Influence of initial conditions on the large-scale dynamo growth rate

TL;DR

This study explores how initial energy and helicity conditions influence the growth rate of large-scale magnetic fields in MHD systems, revealing dependencies and interactions that affect dynamo processes.

Contribution

It demonstrates that initial large-scale magnetic energy and helicity proportionally affect the growth rate, and highlights the role of nonhelical fields and velocity field dynamics in early dynamo stages.

Findings

Growth rate depends on initial large-scale magnetic energy and helicity.

Nonhelical magnetic fields can suppress velocity fields via Lorentz force.

Kinetic energy migrates backward during early dynamo stages.

Abstract

To investigate the effect of energy and helicity on the growth of magnetic field, helical kinetic forcing was applied to the magnetohydrodynamic(MHD) system that had a specific distribution of energy and helicity as initial conditions. Simulation results show the saturation of a system is not influenced by the initial conditions, but the growth rate of large scale magnetic field is proportionally dependent on the initial large scale magnetic energy and helicity. It is already known that the helical component of small scale magnetic field(i.e., current helicity $<{\bf j}\cdot {\bf b}>$) quenches the growth of large scale magnetic field. However, $<{\bf j}\cdot {\bf b}>$ can also boost the growth of large scale magnetic field by changing its sign and magnitude. In addition, simulation shows the nonhelical magnetic field can suppress the velocity field through Lorentz force. Comparison of the profiles of evolving magnetic and kinetic energy indicates that kinetic energy migrates backward when the external energy flows into the three dimensional MHD system, which means the velocity field may play a preceding role in the very early MHD dynamo stage.