The acceleration of energetic particle fluxes in shock fronts in interplanetary space

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Abstract
On six occasions during 1966, enhanced fluxes of > 7.5-MeV energetic particles have been observed coincident with the onset of a Forbush decrease which was initiated by a solar flare. The anisotropic character of the particle fluxes indicates that the particles were not trapped and that particle acceleration was occurring in the region associated with the onset of the Forbush decreases. Subsequent to the onset of the Forbush decreases, bidirectional anisotropies of 7.5–45 MeV-protons were observed. It will be shown that these observations provide strong evidence for the shock-wave model for the Forbush decrease. It is suggested that particle acceleration in interplanetary shocks is a commonly occurring phenomenon, and, in particular, it is suggested that recurrent ≈1-MeV proton events are due to particle acceleration in corotating shock fronts.