A comprehensive survey of plasmasphere refilling at geosynchronous orbit

Abstract

Plasmasphere refilling is one of the most fundamental examples of ionosphere‐magnetosphere coupling. Geosynchronous low‐energy ion measurements are particularly useful in addressing this issue. In this survey we collect almost 11 satellite years of data, from January 1990 to September 2000, from Los Alamos magnetospheric plasma analyzers on board five satellites. The data are sorted according to the level of magnetospheric activity, local time, season, and solar cycle phase for a comprehensive examination of the early‐time refilling rate. These statistical results indicate that the early‐time refilling rate increases with increasing geomagnetic activity. Additionally, the early‐time refilling rate at low solar activity is about a factor of 1.5 – 2 higher than that at high solar activity. This may be related to the higher ionospheric neutral hydrogen density at solar minimum. Moreover, when the geomagnetic activity index is higher than 3, the early‐time refilling rate is observed to be slightly higher during the seasonal equinox periods than at solstice, but no seasonal variation is found at low geomagnetic activity. The late‐time refilling rate is also examined during intervals of prolonged geomagnetic quiet. The refilling process is not linear as a function of time: The early‐time refilling rate ranges between 2.5 and 6.5 cm−3 d−1, while the late‐time refilling rate is found to be ∼ 10 – 25 cm−3 d−1. The late‐time refilling rate is also anticorrelated with solar activity. The plasmasphere at geosynchronous orbit is observed to reach a maximum density at ∼ 100 – 200 cm−3.