Rotating Parametric Instability in Earth's Core and the Geodynamo

Records of paleomagnetic intensity from ocean sediment cores reveal cycles of growth and decay. This transient nature of the geomagnetic field is consistent with sequential excitation and collapse of rotational parametric instability (RPI) in the fluid core. Linear stability theory predicts RPI will develop in rotating contained fluids provided dissipation does not outpace the pure or ideal growth. Small amplitude, periodic strains from tides and precession provide elliptical and shear deformation of streamlines that couple with the core's stable rotational modes to drive the development of RPI in the fluid. The continuous action of these small strains on the fluid result in the build up of RPI through the linear regime into finite amplitude and inevitable collapse. Growth and decay of intensity are a measure of the dynamo action in the core which would be driven by turbulent flow due to RPIs. Alternating growths and decays have been recovered from records including single site records from the West Caroline Basin, and the North Atlantic, as well as the NAPIS-75 and GLOPIS-75 composite records. Neighbouring growth-decay and decay-growth pairs are combined to extract an estimate of the ideal growth of RPIs in Earth's fluid core. This estimate is consistent in the records and compares favorably with the ideal growth of intensity expected from the linear theory of RPI in the fluid core. Decay portions of the records show that elliptical and shear RPIs are a geophysically plausible mechanism to control the geodynamo.