Flare- and Shock-accelerated Energetic Particles in the Solar Events of 2001 April 14 and 15

Abstract

We report heavy-ion composition and spectra for the solar energetic particle (SEP) events of 2001 April 14 and 15, using the combined capabilities of the Advanced Composition Explorer (ACE), Wind, and the Interplanetary Monitoring Platform 8 (IMP-8) to cover the energy range from ∼30 keV nucleon^(-1) to ∼400 MeV nucleon^(-1). These two events are, respectively, the largest impulsive event and the largest ground-level event observed so far in solar cycle 23. These events arose from the same active region and launched into similar interplanetary conditions. Both were associated with large western flares and fast coronal mass ejections (CMEs). However, the two events are distinctly different, thereby providing useful reminders of the fundamental differences between flare- and shock-accelerated SEPs. The detailed observations present challenges for our theoretical understanding of SEP production. Of particular note is the fact that iron has a harder power-law energy spectrum than oxygen above ∼3 MeV nucleon^(-1) in the shock-dominated April 15 event. This spectral difference, which is seen in many other gradual events of various sizes and heliolongitudes, leads to enhanced Fe/O at high energies. Simple shock acceleration models predict the same power-law index for all species. Thus, understanding the origin of this spectral difference will significantly contribute to the resolution of the ongoing debate about the relative roles of CME-driven shocks and flares in producing high-energy solar heavy ions.

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