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 i nterplanetary magnetic field (IMF) B-z and the solar wind dynamic pres sure (P-d) is presented. Hourly averages of 5 years of GOES 6 and 6 ye ars of GOES 7 observations are correlated with IMF B-z and P-d. As pre viously reported, increases in P-d enhance geosynchronous B-z on the d ayside, most strongly around noon, but depress it on the nightside, mo st strongly around midnight. This has been interpreted in terms of inc reases in the cross-tail and magnetopause currents. Our study shows th at the dayside geosynchronous magnetic field decreases with IMF B-z, p articularly during periods of southward IMF. During periods of northwa rd 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, duri ng 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 d awn and dusk, apparently because of enhanced cross-tail current. Durin g periods of southward IMF, geosynchronous B-x depends most strongly o n IMF B-z near dawn, dusk, and midnight. The dawn and dusk correlation s can be attributed to the enhanced region 1 Birkeland currents, and t he midnight correlation can be attributed to enhanced cross-tail curre nt. Geosynchronous B-y has the best correlations with IMF B-z near 200 0 LT and 0400 LT, which can be attributed to enhanced region 1 Birkela nd and/or cross-tail currents. Also, we show that these magnetic field perturbations can be interpreted in terms of fast rarefaction waves a nd merging at equatorial regions.