Anisotropies in relativistic cosmic rays from the invisible disk of the Sun

Abstract

The most recent (September 1–2, 1971) of the rare ground level events signaling the arrival at the earth of relativistic solar cosmic rays arising from a flare on the sun's invisible hemisphere displayed marked anisotropy for several hours. Application of newly developed analytical procedures has made it possible to study in detail the evolution of the anisotropic pitch angle distribution. Throughout the anisotropic phase the solar particle flux is proportional to the cosine of the angle between the weighted mean asymptotic direction of viewing and an axis of symmetry corresponding to the spiral interplanetary magnetic field line that connects the sun to the earth. Furthermore, the degree of anisotropy deduced from the ratio of the fluxes in the garden hose direction toward and away from the sun decays exponentially. The results of this analysis and the reexamination of the three previously observed invisible hemisphere ground level events have demonstrated that propagation along the spiral interplanetary field line connecting the sun to the earth occurs even when the source lies far beyond the limb of the sun. In one case (January 28, 1967) the anisotropy near onset was 100% (i.e., no particles traveling toward the sun), a situation that had only been reported twice previously and then in association with visible flares that occurred within 10° of the foot of the nominal garden hose field line. These results bespeak the operation of an efficient transport mechanism in the solar corona that enables relativistic protons to travel over distances at least as great as 70° in heliolongitude in about 15 min.

Relativistic solar cosmic rays

“…McCracken [1962b] has demonstrated that the flux distribution in space is symmetric with respect to an axis which is coincident with the nominal garden hose field line. Several authors have reported that this symmetry in the flux distribution holds for all types of events including those for which the parent solar flares are located on the invisible disk of the sun [Lockwood, 1968; Mathews and Wilson, 1970;Duggal et al, 1971;Maurer et al, 1973;Duggal and Pomerantz, 1973a]. In view of this important result the attempt in recent years has been to determine the exact nature of the angular dependence of the flux with respect to the garden hose field lines [Duggal, 1975].…”

Interplanetary acceleration of solar cosmic rays to relativistic energy

“…The maximum intensity was reached 2-3 hours after the onset, and the duration of the GLE was about 8 hours. Analysis of data from several high-latitude neutron monitors by the method outlined by Duggal and Pomerantz [1973] indicates that the pitch For the GLE of August 4, 1972, this procedure leads to the conclusion that t, • 1200 UT. Thus it appears that the acceleration of relativistic particles occurred 2 hours after the termination of the optical activity of the 313 solar flare (Table 1).…”