Collisionless relaxation of a disequilibrated current sheet and implications for bifurcated structures*

Current sheets are ubiquitous plasma structures that are important in magnetospheric contexts. Although numerous current sheet equilibrium solutions have been found, the process in which an initially disequilibrated sheet collisionlessly relaxes or equilibrates remains unknown. It is first shown that particle orbits in a sheared magnetic field profile can be classified into four orbit classes. Phase-space distributions of the classes are then analyzed and compared to particle-in-cell simulations. It is found that a current sheet equilibrates through collisionless transitions among the orbit classes. Bifurcated current sheets, which are frequently observed in geospace but whose origins have been elusive, naturally arise from this process. It is thus suggested that current sheet equilibration is responsible for such bifurcated structures; comparisons of particle-in-cell simulations to spacecraft observations support this fact.