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Energy-dependent SEP Fe/O abundances during the May 2024 superstorm

G. D. Muro, C. M. S. Cohen, Z. Xu, R. A. Leske, A. C. Cummings, S. Bale, G. D. Berland, E. R. Christian, M. E. Cuesta, M. I. Desai, F. Fraschetti, J. Giacalone, L. Y. Khoo, A. Labrador, D. J. McComas, J. G. Mitchell, M. Pulupa, N. A. Schwadron, M. M. Shen

Abstract

During mid-May 2024, active region (AR) 13664 produced a series of M- and X-class flares along with several coronal mass ejections (CMEs) that resulted in exceptionally strong aurora at Earth. This study presents in-situ solar energetic particle (SEP) ion composition data from Solar Terrestrial Relations Observatory Ahead (STA), Advanced Composition Explorer (ACE), and Parker Solar Probe (PSP) as their magnetic connectivity to AR 13664 varied throughout the event period. Between 08 to 24 May, STA was separated by 12° in longitude from ACE at 0.96 AU. SEP intensities rose gradually due to merged CMEs from AR 13664. On 13 May, an M6 flare was followed by a rapid-onset SEP event at STA, although velocity dispersion analysis yielded no clear path length or release time. PSP, 95° longitudinally separated from Earth at 0.74 AU, observed gradually increasing SEP intensities beginning 11 May, followed by a jump in both SEP intensity and magnetic field (>100 nT) on 16 May. These early event intervals display stepwise SEP increases, consistent with the passage of successive CMEs. On 20 May, an X16.5 flare from AR 13664 produced an Fe-rich SEP event observed at all three spacecraft despite their wide longitudinal separations. Throughout the period, Fe/O ratios ranged from <0.01 to >0.8 and increased with energy between 1 to 100 MeV/nuc. This trend deviates from the typical energy-dependent decrease expected from diffusive shock acceleration and suggests more complex scenarios, possibly involving variable suprathermal seed populations or species-dependent transport.

Energy-dependent SEP Fe/O abundances during the May 2024 superstorm

Abstract

During mid-May 2024, active region (AR) 13664 produced a series of M- and X-class flares along with several coronal mass ejections (CMEs) that resulted in exceptionally strong aurora at Earth. This study presents in-situ solar energetic particle (SEP) ion composition data from Solar Terrestrial Relations Observatory Ahead (STA), Advanced Composition Explorer (ACE), and Parker Solar Probe (PSP) as their magnetic connectivity to AR 13664 varied throughout the event period. Between 08 to 24 May, STA was separated by 12° in longitude from ACE at 0.96 AU. SEP intensities rose gradually due to merged CMEs from AR 13664. On 13 May, an M6 flare was followed by a rapid-onset SEP event at STA, although velocity dispersion analysis yielded no clear path length or release time. PSP, 95° longitudinally separated from Earth at 0.74 AU, observed gradually increasing SEP intensities beginning 11 May, followed by a jump in both SEP intensity and magnetic field (>100 nT) on 16 May. These early event intervals display stepwise SEP increases, consistent with the passage of successive CMEs. On 20 May, an X16.5 flare from AR 13664 produced an Fe-rich SEP event observed at all three spacecraft despite their wide longitudinal separations. Throughout the period, Fe/O ratios ranged from <0.01 to >0.8 and increased with energy between 1 to 100 MeV/nuc. This trend deviates from the typical energy-dependent decrease expected from diffusive shock acceleration and suggests more complex scenarios, possibly involving variable suprathermal seed populations or species-dependent transport.

Paper Structure

This paper contains 15 sections, 11 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Spacecraft configuration and solar activity
  3. In-situ SEP instruments
  4. Energetic particle measurements
  5. Solar interval partitioning
  6. Ion composition measurements
  7. Fe/O analysis
  8. Discussion
  9. Time profiles and evolving magnetic connection
  10. Spectral shapes
  11. Fe/O energy-dependent variation
  12. Stepwise increases in intensity as merged CMEs
  13. Flare contributions and shock geometries
  14. UserColor Effect of transport processes
  15. Conclusion

Figures (11)

  • Figure 1: Spacecraft configuration of ACE in grey, STA in red, and PSP in violet from the Solar-MACH model gieseler2023 during mid-May 2024. The black reference arrows designate the longitude of AR 13664. All nominal Parker spiral field lines are UserColor mapped back from the observer at (left) 00:00 on 8 May, (middle) 00:00 on 13 May and (right) 00:00 on 20 May, based on 400 $km s^{-1}$ solar wind speed, and colored to match their associated spacecraft, with the dotted line corresponding to the AR field line. See Table \ref{['tab-solarmach']} for UserColor coordinates .
  • Figure 2: Ultraviolet image from GOES/SUVI 131 on 8 May 2024, enhanced with multi-scale Gaussian normalization.
  • Figure 3: (Top) Proton measurements from STA/HET and PSP/HET, partitioned into three separate periods of solar intervals, from 8 to 24 May 2024. Between them is a visual representation of the timings for CMEs as a black cross and X-class flares as a red X generated by AR 13664, adapted from hayakawa2025 and jaswal2025. (Bottom) Magnetic field measurements from ACE, STA, and PSP.
  • Figure 4: Intensity plots measured by STA/LET A for: (1) protons, (2) helium, (3) oxygen, and (4) iron at 1 minute cadence.
  • Figure 5: Intensity plots measured by ACE/SIS for: (1) helium, (2) oxygen, and (3) iron at 256 second cadence.
  • ...and 6 more figures