Plasma heating and flow in an auroral arc

We report direct observations of the three‐dimensional velocity distribution of selected topside ionospheric ion species in an auroral context 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‐Maxwellian, at one point exhibiting an apparent ring‐like shape. However, these observations are shown to be aliased within the auroral arc by temporal variations that are not well‐resolved by the core plasma instrument. The dc electric field measurements reveal superthermal plasma drifts that are consistent with passage of the payload through a series of vortex structures or a larger scale circularly polarized hydromagnetic wave structure within the auroral arc. The dc electric field also shows that impulsive solitary structures, with a frequency spectrum in the ion cyclotron frequency range, occur in close correlation with the tail heating events. The drift and core heating observations lend support to the idea that core ion heating is driven at low altitudes by rapid convective motions imposed by the magnetosphere. Plasma wave emissions at ion frequencies and parallel heating of the low‐energy electron plasma are observed in conjunction with this auroral form; however, the conditions are much more complex than those typically invoked in previous theoretical treatments of superthermal frictional heating. The observed ion heating within the arc clearly exceeds that expected from frictional heating for the light ion species H + and He + , and the core distributions also contain hot transverse tails, indicating an anomalous transverse heat source.