The Seed Population for Heavy Ions in Large Solar Energetic Particle Events

Details The Seed Population for Heavy Ions in Large Solar Energetic Particle Events The Seed Population for Heavy Ions in Large Solar Energetic Particle Events

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsh41b..02d&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #SH41B-02

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2101 Coronal Mass Ejections (7513), 2114 Energetic Particles (7514), 2139 Interplanetary Shocks, 7519 Flares

Scientific paper

Large, CME shock-associated solar energetic particle (LSEP) events have long been assumed to accelerate coronal or solar wind material, and have been used to infer the composition of these material reservoirs. However, recent measurements on ACE have revealed that surprisingly, there are complex and unsystematic differences between the LSEP ion abundances and the solar wind, re-opening the question of the identity of the particle population being energized in these events. Using high-resolution mass spectrometers ULEIS and SIS on board ACE, we have surveyed the ion composition in 64 LSEP events of solar cycle 23. Our results show the following: (1) The Fe spectra steepen at lower energies than those of O. (2) The rare isotope 3He is found in 46% of the events in quantities greatly enhanced over the corona or the solar wind values. (3) The low energy average Fe/O ratio is enhanced over coronal or solar wind values between factors of ~2-40 from one event to another. (4) Heavy ion abundances from C-Fe show systematic M/Q-dependent enhancements when compared with those measured in the quiet corona. Taken together, these results confirm the role of CME-driven shocks in energizing the particles up to ~60 MeV/nucleon, but also show that the seed population is not dominated by ions originating from the bulk corona or the thermal solar wind. Rather, it appears that the source material for CME-associated LSEP events originates predominantly from a suprathermal population with enriched heavy ion abundances that are remarkably similar to those seen in the smaller impulsive flare-related SEP events. These new results indicate that current LSEP models must take account of this dynamic suprathermal seed population as a critical pre- cursor to the CME shock acceleration processes.

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