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 th e poleward boundary of the auroral oval in the 2200-0300 MLT sector at alti tudes of similar to 2.5-3 Earth's radii reveal the frequent occurrence of t hermal and super thermal H+ ion outflows. These events are strongly correla ted with suprathermal electron fluxes and broadband electromagnetic ULF wav es. The pitch angle distributions give evidence of transverse heating occur ring 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, exhib iting 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 fie ld-aligned direction at lower latitudes, The data analysis suggests that io n cyclotron resonance interaction with ULF electromagnetic turbulence can a ccount for the observed heating, even if we cannot totally exclude that tra nsverse velocity shears and nonresonant stochastic transverse acceleration sometimes contribute to the ion energization in view of the de electric fie ld fluctuations commonly observed at the same times, During the expansion p hase of substorms the region of transverse heating at the poleward boundary of the discrete auroral oval exhibits a latitudinal structure characterize d by an alternate occurrence of latitudinally narrow regions of intense and weal; ion fluxes, These latitudinal variations are associated with magneti c fluctuations at a frequency of similar to 2x10(-2) Hz, interpreted in ter ms of hydromagnetic Alfven waves. Equatorward of the hearing region, the en ergy spectrograms recorded during the same events exhibit an energy-latitud e dispersion signature with energy decreasing as latitude decreases, This d ispersion is the result of the velocity filter effect due to the large-scal e convection and of the poleward motion of the ion heating source associate d 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 cri terion for identifying the poleward boundary of the soft electron layer lyi ng at the high-latitude edge of the plasma sheet boundary layer.