SIMULTANEOUS PRENOON AND POSTNOON OBSERVATIONS OF 3 FIELD-ALIGNED CURRENT SYSTEMS FROM VIKING AND DMSP-F7

The spatial structure of dayside large-scale field-aligned current (FA G) systems is examined by using Viking and DMSP-F7 data. We focus on f our events in which the satellites simultaneously observed postnoon an d prenoon three FAC systems: the region 2, the region 1, and the mantl e (referred to as midday region 0) systems, from equatorward to polewa rd. These events provide the most solid evidence to date that the midd ay region 0 system is a separate and unique FAC system, and is not an extension of the region 1 system from other local times. The events ar e examined comprehensively by making use of a multi-instrument data se t, which includes magnetic field, particle flux, electric field, auror al UV image data from the satellites, and the Sondrestrom convection d ata. The results are summarized as follows: (1) Region 2 currents flow mostly in the CPS precipitation region, often overlapping with the BP S at their poleward edge. (2) The region 1 system is located in the co re part of the auroral oval and is confined in a relatively narrow ran ge in latitude which includes the convection reversal. The low-latitud e boundary layer, possibly including the outer part of the plasma shee t, and the external cusp are the major source regions of dayside regio n 1 currents. (3) Midday region 0 currents flow on open field lines an d are collocated with the shear of antisunward convection flows with v elocities decreasing poleward. On the basis of these results we suppor t the view that both prenoon and postnoon current systems consist of t he three-sheet structure when the distortion of the convection pattern associated with interplanetary magnetic field (IMF) B-y is small and both morningside and eveningside convection cells are crescent-shaped. We also propose that the midday region 0 and a part of the region 1 s ystems are closely coupled to the same source.