Cluster observations of velocity space-restricted ion distributions near the plasma sheet

We present Cluster ion observations obtained at 18 RE in the magnetotail on 1 October 2001. According to a recent analysis, the quartet encountered a reconnection region and a tailward‐moving neutral line. We examine in detail selected 3‐D ion distributions, which through much of the hour following 0925 UT were non‐gyrotropic. B‐perpendicular slices of velocity space showed crescent‐shaped regions. Occupied gyrophases were consistent over a wide range of parallel velocities, stable over time, and occurred unaccompanied by strong ion gyrofrequency waves. We interpret these observations as signatures of remote sensing near sharp particle gradients. In this view, distributions obtained simultaneously while Cluster straddled the current sheet are simply explained. Additionally, the computed first moments can have large transverse ( × B) components (, a unit boundary normal), without net plasma transport. We infer separate O+ layers above and below the current sheet.