Quiet time variability of the geosynchronous magnetic field and its response to the solar wind

We present a survey of the variability of the geosynchronous magnetic field strength on the dayside using observations by the GOES satellites over a period exceeding 4 years. Only intervals of reduced geomagnetic activity, as defined by Dst > -20 nT, were considered in this study. The magnetic field strength data were filtered with a passband of 1.7 mHz to 17 mHz (1-10 minutes), a process that eliminates the diurnal variation of the field strength and the effects of most of the higher frequency (> 17 mHz) ultralow-frequency (ULF) waves. The geosynchronous field strength appears to exhibit the greatest variability in the prenoon sector for spiral interplanetary magnetic fields (IMF) and in the postnoon sector for orthospiral IMF, suggesting that pressure pulses generated in the foreshock/bow shock region may have a significant influence on the geosynchronous field. The seasonal dependence of the variability was determined to be positively correlated to the seasonal dependence of ground-based observations of magnetic impulse events. The response of the variability of the geosynchronous magnetic field strength around local noon to solar wind parameters was also studied. Here, we observed that the variability was strongly affected by changes in the solar wind dynamic pressure but was seemingly independent of the northward/southward direction of the interplanetary magnetic field. However, for high solar wind dynamic pressures, the variability was found to be greater for northward IMF than for southward IMF.