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Paper: Ground-Level Enhanced (GLE) Solar Particle Events at Solar Minimum
Volume: 428, SOHO-23: Understanding a Peculiar Solar Minimum
Page: 329
Authors: Tylka, A. J.; Dietrich, W. F.
Abstract: Ground-Level Enhanced (GLE) solar particle events, in which proton energies exceed ∼400 MeV and produce measurable rate increases in terrestrial neutron monitors, are rare. GLEs at solar minimum are even rarer. Since the creation of the world-wide neutron monitor network in 1955, 66 GLEs have been reported. Only two of these, on 1976 April 30 and 2006 December 13, occurred at (or nearly at) solar minimum conditions, when the monthly smoothed sunspot number was less than 15. The 1976 event was also observed by particle instruments on the IMP8 spacecraft; the 2006 event was observed by instruments on Wind, ACE, SOHO, GOES, and STEREO. We examine the particle characteristics of these two events at satellite and neutron monitor energies. In general, these two GLEs are surprisingly similar, both to each each other and to GLEs that occur at other times during the solar cycle. The most noteworthy difference between the two GLEs is their relative sizes. This difference suggests that contributions to the coronal suprathermal seed population from activity in the days before the event play a critical role in the production of GLEs.
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