2. Strengthening the foundations of Space science and technology.
Below we address in turn each of these three goals, and we refer in quoted italic passages taken directly from the document European Commission C (2009) 5893 of 29 July 2009.
1.1 Relevance of our project to the aims of the EC theme 9 space activity on research into reducing the vulnerability of space assets
As noted in the work programme: “Space weather gives us displays of the aurora, or northern lights. Aside from this visible effect on the atmosphere, solar activity modulated effects on the Sun and in the helio- and magnetosphere affect the entire Earth environment from the magnetosphere down to the ionosphere and even to the lower atmosphere climate system. At its worst, space weather is a natural hazard which does not only modify the atmosphere but also can catastrophically disrupt the operations of many technological systems, thus causing disruption to people’s lives and jobs. Space storms (particles, plasma or electromagnetic) are a recognised aerospace hazard and can cause major failures, e.g. onboard spacecraft, in electrical power grids, in telecommunications links (satellite, launcher and groundbased) and in navigation systems. During the current approach of the next solar maximum (around 2012), more accurate prediction, assessment and early-warning capabilities of disruptive events that are to be expected as part of this cyclical phenomenon are particularly poignant”
The space weather studies supported by SWIFF addressed this fundamental need expressed in the work programme. In particular we focused on the fact that “Collaborative Research projects are invited to focus on research areas such as:
– early warning and forecasting methods to allow for a mitigation of space weather effects on humans in aerospace vehicles and on vulnerable technologies in space (in particular satellites, communication and navigation systems) and on the ground (communication and power nets)...
– space weather models to improve specification and prediction capabilities, with emphasis on the linkage of the different physical processes that occur simultaneously or sequentially in many domains.”
SWIFF is exactly this: a collaborative effort among some of the prime centres in Europe with expertise on producing space weather models focusing especially on the linkage (coupling) of different physical processes that occur simultaneously and sequentially with the goal of producing a integrated framework for forecasting and early warning.
The whole structure of SWIFF is centred around one the development of model and software (WP2 – Multiphysics and multiscale modelling for space weather) designed with in mind specifically the multiscale and multiphysics linkage present in space weather from its source on the Sun (WP3 - Coupling at the solar surface: photosphere, chromosphere and corona) in its evolution in space up to the Earth (WP 4 - Coupling in space: solar wind-magnetosphere) and its impact on geophysics and on human activities in space and on the ground (WP5 - Coupling at the Earth: magnetosphere-ionosphere-thermosphere coupling).
The focus of SWIFF was not only to provide a physics-based description of space weather events for the interested academicians and scientist, but it to produce methods and software tools directly aimed at the users with emphasis on being able to produce services that can be practical in forecasting activities to allow the mitigation of the impact on humans and technology. We aimed at taking at heart the requirement that: “Collaborative Research Projects are expected to significantly contribute to the European capacity to prevent damage / protect space assets from space weather events. Projects are expected to significantly contribute to both improving forecasts and predictions of disruptive space weather events. Projects are also expected to identify best practices to limit the impacts on space- and ground-based infrastructures and their data provision.” We addressed this by focusing on physics-based models that bring in the underlying physics at the kinetic level that was missed by the pre-existing fluid models. The processes at the center of space weather are fuelled by processes of energy release (wave-particle interaction, shock, magnetic reconnection) that the fluid approach cannot represent with fidelity, requiring particle-based kinetic models. In particular, particle acceleration behind SEP events requires kinetic treatment. The development of suitable fluid and kinetic models that can be linked and used concurrently has been the crown achievement of SWIFF. This is a great achievement well beyond space science: the same tools can and are already being picked up in other fields: industrial plasma applications and nuclear fusion research.
The foreground produced by SWIFF is in fact of general use and availability as described below in our dissemination plan and as requested in the often-mentioned EC document:
“The work programme will support European coordination activities both to ensure the open exchange of information on emergencies that may have been caused by space weather events, with the goal of structuring international and European research efforts.“
1.2 Complementarity with ESA activities and in particular the ESA Space Situational Awareness Preparatory Programme (SSA)
The SWIFF team operated under the inspiring guidance of the words of the document European Commission C (2009) 5893 of 29 July 2009: “It is expected that the project will be complementary to and clearly demonstrates an added value to the efforts already carried out in this field by ESA or national agencies.”
Our effort was designed with complementarity in mind and with the aim of not replicating any effort currently planned in the First phase: “SSA preparatory Programme” of ESA.
The plans for SSA call for Space Weather Service Elements that call for:
• Space environment modelling for spacecraft design
• Space environment modelling for SSA survey and tracking
• Data service (NOAA SWPC type – including integrated numerical modelling element).
Especially for phase 1 of the planning, the SSA plans called for the establishment of a Virtual Space Weather Modelling Centre (VSWMC). This SSA activity is currently ongoing and was awarded to a Begian team coordinated at KULeuven (by Poedts), two doors down the same corridor where SWIFF is coordinated (by Lapenta). This served as a fantastic opportunity that we did not let slip from our hands. The software developed by SWIFF is being deployed into the VSWMC and the expertise gained by SWIFF is equally benefitting the VSWMC. OF course, in the inclusive spirit of considering all developments in Europe and internationally.
SWIFF has been ideally suited for making the groundwork for this effort. Most of the members of SWIFF belong to member states who are also ESA member state who support the Space Weather Element of SSA. SWIFF was a stepping stone to gather the competence and the basic integrated framework that will allow to refine progressively more the forecasting ability of SSA.
SWIFF did not have the goal of developing a full package for space weather modelling. That would have been a wasteful repetition the effort ESA is doing. We formed the basic mathematical, computational and software know-how and base of tools able to study the space weather problems. The key missing piece before SWIFF was the inability of previous models to sufficiently account for the linkage (or coupling) of different physics developing on different scales. This is at the core of all space weather: in the solar corona, at the interaction of the solar wind with the Earth magnetosphere and at the interaction of the magnetosphere with the Earth atmosphere, the scales of the microphysics and macrophysics are tightly coupled and different physical processes interact tightly. SWIFF filled this gap by developing and deploying the methods and software to handle this. Without this modelling space weather based on the fundamental physics understanding would have been impossible.
The SWIFF actions have been perfectly complementary with ESA, the SSA will collect the existing expertise and the new products developed by SWIFF not only in the ESA Virtual Space Weather Modelling Centre but also in the subsequent activities that are following. The efforts do not overlap wastefully but complement each other synergistically: we develop the necessary tools, SSA will develop the final forecasting system.
1.3 Complementary with other European Commission funded research
Besides striving for complementarity with ESA funded project, SWIFF worked on situating itself in a synergistic relationship with other EC-FP7 funded projects. SWIFF was perhaps unique in its goal of addressing the fundamental need to develop methods to do a physics-based forecast of space weather events. This situated SWIFF in the central position of needing from one side input from data-focused projects to provided the needed observational input for our models and from the other side of providing new tools to be used by more application oriented projects.
For the first task, SWIFF found itself in the lucky situation of gathering help from the former FP7 project SOTERIA that focused on data archives and product specifically focusing on space weather. SOTERIA and SWIFF were both coordinated by Lapenta, although no other partner of one was member of the other. Nevertheless the common coordination and the focus to being open to the public of both projects provided fantastic opportunities fully exploited by SWIFF.
For the second task, the difficulties to overcome where more severe because of the notorious “not invented here” syndrome: the reluctance of groups in using tools developed by other tools. But SWIFF was not scared by this well-known difficulty and confronted the situation head on. We engaged in several activities:
• Building of teams across FP7 projects, such as those sponsored on themes of WP5 by the ISSI institute of Bern to discuss radiation belts
• Organization of events to foster cross fertilization, such as the session on space weather modelling organized by Lapenta and Aylward at the 2012 european space weather week
• Personal contact with other FP7 projects and with other teams in Europe working on space weather
• Participation to special issues and common review documents aimed at disseminating the activities of the different FP7 projects on space weather funded by the same EC call.
All these activities built progressively more momentum for the use and dissemination of SWIFF tools and to provide SWIFF with the needed input on the needs of other projects and of end users.
1.4 Relevance to other aims of the activity 9.2 of the EC theme Space
Besides focusing on reducing the vulnerability to space weather events by focusing on the two activities above of the EC and of ESA, we also made great contributions to the wider goals of the European research as indicated in the activity 9.2 (strengthening the foundations of space science and technology), as laid out in the work programme for cooperation in Space. Quotations in italics in the text below are from the work programme on theme 9 -Space, document: European Commission C (2009) 5893 of 29 July 2009.
The work programme aims at “Providing R&D support to the foundations of Space science, exploration, space transportation and space technology through synergies with initiatives of ESA or other European, national or regional entities.” SWIFF focused on bringing to bear in a integrated plan activities that are currently funded by local governments, expanded them and connected them to form a collaborative network where the parts synergistically produce a sum that is able to address a problem not previously possible without a pan-European scope: creating the basis for a complete space weather forecasting model and software platform. SWIFF produced software and methods of general interest beyond just addressing the vulnerability of space assets. We did that, but we did it following the path of great science that will produce along the way new and useful products (models and software) whose scope reaches every aspects of space science.
