The role of the Yoshizawa effect in the Archontis dynamo

TL;DR

This paper investigates how the Yoshizawa effect, driven by cross helicity in a simplified flow, can generate large-scale magnetic fields through a dynamo mechanism, with implications for solar wind turbulence.

Contribution

It demonstrates that the Yoshizawa effect can cause exponential magnetic field growth and is less constrained than traditional dynamos, expanding understanding of magnetic field generation.

Findings

Yoshizawa effect leads to exponential magnetic field amplification.

The effect is proportional to the mean magnetic field.

It is not significantly limited by conservation laws.

Abstract

The generation of mean magnetic fields is studied for a simple non-helical flow where a net cross helicity of either sign can emerge. This flow, which is also known as the Archontis flow, is a generalization of the Arnold--Beltrami--Childress flow, but with the cosine terms omitted. The presence of cross helicity leads to a mean-field dynamo effect that is known as the Yoshizawa effect. Direct numerical simulations of such flows demonstrate the presence of magnetic fields on scales larger than the scale of the flow. Contrary to earlier expectations, the Yoshizawa effect is found to be proportional to the mean magnetic field and can therefore lead to its exponential instead of just linear amplification for magnetic Reynolds numbers that exceed a certain critical value. Unlike $\alpha$ effect dynamos, it is found that the Yoshizawa effect is not noticeably constrained by the presence of a conservation law. It is argued that this is due to the presence of a forcing term in the momentum equation which leads to a nonzero correlation with the magnetic field. Finally, the application to energy convergence in solar wind turbulence is discussed.