Parallel Electron Heating by Tangential Discontinuity in the Turbulent Magnetosheath

Energetic electrons exist widely in the turbulent magnetosheath, but how they are generated remains unclear. Here we report a new structure, at which electrons are efficiently accelerated in the direction parallel to the magnetic field. Such a structure, formed at the edge of a high-speed jet (HSJ), is a tangential discontinuity (TD) in the MHD regime, but exhibits impulsive fine structures in the kinetic-scale regime. The pulsation of the TD, caused by time-varying size of the HSJ, leads to the energization process: when the transverse section of the HSJ increases, a magnetic mirror is formed and subsequently electrons are trapped and accelerated via the Fermi mechanism; when the transverse section of the HSJ decreases, the magnetic mirror disappears and subsequently electrons escape. Such parallel electron heating can lead to three times of parallel-temperature increase; it can shed light on the study of electron heating in the solar wind, where TDs exist extensively.