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Ion temperature anisotropy across a magnetotail reconnection jet.

ABSTRACT: A significant fraction of the energy released by magnetotail reconnection appears to go into ion heating, but this heating is generally anisotropic. We examine ARTEMIS dual-spacecraft observations of a long-duration magnetotail exhaust generated by antiparallel reconnection in conjunction with particle-in-cell simulations, showing spatial variations in the anisotropy across the outflow far (>100di ) downstream of the X line. A consistent pattern is found in both the spacecraft data and the simulations: While the total temperature across the exhaust is rather constant, near the boundaries Ti,|| dominates. The plasma is well above the firehose threshold within patchy spatial regions at |BX |?[0.1,0.5]B0, suggesting that the drive for the instability is strong and the instability is too weak to relax the anisotropy. At the midplane ( |BX|?0.1B0), Ti,?>Ti,|| and ions undergo Speiser-like motion despite the large distance from the X line.

SUBMITTER: Hietala H

PROVIDER: S-EPMC4950132 | biostudies-literature | 2015 Sep

REPOSITORIES: biostudies-literature

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Ion temperature anisotropy across a magnetotail reconnection jet.

Hietala H H Drake J F JF Phan T D TD Eastwood J P JP McFadden J P JP

Geophysical research letters 20150918 18

A significant fraction of the energy released by magnetotail reconnection appears to go into ion heating, but this heating is generally anisotropic. We examine ARTEMIS dual-spacecraft observations of a long-duration magnetotail exhaust generated by antiparallel reconnection in conjunction with particle-in-cell simulations, showing spatial variations in the anisotropy across the outflow far (>100di ) downstream of the X line. A consistent pattern is found in both the spac ...[more]

PMID: 27478283

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Project description:Abstract Fast divergent flows of electrons and ions in the magnetotail plasma sheet are conventionally interpreted as a key reconnection signature caused by the magnetic topology change at the X‐line. Therefore, reversals of the x‐component (Vx⊥) of the plasma flow perpendicular to the magnetic field must correlate with the sign changes in the north‐south component of the magnetic field (Bz). Here we present observations of the flow reversals that take place with no correlated Bz reversals. We report six such events, which were measured with the high‐resolution plasma and fields instruments of the Magnetospheric Multiscale mission. We found that electron flow reversals in the absence of Bz reversals (a) have amplitudes of ∼1,000–2,000 km s−1 and durations of a few seconds; (b) are embedded into larger‐scale ion flow reversals with enhanced ion agyrotropy; and (c) compared with conventional reconnection outflows around the electron diffusion regions (EDRs), have less (if ever) pronounced electron agyrotropy, dawnward electron flow amplitude, and electric field strength toward the neutral sheet, although their energy conversion parameters, including the Joule heating rate, are quite substantial. These results suggest that such flow reversals develop in the ion‐demagnetization regions away from electron‐scale current sheets, in particular the EDRs, and yet they play an important role in the energy conversion. These divergent flows are interpreted as precursors of the flow‐driven reconnection onsets provided by the ion tearing or the ballooning/interchange instability. Key Points Transient electron flow reversals embedded into a more gradual ion flow reversal in the magnetotail occur without magnetic topology change Properties of the discovered divergent flows are distinctly different from those of reconnection outflows in the electron diffusion region Divergent flows are interpreted as flow‐driven reconnection onsets due to the ion tearing or the ballooning/interchange instability

Dataset Information

Ion temperature anisotropy across a magnetotail reconnection jet.

Publications

Ion temperature anisotropy across a magnetotail reconnection jet.

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