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.
Highlights
The magnetic fields and zonal wind structure of Jupiter and Saturn can be modeled using the anelastic MHD spherical dynamo equations
To compare the emerging solutions to end-member scenarios, a pure hydrodynamic simulation with no magnetic fields and a fully conducting model where the electrical conductivity is constant along radius are added
This is in line with the classic picture of the zonal flow created by a consistent tilt in the convective columns which further enhance the differential rotation (Busse, 2002)
Summary
The magnetic fields and zonal wind structure of Jupiter and Saturn can be modeled using the anelastic MHD spherical dynamo equations. A key feature distinguishing Saturn from Jupiter is the depth where the transition between molecular hydrogen and its high-pressure metallic phase occurs (Lorenzen et al, 2011) Jupiter models, such as Jones (2014) or Duarte et al (2013), which successfully reproduce the magnetic field dipolarity and dipole tilt, have a shallow hydrodynamic layer and a thick dynamo region deeper down. The nonlinear interaction and relative importance of the governing forces (buoyancy, Coriolis, Lorentz, dissipation) leads to characteristic phenomena such as predominantly columnar convective flows, deepreaching zonal wind systems, and dynamo-generated magnetic fields. The ratios of those forces are typically quantified by a set of nondimensional numbers, e.g. the Ekman, Rayleigh, hydrodynamic and magnetic Prandtl number. We must use enhanced diffusivities and surface heat flux, hoping that the small scales they eliminate are not so important in determining the larger scale flows and fields we are most interested in
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