The general pattern of auroral particle precipitation

Abstract

Gross statistical data on optical- and radio-auroral phenomena are examined to establish the latitude-versus-time pattern for the causal particle precipitation. Although these phenomena may occur at any time of day at auroral-zone latitudes, they are shown to occur most frequently at about 08 and 22 hours geomagnetic time. These two dominant diurnal maxima exhibit radically different characteristics, indicating that two distinct particle populations are involved on the average. Further, the two types of precipitation are recognizable at other times of day in the statistical data, although at a reduced intensity, leading to a two-zoned precipitation pattern in which there is a clear separation between the zones during most of the day. The most prominent features of the precipitation in the zone with the night-time maximum are shown to be the impulsive character of the precipitation and its steep energy spectrum, whereas the zone with the morning maximum is best described as having a continuous influx with a much flatter particle spectrum. On the basis of detailed studies of these characteristics, of observations on other related ionospheric parameters, and of direct particle measurements made with rockets and satellites it seems plausible to associate the two particle populations with different regions of the magnetosphere. This relationship to the magnetospheric structure and to the processes therein is discussed for both the ‘splash’ and ‘drizzle’ types of precipitation.