Turbulent Magnetic Diffusivity β Effect in a Magnetically Forced System

Abstract

We have studied the large-scale dynamo forced with helical magnetic energy. Compared to the kinetic forcing process, the magnetic process is not clearly observed nor intuitive. However, it may represent the actual B field amplification in the stellar corona, accretion disk, plasma lab, or other magnetically dominated systems where the strong kinetic effect does not exist. The interaction between the magnetic field and the plasma is essentially nonlinear. However, when the plasma system is driven by helical energy, whether kinetic or magnetic, the nonlinear process can be linearized with pseudotensors a , β and the large-scale magnetic field $\overline{{\boldsymbol{B}}}$ . Conventionally, the α effect is thought to be the main dynamo effect converting kinetic energy into magnetic energy and transferring it to the large-scale regime. In contrast, β effect has been thought to diffuse magnetic energy. However, these conclusions are not based on the exact definition of α and β . In this paper, instead of the analytic definition of α and β , we derive a semi-analytic equation and apply it to the simulation data. The half analytic and numerical result shows that the averaged α effect is not so important in amplifying the $\overline{{\boldsymbol{B}}}$ field. Rather, it is the negative β effect combined with the Laplacian (∇ ^2 → − k ^2 ) that plays a key role in the dynamo process. Further, the negative magnetic diffusivity accounts for the attenuation of the plasma kinetic energy ${\overline{E}}_{V}$ in large scales. We discuss this process using the theoretical method and the intuitive field structure model.

Keywords

• Solar dynamo
• Magnetohydrodynamics
• Magnetic fields
• Plasma astrophysics