We describe strong interplanetary magnetic field (IMF) and seasonal effects in the convection of nightside ionospheric plasma. The findings are based on a statistical analysis of observations made with the JHU/APL HF radar located at Goose Bay, Labrador. For positive sign of the IMF dawn-dusk component, i.e., By>0, the dawn cell is more crescent-shaped and the dusk cell more round while for By0. The return flows associated with the crescent-shaped cell dominate at midnight MLT (Magnetic Local Time); the reversal in the zonal velocity in the 67¿--69¿ &Lgr; (magnetic latitude) interval occurs 21/2 hr earlier for By>0.

The seasonal dependence of nightside convection resembles in important respects the By dependence. Greater latitudinal velocity shears occur in the morning/afternoon sector for summer/winter and the return flow of this sector dominates at midnight. The zonal flow reversal occurs 21/2 hr earlier in summer than in winter. The maximum effects are obtained on the nightside for the pairings [By>0, summer> and [By<0, winter>; the first produces the more structured cell in the morning, the second in the evening, and this cell dominates the return flow at midnight. The difference in the zonal flow reversals for these pairings exceeds 4 hr in MLT. ¿American Geophysical Union 1995