IONOSPHERIC CONVECTION AT CASEY, A SOUTHERN POLAR-CAP STATION

A digital ionosonde (Digisonde Portable Sounder 4) located at Casey, A ntarctica (66.3 degrees S, 110.5 degrees E, -80.8 degrees corrected ge omagnetic latitude) has been operational since early 1993 and has accu mulated 3 years of plasma drift measurements, providing an excellent d ata set for studying the characteristics of ionospheric convection flo w at a southern polar cap station. The purpose of this study is to inv estigate the influence of the IMF on the F region ionospheric convecti on over Casey and to compare it to the Heppner-Maynard satellite-deriv ed electric field models. We find clear dependencies in the drift on t he sign and strength of the IMF B-y and B-z components and with Kp. An tisunward flow dominates during B-z south conditions, turning to have a sunward component around noon when B-z is northward. The B-y compone nt causes the entire convection system to rotate and distorts the days ide flow in the proximity of the throat, with a dawnward (duskward) co mponent for B-y negative (positive). Comparison with the B-z south Hep pner-Maynard BC, DE, and A patterns is favorable at most times, althou gh we predict a rounder, more dominant dusk (dawn) cell and a smaller crescent-shaped dawn (dusk) cell for B-y < 0 (B-y > 0). There is a dep endence on Kp when B-z is south in both the model and the drifts, flow directions becoming more antisunward and velocities becoming higher o n the dayside as Kp increases. This implies the polar cap is expanding under conditions of enhanced reconnection. When B-z is north, the F r egion drift agreement with the BCP(P) and DEP(P) models is excellent o n the dawn (dusk) side for B-y < 0 (B-y > 0) but diverges on the oppos ite side as the pattern flow lines twist sunward. Separation of the dr ifts into B-z weakly (<3 nT) and strongly (>3 nT) northward cases did not reveal any appreciable difference in the observed drift velocities .