Polarity reversals in geodynamo models with core evolution

Numerical dynamos with time-variable control parameters that simulate the secular evolution of the core are used to interpret long-term trends in geomagnetic field behavior, including polarity reversal frequency. Dynamos with incremental changes in convective forcing and rotation rate show continuous trends in dipole field intensity and fluid velocity, with systematic variations in polarity chrons in some cases. Otherwise similar dynamos with constant forcing and rotation rate 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 in a non-reversing state and subject to an increasing (anomalous) inner core growth rate and constant rotation evolves over 20 Myr to a reversing state with substantial trends in dipole intensity and 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 frequency spectra are qualitatively similar to estimates of the geomagnetic spectrum at very low frequencies.