Radar and optical measurements of ionospheric processes associated with intense subauroral electric fields

Abstract

Observations of very large poleward directed electric fields were obtained with a clustered set of instrumentation that included the Millstone Hill incoherent scatter radar, the Boston University Mobile Ionospheric Observatory, and the HILAT and Defense Meteorological Satellite Program (DMSP) F6 and F7 satellites. In this paper we concentrate on data from the Millstone Hill incoherent scatter radar which was operated on selected evenings in a rapid azimuthal scan, centered on magnetic west. The mode was designed with the express purpose of measuring line‐of‐sight drift velocity and electron density as a function of latitude during events with large localized electric fields. During this same period, the Defense Nuclear Agency HILAT satellite made northern hemisphere measurements every 100 min of ion drift, density, and field‐aligned currents across the equatorial boundary of the auroral oval. A detailed study of optical data in this region is provided in a companion paper. On the evenings of April 20 and 21, 1985, during an intense magnetic storm (Kp > 8+), large ionospheric electric fields (E > 80 mV/m) were detected along the edge of the auroral oval with the Millstone Hill incoherent scatter radar. These constitute the first definitive incoherent scatter observations of this phenomenon. An L shell‐aligned (zero order) deep trough in electron density was colocated with these large electric fields at L shells as low as L = 2.8. These data indicate that the trough develops much more quickly than present theories predict, at least near the F peak. We also report elevated ion and electron temperatures in the trough and conjecture that these may contribute to the rapid decay. We also show that the associated field‐aligned currents are very weak, as predicted by Banks and Yasuhara (1978) but that it is the F region structure which dominates the conductivity gradient rather than the E region emphasized by the earlier work. We also discuss the data set in light of competing theories for the production of large electric fields and for undulations of the edge of the diffuse aurora. In particular we discuss the importance of large radial ion temperature gradients indicated by the DMSP data we present.