COHERENT STRUCTURES AND TURBULENT CASCADES IN 2-DIMENSIONAL INCOMPRESSIBLE MAGNETOHYDRODYNAMIC TURBULENCE

Numerical solutions of decaying two-dimensional incompressible magneto hydrodynamic turbulence reach a long-lived self-similar state which is described in terms of a turbulent enstrophy cascade. The ratio of kin etic to magnetic enstrophy remains approximately constant, while the r atio of energies decreases steadily. Although the enstrophy is not an inviscid invariant, the rates of enstrophy production, dissipation, an d conversion from magnetic to kinetic enstrophy are very evenly balanc ed, resulting in smooth power law decay. Energy spectra have a k(-3/2) dependence at early times, but steepen to k(-5/2). Local alignment of the intermediate and small-scale fields grows, but global correlation coefficients do not, The spatial kurtosis of current grows and is alw ays greater than the vorticity kurtosis. Axisymmetric monopole pattern s in the current (magnetic vortices) are dominant structures; they typ ically have a weaker concentric, nonmonotonic vorticity component. Fas t reconnection or coalescence events occur on advective and Alfven tim e scales between close vortices of like sign. Current sheets created d uring these coalescence events are local sites of enstrophy production , conversion, and dissipation. The number of vortices decreases until the fluid reaches a magnetic dipole as its nonlinear evolutionary end- state. (C) 1995 American Institute of Physics.