Abstract
Abstract. An energetic ion (E≤40) event observed by the CLUSTER/CIS experiment upstream of the Earth's bow shock is studied in detail. The ion event is observed in association with quasi-monochromatic ULF MHD-like waves, which we show modulate the ion fluxes. According to three statistical bow shock position models, the Cluster spacecrafts are located at ~0.5 Re from the shock and the averaged bow shock θBn0 is about ~30°. The analysis of the three-dimensional angular distribution indicates that ions propagating roughly along the magnetic field direction are observed at the onset of the event. Later on, the angular distribution is gyrophase-bunched and the pitch-angle distribution is peaked at α0~θBn0, consistent with the specular reflection production mechanism. The analysis of the waves shows that they are left-handed in the spacecraft frame of reference (right-handed in the solar wind frame) and propagate roughly along the ambient magnetic field; we have found that they are in cyclotron-resonance with the field-aligned beam observed just upstream. Using properties of the waves and particles, we explain the observed particle flux-modulation in the context of θBn changes at the shock caused by the convected ULF waves. We have found that the high count rates coincide with particles leaving the shock when θBn angles are less than ~40°, consistent with the specular reflection hypothesis as the production mechanism of ions.
Highlights
Several types of ion populations have been observed to emerge from the Earth’s bow shock into the upstream regions
We have examined the particle count rate modulation observed upstream of the bow shock by the Cluster-CIS experiment
The parallel and the perpendicular velocity components are consistent with the specular reflection hypothesis, placing the production source at the shock
Summary
Several types of ion populations have been observed to emerge from the Earth’s bow shock into the upstream regions. Gosling et al (1982) reported the first evidence of specular reflection of a portion of the solar wind using the ISEE-FPE experiment, at a time when ISEE was just upstream of the shock Another event consistent with specular reflection observed at ∼4 RE, and in association with ULF waves, was studied by Thomsen et al (1985). In the presence of large amplitude ULF waves near the shock, which are always observed in association with gyrating ions, the local bow shock θBn varies substantially. 6 we demonstrate that the intermittently observed gyrating particle distributions are consistent with changing local shock geometries modulated by the large-amplitude waves.
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