GROWTH AND EVOLUTION OF A PLASMOID ASSOCIATED WITH A SMALL, ISOLATED SUBSTORM - IMP-8 AND GEOTAIL MEASUREMENTS IN THE MAGNETOTAIL

A tailward-moving plasmoid was observed at the GEOTAIL satellite at a GSM position (-73.3, 18.1, -1.1) R(E) on September 16, 1993, at 0417 U T, when the IMP 8 satellite was at (-37.5, -2.5, 1.7) R(E) at the midn ight plasma sheet/lobe interface. The first indication of the plasmoid formation a few minutes after the negative bay onset of a small, loca lized auroral substorm was the onset of tailward beams of energetic io ns and electrons at GEOTAIL well within the plasma sheet. Earthward-st reaming energetic ions observed at IMP 8 a few minutes later suggest t hat the plasmoid evolved from within the plasma sheet to encompass the flux of nearly the entire thickness of the plasma sheet. The opposite direction of the anisotropies at IMP 8 and GEOTAIL suggest that the p article acceleration region was between X = -37.5 and -73 R(E) at that time. The isolated substorm associated with this plasmoid started equ atorward of 67 degrees latitude at a location which we map to near-Ear th nightside plasma sheet (\X\ < 15 R(E)) based on ground observations of a field line resonance. The active electrojet did not expand polew ard until at least 10 min after the detection of the acceleration regi on tailward of IMP 8 and at least several minutes after the core of th e plasmoid had moved tailward of GEOTAIL. These observations reinforce by means of in situ, concurrent, multipoint measurements the attitude expressed recently by several researchers that the locations of lobe reconnection and equatorial projection of electrojet intensification d uring substorm expansion are distinctly different from each other.