The Disappearance of Large, Fe-Rich Solar Energetic Particle Events in the Declining Phase of Cycle 23: Implications for the Role of Flares

Details The Disappearance of Large, Fe-Rich Solar Energetic Particle Events in the Declining Phase of Cycle 23: Implications for the Role of Flares The Disappearance of Large, Fe-Rich Solar Energetic Particle Events in the Declining Phase of Cycle 23: Implications for the Role of Flares

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsh41b..03d&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #SH41B-03

Physics

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

Scientific paper

Solar energetic particle (SEP) events are generally divided into two categories, "gradual" and "impulsive". corresponding, respectively, to acceleration by shocks driven by fast coronal mass ejections (CMEs) or acceleration at sites associated with flares, probably through resonant wave-particle interactions following magnetic reconnection. One of the defining distinctions between the two types is the event-integrated Fe/O ratio, with gradual events at a few MeV/nucleon exhibiting typical coronal values while impulsive events generally show strong enhancements. But the precise, comprehensive observations from a fleet of new spacecraft at the start of Cycle 23 immediately challenged this neat picture: Fe/O ratios generally varied with energy, and a large fraction of the nominally "gradual" events, when observed at energies above the few MeV/nucleon where the two categories were originally developed, showed enhanced Fe/O ratios approaching those typically associated with impulsive events. In 1997-2002, 13 out of the 38 very large SEP events (identified by >30 MeV proton fluence above 2 x 105/cm2-sr) had an Fe/O ratio above 30 MeV/nucleon that was at least four times the nominal coronal value. But in 2003-2005, zero out of the 20 events satisfying the same selection criterion displayed comparably large Fe/O enhancements. This dramatic shift clearly indicates that the condition(s) that allow flares to contribute to large SEP events have changed in some fundamental way in the declining phase of Cycle 23. In particular, three hypotheses (direct-flare; shock- acceleration of escaping suprathermals from the accompanying flare; or shock acceleration of remnant suprathermals from previous flare activity) have been proposed in order to explain the flare-like composition seen at high energies in some large gradual events. Based on comparisons of the reported flare, CME, and suprathermal characteristics in the two time periods, we show that the third hypothesis appears to be most likely to be able to accommodate the late-Cycle disappearance of the large Fe-rich events. We also examine open issues surrounding this explanation that Sentinels will be able to address.

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