WHAT IS THE SOURCE OF OBSERVED ANNUAL VARIATIONS IN PLASMASPHERIC DENSITY

Plasmaspheric densities have been observed previously to be higher in December than in June, with the ratio varying between 1.5 and 3.0 and with larger variations at lower L shells. In order to search for the c ause of the observed annual variations, we have modeled plasmaspheric density, using a time-dependent hydrodynamic model. On an L = 2 field line with geomagnetic longitude equal to 300 degrees, the modeled plas maspheric densities were a factor of 1.5 times higher in December than in June. The modeled December to June density ratio was found to incr ease slightly with L shell, in contrast to observations; this discrepa ncy may be due to the fact that outer plasmaspheric flux tubes are nev er completely full. In addition, for an L = 2 field line with geomagne tic longitude equal to 120 degrees, the modeled plasmaspheric density was higher in June than in December by a factor of about 1.2. Various numerical tests were also performed in order to examine the sensitivit y of plasmaspheric density to various parameters. In particular, a lar ge vertical neutral wind was applied in order to raise the O+ profile, which had the effect of raising plasmaspheric density by a factor of 6. This in conjunction with a theoretical analysis suggests that plasm aspheric density levels are very sensitive to O+ levels in the upper i onosphere. We conclude that annual variations in plasmaspheric density are due to similar variations in ionospheric O+.