Ion upflow and downflow at the topside ionosphere observed by the EISCAT VHF radar

We have determined the MLT distribution and Kp dependence of the ion upflow and downflow of the thermal bulk oxygen ion population based on a data ana lysis using the EISCAT VHF radar CP-7 data obtained at Tromso during the pe riod between 1990 and 1996: (1) both ion upflow and downflow events can be observed at any local time (MLT), irrespective of dayside and nightside, an d under any magnetic disturbance level, irrespective of quiet and disturbed levels; (2) these upflow and downflow events are more frequently observed in the nightside than in the dayside; (3) the upflow events are more freque ntly observed than the downflow events at any local time except midnight an d at any K-P level and the difference of the occurrence frequencies between the upflow and downflow events is smaller around midnight; and (4) the occ urrence frequencies of both the ion upflow and downflow events appear to in crease with increasing K-P level, while the occurrence frequency of the dow nflow appears to stop increasing at some K-P level.