Geomagnetic Jerks
Geomagnetic jerks are sudden changes in the second time-derivative in the Earth's magnetic field observed at the surface. I co-authored a study showing that jerks can be explained by rigid zonal flows acting on the magnetic field at the top of the core. One important result of this study is that, while rigid zonal flows are global, whether a jerk occurs or not depends on the local morphology of the magnetic field (see Figure 1 below). While this captures some aspects of jerks, more recent work has shown that this only offers a partial explanation to jerks.
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Paper on this topic
Bloxham, J., Zatman, S. and Dumberry, M., 2002, The origin of geomagnetic jerks, Nature, 420, 65-68.