Direct Observation of Acceleration and Thermalization of Beam Electrons Caused by Double Layers in the Earth's Plasma Sheet

Abstract

We report direct observations of the detailed processes for the acceleration and thermalization of beam electrons in the plasma sheet (PS) of the Earth's magnetotail. With observations from the four magnetospheric multiscale (MMS) satellites, the potential of a double layer (DL) structure is found to be mostly consistent with enhancements of the averaged parallel energy of the beam electrons passing through the DL structure, demonstrating that beam electrons are accelerated by the DL structure. Thereafter, Debye‐scale parallel electric field turbulences and an increase of the parallel temperature are simultaneously observed, implying that electrostatic parallel turbulences thermalize beam electrons. These observations imply that the DL is a key process controlling the acceleration and thermalization of beam electrons in the PS of the Earth's magnetotail.

Double Layers in the Earth's Bow Shock

“…Our interferometry analysis showed that double layers in the Earth's bow shock have plasma frame speed in the range of 0.5-2c IA , where we used a rough estimation of the local ion-acoustic speed, 𝐴𝐴 𝐴𝐴IA = (𝑇𝑇𝑒𝑒‖∕𝑚𝑚𝑝𝑝) 1∕2 , because its exact value depends on non-Maxwellian features of electron and ion distribution functions (e.g., Vasko et al (2017)). These double layers are of ion-acoustic type and, thus, identical with double layers in the auroral region (Andersson et al, 2002;Ergun et al, 2001) and plasma sheet (Yuan et al, 2022). Similarly to previous observations, the double layers are associated with high-frequency electric field fluctuations (Figures 1 and 4), though that is not a universal feature (Figure 4a).…”

“…Double layers presented in this letter are electrostatic and they fundamentally differ from electric field spikes reported in the foot of a quasi‐perpendicular Earth's bow shock (Chen et al., 2018); those spikes were nonlinear whistler mode structures (Balikhin et al., 1997; Walker et al., 1999). Similar electrostatic double layers were previously reported in the auroral region (Andersson et al., 2002; Ergun et al., 2001), plasma sheet (Ergun et al., 2009; Yuan et al., 2022), inner magnetosphere (Deng et al., 2010; Malaspina et al., 2014), reconnection current sheet (Wang et al., 2014; Øieroset et al., 2021), and Venusian bow shock (Malaspina et al., 2020). In rare cases the double layer propagation velocity was revealed by interferometry (Andersson et al., 2002; Ergun et al., 2001; Yuan et al., 2022), while typically it was assumed to coincide with the local ion‐acoustic speed.…”

“…Similar electrostatic double layers were previously reported in the auroral region (Andersson et al., 2002; Ergun et al., 2001), plasma sheet (Ergun et al., 2009; Yuan et al., 2022), inner magnetosphere (Deng et al., 2010; Malaspina et al., 2014), reconnection current sheet (Wang et al., 2014; Øieroset et al., 2021), and Venusian bow shock (Malaspina et al., 2020). In rare cases the double layer propagation velocity was revealed by interferometry (Andersson et al., 2002; Ergun et al., 2001; Yuan et al., 2022), while typically it was assumed to coincide with the local ion‐acoustic speed. Our interferometry analysis showed that double layers in the Earth's bow shock have plasma frame speed in the range of 0.5–2 c IA , where we used a rough estimation of the local ion‐acoustic speed, $${c}_{\text{IA}}={\left({T}_{e\Vert}/{m}_{p}\right)}^{1/2}$$, because its exact value depends on non‐Maxwellian features of electron and ion distribution functions (e.g., Vasko et al.…”

“…• The first identification of ion-acoustic double layers and electron temperature variation across them in the Earth's bow shock • Double layers have typical spatial width around 10 Debye lengths or one tenth of electron inertial length • Typical potential drop across a double layer is 2%-7% of the cross-shock potential in the de Hoffmann-Teller frame et al, 1982), while later studies showed they had plasma frame speeds around local ion-acoustic speed and spatial widths of about 10 Debye lengths (Andersson et al, 2002;Ergun et al, 2001). Similar ion-acoustic double layers were observed around plasma injection fronts in the inner magnetosphere (Deng et al, 2010;Malaspina et al, 2014), fast plasma flows in the plasma sheet (Ergun et al, 2009;Yuan et al, 2022), and reconnection current sheets in the magnetosphere (Wang et al, 2014;Øieroset et al, 2021). Parker Solar Probe measurements have recently revealed double layers, presumably of ion-acoustic type, in the Venusian bow shock (Malaspina et al, 2020).…”

“…Parker Solar Probe measurements have recently revealed double layers, presumably of ion‐acoustic type, in the Venusian bow shock (Malaspina et al., 2020). The spacecraft measurements showed that ion‐acoustic double layers can indeed accelerate or heat electrons (Ergun et al., 2009; Yuan et al., 2022). The presence of electrostatic double layers in the Earth's bow shock was pointed out in the past (Hobara et al., 2008; Goodrich et al., 2018), but no in‐depth analysis of their properties and the associated electron heating was carried out.…”