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Local Particle Acceleration in an ICME-in-Sheath Structure Observed by Solar Orbiter

Xiaomin Chen, Chuan Li, Zigong Xu, Georgios Nicolaou, Alexander Kollhoff, George C. Ho, Robert F. Wimmer-Schweingruber, Christopher J. Owen

TL;DR

This study analyzes local particle acceleration within an ICME-in-Sheath (IIS) structure formed during a twin-CME event on 2024 March 23 using coordinated Solar Orbiter and Wind measurements. By combining in-situ particle data (electrons and ions) across 0.4 AU to 1 AU with solar-imaging context, the authors identify electron acceleration at the IIS boundary and characterize ion spectra that largely follow diffusive shock acceleration, with Fe and O showing a double power-law and a flare-seed enhancement in the IIS. The electron spectrum in the IIS exhibits a common solar-origin spectral index of $\sim$2.2 across the shock sheath, IIS, and primary ICME, while Fe/O shows energy-dependent behavior consistent with seed particle contributions and reduced high-rigidity ion acceleration. The work proposes two plausible magnetic-field evolution scenarios for IIS during propagation and highlights the importance of CME–CME interactions in shaping SEP acceleration, with implications for space-weather forecasting and our understanding of particle acceleration in complex interplanetary structures.

Abstract

Local particle acceleration in the shock sheath region formed during the interaction between multiple coronal mass ejections (CMEs) is a complicated process that is still under investigation. On March 23, 2024, the successive eruption of two magnetic flux ropes (MFRs) from the solar active region 3614 produced twin CMEs, as identified in coronagraph images. By analyzing in-situ data from Solar Orbiter and Wind, it is found that the primary ICME-driven shock overtook the preceding ICME, trapping it in the sheath between the shock and the primary ICME, forming the ICME-in-sheath (IIS) structure. Using Solar Orbiter observations, we show that both electrons and ions are accelerated within the IIS. A clear enhancement of suprathermal electrons was observed at the IIS boundary, where strong flow shear and large magnetic field variation suggest possible local electron acceleration. Electrons (>38 keV) exhibit a long-lasting enhancement in the IIS with a spectral index of ~2.2, similar to that in the shock sheath and the primary ICME, indicating a similar solar origin. Inside both the sheath and IIS, spectra of proton and 4He are generally consistent with the prediction of the diffusive shock acceleration, whereas Fe and O present a double power-law shape. Additionally, the Fe/O ratio in the IIS is higher than that in the sheath, and more close to the abundance of the flare-related particles, suggesting the remnant particles of flare confined in the IIS.

Local Particle Acceleration in an ICME-in-Sheath Structure Observed by Solar Orbiter

TL;DR

This study analyzes local particle acceleration within an ICME-in-Sheath (IIS) structure formed during a twin-CME event on 2024 March 23 using coordinated Solar Orbiter and Wind measurements. By combining in-situ particle data (electrons and ions) across 0.4 AU to 1 AU with solar-imaging context, the authors identify electron acceleration at the IIS boundary and characterize ion spectra that largely follow diffusive shock acceleration, with Fe and O showing a double power-law and a flare-seed enhancement in the IIS. The electron spectrum in the IIS exhibits a common solar-origin spectral index of 2.2 across the shock sheath, IIS, and primary ICME, while Fe/O shows energy-dependent behavior consistent with seed particle contributions and reduced high-rigidity ion acceleration. The work proposes two plausible magnetic-field evolution scenarios for IIS during propagation and highlights the importance of CME–CME interactions in shaping SEP acceleration, with implications for space-weather forecasting and our understanding of particle acceleration in complex interplanetary structures.

