Thermal electron densities and temperatures in the dayside cusp region

Abstract

Measurements of the density and temperature of electrons in the energy range 0–2 eV henceforth referred to as thermal electrons, observations of particle fluxes in the energy range 50 eV–15 keV and measurements of the d.c. electric field on board the INJUN V satellite have been investigated in the dayside cusp region and over the polar cap. Limiting the observations to the 14–15 MLT interval we find the following features at altitudes between 2000 and 2500 km. On the dayside there is a region typically three to nine degrees wide in which the thermal charged particles are influenced by magnetosheath particle fluxes. This region may be subdivided into two parts. Region A: An equatorial edge, which is characterized by an increase in poleward direction in temperature and often a decrease in density of thermal electrons, a plasma trough. Within this region, which is of the order of two degrees in width, there is usually observed an electric field reversal and the highest intensity of particle fluxes in the energy range 50 eV–15 keV. Region B, which extends poleward of Region A: In this region the density of thermal electrons is enhanced, while their temperature is close to that in the surrounding region and in particular lower than in Region A. The width of Region B is highly variable. The average width observed in the 35 cases selected for this study is six degrees. The equatorial borders of Regions A and B move equatorward with increasing magnetic activity measured by Kp. The increase of the density of thermal electrons in Region B could be due to the redistribution of ionization resulting from the heating of the F-region by magnetosheath particles precipitated in Region A and successive poleward convection. There could also be a contribution from charged particles with energies below the particle detector threshold of 50 eV on INJUN V produced above the F-Layer maximum in Region B.