Mass density at geostationary orbit and apparent mass refilling

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 F-10.7 EUV index, magnetic local time, 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.