High-Energy Insights from an Escaping Coronal Mass Ejection

(Solar Orbiter Nugget #46 by Laura A. Hayes (1,*), Säm Krucker (2,3), Hannah Collier (2,4), and Daniel Ryan(2))

1. Introduction

Solar eruptive events, consisting of both solar flares and coronal mass ejections (CMEs), represent some of the most energetically dynamic phenomena in our solar system. These events involve the rapid release of magnetic energy, which is converted into plasma heating, particle acceleration, and kinetic energy driving the CME. Understanding how this energy is partitioned remains a central challenge in solar physics. Hard X-ray observations provide important insights into these processes, targeting accelerated electrons and hot plasma (>10 MK). However, high-altitude X-ray emissions linked to CMEs are often too faint to detect compared to the intense brightness of flare footpoints in the lower atmosphere. Occulted events, where the bright flare is hidden behind the solar limb, offer a unique opportunity to isolate and study these emissions [e.g. 1]. The February 15, 2022, solar eruption presented such an opportunity. Observed simultaneously by multiple spacecraft, including Solar Orbiter, this event enabled the detailed study of high-altitude X-ray sources, providing valuable insights into the energy distribution and particle acceleration within CMEs.

2. Event Overview

On February 15, 2022, a large filament eruption occurred on the Sun’s eastern limb, resulting in a fast CME, a widespread interplanetary shock, and a solar energetic particle event (see also [2, 3])]. From Solar Orbiter’s vantage point, the eruption was viewed as an occulted event, with the bright flare emission obscured by the solar limb. It was estimated that the source flare associated with the eruption was ~45° behind the limb as seen from Solar Orbiter (see Figure 1). The Spectrometer/Telescope for Imaging X-rays (STIX) captured high-energy X-ray emissions from the eruption, revealing both thermal and non-thermal components. Simultaneously, the Full Sun Imager (FSI) of the Extreme Ultraviolet Imager (EUI) provided complementary observations of the filament's evolution from the Solar Orbiter perspective, and STEREO-A’s Extreme Ultraviolet Imager (EUVI) provided multi-viewpoint observations (see Figure 2). These multi-instrument observations allowed for a comprehensive analysis of the CME’s energetic properties. 

Figure 1. The spacecraft position (left hand panel) and X-ray and radio observations of the eruption as observed by Earth, Solar Orbiter, and PSP (right). The black arrow in the orbit plot denotes the location of the eruption. The GOES X-ray light curves are shown in the top of the right hand panel, where no X-ray emission is detected - by the time the source came into view of Earth the emission was too faint. The middle panel shows the STIX X-ray lightcurves as seen from Solar Orbiter, and the bottom panel shows the dynamic spectra from PSP/Fields - where type-III radio bursts associated with the eruption can be seen.

Figure 2. The spacecraft position (left hand panel) and X-ray and radio observations of the eruption as observed by Earth, Solar Orbiter, and PSP (right). The black arrow in the orbit plot denotes the location of the eruption. The GOES X-ray light curves are shown in the top of the right hand panel, where no X-ray emission is detected - by the time the source came into view of Earth the emission was too faint. The middle panel shows the STIX X-ray lightcurves as seen from Solar Orbiter, and the bottom panel shows the dynamic spectra from PSP/Fields - where type-III radio bursts associated with the eruption can be seen.

3. Key Science Highlights

With STIX we could image and spectrally analyse the X-ray emission associated with the CME. The key results of this study are presented in [4] and are summarised below:

• High-altitude X-ray emissions: STIX detected X-ray emissions originating at altitudes exceeding 0.3 solar radii above the Sun’s surface associated with the CME filament eruption. Spectral analysis found these emissions featured both a hot thermal plasma at 17 MK and non-thermal components with accelerated electron energies above 11 keV, with an electron spectral index of 3.9 (see Figure 3).
• Multi-Instrument and multi-vantage analysis: Combining STIX X-ray data with EUV observations from Solar Orbiter’s EUI/FSI and STEREO-A/EUVI, we tracked the filament’s evolution. X-ray imaging analysis found the X-ray source associated with the CME spatially coincided with the EUV-bright structures observed in 304 Å and 174 Å. The X-ray source grew larger and outwards until the source grew to a size exceeding the STIX imaging limit (180”), see Figure 4.
• X-ray diagnostic of energetics: The thermal X-ray source contained an estimated 3.8x10^37 electrons, with a total thermal energy of ~2.7x10^29 ergs. The non-thermal population comprised approximately 7x10^35 electrons,  and contributed ~2x10^28 erg to the energy budget, which is approximately 7% of the estimated thermal energy.
• Particle acceleration in solar eruptive events: Non-thermal emissions suggest that electrons were accelerated and trapped within the filament structure. This aligns with theoretical models proposing electron injection into complex magnetic flux ropes during CME eruptions.

Figure 3. The left hand panel shows the thermal and non-thermal X-ray sources imaged and plotted on-top of a running difference 174 Å FSI image. The right hand panel shows the X-ray spectral analysis where the thermal (red) and non-thermal (blue) spectral components are fitted. These fitting parameters are used to derive energetics of the X-ray sources.

Figure 4. The top panel shows the time sequence STIX images in 4-10 keV over the peak reconstructed using the CLEAN algorithm with a 30 s time integration, with the green colors and contours showing the X-ray intensities and the black circle showing the forward fit gaussian fit of the source, up-to the 180” limit of STIX imaging (dashed black circle).  The corresponding EUI/FSI maps for the two time intervals for which observations are available are plotted below, with the STIX contours over-plotted on top.

4. Conclusions

This study highlights the importance of occulted solar events for revealing high-altitude X-ray coronal processes. The findings demonstrate Solar Orbiter’s unique capability to bridge observational gaps, particularly in understanding the energetic interplay between flares and CMEs. Multi-vantage point observations are crucial, revealing important details that would remain obscured from a single viewpoint.

Looking ahead, the combination of STIX with instruments like ASO-S’s Hard X-ray Imager and Parker Solar Probe’s in-situ measurements promises to further insights into the open questions in processes occurring in solar eruptive events. Such coordinated efforts will refine our understanding of particle acceleration, magnetic reconnection, and energy transfer in the Sun’s dynamic atmosphere.
 

Affiliations

1. European Space Agency (ESA), ESTEC), Keplerlaan 1, 2201 AZ Noordwijk
* Now at the Astronomy & Astrophysics Section, School of Cosmic Physics, Dublin Institute for Advanced Studies (DIAS), Ireland
2. University of Applied Sciences and Arts Northwestern Switzerland, University of California, ETH Zürich.
3. Space Sciences Laboratory, University of California, 7 Gauss Way, 94720 Berkeley, USA
4. ETH Zürich, Rämistrasse 101, 8092 Zürich, Switzerland
 

References

[1] Krucker, S., White, S. M., & Lin, R. P. 2007, ApJ, 669, L49
[2] Mierla, M., Zhukov, A. N., Berghmans, D., et al. 2022, A&A, 662, L5
[3] Palmerio, E., Carcaboso, F., Khoo, L. Y., et al. 2024, ApJ, 963, 108
[4] Hayes, L. A., Krucker, S., Collier, H, Ryan, D. et al. 2024, A&A 691 (2024): A190.