Interplanetary acceleration of solar cosmic rays to relativistic energy

Abstract

Although the August 1972 cosmic ray storm (i.e., a rapid succession of Forbush decreases) was the greatest in more than 3 decades of continuous observations, halving the flux of galactic cosmic rays above 1 GeV, its basic characteristics were not unusual despite its unprecedented magnitude and complexity. However, the associated ground level event of August 4, representing the arrival of relativistic solar particles, displayed abnormal features. It is suggested that the observed nucleonic intensity enhancement was a consequence of the acceleration of ambient lower-energy solar protons to relativistic energies by reflection between two shocks moving with respect to each other in the interplanetary medium. A reexamination of the only other recorded disturbance that approached the magnitude of the recent cosmic ray storm shows that the ground level event of July 19, 1959, may have been similarly produced by this interplanetary acceleration mechanism.