Evidence for in situ particle energization during the May-2024 event based on ASPEX instrument on board Aditya-L1

TL;DR

This paper tackles how ICME–ICME interactions modify particle energization and solar-wind structure by analyzing directionally resolved energy flux spectra from two orthogonal planes on Aditya-L1’s ASPEX SWIS (THA-1 and THA-2) and energetic particle data from STEPS during the May 2024 event. The authors identify two complex ejecta (CE1 and CE2) and focus on Interval B, where a forward shock propagates into the trailing magnetic cloud of CE1, creating a long-lived downstream energized region and pronounced cross-plane transport of protons and alpha particles. Velocity distributions reveal both magnetic-cloud-like bimodality (in intervals A and A') and strong shock-driven broadening and species merging in Interval B, consistent with turbulence-driven heating and wave–particle interactions. The results provide direct, plane-resolved evidence of in situ particle energization in ICME–ICME interactions and emphasize the importance of multi-plane measurements for understanding energy partitioning and transport in the inner heliosphere.

Abstract

The interaction between interplanetary Coronal Mass Ejection (ICME) structures can alter the geo-effectiveness of the ICME events in myriad ways. Many aspects of these interaction processes are not well-understood till date. Using the energy spectra measured in two mutually orthogonal top hat analyzers (THA 1 and 2), which are part of the Solar Wind Ion Spectrometer (SWIS) subsystem of the Aditya Solar Wind Particle EXperiment (ASPEX) on board India's Aditya L1 mission, we gain insights into intricate features of ICME ICME interactions during May 2024 solar event. We report here an unprecedented two-orthogonal-plane perspective of ICME ICME interactions for the first time from the L1 point. The investigation reveals a special interaction region formed by the propagation of the forward shock driven by complex ejecta in the preceding ICME. The interaction causes the formation of a downstream region spanning over 13 hours, which propagates in the interplanetary medium. The observations reveal that this region serves as a site for proton and alpha particle energization, and the particles within this region get distributed from one plane to the other. The presence of forward shock and particle energization is confirmed by the energetic particle flux measurements by the SupraThermal and Energetic Particle Spectrometer (STEPS) of ASPEX. These observations provide an unprecedented perspective on how solar wind ions become energized and distributed in an ICME-ICME interaction region.

Figures (4)

• Figure 1: Energy flux spectrograms from AL1-ASPEX-SWIS from 10 May 2024 to 15 May 2024. The panels (a) and (b) show the energy flux spectrograms for THA-1 and THA-2 of AL1-ASPEX-SWIS. The color bar on the right of the upper two panels a and b represents the order of energy flux $\left(\mathrm{eV}\,\mathrm{cm}^{-2}\,\mathrm{sr}^{-1}\,\mathrm{s}^{-1}\,\mathrm{eV}^{-1} \right)$, whereas the x-axis represents the time in UTC and the y-axis represents the Energy values in E(eV)/q. Panels (c) represent the velocity components $V_{x}$ (green), $V_{y}$ (blue), and $V_{z}$ (red) in km/s. Panel (d) illustrates the plasma temperature (red, in eV) and plasma beta (blue). Panels (e) represent the magnetic field components $B_{x}$ (green), $B_{y}$ (blue), and $B_{z}$ (red) in km/s. The vertical black solid lines in all the panels represent the boundaries of the complex ejecta 1 and 2, respectively. The two complex ejecta structures: Complex Ejecta 1 (CE1) comprises the UTC time interval shown by shaded blue and grey regions in panels b, c and d, while Complex Ejecta 2 (CE2) comprises the UTC time interval shown by the shaded red region in panels c, d, and e. The grey region, which is part of CE1, lies between the blue and red shaded intervals, corresponds to Interval B. The region between the first dotted line within CE1 and the dashed line represents interval A, which is followed by interval B (gray shaded). The region between the solid boundary of CE2 and the dotted line inside CE2 corresponds to interval $A'$. The dashed line inside the CE1 represents the shock structure preceding the interval B.
• Figure 2: (a) Energy-resolved ion flux measurements from the STEPS instrument onboard AL1-ASPEX during 10–13 May 2024. The four panels display differential ion fluxes from the STEPS-PS, STEPS-IM, STEPS-NP, and STEPS-EP detectors across energy channels ranging from $\sim$0.12 to $\sim$1.9 MeV, with color-coded lines denoting different energy bands. The interval B lies between the vertical red dash-dot (mark the forward shock arrival (08:47 UTC on 12 May) ) and blue dash-dot lines. (b) Time-energy spectrograms of particle fluxes measured by THA-1 (top panels) and THA-2 (bottom panels) on 12 May 2024, shown for selected angular sectors. The color scale represents the logarithm of the particle flux $[\mathrm{eV}/\mathrm{sr \cdot s \cdot cm^2 \cdot eV}]$, indicating variations in intensity as a function of energy and time. Vertical dashed lines denote boundaries encompassing Interval 3. For THA-1, Sector 10 is oriented sunward (approximately along the $-Y_{\mathrm{GSE}}$ direction), while for THA-2, Sector 17 is sunward (approximately along the $+Z_{\mathrm{GSE}}$ direction). The blue arrows indicate the approximate GSE coordinate orientation of the sectors shown in each panel, providing context for the directional information of the measured fluxes.
• Figure 3: Temporal evolution of particle fluence spectra and velocity distribution functions (VDFs) observed by THA-1 and THA-2 during the three distinct intervals A, B, and $A'$ on 12--13 May 2024. The panels in the first, middle, and rightmost columns correspond to intervals A, B, and $A'$, respectively. The fluence spectra for THA-1 are shown in Figure 3, panels (a)--(c), and for THA-2 in panels (g)--(i), representing fluence as a function of energy (eV) for the intervals 01:06--08:44 UTC, 08:47--22:00 UTC, and 22:04 (12 May)--04:37 (13 May) UTC, respectively. The corresponding 2D velocity distribution functions are displayed in panels (d)--(f) for THA-1 and panels (j)--(l) for THA-2, shown in the $v_x$--$v_y$ and $v_x$--$v_z$ planes, respectively, with color scales indicating $VDF~[s^3/cm^6]$. The fluence plots highlight the evolution of energetic particle populations, while the VDFs reveal anisotropies and velocity-space structures that illustrate the dynamic processes of particle acceleration and transport during each interval.
• Figure 4: Schematic representation of the ICME--ICME interaction sequence during May 2024. The illustration shows the propagation of Complex Ejecta 1 (CE1) and Complex Ejecta 2 (CE2) originating from Active Region 13664 on the Sun. The forward shock driven by CE2 impacts the trailing part of ICME 4 (associated with CE1), resulting in plasma compression and localized heating. The region labeled Interval B marks the site of enhanced proton and alpha-particle energization observed at L1.