Asymmetric Dipole Trends in Geodynamo Models and the Paleomagnetic Field

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. Paleomagnetic observations suggest that the dipole field grows and decays at different rates. Abrupt growth of the dipole is often followed by slower decay, particularly when averaged over short-period fluctuations. Geodynamo models are capable of reproducing this behavior, but only with a restricted set of the boundary conditions. Many features of the geodynamo models are captured in stochastic models when the dipole generation is represented as a series of cyclonic convection events. The average recurrence time for the cyclonic convection events is the main factor in setting the level of asymmetry. Extending these results to the paleomagnetic field implies a recurrence time of several kyr. Such long recurrence times cannot be attributed to individual convection events when the timescale for fluid to rise through the core is only 100 years. Instead, we attribute the long recurrences times to magnetic-flux expulsion. Large flux-expulsion events occurring on kyr timescales are sufficient to account for the observed asymmetry. Skewed dipole trends are predicted in geodynamo models under a restricted set of boundary conditionsSkewness in the models is correlated with fluid upwelling and magnetic-flux expulsion at the core-mantle boundaryTrends in the paleomagnetic field imply large flux-expulsion events on millennial timescales

Automated summary

Temporal trends in the paleomagnetic dipole moment exhibit positive skewness, with larger positive trends occurring less frequently than negative trends. Numerical geodynamo models suggest that this skewness is associated with fluid upwelling and magnetic-flux expulsion at the core-mantle boundary.