Localized Origin at the Core‐Mantle Boundary of the 1969 Geomagnetic Impulse

The 1969 geomagnetic impulse provided the first compelling evidence for rapid changes in Earth's core-generated magnetic field, taking place on timescales of a few years or less. We show here it originated at the core-mantle boundary (CMB) largely as a localized change in the field acceleration under north and central America. We find the impulse events in 1969 and 2017 involved similar amplitudes of field acceleration change with similar localized dipole structures. However in 1969 the acceleration change pattern was north-south rather than east-west oriented, and it propagated poleward rather than westward. Moreover, there was a distinctive local surge in the CMB secular variation leading up to the 1969 event. We propose the 1969 impulse resulted from hydromagnetic waves arriving at the CMB that were triggered by a convective burst near to the core surface; this event involved localized flux expulsion and inward propagating waves.