On the generation of auroral radio emissions at harmonics of the lower ionospheric electron cyclotron frequency: X, O and Z mode maser calculations

Abstract

Electron cyclotron maser emission has been suggested as a possible explanation of ground‐based observations of auroral radio waves at twice and three times the lower ionospheric electron cyclotron frequency (2ƒce and 3ƒce). Preliminary theoretical work concluded that direct maser excitation at the first and second harmonics of the free space extraordinary mode (X2 and X3) could exceed collisional ionospheric damping rates, although growth rates were low. Recently, however, ground‐based observations indicate that these emissions are predominantly O mode polarized. Therefore a reassessment of the lower ionospheric cyclotron maser as an emission mechanism is necessary. In this paper, kinetic cyclotron maser instability calculations are extended to include the trapped Z mode for varying thermal energies and auroral potential drops within the context of a realistic ionospheric electron distribution model. It is found that when the local upper hybrid frequency is in the close vicinity of 2ƒce or 3ƒce, harmonic Z mode (Z2 and Z3) growth rate is shown to be exceedingly high. This finding suggests that an indirect mechanism that involves linear or nonlinear mode conversion of the excited harmonic Z mode into the ordinary (O) mode is a possible emission mechanism.