Dynamos in giant planets and brown dwarfs

Global-scale numerical simulations of magnetic field generation

All giant planets in the solar system possess magnetic fields. X-ray emissions from brown dwarfs suggest that they also generate strong fields by a dynamo process that is driven by convection in the electrically conducting interior. Common to both classes of objects is that the electrical conductivity is a strong function of radius. In giant planets it drops towards the surface to values that make the outer layers practically an insulator. This gives rise to different dynamical regimes with very strong differential rotation near the surface and more benign motions in the dynamo region. In brown dwarfs the conductivity also drops sharply towards the surface, but maintains a high enough value for magnetic processes playing a role in the top layers. The consequence for magnetic field generation and differential rotation are unexplored.

In this project global-scale numerical simulations of convection-driven flow and magnetic field generation in spherical shells with stratified density and electrical conductivity will be used to study the transition from giant planets to brown dwarfs. The aim is to develop an understanding of magnetic field strength and topology and of the surface rotation pattern as function of net rotation rate and effective surface temperature. Observations of magnetic fields on brown dwarfs by spectroscopy are expected for the near future and will allow a comparison with the model results.

Results from a dynamo simulation for Jupiter, showing the complex magnetic field structure in the deep interior contrasting with the simple dipolar field in the non-conducting outermost layer. (Gastine et al., Geophysical Research Letters, 2014).

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