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

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