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

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 ACE, Wind, and IMP8 to cover the energy range from ~30keV/nucleon to ~400 MeV/nucleon. These two events arose from the same active region and launched into similar interplanetary conditions. 

Both events were associatied with large western flares and fast coronal mass ejections (CMEs). Nevertheless, the two events are distinctly different, thereby illustrating fundamental differences between flare- and shock- accelerated SEPs. The 2001 April 14 event shows very similar Fe and O energy spectra and thousand-fold enhancements over coronal abundances for 3He/4He and trans-Fe elements. 

These characteristics suggest that flare-acceleration processes dominate in the 2001 April 14 event, making it the largest impulsive event of Solar Cycle 23. The 2001 April 15 event, which is the largest ground-level event (GLE) so far in this Cycle, lacks these strong abundance distortions. 

However, the Fe and O energy spectra are different, with iron having a harder power-law energy spectrum than oxygen. The CME-driven shock is presumably the primary accelerator in the 2001 April 15 event. But the spectral differences between Fe and O, which are also seen in many other gradual events of various sizes and heliolongitudes, present new challenges for the application of shock theory to SEP production.