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Finding the Origin of the Slow Solar Wind

Periodic Reporting for period 2 - SLOW_SOURCE (Finding the Origin of the Slow Solar Wind)

Reporting period: 2020-10-01 to 2022-03-31

This project studies the mechanisms that produce the solar wind, this beam of supersonic charged particles (mainly composed of ionized hydrogen and helium) expelled continuously by the Sun. How stars produce their winds are long-lasting open challenges in astrophysics. Yet stellar winds play a fundamental role in the long-term evolution of stars, the properties of the interstellar medium located between stars and perhaps most importantly the habitability of their orbiting bodies such as our planet Earth. The energy transported by the solar wind - and the occasional solar storms that propagate through the wind - can drive ‘space weather events’ at Earth (so-called geomagnetic storms) that can cause significant perturbations to satellite systems, to ground-based assets such as example electric power lines and general radio transmissions (global positioning, high-frequency communication). Reliable predictions of the occurrence and magnitude of geomagnetic storms depends sensitively on an accurate modelling of the solar wind which in turns requires a better understanding of how it forms and expands in the solar atmosphere.
The SLOW_SOURCE project aims at developing numerical models capable of describing a comprehensive set of complex physical processes at play during the formation of the solar wind. The latter is observed in at least two states, a fast wind blowing with speeds greater than 500 kilometres per second (km/s) and slower wind with speeds less than 450 km/s. These two winds differ not only in their relative speeds but also in their composition, i.e. the amount of heavy elements such as iron, oxygen and magnesium carried by the wind. The slow wind in particular is highly enriched in iron and magnesium compared to the fast wind. Since the solar wind composition is regulated very close to the solar surface it is thought these composition changes are directly related to the processes that produce the solar wind. The goal of the SLOW_SOURCE project is to explain the bulk properties and peculiar composition of the slow solar wind. One theory relates this varying composition to diffusion processes and/or turbulence developing along an otherwise stationary solar wind. The other theory supposes that the magnetic field of the solar corona is continually reconfigured to allow matter that should normally be confined to the corona (on magnetic loops) into the solar wind. These two theories have defined the structure of the project and basically feed two overarching questions:

Q1 “Can the bulk properties and composition of the slow solar wind be explained by stationary solar wind models?” To address this question we have to first develop novel multi-species model of the solar wind capable of simulating the interactions between the major and minor constituents of the solar atmosphere that ionize with distance away from the Sun from a predominantly neutral atoms to charged particles. The key objectives are here to deliver some realistic simulations of the way heavy ions are extracted from the layers of the dense atmosphere (chromosphere) into the solar corona and the supersonic solar wind. These models can be compared with observations from the recently launched Parker Solar Probe and Solar Orbiter mission.

Q2“Does the variable composition of the slow solar wind composition imply that time-dependent transient processes are involved in the formation process of the slow wind?” To address this question we have to develop models of the solar atmosphere that can treat the dynamism of the solar corona where the solar forms. This reconfiguration is driven by the magnetic field because in this region of the solar atmosphere magnetic fields impose the dominant forces. Hence we have to develop models capable of not only treating the challenging problem of composition addressed in the previous question (Q1) but also the dynamism of the solar atmosphere.
The first period of the project has produced several important scientific results, they involve the development and exploitation of new solar atmospheric models as well as the analysis of the exceptional data recently acquired by the Parker Solar Probe and Solar Orbiter.

During the first part of SLOW_SOURCE we put in place two types of numerical models to simulate the origin and properties of the slow solar wind. The first approach is a 1-D multi-species model capable of addressing the complex interactions that occur between neutrals and charged particles (neutral hydrogen, electrons, protons and heavier ions) at the boundary between the dense chromosphere and the tenuous corona. This model can now simulate the transport of particles along open magnetic field lines (solar wind plasma) and magnetic loops (trapped plasma) as well a pre-defined transition between a closed to open geometry. The first step has been to compare with previous solar wind and loop models. For the first time our model is capable, for realistic coronal temperatures and densities, of reproducing the ionization states of heavy ions measured in the fast and slow solar winds.

The second modelling approach treats interactions between the plasma and the magnetic field more self-consistently through a magneto-hydrodynamic approach. This type of model is capable of simulating the time-dependent evolution of the magnetic fields that are continuously reconfigured at the source of the slow solar wind in what are called ‘helmet streamers’. These reconfigurations are expected to be an important source of variability of the slow solar wind. The project has modelled, for the first time and for realistic coronal and solar wind conditions, the formation in 3-D of small flux ropes that are continually released by helmet streamers. These novel simulations have allowed us to identify the sequence of physical mechanisms at play during the formation of these transients involving ballooning and tearing instabilities. This has allowed us to identify regions of the solar corona where these types of transients are likely to be produced frequently.

The project has also contributed to the first analysis of the data acquired by the recently launched Parker Solar Probe and Solar Orbiter missions. These first papers confirm that the slow solar wind has multiple complex components. We have shown in a first series of papers that there is a quiet component to the wind related to a quasi-stationary wind-formation process. Many features of this quiet component are reproduced very well by our 1-D multi-species modelling reinforcing the idea that the slow and fast winds can share similar formation mechanisms. The other component of the slow wind measured by Parker Solar Probe and Solar Orbiter is more variable and more complex than previously thought, this component could form through dynamic reconfigurations of the solar corona. Using the project’s time-dependent 3-D modelling approach, we showed that a subset at least of this variability originates in helmet streamers since our model was successful at reproducing the medium-scale variability of the slow solar wind observed by these two missions. The revolutionary data measured by Parker Solar Probe inside the solar corona has revealed that a significant fraction of the solar wind contains also smaller-scale structures marked by inversions in the magnetic field direction (called ‘switchbacks’). Our team has shown that the temporal scales over which these structures are emitted from the solar corona are compatible with transient phenomena occurring at the solar supergranular scale (30Mm).
Based on the achievements and conclusions of the first step of the project, we plan to pursue two important axes of research during the second part of the project. A first goal will be to determine the processes that drive the formation, release and propagation of the magnetic switchbacks discovered by Parker Solar Probe in the slow solar wind during the first year of the present project. This work will exploit our 3-D dynamic model to simulate flux emergence and its interactions with the pre-existing solar magnetic field. For different insertions processes, our model will simulate the propagation of these transient structures self-consistently from the Sun to the probes taking in situ measurements. We will be able to compare simulations and observations directly for different solar encounters, this will provide a direct evaluation of our model.

A second scientific goal will be to determine the mechanisms that regulate the abundance of heavy ions in the slow solar wind by combining our two modelling approaches discussed previously. In the first part of the project we simulate the effect of turbulence and waves on the extraction of heavy ions from the chromosphere to the solar corona but along undisturbed magnetic loops. In this second part we want to make our simulations more realistic and allow closed loops to open up to the heliosphere. To achieve that we plan to run our novel 1-D multi-species transport model on the output of our 3-D dynamic coronal model. The 3-D model will provide the time-dependent evolution of magnetic field lines as they progress inside streamers from an initially closed to an open topology. This opening of previously trapped solar plasma is expected to release heavy ions into the slow wind and to regulate its composition. These unprecedented simulations will be compared with data from Parker Solar Probe and Solar Orbiter, the latter mission in particular is equipped with a comprehensive set of instrumentation capable of measuring plasma composition in the solar corona and in the solar wind.
Left: Cut of simulated solar streamer. Right: 3-D flux rope released.