Cluster-CIS observations of bow shock-reflected ions

We present observations of E <= 40 keV/e ions measured by the Cluster Ion Spectrometry experiment at ~ 0.5 R_E upstream of the bow shock. The ions were simultaneously observed by three of the spacecraft. The proton and helium ion distributions are analyzed at 4-second time resolution. These ion distributions are observed in association with low-frequency waves having substantial amplitudes (Δ B/B ~ 1). Both proton and helium intensities are modulated in correlation with the angle θ _Bn, between the instantaneous magnetic field direction and the local shock normal. Because of the presence high amplitude magnetic fluctuations, the shock switches periodically from the quasi-parallel (θ _Bn < 45^o) to the quasi-perpendicular (θ _Bn > 45^o) regime. The ion intensity is higher by one order of magnitude for ions observed at the quasi-parallel shock, compared to those observed at the quasi-perpendicular shock. However, the peak of the energy spectrum is located at a higher energy in the quasi-perpendicular shock regime. Moreover, the the angular distributions indicate that the ions are gyrophase-bunched with one or two gyrophase domains, depending upon the instantaneous value of θ _Bn. Using CIS data from three of the spacecraft, we investigate the possibility that the ion distributions are produced by a specular reflection at the shock. We also examine how the presence of the low-frequency waves affects the observed properties of the ion distributions.