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

Abstract

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

Cosmic ray detector response to transient solar modulation: Forbush decreases

“…The magnetic field configuration of the accompanying shock [Lockwood et al, 1975] led to an intensity decrease in MTs deep underground [Swinson, 1973] and in IC and underground MTs at Yakutsk [Filippov et al, 1973] as well as the multidirectional MTs at Nagoya [Sekido et al, 1973]. Agrawal et al [1974] report on FD recorded by a global network of NMs and surface MTs (see their Figure 4). Medrano et al [1975] discuss a model for the associated ICME.…”

Particle trapping and acceleration during the August 1972 event

“…Vernov et al (1970a, b) during other events observed low energy particles accumulated behind shocks. For this 5 August event Agrawal et al (1974) give a similar explanation and for this particular event such a possibility was tested by Otaola et al (1977a) and Perez-Enriquez (private communication). When the field is perturbed, the particle mean free path is very small and so particles are convected by the solar wind, i.e.…”

“…It reaches the minimum of the F.d. at 1 or 2 UT, of 5 August, depending not only on the asymptotic direction of the station (Agrawal et al, 1974) but also on the rigidity of the particles. Note that all stations with cutoff rigidities Pc < 1.4 GV which observed the GLE, i.e.…”

“…The spectral exponent of the cosmic rays was found to be the same during the deep F.d. and the following increase according to the Agrawal et al (1974) calculations. This implies that galactic cosmic rays were filtered in the same way and the Earth was shielded with the magnetic barrier or configuration of the shocks, as before.…”

Modulation of cosmic rays at different cutoff rigidity

“…A number of investigators have reported the short-term characteristics of the daily variation, where they have selected continually occurring days of high and low amplitudes of diurnal variation (Tiwari, 1994;Agrawal et al, 1974, Tiwari, 1995. These results have pointed out significant departures in the time of maximum as well as their association with higher harmonics.…”