Lúcia D. V. Duarte, Thomas Gastine, Johannes Wicht
Observations of the gas giants show that both planets have dipolar magnetic fields: Jupiter's is very similar to the Earth's and Saturn's is very axisymmetric. We aim to construct realistic numerical models that explain these features. While the small density jump across terrestrial iron cores allows to use the Boussinesq approximation, the picture is different for the gas giants. Here, the density decreases around 5000 from the deep interior to the surface. Though most of this density jump is accommodated in the outer molecular envelopes, it may still be significant in the metallic dynamo region. Among other properties, the electrical conductivity varies significantly with radius, being roughly constant in the metallic hydrogen region and decaying exponentially in the molecular envelope. We solve an anelastic numerical dynamo model to explore the effects of density stratification and electrical conductivity variation on magnetic field generation. We use an anelastic version of the MHD code MagIC with density jumps up to 245 and an electrical conductivity that decays exponentially in the outer 5-30% of the simulated shell. Past simulations using constant conductivity showed that dipole-dominated magnetic fields are only found up to a density jump of 6. An increasing stratification progressively confines the most active convective region close to the outer boundary equator. Mean field models have shown that such a configuration prefers non-axisymmetric modes. The exponential conductivity decay helps by separating magnetic field generation from the dominant convective region. For intermediate stratifications (6< density jump <148), the dipole component dominates during short periods. Stable strongly dipolar solutions are found either when a large stratification, at E=10^-4, more clearly separates the dynamo from the dominant convective region, or when lower Ekman number is considered.
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http://arxiv.org/abs/1210.3245
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