Reconnecting Compressed Current Sheets: Impact of Highly Sheared Flows and Resulting Turbulence

We investigate kinetic-scale current sheets with a magnetic field reversal that are formed by compressing a larger scale fluid current sheet down to kinetic scales. The compression forces the current sheet to generate an intense localized ambipolar electric field in the Hall direction. As a result, the ions do not fully respond to the electric field, while the electrons develop an intense sheared ExB flow, which carries a current and alters the structure of the current sheet, giving it multiple spatial scales and enabling bifurcated or embedded current sheets to form depending on the level of compression. The localized electric field also modifies the particle orbits and the equilibrium distribution functions leading to non-gyrotropic distributions by renormalizing the gyromotion. We model this physics via an exact Vlasov equilibrium [1] which captures the essential physics of compression and through asymptotic perturbation theory. This equilibrium is shown, through a non-local linear analysis, to be unstable to lower-hybrid waves due to the electron-ion hybrid (EIH) instability [2]. The analysis shows that the electrostatic EIH develops eigenmodes which are confined to the central portion of the current sheet near the magnetic field reversal point and can lead to substantial anomalous dissipation, both resistivity and viscosity, which can impact reconnection. We model this physics through rigorous eigenmode analysis and through first principles particle-in-cell simulations. With the resulting models and simulations, we are able to make direct comparisons with observations from NASA’s MMS mission [3, 4].