Abstract

Local particle acceleration in the shock sheath region formed during the interaction between multiple coronal mass ejections (CMEs) is a complicated process that is still under investigation. On March 23, 2024, the successive eruption of two magnetic flux ropes (MFRs) from the solar active region 3614 produced twin CMEs, as identified in coronagraph images. By analyzing in-situ data from Solar Orbiter and Wind, it is found that the primary ICME-driven shock overtook the preceding ICME, trapping it in the sheath between the shock and the primary ICME, forming the ICME-in-sheath (IIS) structure. Using Solar Orbiter observations, we show that both electrons and ions are accelerated within the IIS. A clear enhancement of suprathermal electrons was observed at the IIS boundary, where strong flow shear and large magnetic field variation suggest possible local electron acceleration. Electrons (>38 keV) exhibit a long-lasting enhancement in the IIS with a spectral index of ~2.2, similar to that in the shock sheath and the primary ICME, indicating a similar solar origin. Inside both the sheath and IIS, spectra of proton and 4He are generally consistent with the prediction of the diffusive shock acceleration, whereas Fe and O present a double power-law shape. Additionally, the Fe/O ratio in the IIS is higher than that in the sheath, and more close to the abundance of the flare-related particles, suggesting the remnant particles of flare confined in the IIS.
Paper Structure (8 sections, 5 figures, 1 table)

This paper contains 8 sections, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Instrumentation
  3. Observations and Results
  4. Overview of the solar eruption
  5. Structure of the shock sheath from 0.4 AU to 1 AU
  6. Electron acceleration at the boundary layer of the IIS
  7. Protons and heavy ions in the shock sheath
  8. Conclusion and discussion

Figures (5)

  • Figure 1: Overview of the solar eruption on 2024 March 23. (a) PFSS modeling of magnetic fields overlaid on the AIA 94Å image, with closed and open field lines shown in white and yellow, respectively. (b) Base-difference AIA 94Å image, with the magnetic flux rope outlined by a yellow dashed line. (c) AIA 304Å image, highlighting the erupting filament in green. (d) Positions of Solar Orbiter and Earth on the ecliptic plane with IMF Parker spirals. The solar wind speed is averaged over 00:00–01:00 UT and indicated in the legend. (e) LASCO-C2 white-light and CHASE H$\alpha$ images. The CHASE H$\alpha$ image was taken at 01:12 UT during the impulsive phase of the flare. (f) Same as (e), but for the second CME.
  • Figure 2: In-situ measurements of the ICME and shock sheath structures. (a) Solar Orbiter measurements, showing, from top to bottom, the magnetic field in RTN coordinates, $\phi_B$, $\theta_B$, solar wind velocity, density, temperature, and PADs of strahl electrons ($\textgreater$70 eV). The five vertical dashed lines mark the shock arrival, the boundary layer of the IIS, a sharp rotation of magnetic field, the entering of the IIS, and the primary ICME. (b) Shock sheath parameters from Wind in RTN coordinates, with the last panel showing PADs of strahl electrons (121 eV). Three vertical dashed lines indicate the shock arrival, the IIS boundary, and the boundary between the IIS and the primary ICME. Panels (c) and (d) illustrate the crossings of Solar Orbiter and Wind of the shock sheath, the IIS and the ICME structures in the ecliptic plane.
  • Figure 3: Magnetic fields observed by MAG and electron observation by EAS and EPT. (a) Magnetic field in RTN coordinates. (b) wavelet PSD of magnetic field fluctuations. (c) PADs of strahl electrons ($\textgreater$70 eV) (d) PADs of electrons over 1.14-5.29 keV. (e) PAs of EPT dectectors (f) PADs of electrons at 38.6 keV. (g) temporal profiles of electrons over 38-186 keV averaged in four directions.
  • Figure 4: Panel (a) shows the electron VDFs observed by EAS. Panel (b) shows the electron spectra observed by EPT.
  • Figure 5: Panel (a) presents temporal profiles of H, $^4$He, Fe and O observed by SIS averaged over a and b directions. Panel (b) shows the particle spectra in the shock sheath. Panel (c) shows the particle spectra in the IIS.