Violation of the gyrotropic pressure closure due to a velocity shear in a magnetised plasma
Résumé
Kinetic processes related to quadrupolar deformations of a plasma distribution function, notably mechanisms of pressure anisotropisation, can be described by a fluid model which retains the full pressure tensor dynamics [1]. In this framework we show that the momentum anisotropy in a shear flow can be transferred to a pressure anisotropy due to the action of the stress tensor (and in particular of its symmetric part) on the second order velocity moment of the transport equation (i.e., the pressure tensor) [2]. This purely dynamical mechanism induces the anisotropisation of an initially isotropic pressure tensor on a time scale of the order of the inverse of the velocity gradients, when this becomes non-negligible with respect to the cyclotron frequency. Pressure anisotropy this way generated is both gyrotropic and non-gyrotropic and can explain direct observations made in the solar wind or in simulations of Vlasov turbulence [3]. In particular, the velocity shear associated to vorticity sheets allows us to interpret [4] the correlation between pressure anisotropy and fluid vorticity which has been observed in numerical simulations. The generation of non-gyrotropic anisotropy corresponds to a loss of conservation of the average particle magnetic moment, for which we provide an evolution equation in a fluid description.
Origine : Fichiers éditeurs autorisés sur une archive ouverte
Loading...