Height-dependent ionospheric variations in the vicinity of nightside poleward expanding aurora after substorm onset

High-latitude ionospheric variations at times near auroral substorms exhibit large temporal variations in both vertical and horizontal extents. Statistical analysis was made of data from the European Incoherent Scatter UHF radar at Tromso, Norway, and International Monitor for Auroral Geomagnetic Effects magnetometer for finding common features in electron density, ion and electron temperatures and relating these to currents and associated heating. This paper particularly focused on the height dependencies. Results show clear evidences of large electric field with corresponding frictional heating and Pedersen currents located just outside the front of the poleward expanding aurora, which typically appeared at the eastside of westward traveling surge. At the beginning of the substorm recovery phase, the ionospheric density had a large peak in the E region and a smaller peak in the F region. This structure was named as C form in this paper based on its shape in the altitude-time plot. The lower altitude density maximum is associated with hard auroral electron precipitation probably during pulsating aurora. We attribute the upper F region density maximum to local ionization by lower energy particle precipitation and/or long-lived plasma that is convected horizontally into the overhead measurement volume from the dayside hemisphere.