Wave-driven dynamo action in spherical MHD systems

TL;DR

This study demonstrates that a mechanically forced two-vortex flow inside a sphere can generate magnetic fields through wave-driven dynamo action, with resonance effects and cyclic self-killing behavior, relevant to experimental dynamo mechanisms.

Contribution

It reveals a novel wave-driven dynamo mechanism in spherical MHD systems, highlighting resonance effects and cyclic behavior not previously documented.

Findings

Flow acts as a dynamo despite non-dynamo snapshots

Magnetic field growth rate shows resonance dependence on wave frequency

Identifies a cyclic self-killing and self-recovering dynamo mechanism

Abstract

Hydrodynamic and magnetohydrodynamic numerical studies of a mechanically forced two-vortex flow inside a sphere are reported. The simulations are performed in the intermediate regime between the laminar flow and developed turbulence where a hydrodynamic instability is found to generate internal waves with a characteristic m=2 zonal wave number. It is shown that this time-periodic flow acts as a dynamo although snapshots of the flow as well as the mean flow are not dynamos. The magnetic fields' growth rate exhibits resonance effects depending on the wave frequency. Furthermore, a cyclic self-killing and self-recovering dynamo based on the relative alignment of the velocity and magnetic fields is presented. The phenomena are explained in terms of a mixing of non-orthogonal eigenstates of the time dependent linear operator of the magnetic induction equation. The potential relevance of this mechanism to dynamo experiments is discussed.