Planetary dynamos driven by semi-convection in stratified layers

ISTerre, Université Grenoble Alpes, 38610 Gières, France

^★ Corresponding author: pruzinap@univ-grenoble-alpes.fr

Received: 27 June 2025
Accepted: 1 September 2025

Abstract

Stably stratified fluid layers are common in gaseous planets, stellar interiors, and planetary cores, and have long been considered incapable of sustaining dynamo action. Here, we show that semi-convection – driven by a destabilising thermal gradient within an overall stably stratified medium – can, in fact, give rise to self-sustained magnetic fields. Motivated by recent models suggesting that large portions of Jupiter and Saturn may be semi-convective, we performed direct numerical simulations in spherical shells, operating in the planetary-relevant regime of low magnetic Prandtl numbers. From a primary semi-convection instability, a layered convection state spontaneously develops, consisting of a convective region beneath a stably stratified layer of comparable thickness. Fluid motions in this convective region are strong enough to produce magnetic fields with key features observed in planetary dynamos, including strong dipolarity, realistic field strengths, and spectral characteristics. These results provide the first direct evidence that semi-convection can drive dynamo action in stably stratified regions of gas giants and stellar interiors, with important implications for understanding astrophysical magnetic field generation.