Modulation of the auroral acceleration region by Alfven waves: A new mechanism for ULF wave modulation of electron acceleration

We consider the interaction of Alfven waves with the auroral acceleration region. The AAR is characterized by an electric potential drop that supports a field-aligned upward current and the acceleration of 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 particle acceleration. Analytical estimates of the potential drop modulation by Alfven waves are provided within the "thin" AAR approximation, which is valid for small-scale disturbances. The topside ionosphere between the bottom boundary of the AAR and the E-layer of the ionosphere forms a resonator at auroral latitudes, which can trap Alfvenic small-scale disturbances with frequencies ~ 0.2 Hz. Occurrence of the AAR-associated resonator may cause fine spectral features of modulation of electrons accelerated in the AAR. We outline critical tests for the verification of the proposed mechanism in simultaneous observations at magnetometer and riometer stations at auroral latitudes.