Three‐dimensional observations of gyrating ion distributions far upstream from the Earth's bow shock and their association with low‐frequency waves

Abstract

This report discusses the nature of gyrating ion distributions observed on board the Wind spacecraft by the three‐dimensional ion electrostatic analyzer with high geometrical factor (3DP PESA‐High). The gyrating ion distributions are observed near the inner ion beam foreshock boundary at distances between ∼9 and ∼83 RE. Our upstream measurements confirm several features previously reported using two‐dimensional measurements. These distributions are observed in association with low‐frequency waves with substantial amplitude (|δB|/B > 0.2). The analysis of the waves shows that they propagate in the right‐hand mode roughly along the background magnetic field. The ions are bunched in gyrophase angle when the associated waves are quasi‐monochromatic and high in amplitude. The peak of the ion distribution function rotates in the gyrophase plane. If the wave train is nonmonochromatic, the particle phase angle distribution is extended over a larger range, suggesting the occurrence of a phase mixing effect or a source at the shock. The phase angle distribution also seems to be energy‐dependent, and no gyrophase rotation is observed in this case. Furthermore, we have characterized the ion distributions by computing their densities as well as parallel and perpendicular velocities. The results clearly indicate that the waves are cyclotron‐resonant with the field‐aligned beams observed just upstream. The resonance condition strongly suggests the local production of these gyrating ions in a field‐aligned‐beam disruption. Such a resonant wave‐particle interaction may be a dominant characteristic of the back‐streaming ion population in the foreshock at large distances from the Earth's bow shock.