Distinct types of electron velocity distributions in magnetotail reconnection exhausts

Abstract:

Electron distributions in the exhaust region of magnetotail reconnection with negligible guide field have been reported to be hot and isotropic in previous studies. Here we examined the electron distributions in 32 exhaust regions encountered by the Cluster spacecraft in the magnetotail and document various types of anisotropy. Field-aligned beams are observed in nearly all events. The dominant exhaust distribution is hot and isotropic in 17 events, cold and isotropic in seven events, and hybrid in eight events. The hybrid distribution consists of field-aligned lower energy electrons and isotropic higher energy electrons. Counter-streaming beams are dominant in four of these hybrid events. High energy (>2keV) populations with T_e_perp > T_e_para in addition to counter-streaming beams are frequently observed when the spacecraft are close to the neutral plane (Bx ~ 0), indicating perpendicular heating near the magnetic pile-up region. In exhaust regions with hot and isotropic electron distributions, the peak ion outflow speed is significantly larger and density lower than that in the exhaust with cold and isotropic electrons. We suggest that the cold isotropic exhaust distributions are from the early stages of reconnection, the hot isotropic exhaust distributions are from the well-developed stage, and the hybrid exhaust distributions may indicate a different type of reconnection with the upstream source in the plasma sheet rather than the lobe.