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

TL;DR

This study analyzes energy-dependent Fe/O abundances in solar energetic particles during the May 2024 superstorm, revealing complex composition trends that challenge standard acceleration models and suggest diverse transport or seed populations.

Contribution

It provides the first detailed multi-spacecraft analysis of SEP Fe/O ratios during a major solar storm, highlighting deviations from expected energy dependence and proposing new interpretations.

Findings

Fe/O ratios varied from <0.01 to >0.8 across energies.

Fe/O ratios increased with energy, contrary to typical models.

SEP events were observed across wide longitudinal separations.

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{\deg} 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{\deg} 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.