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This paper examines the relative importance of ionospheric and thermospheric densities and temperatures in producing the annual variation in the plasmaspheric electron density (Neq). In the Eastern American sector, whistler observations show a factor of 2–3 variation in Neq at low solar activity with a minimum in June and a maximum in December. The field line interhemispheric plasma (FLIP) model also shows an annual Neq variation in the American sector at solar minimum, but the model densities are about 30% higher. By using the ability of the FLIP model to be constrained by the measured hmF2, NmF2, and Te, we are able to show that, contrary to previous studies, it is the plasmaspheric thermal structure, and not ionospheric density that plays the key role in producing the annual variation in the model Neq at solar minimum. We find that the model annual Neq variations in the American and Australian sectors are out of phase by six months as would be expected from the opposite tilt of the Earth's magnetic field. This study also shows that the plasmaspheric electron density is anticorrelated with magnetic and solar activity due to changes in neutral hydrogen density. Thermospheric neutral winds appear to play only a small role in the annual Neq variations because longitudinal variations in the measured hmF2 are small. However, magnetic storm induced variability in ionospheric density through changes in thermospheric winds and neutral densities do contribute to the day-to-day variability of Neq. At solar maximum in November 1989 the model densities are generally in good agreement with both whistler and satellite observation in both the American and New Zealand sectors. However, the standard model is unable to reproduce the relatively small annual variation in equatorial electron density that was observed from Dunedin, New Zealand at solar maximum in 1990. We show that the low measured densities at solar maximum may be explained by magnetic storm induced depletion of the plasmasphere by convection electric fields. |
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