Abstract
Sixty-nine ground level events (GLEs) caused by relativistic solar protons have been observed from 1942 to 2005. GLEs are characteristically associated with intense solar flares [having peak ~9 GHz flux densities SP(9 GHz) > 103 sfu] and fast (>1000 km s-1) coronal mass ejections (CMEs). The small GLEs on 1979 August 21 and 1981 May 10 provide an exception to these rules of thumb. In comparison with other GLEs, they were associated with significantly weaker flares [SP(9 GHz) 1000 times the normal background because of preceding SEP events originating in active regions that were located in each case ~100° east of the active region responsible for the GLE. We suggest that the relativistic solar protons observed in these two events resulted from CME-driven shock acceleration of an elevated coronal seed population, reflecting the enhanced background proton intensity at 1 AU. For this scenario, the timing onset of the relativistic protons in the two events indicates that the shocks had access to the energetic seed particles within ~2-5 R☉ of the solar surface. While an elevated ~10 MeV proton background at Earth is a favorable/common condition for GLE occurrence, it is not a requirement.
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