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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. A8, 1201, doi:10.1029/2001JA900113, 2002

Evidence for kinetic Alfvén waves and parallel electron energization at 4–6 R_E altitudes in the plasma sheet boundary layer

J. R. Wygant

School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota, USA

A. Keiling

School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota, USA

C. A. Cattell

School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota, USA

R. L. Lysak

School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota, USA

M. Temerin

Space Sciences Laboratory, University of California, Berkeley, California, USA

F. S. Mozer

Space Sciences Laboratory, University of California, Berkeley, California, USA

C. A. Kletzing

Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, USA

J. D. Scudder

Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, USA

V. Streltsov

Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire, USA

W. Lotko

Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire, USA

C. T. Russell

Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA

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 R_E. These structures coincide with larger-scale Alfvénic 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 Alfvén waves have periods of ∼20–60 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 ∼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 (with an electron inertial length c/ω_pe∼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 ∫ Edl associated with small-scale kinetic Alfvén wave structures. The energy flux in the electron “beams” is ∼0.7 ergs cm⁻² s⁻¹. 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 that 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 0.5 and 2 R_E, there are important heating and acceleration mechanisms operating at these higher altitudes in the plasma sheet.

Published 24 August 2002.

Index Terms: 2712 Magnetospheric Physics: Electric fields (2411); 2748 Magnetospheric Physics: Magnetotail boundary layers; 2752 Magnetospheric Physics: MHD waves and instabilities; 2704 Magnetospheric Physics: Auroral phenomena (2407).

Subscriber Access to Full Article (Nonsubscribers may purchase for $9.00, Includes print PDF, file size: 915336 bytes)

Citation: Wygant, J. R., et al. (2002), Evidence for kinetic Alfvén waves and parallel electron energization at 4–6 R_E altitudes in the plasma sheet boundary layer, J. Geophys. Res., 107(A8), 1201, doi:10.1029/2001JA900113.

Copyright 2002 by the American Geophysical Union.