Abstract
Wave measurements in planetary foreshocks and cometary environments show the sporadic occurrence of magnetic spectra with harmonic structure related to ion cyclotron frequencies. Dilute populations of anisotropic and/or drifting charged particles can excite obliquely propagating modes with spacecraft frequencies close to the observed harmonics. We extend previous analyses of this generation mechanism to drifting and nondrifting loss‐cone type distributions of heavy ions in a dense hydrogen magnetoplasma, characterizing the complex (real frequency and growthrate) dispersion, polarization and compressibility of the unstable cyclotron harmonic waves. Solution of the full kinetic dispersion equation shows that it is possible to attain harmonic excitation, both in the drifting and nondrifting regimes. However, the bandwidth inherent to frequency Doppler shifts of obliquely propagating waves might preclude the observation of spectral structure in the spacecraft frame. The Giotto observations in the upstream region of comet Halley provide a reference to discuss the results.
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