DAYSIDE ION FRICTIONAL HEATING - EISCAT OBSERVATIONS AND COMPARISON WITH MODEL RESULTS

An extended period of intense dayside ion frictional heating was obser ved on 3 April 1992 with the UHF radar of the European Incoherent Scat ter (EISCAT) facility during a run of the common program CP-1-J. The e levated F-region ion temperature resulted from an enhancement in the i on drift velocity, principally in the field-orthogonal zonal direction , which steadily increased, in a westward direction, to a value in exc ess of 2 km s(-1) during a 4 h interval commencing at approximately 10 UT (12 MLT). The maximum enhancement in the field-parallel ion temper ature measured at 300 km altitude exceeded 700 K. The electron concent ration in the F-region was substantially depleted during the interval of ion frictional heating, which further resulted in an increase in th e F-region electron temperature as, at the local time of the event, th e plasma was solar illuminated. A zonal E boolean AND B velocity signa ture modelling that observed over EISCAT during the aforementioned int erval was imposed on the Sheffield University plasmasphere and ionosph ere model (SUPIM), previously used to study the effects of sub-auroral ion drifts (SAID). The model yields densities, temperatures and field -aligned velocities for the six major ion species and for the electron s, and has recently been modified to account for O+ temperature anisot ropy. The plasma parameters modelled at F-region altitudes were compar ed with those measured by the EISCAT radar and, although they tended t o exhibit the same general trends, the model results predicted substan tially larger field-parallel ion temperature enhancements than were ob served. However, the inclusion of a time-dependent calculated neutral wind, caused by ion drag, reduced the modelled field-parallel ion temp erature to the values measured by EISCAT without severely affecting th e other parameters. During extended periods of high ion flow, particul arly on the dayside, an enhanced neutral wind becomes highly significa nt in determining the extent of frictional heating of the ion populati on.