Dynamo in a rotating ellipsoidal fluid cores subjected to libration, tides and precession

K. Sandeep Reddy, B. Favier and M. Le Bars

We investigate the magnetic field generation in rotating ellipsoidal fluid cores subjected to libration, tides and precession. Dynamo action in planetary cores has been extensively studied in the context of convectively-driven flows. From our results we show that mechanical forcings, namely tides, libration and precession, are also able to kinematically sustain a magnetic field against ohmic diffusion. Previous attempts published in the literature focused on the laminar response or considered idealized spherical configurations. In contrast, we focus here on the developed turbulent regime and we self-consistently solve the MagnetoHydroDynamics (MHD) equations in an ellipsoidal container.

Pictures of (a, b, c) meridional (x,z) slices of the magnitude of the vorticity |ω|, (d, e, f) meridional (x,z) slices of the magnitude of the magnetic field |B|, and (g, h, i) amplitude of the normal component of the magnetic field at the inner domain boundary (i.e. the CMB) Bn at CMB, for dynamos driven by tides, libration, and precession in the first, second and third columns, respectively.

Pictures of

(a) Evolution of the vertical kinetic and magnetic energies of the tidally driven dynamo for Pm=2.

(b), (c) Growth rates σ of the magnetic field for kinematic dynamo driven by tides and libration, respectively, as a function of the magnetic Prandtl number.

(d) growth rates σ of the magnetic field for precession dynamo as a function of the equatorial ellipticity β of the domain.

Reference:

K. Sandeep Reddy, B. Favier, M. Le bars, (2018) "Turbulent kinematic dynamos in ellipsoids driven by mechanical forcing", Geophysical Research Letters, 45, 1741–1750.

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