Dependence of cyclotron maser instability growth rates on electron velocity distributions and perturbation by solitary waves

[1] We calculate growth rates and corresponding gains for right-hand circularly polarized extraordinary (RX) and left-hand ordinary circularly polarized (LO) mode radiation associated with the cyclotron maser instability (CMI) 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 90 degrees for significant CMI growth. The growth rate is sharply peaked for perpendicular radiation (k(parallel to) = 0), with a full width at half maximum 1.7 degrees, in good agreement with observed k-vector orientations and numerical simulations. The fractional bandwidth varied between 10(-4) and 10(-2), depending most strongly on propagation direction. This range encompasses nearly all observed fractional auroral kilometric radiation (AKR) burst bandwidths. We find excellent agreement between the computed RX mode emergent intensities and observed AKR intensities assuming convective growth length L-c approximate to 20 - 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 strong electromagnetic ion cyclotron waves.