Geosynchronous magnetic field temporal response to solar wind and IMF variations

We investigate the time-dependent response of the geosynchronous magnetic field to variations in the interplanetary magnetic field (IMF) orientation and sudden changes in the solar wind dynamic pressure. Because IMF discontinuities drape about the magnetopause, we expect the response to southward IMF turnings to occur nearly simultaneously over a wide range of local times. By contrast, the pressure variations launched by solar wind tangential discontinuities aligned with the spiral IMF into the magnetosheath should keep pace with the solar wind and generally strike the postnoon magnetopause first. Consistent with these predictions, we find that the magnetospheric magnetic field strength begins to diminish at all dayside local times within 4 to 5 min following the estimated arrival time of southward IMF turnings at the subsolar magnetopause, and that the nightside field begins to reconfigure within 12 min. By contrast, the response to pressure variations generally appears some 2.2 min after the arrival of the discontinuity in the noon-dusk sector, 4.4 min later in the noon-dawn sector, and 8.8 min later on the nightside.