Publication Date:
2013-01-17
Description:
[1] On 12 October 2011 the two ARTEMIS probes, in lunar orbit ~9 R E north of the neutral sheet, sequentially observed a tailward-moving, expanding plasmoid. Their observations reveal a multi-layered plasma sheet composed of tailward-flowing hot plasma within the plasmoid surrounded by earthward-flowing, less energetic plasma. Prior observations of similar earthward flows ahead of or behind plasmoids have been interpreted as earthward outflow from a continuously active distant-tail neutral line (DNL) opposite an approaching plasmoid. No evidence of active DNL reconnection was observed by the probes, however, as they traversed the plasmoid's leading and trailing edges, penetrating to above its core. We suggest an alternate interpretation: compression of ambient plasma by the tailward-moving plasmoid proper propels the plasma lobeward and earthward, i.e., above and below the plasmoid proper. Using the propagation velocity obtained from timing analysis, we estimate the average plasmoid proper size in its propagation direction to be 9 R E and its expansion rate to be ~7 R E /min at the observation locations. This observation of plasmoid expansion made at the plasmoid boundary is interpreted as plasmoid growth in both the X GSM and the Z GSM directions due to near-Earth-neutral-line (NENL) reconnection on closed plasma sheet field lines. The velocity inside the plasmoid proper was found to be non-uniform; the core likely moves as fast as 500 km/s, yet the outer layers move more slowly (and reverse direction). The absence of lobe reconnection, in particular on the earthward side, suggests that plasmoid formation and expulsion both result from closed plasma sheet field-line reconnection.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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