Evidence for kinetic Alfven waves and parallel electron energization at 4-6 RE altitudes in the plasma sheet boundary layer -: art. no. 1201

We present evidence based on measurements from the Polar spacecraft for the existence of small-scale, large-amplitude kinetic Alfven waves/spikes at the plasma sheet boundary layer (PSBL) at altitudes of 4-6 R-E. These structures coincide with larger-scale Alfvenic waves that carry a large net Poynting flux along magnetic field lines toward the Earth. Both structures are typically observed in the PSBL but have also been observed deeper in the plasma sheet. The small-scale spikes have electric field amplitudes up to 300 mV m(-1) and associated magnetic field variations between 0.5 and 5 nT. Previous analysis has shown that the larger-scale Alfven waves have periods of similar to20-60 s and carry enough Poynting flux to explain the generation of the most intense auroral structures observed in the Polar Ultraviolet Imager data set. In this paper it is shown that the smaller-scale waves have durations in the spacecraft frame of 250 ms to 1 s (but may have shorter time durations since the Nyquist frequency of the magnetic field experiment is similar to4 Hz.). The characteristic ratio of the amplitudes of the electric to magnetic field fluctuations is strong evidence that the waves are kinetic Alfven waves with scale sizes perpendicular to the magnetic field on the order of 20-120 km (with an electron inertial length c/omega(pe)similar to10 km and an ion gyroradius similar to20 km). Theoretical analysis of the observed spikes suggests that these waves should be very efficient at accelerating electrons parallel to the magnetic field. Simultaneously measured electron velocity space distribution functions from the Polar Hydra instrument include parallel electron heating features and earthward electron beams, indicating strong parallel energization. The characteristic parallel energy is on the order of similar to1 keV, consistent with estimates of the parallel integral Edl associated with small-scale kinetic Alfven wave structures. The energy flux in the electron 'beams'' is similar to0.7 ergs cm(-2) s(-1). These observations suggest that the small-scale kinetic Alfven waves are generated from the larger-scale Alfven waves through one or more of a variety of mechanisms that have been proposed to result in the filamentation of large-amplitude Alfven waves. The observations presented herein provide strong evidence that in addition to the auroral particle energization processes known to occur at altitudes between 0.5 and 2 R-E, there are important heating and acceleration mechanisms operating at these higher altitudes in the plasma sheet.