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 typ ical when studying the effects of smaller-scale fields on larger-scale ones , namely, the small-scale MHD fields are assumed to be sufficiently weak, j ointly stationary, homogeneous, and maintained by external sources. The cri teria of large-scale field generation due to negative viscosities are deriv ed for various special forms of isotropic small-scale fields as well as ani sotropic 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].