ISEE 1 and 2 observations of Birkeland currents in the Earth's inner magnetosphere

Abstract

Data from the ISEE 1 and 2 magnetometers were examined for evidence of Birkeland currents in the earth's inner magnetosphere, from 2.4‐ to 7‐RE radial distance. This survey reports on the two spacecraft measurements of the properties of the Birkeland currents. Both line and sheetlike currents were observed. Large‐scale Birkeland current signatures were observed throughout the 18 hours of local time in which the data coverage was sufficient. In local time this coverage was from local noon through dusk to local midnight to the dawn meridian. In approximately 27% of the spacecraft's outbound passes, large‐scale current structures were identifiable as parts of the region 1 or region 2 current systems. In approximately 19% of the outbound passes, no distinguishable region 1 or region 2 Birkeland currents were observed. These cases occurred during periods of low magnetic activity. In the remaining 54%, both multiple current structures and ambiguous magnetic signatures were observed. The multiple structures were generally of smaller scale than the structures identified as the region 1 and 2 current structures, and the ambiguous cases had no clear sources or causes. For these cases the spatial and temporal variations were not easily separated. Two passes through the dayside magnetosphere showed Birkeland current layers at the interface between the outer plasma sheet convection boundary and a plasma mantlelike boundary region, located inside the postnoon magnetopause. These two cases provided clear examples of region 1 Birkeland current flow in the dayside magnetosphere.