A specific strength of SWIFF has been to address the need to gather competences and existing data gathering and existing software from old and new EU member states. The coverage is not merely geographical: each team from each country contributed a crucial aspect of the understanding of space weather processes, contributing a portion of the methods, of the observations and of the expertise in for which they are the apex of competence. As noted in the work programme, “a particular added value is also seen in contributions which the new EU Member States and the international community can make.”
An additional asset of the SWIFF project was its link to international activities beyond the European union and beyond Europe. Connections by partners of the SWIFF collaborative network have been supported with similar efforts in the United States of American, China and Japan. While no funding exchange was executed, the SWIFF consortium included expertise from international collaborations. Satellite data and ground-based data from around the world were used in our analysis in collaboration with international partners. An example of such activities is the involvement of the Coordinator of SWIFF with the Colorado and UCLA teams of the Magnetospheric Multiscale Mission of NASA, where he is a co-investigator. Similar examples are present in all partners. Great synergies have been achieved by sharing tools and scientific insight with international projects.
The breath of the international partnership of the proponents of SWIFF directly addresses the work program request: “In the context of International cooperation, a diversified approach is a key element in Europe’s space policy. Candidates for cooperation among other established or emerging space powers are the United States, Russia, Canada, People’s Republic of China, India, and Ukraine.“
Furthermore attention is paid to the requirement of ”promoting the contribution of space assets to the scientific and technological knowledge and foster its transfer to educational bodies”.
List of Websites:
www.swiff.eu
Contact: Giovanni Lapenta
Dept Wiskunde - KU Leuven
Celestijnenlaan 200b - bus 2400
3001 Leuven
Belgium
tel. +32 16 32 79 65
Space weather refers to a complex condition state resulting from the Sun activity, propagating in the interplanetary space and possibly affecting the Earth space environment, which all together can influence, not only the performance and reliability of space-borne and ground-based technological systems, but more significantly can endanger human life or health. Adverse conditions in the space environment can cause disruption of satellite operations, communications, navigation, and electric power distribution grids, leading to a variety of socioeconomic losses. In addition, the space weather influences the Earth climate. It is well known that the Sun activity affects the total amount of heat and light reaching the Earth and the amount of cosmic rays arriving in the atmosphere, directly contribute to a phenomenon responsible for the amount of cloud cover and precipitation. Given these crucial impacts on society, the space weather is attracting a growing worldwide attention and progressively increases presence within international research projects.
The actors of space weather are the Sun, the Earth and the vast space in between. Like any star, the sun is made of a highly energetic and highly conductive gas, called plasma. In plasmas, the atoms have been broken into their nuclei and their electrons that become free to move. The hot plasma of the Sun is confined by gravity and moves in complex patterns that produce large currents and large magnetic fields. The gravitational confinement is not perfect and a highly varying outflow of plasma, called solar wind, is emitted from the Sun to permeate the whole solar system, reaching the Earth. The Earth has itself a magnetic field. The Earth magnetic field makes compasses point towards the north pole and allows many species of animals to find their way during migrations. This very same field protects the Earth from the incoming solar wind and its disturbances. Only a small part of the particles can reach the Earth surface, the so-called cosmic rays. Most of the incoming plasma is stopped and deflected, reaching only high strata of the atmosphere at the polar regions and causing the aurora seen by the people of the northern latitudes as the northern lights.
To describe and predict such space weather processes, both electromagnetic fields and plasma particles need to be modelled and simulated. The nuclei (mostly protons, the nuclei of hydrogen) and the electrons of the plasma are loosely coupled, where each species has its own typical scales. The electromagnetic fields keep the species coupled forming a very non-linear and multi-scale system.
SWIFF (Space Weather Integrated Forecasting Framework, swiff.eu) aims at addressing the crucial challenges in the modelling and forecasting space weather: the presence of multiple scales and multiple physics.
Multiple physics is present because the conditions found in space are widely different in different regions spanning many orders in density range, intensity of the magnetic field and temperature. High density relatively cold plasmas (e.g. the photosphere and the ionosphere) are collisional and are best described by fluid models. Dilute and hot plasmas (e.g. the Earth magnetopshere) are collisionless and require a kinetic treatment. Additionally processes of radiative emission, chemical reactions and many others need to be included in certain regions, making the need to handle multiple physics models interacting with each other a key requirement for space weather.
Swiff wants to join in a integrated framework fluid and kinetic models under the same roof able to work together to provide a full description of all coupled scales.
Project Context and Objectives:
The Sun-Earth connection is a much more dynamic and eventful system than the common-life experience of looking at the sky can suggest. The Sun is far from a stationary ball emitting light. Instead it produces a highly diverse and varying flow of plasma, called solar wind, and a magnetic field that is ejected towards the rest of the solar system, interacting actively with the Earth. The term "Space weather" refers to conditions on the Sun and in the solar wind, magnetosphere, ionosphere, and thermosphere that can influence the performance and reliability of space-borne and ground-based technological systems and can endanger human life or health. Adverse conditions in the space environment can cause disruption of satellite operations, communications, navigation, and electric power distribution grids, leading to a variety of socioeconomic losses (Definition used by the U.S. National Space Weather Plan (http://www.ofcm.gov/nswp-sp/text/a-cover.htm)).
The study of space weather has become key in our technologically advanced society. Space weather disturbances can adversely affect the health of humans in space and in high altitude commercial flights (especially those on polar orbits flying over northern Canada and the north pole but also many of those over Europe as well). The astronauts on the space station are subject to a high risk even while partially shielded by the Earth 's magnetic field (so much so that the space station has a specially shielded area to seek refuge during space weather storms). But future missions to the Moon or Mars are critically affected by space weather effects, to a degree that it is currently expected that, without proper actions, the life of at least one astronaut would be lost due to radiation exposure generated by space weather events. Technology is also very susceptible. Satellites have been lost to space weather storms and critical land infrastructures (power lines, pipelines) have been disrupted during particularly severe space weather storms.
There is critical realisation at the political and scientific level that action should be taken to address space weather issues. If meteorological predictions are a good paradigm, to achieve the same level of predictive capability for space weather two directions need to be followed. Modern weather forecasts, shown for example on TV before/after the newscast, are based on two critical developments: the existence of a wide network of measuring stations and of meteosats, and on the existence of sophisticated computer models to use the observational information to project conditions in the future. This is a great advance over what was typical a generation ago when General Eisenhower decided the day of D-day based on the educated guess of expert meteorologists that could not have any meteosat or computer model to help. While great progress has been made in space science, we are comparatively, for space weather forecasting, still in the modus operandi of meteorology in the days of General Eisenhower.
However unlike those days, Europe is relatively affluent and at peace. A plan is in the making to understand and control space weather: space situational awareness (SSA) now well under way. Of the various goals of the SSA, the present project developed the cloest relation specifically with the virtual space weather modelling (VSWMC) effort. Numerical modelling is one of the two pillars that have advanced regular weather models. A number of advances numerical algorithms at centres around the world have been created to model the evolution of the atmosphere and oceans in order to predict the weather of the following few days. The eventual goal is to have the same capability for space weather.
Here, we are not trying to precede the SSA effort. ESA will lead the way for the creation of a space weather modelling effort under its virtual space weather modelling effort. Instead, we aimed at preparing the ground and show the way to follow in establishing a framework for that effort. This has been our goal:
1. Zero-in on the physics of all aspects of space weather and design mathematical models that can address them.
2. Develop specific computational models that are especially suited to handling the great complexity of space weather events where the range of time evolutions and of spatial variations are so much more challenging than in regular meteorological models.
3. Develop the software needed to implement such computational models on the modern supercomputers available now in Europe.
Other similar efforts are underway in the rest of the world. The US, even leaving aside the numerous and important military programs, funds several centres to model space weather (e.g. the NOAA space weather forecasting centre in Boulder, the CISM centred in Boston and funded by NSF, the CCMC funded by NASA and the SWMC in Michigan to name just perhaps some of the most important). Japan has recently initiated one similar centre following the example of the Americans.
Europe needed to act in this direction too. We implemented with Swiff a plan that, while taking full advantage of the American experience, intended to follow a different path, designed especially to combine synergistically the current strengths of Europe while avoiding any weakness to become a hindrance in reaching our goal. But before outlining our plan, we would like to stress our admiration and support to other approaches followed elsewhere. The coordinator has worked in the US system until 2 years ago and has learnt there to appreciate the great advances of the American research in this field. We fully intend to continue our collaborations in the US. While explicitly not part of this effort, we count on our many sources of input, support and collaboration with the US. However, as explained above, we will follow a different path.
