The spatial relationship between the evening radar aurora and field-aligned currents

Abstract

In recent years the diffuse radar aurora has been shown to be closely associated with the auroral electrojets. Greenwald et al. (1973, 1975) showed that the range-integrated amplitude of the diffuse radar aurora is linearly proportional to the horizontal component of the magnetic perturbations found beneath the echoing region. Tsunoda et al. (1974) showed that the evening diffuse radar aurora was characterized by a distinct poleward boundary that was usually aligned along a magnetic L shell. They hypothesized that such a boundary could be produced by a latitudinal gradient in either the ionospheric electric field or the conductivity. In either case, field-aligned currents might be expected to flow in such a region. An initial study was therefore undertaken to examine the spatial relationship of the radar aurora to the field-aligned currents observed by virtue of the transverse magnetic disturbances detected by the Triad satellite. By utilizing a 398-MHz phased array radar located at Homer, Alaska, event-by-event comparisons were made between the evening radar aurora and the field-aligned currents in the 1700–2100 MLT sector. The results of this study include the following findings: (1) the downward field-aligned currents in the evening sector are closely associated with the eastward electrojet; (2) the upward field-aligned currents in the same time sector are associated with the visual aurora; (3) there is a poleward-directed electric field across the oppositely directed field-aligned currents, implying at least partial closure through the ionosphere via a Pedersen current; and (4) the downward field-aligned currents are associated with the diffuse particle precipitation responsible for the production of the diffuse radar aurora (and hence the eastward electrojet). However, the downward field-aligned currents must be carried by precipitating protons and/or upward-moving low-energy electrons.