Hannah F. Rogers
Global magnetic models are used for a variety of different purposes in everyday life (e.g. navigation in your smartphone) but we do not fully understand how the magnetic field is changing in time. We assume secular variation (the magnetic field change) of the Earth is linked to the motion of liquid iron flowing on the surface of the outer core. These core surface flow models are known to be under-determined and thus require assumptions to produce feasible flows when inverting from secular variation. However, some regions of the outer core surface remain uncertain when applying assumptions based on core flow dynamics. Equally, there are regions where the flow is thought to operate in different ways, such as under seismic anomalies at the base of the mantle or in line with the inner core at the north and south pole.
This poster introduces properties of the outer core and the basics of inverting secular variation data to core surface flow, before describing my research. My research uses a mathematical function (spherical Slepian functions) to separate core flow models, confining the flow to either inside or outside a region of interest. This short introduction shows the main outcome of the research and points to additional sources for further questions.
By Hannah Rogers, University of Edinburgh
Supervisors: Kathy Whaler, Ciarán Beggan, Tetsuya Komabayashi
WHY DOES THE EARTH’S MAGNETIC FIELD CHANGE?
The Earth’s core consists largely of iron, and is very hot from when the Earth formed. Pressure keeps the inner core solid, but the outer part is liquid, about as runny as water. A magnetic field is generated in the outer core by complex motions of the liquid. We want to know how the liquid in the outer core is moving and use it to forecast changes to the magnetic field. This is useful for multiple reasons, such as navigation and satellite orientation. The International Geomagnetic Reference Field (IGRF) model is updated every 5 years but, due to our poor understanding of how the core flow is changing, the 2015-2020 forecast error was larger than expected.
IS THE MAGNETIC FIELD CHANGE RELATED TO FLOW IN THE OUTER CORE?
One way we can measure how the liquid in the outer core is moving is by observing how magnetic field lines change. However, there are ambiguities so we have to make assumptions in order to produce sensible models. These assumptions are generally based on the physical properties of the way that fluids move in rotating spheres. These provide a reasonable solution over the majority of the core-mantle boundary but not the entire surface.
To solve for core surface flow, we invert the change of the magnetic field strength (known as secular variation). As the liquid iron moves along the surface of the Earth’s outer core, the field lines are dragged along with it. Flows on the core surface generally show three main features of 1) strong westward flow in the Atlantic Ocean, 2) much weaker flow under the Pacific Ocean, and 3) an eccentric gyre the region of the CMB below eastern Russia, across Africa and up to the north pole over eastern United States. The average flow speed is ~15 km/yr (about as quick as a snail moves).
WHY INVESTIGATE REGIONAL FLOWS?
We want to improve forecasts of magnetic field change by better understanding individual regions. These individual regions are motivated by:
• some assumptions applied when inverting from field to flow define the flow over only part of the core surface, e.g. tangentially geostrophic flow (Le Mouël, 1984)
• investigating different fluid motions in different regions of the sphere, e.g. the column above and below the inner core – the tangent cylinder (Amit & Pais, 2013)
• spatially uneven data collection (Kim & von Frese, 2017)
• investigating how features on the base of the mantle affect flow on the outer core surface, e.g. seismic velocity anomalies
The technique we use is called spherical Slepian functions, which we are applying to core flows for the first time (Simons, 2010).
AN EXAMPLE OF REGIONAL FLOW USING SPHERICAL SLEPIAN FUNCTIONS
Large low shear velocity provinces (LLSVPs) are features on the base of the mantle which slow any seismic waves passing through these regions (e.g. Garnero et al, 2016). There is debate about how LLSVPs formed and the impact that they have on the Earth’s system.
Despite spherical Slepian functions being a promising technique, we have generated significant unwanted signal at the edge of the LLSVPs caused by the limited bandwidth of core surface flows (maximum spherical harmonic degree = 20). The energy spectra show than the flow energy within the LLSVP is weaker than the input at low spherical harmonic degrees but significantly stronger than the input at higher degrees. We are investigating ways of overcoming this, and other aspects of the technique applied to core surface flows.