Parallel Electron Heating by Tangential Discontinuity in the Turbulent Magnetosheath

Abstract

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.

Characteristics of Interplanetary Discontinuities in the Inner Heliosphere Revealed by Parker Solar Probe

“…The rotational and tangential discontinuities (RDs & TDs) are the most common types observed in the solar wind (Smith 1973;Mariani et al 1983;Paschmann et al 2013). TDs can be treated as stationary boundaries in the plasma rest frame separating distinct plasmas (Burlaga & Ness 1969;Liu et al 2019;Fu et al 2012). RDs, however, are propagating kinks in the magnetic and flow fields (Smith 1973).…”

Particle energization in space plasmas: towards a multi-point, multi-scale plasma observatory

“…In near-Earth space the most important examples are jets generated in the magnetopause and magnetotail current sheets as well as jets downstream of the shock. As jets propagate and interact with the ambient plasma, particles are heated and accelerated at the jet boundary, often referred to as plasma jet front [8,44,[132][133][134][135]. Jets eventually stop in the jet braking regions upon interaction with obstacles, such as the Earth's magnetic dipolar field [136,137], where particles can be efficiently accelerated [138,139] and later injected into the inner magnetosphere [140].…”

“…As jets propagate in the magnetosheath, they interact with and modify the ambient plasma [155,156]. This interaction creates sites where particles can be efficiently energized, such as magnetic bottles where Fermi acceleration operates [135] and kinetic-scale current sheets [157] and shocks [158]. All these sites have a nonlinear and nonstationary structure whose spatial and temporal evolution cannot be resolved with current four-point measurements even at one given scale.…”

First Topology of Electron‐Scale Magnetic Hole

“…Later, the electron‐scale magnetic holes (ESMHs), whose sizes are less than or of the order of ion gyroradius or ion inertial length, are reported in both observations (Ge et al, 2011; Gershman et al, 2016; Goodrich et al, 2016; Huang, Du, et al, 2017; Huang, Sahraoui, et al, 2017; Huang et al, 2018, 2019; Y. Y. Liu, Fu, Liu, et al, 2019; H. Liu, Zong, Zhang, et al, 2019; Shustov et al, 2019; Yao et al, 2016, 2017) and particle‐in‐cell (PIC) simulations (Haynes et al, 2015; Roytershteyn et al, 2015). The PIC simulation shows that ESMHs can form in decaying turbulent plasmas, and their formation is due to the vortices of trapped electrons with petal‐like orbits (Haynes et al, 2015).…”

“…Magnetic holes (MHs), sometimes also referred to as magnetic depressions (MDs) or magnetic cavities, are characterized by the depressions of magnetic field magnitude. Such structures are first reported by Turner et al (1977) in the solar wind and then widely observed in the interplanetary space (Xiao et al, 2014, 2010; Zhang et al, 2008, 2009), the magnetosphere of comet (Russell et al, 1987), and the terrestrial magnetosheath/plasma sheet (Balikhin et al, 2012; Fu, Zhao et al, 2020; Joy et al, 2006; Y. Y. Liu, Fu, Liu, et al, 2019; Lucek et al, 1999; Nowada et al, 2009; Shi et al, 2009; Soucek et al, 2008; Sun et al, 2012; Tsurutani et al, 1982; Zhima et al, 2015). The spatial scales of MHs range from several to thousands of proton gyroradii ρ i (Tsurutani et al, 2011).…”