SINS of Viscosity Damped Turbulence

The problems with explaining the Small Ionized and Neutral Structures (SINS) appealing to turbulence stem from inefficiency of the Kol-mogorov cascade in creating large fluctuations at sufficiently small scales. However , other types of cascades are possible. When magnetic turbulence in a fluid with viscosity that is much larger than resistivity gets to a viscous damping scale, the turbulence does not vanish. Instead, it gets into a different new regime. Viscosity-damped turbulence produces fluctuations on the small scales. Magnetic fields sheared by turbulent motions by eddies not damped by turbulence create small scale filaments that are confined by the external plasma pressure. This creates small scale density fluctuations. In addition, extended current sheets create even stronger density gradients that accompany field reversals in the plane perpendicular to mean magnetic field. Those can be responsible for the SINS formation. This scenario is applicable to partially ionized gas. More studies of reconnection in the viscosity dominated regime are necessary to understand better the extend to which the magnetic reversals can compress the gas. Turbulence can be viewed as a cascade of energy from a large injection energy scale to dissipation at a smaller scale. The latter is being established by equating the rate of turbulent energy transfer to the rate of energy damping arising, for instance, from viscosity. Naively, one does not expect to see any turbulent phenomena below such a scale. Such reasoning may not be true in the presence of magnetic field, however. Consider magnetized fluid with viscosity ν much larger than magnetic diffusivity η, which is the case of a high magnetic Prantl number P r fluid. The partially ionized gas can serve as an example of such a fluid up to the scales of ion-neutral decoupling (see a more rigorous treatment in Lazarian, Vishniac & Cho 2004, henceforth LVC04). Fully ionized plasma is a more controversial example. For instance, It is well known that for plasma the diffusivities are different along and perpendicular to magnetic field lines. Therefore, the plasma the Prandtl number is huge if we use the parallel diffusivity ν ≫ ν ⊥. A treatment of the fully ionized plasma as a high Prandtl number medium is advocated in Schechochihin et al (2004, henceforth SCTMM). The turbulent cascade in a fluid with isotropic η proceeds up to a scale at which the cascading rate, which for the Kolmogorov turbulence, i.e. v l ∼ l 1/3 , …