The rate of merging and the strength of the region 1 Birkeland currents increase during periods of southward interplanetary magnetic field (IMF). Fringe fields of the Birkeland currents depress dayside magnetospheric magnetic field strengths and remove magnetic flux from the dayside magnetosphere, thereby allowing the dayside magnetopause to move inward and the cusp equatorward. We use previously derived fits to the magnetopause location as a function of IMF Bz, the condition of pressure balance at the magnetopause, and an idealized model of region 1 Birkeland currents to estimate that strong southward IMF turnings will produce ~13- to 26-nT depressions in the geosynchronous magnetic field strength over periods of 30--60 min. We then present three case studies of geosynchronous magnetic field strength variations during periods of nearly constant solar wind dynamic pressure and southward IMF. The dayside magnetospheric magnetic field strength was depressed ~10 nT during a period of strongly southward IMF (Bz=-6 nT), but only ~5 nT during two more typical periods of slightly southward IMF (Bz=-2 to -3 nT). The depressions correspond to periods of enhanced AL index, which we interpret as evidence for directly driven solar wind--magnetosphere interaction rather than the unloading of energy stored within the magnetotail. Dayside geosynchronous magnetic field strengths are weakly correlated with IMF Bz. ¿ American Geophysical Union 1994