Low‐frequency modulation of plasmas and soft electron precipitation near the dayside cusp

Abstract

On December 2, 1983, the HILAT satellite trajectory was approximately aligned with the noon‐midnight meridian while crossing the northern polar cusp/cleft region. During that time we observed (1) an increase in scintillations of radio signals, (2) strongly variable fluxes of precipitating low‐energy electrons, (3) gradients in the thermal ion density, (4) fluctuations in the east‐west component of the ion drift velocities superimposed on the noon sector convection pattern, and (5) field‐aligned currents. In the region corresponding to the highest level of radio scintillations we also detected fluctuations in electron fluxes on a time scale of half a second superimposed on a low‐energy inverted‐V event. The analysis of these measurements, which provide information on both large‐ and small‐scale dynamics of the dayside cusp, leads to the following conclusions: (1) the large‐scale convection pattern is characteristic for the northern hemisphere high‐latitude ionosphere when the interplanetary magnetic field has negative Y and Z components; (2) the most intense scintillations and the cusp field‐aligned currents occur on recently opened magnetic field lines; (3) plasma created by increased fluxes of precipitating low‐energy electrons convects antisunward with velocity of ∼0.5 km/s; (4) enhanced chemical reactions due to the Joule heating associated with large electric fields maximize at the equatorward edge of the cusp, leading to the net depletion in total electron content and plasma density measurements; (5) plasma irregularities throughout the F region are formed due to a combined effect of structured soft electron precipitation and current‐convective instabilities; and (6) if the subsecond fluctuations of the ion drift measurements seen during the inverted‐V event are due to static variations in the ambient electric field, there are potential drops of a few tens of volts between the altitude of the satellite (∼830 km) and the lower ionosphere.