Interplanetary Deceleration of Solar Cosmic Rays

Abstract

Observations of solar-flare proton fluxes of low energies (1-10 MeV) during the 7 June 1969 event have been used to study the effects of energy-change processes on particles propagating in interplanetary space. It is found that the proton energies are decreasing with time at a rate which is consistent with an exponential-decay time constant of 210±10 h. Since adiabatic deceleration in a uniform solar wind would result in a faster decay (78±4 h), additional processes such as Fermi acceleration or a more general deceleration process must be considered.