Motions at the core surface derived from SV data

Summary. Examples of core motions which generate the observed secular variation field – as given by various models for 1970 and 1980 – from the main field have been computed in the frozen flux approximation, assuming that the spectrum of the motion is of low degree and decreases with wave-number. No mode of degree > 4 in the expansion of the motion can be derived with any degree of confidence. Among the low degree modes, some appear to be stable (they come out with the same magnitude whatever the inversion scheme used). The flow made of these stable modes is then examined. An outstanding feature of the flow is the body westward drift. But it seems necessary, if one looks for such a regular flow, to consider both toroïdal and poloïdal components, which would imply upwelling and down-welling in the upper layers of the core. The toroïdal part of the flow appears to be enhanced by the 1969 impulse, although its geometry is nearly unchanged. On the contrary the geometry of the computed poloïdal part is different in 1980 and in 1970;