Cosmic-ray propagation processes. 1. A study of the cosmic-ray flare effect

Abstract

Observations of the cosmic radiation generated by solar flares are used to study the propagation of cosmic rays through the interplanetary magnetic field. The observations were made by cosmic-ray detectors flown on two identical interplanetary spacecraft, Pioneers 6 and 7. The detectors determined the directional, energy, and temporal characteristics of the cosmic radiation of energy greater than 7.5 Mev. It was found that cosmic radiation of solar origin is normally extremely anisotropic, the direction of maximum flux being aligned parallel to the interplanetary magnetic field vector at early times during flare effects. At late times a persistent anisotropy of amplitude 5-10% is observed, the maximum flux arriving from the sun-spacecraft direction. It is argued that at late times the cosmic radiation is in diffusive equilibrium, and an explanation for the persistent anisotropy is advanced in terms of expulsion of the cosmic radiation from the solar system by the combined action of the solar wind and a cosmic-ray density gradient. It is shown that the cosmic-ray fluxes and their anisotropic characteristics are sometimes very greatly influenced by interplanetary conditions, and may exhibit strong spatial inhomogeneities. It is shown that the low-energy cosmic rays tend to remain identified with the magnetic tube of force onto which they are initially injected, this being the equivalent to saying that they corotate with the sun. Under some circumstances, the propagation of cosmic rays from the sun to the orbit of earth is completely dominated by a 'bulk motion' propagation mode, in which the cosmic rays do not gain access to the spacecraft until the magnetic regime into which the cosmic rays are injected envelops the spacecraft. This bulk motion propagation mode applies both to magnetic regimes initiated by solar flares and to the regimes associated with recurring Forbush decreases, the latter being long-lived field structures imbedded in the normal solar wind. In two cases, it is shown that the anisotropy directions and cosmic-ray times of flight are such as to indicate diffusion of the cosmic rays to a point on the western portion of the solar disk before injection onto the field line that carried them to the spacecraft. The temporal decay of a cosmic-ray flare effect subsequent to impulsive injection by a solar flare is studied, an example being presented in which the decay was essentially exponential over a period of time equal to 9.2 decay time constants. Simultaneous observations by both spacecrafts when separated by about 54° in solar azimuth indicate azimuthal cosmic-ray density gradients in excess of two orders of magnitude per 60° for extended periods during the initial phases of flare effects. By studying the rate at which a known cosmic-ray flux propagating away from the sun was backscattered so as to return to the spacecraft from the antisun direction, the mean free path for large-angle scattering in the inner solar system is shown to be of the order of 1.0 AU. The application of an isotropic diffusion model to the same data is shown to yield much lower values of mean free paths, and it is argued that this latter method is, in most cases, inapplicable.