The American way has been to leverage on their superior base of existing tools to mesh them together, by making different codes, developed for other related activities, talk to each other. This has of course the advantage of leading to a quick development. However the experience has been that the models and codes present serious challenges in their coupling and models can be so fundamentally incompatible as to make the progress difficult and uncertain. For this reason, our approach was different and followed the steps already mentioned above.
1. A strength of the European research is that not having had the tools and the shear financial size of the American system we have not crystallised into established widely used software tools and we have left more room to the investigation of different and more risky options. Instead of basing the software on the methods and knowledge of the 60's and 70's like some of the national labs in USA have tended to do, more research has gone into new innovative models and computational methods. Not that research has not gone in this direction in the US also, and indeed one of the key tools we will use was developed there (by the coordinator). But the early establishment of huge gorilla-codes has hindered the acceptance of new methods.
2. The existence of a wide range of new ideas and new methods in Europe was used as a springboard to design space weather models from the bottom up. By analysing
• at the physics present
• at the mathematical models needed to represent it
• at the algorithms best suited to simulate the models on computers
• at the best software to implement the models on modern supercomputers.
Instead of trying to piece together relatively incompatible software originally designed for other purposes, we designed models and tools from the beginning to be a seamless description of space weather. Nothing needed this more acutely than the coupling of large scale processes best described by macroscopic methods (fluid approach) with small scale description of microscopic processes (kinetic approach). These are the regions where the American software plug-in approach has found the most difficulties and needs a new fresh perspective. We provided such new perspective by redesigning the modelling with the focus immediately on these regions of coupling. The task of Swiff has been to develop methods to couple and analyse multiple methods and codes to treat compled multiscale systems.
The project has been structured around:
• developing of models and software for space weather (WP2)
• studying the coupling at the Sun (WP3)
• studying the coupling in space between the solar wind and the magnetosphere (WP4)
• studying the coupling between the Earth's atmosphere and the Earth-space environment (WP5)
A management work package (WP1) and a dissemination work package (WP6) complete the overview. The management and dissemination WP are described in Section B2 and B3, respectively. The other science WPs are described below.
Project Results:
The fundamental goal of the SWIFF project has been to progress beyond the state of the art in two ways.
First, in the development of mathematical models and computational methods and software especially designed to handle the multiple physics and the multiple scales characteristic of space weather phenomena. The project intended to produce an integrated forecasting framework able to model space weather events from their solar origin to the impact on Earth and its space environment, focusing on the effect on humans and technology in space and on the ground infrastructure. This is the goal of WP2.
Second, we intended to demonstrate for practical space weather events and processes the validity and usefulness of the SWIFF integrated space weather forecasting framework. The WP 3 to 5 include all phases of space weather events and have pushed forward the state of the art in modelling specific space weather events and processes.
Below the progress in both directions is described in detail.
1.2.1 Multiscale modelling for space weather (WP2)
Computationally, both fluid and kinetic approaches have been dominated by explicit methods. Explicit techniques are well-known for being conceptually simple and of straightforward implementation. In the context of multiple scale modelling where the kinetic scale has to be considered, the limitations arising with the use of explicit methods in the treatment of fluid models, become a minor consideration when compared with the limits imposed by the explicit discretisation of the kinetic level of description. For kinetic modelling, two constraints need to be satisfied. First, they need to resolve the fastest time scale (typically the electron plasma frequency for space plasma simulations) supported by the model, which may be orders of magnitude faster than the dynamical time scale of interest. Furthermore, explicit methods need to resolve also the smallest spatial scale, the Debye length that is typically several orders of magnitude smaller than the other scales of interest. Given these limitations, the fact is that 3D explicit fully kinetic simulations for realistic choice of parameters will remain out of reach in the foreseeable future. Therefore, we have taken a different approach which serves as the motivation for the proposed work.
A well known technique for handling multi-scale problems is Adaptive Mesh Refinement (AMR). However, an explicit full particle code with AMR capability will still keep the problems of space weather out of reach for realistic parameters. The reason is that the time step limitations suffered by explicit methods will still be present in any AMR explicit PIC code. This limits the advantage that AMR can offer. As a result, we propose to couple the use of implicit kinetic methods which removes the restriction of Debye length resolution and of time step resolution, so opening the possibility to take full advantage of AMR, resulting in performance advantage of several orders of magnitude over uniform-grid explicit schemes. In this context, instead, it is not crucial to rely on implicit modelling of the fluid description, as the time scales involved in that are already much longer than the kinetic scales and typically need to be resolved to achieve sufficient accuracy.
Implicit methods allow one to handle fast time scales and small length scales to resolve the dynamical scales of interest. Implicit PIC methods have been shown to be free from both spatial and temporal limitations, and as a consequence have been able to push the limits of kinetic plasma physics simulation to regimes where explicit methods have not been able to reach, even with the use of powerful supercomputers. Thus, implicit PIC approaches not only deal with multiple time scales, but are by design ideally suited to deal with multiple length scales in a reliable manner. Fluid simulations have also benefited from fully implicit approaches, as demonstrated in resistive and Hall MHD. Pioneering work in implicit MHD treatments show that it is possible to step over fast time scales without compromising accuracy or efficiency, reporting CPU speed-ups of implicit versus explicit methods of an order of magnitude.
Implicit kinetic plasma simulation has taken essentially two lines of investigation that over the years have converged to a similar framework and the distinction has become largely of historical interest. The two approaches are: the direct implicit method [Hewett, Langdon et al.] and the implicit moment method. Both led to well established codes with a wealth of applications. The first method has been developed primarily at Lawrence Livermore National Laboratory (LLNL) and the second has been developed primarily at the Los Alamos National Laboratory (LANL). The present effort takes full advantage of all the progress made to date on each of the methods, but relies primarily on the implicit moment method for two reasons. The first is practical, one of the proponents has been long involved in the development of the method and of the modern codes based on them. The second is theoretical: the implicit moment method is based on solving the kinetic equations by coupling them with the moment equations (i.e. with fluid equations): since it speaks both the kinetic and the fluid language, it is naturally suited to handle the task of using fluid equations in some regions and kinetic equations in other.
The major application for the hybrid multi-scale simulation tool is to address magnetic reconnection, going beyond the detailed inter-code comparisons in the context of the well documented GEM and Newton challenges. The challenges are to revisit the many recent insights obtained by the various teams involved in this project, realising both 2D and 3D multi-scale simulations in especially collisionless magnetic reconnection regimes. We intend to perform detailed simulations tailored to plasma configurations relevant for coronal mass ejections in the solar corona, heliospheric current sheet conditions, and magnetospheric reconnection sites. In all of these settings, the role of cospatial shear flow combined with magnetic shear is to be explored in detail. Using the hybrid simulation suite, we aim to quantitatively address the role of incorporating ideal to Hall-MHD, single to multi-fluid, as well as fluid to kinetic aspects under realistic and reproducible initial and boundary conditions.
A major challenge in modelling many space and laboratory reconnection events is to reproduce the observed (high) reconnection rates. For this it is crucial to know if, at the small scales, the process is laminar or turbulent. At these small scales, fluid modelling has to be complemented with kinetic modelling of the charged particle velocity distributions. While the plasma fluid turbulence exhibits vortices and shear flows in 2D or 3D, the energy dependent particle trajectories in the kinetic model effectively extend the turbulent fluctuations to a higher-dimensional phase space, which may lead to enhanced reconnection or to collisionless damping due to phase space mixing. Using the many (hybrid, PIC and grid-adaptive) codes that we jointly share at the start of this project, we could immediately perform a first detailed comparison of the different algorithmic approaches within the first half year of our efforts, and swiftly moved on to develop and integrate this knowledge in the targeted multi-scale approach.
1.2.2 Coupling at the solar surface: photosphere, chromosphere and corona (WP3)
Models of the source regions of solar energetic events can be developed along two paths; 1) they can be based on empirically deduced magnetic field arrangements and topologies, or 2) they can be based on realistic numerical models of solar active regions. We plan to use a combination of these two approaches; while gradually developing increasingly realistic models of solar active regions we indeed in any case need to be able to check if the magnetic flux emergence and magnetic stress build-up in the models is sufficiently similar to observed situations. In particular, we need to evaluate if the models generate similar statistics with respect to the frequency and severity of the generated energetic events, and if these events in turn are able to drive space weather with statistics similar to the observed space weather.
CMEs can be triggered by newly emerging magnetic fields that destabilise the overlying coronal fields. The state-of the art models for the initiation of CMEs use either an imposed shearing motion at the photosphere to create the conditions for an eruption or an imposed time evolution of the radial component of the magnetic field at the computational base to simulate the emergence of new magnetic fields. Simulations of flux emergence have shown that the shearing motions and high-density eruptions can occur in a self-consistent manner in response to the Lorentz force generated by emerging magnetic fields and the interaction with the over-lying coronal field. These simulations can provide the appropriate boundary conditions on magnetic field components and horizontal velocity as input to the solar wind codes.
