A New Perspective on Magnetotail Electron and Ion Divergent Flows: MMS Observations

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 (V-x perpendicular to) of the plasma flow perpendicular to the magnetic field must correlate with the sign changes in the north-south component of the magnetic field (B-z). Here we present observations of the flow reversals that take place with no correlated B-z 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 B-z reversals (a) have amplitudes of similar to 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.