Fluid and kinetics signatures of reconnection at the dawn tail magnetopause:: Wind observations

We report an accelerated plasma flow event detected by Wind at the low-latitude dawn tail magnetopause (x(GSE) approximate to -10 R-E) when the local magnetic shear across the magnetopause was high (similar to 180 degrees) and the GSM y, component of the interplanetary magnetic field was positive and much larger than the z component. High time resolution (3 s) three-dimensional ion and electron distributions were obtained for this event. We have performed rigorous tests of fluid and particle predictions of reconnection. Consistent with reconnection, we observed at the magnetopause (I) jetting of plasma, with a flow speed, measured in the deHoffmann-Teller frame, at 98% of the Alfven speed; (2) mixing of a nearly isotropic hot plasma sheet ion distribution with a field-aligned magnetosheath distribution having a low-energy cutoff at the predicted deHoffmann-Teller velocity; (3) opposite streaming of magnetosheath and plasma sheet electrons consistent with open field topology; and (4) a finite inward (earthward) directed normal magnetic field (B-N < 0) at and an associated earthward plasma flow (at 94% of the normal Alfven speed) across the magnetopause. The combined fluid and particle signatures provide a comprehensive set of evidence for reconnection at the magnetopause, These reconnection signatures were observed in the tail flank magnetopause, where the presence of fast plasma flow (at almost twice the local Alfven speed) in the adjacent magnetosheath has been predicted to suppress reconnection. The sense of the flow enhancement, the direction of the electron heat flux, and the polarity of BN are all consistent with each other and with the spacecraft being located tailward and northward of the reconnection site. Our analysis places the reconnection site similar to7 Earth radii south of the magnetic equator, The dimensionless reconnection rate at this flank magnetopause is estimated to be in the range of 0.1 to 0.2, which is similar to values reported for the subsolar region. In essence, the present event provides unambiguous evidence for reconnection and shows that reconnection signatures at the tail flank magnetopause are not noticeably different from those predicted and observed in the subsolar region.