A generation mechanism for radio waves in the frequency range 150--700 kHz observed by ground facilities is suggested in terms of an electromagnetic electron cyclotron instability driven by auroral electrons. The excited waves can propagate downward along the ambient magnetic field lines and are thus observable with ground facilities. The trapped auroral electrons are supposed to play an important role in the generation process, because they give rise to a thermal anisotropy which consequently leads to the instability. The present work is a natural extension of the theory proposed earlier by Wu et al. [1983> which was discussed in a different context but may be used to explain the observed waves originated at low altitudes (<0.5 RE). This paper presents a possible wave generation mechanism valid in the entire auroral field line region of interest. ¿ American Geophysical Union 1989