Phase-space distribution and relaxation of fundamental plasma structures at kinetic scales

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Although equilibrium solutions of fundamental plasma structures such as current sheets and flux ropes have been extensively studied, they form in general from initially non-equilibrium states. Here, we summarize how non-equilibrium structures relax to an equilibrium in a collisionless manner at kinetic scales. Particle orbits in the underlying electromagnetic fields can be classified into distinct classes, which in turn leave corresponding footprints in phase-space. A linear Vlasov analysis shows that the structures respond in such a way that particle orbit class transitions are induced that pinch and heat the current sheet. These transitions are directly confirmed by tracking real-time particle trajectories in kinetic simulations of pinching non-equilibrium structures. The resultant kinetic equilibria are much more likely to be the underlying structures in various plasma phenomena, instead of idealized, Maxwellian equilibria such as the Harris sheet. Further implications are discussed.