Large-scale planetary or stellar magnetic fields generated by a dynamo effect
are mostly attributed to flows forced by buoyancy forces in electrically
conducting fluid layers. However, these large-scale fields may also be
controlled by tides, as previously suggested for the star τ-boo, Mars or
the Early Moon. By simulating a small local patch of a rotating fluid,
\cite{Barker2014} have recently shown that tides can drive small-scale dynamos
by exciting a hydrodynamic instability, the so-called elliptical (or tidal)
instability. By performing global magnetohydrodynamic simulations of a rotating
spherical fluid body, we investigate if this instability can also drive the
observed large-scale magnetic fields. We are thus interested by the dynamo
threshold and the generated magnetic field in order to test if such a mechanism
is relevant for planets and stars. Rather than solving the problem in a
geometry deformed by tides, we consider a spherical fluid body and add a body
force to mimic the tidal deformation in the bulk of the fluid. This allows us
to use an efficient spectral code to solve the magnetohydrodynamic problem. We
first compare the hydrodynamic results with theoretical asymptotic results, and
numerical results obtained in a truely deformed ellipsoid, which confirms the
presence of the elliptical instability. We then perform magnetohydrodynamic
simulations, and investigate the dynamo capability of the flow. Kinematic and
self-consistent dynamos are finally simulated, showing that the elliptical
instability is capable of generating dipole dominated large-scale magnetic
field in global simulations of a fluid rotating sphere.Comment: Astrophysical Journal Letters In press, (accepted) (2014) (accepted
