ION-CYCLOTRON WAVE EMISSION AT THE QUASI-PERPENDICULAR BOW SHOCK

The power spectra of magnetic fluctuations occurring close to the ramp of the quasi perpendicular, low-beta bow shock indicate the presence of obliquely propagating electromagnetic waves with frequencies above the ion cyclotron frequency, OMEGA(i). These waves appear to be associ ated with ion distributions consisting of a bi-Maxwellian core and an energetic, approximately gyrotropic ring. We investigate the generatio n of ion cyclotron waves by distributions of this type, using particle and wave data from the AMPTE/IRM spacecraft. In the case of a monoene rgetic ring, instability is possible over a broad range of frequencies omega > OMEGA(i), with the highest growth rates occurring at propagat ion angles of typically 50-degrees - 80-degrees. As the velocity sprea d of the ring vr increases, the growth rate of perpendicular-propagati ng waves falls, complete stabilization occurring when v(r) is greater than about 20% of the mean ring speed u. The parallel-propagating Alfv en ion cyclotron mode can be excited if the core is anisotropic, with T(perpendicular-to) congruent-to 3T(parallel-to). The maximum growth r ate is obtained when v(r) is comparable to the core ion parallel therm al speed. However, if v(r) much less than u, the growth rate is much s maller than OMEGA(i). Using these results, we show that certain qualit ative features of the AMPTE/IRM wave data can be understood in terms o f a nearly monoenergetic ion ring bearn at the shock ramp, evolving in to an extended ring beam, and then merging with a quasi bi-Maxwellian ion core as it moves downstream.