OBSERVATIONS OF IMF AND SEASONAL EFFECTS IN HIGH-LATITUDE CONVECTION

We describe strong interplanetary magnetic field (IMF) and seasonal ef fects 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., B-y>0, the dawn cell is more crescent-s haped and the dusk cell more round while for B-y<0 these pairings of s ize and shape are reversed. The more extreme crescent/round cell dicho tomy is obtained for B-y>0. The return flows associated with the cresc ent-shaped cell dominate at midnight MLT (Magnetic Local Time); the re versal in the zonal velocity in the 67 degrees-69 degrees boolean AND (magnetic latitude) interval occurs 2 1/2 hr earlier for B-y>0. The se asonal dependence of nightside convection resembles in important respe cts the By dependence. Greater latitudinal velocity shears occur in th e morning/afternoon sector for summer/winter and the return flow of th is sector dominates at midnight. The zonal flow reversal occurs 2 1/2 hr earlier in summer than in winter. The maximum effects are obtained on the nightside for the pairings [B-y>0,summer] and [B-y<0,winter]; t he first produces the more structured cell in the morning, the second in the evening, and this cell dominates the return flow at midnight. T he difference in the zonal flow reversals for these pairings exceeds 4 hr in MLT.