Unusual geomagnetic response to the solar wind dynamic pressure pulse during storm-time

Mid-to-low-latitude ground magnetometers located on the dayside observe an unusualnegative bay in the H-component as a direct response to the solar wind dynamic pressure (SWPd) pulse occurring during period of steadily intense southward IMF in the course of the magnetic storm main phase. Occurrence of geomagnetic depression maximizes in the postnoon sector. Geosynchronous satellite also observes a decrease of the dayside magnetic field. Simultaneous high-latitude magnetic disturbances are likely indicative oflarge strengthening of Region 1 field-aligned current (FAC). Observations seem to confirm the theoretical prediction that Chapman-Ferraro dynamics is replaced by Region 1 FACdynamics when the IMF is strongly southward and SWPd is high.