No. 32: Mar-Apr 1984
"It now seems almost certain that around 1969 a spectacular change took place in the geomagnetic field. The change was almost synchronous over the whole of the Earth's surface, took place in less than two years, and is now known to have consisted of a 'jerk': a step change in secular acceleration of the magnetic field that has its origin inside the Earth."
(Whaler, K.A.; "Geomagnetic Impulses and Deep Mantle Conductivity," Nature, 306:117, 1983.)
Comment. No one really knows just how a "jerk" in the magnetic field is initiated; in fact, the origin of the geomagnetic field as a whole is not well-understood.