The Generation of Magnetic Fields through Driven Turbulence

We have tested the ability of driven turbulence to generate magnetic fieldstructure from a weak uniform field using three dimensional numericalsimulations of incompressible turbulence. We used a pseudo-spectral code with anumerical resolution of up to $144^3$ collocation points. We find that themagnetic fields are amplified through field line stretching at a rateproportional to the difference between the velocity and the magnetic fieldstrength times a constant. Equipartition between the kinetic and magneticenergy densities occurs at a scale somewhat smaller than the kinetic energypeak. Above the equipartition scale the velocity structure is, as expected,nearly isotropic. The magnetic field structure at these scales is uncertain,but the field correlation function is very weak. At the equipartition scale themagnetic fields show only a moderate degree of anisotropy, so that the typicalradius of curvature of field lines is comparable to the typical perpendicularscale for field reversal. In other words, there are few field reversals withineddies at the equipartition scale, and no fine-grained series of reversals atsmaller scales. At scales below the equipartition scale, both velocity andmagnetic structures are anisotropic; the eddies are stretched along the localmagnetic field lines, and the magnetic energy dominates the kinetic energy onthe same scale by a factor which increases at higher wavenumbers. We do notshow a scale-free inertial range, but the power spectra are a function ofresolution and/or the imposed viscosity and resistivity. Our results areconsistent with the emergence of a scale-free inertial range at higher Reynoldsnumbers.Comment: 14 pages (8 NEW figures), ApJ, in press (July 20, 2000?