Small-scale Dynamo with Nonzero Correlation Time

Abstract

The small-scale dynamo is typically studied by assuming that the correlation time of the velocity field is zero. Some authors have used a smooth renovating flow model to study how the properties of the dynamo are affected by the correlation time being nonzero. Here, we assume the velocity is an incompressible Gaussian random field (which need not be smooth), and derive the lowest-order corrections to the evolution equation for the two-point correlation of the magnetic field in Fourier space. Using this, we obtain the evolution equation for the longitudinal correlation function of the magnetic field ( M _L ) in nonhelical turbulence, valid for arbitrary Prandtl number. The nonresistive terms of this equation do not contain spatial derivatives of M _L of order greater than 2. We further simplify this equation in the limit of high Prandtl number, and find that the growth rate of the magnetic energy is much smaller than previously reported. Nevertheless, the magnetic power spectrum still retains the Kazantsev form at high Prandtl number.

Keywords

• Magnetohydrodynamics
• Astrophysical magnetism
• Perturbation methods