Skip to main content

Unravelling The Structure and Evolution of Solar Magnetic Flux Ropes and Their Magnetosheaths

Periodic Reporting for period 3 - SolMAG (Unravelling The Structure and Evolution of Solar Magnetic Flux Ropes and Their Magnetosheaths)

Reporting period: 2020-06-01 to 2021-11-30

This project studies Coronal Mass Ejections (CMEs) – gigantic eruptions of plasma and magnetic field from the Sun in the form of magnetic flux ropes - and their turbulent sheath regions. CME flux ropes and their sheaths are key agents of driving strong space weather storms at Earth that can have significant consequences for our space assets in orbit and their related applications (e.g. communication) and for many ground technological systems with substantial economic losses. The key parameter to determine a CME’s ability to disturb our near-Earth space is its magnetic field (direction and magnitude). The magnetic field, however, cannot currently be determined in advance and must be measured directly. This severely limits the current accuracy of space weather forecasting and the understanding of the physics behind the initiation and evolution of CMEs. Another big issue is that the turbulent sheath ahead of CMEs is currently poorly understood. In addition, CME‐driven sheaths present a unique natural environment to study fundamental plasma physical processes.

The SolMAG project aims to derive realistic estimates of the magnetic field in both CME flux ropes and sheaths from Sun to Earth. The project Work Packages (WPs) and their key overall objectives are

WPA “Quantification of Magnetic Structure and Evolution of CME Flux Ropes” analyses CME flux ropes from the point of their formation at the Sun until sampled in-situ. The key objective of WPA is to deliver realistic information on the intrinsic magnetic structure of CMEs at the time of the eruption and its early evolution in the low corona (few first solar radii).

WPB “Comprehensive Characterization of CME Sheaths Regions” studies CME sheaths using a comprehensive in-situ analysis and simulations. Sheaths accrete gradually as CMEs plough supersonically through interplanetary space and physical processes at the shock and at the flux rope leading edge lead to highly irregular and complex structure. The key objectives of WPB are to quantify the occurrence of various plasma waves in CME sheaths and the nature of turbulence.

WPC “Sheath and CME Interactions” studies coupling of turbulent sheaths to Earth’s magnetosheath and one of the most complex processes related to CMEs - the merging of multiple eruptions. The key objectives are to detail how multi-scale CME sheath fluctuations transfer to the Earth’s magnetosheath and use our data-driven simulations and in-situ observations to study CME-CME interactions with flux ropes that have different magnetic structure.
The first period of the project has produced several important scientific results.

We have performed the first fully data-driven and time-dependent simulations of erupting flux ropes in the inner corona using realistic boundary conditions. This approach thus provides self-consistently an estimate of the magnetic structure of the CME flux rope, only magnetograms from the surface of the Sun are used to drive the model. Our method is computationally efficient, and thus, has the potential to be used in near-real time space weather forecasting. The project has also resulted in a novel scheme to estimate the magnetic structure of flux ropes using a synthesis of several proxies based on remote-sensing solar observations. In particular, our simulation/observational results can constrain CME flux ropes in state-of-the-art numerical space weather simulations (e.g. European space weather model EUHFORIA). Our study using direct solar wind measurements from pairs of lined-up spacecraft revealed that the magnetic field structure of CME flux ropes typically does not significantly change in interplanetary space (after CME has propagated to about one-third of its Sun-Earth journey). This means that a spacecraft placed between the Sun and Earth could produce accurate early‐warning space weather forecasts of Earth-impacting CMEs.

The interaction between CMEs can however result in dramatic changes in flux rope properties over relatively short heliospheric distances. We analysed a case where two CMEs were still detached from each other at the orbit of Venus, but had merged into a single coherent flux rope while they reached the Earth. The outcome of such interaction depends strongly on the magnetic structure of CMEs and our study also highlights the importance of having realistic information of intrinsic CME magnetic fields, accurate modelling of CME propagation in the heliosphere and multi-spacecraft observations.

Our comprehensive analysis showed that certain types of plasma waves (mirror mode and ion acoustic waves) are ubiquitous in CME sheaths, in particular close to the CME-driven shock, and they form at the early stages of CME evolution. Our most recent studies have focused on analyzing the properties of turbulence in CME-driven sheaths and its variations from the shock to the CME leading edge. These studies are important for understanding how CME sheaths form and evolve. Characterizing turbulence and plasma waves in sheaths is also highly important for space weather forecasting. Our project studies have also demonstrated turbulent CME-sheaths perturb particularly strongly the Van Allen radiation belts that circle the Earth. High-energy electrons in the belts are a significant threat to satellites orbiting in that region.
In the second part of the project we will couple our inner coronal simulation to outer coronal simulation. With this coupled model we will perform simulations of CME flux ropes through the corona and can capture also the eruption process. We will explore how magnetic helicity and free magnetic energy dictate when the eruption will happen and the best ways to estimate the non-inductive electric field component that is largely responsible for energizing the coronal magnetic field. One issue we need to resolve is how to best use our data-driven model to constrain CME flux ropes in numerical space weather simulations at the outer edge of the corona (about 20 solar radii away from the Sun). Only very recently have numerical space weather simulations (e.g. EUHFORIA, ENLIL) reached the maturity to include CME flux ropes. Before, they have been driven with “cone-models” only, where CMEs do not have a magnetic structure. Driving these models with realistic CME initial magnetic fields would be a major improvement to space weather forecasting. Exploring interactions of CMEs using the information from data-driven coronal models with realistic electric fields as boundary conditions goes also clearly beyond the current state-of-the-art.

Our sheath studies will deepen into a more detailed analysis of turbulence in different parts of the sheaths and coherence of magnetic field structures in sheaths. Our most recent findings also underline the importance of comprehensive understanding of the magnetic field in the sheath. Thus, they support any suggestions on a multi-spacecraft mission launched into interplanetary space, which could be used to investigate spatial extent of structures in the sheath. These studies will also include the analysis of sheath turbulence in the inner heliosphere when encountered by Parker Solar Probe and Solar Orbiter. We can immediately use our tools for the data of these spacecraft when available. By the end of the project, we expect to have gained major new insight on both large- and small-scale sheath properties, including how these fluctuations transfer into the Earth’s magnetic environment.
flux rope in TMFM simulation