Wave excitation downstream of the low‐β, quasi‐perpendicular bow shock

Abstract

We interpret the low‐frequency wave observations of the AMPTE/IRM emission in the downstream region of a marginally supercritical, quasi‐perpendicular bow shock. A companion paper on ion heating comprehensively describes the corresponding plasma and wave magnetic field measurements. A simple model of the shocked plasma consisting of a background hydrogen magnetoplasma with anisotropic core protons permeated by a hotter dilute beam of anisotropic protons reproduces the basic features of the wave polarization and frequency spectra: wave activity below the proton cyclotron frequency, sometimes displaying a double‐humped structure, with unusual (in space plasmas outside the magnetosphere) dominance of (plasma frame) left‐hand‐polarized modes. The core and beam anisotropies stimulate electromagnetic (left‐hand) ion cyclotron waves in distinct spectral regions, whereas the large downstream Alfvén speed (with respect to the beam drift speed) precludes an efficient cyclotron resonant generation of the right‐hand magnetosonic modes.