MMS Observations of a Compressed Current Sheet: Importance of the Ambipolar Electric Field

Development of the ambipolar electric field in a compressed current sheet and the impact on magnetic reconnection

“…It is not clear if this transport can be sustained in a strongly sheared magnetic field geometry formed when a guide field is present. We find that E × B velocity shear due to the perpendicular ambipolar electric field can naturally arise in the centre of current sheets around the magnetic reversal and can drive lower hybrid waves (DuBois et al 2022). These waves, which in the absence of velocity shear reduce to the modified two stream instability for a non-zero wave vector along the magnetic field (Mcbride et al 1972), are consistent with the local plasma conditions at the current sheet centre and require no extraordinary arguments to justify their existence.…”

“…The resulting turbulence affects the current sheet dynamics through anomalous resistivity, viscosity and cross-field diffusion. These inevitable gyro-scale current sheet phenomena have not been comprehensively studied but, based on recent studies (Ganguli et al 2020;DuBois et al 2022), could be quite consequential.…”

“…Current sheets are ubiquitous in space plasmas and are an important factor in how the Sun controls the near-Earth plasma environment (Chen 1993;Yamada, Kulsrud & Ji 2010;Petrukovich et al 2015;Chen, Liu & Hu 2021;Chitta, Priest & Cheng 2021;Wilson et al 2021). For example, during geomagnetically active periods, the solar wind compresses the magnetosphere creating ion-scale current sheets (Takahashi & Hones 1988;Schindler & Hesse 2008Ganguli et al 2020) in the magnetotail that have substructures embedded in the current density (McComas et al 1986;Schindler & Birn 1993;Sergeev et al 1993;Sanny et al 1994;Hoshino et al 1996;Sitnov, Guzdar & Swisdak 2003;Asano et al 2004;Runov et al 2004Runov et al , 2006Petrukovich et al 2011;DuBois et al 2022). These gyro-scale current sheets are dominated by ambipolar effects that lead to unique physics such as multi-scale substructures and sheared E × B flows that have recently been observed (DuBois et al 2022).…”

“…For example, during geomagnetically active periods, the solar wind compresses the magnetosphere creating ion-scale current sheets (Takahashi & Hones 1988;Schindler & Hesse 2008Ganguli et al 2020) in the magnetotail that have substructures embedded in the current density (McComas et al 1986;Schindler & Birn 1993;Sergeev et al 1993;Sanny et al 1994;Hoshino et al 1996;Sitnov, Guzdar & Swisdak 2003;Asano et al 2004;Runov et al 2004Runov et al , 2006Petrukovich et al 2011;DuBois et al 2022). These gyro-scale current sheets are dominated by ambipolar effects that lead to unique physics such as multi-scale substructures and sheared E × B flows that have recently been observed (DuBois et al 2022). Other features that are not captured in the Harris equilibrium (Harris 1962), such as unequal sheared flows in electrons and ions, intense electrostatic lower hybrid emissions in the centre of the current sheet where the density gradient is negligible, non-gyrotropic electron distribution functions, etc., are also observed in gyro-scale current sheets (DuBois et al 2022).…”

“…Other features that are not captured in the Harris equilibrium (Harris 1962), such as unequal sheared flows in electrons and ions, intense electrostatic lower hybrid emissions in the centre of the current sheet where the density gradient is negligible, non-gyrotropic electron distribution functions, etc., are also observed in gyro-scale current sheets (DuBois et al 2022). In this article, we attempt to explain the features observed in a gyro-scale current sheet prior to magnetic reconnection from in situ data using our kinetic current sheet model (Ganguli et al 2020;DuBois et al 2022) in which the ambipolar electric field is self-consistently generated at the gyro-scale by compression. We find that the global compression is the ultimate common physical origin for these seemingly unrelated observational features.…”