Stable stratification promotes multiple zonal jets in a turbulent Jovian dynamo model. (arXiv:2012.06438v2 [astro-ph.EP] UPDATED)

Stable stratification promotes multiple zonal jets in a turbulent Jovian dynamo model. (arXiv:2012.06438v2 [astro-ph.EP] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Gastine_T/0/1/0/all/0/1">T. Gastine</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wicht_J/0/1/0/all/0/1">J. Wicht</a>

The ongoing NASA’s Juno mission puts new constraints on the internal dynamics
of Jupiter. Data gathered by its onboard magnetometer reveal a dipole-dominated
surface magnetic field accompanied by strong localised magnetic flux patches.
The gravity measurements indicate that the fierce surface zonal jets extend
several thousands of kilometers below the cloud level before rapidly decaying
below $0.94-0.96,R_J$, $R_J$ being the mean Jovian radius at the one bar
level. Several internal models suggest an intricate internal structure with a
thin intermediate region in which helium would segregate from hydrogen, forming
a compositionally-stratified layer. Here, we develop the first global Jovian
dynamo which incorporates an intermediate stably-stratified layer between
$0.82,R_J$ and $0.86,R_J$. Analysing the energy balance reveals that the
magnetic energy is almost one order of magnitude larger than kinetic energy in
the metallic region, while most of the kinetic energy is pumped into zonal
motions in the molecular envelope. Those result from the different underlying
force hierarchy with a triple balance between Lorentz, Archimedean and
ageostrophic Coriolis forces in the metallic core and inertia, buoyancy and
ageostrophic Coriolis forces controlling the external layers. The simulation
presented here is the first to demonstrate that multiple zonal jets and
dipole-dominated dynamo action can be consolidated in a global simulation. The
inclusion of an stable layer is a necessary ingredient that allows zonal jets
to develop in the outer envelope without contributing to the dynamo action in
the deeper metallic region. Stable stratification however also smooths out the
small-scale features of the magnetic field by skin effect. These constraints
suggest that possible stable layers in Jupiter should be located much closer to
the surface ($0.9-0.95,R_J$).

The ongoing NASA’s Juno mission puts new constraints on the internal dynamics
of Jupiter. Data gathered by its onboard magnetometer reveal a dipole-dominated
surface magnetic field accompanied by strong localised magnetic flux patches.
The gravity measurements indicate that the fierce surface zonal jets extend
several thousands of kilometers below the cloud level before rapidly decaying
below $0.94-0.96,R_J$, $R_J$ being the mean Jovian radius at the one bar
level. Several internal models suggest an intricate internal structure with a
thin intermediate region in which helium would segregate from hydrogen, forming
a compositionally-stratified layer. Here, we develop the first global Jovian
dynamo which incorporates an intermediate stably-stratified layer between
$0.82,R_J$ and $0.86,R_J$. Analysing the energy balance reveals that the
magnetic energy is almost one order of magnitude larger than kinetic energy in
the metallic region, while most of the kinetic energy is pumped into zonal
motions in the molecular envelope. Those result from the different underlying
force hierarchy with a triple balance between Lorentz, Archimedean and
ageostrophic Coriolis forces in the metallic core and inertia, buoyancy and
ageostrophic Coriolis forces controlling the external layers. The simulation
presented here is the first to demonstrate that multiple zonal jets and
dipole-dominated dynamo action can be consolidated in a global simulation. The
inclusion of an stable layer is a necessary ingredient that allows zonal jets
to develop in the outer envelope without contributing to the dynamo action in
the deeper metallic region. Stable stratification however also smooths out the
small-scale features of the magnetic field by skin effect. These constraints
suggest that possible stable layers in Jupiter should be located much closer to
the surface ($0.9-0.95,R_J$).

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