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Precessing spherical shells: flows, dissipation, dynamo and the lunar core


Precession of planets or moons affects internal liquid layers by driving flows, instabilities and possibly dynamos. The energy dissipated by these phenomena can influence orbital parameters such as the planet's spin rate. However, there is no systematic study of these flows in the spherical shell geometry relevant for planets, turning any extrapolation to celestial bodies to pure speculation. We have run more than 900 simulations of fluid spherical shells affected by precession, to systematically study basic flows, instabilities, turbulence, and magnetic field generation. We observe no significant effects of the inner core on the onset of the instabilities. We obtain an analytical estimate of the viscous dissipation. We propose theoretical onsets for hydrodynamic instabilities. We extend previous precession dynamo studies towards lower viscosities, at the limits of today's computers. In the low viscosity regime, precession dynamos rely on the presence of large-scale vortices, and the surface magnetic fields are dominated by small scales. Interestingly, intermittent and self-killing dynamos are observed. Our results suggest that large-scale planetary magnetic fields are unlikely to be produced by a precessing dynamo in a spherical core. But this question remains open as planetary cores are not exactly spherical. Moreover, the fully turbulent dissipation regime has barely been reached. Finally, we apply our results to the Moon, showing that turbulence can be expected in its liquid core during its whole history. Using recent experimental results, we propose updated formulas predicting the precession driven flow and dissipation in both the laminar and the turbulent regimes.
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Dates and versions

insu-01874075 , version 1 (13-09-2018)
insu-01874075 , version 2 (05-02-2019)


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David Cébron, Raphaël Laguerre, Jerome Noir, Nathanaël Schaeffer. Precessing spherical shells: flows, dissipation, dynamo and the lunar core. 2018. ⟨insu-01874075v1⟩
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