Inferring core processes using stochastic models of the geodynamo

Recent studies have represented time variations in the Earth's axial
magnetic dipole field as a stochastic process, which comprises both
deterministic and random elements. To explore how these elements are
affected by the style and vigor of convection in the core, as well as the
core-mantle boundary conditions, we construct stochastic models from a set
of numerical dynamo simulations at low Ekman numbers. The deterministic
part of the stochastic model, the drift term, characterises the slow
relaxation of the dipole back to its time average. We find that these
variations are predominantly accommodated by the slowest decay
mode, enhanced by turbulent diffusion to enable a faster
relaxation. The random part—the noise term—is set by the amplitude and
timescale of variations in dipole field generation, including
contributions from both velocity and internal magnetic field
variations.  Applying these interpretations to the paleomagnetic
field suggests that reversal rates are very sensitive to rms variations in
the source. Less than a 50 percent reduction in rms source variations is
sufficient to prevent reversals for the recent magnetic field.