On the origin of field-aligned beams at the quasi-perpendicular bow shock: multi-spacecraft observations by Cluster

摘要

Two distinct populations of reflected and accelerated ions are known tooriginate from quasi-perpendicular shocks, gyrating ions and reflected ionbeams. Recent observations under such bow shock conditions with Cluster haveshown strong evidence that both particle distributions appear to emerge fromthe same reflection process. In this paper the basic production mechanism offield-aligned beams has been investigated by using CLUSTER multi-spacecraftmeasurements. We have analyzed several quasi-perpendicular shocks with theCluster Ion Spectrometry experiment (CIS) and followed the spatial andtemporal evolution of the reflected and transmitted ion populations acrossthe shock. These observations show that the field-aligned beams most likelyresult from effective scattering in pitch angle during reflection in theshock ramp. Investigating a low Mach number shock, leakage of a fraction ofthe thermalized ion distribution in the downstream region does not appear tobe the source as the volume in phase space occupied by beam ions is emptydownstream of the shock ramp.