http://arxiv.org/abs/2102.03500
We study the large scale dynamo process in a system forced by helical magnetic energy. The dynamo process is basically nonlinear, but can be linearized with pseudo scholars $\alpha$ & $\beta$ and large scale magnetic field ${\overline{\bf B}}$. A coupled semi-analytic equations based on statistical mechanics are used to investigate the exact evolution of $\alpha$\&$\beta$. This equation set requires only magnetic helicity and magnetic energy. They are fundamental physics quantities that can be obtained from the dynamo simulation or observation without any artificial modification or assumption. $\alpha$ effect is thought to be related to magnetic field amplification. However, in reality it converges to $zero$ very quickly without a significant contribution to ${\overline{\bf B}}$ field amplification. Conversely, $\beta$ effect for the magnetic diffusion maintains a negative value, which plays a key role in the amplification with Laplacian $\nabla^2\rightarrow -k^2$. In addition, negative magnetic diffusion accounts for the attenuation of plasma kinetic energy when the system is saturated. The negative magnetic diffusion is from the interaction of advective term $-{\bf U}\cdot\nabla {\bf B}$ and the strongly helical field. When plasma velocity field $\bf U$ is divided into the poloidal component ${\bf U}{pol}$ and toroidal one ${\bf U}{tor}$ in the absence of reflection symmetry, they interact with ${\bf B}\cdot\nabla {\bf U}$ and $-{\bf U}\cdot\nabla {\bf B}$ to produce $\alpha$ effect and (negative) $\beta$ effect, respectively. We discussed this process using the theoretical method and intuitive field structure model.
K. Park and M. Cheoun
Tue, 9 Feb 21
21/87
Comments: N/A
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