Interhemispheric plasma flows in the equatorial topside ionosphere

Latitudinal, longitudinal, and seasonal variations in the field-aligned and perpendicular flows measured at an altitude near 830 km, by the Defense Meteorological Satellite Program F10 satellite are examined. These profiles are studied during the night;time (2100 magnetic local time) for solstice periods in 1991 when the solar activity is high. Latitude and longitude variations show the influence of F region winds in modulating the observed field-aligned flows. At night, large downward field-aligned flows of the order 400-600 ms(-1) are observed in the winter hemisphere and coincide in longitude with the location of previously identified adiabatic heating effects studied by Venkatraman and Heelis [1999a]. Interhemispheric flows at 2100 hours local time are seen to extend up to apex heights of similar to 1000 km during times of high solar activity. Perpendicular drifts, however are relatively invariant with latitude as expected.