Decay rates of magnetic modes below the threshold of a turbulent dynamo

TL;DR

This study measures magnetic mode decay rates in turbulent liquid sodium flows below the dynamo threshold, revealing how turbulence and flow shape influence mode stability and reversals, supporting recent dynamo models.

Contribution

It introduces a method to measure decay rates of different magnetic modes independently in turbulent flows, and links mode behavior to flow geometry and dynamo theory.

Findings

Decay rates depend on propeller shape.

Modes become nearly marginal during reversals.

Turbulence induces energy transfer between modes.

Abstract

We measure the decay rates of magnetic field modes in a turbulent flow of liquid sodium below the dynamo threshold.We observe that turbulent fluctuations induce energy transfers between modes with different symmetries (dipolar and quadrupolar). Using symmetry properties, we show how to measure the decay rate of each mode without being restricted to the one with the smallest damping rate.We observe that the respective values of the decay rates of these modes depend on the shape of the propellers driving the flow. Dynamical regimes, including field reversals, are observed onlywhen the modes are both nearly marginal. This is in linewith a recently proposed model.