Ionospheric current signatures of transient plasma sheet flows

The plasma flow in the central plasma sheet of the magnetospheric tail often includes short impulsive bursts. Here we investigate the ionospheric signatures of such bursts. The Wind satellite recorded several transient fast flows in the plasma sheet (at geocentric distances of similar to 12 R-E) on December 21-22, 1995. The data are compared with magnetic field observations made in the Scandinavian sector, at the ionospheric conjugate point of the satellite. Superposed epoch analysis of the satellite data suggests that most of the events are Earthward flow bursts accompanied by magnetic dipolarizations, increases in the convection electric field, and drops in the plasma pressure and density. Occasionally, also isolated tailward flow bursts within closed flux tubes can be observed. We demonstrate that in both cases the transient plasma sheet flows are systematically associated with distinct ground magnetic field variations which (after 90 degrees rotation) have specific vortex-like spatial distributions. The vortex patterns have similar duration to that of the flows at Wind and their longitudinal extent (less than or equal to 1 hour in local time) is consistent with the azimuthal scale sizes (similar to 3 R-E) Of the transient flows reported earlier. In many cases the sense of flow rotation observed at Wind and at Wind's ionospheric footpoint agree with our expectation. Despite the care taken in accounting for the instantaneous and local currents that affect the mapping, uncertainties in the footpoint location may still be responsible of the absence of a higher degree for compliance with theory.