Observations of persistent dayside F region electron temperature enhancements associated with soft magnetosheathlike precipitation

Abstract

A series of experiments with the Sondrestrom incoherent scatter (IS) radar (66.99°N latitude, 50.95°W longitude) were designed to examine F region structure in the dayside auroral oval in order to search for plasma signatures from magnetospheric regions such as the cusp, boundary plasma sheet, low‐latitude boundary layer, and mantle. This IS radar mode, optimized to search for ionospheric features which remain fixed in local time, was coordinated with overflights of the DMSP F‐10 satellite on 2 days in September 1992. For both study days, IS radar data show persistent (∼ 7 hour), enhanced Te regions at 300 to 500 km. These enhanced Te regions evolve during periods in which relatively unstructured, laminar Ne densities are observed and thus are not merely the result of a structured suppression of electron cooling. The cores of these Te enhancements were observed at latitudes and magnetic local times corresponding to DMSP satellite measurement of soft (< 100 eV) cusplike precipitation. These Te hotspots move systematically equatorward with increasing geomagnetic activity and display a sharp field‐aligned equatorward edge at the location of satellite cusp detection. Unlike prior IS radar/satellite cusp investigations, no significant Ne enhancements were measured coincident with Te hotspots. A simple ionospheric model is invoked to confirm that such soft cusplike precipitation does not significantly alter the magnitude of the ambient plasma density, and we argue that cusp detection based on collocated Ne and Te enhancements is seldom possible. The local time persistence of the Te enhancements, beyond the typical cusp widths, suggests an association with additional dayside magnetospheric regimes such as the low‐latitude boundary layer. Both latitudinal and vertical Te gradients maximize at the location of satellite cusp detection, suggesting that the heat source is a divergence of magnetospheric heat flux on freshly reconnected geomagnetic field lines.