Electron Pitch Angle Distributions in Magnetotail Tailward Flows

Electron pitch angle distributions (PADs) are crucial to revealing electron acceleration and heating in geospace. The electron PADs in the Earth's magnetotail have been widely studied by statistical or case analysis. However, the statistical features of electron PADs in tailward flows are still unclear. We survey data from the Magnetospheric Multiscale (MMS) satellite to statistically investigate electron PADs in tailward flows in the Earth's magnetotail for the first time. We find that for most (66.3%) tailward flows, electrons with energy from 0.1 to 30 keV are mainly field-aligned. The PADs of suprathermal (>2 keV) electrons in a part of tailward flows (26.3%) are mostly isotropic. Additionally, the perpendicular-dominated PADs of suprathermal electrons only exist in a few (7.4%) tailward flows. According to the different dominant PADs of suprathermal electrons, the tailward flows are classified into three types: field-aligned-dominated type, isotropic-dominated type and perpendicular-dominated type flows. The dependence of electron PADs on plasma environmental parameters is investigated in these three types of flows. For field-aligned-dominated type flows, the field-aligned anisotropy decreases toward the central plasma sheet (CPS) and with the increase of the energy. In addition, it is found that the field-aligned anisotropy in the field-aligned-dominated tailward flows reduces noticeably when the |V_ix| exceeds 800 km/s. In isotropic-dominated type flows, the field-aligned anisotropy of electrons with energy ≥10 keV also shows a slight decreasing trend toward the CPS. For perpendicular-dominated type flows, the perpendicular anisotropy exhibits two non-monotonic processes, initially increasing and then decreasing with the increase of distance from the CPS; while becomes more evident for higher energy electrons. Our statistical results indicate that in tailward flows, electron anisotropy depends on the distance from the CPS, energy and plasma flow velocity. Tailward flows are associated with the transport of plasma toward the distant tail, and the statistical analysis of electron PADs in these flows can improve our understanding of the related electron dynamics in the magnetotail.