On dynamo action produced by boundary thermal coupling

Binod Sreenivasan (1)

1 University of Leeds

Rotating dynamos controlled by laterally varying thermal conditions at the bound- ary are investigated in this paper. A quasi-stationary, locked dynamo solution is obtained when the thermal winds produced by the non-axisymmetric lateral varia- tions come into an approximate balance with the Coriolis forces. This force balance is verified numerically for both equatorially symmetric and antisymmetric bound- ary variations. The introduction of lateral variations at the boundary can excite dynamo action in a weakly convective regime that does not otherwise sustain a magnetic field with homogeneous boundary heating. A sufficiently large lateral vari-ation drives strong radial and axial uid motions near the equatorial plane; these ows in turn generate the helicity required for dynamo action. It is shown that a boundary-locked dynamo operates in a state of equipartition between the velocity and magnetic fields. The departure from equipartition in a partially locked dynamo allows the magnetic energy to be greater than the kinetic energy. As the balance of forces in a locked dynamo is different from that in a convection-driven dynamo, lower-mantle coupling could have a marked effect on the structure and dynamics of convection in the Earth's core.

Mots clés: Geodynamo; Core-mantle coupling; Locking; Boundary-driven thermal winds; Equipartition solution; Kinetic helicity; Dynamo action

Binod Sreenivasan. On dynamo action produced by boundary thermal coupling. Physics of the Earth and Planetary Interiors, 2009, 177 (3-4), pp.130. ⟨10.1016/j.pepi.2009.08.007⟩. ⟨hal-00592585⟩