Many models of the coronal magnetic field are constructed at a single instant in time through either potential or linear force-free extrapolations from observed photospheric magnetograms. However, these instantaneous descriptions of the coronal field do not take into account any previous history of magnetic interactions and the slow build up of non-potential effects. To take these into account over long periods of time (months to years), the evolution and injection of electric currents due to flux emergence, differential rotation and the meridional flow must be considered. Such processes have been developed in the quasi-static, non-linear, force free model of Mackay and van Ballegooijen (2006) that uses observed magnetograms. To date this model has successfully explained the large-scale transport of helicity across the Sun and the associated chirality of solar filaments. In addition, it has a 50% success rate in predicting the sites of onset of CMEs (far exceeding any other model). The aim is to use the magnetic field distributions produced by this code as input into full 3D MHD simulations, allowing us to follow the dynamical evolution of CMEs as they are ejected from the Sun.
In order to produce realistic simulations of the Sun, realistic boundary conditions must be applied to the numerical models. In many cases this involves applying the magnetic field and velocity on the model solar surface. These can be supplied through the use of magnetic feature tracking algorithms that can extract the important elements of observed photospheric magnetograms.
To identify and understand the fundamental physics processes that are behind specific plasma behaviours requires the use of specially designed focused analysis tools. Members of the St Andrews node have been focusing on developing such tools and have tested a state-of-the-art suite of algorithms designed to work on general numerical data to determine the magnetic skeleton (and quasi-skeleton) of the magnetic field. Knowing these sites is important as they enable many of the key locations of magnetic reconnection to be identified and also the locations at which waves may be focused and hence dissipate.
The possibility and means to develop realistic models of solar active regions have emerged over the last few years, as a consequence of the ever increasing computational capacity. This has led, on the one hand to the possibility of modelling the structure of individual sunspots and pairs of sunspots, as done by Rempel et al (2008), and on the other to the possibility of modelling the multi-scale structure of convective motions that probably are the cause of the near self-similarity of magnetic field structures emerging through the solar surface, as done by Stein, Lagerfjärd and Nordlund (2009). These models needed to be extended to larger scales, and studies needed to be made with respect to the amount of spatial resolution that is necessary to obtain quasi-realistic results.
The sub-surface and photospheric models then needed to be coupled to models that incorporate chromospheric and coronal physics. Such models have been developed in recent years by Gudiksen and Hansteen and collaborators in Oslo, and the methods needed to represent the chromospheric and coronal physics with reasonable accuracy are by now well developed. As part of WP3 we developed and carried these investigations forward, reaching the goal of modelling energetic, space weather driving events ab initio.
1.2.3 Coupling in space: solar wind-magnetosphere (WP4)
The solar wind-magnetosphere coupling strongly depends on the solar wind properties and their variability. The connection between the solar wind and magnetosphere is mediated through the magnetosheath and magnetopause boundaries. The properties of the magnetosheath shocked plasma and its electromagnetic field near the magnetopause determine the magnetopause location and properties including the position of the reconnection regions and its stability.
Satellite measurements have supplied clear evidence of rolled-up vortices at the flank of the Magnetopause. In this region, the Kelvin - Helmholtz instability (KHI) has been proved to play a crucial role in the interaction between the solar wind and the Earth's magnetosphere and to provide a mechanism by which the solar wind enters the Earth's magnetosphere. The understanding of the mechanisms responsible for the solar wind entering in the Earth's magnetosphere is of great importance in the context of the physics of the magnetosphere and, as a consequence, for space weather phenomena such as magnetic storms and aurorae.
The KHI has been invoked as a possible mechanism to account for the increase of the plasma transport since during northward magnetic field periods, when magnetic reconnection is considered as inefficient, a relevant mixing between the solar wind and the magnetospheric plasma is observed, even larger than during southward configurations. KHI instability driven by a velocity shear can grow at low latitude since the nearly perpendicular magnetic field does not inhibit instability developments. This provides an efficient mechanism for the formation of a mixing layer and for the entry of the solar plasma into the magnetosphere, explaining the efficient transport during northward solar wind periods. Several observations support this explanation and show that the physical quantities observed along the magnetopause flank at low latitude are compatible with a KH vortex. The vortex formation process drags the magnetic field component parallel to the solar wind direction into the flow. The magnetic field is thus more and more stretched inside the vortices until it reconnects, redistributing the initial kinetic energy into accelerated particles, heating, etc., but also playing a significant role in the transport properties of the plasma.
Magnetic Reconnection is one of the most important basic processes of magnetised plasmas and is the only process able to reorganize the large-scale topology of the magnetic field as well as to influence the energetics of the system. We underline that magnetic reconnection is considered to be the driver of the most important energetic phenomena observed in the solar atmosphere and in many regions of the (outer) Earth Magnetosphere.
The research effort in the understanding of the physics underlying magnetic reconnection is impressive and has reached remarkable progress in the last ten years. However, magnetic reconnection still remains a formidable challenge, in particular on the problem of the conditions for the system to spontaneously lead to reconnection. Theoretically, two different approaches are used to investigate magnetic reconnection: either as an initial value problem within which a specific magnetic configuration is considered such that the instability develops by itself or as a boundary value problem in a "stable" magnetic configuration where reconnection is forced to develop by an external driver. In both cases, the system is ad hoc prepared to develop reconnection.
In the low latitude magnetopause context, another important secondary vortex instability that can develop on the shoulder of KH vortices is the Rayleigh-Taylor (RT) instability driven by the initial density jump between the solar wind and the magnetosphere since the vortex centrifugal acceleration acts as an efficient gravity making the vortex arms RT unstable. The process stops the vortex pairing process and leads finally to the generation of a turbulent mixing layer. In a simplified 2D geometry, the linear and early nonlinear evolution of large-scale (as compared to the ion skin depth di) vortices is essentially a MHD process depending only on the initial velocity and magnetic field, regardless of the specific small-scale physics. If the value of the in-plane magnetic field parallel to the initial flow is “low” enough, the motion of the vortices advects, stretches and rolls-up the magnetic field lines leading in a natural way to magnetic inversion layers where the process of magnetic reconnection can develop. Thus, as already stated, magnetic reconnection acts as a secondary instability on the primary KH instability. Although large-scale vortices are essentially MHD structures, their motion is not only capable of creating favourable conditions for reconnection to act, but in particular to build up sub-di current sheets within which two-fluid dynamics develops. Transition to “fast” reconnection is observed. A very important result has been to demonstrate that reconnection, despite being a very localised process, is capable of changing the large-scale magnetic topology as established by the flow evolution. This global variation is due to the small-scale physics generated dynamically by the large-scale evolution. The possibility of changing the magnetic topology is of fundamental importance concerning the transport properties of the low latitude magnetosphere system.
The solar wind-magnetosphere coupling also manifests itself in the presence of strongly turbulent regions. In particular, in situ observations clearly demonstrated that the magnetosheath region is significantly turbulent. The plasma turbulence originates in the solar wind, is transformed (typically amplified) by the collisionless shock which itself is a source of turbulence/wave activity driven by large departures of the particle distribution functions from Maxwellian ones. Further fluctuating wave energy is injected to the system as the magnetosheath plasma flows around the magnetospheric cavity leading to plasma compression/expansion and stretching of magnetic field lines as they are draped over the magnetopause. Such processes typically lead to the ion perpendicular temperatures larger than the parallel ones. For lower betas this temperature anisotropy generates Alfven ion cyclotron waves whereas for higher beta it generates mirror mode waves. The mirror instability leads to formation of coherent structures with the form of pressure-balanced non propagating magnetic enhancements or depressions. The mirror mode structures are very robust and survive to regions which are linearly stable with respect to the mirror instability. Recent CLUSTER observations indicate that the mirror structures influence or even drive the magnetic reconnection at the magnetopause and consequently they influence the transport of the solar wind plasma to the magnetosphere. Similarly it is expected that important plasma turbulence/wave activity have an impact on the magnetopause properties and plasma transport. The nonlinear physics of plasma turbulence and kinetic instabilities is only poorly understood and even less is known about their interaction with the magnetopause (we note that a relevant phenomenon called impulsive penetration is considered as an alternative to magnetic reconnection). For the mirror mode instability in situ observations indicate that magnetic enhancements/humps are present close to the bow shock whereas the magnetic depressions/holes are observed further downstream, even close to the magnetopause in low beta plasmas. This behaviour may be related to theoretical work indicating a subcritical bifurcation (leading to bistability behaviour) of the mirror instability near threshold. However, in reality the magnetosheath plasma is typically far from the mirror threshold where the theoretical approach fails. Local hybrid simulations (standard ones and simulations using expanding box models to investigate an influence of plasma property changes as it flows around the magnetopause) exhibited properties similar to the observed ones. In these simulations the mirror instability saturated in the form of the magnetic humps which transformed to magnetic holes later on. The quasi-parallel portion of the bow shock and proton foreshock is a source of large-amplitude, almost isotropic magnetic fluctuations/turbulence essentially of the kinetic nature. Their properties and generating mechanism is not well understood and even less is known about their evolution further downstream and their interaction with the magnetopause. Large global (2D and 3D) kinetic simulations of the solar wind/magnetosheath/magnetopause were needed and carried out by Swiff (complemented by detailed local simulations of relevant phenomena) in order to investigate the role of these processes in the transport of particles to the magnetosphere and their consequences on the space whether.
