Plasmoid growth and expulsion revealed by two‐point ARTEMIS observations

Abstract

[1] On 12 October 2011, two ARTEMIS probes, in lunar orbit~9 R E 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 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 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.

Azimuthal extent and properties of midtail plasmoids from two‐point ARTEMIS observations at the Earth‐Moon Lagrange points

“…When traversing the plasmoid from and back to the lobe region, P1 observed mostly tailward flow (except for a transient earthward flow interval around 14:08:50). Probe 2, however, observed a multilayer plasmoid structure similar to the structure observed in a previous case study in which the plasmoid was ejected before the reconnection had proceeded to encompass lobe field lines [ Li et al ., ]. Unfortunately, during this period only slow‐survey data are available, and particle full distributions are not at sufficiently high time resolution to help identify the source of the earthward preplasmoid flow.…”

“…Unfortunately, during this period only slow‐survey data are available, and particle full distributions are not at sufficiently high time resolution to help identify the source of the earthward preplasmoid flow. The earthward flow could be reconnection outflow from the distant neutral line (DNL) [ Nishida et al ., ] or ambient plasma sheet flow caused by compression of the tailward propagating plasmoid [ Li et al ., ]. In the reconnection outflow scenario, the DNL would have been active at P2's azimuthal sector ( Y AGSM = 14.7 R E ) and inactive at P1's location ( Y AGSM = −1.5 R E ), which suggests that the DNL is either localized or discontinuous.…”

Three‐Dimensional X‐line Spreading in Asymmetric Magnetic Reconnection

“…On the Sun, solar flare ribbons are observed to spread along the magnetic polarity inversion line unidirectionally or bidirectionally (Cheng et al, ; Fletcher et al, ; Isobe et al, ; Lee & Gary, ; Liu et al, ; Qiu, ; Qiu et al, ). In the Earth's magnetotail and magnetopause, a wide range of X‐line extents, from a few Earth radii to longer than 10 , along the current (out‐of‐plane) direction exists (Fuselier et al, ; Li et al, ; Nakamura et al, ; Phan et al, ; Zou et al, ). In the solar wind, a long extended X‐line of over hundreds of has also been reported (Phan et al, ).…”

Hot flow anomaly remnant in the far geotail?

“…When a plasmoid passes, the B z component of the magnetic field changes its sign and helicity can be seen in the magnetic field related to a flux rope. Furthermore the density increases in the far tail plasmoid observations (see Kiehas et al, 2012;Li et al, 2013;Vörös et al, 2014, Figure 2, 5, Figure 3 and Figure 11, respectively). The density in the plasmoids does not change (Ieda et al, 1998) or increases (Moldwin and Hughes, 1992) when the plasmoids move downtail.…”