  Extreme variations of Earth's magnetic field occurred in the Levant region
around 1000 BC, when the field intensity rapidly rose and fell by a factor of
2. No coherent link currently exists between this intensity spike and the
global field produced by the core geodynamo. Here we show that the Levantine
spike must span > 60 degrees longitude at Earth's surface if it originates from
the core-mantle boundary (CMB). Several low intensity data are incompatible
with this geometric bound, though age uncertainties suggest these data could
have sampled the field before the spike emerged. Models that best satisfy
energetic and geometric constraints produce CMB spikes 8-22 degrees wide,
peaking at O(100) mT. We suggest that the Levantine spike reflects an intense
CMB flux patch that grew in place before migrating northwest, contributing to
growth of the dipole field. Estimates of Ohmic heating suggest that diffusive
processes likely govern the ultimate decay of geomagnetic spikes.
