The geodynamo, past, present and future

The geodynamo simulation of Glatzmaier and Roberts (1996, Physica D97, 81) is driven by the cooling of the model Earth, which releases latent heat and light components of core fluid at the freezing surface of the inner core as it advances outwards. At some time in the past, the inner core was only a quarter of its present size and at some time in the future it will be twice its present size. The geodynamo operating during those epochs are studied here, the three models (past, present and future) being tied together in an evolutionary sense. The time taken for the models to evolve from past to future depends on the cooling rate, which is controlled by the dynamics of the mantle and is not studied here. All three models generate external fields of comparable strength and all three appear to be close to Taylor states. Unexpectedly, the future model showed considerable variability in time, while the past model does not. Deviations from axisymmetry in the external field increase with inner core radius and the relative predominance of the centered dipole over other multipole components declines.