MAGNETOHYDRODYNAMIC EQUATIONS UNDER ANISOTROPIC CONDITIONS

Scaling analysis is used to derive approximations of magnetohydrodynam ics with self-consistent leading-order dynamics under general conditio ns of anisotropy. Both incompressible and weakly compressible limits a re considered. The horizontal magnetic and velocity fields obey dynami cs given by a reduced closed set of equations, but the vertical compon ents have different decoupled dynamics in the different regimes. Conse rvation laws are also discussed. It is shown that the reduced equation s are not self-consistent unless either the ratio of vertical to horiz ontal length scales is large or the fluid is gravitationally stratifie d and the ratio of length scales is small. New equations are derived f or a rotating stratified plasma, which are an extension of the quasige ostrophic equations of neutral fluids.