Coherent structures and turbulent cascades in two-dimensional incompressible magnetohydrodynamic turbulence
- National Center for Atmospheric Research, Boulder, Colorado 80307 (United States)
- Institute for Fusion Studies, University of Texas at Austin, Austin, Texas 78712 (United States)
Numerical solutions of decaying two-dimensional incompressible magnetohydrodynamic turbulence reach a long-lived self-similar state which is described in terms of a turbulent enstrophy cascade. The ratio of kinetic to magnetic enstrophy remains approximately constant, while the ratio of energies decreases steadily. Although the enstrophy is not an inviscid invariant, the rates of enstrophy production, dissipation, and conversion from magnetic to kinetic enstrophy are very evenly balanced, resulting in smooth power law decay. Energy spectra have a {ital k}{sup {minus}3/2} dependence at early times, but steepen to {ital k}{sup {minus}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 always greater than the vorticity kurtosis. Axisymmetric monopole patterns in the current (magnetic vortices) are dominant structures; they typically have a weaker concentric, nonmonotonic vorticity component. Fast reconnection or coalescence events occur on advective and Alfven time scales between close vortices of like sign. Current sheets created during 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. {copyright} {ital 1995} {ital American} {ital Institute} {ital of} {ital Physics}.
- OSTI ID:
- 165745
- Journal Information:
- Physics of Plasmas, Vol. 2, Issue 10; Other Information: PBD: Oct 1995
- Country of Publication:
- United States
- Language:
- English
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