Beam‐driven ion cyclotron harmonic resonances in the terrestrial foreshock

We report an observation of low‐frequency terrestrial upstream waves which exhibit enhancements in the magnetic field power and polarization spectra at harmonics of the proton cyclotron resonance. These magnetic fluctuations exist concurrent with a “diffuse” suprathermal proton population, but a field rotation immediately following the wave observation reveals a “reflected” suprathermal distribution. We also compute instability growth rates due to this highly anisotropic “reflected” component and find relative maxima at frequencies corresponding to the enhancements in the observed spectra. We thus infer that the multiple spectral peaks are due to resonant, electromagnetic ion beam instabilities and conclude that the observed ion beam is the source of the observed upstream waves. Spectral analysis of the polarization, magnetic helicity, and minimum variance directions supports this interpretation. We find observational and theoretical evidence for both right‐hand polarized waves propagating parallel to the mean magnetic field and left‐hand polarized waves propagating oblique to the mean field.