1.2.4 Coupling at the Earth: magnetosphere-ionosphere-thermosphere coupling (WP5)
Kinetic models for space weather have been developed to understand the physical mechanisms responsible of their time variations during disturbed periods. For each region of the inner magnetosphere, forecasting models has been developed on the basis of these physical kinetic models. The models have been coupled to take into account the interactions between the magnetospheric regions.
Concerning the terrestrial plasmasphere, where the cold plasma is trapped by the magnetic field in the inner magnetosphere, a three dimensional kinetic dynamic model has been developed at BISA [Pierrard and Stegen, JGR, 113, A10209, 2008 and reference therein]. The core of the plasmasphere is obtained from the kinetic exospheric approach assuming a kappa velocity distribution function for the particles. The position of the plasmapause, the limit of the plasmasphere, is determined by the interchange instability mechanism and an empirical model of the convection electric field. The model that gives the position of the terrestrial plasmapause as a function of the geomagnetic activity level can be run on the space weather portal: http://www.spaceweather.eu. Plasmaspheric exploration got a major boost since 2000, when IMAGE and CLUSTER satellites were launched. These observations allow us to improve the physics-based model by using empirical data.
The plasmasphere is the high altitude extension of the ionosphere at low and mean latitudes, so that a coupling with this region would be highly useful, especially to determine the variability of the total electron content that influences the radio wave propagation and Global Positioning System (GPS). For the ionosphere, the IRI (International Reference Ionosphere) model has been used. This model describes the number density and temperature of the ionized particles as a function of the altitude (less than 700 km), the time and date. The transition region between 700 km (ionospheric model) and 2000 km (plasmaspheric model) requires specific investigations for coupling.
For radiation belts, we started with the database of satellite observations and empirical models available on SPENVIS (www.oma.spenvis.be) developed at BISA. Analyses of SAMPEX, CRRES, Equator-S, CLUSTER and SAC-C observations has been made to improve the understanding of radiation variability. The satellite observations available in the SPENVIS database has been used to develop a dynamic model of the space radiations. Swiff went beyond the pre-existing static radiation models by introducing dynamic features based on observations of transient flux events. This approach facilitates the investigation of the physical processes involved in the flux variations in the magnetosphere.
Moreover, radiation belt populations are very sensitive to the core plasmasphere distribution and specifically to the position of the plasmapause. There is a relationship between the position of the plasmapause and the internal border of the external radiation belt. This relationship has been investigated in more detail to couple the plasmaspheric model to the radiation belts variability.
The polar wind corresponds to the plasma outflowing along open magnetic field lines and escaping from the ionosphere at high latitudes. To obtain forecasting in this region, we started from the kinetic models developed at BISA [Lemaire and Pierrard, Astrophys. Space Sci., 277, 2, 169-180, 2001] that especially emphasise the effects of non-Maxwellian velocity distributions. The presence of suprathermal particles in the tail of the distribution, as often observed in space plasmas, leads to a significant increase of the escape flux. The relationship existing between the current and the voltage has also been used in the auroral regions.
The WP5 had the opportunity to validate the physical models and to constrain them by empirical observations to determine precisely the effects of the solar and geomagnetic activity. The models have been used as tools for forecasting and nowcasting of space weather. By coupling the different regions of the magnetosphere (and with solar wind disturbance), it has been possible to obtain a full description of the magnetospheric perturbations.
S&T Approach
The fundamental challenge in the modelling of space weather is the need to account for multiple physics processes developing simultaneously in the same region of space or progressing in a cascade in different regions, and characterised by widely different spatial and temporal scales. For each process and for each scale the correct and most efficient method is different. In the course of a storm or a substorm, the Earth magnetic tail (the region of the magnetotail on the night side of the Earth opposite to the Sun) becomes active with many waves and instabilities developing and ultimately affecting the Earth ionosphere and even (indirectly) the ground.
The agents are different: electrons, ions and electromagnetic fields (here gravity is not important but on the Sun it is adding one more worry for the method developer). Each agent evolves on very different spatial and temporal scales, spanning times from the tens of microseconds to the hours and from hundreds of meters to millions of kilometres. The fast small processes are best treated by the study of single particle trajectories or by kinetic models, where one studies the statistical probability of finding electrons or ions with a certain velocity in a certain position. But such models are not best suited for larger scales, as the cost of the simulation would be prohibitive. In that case one need to resort to fluid models where the information on the statistical distribution is lost and the model treats only average quantities (e.g. density, average flux or temperature).
Moreover different populations of particles are present. For example in space weather events, to the "usual" solar wind, population of high energy, even relativistic, particles are present travelling at much faster speeds, close to the speed of light. Such high energy particles are one of the main threats for humans and technology in space. Of course the physical model for such particles is often different from that of other type of particles. Another challenge is that the conditions in different regions can be so different that model that work in one region would not work in another: for example the lower strata of the Sun's surface and atmosphere (photosphere, chromosphere, corona) are much more collisional than higher strata, and similarly the upper Earth atmosphere and ionosphere is more collisional than the magnetosphere and the physical processes are largely different.
The summary of this short description left out on purpose some aspects for the interest of brevity, but not because such processes would not be considered (see the WP description to find a complete list of all aspects considered). Yet from this summary one can immediately understand that SWIFF had to achieve two goals that critical for the space weather community:
1. The ability to treat concurrently in a single integrated framework microphysics (treated kinetically) and macrophysics (treated with fluid methods). The need to couple fluid and kinetic models is without doubt the most burning and crucial challenge not just for space weather modelling but also for many other aspects of computational science.
2. The ability to couple different physics in the same region at the same time or in cascade in the same or different regions. This effort is explicitly mentioned as a key goal of the present call from the European Commission. The document European Commission C (2009) 5893 of 29 July 2009 explicitly asks for "space weather models to improve specification and prediction capabilities, with emphasis on the linkage of the different physical processes that occur simultaneously or sequentially in many domains.” (quoted directly from the document). This is exactly what the WP 3 to 5 in close connection with the modelling efforts of WP2 achieved. We focused solely on developing a software framework to simulate in the modern supercomputers the linkage (or coupling) of different physical processes. And we do so not in general but with the eye fixed on the specific key areas of linkage, as summarised by each WP:
• studying the coupling at the Sun (WP3)
• studying the coupling in space between the solar wind and the magnetosphere (WP4)
• studying the coupling between the Earth's atmosphere and the Earth-space environment (WP5)
In the next section, we describe for each science WP how this approach is implemented in a specific strategy. The WP1 and WP6 are described respectively in Section B2 and B3.
1.3.2 Multiscale modelling for space weather (WP2)
Within this work package, we started our activities with an immediate intra-comparison of all codes available to the teams of this project. All teams had state-of-the-art simulation tools developed in house, and we carefully selected a comparison on idealized problem setups that combine the main physics challenges that recur in the later work packages. In particular, reconnection is a recurring theme of interest to all later applications, and we wish to identify the main advantages and disadvantages of using particular algorithms and discretizations on identical problems on the one hand, while on the other hand better identifying the role of ever more complex physics in addressing reconnection effects in detail. For this purpose, GEM-type 2D and 3D setups need to be revisited with all (grid-adaptive) fluid, hybrid, PIC and multi-physics solvers already available at the start of the consortium. This helped us select the best strategy for consecutive coupling and integration efforts, where truly multi-physics modelling is pursued. In 3D reconnection, the identification of the magnetic skeleton is a useful aid to unravel the complex topological changes developing in global MHD models. We started with identifying the potential role of using different discretization schemes in global (grid-adaptive) MHD models on the skeleton evolution. This must give us the detailed insight needed for interpreting the large-scale effects in CME initiation studies from WP3. In a similar spirit, we repeated this comparison exercise for shear flow dynamics, as known to be important for local magnetopause conditions targeted in WP4. Again, running similar idealized configurations using both single to multi-fluid models confirm or identify the main physical ingredients, and help separate between physically versus algorithmically driven details of the ensuing dynamics.
