Low-energy upflowing ion events at the poleward boundary of the nightside auroral oval: High-altitude Interball-Auroral probe observations

Ion data acquired by the Interball-Auroral satellite during crossings of the poleward boundary of the auroral oval in the 2200-0300 MLT sector at altitudes of similar to 2.5-3 Earth's radii reveal the frequent occurrence of thermal and super thermal H+ ion outflows. These events are strongly correlated with suprathermal electron fluxes and broadband electromagnetic ULF waves. The pitch angle distributions give evidence of transverse heating occurring in a latitudinally narrow layer at the boundary between the polar cap and the plasma sheet boundary layer, over a broad altitude range extending up to the satellite altitude. The distributions evolve with latitude, exhibiting fluxes maximizing at pitch angles close to 90 degrees at the poleward edge of the outflow structure and at pitch angles closer to the upward field-aligned direction at lower latitudes, The data analysis suggests that ion cyclotron resonance interaction with ULF electromagnetic turbulence can account for the observed heating, even if we cannot totally exclude that transverse velocity shears and nonresonant stochastic transverse acceleration sometimes contribute to the ion energization in view of the de electric field fluctuations commonly observed at the same times, During the expansion phase of substorms the region of transverse heating at the poleward boundary of the discrete auroral oval exhibits a latitudinal structure characterized by an alternate occurrence of latitudinally narrow regions of intense and weal; ion fluxes, These latitudinal variations are associated with magnetic fluctuations at a frequency of similar to 2x10(-2) Hz, interpreted in terms of hydromagnetic Alfven waves. Equatorward of the hearing region, the energy spectrograms recorded during the same events exhibit an energy-latitude dispersion signature with energy decreasing as latitude decreases, This dispersion is the result of the velocity filter effect due to the large-scale convection and of the poleward motion of the ion heating source associated with the poleward motion of the high-latitude edge of the active auroral region. The poleward edge of the low-energy ion structure marked by a sharp latitudinal gradient of the ion flux appears as a reliable midaltitude criterion for identifying the poleward boundary of the soft electron layer lying at the high-latitude edge of the plasma sheet boundary layer.