ON THE GENERATION OF AURORAL RADIO EMISSIONS AT HARMONICS OF THE LOWER IONOSPHERIC ELECTRON-CYCLOTRON FREQUENCY - X-MODE, O-MODE AND Z-MODEMASER CALCULATIONS

Electron cyclotron maser emission has been suggested as a possible exp lanation of ground-based observations of auroral radio waves at twice and three times the lower ionospheric electron cyclotron frequency (2f (ce) and 3f(ce)). Preliminary theoretical work concluded that direct m aser excitation at the first and second harmonics of the free space ex traordinary mode (X2 and X3) could exceed collisional ionospheric damp ing rates, although growth rates were low. Recently, however, ground-b ased observations indicate that these emissions are predominantly O mo de polarized. Therefore a reassessment of the lower ionospheric. cyclo tron maser as an emission mechanism is necessary. In this paper, kinet ic cyclotron maser instability calculations are extended to include th e trapped Z mode for varying thermal energies and auroral potential dr ops within the context of a realistic ionospheric electron distributio n model. It is found that when the local upper hybrid frequency is in the close vicinity of 2f(ce) or 3f(ce), harmonic Z mode (Z2 and Z3) gr owth rate is shown to be exceedingly high. This finding suggests that an indirect mechanism that involves linear or nonlinear mode conversio n of the excited harmonic Z mode into the ordinary (O) mode is a possi ble emission mechanism.