DE 1 observations of return current regions in the nightside auroral oval

Abstract

Data from the Dynamics Explorer 1 high altitude plasma instrument (HAPI) and magnetometer (MAG‐A) are used to examine particle acceleration and Birkeland current phenomena in the mid‐altitude (10,000–20,000 km) nightside auroral region between 2000 and 2200 hours MLT. Field‐aligned current densities as derived from the two instruments are compared, generally resulting in good agreement. The latitudinal distribution of upward and downward field‐aligned currents is examined, both on a large scale with respect to regional boundaries and on a local scale with respect to individual inverted‐V events. The charge carriers of the downward currents are found to be cold ionospheric electrons accelerated into upward beams by potential drops of a few tens of volts located at altitudes near 1 Earth radius. In several instances, large potential differences below the satellite are indicated by energetic upward ion beams with no (or very small) potential drops above the satellite. In these cases, very small Birkeland currents (≲10−8 A/m²) are observed above the inverted V, suggesting the possibility that the field‐aligned current systems associated with individual inverted‐V events may close at relatively low altitudes above the auroral oval, or that field‐aligned potential differences may be generated without significant Birkeland currents.