Negative magnetic viscosity in two dimensions

The occurrence of "negative viscosities" is studied within the framework of two-dimensional magnetohydrodynamics MHD. We use assumptions which are typical when studying the effects of smaller-scale fields on larger-scale ones, namely, the small-scale MHD fields are assumed to be sufficiently weak, jointly stationary, homogeneous, and maintained by external sources. The criteria of large-scale field generation due to negative viscosities are derived for various special forms of isotropic small-scale fields as well as anisotropic ones; the latter can be regarded as MHD stochastic analogs of the known Kolmogorov flow. (C) 1999 American Institute of Physics. [S1063-7761(99)01010-0].