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

*J. Geophys. Res., 91*, 3152 - 3166, 1986.

(Received August 8, 1985;
revised November 8, 1985;
accepted December 2, 1985.)

Copyright 1986 by the American Geophysical Union.

Paper number 5A8825

The effect of including relativistic corrections in the dispersion
relation for a low-density plasma consisting of moderately energetic electrons
(energy ~ 2.5 keV) is investigated. Such a low-density plasma is presumed to
exist at those altitudes on auroral zone field lines where auroral kilometric
radiation is generated. Two different types of dispersion relation are employed
for the purpose of studying the wave dispersion. The simpler of these is a "ring"
distribution, in which both ambient and hot electrons are assumed to be cold
(i.e., have no thermal spread associated with them). Using this dispersion
relation, we find that for sufficiently low densities, the most unstable mode is
a "trapped" mode that is decoupled from the freely propagating *R-X* mode found in
a cold plasma. Since the dispersion relation neglects the effects of temperature,
we also study wave dispersion using a spherically symmetric Dory-Guest-Harris
(DGH) distribution. This "shell" distribution shows that provided the peak
momentum of the DGH distribution is larger than the thermal spread, the most
unstable mode at low wave vectors is still the trapped mode. Our analysis
indicates that the trapped mode is usually the most unstable mode and the
freely propagating *R-X* mode is not driven unstable, when the wave dispersion
is assumed to be given by a weakly relativistic dispersion relation. We show
that relativistic effects may be important for typical auroral electron
distributions, and that there is insufficient evidence from particle data to
justify the usual assumption that the cold plasma approximation is adequate when
describing wave dispersion near the gyrofrequency on auroral zone field lines.

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Contents

1. Introduction

2. Ring Distribution

3. Finite Temperature Distribution

4. Extension to Finite Perpendicular Wave Vector

5. Applicability to the Auroral Zone

6. Summary and Conclusions

References

Figure Captions

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