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J. Cho (UTexas/JHU), E.T. Vishniac (JHU)
We have studied the effects of uniform external magnetic fields on driven magnetohydrodynamic (MHD) turbulence, including, as a limit, the case of zero external field. We have two main results. First, as the strength of the external field is increased, the kinetic energy density drops by roughly the product of the rms velocity (V) and the strength of the external field (B0). The magnetic energy density rises by roughly the same amount. Consequently, the rms magnetic field is always much greater than the external field. Second, when the external magnetic field (in velocity unit) is not very strong (say, less than 0.1 times the rms velocity) the induced magnetic field remains almost isotropic, i.e. there is no apparent anisotropy of order B0/V. We discuss implications of these results on some recent work on mean-field dynamo theories. To explain the dependence of the kinetic and magnetic energy densities on the mean field external field, we present a simple model based on the assumption of fast turbulent diffusion processes. We have also confirmed our previous result that the magnetic fields are amplified through field line stretching at a rate proportional to the difference between the velocity and the magnetic field strength times a constant.
This work was partially supported by National Computational Science Alliance under CTS980010N and utilized the NCSA SGI/CRAY Origin2000.