The unusual relativistic solar proton events of 1979 August 21 and 1981 May 10

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 similar to 9 GHz flux densities S-P(9 GHz) > 10(3) 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 [S-P(9 GHz) < 30 sfu vs. a median value of similar to 8000 sfu for all GLEs] and slower CMEs (plane-of-sky speeds similar to 800 km s(-1) vs. a median of similar to 1600 km s(-1)). The sunspot groups in which these two events originated ranked near the bottom of GLE-parent regions in terms of sunspot area (similar to 100 millionths of a solar hemisphere [msh] vs. a median of similar to 850 msh). What enabled these two otherwise commonplace solar eruptions to accelerate protons to GeV energies? In both cases, intense, long-duration, metric type II bursts were observed. In addition, both of these GLEs occurred when the background similar to 10 MeV proton intensity at 1 AU was > 1000 times the normal background because of preceding SEP events originating in active regions that were located in each case similar to 100 degrees 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 similar to 2-5 R-circle dot of the solar surface. While an elevated similar to 10 MeV proton background at Earth is a favorable/common condition for GLE occurrence, it is not a requirement.