We next pursued optimal coupling strategies, in particular in due preparation for the science goals of WP3: starting from detailed flux emergence simulations and/or observationally based extrapolations of the magnetic topology, we wish to make more realistic space weather event initiation models. That required to improve on current strategies, where e.g. in global MHD simulations the static potential background field is subtracted and the problem is reformulated in a (nonlinear) perturbative sense about this equilibrium, to one where the initial conditions from modern extrapolation codes are incorporated much more accurately (possibly designing more sophisticated splittings about force-free states). In the same spirit, we needed to couple flux emergence models to grid-adaptive CME initiation modelling, and that ultimately means that we must be able to handle differing discretizations in various spatially distinct regions of our simulation domain. Indeed, the most advanced flux emergence simulations typically employ high order finite difference treatments, ideally suited to also include more complex, radiation-related physics, while the CME models to date mostly build on modern shock-capturing techniques employing approximate Riemann solver methodologies in finite volume solvers. Having the capability to combine these in a scale-encompassing simulation brought the CME initiation models to the next level of realism. This has been compared to strategies where a much looser coupling is used (currently in effect in some of the US based frameworks), namely one where differing codes get coupled through mutual boundary condition prescriptions.
In a next level of complexity, we deployed differing physics solvers per grid level in AMR simulations. In the first subtask of the work package, we have identified the best hybrid approach, and it is in this stage that we like to bring these together in the same grid-adaptive, multi-physics simulation. This has been done separately for coupled fluid treatments on the one hand (from single fluid MHD, over Hall-MHD, to two-fluid descriptions), and in a PIC type framework exploiting the implicit moment method as pioneered in Parsek3D (initiated by the project coordinator) and related code efforts. In the latter PIC-AMR code, we demonstrated seamless multi-level simulations where the grid changes (in the computation of the electromagnetic fields) and the particle movers/splitters have no adverse effects on the simulated dynamics. Finally, the ultimate goal of this workpackage was to design and implement a novel approach that handles both fluid and kinetic treatments in multi-scale simulations. This combined the AMR strategies for PIC treatments alone on the one hand, and for coupled fluid treatments on the other hand. The implicit moment method is again very suited in this aim, since we readily were able to compute the needed moments and fluxes that connect both descriptions accurately. Ultimately, we then incorporated also the detailed insights gained from kinetic models that go down to thermospheric layers (from WP5) in global CME initiation to arrival modelling. A demonstration was given in Deliverable 2.6 on selected, well-documented space weather events, where multi-spacecraft (especially CLUSTER) info was used for validation.
The codes developed in this work package have been applied in the following application-oriented tasks, and has been delivered to the scientific community as opensource software, complete with detailed documentation on their usage and typical range of validity.
1.3.3 Coupling at the solar surface: photosphere, chromosphere and corona (WP3)
A key objective of global, long-term solar simulations is to determine the 3D coronal magnetic field configurations in which CMEs can occur. For example, one model for the initiation of CMEs is the flux cancellation model, which produces flux ropes that lose equilibrium and are ejected from the corona. To determine the location of formation and subsequent ejection of flux ropes, the global non-linear force-free field simulations has been used. These simulations used realistic photospheric magnetic field distributions (derived from observations) to model the slow evolution of the large-scale coronal field through a series of quasi-static, non-linear force free field equilibria over periods of weeks to months. New magnetic flux in the form of emerging bipoles has been added in accordance with observations to maintain the accuracy of both photospheric and coronal fields. Through this process, the gradual formation of magnetic flux ropes (and their subsequent ejection) has been studied. Once ejections are found, the 3D magnetic field configuration just prior to the onset of ejections has been taken and used as the initial condition for full 3D MHD simulations. The MHD simulations then consider for the first time the dynamics of the eruptions over short times-scales derived from observations. The resulting structures and properties then were compared with observations.
Efficient magnetic reconnection is required to eject structures as large as CMEs and it is extremely likely that the reconnection does not occur at a single site. Instead large-scale current sheets fragment and reconnection occurs at many localised sites. In order to properly understand the energetic consequences of flares and CMEs, it is important to be able to determine these local sites of energy release and to be able to understand the behaviour of the plasma and magnetic fields at these sites on the micro-scale. However, to see the overall global consequences of reconnection one needs to know how the local reconnection sites are linked. It has recently been shown that in a number of situations the magnetic field may reconnect not just once, but many times at neighbouring local sites enabling much more energy to be released than one might initially imagine. (This extra energy comes from a prolonged injection of Poynting flux that is maintained as the magnetic field does not take the quickest route to its lower energy state.) Moreover, this energy release has a longer time duration and a wider spatial area than initially anticipated. A key to understanding CMEs and, in particular, their consequences for space weather, has been to establish how significant a role this type of behaviour plays in the reconnection involved in CME initiation and whether the extra magnetic energy released is converted in to bulk plasma flows or heating. To do this MHD models of CMEs must be initiated from non-linear force free magnetic fields that are driven in order to create a CME, rather than simply starting from a marginally stable magnetic configuration with an initial imposed stressed configuration.
From the empirically based 3D MHD models of the solar (interior and) atmosphere that produce CMEs, we used the new suite of analysis tools in order to determine the key characteristics required for CME and flare initiation and to establish whether there are a range of magnetic configurations and velocity flow patterns that can produce CMEs or whether there is one preferred configuration.
We also studied initialization of energetic solar events based on ab initio models of solar active regions. Such models are just now becoming possible, based upon large scale MHD simulations stretching down at least 20 Mm below the solar surface. When sufficient amounts of magnetic flux are injected at the lower boundary of such models the interaction of this magnetic flux with multi-scale convection can yield realistic models of solar active regions. As in the case of real solar active regions an infinite number of dynamic arrangements is possible, and one of the main challenges of this approach was to learn how to seed the sub-surface magnetic field in such a way that energetic events result in the overlying solar corona when the magnetic fields emerge through the solar surface.
1.3.4 Coupling in space: solar wind-magnetosphere (WP4)
Our final goal was to model the solar wind plasma entry into the magnetosphere by a “global” simulation including large and small scale physics. Such a problem involves an impressive range of scales and frequencies, at least of the order of four orders of magnitude. Until now, the work concerning the low latitude magnetosphere system dynamics has been focused on the basic processes as magnetic reconnection effects, assuming a constant density profile, or as RT effects induced by the initial density jump, assuming a purely perpendicular initial magnetic field. In the “real” case these two effects and the pairing process compete and are strongly influenced by the specific vortex configuration process generated by the KH instability in a magnetised plasma where initially practically all the macroscopic quantities are inhomogeneous and varies not necessarily on the same spatial scale length. It was therefore crucial to establish the role of secondary instabilities on the dynamics of the "real" system, since they strongly influence the increase of the width of the mixing layer and its internal dynamics that are the most important factors for the evolution at the flank of the Earth's Magnetosphere.
From this point of view, the possibility to access CLUSTER and THEMIS satellite data is of crucial importance. This has been made by means of a well established collaboration with the colleagues of the LPP laboratory at « Ecole Polytechnique » in France. We note that at LPP, Anna Tenerani has just started a PhD thesis in partnership (supervisors F. Califano and O. Le Contel) and that Matteo Faganello that made most of the work on this subject in Pisa during his PhD thesis has now a post-doc position at LPP, thus giving to the Pisa group and all other teams here the possibility to access THEMIS and CLUSTER data from the magnetosphere. Furthermore, a realistic model should consider kinetic effects, as for example Finite Larmor Radius effects, particle resonances, wave - particle interaction. Due to the strong difficulties of studying numerically the full physics and geometry of this problem, we need first a description “simple”' enough to follow the inhomogeneous and geometrically complex large-scale motion. At the same time, any model must be capable of describing the small-scale microphysics including effects allowing for “fast” reconnection.
For these reasons, we continued to adopt in the first part of the project a collisionless, two-fluid description of the plasma that neglects completely kinetic effects but allows us to take into account a relatively large range of scales and frequencies, from MHD to electron-MHD. In the second part of this project, we started a kinetic study of the full problem by making use Eulerian, Vlasov low noise, codes for an "accurate" investigation of selected kinetic processes from the full system evolution, as for example the formation of coherent kinetic structures. A Hybrid approach (kinetic ions, fluid electrons) in Eulerian and Lagrangian formulations has been also used as a complementary tool to reduce the computational cost of kinetic simulations.
In summary, the main points that we developed are:
1. Nonlinear 2D evolution of the two-fluid model. We used "realistic" initial conditions of the macroscopic fields as extrapolated from satellite data. We investigated the role of the main parameters of the system and study the influence of the magnetic tension on the onset of the RT instability that, in turns, could force magnetic field lines to reconnect.
2. Nonlinear 3D evolution of the two-fluid model. The first relevant 3D effect studied is the effect of a small equilibrium variation along the perpendicular direction that can affect the pairing mechanism. Furthermore, the 3D geometry can change the large-scale flow plasma compression responsible for the formation of sub-di current sheets structures. Finally, a 3D configuration allowed us to study the role of the connection of magnetospheric field lines to the Earth.
