University of Minnesota
School of Physics & Astronomy
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John Wygant

Evidence for Kinetic Alfven Waves and parallel electron energyization at 4-6 Re altitudes in the plasmasheet boundary layer
J. R. Wygant, A. Keiling, C. A. Cattell, R. L. Lysak, M. Temerin, C. A. Kletzing and J. D. Scudder, F. S. Mozer,V. Streltsov, W. Lotko, C. T. Russell, J. of Geophys. Res., 2002

Abstract

We present evidence based on measurements from the Polar spacecraft for the existence of small-scale, large amplitude kinetic Alfvén waves/spikes at the plasma sheet boundary layer (PSBL) at altitudes of 4-6 Re. These structures coincide with larger scale Alfvénic waves which carry a large net Poynting flux along magnetic field lines towards 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 and associated magnetic field variations between 0.5 nT to 5 nT. Previous analysis has shown that the larger scale Alfvén waves have periods of ~20-60 seconds 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 1s (but may have shorter time durations since the Nyquist frequency of the magnetic field experiment is ~4 Hz.). The characteristic ratio of the amplitudes of the electric to magnetic field fluctuations is strong evidence that the waves are kinetic Alfvén waves with scale sizes perpendicular to the magnetic field on the order of 20-120 km (where an electron inertial length, c/wpe~10 km, and an ion gyroradius ~20 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 ~1 keV, consistent with estimates of the parallel Ú E?dl associated with small-scale kinetic Alfvén wave structures. The energy flux in the electron "beams" is about 0.7 ergs/cm2s. These observations suggest that the small-scale kinetic Alfvén waves are generated from the larger-scale Alfvén waves through one or more of a variety of mechanisms which have been proposed to result in the filamentation of large amplitude Alfvén waves. The observations presented herein provide strong evidence that, in addition to the auroral particle energization processes known to occur at altitudes between .5 Re and 2 Re, there are important heating and acceleration mechanisms operating at these higher altitudes in the plasma sheet.