Blanc SIMI 6 - Blanc - SIMI 6 - Système Terre, environnement, risques

Turbulence and dynamo in planetary cores – TuDy

Two laboratory experiments DTS-Omega and ZoRo will shed some light on turbulence and dynamo processes in planetary cores.

Numerical dynamo simulations have brought a lot about the role of rotation in the generation of magnetic fields in <br />planets and stars. <br />Nevertheless, the organization of small-scale fluctuations remains out of reach of these simulations. If we try <br />to build plausible scenarios of the cascade of kinetic and magnetic energies from the measured large-scales <br />down, we see that classical ideas on magnetohydrodynamic turbulence would yield unrealistic estimates of dissipation in the Earth’s core.

Explore the role of rotation and magnetic field on the organization and dissipation of turbulence.

If we try <br />to build plausible scenarios of the cascade of kinetic and magnetic energies from the measured large-scales <br />down, we see that classical ideas on magnetohydrodynamic turbulence would yield unrealistic estimates of <br />viscous dissipation in the Earth’s core. We think that the resolution of this paradox lies in the strong role of <br />rotation on the dissipation of the dynamo. To explore this idea, we propose a project organized around two <br />Lab experiments, complemented and extended by numerical simulations.

The first experiment, called DTS-Omega, is devoted to the study of magnetostrophic turbulence. It is an extension
of the DTS magnetized spherical Couette experiment and builds upon the unique expertise we have acquired
in running liquid sodium experiments. Early runs of the DTS experiment suggest that turbulence is strongly
reduced when both rotation and a magnetic field are present. The experiment we propose will allow a much
deeper understanding of this phenomenon. It takes advantage of an experimental breakthrough we have
prepared over the past year, which consists in an embarked electronic hardware DTSNum that allows the
recording of more than 200 data channels at sampling rates up to 10kHz.

The second experiment, called ZoRo, is intended to shed light on the generation of zonal motions within
convective planetary cores. New exciting ideas based upon the mixing of potential vorticity have emerged in
the past years. However, they have been explored mostly in the context of thin layers, such as the
atmospheres and oceans, at the surface of a sphere. The fact that zonal motions and quasi-geostrophic
columns can extend all the way across the fluid layer in planetary cores modifies the picture. The experiment
we propose is challenging, as we want to achieve strong enough convection and large enough rotation in
order to form a large number of zonal bands. Measurements are far from trivial in such a situation, but we
hope to get novel results, making use of a modified version of the DTSNum hardware.

Numerical simulations will help us link these two fundamental topics to the modelling of the geodynamo. In
particular, we will extend the results of the ZoRo experiment by developing a hybrid quasi-geostrophic
convective dynamo model.

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Over the past decade, we have learnt a lot about the role of rotation in the generation of magnetic fields in planets and stars. Numerical convective dynamo simulations have played a major role in this revolution. Nevertheless, the organization of small-scale fluctuations remains out of reach of these simulations. If we try to build plausible scenarios of the cascade of kinetic and magnetic energies from the measured large-scales down, we see that classical ideas on magnetohydrodynamic turbulence would yield unrealistic estimates of viscous dissipation in the Earth’s core. We think that the resolution of this paradox lies in the strong role of rotation on the dissipation of the dynamo. To explore this idea, we propose a project organized around two Lab experiments, complemented and extended by numerical simulations.

The first experiment, called DTSOmega, is devoted to the study of magnetostrophic turbulence. It is an extension of the DTS magnetized spherical Couette experiment and builds upon the unique expertise we have acquired in running liquid sodium experiments. Early runs of the DTS experiment suggest that turbulence is strongly reduced when both rotation and a magnetic field are present. The experiment we propose will allow a much deeper understanding of this phenomenon. It takes advantage of an experimental breakthrough we have prepared over the past year, which consists in an embarked electronic hardware DTSNum that allows the recording of more than 200 data channels at sampling rates up to 10kHz.

The second experiment, called ZoRo, is intended to shed light on the generation of zonal motions within convective planetary cores. New exciting ideas based upon the mixing of potential vorticity have emerged in the past years. However, they have been explored mostly in the context of thin layers, such as the atmospheres and oceans, at the surface of a sphere. The fact that zonal motions and quasi-geostrophic columns can extend all the way across the fluid layer in planetary cores modifies the picture. The experiment we propose is challenging, as we want to achieve a strong enough convection and large enough rotation in order to form a large number of zonal bands. Measurements are far from trivial in such a situation, but we hope to get novel results, making use of a modified version of the DTSNum hardware.

Numerical simulations will help us link these two fundamental topics to the modelling of the geodynamo. In particular, we will extend the results of the ZoRo experiment by developing a hybrid quasi-geostrophic convective dynamo model.

Our project relies upon the expertise of a single partner: the geodynamo team of ISTerre. It will greatly benefit from the strong international collaborations we have established with several partners. The topic we investigate is very interdisciplinary, and our long-established contacts with colleagues from other communities (physics, fluid mechanics, astrophysics, engineering, etc) are essential.

Project coordinator

Monsieur Henri-Claude NATAF (Institut des Sciences de la Terre)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

ISTerre-CNRS Institut des Sciences de la Terre

Help of the ANR 276,400 euros
Beginning and duration of the scientific project: September 2013 - 48 Months

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