Negative Turbulent Magnetic Diffusivity $\beta$ effect in a Magnetically Forced System

TL;DR

This study investigates the magnetically driven large-scale dynamo process, revealing that negative turbulent magnetic diffusivity (beta effect) plays a crucial role in amplifying large-scale magnetic fields, challenging traditional views of magnetic field diffusion.

Contribution

The paper provides a semi-analytic solution for the alpha and beta effects, demonstrating the dominant role of negative beta in large-scale magnetic field amplification.

Findings

Beta effect remains negative and facilitates magnetic field amplification.

Alpha effect decreases before significantly contributing to magnetic field growth.

Negative diffusivity accounts for plasma kinetic energy attenuation.

Abstract

We have studied the large scale dynamo process forced with helical magnetic energy. The magnetically driven dynamo is not so well studied as kinetically forced dynamo. It has been thought to represent the amplification of magnetic field in the stellar corona, accretion disk, or plasma lab. However, the interaction between the helical magnetic field and plasma is a more fundamental phenomenon that can be extended to the early Universe. The scale-invariant helical magnetic field not only explains the currently observed large scale astrophysical magnetic fields but also has information on the horizon scale in the early Universe. The interaction between magnetic field and plasma is inherently non-linear, making its mechanism difficult to understand. But, if the plasma system is driven with helical field, the process can be linearized with alpha&betaand large scale magnetic field. Conventionally, alpha effect is thought to transfer magnetic field to the large scale regime, and betaeffect diffuses magnetic field. However, these conclusions are based on the incompletely derived alpha&beta. To get the exact profiles of evolving alpha&\b{eta}, we solved a coupled semi-analytic equation set and applied the result to simulation data for the large scale magnetic helicity and magnetic energy. Our result shows that the averaged alpha effect decreases before making a significant contribution to the amplification of large scale B field. Rather, betaeffect, which keeps negative, de facto plays a key role in the amplification of large scale B field with Laplacian. And, this negative diffusivity accounts for the attenuation of plasma kinetic energy