We calculate growth rates and corresponding gains for RX and LO mode
radiation associated with the cyclotron maser instability for parameterized
horseshoe electron velocity distributions. The velocity distribution function
was modeled to closely fit the electron distribution functions observed in the
auroral cavity. We systematically varied the model parameters as well as the
propagation direction to study the dependence of growth rates on model
parameters. The growth rate depends strongly on loss cone opening angle, which
must be less than 90o for significant CMI growth. The growth rate is
sharply peaked for perpendicular radiation (kβ₯β=0), with a
full-width at half-maximum 1.7o, in good agreement with observed k-vector
orientations and numerical simulations. The fractional bandwidth varied between
10β4 and 10β2, depending most strongly on propagation direction. This
range encompasses nearly all observed fractional AKR burst bandwidths. We find
excellent agreement between the computed RX mode emergent intensities and
observed AKR intensities assuming convective growth length Lcββ20-40 km
and group speed 0.15c. The only computed LO mode growth rates compatible
observed LO mode radiation levels occurred for number densities more than 100
times the average energetic electron densities measured in auroral cavities.
This implies that LO mode radiation is not produced directly by the CMI
mechanism but more likely results from mode conversion of RX mode radiation. We
find that perturbation of the model velocity distribution by large ion solitary
waves (ion holes) can enhance the growth rate by a factor of 2-4. This will
result in a gain enhancement more than 40 dB depending on the convective growth
length within the structure. Similar enhancements may be caused by EMIC waves.Comment: 21 pages, 11 figures. J. Geophys. Res. 2007 (accepted