3. Role of kinetic effects. A) We modelled some of the most relevant kinetic effects (ex.: FLR effects), still adopting a fluid approach. B) We used results of two-fluid simulations in order to localise the regions where, locally, kinetic effects could be relevant and to extrapolate the initial conditions for reproducing the same dynamics, but using now a full kinetic Vlasov approach. A typical example is the formation of magnetic islands by the secondary magnetic reconnection instability observed in two-fluids simulations. C) We made use of 2D and 3D PIC codes. This is probably the most important step concerning the possibility of considering a large-scale system but including all kinetic effects. Such simulations are obviously computationally heavy to be performed and the analysis of the corresponding data a complex task. Therefore, this approach has been performed after a first screening made by 2D and 3D two-fluids investigations.
4. We applied the mathematical - numerical global models developed in WP2 to obtain a self-consistent fluid to kinetic description of Solar Wind interaction with the Magnetosphere at low latitude focusing on transport problem. This allowed us to introduce our results in the context of the space whether forecasting problem.
1.3.5 Coupling at the Earth: magnetosphere-ionosphere-thermosphere coupling (WP5)
To investigate the chain of processes in the inner magnetosphere during quiet and disturbed periods, we started from physical kinetic models developed at BISA. These models were mainly developed to study the physical mechanisms implicated in the dynamics of the magnetospheric regions. In the present project, they has been improved by using constraints coming from empirical satellite observations. They has been used to develop nowcasting and forecasting models that have a direct application for space weather. The solar wind-magnetosphere coupling models developed in WP4 has been used as inputs to predict levels of geomagnetic activity.
Concerning the plasmasphere, the 3D kinetic model of the plasmasphere developed by Pierrard and Stegen (2008) has been improved by including the temperature and the ion composition, as well as the dynamic variability of the plasmaspheric density and plasmapause location as a function of day, season, solar-cycle, and storm-time variability.
The model has been constrained by empirical observations used mainly as boundary conditions. The model has been validated by comparisons with recent observations, mainly from CLUSTER and IMAGE satellites. EUV from IMAGE gives a global view of the Earth’s plasmasphere when the spacecraft flights above the North Pole. This is especially useful to determine the dynamics of the plasmaspheric region.
A forecasting and nowcasting model has been developed for quiet and disturbed periods: the user will introduce a date (the time), a point or an orbit in space. Deducing the geomagnetic activity index Kp, the model will predict the number density of electrons, protons and other particle species, the temperature and the position of the plasmapause as a function of the magnetic local time.
The model of plasmasphere has then been coupled with the ionosphere. The empirical International Reference Ionosphere (IRI) has been used for this task. This extend the model to lower altitudes. The IRI model gives values from 200 to 700 km. The plasmaspheric model gives values from 2000 km of altitude to 9 Re. The coupling in the transition region from 700 km to 2000 km requires specific investigations. The model has been used to determine the Total Electron Content (TEC) and its large scale variations that affect GPS and radio signals. The effects of plasmaspheric plumes formed during geomagnetic storms on the dynamics of the magnetosphere especially affects the TEC. The coupling with the thermosphere and especially the heating has been investigated with the MSIS empirical model that provides thermospheric temperature and density based on in-situ data from seven satellites and numerous rocket probes. The model coupling the plasmasphere, the ionosphere and the thermosphere has been implemented on a space weather website as a tool for the scientific community.
The energetic particles of the radiation belts are especially important for space weather due to their effects on spacecraft and the health of astronauts. We studied the dynamic of the radiation belts by using SAC-C, CLUSTER and other satellite observations provided on SPENVIS (www.spenvis.oma.be). This improved the steady state average empirical models generally developed to estimate the radiation flux encountered by spacecraft along their orbit.
The goal was to determine the flux enhancement and the decay time after a geomagnetic storm. The flux enhancement following the recovery phase of a geomagnetic storm depends on the driver by a Corotating Interaction Regions (CIR) or a Coronal Mass Ejection (CME). The solar wind parameters are thus important input for forecasting. We determined the most appropriate solar wind parameters to be used for prediction (solar wind velocity, number density, pressure, magnetic field direction…). The plasmasphere-radiation belt coupling has been provided by investigating the relationship between the position of the plasmapause and the internal border of the external radiation belt.
A forecasting model for space radiations has been developed: the user introduces a point, respectively an orbit, and the time. Regarding the solar parameters and/or geomagnetic indices, the model calculates the real-time fluxes or forecasts. Such predictions are useful for flying spacecraft since the energetic particles of space damage electronic circuits and solar panels of satellites. The radiation model has been implemented on a space weather website as a tool for the scientific community.
The description of the inner magnetosphere has been extended at high latitudes with a model for the terrestrial polar wind. Based on the kinetic models developed at BISA, a study of the photoelectrons has been provided to determine more precisely the flux of ions escaping at high latitudes. The model has been compared to empirical observations. Moreover, the polar wind escape is in some ways similar to the solar wind escape, so that their theoretical studies followed the same historical evolution. The comparisons with solar wind models and especially between models based on the kinetic and the MHD approaches improved our understanding of these phenomena.
The forecasting model of the polar wind uses time as an input. Determining the geomagnetic disturbance, it predicts the escape flux of light ions like hydrogen and helium. The polar wind model has been implemented on a space weather website as a tool for the scientific community.
Potential Impact:
Space weather refers to conditions on the Sun, in the interplanetary space and in the Earth space environment that can influence the performance and reliability of space-borne and ground-based technological systems and can endanger human life or health. Adverse conditions in the space environment can cause disruption of satellite operations, communications, navigation, and electric power distribution grids, leading to a variety of socioeconomic losses. The conditions in space are also linked to the Earth climate. The activity of the Sun affects the total amount of heat and light reaching the Earth and the amount of cosmic rays arriving in the atmosphere, a phenomenon linked with the amount of cloud cover and precipitation. Given these great impacts on society, space weather is attracting a growing attention and is the subject of international efforts worldwide.
SWIFF aimed at forming a wide collaboration between teams dealing with different fields of solar- space- and geophysics. The main goal is to create a framework for space weather modelling in the form of: a) mathematical models suitable to represent the physics processes behind space weather events, b) computational methods to solve such models, c) software to form a integrated framework to model space weather events. The outcome of the SWIFF research activities will led to an improvement of the methods that can be used to predict space weather events, and their near Earth space and terrestrial impact (geoeffectiveness). The achieved developments and outputs are expected to contribute to the future development of space weather science and space weather forecasting, with a legacy continuing long after the end of the project.
Our activity was developed with the goal of impacting two major aspects of the European research in space weather:
1. Addressing the needs expressed by the document European Commission C (2009) 5893 of 29 July 2009, under theme 9 – Space, with specific focus on the Area 9.2.3: Research into reducing the vulnerability of space assets as expressed in the call: SPA.2010.2.3-01 Security of space assets from space weather events.
2. Forming the fertile ground for the future activities of ESA in the area of Space Situational Awareness (SSA), as expressed in the plans of the First phase: “SSA preparatory Programme” of ESA presented at the Space Weather Working Team meeting in Brussel in September and in documents presented there (http://www.esa-spaceweather.net/index.html).
Additionally SWIFF addressed fundamental issues and developed tools of general interest to space research as described in the same EC document mentioned above under the other areas of the Activity: 9.2. Strengthening the foundations of Space science and technology.
Below we address in turn each of these three goals, and we refer in quoted italic passages taken directly from the document European Commission C (2009) 5893 of 29 July 2009.
1.1 Relevance of our project to the aims of the EC theme 9 space activity on research into reducing the vulnerability of space assets
As noted in the work programme: “Space weather gives us displays of the aurora, or northern lights. Aside from this visible effect on the atmosphere, solar activity modulated effects on the Sun and in the helio- and magnetosphere affect the entire Earth environment from the magnetosphere down to the ionosphere and even to the lower atmosphere climate system. At its worst, space weather is a natural hazard which does not only modify the atmosphere but also can catastrophically disrupt the operations of many technological systems, thus causing disruption to people’s lives and jobs. Space storms (particles, plasma or electromagnetic) are a recognised aerospace hazard and can cause major failures, e.g. onboard spacecraft, in electrical power grids, in telecommunications links (satellite, launcher and groundbased) and in navigation systems. During the current approach of the next solar maximum (around 2012), more accurate prediction, assessment and early-warning capabilities of disruptive events that are to be expected as part of this cyclical phenomenon are particularly poignant”
The space weather studies supported by SWIFF addressed this fundamental need expressed in the work programme. In particular we focused on the fact that “Collaborative Research projects are invited to focus on research areas such as:
– early warning and forecasting methods to allow for a mitigation of space weather effects on humans in aerospace vehicles and on vulnerable technologies in space (in particular satellites, communication and navigation systems) and on the ground (communication and power nets)...
– space weather models to improve specification and prediction capabilities, with emphasis on the linkage of the different physical processes that occur simultaneously or sequentially in many domains.”
SWIFF is exactly this: a collaborative effort among some of the prime centres in Europe with expertise on producing space weather models focusing especially on the linkage (coupling) of different physical processes that occur simultaneously and sequentially with the goal of producing a integrated framework for forecasting and early warning.
