Turbulent Magnetohydrodynamic Elasticity: I. Boussinesq-like   Approximations for Steady Shear
  We re-examine the Boussinesq hypothesis of an effective turbulent viscosity within the context of simple closure considerations for models of strong magnetohydrodynamic turbulence. Reynolds-stress and turbulent Maxwell-stress closure models will necessarily introduce a suite of transport coefficients, all of which are to some degree model-dependent. One of the most important coefficients is the relaxation time for the turbulent Maxwell stress, which until recently has been relatively ignored. We discuss this relaxation within the context of magnetohydrodynamic turbulence in steady high Reynolds-number, high magnetic-Reynolds-number shearing flows. The relaxation time for the turbulent Maxwell stress is not limited by the shear time scale, in contrast with Reynolds-stress closure models for purely hydrodynamical turbulence. The anisotropy of the turbulent stress tensor for magnetohydrodynamic turbulence, even for the case of zero mean-field considered here, can therefore not be neglected. This shear-generated anisotropy can be interpreted as being due to an effective turbulent elasticity, in analogy to the Boussinesq turbulent viscosity. We claim that this turbulent elasticity should be important for any astrophysical problem in which the turbulent stress in quasi-steady shear has been treated phenomenologically with an effective viscosity. 
Peter T. Williams