Bow shock specularly reflected ions in the presence of low-frequency electromagnetic waves:: a case study

An energetic ion (E less than or equal to40 keV) 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 similar to0.5 R-E from the shock and the averaged bow shock theta(Bno) is about similar to30degrees. 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 alpha(o)similar totheta(Bno), 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 theta(Bn) 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 theta(Bn) angles are less than similar to40degrees, consistent with the specular reflection hypothesis as the production mechanism of ions.