Time evolution of the fluid flow at the top of the core.: Geomagnetic jerks

The knowledge of the geomagnetic field and its secular variation allows us to compute the fluid flow at the core surface. The poloidal and toroidal components of the fluid flow at the core-mantle boundary (CMB) have been calculated every year from the Bloxham and Jackson model (1992) and plotted at 50 year intervals over the last three centuries. The flow patterns conserve some broad features over this whole time-span. The time constant of the degree 1 component of the motion is larger than the time constant of the rest of the flow. The average motion over 300 years appears to be in large part symmetrical with respect to the equator. This average flow can be represented by the sum of a few geostrophic vectors. The acceleration fields corresponding to the well documented jerks of 1969, 1979, 1992 have also been computed. The geometry of these acceleration fields is the same, within a change of sign, for the three events. Moreover, this geometry has close connections with the geometry of the flow itself. The spatial and temporal variations of the flow field can be simply described, in a first approximation; it is possible to give an analytical schematic representation of the flow field during the last three decades. Some characteristics of the decadal length-of-day variations follow if the coupling torque between core and mantle is topographic.