Excitability in the vicinity of a saddle-node bifurcation: a mechanism for reversals

TL;DR

This paper introduces a new mechanism involving saddle-node bifurcations and eigenmode interactions to explain magnetic field reversals in dynamo systems, aligning with experimental and planetary observations.

Contribution

It proposes a novel reversal mechanism based on eigenmode collision near a saddle-node bifurcation, accounting for turbulence-induced reversals in magnetic fields.

Findings

Reversals occur near saddle-node bifurcations of dynamo eigenmodes.

Small turbulence fluctuations can trigger reversals.

The mechanism explains magnetic reversals in experiments and planetary fields.

Abstract

We present a new mechanism for oscillatory or random reversals of the magnetic field that occur from a stationary dynamo state. The basic ingredient is the existence of two nearly critical dynamo eigenmodes, a stable and an unstable one, that collide during a saddle-node bifurcation. Above this bifurcation, a finite amplitude limit cycle is generated. In the neighbourhood of the bifurcation, even a small amount of turbulent fluctuations is enough to generate random reversals of the magnetic field. When the two competing eigenmodes have different symmetries, this scenario requires a broken symmetry of the flow that generates the dynamo. This is in good agreement with the observations of the VKS experiment. It can also explain the dynamics of planetary or stellar magnetic fields and can be used to understand reversals of large scale flows that often develop on a turbulent background.