This collaborative research combines measurements of the Earth's ancient magnetic field, information on deep Earth structure, and numerical models of the geodynamo, to investigate the causes and consequences of geomagnetic polarity reversals. The broad objectives of this research are to establish the physical mechanisms in the liquid outer core by which the geodynamo reverses its magnetic polarity, and identify other geophysical controls on the polarity reversal process, with special emphasis on how the Earth's solid mantle affects the frequency of polarity reversals through geologic time.
The investigation uses first-principles numerical models of the dynamo process. These models solve the equations of fluid mechanics, heat and mass transport, and electromagnetism simultaneously in a rotating, electrically-conducting sphere with the same radial structure as the Earth's core, subject to a wide choice of boundary conditions. The results of the numerical models are being scaled to Earth's core conditions using previously-established scaling relationships. The investigators are comparing the time-variable output of the numerical dynamo models with the record of the paleomagnetic field during polarity changes and excursions. The dynamo model reversal behavior is being used to identify the fluid dynamical conditions in the liquid outer core and solid inner core that produce characteristic paleomagnetic signature of rapid polarity reversals separated by long stable polarity epochs with a relatively uniform intensity, dipole-dominant magnetic field.
The first part of the project is to delineate the polarity reversal mechanism of a standard, benchmark numerical dynamo with homogeneous (uniform) conditions at the core-mantle boundary. The second part of the project is to determine the sensitivity of the dynamo model reversals to varying boundary conditions that simulate changes in the mantle through time. In particular, the investigators are testing heterogeneous core-mantle boundary thermal conditions that correspond to competing interpretations of the seismically-imaged three dimensional structure of the lower mantle. This phase of the research is aimed at establishing the geophysical factors that cause the geomagnetic field to alternate between reversing and nonreversing (superchron) states. The third part aims to bring the numerical model closer to the geodynamo, by determining the changes in magnetic field structure and polarity reversal behavior as the vigor of convection, the rate of rotation, diffusivities, and other model parameters are progressively increased toward realistic values for the Earth's core.
In developing a reversing dynamo model that is consistent with seismic, paleomagnetic, geomagnetic observations, this study is providing a first-principles-based tool for driving magnetospheric and atmospheric models that can be used to predict environmental effects associated with gross changes in the geomagnetic field, past and future.
