Alfven waves, density cavities and electron acceleration observed from theFAST spacecraft

Inertial Alfven waves propagating in regions of auroral electron accelerati on are observed from the FAST (Fast Auroral Snapshot) spacecraft over its e ntire altitude range (350-4180 km). These electron skin depth sized field s tructures are dispersive and carry an electric field component parallel to the geomagnetic field leading to a variety of non-linear effects including the formation of magnetic field-aligned density cavities and electron accel eration. High resolution measurements show that the electron distribution i nside the impulsive wave field envelope or density cavity consists of a col d ionospheric component and an accelerated and heated field-aligned compone nt comprised of downgoing and reflected ionospheric and magnetosheath (sola r wind like) electrons. In large amplitude examples the plasma within the w ave may be dominated by the accelerated component and the depletion of plas ma within the cavity approaches 100%. Comparison of the observed density de pletion with the predictions of ponderomotive density cavity Formation in a ssociation with electron heating shows good agreement. Furthermore, simulat ions show that most of the observed features of the accelerated component c an be explained through Landau resonance of the cold ionospheric and magnet osheath electrons with the inertial Alfven wave as it propagates through an altitude dependent density profile.