Geomagnetic field analysis -- II. Secular variation consistent with a perfectly conducting core

Secular variation of the Earth's magnetic field is attributed to motion of fluid in the core, which can be taken as a perfect electrical conductor for sufficiently short time periods. This assumption is tested by direct calculation of models of secular variation that are consistent with a perfectly conducting core and that satisfy annual mean data from magnetic observatories for 1959 onwards, and found to be acceptable. Models for 1959, 1964, 1969, 1974 and 1979 are used to show evidence of a complex change in core motions associated with the secular acceleration impulse or ‘jerk’ that occurred in 1969.