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

There are recent observational indications (lack of convergent electric fie ld signatures above the auroral oval at 4 R-E altitude) that the U-shaped p otential 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 reso lve 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 con e and up an electron potential energy hill at similar to 3-4R(E) altitude r ange. 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 Aux because the electrons can stay in the resonant velocity range for a longer time if a downward electr ic field decelerates the electrons at the same time when waves accelerate t hem in the parallel direction. The lower part of the O-shaped potential wel l is essentially the same as in the U-shaped model. The electron energizati on comes from plasma waves in this model, but the final low-altitude fluxes are produced by electrostatic acceleration. Thus. the transfer of energy f rom waves to particles cakes places in an "energization region", which is a bove the acceleration region. In the energization region the static electri c field points downward while in the acceleration region it points upward. The model is compatible with the large body of low-altitude observations su pporting the U-shaped model while explaining the new observations of the la ck of electric field at high altitude.