PLASMA-HEATING AND FLOW IN AN AURORAL ARC

We report direct observations of the three-dimensional velocity distri bution of selected topside ionospheric ion species in an auroral conte xt between 500 and 550 km altitude. We find heating transverse to the local magnetic field in the core plasma, with significant heating of O +, He+ and H+, as well as tail heating events that occur independently of the core heating. The O+ velocity distribution departs from bi-Max wellian, at one point exhibiting an apparent ring-like shape. However, these observations are shown to be aliased within the amoral are by t emporal variations that are not well-resolved by the core plasma instr ument. The de electric field measurements reveal superthermal plasma d rifts that are consistent with passage of the payload through a series of vortex structures or a larger scale circularly polarized hydromagn etic wave structure within the auroral are. The de electric field also shows that impulsive solitary structures, with a frequency spectrum i n the ion cyclotron frequency range, occur in close correlation with t he tail heating events. The drift and core heating observations lend s upport to the idea that core ion heating is driven at low altitudes by rapid convective motions imposed by the magnetosphere. Plasma wave em issions at ion frequencies and parallel heating of the low-energy elec tron plasma are observed in conjunction with this amoral form; however , the conditions are much more complex than those typically invoked in previous theoretical treatments of superthermal frictional heating. T he observed ion heating within the are clearly exceeds that expected f rom frictional heating for the light ion species H+ and He+, and the c ore distributions also contain hot transverse tails, indicating an ano malous transverse heat source.