Princeton Center for Heliophysics Seminar

Electron-only reconnection is a variant of magnetic reconnection without significant ion involvement. Such reconnection has recently been observed in the terrestrial magnetosheath and has received considerable attention due to its likely role in the energy cascade of turbulent magnetized plasmas. Different electron energization mechanisms usually favor electron acceleration along different directions and occur in different regions of reconnection. Therefore, spatially resolved electron velocity distribution function (EVDF) measurements along specific directions are required to elucidate the underlying electron energization mechanisms during electron-only reconnection. 

In the PHAse Space MApping (PHASMA) facility, a first-of-its-kind multi-dimensional incoherent Thomson scattering system enables 3D EVDF measurements by employing two injection paths and two collection paths. In this seminar, I will present 3D local electron heating measurement around the separatrix during reconnection. The measured heating corresponds to 70% of the incoming Poynting flux [Shi et al., Phys. Rev. Lett. 128, 025002 (2022)]. The electron temperature is clearly anisotropic, with the ratio between parallel and perpendicular electron temperature Te|| / Te⟂ ~ 1.5. This preferential parallel electron heating reveals the important role of the reconnecting electric field in electron energization for strong guide field reconnection. In addition, non-Maxwellian EVDFs, composed of a warm bulk population and a cold beam, were measured. Oppositely directed electron beams that appear on either side of the X-point are clear evidence of electron acceleration in magnetic reconnection. Interestingly, parallel EVDFs with either a flat top or beam component are also observed around two separatrices, reflecting the removal or enhancement of some electron portion of the electron velocity phase space. This agrees well with the field-particle correlation signatures reported in gyrokinetic numerical simulations [McCubbin et al., Phys. Plasmas 29, 052105 (2022)].