Parallel‐dominant and perpendicular‐dominant components of the fast bulk flow: Comparing with the PSBL beams

Abstract

AbstractUtilizing multipoint observations by the Cluster satellites, we investigated the ion distributions of the fast bulk flows (FBFs) in the plasma sheet. Simultaneous observation by C1 and C3 revealed that parallel‐dominant and perpendicular‐dominant components of the flows coexist and correspond to Bx‐dominant and Bz‐dominant magnetic field regions within the FBFs, respectively. In both cases, the ions distributions are characterized by a single‐beam/crescent shape. In particular, no reflected ions are found within the FBFs. Statistical analysis showed that within the FBFs, the strength of the Bx component is typically less than 5 nT for Bz‐dominant regions and above 10 nT for Bx‐dominant regions. To distinguish between the parallel‐dominant component of the FBFs and the field‐aligned beams in the plasma sheet boundary layer (PSBL), we further statistically analyzed the tailward parallel flows (TPF) with positive Bz in the plasma sheet. The results indicated that the FBFs tend to have higher velocity, weaker B, and higher magnetic tilt angle (θMTA) than the TPFs/PSBL beams. Statistically, in the region of B > 30 nT (θMTA > 10°), only PSBL beams can be observed, while in the region of B < 10 nT (θMTA > 30°), the FBFs are dominant. In the intermediate region (10° < θMTA < 30°) of the plasma sheet, the FBFs and the PSBL beams cooccur. These Cluster observations suggest that the X line can produce both perpendicular flow in central plasma sheet and parallel flow in the PSBL. In addition, the parallel‐dominant component of the FBFs could be an important origin for the PSBL beams.