Walen 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 similar to 800 km s(-1) in the plasma sheet at X-GSE similar to - 60 RE during late March and early April 1999. Both earthward and tailward f lows were observed. The high-speed flows had a duration of several hours, u nlike the shorter-lasting (tens of minutes) bursty bulk flows, which are ty pically observed closer to the Earth. We have analyzed in detail a similar to 10 hour interval of high-speed flows detected during rather low geomagne tic activity and northward interplanetary magnetic field. Our analysis indi cates that the fast flows are produced by magnetic reconnection and that th e observed flow reversals are consistent with the passage of a reconnection X line. First, the results of the sheer stress balance test (the Walen tes t) indicate that the flow in the deHoffmann-Teller frame, which is aligned with the magnetic field, has an average flow speed that is similar to 60% o f the Alfven speed, consistent with the presence of slow shocks in the magn etotail reconnection layer. Furthermore, the slope of the Walen regression line switches sign at the flow reversals, as expected from reconnection. Co nsistent with the passage of a reconnection X line, the magnetic field comp onent normal to the neutral sheet also reverses sign at the flow reversals. For this event the tailward flowing plasma is hotter than the earthward fl owing plasma, consistent with the two plasmas being magnetically disconnect ed. Our observations imply that quasi-steady reconnection can occur in the midmagnetotail region during periods of persistent northward interplanetary magnetic field.