Growth and evolution of a plasmoid associated with a small, isolated substorm: IMP 8 and GEOTAIL measurements in the magnetotail

Abstract

A tailward‐moving plasmoid was observed at the GEOTAIL satellite at a GSM position (−73.3, 18.1, −1.1) RE on September 16, 1993, at 0417 UT, when the IMP 8 satellite was at (−37.5, −2.5, 1.7) RE at the midnight plasma sheet/lobe interface. The first indication of the plasmoid formation a few minutes after the negative bay onset of a small, localized auroral substorm was the onset of tailward beams of energetic ions and electrons at GEOTAIL well within the plasma sheet. Earthward‐streaming energetic ions observed at IMP 8 a few minutes later suggest that 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 particle acceleration region was between X=−37.5 and −73 RE at that time. The isolated substorm associated with this plasmoid started equatorward of 67° latitude at a location which we map to near‐Earth nightside plasma sheet (|X| < 15 RE) based on ground observations of a field line resonance. The active electrojet did not expand poleward until at least 10 min after the detection of the acceleration region tailward of IMP 8 and at least several minutes after the core of the 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 during substorm expansion are distinctly different from each other.