Altitude variation of middle latitude topside ionospheric electron-density profiles

Mid-latitude topside ionospheric electron-density (N_e) altitude profiles are currently being scaled from ISIS (International Satellites for Ionospheric Studies) topside-sounder data that cover an 18-year time interval. These comprise a valuable data set that can be used to investigate secular topside ionosphere variations over more than a solar cycle. At middle latitudes, the electron temperature (T_e) and the relative H^+ to O^+ composition change across the low-latitude walls of the mid-latitude N_e and light ion troughs. At low altitudes local ionospheric processes dominate the N_e/ion composition variations in the trough boundaries. As the altitude increases, the T_e gradients change and H^+ becomes significant. Under these conditions high-altitude energy sources and field aligned plasma transport processes, associated with the physics of the plasmapause, become more important. These extra-ionospheric perturbations have consequences for the altitude variation of the location and shape of the N_e trough wall. Limited attention has been given to the detailed variations of the mid-latitude topside ionosphere as a function of altitude. We are investigating these variations by using a recently developed algorithm for relating the altitudinal changes in the vertical scale heights of topside N_e profiles to plasma temperature and composition changes. The ultimate objectives are to (1) determine the dependence of the mid-latitude topside ionospheric N_e altitude distributions on long-term solar-cycle variations and short-term solar wind and magnetic disturbances and (2) provide a framework for delineating the altitude dependence of mid-latitude ionospheric structures associated with outer plasmasphere structures.