Collisional dissipation in turbulent weakly-collisional plasmas.

Pezzi O., Perrone D., Servidio S., Valentini F., Sorriso-Valvo L., Greco A., Matthaeus W.H., Veltri P.

II - Fisica della materia
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The comprehension of the heating of turbulent, weakly-collisional plasmas is a puzzling problem in plasma physics. Indeed, in these systems, the energy is usually transferred by turbulent nonlinear couplings from large to small scales, where energy dissipation eventually occurs. Several collisionless mechanisms have been proposed to explain the energy dissipation. These processes neglect collisions and, hence, lack the final part of the heating process description, related to the irreversible degradation of the information. On the other hand, the presence of distorted far-from-equilibrium structures in velocity space --- routinely observed in both in situ spacecraft measurements and numerical simulations --- enhances the role of inter-particle collisions, thus enabling collisions to play a significant role in degrading the free energy of the particle distribution function into heat. Here we report evidence that fine velocity structures are effectively dissipated by collisions in a time much shorter compared to global non-Maxwellian features, such as temperature anisotropies. To estimate the collisions role in a turbulent scenario, we discuss the results of a numerical campaign of hybrid Vlasov-Maxwell simulations. In these simulations, collisions are initially neglected to generate the kinetic turbulent state, characterized by the presence of coherent structures (vortices and current sheets) and of distorted proton distribution functions. Then, collisions are turned on to describe the inter-play of collisions, which tend to restore the thermal equilibrium, and other collisionless, dynamical processes.

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