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Journal of Geophysical Research


We used the inferred equatorial mass density rho_{m,eq} based on measurements of Alfven wave frequencies measured by the GOES satellites during 1980--1991 in order to construct a number of different models of varying complexity for the equatorial mass density at geostationary orbit. The most complicated models are able to account for 66% of the variance with a typical variation from actual values of a factor of 1.56. The factors that influenced rho_{m,eq} in the models were, in order of decreasing importance, the F10.7 EUV index, magnetic local time, MLT, the solar wind dynamic pressure P_{dyn}, the phase of the year, and the solar wind B_Z (GSM Z direction). During some intervals, some of which were especially geomagnetically quiet, rho_{m,eq} rose to values that were significantly higher than those predicted by our models. For 10 especially quiet intervals, we examined long-term (> 1 day) apparent refilling, the increase in rho_{m,eq} at a fixed location. We found that the behavior of rho_{m,eq} varies for different events. In some cases, there is significant apparent refilling, whereas in other cases rho_{m,eq} stays the same or even decreases slightly. Nevertheless, we showed that on average rho_{m,eq} increases exponentially during quiet intervals. There is variation of apparent refilling with respect to the phase of the solar cycle. On the third day of apparent refilling, rho_{m,eq} has on average a similar value at solar maximum or solar minimum, but at solar maximum, rho_{m,eq} begins with a larger value and rises relatively less than at solar minimum.