On the source region and orientations of nightside auroral arcs

Abstract

Using large data base of stable auroral arcs observed at Gillam we identified those passes of POES spacecraft which were magnetically conjugate to these arcs. We studied statistically arc locations with respect to the energetic particle isotropy boundaries (IBs), which are the well-known markers of the magnetotail regions. In the dusk and night sectors we confirm previous finding that arcs tend to appear close to or poleward of the electron isotropy boundary (IBe), indicating the location of the arcs in the small BZ (large magnetic field curvature) region of the tail current sheet. Different from that, in the dawn sector significant fraction of arcs appears deeper in the magnetosphere, between electron and proton IBs, in the transition region between the current sheet and dipole-like magnetosphere. Addressing the orientations of stable arcs we use the empirical magnetospheric models to demonstrate that nearly sun-aligned plasma sheet structures in the tail are significantly rotated toward azimuthal alignment when mapped from the tail equator to the ionosphere. We discuss how the flow braking/rotation can provide (in direct or secondary/way) the E-W aligned arcs. Based on these results we argue that a significant fraction of the highlatitude stable auroral arcs could be formed by the fast flow convection channels (BBFs); particularly the observed dawn-dusk asymmetry in the relative location of arcs and isotropy boundary can be contributed by this mechanism.