Ionospheric erosion by Alfvén waves

Abstract: Using observations from the FAST small explorer spacecraft we present fields and plasma observations above the dayside auroral oval showing the erosion of ionospheric plasmas from the topside ionosphere by the action of Alfvén waves. Using interferometric techniques the waves are shown to approximately obey the expected dispersion for Alfvén waves with transverse scales extending from greater than electron inertial lengths down to ion gyro-radii. Measurements of the plasma density where these waves are observed show that over latitudinal widths exceeding 100 km total depletion of the cold ionospheric plasma can occur. The plasma within these depleted regions or cavities is composed of magnetosheath ion and electron distributions and upgoing transversely accelerated ions and downgoing field-aligned electrons distributed as conics and field-aligned beams respectively. Poynting flux observations on the density gradients comprising the cavity walls show that these waves are directed downwards and focused inwards towards regions of lower density. The wave phase velocity measurement in the plasma frame, while subject to significant uncertainty, is directed transversely outwards from the cavity. These observations suggest a feedback model for Alfvén wave focusing and ion heating on density gradients that can lead to intense ion outflow from the ionosphere and subsequent depletion of ionospheric plasmas.