A dynamo driven by zonal jets at the upper surface: Applications to giant planets

TL;DR

This paper proposes a new dynamo mechanism driven by unstable zonal jets in a rotating spherical shell, which could explain the magnetic field structures of giant planets like Jupiter and Neptune.

Contribution

It introduces a dynamo model based on shear instability of zonal jets and explores how jet width influences magnetic field topology in giant planets.

Findings

Self-sustained magnetic fields at magnetic Reynolds numbers >1000.

Rossby wave propagation is essential for dynamo action.

Magnetic field topology depends on zonal jet width.

Abstract

We present a dynamo mechanism arising from the presence of barotropically unstable zonal jet currents in a rotating spherical shell. The shear instability of the zonal flow develops in the form of a global Rossby mode, whose azimuthal wavenumber depends on the width of the zonal jets. We obtain self-sustained magnetic fields at magnetic Reynolds numbers greater than 1000. We show that the propagation of the Rossby waves is crucial for dynamo action. The amplitude of the axisymmetric poloidal magnetic field depends on the wavenumber of the Rossby mode, and hence on the width of the zonal jets. We discuss the plausibility of this dynamo mechanism for generating the magnetic field of the giant planets. Our results suggest a possible link between the topology of the magnetic field and the profile of the zonal winds observed at the surface of the giant planets. For narrow Jupiter-like jets, the poloidal magnetic field is dominated by an axial dipole whereas for wide Neptune-like jets, the axisymmetric poloidal field is weak.