RevEarth : Realistic modelling of Earth’s magnetic field reversals

Presentation

The magnetic field of the Earth (the geomagnetic field) forms a protective envelope against the solar wind, which may otherwise erode the atmosphere, as it happened on Mars. As such, strong planetary magnetic fields are arguably necessary for life.

The geological record shows that the geomagnetic field reversed its polarity hundreds of times in the past, at a highly variable rate. In the recent geological history (for the past 25 million years), reversals occurred approximately 4 to 5 times per million year. Further in the past, there exists periods of stable polarity (termed chrons) that lasted for several tens of millions of years. Paleomagnetic data further indicate that reversals are very short events ; even if the question remains to be settled, they do not last for more than a few thousands years, which make them almost instantaneous on a geological time scale. This further complicates the detailed description of the properties of the geomagnetic field during a reversal (the transitional field henceforth).

Due to the scarcity of associated data, reversals remain largely unexplained and their consequences uncertain. Numerical simulations appear as a welcome tool to complement this partial understanding.

The magnetic field of our planet is generated and sustained by the circulation of liquid metal deep inside its core. Realistic simulations of this process (termed the geodynamo) are currently restricted to integration intervals short in comparison to the typical duration of a chron in the recent geological record (millennia as opposed to hundreds of millennia, say). By improving an existing state-of-the-art simulation code, combined with a rare event algorithm, we will reach geophysically relevant regimes in reversing geodynamo simulations over an adequate integration interval for the first time.

We plan to build a database of at least 1000 reversals and excursions (aborted reversals), for further analysis in the framework of this project and beyond. Using our low-viscosity, strong magnetic field simulations, we will investigate the effect of several key factors of the evolution of Earth on the reversal rate and the properties of the transitional field. Particular attention will be paid to the presence and size of the inner-core, and the heterogeneities in the heat flux extracted from the core by the surrounding silicate mantle. Even if these questions have already been addressed in previous high-viscosity studies, the new regime in which our reversing geodynamo simulations operate is currently uncharted territory.

Upon completion of a parametric survey, we will produce a geodynamo simulation spanning 300 million years, subject to a realistic, heterogeneous, and time-varying heat flux extracted by the mantle, using the output of a high-fidelity general circulation model of mantle convection.
In addition, we will combine observations and simulations via data assimilation to assess and calibrate a series of reduced, low-dimensional, models of geomagnetic reversals and excursions.

We will use these simulations to shed light on the physical processes leading to a reversal, to seek robust precursors to reversals, to characterize the typical magnetic field shape and intensity during a reversal, and to estimate the consequences of a reversal for society.