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

Abstract

[1] 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 À1 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/w 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 R Edl associated with small-scale kinetic Alfvén wave structures. The energy flux in the electron ''beams'' is $0.7 ergs cm À2 s À1 . 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.

Development of parallel electric fields at the plasma sheet boundary layer

“…At higher altitudes where the warm plasma effects are important, Landau damping can lead to enhanced parallel electric fields that can accelerate auroral electrons, as in the observations of Wygant et al [2002]. The simulation results show that parallel electric field structures with scale sizes of a few tens of kilometers can result from this process, as observed by Knudsen et al [2001].…”

“…The conversion of a fraction of this Poynting flux to electron fluxes was observed by Dombeck et al [2005] by comparing Polar and FAST data. Furthermore, Wygant et al [2002] observed the field-aligned acceleration of electrons at Polar in the 4-6 R E altitude range. These fieldaligned electrons had pitch angles within the loss cone, approximately 5°at Polar altitudes.…”

“…[4] In addition to these observations at low altitude, Polar observations have indicated that strong Alfvénic Poynting flux is often observed at the plasma sheet boundary layer [e.g., Wygant et al, 2000Wygant et al, , 2002Keiling et al, 2000Keiling et al, , 2001Keiling et al, , 2002. This Poynting flux, mapped to the ionosphere, was sufficient to provide enough energy for auroral arcs.…”

AKR modulation and global Pi2 oscillation

“…They pointed out that the ability of an inertial Alfvén wave to accelerate electrons is well established theoretically, much less is known observationally. That is to say, the Alfvénic process has not been established observationally, though some case studies suggested auroral particle acceleration by the inertial or kinetic Alfvén wave [e.g., Wygant et al, 2002]. Wygant et al [2002] suggested that small-scale kinetic Alfvén waves are generated from the large-scale Alfvén wave structures.…”

“…That is to say, the Alfvénic process has not been established observationally, though some case studies suggested auroral particle acceleration by the inertial or kinetic Alfvén wave [e.g., Wygant et al, 2002]. Wygant et al [2002] suggested that small-scale kinetic Alfvén waves are generated from the large-scale Alfvén wave structures. The midlatitude and high-latitude Pi2 of the present event exhibited the signature of the oscillating SCW (dB SCW ), which is considered as a manifestation of the Alfvénic perturbation of the field-aligned current (dj k ) along the auroral field line.…”