Evidence for Solar-Cycle Changes in the Inner-Belt Protons

Abstract

Observations of high-energy photons geomagnetically trapped in the inner Van Allen belt have been carried out since September 1962. Between November 1962 and June 1966, the omnidirectional proton flux at 63 MeV remained constant to within ±7.6%. However, since August 1966 we have obtained substantial evidence that the proton flux is decreasing in a manner which is attributable to solar-cycle variations in the density of the upper atmosphere.