F-REGION PLASMA DRIFTS OVER ARECIBO - SOLAR-CYCLE, SEASONAL, AND MAGNETIC ACTIVITY EFFECTS

We have used Arecibo incoherent scatter measurements from 1981 to 1990 to determine the characteristics of low-latitude F region plasma drif ts. The measurements show large day-to-day variability even during mag netically quiet periods. The average poleward/perpendicular plasma dri fts do not change significantly with season and solar cycle except in the midnight-morning sector. The zonal drifts show clear solar cycle a nd seasonal effects. The afternoon-nighttime eastward drifts increase with solar flux; the westward drifts in the early morning-afternoon se ctor show a large increase from summer to winter but are independent o f solar activity. The two perpendicular velocity components also respo nd differently to magnetic activity. The average northward/perpendicul ar drifts decrease with magnetic activity during the day but do not ex hibit a systematic response at night. The zonal component shows increa sed westward drifts occurring predominantly at night. The plasma drift s along the magnetic field lines exhibit large altitudinal and seasona l variations, particularly near solar minimum, and are generally antic orrelated with the perpendicular/north drifts. The drift patterns obse rved by the Arecibo and the middle and upper atmosphere radars have si gnificantly different seasonal dependence. This can be explained by el ectrodynamic effects in the corresponding local and conjugate ionosphe res. The large longitudinal variation of the quiet time F region plasm a drifts results from the displacement between the geographic and dip equators and from magnetic field declination effects. In general, the longitudinal variation should also depend on magnetic activity.