Simulation of Asymmetric Magnetic Reconnection in the Magnetosphere Starting from Fully Kinetic Current-Sheet Equilibria

The well-known Harris equilibrium has long served as a starting point for kinetic simulations of symmetric magnetic reconnection, such as in Earth's magnetotail. The Harris equilibrium, however cannot be easily generalized to model asymmetric cases where the magnetic field and plasma density differ on the two side of the current sheet, as is the case for reconnection at the magnetopause. A generalization of the Harris equilibrium suggested by Yamada et al. [Phys. Plasmas, V.7, 1781 (2000)], which introduced electric fields normal to the current sheet, serves as the starting point for the new family of asymmetric current-sheet equilibria described here. The key new feature is to associate the self-consistent electrostatic fields normal to the current sheet with 'potential barriers' that can segregate differing nonthermal populations of electrons and/or ions on opposite sides of the current sheet - with similarities to the features of nonlinear electrostatic structures such as double layers. Equilibria from this family have been used to initialize implicit PIC simulations - both with and without local perturbations to seed reconnection onset. In the absence of such perturbations, the plasma evolves as an equilibrium subject to a slow tearing-like instability that has the expected dependence on initial sheet thickness. Reconnection simulations seeded by an initial perturbation of the current sheet have been performed for varying guide-field strengths. For example, simulations with a moderate guide field reveal strong asymmetries in the evolved density, parallel electron flow, and the bipolar parallel electric-field signatures of electron holes (see accompanying figure). Snapshots of (a) ion density, (b) x-component of the electron flow velocity, and (c) parallel electric field in a small region near the nominal x-point from an implicit PIC simulation of asymmetric reconnection. The initial guide field was approximately half of the average of the asymptotic |B| on the two sides of the current sheet.