Alfven resonator in the topside ionosphere beneath the auroral acceleration region

Abstract

The possible occurrence of a new resonator in the topside ionosphere at auroral latitudes is proposed. This resonator is formed between the E‐layer of the ionosphere and the bottom boundary of the auroral acceleration region (AAR), which has a localized field‐aligned potential drop. The AAR is shown to effectively reflect Alfven waves with transverse scales less than the Alfven transit scale λA. The proposed resonator can trap and accumulate Alfvenic small‐scale disturbances with periods from a few seconds to a few tens of seconds, and with transverse scales from a kilometers to a few tens of kilometers. The eigenfrequencies of the AAR‐associated resonator are estimated to be lower than that of the ionospheric Alfven resonator. The small‐scale Alfvenic structures commonly observed by satellites can be generated by a nonsteady field‐aligned current transported by precipitating electrons and trapped in the AAR‐associated resonator. Observations by ground‐based magnetometers at auroral latitudes often show the occurrence of intensifications in the dynamic spectra of Pi1 pulsations in accordance with the predictions of the model.