Localized origin at the core-mantle boundary of the 1969 geomagnetic impulse
The 1969 geomagnetic impulse was the first identified instance of rapid change in Earth's core-generated magnetic field. We show here that its origin at the core-mantle boundary (CMB) was a localized change in the radial field acceleration under the Americas that, thanks to the Green's function linking CMB and surface fields, accounts for prominent signatures in $B_r$ and $B_\theta$ in Central and North America, and in $B_\phi$ in Europe. We find the 1969 and 2017 impulses had similar amplitudes and localized dipole structures. However in 1969 the pattern was north-south rather than east-west oriented, and field acceleration patterns propagated poleward not westward. Furthermore, a local surge in the CMB secular variation was seen leading up to the 1969 event. We propose the 1969 impulse is a result of hydromagnetic waves arriving at the core surface but triggered near the CMB, so associated with localized flux expulsion and inward propagating waves.