Geodynamo Models with Core Evolution

Numerical dynamos with time-variable parameters that simulate the secular evolution of the core are used to interpret long-term trends in geomagnetic field behavior. We find dynamos with slow variations in convective forcing and decreasing rotation rate show trends in average dipole field intensity and fluid velocity, sometimes with systematic variations in polarity chrons. Similar dynamos driven by constant forcing and constant rotation have statistically stationary dipole field intensity and fluid velocity, with random polarity chrons. A reversing dynamo with steady rotational deceleration constrained by tidal friction, and a decreasing (regular) inner core growth rate constrained by core thermal history, evolves over 10 ~Myr with minor trends in average dipole intensity, fluid velocity, and polarity chron length. In contrast, a dynamo started from a non-reversing initial state and subject to an increasing (anomalous) inner core growth rate and constant rotation evolves in 20 ~Myr to a reversing state with a secular increase in variability, decrease in dipole intensity, and decrease in average polarity chron length. The dispersion of polarity chron lengths in the dynamo model with anomalous evolution is qualitatively similar to the observed dispersion of geomagnetic polarity chrons since the end of the Cretaceous Normal Superchron, and the model dipole field spectra are qualitatively similar to recent estimates of the geomagnetic spectrum at very low frequencies. Regular monotonic core evolution tends to produce slower (possibly neglgible) dynamo evolution, which may account for the observed stationarity of the dipole moment intensity over the last several hundred million years. The regular evolution dynamo also shows nearly constant reversal statistics, implying that the regime boundary separating reversing and non-reversing dynamo behavior is also regular, so that an episode of anomalous evolution may be required to initiate or end a magnetic superchron.