Rapid Pitch Angle Evolution of Suprathermal Electrons Behind Dipolarization Fronts

Abstract

The pitch angle distribution (PAD) of suprathermal electrons can have both spatial and temporal evolution in the magnetotail and theoretically can be an indication of electron energization/cooling processes there. So far, the spatial evolution of PAD has been well studied, leaving the temporal evolution as an open question. To reveal the temporal evolution of electron PAD, spacecraft should monitor the same flux tube for a relatively long period, which is not easy in the dynamic magnetotail. In this study, we present such an observation by Cluster spacecraft in the magnetotail behind a dipolarization front (DF). We find that the PAD of suprathermal electrons can evolve from pancake type to butterfly type during <4 s and then to cigar type during <8 s. During this process, the flow velocity is nearly zero and the plasma entropy is constant, meaning that the evolution is temporal. We interpret such temporal evolution using the betatron cooling process, which is driven by quasi‐adiabatic expansion of flux tubes, and the magnetic mirror effect, which possibly exists behind the DF as well.

Statistical Properties of Sub‐Ion Magnetic Holes in the Dipolarized Magnetotail: Formation, Structure, and Dynamics

“…Note that adiabatic heating is expected to be the dominant energization process for high‐energy electrons that are not significantly affected by nonadiabatic wave‐particle interactions around the front (Fu et al, ; Fu et al, ). Although, for this population, the adiabatic heating model agrees well with spacecraft observations (e.g., Liu, Fu, Xu, Wang, et al, ; Liu, Fu, Xu, Cao, et al, ; Liu, Fu, Cao, Xu, Yu, et al, ), such high‐energy electrons likely cannot be trapped in MHs. Thus, we consider here the thermal electron population (dominating in the electron temperature; heating of this population has been shown to scale with | B z |; see Runov et al, ).…”

Electron Pitch‐Angle Distribution in Earth's Magnetotail: Pancake, Cigar, Isotropy, Butterfly, and Rolling‐Pin

“…The earthward transport of mass and magnetic flux, usually accompanied by intermittent coherent structures, such as bursty bulk flows (BBF) (e.g., Angelopoulos et al, 1992; Cao et al, 2006, , 2013) and dipolarization fronts (e.g., Fu, Khotyaintsev, Vaivads, André, & Huang, 2012a, 2012b; Fu, Cao, et al, 2013; Liu et al, 2013; Liu, Fu, Xu, et al, 2018; Liu, Fu, Vaivads, et al, 2018; Nakamura et al, 2002; Runov et al, 2009; Sergeev et al, 2009; Yao et al, 2017), ultimately provides sufficient energy for the auroral substorms (e.g., Angelopoulos et al, 2008; Liu et al, 2014; Liu, Fu, Xu, et al, 2018; Yao et al, 2012). One typical signature associated with such energy transport is the increase of suprathermal (from tens to hundreds of keV) electron flux in the terrestrial magnetotail (e.g., Asano et al, 2010; Kronberg et al, 2017; Pan et al, 2014; Turner et al, 2016), which results from local acceleration at reconnection region (e.g., Chen, Fu, Zhang, et al, ; Drake et al, 2006; Egedal et al, 2010; Hoshino et al, 2001; Huang, Vaivads, et al, 2012; Fu, Xu, et al, 2019; Øieroset et al, 2002; Vaivads et al, 2011; Fu et al, ) and dipolarization fronts (DFs) (e.g., Ashour‐Abdalla et al, 2011; Birn et al, 2014; Duan et al, 2014; Fu et al, 2011; Fu, Khotyaintsev, et al, 2013; Fu, Grigorenko, et al, 2020; Gabrielse et al, 2016; Huang, Zhou, et al, 2012; Khotyaintsev et al, 2011; Liu, Fu, Xu, Wang, et al, 2017; Liu, Fu, Cao, Xu, et al, 2017; Liu, Liu, Xu, et al, 2018; Liu et al, 2019; Lu et al, 2016; Pan et al, 2012; Runov et al, 2013; Wu et al, 201...…”

Energy Range of Electron Rolling Pin Distribution Behind Dipolarization Front

“…So far, there have been several types of electron PAD reported behind DFs. For instance, the pancake distribution (showing electron pitch angles primarily at 90°) has been observed inside growing FPRs (Fu et al, ; Liu, Fu, Cao, et al, ) or near the neutral sheet (Birn et al, ; Runov et al, ); the cigar distribution (showing electron pitch angles primarily at 0° and 180°) has been observed inside decaying FPRs (Fu et al, ; Liu, Fu, Xu, Cao, & Liu, ) or away from the neutral sheet (Birn et al, ; Runov et al, ); the isotropic distribution (showing electron pitch angles uniformly from 0° to 180°) has been observed inside steady FPRs (Fu, Khotyaintsev, Vaivads, André, Sergeev, et al, ) or associated with strong wave emissions (Deng et al, ; Fu et al, ; Huang et al, ; Hwang et al, ; Yang et al, ); the butterfly distribution (showing electron pitch angles primarily at 45° and 135°) has been observed inside expanding flux tubes behind the DFs (Liu, Fu, Cao, et al, ). These four types of electron PAD have been well discussed in previous literatures.…”