The topside ionosphere at mid-latitudes during local sunrise

Abstract

Electron-density and scale height distributions obtained from more than 1000 Alouette I ionograms have been used to study the structure and variations of the mid-latitude topside ionosphere during the sunrise period from 21 May to 4 July 1963. With the assumption that the ionosphere is in a state of diffusive equilibrium, the scale heights are interpreted in terms of combined electron and ion temperature distributions and relative ion concentrations. During the overall sunrise period, there was a strong latitudinal variation of the electron density and scale height altitude distributions. It was the latitudinal scale height distribution which determined the effects of solar zenith angle variations at specific latitudes on the temperature and ion mass distributions. For latitudes south of ~55°N geomagnetic latitude, scale height variations due to solar zenith angle changes are interpreted in terms of temperature increases and mean ion mass changes. For latitudes north of ~55°N, however, these variations are characterized by temperature increases mostly. The electron densities at fixed topside altitudes decreased with increasing illumination at all altitudes and latitudes until about ground sunrise.