High-energy cosmic ray intensity increase of nonsolar origin and the unusual Forbush decrease of August 1972

A series of spectacular cosmic ray events, which included two relativistic solar particle enhancements and three major Forbush decreases, was registered by ground-based cosmic ray monitors beginning on August 4, 1972. Among these, the Forbush decrease that occurred on August 4–5 exhibited extremely interesting and complex behavior, the prominent features of which are a preincrease (PI-1) prior to the largest Forbush decrease (FD-2) during the recovery of which an abrupt universal time increase (PI-2) occurred. Large N-S and E-W anisotropies were observed during the entire Forbush decrease event. The rigidity spectra for both FD-2 and PI-2 had practically the same exponent of −1.2±0.2 and an upper cutoff rigidity of about 50–60 GV, and the anisotropy during both PI-1 and PI-2 was from the sunward direction. This paper describes the detailed observational features and presents a unified model to explain these in terms of a transient modulating region associated with the passage of a shock front. In this model, the reflection of particles from the approaching shock front accounts for the preincrease (PI-1), whereas the early onset of FD-2 from the antisun direction is caused by the occultation of particle trajectories reaching the earth from that direction while the detectors looking along the sunward direction are still sampling albedo particles reflected from the shock front. The main Forbush decrease occurs as the shock front, containing tangled magnetic fields with large-scale tangential discontinuities, sweeps past the earth. The particles, diffusing into the cavity as they are swept by the solar wind, get ‘piled up’ behind the tangled field region causing the abrupt increase (PI-2). Evidence from interplanetary plasma, radio, and field measurements is provided in support of the model wherever possible.