Magnetohydrodynamic Turbulence

Magnetohydrodynamic (MHD) turbulence theory is modeled on neutral fluid (Navier‐Stokes) turbulence theory but with some important differences. There have been essentially no repeatable laboratory MHD experiments wherein the boundary conditions could be controlled or varied and a full set of diagnostics implemented. The equations of MHD are convincingly derivable only in the limit of small ratio of collision mean‐free‐paths to macroscopic length scales, an inequality that often goes the other way for magnetofluids of interest. Finally, accurate information on the MHD transport coefficients—and thus, the Reynolds—like numbers that order magnetofluid behavior—is largely lacking; indeed, the algebraic expressions used for such ingredients as the viscous stress tensor are often little more than wishful borrowing from fluid mechanics.