What causes the variability in the properties of energetic storm particle (ESP) events?

Abstract

Energetic storm particle (ESP) events are enhancements above ∼0.05 MeV/nucleon ions near 1 AU in association with the passage of an interplanetary (IP) coronal mass ejection (ICME). The primary candidate of producing these enhancements is diffusive shock acceleration (DSA). ESPs can produce significant increases in the near-Earth particulate radiation and pose severe hazards to astronauts and hardware in space. Physical parameters thought to affect ESP production include IP shock properties (e.g., speed, strength, obliquity) and conditions upstream of the propagating shock (e.g., seed population, ambient plasma conditions). Several theoretical and observational studies tried to relate ESP production to these drivers; however, reliable prediction of ESP properties (e.g. intensities, spectra, abundances), including their event-to-event variability, has so far proven elusive. This indicates an incomplete understanding of how ICME-driven IP shocks accelerate ESPs. Using instruments onboard ACE, we investigate the relations between a large set of parameters (28) that characterize (i) ESP properties, (ii) IP shock and ICME properties, and (iii) the upstream and downstream conditions measured across the IP shock. Ee present a comprehensive correlation matrix between all parameters and those of the ESP properties, in an attempt to identify the dominant parameters that influence ESP production and variability. We find that within the selected dataset, (1) spectral and compositional relations strongly indicate a rigidity-dependent acceleration mechanism, (2) correlations between observations and DSA predictions are poor, (iii) ICME sheath temperature appears to play a role in determining the ESP peak intensities.