Simultaneous satellite and ground observations of transient events near the morningside magnetopause

We present observations of transient events on April 13, 1996, using simult aneous measurements in the solar wind, near the magnetopause, at geosynchro nous orbit, and on the ground at high latitudes. On this day, Geotail passe d outbound through the morningside magnetopause. When Geotail was in the ma gnetosphere, a clear bipolar perturbation in the magnetic field component n ormal to the nominal magnetopause and transient outbound (from the magnetos phere to the magnetosheath) /inbound (from the magnetosheath to the magneto sphere) magnetopause crossings were observed. When Geotail was in the magne tosheath, inbound/outbound crossings were observed. During the bipolar magn etic field signature, Geotail observed asymmetric bipolar (less inward/more outward) flow normal to the nominal magnetopause accompanied by sunward fl ow. Inward flow was observed when the total magnetic field of the event was enhanced, whereas outward flow was observed when the total magnetic field was depressed. We observed that some of the transient outbound/inbound cros sings show significantly different signatures in the plasma flow and magnet ic field, that is, weak inward flow and enhanced magnetic field before the outbound crossing and strong outward flow and decreased magnetic field afte r the inbound crossing. The field and flow variations during the transient outbound/inbound crossings are qualitatively similar to those during the bi polar magnetic field signature. These observations suggest that the bipolar field signature and the transient magnetopause crossings would be similar phenomena on the magnetopause. We discuss whether the observed field and ho w variations during the transient events are caused by inward/outward magne topause motion due to compression/expansion of the magnetosphere by compari ng the data sets obtained from different regions of the solar wind-magnetos phere system.