A lower ionospheric electron cyclotron maser has recently been suggested as a possible source of natural radio emissions that have been observed with ground-based instruments in the auroral zone. According to this theory, the observed emissions are interpreted as X mode electromagnetic waves at 2 and/or 3 times the ionospheric electron cyclotron frequency (fce). In the present study we carry out a more thorough analysis of the cyclotron maser instability, using a more complete growth rate expression and more realistic energetic electron distribution functions. Growth rates of various electromagnetic modes and harmonics are calculated. It is shown that growth rates for 2 fce and 3fce X modes can exceed collisional ionospheric damping rates, confirming the original estimations. However, the present analysis also reveals that O mode waves at the fundamental cyclotron frequency, fce, should also be excited at the source. The propagation of excited electromagnetic waves from the source region to the ground inside a model horizontal density cavity structure is also examined by means of ray tracing technique. It is found that the O mode wave is unable to reach the ground, an explanation of why the emissions at fce are not detected on the ground. ¿ American Geophysical Union 1996