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

This report discusses the nature of gyrating ion distributions observed on board the Wind spacecraft by the three-dimensional ion electrostatic analyz er with high geometrical factor (3DP PESA-High). The gyrating ion distribut ions are observed near the inner ion beam foreshock boundary at distances b etween similar to9 and similar to 83 R-E Our upstream measurements confirm several features previously reported using two-dimensional measurements. Th ese distributions are observed in association with low-frequency waves with substantial amplitude (\ deltaB \ /B > 0.2). The analysis of the waves sho ws 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 d istribution function rotates in the gyrophase plane. If the wave train is n onmonochromatic, the particle phase angle distribution is extended over a l arger range, suggesting the occurrence of a phase mixing effect or a source at the shock. The phase angle distribution also seems to be energy-depende nt, and no,gyrophase rotation is observed in this case. Furthermore, we hav e 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 jus t 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-stre aming ion population in the foreshock at large distances from the Earth's b ow shock.