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. Duringthe declining phase of the current solar cycle, a series of intense solar flares erupted in August 1972 from an active region (McMath plage region 11976) on the solar disk; as a result, severe cosmic ray disturbances on the earth [Pomerantz and Duggal, 1973a] accompanied by quite spectacular visual auroras, geomagnetic storms, radio blackout, and a host of other terrestrial effects were caused. The time

Recurrent Forbush decreases and recurrent geomagnetic disturbances have been attributed to the solar M regions, which are sources of high‐velocity solar plasma streams. A study of recurrent Forbush decreases for the period 1966‐1975 has been made to examine any possible relationship of M regions with solar active regions. It is shown that at the onset of the recurrent Forbush decrease at the earth there is a high probability of encountering a class of active regions at the central meridian of the sun which give rise to flares of importance ≥2B/3N. These active regions are found to be long lasting and to have large areas as well as high Hα intensities. Other active regions, producing flares of lower importance, are distributed randomly on the sun with respect to the onset of a recurrent Forbush decrease. By using the quasi‐radial hypervelocity approximation the base of the leading edge of the high‐velocity stream at the onset of a recurrent Forbush decrease at the earth is traced to the solar longitude about 40° west of the central meridian. From these results it is deduced that M regions are located preferentially to the west of long‐lasting magnetically complex active regions. Earlier studies of the identification of the M regions on the sun have been reexamined and shown to conform to this positional relationship. A possible mechanism of the development of an M region to the west of the long‐lasting magnetically complex active region is also discussed.