THE TEMPORAL EVOLUTION OF THE IONOSPHERIC SIGNATURES OF SUBAURORAL ION DRIFTS

A model of the ionosphere and plasmasphere is used to investigate the effects of an imposed westward plasma drift on O+ and molecular ion behaviour in the nightside ionosphere. A closed subauroral tube of plasma is considered and the velocity input persists for 30 min. The rapid increase in the F-region ion temperature resulting from ion-neutral frictional heating causes an immediate surge in the O+ field-aligned velocity, upwards in the topside ionosphere and downwards below the F2-peak, but after about 10 min into the event the surge in the topside disappears. After the event there is a return flow of O+ from the plasmasphere. The relative abundance of O+ decreases during the event due to the increased rate of conversion of O+ into NO+ and O2+; the decrease is more marked for greater values of the imposed westward ion drift. At a given F-region altitude, a low relative abundance of O+ is more likely under sunspot maximum atmospheric conditions. There are also significant percentages of NO+ in the topside ionosphere during the event. The bearing of these results on satellite observations of SAID (subauroral ion drift events) and on EISCAT incoherent scatter radar observations of ion heating events is discussed.