Ion cyclotron wave emission at the quasi‐perpendicular bow shock

Abstract

The power spectra of magnetic fluctuations occurring close to the ramp of the quasi‐perpendicular, low‐β bow shock indicate the presence of obliquely propagating electromagnetic waves with frequencies above the ion cyclotron frequency, Ωi. These waves appear to be associated with ion distributions consisting of a bi‐Maxwellian core and an energetic, approximately gyrotropic ring. We investigate the generation of ion cyclotron waves by distributions of this type, using particle and wave data from the AMPTE/IRM spacecraft. In the case of a monoenergetic ring, instability is possible over a broad range of frequencies ω > Ωi, with the highest growth rates occurring at propagation angles of typically 50° ‐ 80°. As the velocity spread of the ring υr increases, the growth rate of perpendicular‐propagating waves falls, complete stabilization occurring when υr is greater than about 20% of the mean ring speed u. The parallel‐propagating Alfvén ion cyclotron mode can be excited if the core is anisotropic, with T⊥ ≃ 3T∥. The maximum growth rate is obtained when υr is comparable to the core ion parallel thermal speed. However, if υr « u, the growth rate is much smaller than Ωi. Using these results, we show that certain qualitative features of the AMPTE/IRM wave data can be understood in terms of a nearly monoenergetic ion ring beam at the shock ramp, evolving into an extended ring beam, and then merging with a quasi‐bi‐Maxwellian ion core as it moves downstream.