Effects of interplanetary magnetic field z component and the solar wind dynamic pressure on the geosynchronous magnetic field

A study of the correlation of the geosynchronous magnetic field with interplanetary magnetic field (IMF) B-z and the solar wind dynamic pressure (P-d) is presented. Hourly averages of 5 years of GOES 6 and 6 years of GOES 7 observations are correlated with IMF B-z and P-d. As previously reported, increases in P-d enhance geosynchronous B-z on the dayside, most strongly around noon, but depress it on the nightside, most strongly around midnight. This has been interpreted in terms of increases in the cross-tail and magnetopause currents. Our study shows that the dayside geosynchronous magnetic field decreases with IMF B-z, particularly during periods of southward IMF. During periods of northward IMF, this trend continues but at much slower rate. The results of a multiple regression analysis of GOES B-z as a function of IMF B-z and P-d during periods of northward IMF show that roughly 30% of the IMF B-z uniformly ''penetrates'' the geosynchronous B-z. In contrast, during periods of southward IMF, the effect of IMF B-z on geosynchronous B-z is nonuniform and much larger at all local times, especially near dawn and dusk, apparently because of enhanced cross-tail current. During periods of southward IMF, geosynchronous B-x depends most strongly on IMF B-z near dawn, dusk, and midnight. The dawn and dusk correlations can be attributed to the enhanced region 1 Birkeland currents, and the midnight correlation can be attributed to enhanced cross-tail current. Geosynchronous B-y has the best correlations with IMF B-z near 2000 LT and 0400 LT, which can be attributed to enhanced region 1 Birkeland and/or cross-tail currents. Also, we show that these magnetic field perturbations can be interpreted in terms of fast rarefaction waves and merging at equatorial regions.