Dual spacecraft observations of lobe magnetic field perturbations before, during and after plasmoid release

This study examines a data set returned by IMP 8 and Geotail on January 29, 1995 during a substorm which resulted in the ejection of a plasmoid. The t wo spacecraft (sic) were situated in the north lobe of the tail and both ob served a traveling compression region (TCR). We show that in this instance dual s/c measurements can be used to model all three dimensions of the unde rlying plasmoid and to estimate its rate of expansion. For this event plasm oid dimensions of Delta X similar to 18, Delta Y similar to 30, and Delta Z similar to 10 R-e are determined from the IMP 8 and Geotail observations. Furthermore, a factor of similar to 2 increase in the amplitude of the TCR occurred in the 1.5 min it took to move from IMP 8 to Geotail. Modeled usin g conservation of magnetic flux, this increase in lobe compression implies that the underlying plasmoid was expanding at a rate of similar to 140 km/s . Finally, a reconfiguration of the lobe magnetic field followed plasmoid e jection which moved magnetic flux tubes into the wake behind the plasmoid w here they would become available to feed the reconnection region.