Wednesday 18 Feb 2015Seminar: **11am** Dynamics and Magnetic Field Generation in the Gas Giants

Dr Lucia Duarte - University of Exeter

Physics, 4th floor 11:00-12:00

The atmospheres of the gas giants Jupiter and Saturn are mostly composed of hydrogen and helium and possibly surround a rocky central core. Hydrogen assumes a metallic state at the larger pressures reached in these planets. For Jupiter, the transition happens between 80 and 90% of the radius. The dynamo is classically thought to operate in the lower metallic region while the outer molecular envelope is thought to harbor the observed fierce zonal winds.
Though the density increases by orders of magnitude within the gas giants, most dynamical simulations have neglected this stratification for simplicity. In this work, we present a systematic study of the effects of the stratification on the interior dynamics. The numerical simulations employ the anelastic version of the MHD code MagIC which allows implementing the background density stratification while filtering out the fast sound waves.
The results show that even a mild density contrast already affects the dynamics. The scale of the convective motions decreases with increasing density stratification. Furthermore, these motions are more and more concentrated closer to the outer boundary where the entropy gradient becomes steeper. This localization promotes the production of non-dipolar and thus unrealistic magnetic fields. Another effect that leads to complex multipolar fields is the competition between the strong zonal jets, that tend to develop in the simulations, and large scale Lorentz forces.
More realistic solutions with dipole-dominated magnetic fields are found when not only the density stratification but also the increase of the electrical conductivity with depth is taken into account. The dominant equatorial zonal jet now remains confined to a weaker conducting outer envelope that models the molecular layer. The dynamo action, on the other hand, is mainly confined to the deeper metallic region where the electrical conductivity is high.

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