[ Prof. Gunsu Yun]Collisionless relaxation of a disequilibrated curre…

Caption: (a) Theoretically derived particle orbit classes and their phase-space distributions. (b) Change in the phase-space distribution during the relaxation process of a current sheet, calculated with a particle-in-cell simulation. (c) A bifurcated current sheet detected by the NASA MMS spacecraft in the Earth’s magnetotail (left), and that from the particle-in-cell simulation (right). 

[ Prof.  Gunsu Yun]Collisionless relaxation of a disequilibrated current sheet and implications for bifurcated structures 

Current sheets are ubiquitous plasma structures that play the crucial role of being energy sources for various magnetic phenomena. Although a plethora of current sheet equilibrium solutions have been found, the collisionless process through which a disequilibrated current sheet relaxes or equilibrates remains largely unknown. It is shown, through analyses of phase-space distributions of single-particle orbit classes and particle-in-cell simulations, that collisionless transitions among the orbit classes are responsible for this process. Bifurcated current sheets, which are readily observed in geospace but whose origins remain controversial, are shown to naturally arise from the equilibration process and thus are likely to be the underlying structures in various phenomena; comparisons of spacecraft observations to particle-in-cell simulations support this fact. The bearing of this result on previous explanations of bifurcated structures is also discussed.