Backus has observed that infinite core conductivity implies the vanishing of the time derivative of the magnetic flux through any patch on the core surface bounded by a 'null-flux curve', on which the radial magnetic field vanished. Field model GSFC (12/66) is consistent with this criterion only if features with scales smaller than angular degree 7 are important or if the dipole time derivatives are deleted. Deleting the dipole is reasonable if the dipole decays in its 3rd radial mode or core conductivity is 4 × 4
4
mho/m. If the data admit infinite conductivity, Backus has also shown that there is an infinite-dimensional affine space of 'eligible' surface velocity fields which will produce the observed secular variation from the observed geomagnetic field, but that at any point on any null-flux curve all eligible flows have the same component normal to the curve. Using only secular variation harmonic coefficients with angular degrees 2 to 6, we obtain velocity components normal to the null-flux curves which are compatible with primarily latitude-dependent westward drift, but not with the velocity field recently proposed by Kahle, Ball & Vestine (1967
b
).
Single super-vortex as a proxy for ocean surface flow fields