In this paper we consider a fan instability as a possible source of oblique ion acoustic waves generated by ion beams above the auroral acceleration region. Our model includes a homogeneous plasma with protons as the only ion component. The ion distribution has a suprathermal tail along the ambient magnetic field. A beam with a positive derivative and a tail without a positive derivative in the ion distribution are considered. In the first case, the beam of the ion distribution generates electrostatic ion cyclotron waves and oblique ion acoustic waves. In the second case, the ion cyclotron waves are damped, but ion acoustic waves are still generated by the fan instability. The fan instability mechanism generates waves even when the positive derivative in the ion distribution has disappeared. The paper includes an analytical description and numerical calculations of the fan instability. Particle and wave data from the Viking satellite indicate that the fan instability is important above the auroral acceleration region. ¿ American Geophysical Union 1995.