New model for auroral acceleration: O-shaped potential structure cooperating with waves

There are recent observational indications (lack of convergent electric ®eld signatures above the auroral oval at 4 RE altitude) that the U-shaped potential drop model for auroral acceleration is not applicable in all cases. There is nevertheless much observational evidence favouring the U-shaped model at low altitudes, i.e., in the acceleration region and below. To resolve the puzzle we propose that there is a negative O-shaped potential well which is maintained by plasma waves pushing the electrons into the loss cone and up an electron potential energy hill at 3±4RE altitude range. We present a test particle simulation which shows that when the wave energization is modelled by random parallel boosts, introducing an O-shaped potential increases the precipitating energy ̄ux because the electrons can stay in the resonant velocity range for a longer time if a downward electric ®eld decelerates the electrons at the same time when waves accelerate them in the parallel direction. The lower part of the O-shaped potential well is essentially the same as in the U-shaped model. The electron energization comes from plasma waves in this model, but the ®nal low-altitude ̄uxes are produced by electrostatic acceleration. Thus, the transfer of energy from waves to particles takes places in an ``energization region'', which is above the acceleration region. In the energization region the static electric ®eld points downward while in the acceleration region it points upward. The model is compatible with the large body of lowaltitude observations supporting the U-shaped model while explaining the new observations of the lack of electric ®eld at high altitude.