Plasmoid growth and expulsion revealed by two-point ARTEMIS observations

On 12 October 2011, two ARTEMIS probes, in lunar orbit ~9 RE north of the neutral sheet, sequentially observed a tailward‐moving, expanding plasmoid. Their observations revealed a multilayered 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 opposite an approaching plasmoid. No evidence of active distant neutral line reconnection was observed by the probes, however, as they traversed the plasmoid's leading and trailing edges, penetrating 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 RE and its expansion rate to be ~7 RE/min at the observation locations. This observation of plasmoid expansion made at the plasmoid boundary is interpreted as plasmoid growth in both the XGSM and the ZGSM directions due to near‐Earth‐neutral‐line reconnection on closed plasma sheet field lines. The velocity inside the plasmoid proper was found to be nonuniform; the core likely moves as fast as 500 km/s, yet the outer layers move more slowly (and in the 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. Key points The fast propagating plasmoid was still inside the plasma sheetThe plasmoid was growing due to reconnection on closed field linesThe earthward flow is probably because of compression rather than reconnection