The patterns and sources of high‐latitude particle precipitation

Abstract

We survey, in this paper, the properties of the zones of particle precipitation at high latitudes. We consider the average properties of particle influx from the outer magnetosphere as determined by satellite and rocket observations, as well as ground‐based techniques. The point of view of this paper is to try to associate source regions, and, if possible, precipitation mechanisms with each of the families of precipitating particles that collectively form the auroral zone. The precipitation patterns for both electrons and protons exhibit a diurnal variation. The several zones of particle precipitation can be placed in relationship to each other. The zone of electron precipitation is best described as consisting of a region at low latitudes (60° ⪝ Λ ⪝ 70° near local midnight) where electrons with characteristic energies of tens of kev precipitate; this region merges and overlaps at the higher latitudes with another region (70° ⪝ Λ ⪝ 80° near local midnight) characterized by precipitating electrons of ≈0.5 kev. The limited data available indicate that the zone of proton precipitation spatially overlaps the zone of electron precipitation. For example, the peak of precipitation of > 4‐kev protons near local midnight is found between Λ = 65° and Λ = 70°. Near local noon a separate region of very soft proton precipitation is found in the vicinity of Λ ≈ 80°, in addition to a region of proton influx at lower latitudes. We consider the plasma sheet, the extraterrestrial ring current, the polar cusp, and the outer radiation belt as source regions of precipitating particles. Brief surveys of the precipitation of helium ions and of possible means of artificially inducing particle precipitation are also included.