NSFGEO-NERC: On the origin of extreme variations in Earth's magnetic field

Earth's magnetic field has existed for at least 3.5 billion years and exhibits a complex spectrum of spatial and temporal variations on timescales ranging from less than seconds to millions of years. On average the field is thought to adopt a dipole-dominated configuration, which helps protect the surface environment and low-orbiting satellites from the depredations of the solar wind. Significant variations, e.g., the recent growth of a region of anomalously weak field in the southern Atlantic, and excursions and polarity reversals, may alter the shielding effect provided by the field. These surface observations document a dynamo process operating in the liquid core and provide unique insight into the dynamics and evolution of Earth's deep interior. However, data alone cannot constrain the interactions between magnetic field and flow that occur within the core: that requires an internal view of the dynamo. Understanding past field variations and making predictions about future behaviour therefore requires an intimate link between observations and simulations of the generation process.

The standard picture of geomagnetic secular variation (SV) is provided by time-dependent global models of the historical, Holocene and longer term field. However, paleomagnetic data also provide evidence for Unusually Rapid Geomagnetic Events (URGEs) in the form of rapid geomagnetic intensity spikes, and directional rates of change that greatly exceed values in these models. While these URGEs are not visible in current global field models, we have recently shown that they are comparable to the fastest changes (called extremal events) produced in numerical dynamo simulations and are compatible with the physics of the dynamo process. Our results also reveal that extremal intensity and directional changes arise in different times and places and are associated with migration of distinct magnetic features at the top of the core. These findings link observations and simulations in a new and more complex view of SV, and suggest new approaches for understanding the dynamo process and our ability to predict its future variations.

Progress requires moving beyond simple definitions of extremal events to investigate the spectrum of dynamical behaviour that underpins URGEs. Critical to this goal is using complementary information drawn from paleomagnetic global field models and geodynamo simulations. We propose to develop a new series of global time-dependent geomagnetic field models that can capture rapid changes. In parallel we will produce a new suite of geodynamo simulations accessing the rapidly rotating and vigorously convecting regime thought to describe the dynamics of Earth's core. Synthesis across these approaches will address the following questions:

1. What are the defining spatial and temporal characteristics of URGEs? Do they occur in preferred locations or on systematic timescales?
2. What are the physical origin(s) of URGEs?
3. Are URGEs related to excursions and reversals?
4. Are URGEs related to interactions between the core and mantle and/or stratification at the top of the core?