Magnetic fields in flowing magnetotail plasmas and their significance for magnetic reconnection

Abstract

Results of a statistical analysis of 324 intervals of intense bulk plasma flows in the plasma sheet of the geomagnetic tail, measured by the Imp 6 spacecraft, are presented. Measurements of the magnetic field in the plasma sheet during these flows reveal pronounced dependences of the north‐south magnetic field component Bz(GSM) and the earth‐sun component Bx(GSM) on the flow direction; no such dependence of the By(GSM) component is discerned. Specifically, the fraction of field samples that are oriented southward is significantly greater during intervals of tailward flowing plasma than it is during intervals of earthward flowing plasma. Furthermore, this difference increases markedly as the absolute value of the angle of the magnetic field to the equatorial plane steepens. The total fraction of southward field, even during tailward flows, however, is about one third. As for the Bx component of the field, the distribution of samples in earthward flowing plasma is clearly peaked near zero, but the distribution in tailward flowing plasma is flatter and contains proportionately greater numbers of large positive and negative values of Bx. A moderate increase in ground activity, as determined by the AE index, was observed to be associated with the intervals of plasma flows. These results are reviewed in the context of models of magnetic reconnection in the magnetotail and the geometry of the tail plasma sheet.