Anelastic spherical dynamos with radially variable electrical conductivity

TL;DR

This paper presents numerical simulations of gas giant dynamos considering radially variable electrical conductivity, revealing diverse magnetic field morphologies and dynamical regimes influenced by rotation and conductivity profiles.

Contribution

It introduces a comprehensive model incorporating radially variable electrical conductivity in anelastic dynamo simulations, capturing the transition between different planetary magnetic field regimes.

Findings

Diverse magnetic field morphologies including dipolar and quadrupolar fields.

Identification of two dynamical regimes separated by a magnetic tangent cylinder.

Models with Saturn-like conductivity profiles exhibit long-term oscillations and stronger non-axisymmetric fields.

Abstract

A series of numerical simulations of the dynamo process operating inside gas giant planets has been performed. We use an anelastic, fully nonlinear, three-dimensional, benchmarked MHD code to evolve the flow, entropy and magnetic field. Our models take into account the varying electrical conductivity, high in the ionised metallic hydrogen region, low in the molecular outer region. Our suite of electrical conductivity profiles ranges from Jupiter-like, where the outer hydrodynamic region is quite thin, to Saturn-like, where there is a thick non-conducting shell. The rapid rotation leads to the formation of two distinct dynamical regimes which are separated by a magnetic tangent cylinder - mTC. Outside the mTC there are strong zonal flows, where Reynolds stress balances turbulent viscosity, but inside the mTC Lorentz force reduces the zonal flow. The dynamic interaction between both regions induces meridional circulation. We find a rich diversity of magnetic field morphologies. There are Jupiter-like steady dipolar fields, and a belt of quadrupolar dominated dynamos spanning the range of models between Jupiter-like and Saturn-like conductivity profiles. This diversity may be linked to the appearance of reversed sign helicity in the metallic regions of our dynamos. With Saturn-like conductivity profiles we find models with dipolar magnetic fields, whose axisymmetric components resemble those of Saturn, and which oscillate on a very long time-scale. However, the non-axisymmetric field components of our models are at least ten times larger than those of Saturn, possibly due to the absence of any stably stratified layer.