J. Geophys. Res., 90, 9675 - 9687, 1985.
(Received August 24, 1984;
revised June 4, 1985;
accepted June 5, 1985.)
Copyright 1985 by the American Geophysical Union.
Paper number 4A8293.
/
mc)
is greater than the ratio of the plasma
frequency to the gyrofrequency
(
/
)
wave propagation can be markedly different than the
classical cold plasma limit. We investigate the wave
dispersion in a weakly relativistic electron plasma,
specifically in the context of the generation of
auroral kilometric radiation (AKR). To do this, we have
used a simple electron distribution consisting of a
ring of energetic electrons at constant perpendicular
momentum together with a stationary cold component. It
is found that for such an electron distribution, where
the two electron components have different
gyrofrequencies due to relativistic effects, a new wave
mode is introduced. This wave is trapped between the
two electron gyrofrequencies. We find that the group
velocity for such a wave is small; convective growth
lengths for the wave are calculated to be of the order
of 1 km. Consequently, ray propagation in a simple
model for the density cavity present on auroral zone
field lines is studied. We find that the ray paths tend
to bring waves to altitudes where the growth rate
maximizes; at these altitudes the waves propagate
primarily across the ambient magnetic field. This could
result in substantial amplification of the wave, since
damping is minimized, and the waves may undergo
multiple transitions across the auroral arc. However,
the wave is trapped and must undergo mode conversion to
escape from the auroral arc. Since the wave
polarization is found to be very similar to the R-X
mode polarization, we deduce that coupling to the R-X
mode at the edge of the auroral arc may be reasonably
efficient.
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