The whole structure of SWIFF is centred around one the development of model and software (WP2 – Multiphysics and multiscale modelling for space weather) designed with in mind specifically the multiscale and multiphysics linkage present in space weather from its source on the Sun (WP3 - Coupling at the solar surface: photosphere, chromosphere and corona) in its evolution in space up to the Earth (WP 4 - Coupling in space: solar wind-magnetosphere) and its impact on geophysics and on human activities in space and on the ground (WP5 - Coupling at the Earth: magnetosphere-ionosphere-thermosphere coupling).
The focus of SWIFF was not only to provide a physics-based description of space weather events for the interested academicians and scientist, but it to produce methods and software tools directly aimed at the users with emphasis on being able to produce services that can be practical in forecasting activities to allow the mitigation of the impact on humans and technology. We aimed at taking at heart the requirement that: “Collaborative Research Projects are expected to significantly contribute to the European capacity to prevent damage / protect space assets from space weather events. Projects are expected to significantly contribute to both improving forecasts and predictions of disruptive space weather events. Projects are also expected to identify best practices to limit the impacts on space- and ground-based infrastructures and their data provision.” We addressed this by focusing on physics-based models that bring in the underlying physics at the kinetic level that was missed by the pre-existing fluid models. The processes at the center of space weather are fuelled by processes of energy release (wave-particle interaction, shock, magnetic reconnection) that the fluid approach cannot represent with fidelity, requiring particle-based kinetic models. In particular, particle acceleration behind SEP events requires kinetic treatment. The development of suitable fluid and kinetic models that can be linked and used concurrently has been the crown achievement of SWIFF. This is a great achievement well beyond space science: the same tools can and are already being picked up in other fields: industrial plasma applications and nuclear fusion research.
The foreground produced by SWIFF is in fact of general use and availability as described below in our dissemination plan and as requested in the often-mentioned EC document:
“The work programme will support European coordination activities both to ensure the open exchange of information on emergencies that may have been caused by space weather events, with the goal of structuring international and European research efforts.“
1.2 Complementarity with ESA activities and in particular the ESA Space Situational Awareness Preparatory Programme (SSA)
The SWIFF team operated under the inspiring guidance of the words of the document European Commission C (2009) 5893 of 29 July 2009: “It is expected that the project will be complementary to and clearly demonstrates an added value to the efforts already carried out in this field by ESA or national agencies.”
Our effort was designed with complementarity in mind and with the aim of not replicating any effort currently planned in the First phase: “SSA preparatory Programme” of ESA.
The plans for SSA call for Space Weather Service Elements that call for:
• Space environment modelling for spacecraft design
• Space environment modelling for SSA survey and tracking
• Data service (NOAA SWPC type – including integrated numerical modelling element).
Especially for phase 1 of the planning, the SSA plans called for the establishment of a Virtual Space Weather Modelling Centre (VSWMC). This SSA activity is currently ongoing and was awarded to a Begian team coordinated at KULeuven (by Poedts), two doors down the same corridor where SWIFF is coordinated (by Lapenta). This served as a fantastic opportunity that we did not let slip from our hands. The software developed by SWIFF is being deployed into the VSWMC and the expertise gained by SWIFF is equally benefitting the VSWMC. OF course, in the inclusive spirit of considering all developments in Europe and internationally.
SWIFF has been ideally suited for making the groundwork for this effort. Most of the members of SWIFF belong to member states who are also ESA member state who support the Space Weather Element of SSA. SWIFF was a stepping stone to gather the competence and the basic integrated framework that will allow to refine progressively more the forecasting ability of SSA.
SWIFF did not have the goal of developing a full package for space weather modelling. That would have been a wasteful repetition the effort ESA is doing. We formed the basic mathematical, computational and software know-how and base of tools able to study the space weather problems. The key missing piece before SWIFF was the inability of previous models to sufficiently account for the linkage (or coupling) of different physics developing on different scales. This is at the core of all space weather: in the solar corona, at the interaction of the solar wind with the Earth magnetosphere and at the interaction of the magnetosphere with the Earth atmosphere, the scales of the microphysics and macrophysics are tightly coupled and different physical processes interact tightly. SWIFF filled this gap by developing and deploying the methods and software to handle this. Without this modelling space weather based on the fundamental physics understanding would have been impossible.
The SWIFF actions have been perfectly complementary with ESA, the SSA will collect the existing expertise and the new products developed by SWIFF not only in the ESA Virtual Space Weather Modelling Centre but also in the subsequent activities that are following. The efforts do not overlap wastefully but complement each other synergistically: we develop the necessary tools, SSA will develop the final forecasting system.
1.3 Complementary with other European Commission funded research
Besides striving for complementarity with ESA funded project, SWIFF worked on situating itself in a synergistic relationship with other EC-FP7 funded projects. SWIFF was perhaps unique in its goal of addressing the fundamental need to develop methods to do a physics-based forecast of space weather events. This situated SWIFF in the central position of needing from one side input from data-focused projects to provided the needed observational input for our models and from the other side of providing new tools to be used by more application oriented projects.
For the first task, SWIFF found itself in the lucky situation of gathering help from the former FP7 project SOTERIA that focused on data archives and product specifically focusing on space weather. SOTERIA and SWIFF were both coordinated by Lapenta, although no other partner of one was member of the other. Nevertheless the common coordination and the focus to being open to the public of both projects provided fantastic opportunities fully exploited by SWIFF.
For the second task, the difficulties to overcome where more severe because of the notorious “not invented here” syndrome: the reluctance of groups in using tools developed by other tools. But SWIFF was not scared by this well-known difficulty and confronted the situation head on. We engaged in several activities:
• Building of teams across FP7 projects, such as those sponsored on themes of WP5 by the ISSI institute of Bern to discuss radiation belts
• Organization of events to foster cross fertilization, such as the session on space weather modelling organized by Lapenta and Aylward at the 2012 european space weather week
• Personal contact with other FP7 projects and with other teams in Europe working on space weather
• Participation to special issues and common review documents aimed at disseminating the activities of the different FP7 projects on space weather funded by the same EC call.
All these activities built progressively more momentum for the use and dissemination of SWIFF tools and to provide SWIFF with the needed input on the needs of other projects and of end users.
1.4 Relevance to other aims of the activity 9.2 of the EC theme Space
Besides focusing on reducing the vulnerability to space weather events by focusing on the two activities above of the EC and of ESA, we also made great contributions to the wider goals of the European research as indicated in the activity 9.2 (strengthening the foundations of space science and technology), as laid out in the work programme for cooperation in Space. Quotations in italics in the text below are from the work programme on theme 9 -Space, document: European Commission C (2009) 5893 of 29 July 2009.
The work programme aims at “Providing R&D support to the foundations of Space science, exploration, space transportation and space technology through synergies with initiatives of ESA or other European, national or regional entities.” SWIFF focused on bringing to bear in a integrated plan activities that are currently funded by local governments, expanded them and connected them to form a collaborative network where the parts synergistically produce a sum that is able to address a problem not previously possible without a pan-European scope: creating the basis for a complete space weather forecasting model and software platform. SWIFF produced software and methods of general interest beyond just addressing the vulnerability of space assets. We did that, but we did it following the path of great science that will produce along the way new and useful products (models and software) whose scope reaches every aspects of space science.
A specific strength of SWIFF has been to address the need to gather competences and existing data gathering and existing software from old and new EU member states. The coverage is not merely geographical: each team from each country contributed a crucial aspect of the understanding of space weather processes, contributing a portion of the methods, of the observations and of the expertise in for which they are the apex of competence. As noted in the work programme, “a particular added value is also seen in contributions which the new EU Member States and the international community can make.”
An additional asset of the SWIFF project was its link to international activities beyond the European union and beyond Europe. Connections by partners of the SWIFF collaborative network have been supported with similar efforts in the United States of American, China and Japan. While no funding exchange was executed, the SWIFF consortium included expertise from international collaborations. Satellite data and ground-based data from around the world were used in our analysis in collaboration with international partners. An example of such activities is the involvement of the Coordinator of SWIFF with the Colorado and UCLA teams of the Magnetospheric Multiscale Mission of NASA, where he is a co-investigator. Similar examples are present in all partners. Great synergies have been achieved by sharing tools and scientific insight with international projects.
The breath of the international partnership of the proponents of SWIFF directly addresses the work program request: “In the context of International cooperation, a diversified approach is a key element in Europe’s space policy. Candidates for cooperation among other established or emerging space powers are the United States, Russia, Canada, People’s Republic of China, India, and Ukraine.“
Furthermore attention is paid to the requirement of ”promoting the contribution of space assets to the scientific and technological knowledge and foster its transfer to educational bodies”.
List of Websites:
www.swiff.eu
Contact: Giovanni Lapenta
Dept Wiskunde - KU Leuven
Celestijnenlaan 200b - bus 2400
3001 Leuven
Belgium
tel. +32 16 32 79 65