Alfven wave modulation of the auroral acceleration region

Abstract

We consider the interaction of Alfven waves with the auroral acceleration region (AAR). The AAR is characterized by an electric potential drop that supports a field-aligned upward current and the acceleration of precipitating electrons. An Alfven wave incident on the AAR from the magnetosphere partially reflects back and partially penetrates into the AAR. The rate of wave reflection/transmission is estimated to be critically dependent on the wave transverse scale. Magnetospheric Alfven waves penetrating into the AAR can produce oscillatory variations of the field-aligned potential drop, thus constituting a new mechanism of ULF modulation of electron acceleration. Estimates of the potential drop modulation by Alfven waves are provided within the “thin” AAR approximation, which is valid for a wide range of wave and plasma parameters. The proposed mechanism will produce nearly simultaneous ULF magnetic and riometric variations at auroral latitudes. Occurrence of the AAR-associated resonator in the auroral topside ionosphere between the bottom boundary of the AAR and the E-layer may cause oscillatory frequency dependence of electron acceleration modulations in the range around fractions of a Hz. Another feature of the mechanism considered is the critical dependence of the ratio between the magnetic and riometric signals on the transverse scale of the disturbance. The predicted effects are to be searched for in the simultaneous data of IRIS multi-beam riometers and magnetometers.