Physics
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmsh43c..03t&link_type=abstract
American Geophysical Union, Spring Meeting 2008, abstract #SH43C-03
Physics
2101 Coronal Mass Ejections (7513), 2114 Energetic Particles (7514), 7513 Coronal Mass Ejections (2101), 7514 Energetic Particles (2114), 7519 Flares
Scientific paper
Proton acceleration to energies above ~500 MeV is a controversial and poorly understood aspect of solar energetic particle (SEP) physics, even though these very high-energy events have been observed with neutron monitors in Ground Level Enhancements (GLEs) for more than 50 years. For 40 of the 66 GLEs reported since 1956, we have derived absolutely-normalized event-integrated proton spectra, using data from the world-wide neutron-monitor network and published neutron-yield functions (Clem & Dorman 2000), under the assumption that the proton spectra can be represented as power-laws in rigidity. We believe this analysis to be the most extensive catalogue of GLE spectra ever assembled. As a check on our results, we have compared the fluences derived from our neutron-monitor analyses to satellite measurements at ~300-700 MeV available from IMP8, SAMPEX, and/or GOES for 28 of the events. We generally find very good agreement (i.e., to within ~50%) with the satellite fluence-measurements. We also find reasonable agreement with time-dependent spectral indices that have been published previously for some of the events (e.g., Lovell et al. 1999, Lockwood et al. 2002, and Plainaki et al. 2007). Compared to spectral indices below ~0.4 GV (~100 MeV), we find no evidence for spectral hardening above ~1 GV, something that might be expected if two independent acceleration mechanisms dominated particle production in these two rigidity ranges. We present distributions of event properties above 1 GV, including event size, spectral indices, and the degree of spectral steepening relative to lower energies. These results should be useful constraints in developing and testing theoretical models of proton acceleration in GLEs. We also briefly discuss the implications of these results for the design of astronaut storm shelters, whose efficacy has often been evaluated assuming spectral shapes that are much softer than the ones we derive from the neutron-monitor data. Supported by the Office of Naval Research and by NASA DPR NNG06EC55I.
Dietrich William F.
Tylka Allan J.
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