Correlation of upper-atmospheric Be-7 with solar energetic particle events

A surprisingly large concentration of radioactive Be-7 was observed in the upper atmosphere at altitudes above 320 km on the LDEF satellite that was r ecovered in January 1990. We report on follow-up experiments on Russian spa cecraft at altitudes of 167 to 370 km during the period of 1996 to 1999, sp ecifically designed to measure Be-7 concentrations in low earth orbit. Our data show a significant correlation between the Be-7 concentration and the solar energetic proton fluence at Earth, but not with the overall solar act ivity. During periods of low solar proton fluence, the concentration is cor related with the galactic cosmic ray fluence. This indicates that spallatio n of atmospheric N by both solar energetic particles and cosmic rays is the primary source of Be-7 in the ionosphere.