Walén and variance analyses of high‐speed flows observed by Wind in the midtail plasma sheet: Evidence for reconnection

We have analyzed 4 days of Wind observations of high‐speed convective flows of up to ∼800 km s−1in the plasma sheet at XGSE∼ − 60 REduring late March and early April 1999. Both earthward and tailward flows were observed. The high‐speed flows had a duration of several hours, unlike the shorter‐lasting (tens of minutes) bursty bulk flows, which are typically observed closer to the Earth. We have analyzed in detail a ∼10 hour interval of high‐speed flows detected during rather low geomagnetic activity and northward interplanetary magnetic field. Our analysis indicates that the fast flows are produced by magnetic reconnection and that the observed flow reversals are consistent with the passage of a reconnection X line. First, the results of the shear stress balance test (the Walén test) indicate that the flow in the deHoffmann‐Teller frame, which is aligned with the magnetic field, has an average flow speed that is ∼60% of the Alfvén speed, consistent with the presence of slow shocks in the magnetotail reconnection layer. Furthermore, the slope of the Walén regression line switches sign at the flow reversals, as expected from reconnection. Consistent with the passage of a reconnection X line, the magnetic field component normal to the neutral sheet also reverses sign at the flow reversals. For this event the tailward flowing plasma is hotter than the earthward flowing plasma, consistent with the two plasmas being magnetically disconnected. Our observations imply that quasi‐steady reconnection can occur in the midmagnetotail region during periods of persistent northward interplanetary magnetic field.