Spontaneous Bifurcation of Single Peaked Current Sheets by Chaotic Electron Scattering

Abstract

It is shown that single-peaked collisionless current sheets in a Harris-type equilibrium spontaneously bifurcate as a result of chaotic scattering of electrons at fluctuating magnetic fields near the center of the sheet, as demonstrated by a 2D kinetic particle-in-cell simulation. For this effect to be simulated explicit particle advancing is necessary, since the details of the electron motion have to be resolved. Unlike previous investigations of triggering bifurcated current sheet (BCS) where initial perturbations or external pressure was applied the bifurcation is spontaneous if thermal noise is taken into account. A spontaneous current sheet bifurcation develops quicker than a tearing mode or other plasma instabilities. It is shown that in the course of the current sheet bifurcation the Helmholtz free energy decreases while the entropy increases, i.e. the new, bifurcated current sheet is in a more propable state than the single-peaked one.

Instabilities of collisionless current sheets revisited: The role of anisotropic heating

“…As it was shown in Ref. 28 and confirmed by our simulations, it is mainly driven by a reduction of the electron current dentiy J e,z . This in turn, it is due to a reduction of the electron bulk velocity V e,z , while the electron density distribution n 0e does not vary too much (J e,z = −en 0e V e,z ).…”

“…Temperature anisotropies, generated e.g. in simulations with CIC shape functions, can produce bifurcation in the out-of-plane component of the total current density J z 28 . A comparison for a spe-cific time between the runs CIC-40ppc and TSC-40ppc is shown in Fig.…”

“…Regarding the entropy, in Ref. 28 the relative entropy with respect to the initial velocity distribution function, sometimes called Kullback-Leibler divergence 48 , was used. Let us use instead the conventional definition of information entropy 49 ,…”