The coherent three-wave decay of a linearly unstable electrostatic hydrogen cyclotron (EHC) wave into stable EHC and ion acoustic modes is considered. Possible triads of parent and daughter waves are identified, and the coupling strength between the waves is calculated in two plasma models. The first model is the fluid limit of an electron-hydrogen plasma, and the second is a three-species kinetic model of the plasma in the evening auroral electron acceleration region. It is found that EHC waves commonly observed on auroral field lines are of sufficient amplitude to exceed the threshold for decay to weakly damped EHC modes. Further, frequency and wave number matching can be satisified only if the acoustic mode is oblique to the geomagnetic field. This decay is proposed to be a saturation mechanism for the linearly unstable coherent EHC mode. The temporal dependence of the amplitudes of the three inter-acting modes, as predicted by the four coupled wave kinetic equations, indicates that the coherent decay can act to saturate the parent wave. Further, it indicates that observed EHC waves may actually be linearly marginally stable daughter EHC modes. ¿American Geophysical Union 1987