Abstract. We have determined the MLT distribution and K P dependence of the ion up¯ow and down¯ow of the thermal bulk oxygen ion population based on a data analysis using the EISCAT VHF radar CP-7 data obtained at Tromsù during the period between 1990 and 1996: (1) both ion up¯ow and down¯ow events can be observed at any local time (MLT), irrespective of dayside and nightside, and under any magnetic disturbance level, irrespective of quiet and disturbed levels; (2) these up¯ow and down¯ow events are more frequently observed in the nightside than in the dayside; (3) the up¯ow events are more frequently observed than the down¯ow events at any local time except midnight and at any K P level and the di erence of the occurrence frequencies between the up¯ow and down¯ow events is smaller around midnight; and (4) the occurrence frequencies of both the ion up¯ow and down¯ow events appear to increase with increasing K P level, while the occurrence frequency of the down¯ow appears to stop increasing at some K P level.

Data of three all sky cameras in Kiruna and Tjautjas (Sweden) were used for estimation of altitude of pulsating arc like forms and investigation of so called internal modulation (2 3 Hz) during the ON phase of a few second pulsations. It is found that for two closely spaced arcs, internal modulation took place only in the lowest arc. Based on this finding , an explanation is proposed due to the pulsations of anomalous resistance in the field-aligned current associated with a pulsating structure appart to the traditional mechanism of particle scattering by VLF waves.