C. Gire

Motions at core surface in the geostrophic approximation

Abstract Motions at the top of the core which generate the observed Secular Variation (S.V.) field are computed. To reduce the well known ambiguity of the solution, two constraints are added: the flow is a large scale one and is geostrophic. The computed flow then has a very simple geometry; its poloidal part is roughly axisymmetrical with respect to an equatorial diameter. This geometry is almost unchanged from 1970 to 1980 while the intensity of the velocity is nearly doubled.

Tangentially geostrophic flow at the core-mantle boundary compatible with the observed geomagnetic secular variation: the large-scale component of the flow

Abstract We present a method for determining the large-scale component of a tangentially geostrophic flow beneath the core-mantle boundary compatible with magnetic secular variation observations. We use a tangentially geostrophic basis to ensure the geostrophy of the motion. The fit of the secular variation (SV) generated by the motion to the observed SV (in fact SV models) is adequate, taking into account the existing error level. As in any horizontal geostrophic motion, the flow is expressed as a sum of two independent tangentially geostrophic flows: a zonal component, which is toroidal, and a non-zonal component, which is directly linked with the motions deeper in core. The flow derived for the recent epoch (1970–1985) presents interesting symmetry properties: the non-zonal velocities are the same at two antipodal points, while the zonal velocities are the same at two points symmetrical about the Equator. The non-zonal component of the flow is more vigorous than the zonal one; the consoidal ingredient, though weaker than the toroidal ingredient, is essential and indicates strong vertical motion at depth in low-latitude areas. The SV is actually compatible with a geostrophic motion at the core-mantle boundary and appears to be mainly due to the action of the non-zonal component of the flow.

Geomagnetic secular variation, core motions and implications for the Earth's wobbles

Abstract Motions at the top of the core are known to be responsible for the secular variation of the Earths magnetic field. If this flow is supposed geostrophic, the associated pressure field can have an appropriate geometry to exert a pressure torque upon the elliptical core-mantle boundary and, besides, to alter the elastic products of inertia in such a way as to excite the Earths and core wobbles. We consider some schematic excitation functions and the resulting amplitudes of the Earths and core rotational motions. The proposed mechanism is shown to be efficient for exciting the long-period Markowitz wobble of the rotation axis and also the Chandler wobble if the variations in the pressure field have the right time scales, as indeed suggested by the available secular variation data.

Flow in the fluid core and Earth’s rotation

We address in this paper the possible effects of motions in the fluid core on the Earth’s rotation, both length of day and pole motion. The geomagnetic field is thought to be generated by dynamo action in the conducting core; information on the fluid motion is then obtained through the study of the secular variation of this field.