AUREOL-3 observations of new boundaries in the auroral ion precipitation

Interesting and well-separated structures in the 1-20 keV ion precipitation pattern have been revealed by an analysis of more than 50 crossings of the nightside (21-03 MLT) auroral zone by the AUREOL-3 satellite. First, velocity-dispersed ion structures (VDIS) are crossed near the poleward edge of the oval, and are the best ionospheric signature of ion beams flowing along the plasma sheet boundary layer. Proceeding equatorward, a large majority of VDIS events are bounded by a new and interesting narrow band of strongly reduced precipitation, or a gap, which delineates VDIS from the diffuse precipitation region connected to the CPS. A statistical analysis shows that the gap has an extent of about 1-2 deg, which is almost independent of magnetic activity; its location, about 70 deg ILAT, shifts significantly equatorward with higher magnetic activity levels. Intense electron arcs are observed near the equatorward edge of the gap. An important result is that the overall sequence of VDIS-gap-CPS can be explained in terms of orbital dynamics in the tail. The gap in precipitation appears as the counterpart of the 'wall' regime in the equatorial plane, in which a cross-tail current carried by energetic ions is strongly enhanced between 8 and 12 R(E). This region has important consequences for the development of substorms.