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Temporal trends in the paleomagnetic dipole moment exhibit the property of positive skewness. On average, positive trends are larger and occur less frequently than negative trends over timescales of several tens of kyr. We explore the origin of this property using numerical geodynamo models. A suite of models reveals that skewness arises for a restricted set of boundary conditions. Models driven by heat flow at the top and bottom boundaries exhibit very little skewness, whereas models driven solely by heat flow on the lower boundary produce significant positive skewness. Further increases in skewness occur in the presence of thermal stratification at the top of the core. The level of skewness in the geodynamo models is correlated with estimates of upwelling near the core‐mantle boundary. Sustained upwelling is expected to increase magnetic‐flux expulsion, contributing to higher levels of skewness. Similar behavior is recovered from stochastic models in which the dipole is generated by a random series of cyclonic convection events. Skewness in the stochastic models is quantitatively similar to estimates from the geodynamo models when the average recurrence time of the convection events is 100 years. Extending the stochastic models to the paleomagnetic field implies a longer recurrence time of 1,000 years or more. We interpret this recurrence time in terms of the timing of flux‐expulsion events rather than individual convective events. Abrupt increases in the dipole moment from flux expulsion can produce skewed trends on timescales of tens of kyr.

Fluctuations in the paleomagnetic field suggest that the dipole decay time is shorter than expected, based on current estimates for the molecular magnetic diffusivity in the outer core. Similar behavior is observed in turbulent dynamo simulations, where the short magnetic field decay time cannot be attributed to higher‐order decay modes. We interpret the short decay time as a signature of turbulent diffusion and show that mean‐field theory can quantitatively account for the dynamo results. The predictions depend on the amplitude and length scale of the flow that interacts with the magnetic field. We rely on the pairwise balance between Lorentz/Coriolis and buoyancy/Coriolis forces to identify the relevant part of the flow, and use the resulting flow properties to reproduce results from numerical dynamo simulations within their uncertainties. Upon extending these predictions to the paleomagnetic field, we find that the inferred decay time requires a bulk root‐mean‐square velocity less than 0.8–1.2 mm s⁻¹. Somewhat lower velocities have been estimated at the top of the core from observations of secular variation. These results show that velocities in the interior of the core are constrained by paleomagnetic observations, and that the amplitude of this flow cannot substantially exceed estimates at the core surface.

Observations of relative paleointensity reveal several forms of asymmetry in the time dependence of the virtual axial dipole moment (VADM). Slow decline of the VADM into a reversal is often followed by a more rapid rise back to a quasi‐steady state. Asymmetry is also observed in trends of VADM during times of stable polarity. Trends of increasing VADM over time intervals of a few 10s of kyr are more intense and less frequent than decreasing trends. We examine the origin of this behavior using stochastic models. The usual (Langevin) model can account for asymmetries during reversals, but it cannot reproduce the observed asymmetry in trends during stable polarity. Better agreement is achieved with a different class of stochastic models in which the dipole is generated by a series of impulsive events in time. The timing of each event occurs randomly as a Poisson process and the amplitude is also randomly distributed. Predicted trends replicate the observed asymmetry when the generation events are large and the recurrence time is long (typically longer than 3 kyr). Large and infrequent generation events argue against dipole generation by small‐scale turbulent flow. Instead, the observations favor a mechanism that relies on expulsion of poloidal magnetic field from the core.