Journey Near the Center of the Earth: The Outer Core

Why do our compasses point north? Most people know it is because of our Earth's magnetic field, but how is it created? To answer this, I’d like to take you on a journey roughly 1,800 miles down to the outer core (like going from New York City to Denver). The outer core is home to an important process which scientists refer to as the Geodynamo. This process is responsible for the generation of magnetic fields in the earth's liquid layer, the outer core, which is made up of melted iron and nickel metal. Our Earth's magnetic field protects us and other animals from harmful solar radiation, and is therefore something we cannot live without! By studying the Geodynamo, we can get a better understanding of what happened in earth's past that allowed it to exist as well as what may happen to our field in the future.

A depiction of the Earth's magnetic protecting us from the solar rays. Credit: © ESA/ATG medialab.

To the right: the whispy white loops portray the magnetic field around the Earth, seen here as partially transparent. The outer core is highlighted in fiery red, and the black ball in the center is the inner core. The magnetic field lines gather at the North and South poles of our earth, and this is why we have the Northern and Southern Lights!

The exact mechanisms that occur to bring about this dynamo are not well known, for obvious reasons: inaccessibility to earth’s deep interior, and the complexity and chaotic behavior of magnetic field measurements. We know that the solid metal inner core is almost as hot as the surface of the Sun, so it cannot be the cause of the magnetism. This, along with the discovery of the liquid metal outer core, gave rise to the Geodynamo theory.

Scientists in the 40s and 50s were able to produce a mathematical (or analytical) model for how the Earth's field changes over time. This model, which we refer to as the Induction equation, essentially shows how the fluid stretches and twists the magnetic fields and thereby amplifies them, like water in a tornado. So how the fluid flows has immense implications for whether or not the total magnetic field will grow or decay. But the Induction Equation cannot be solved on paper to determine what type of fluid flows or magnetic fields produce the type of magnetic fields we measure on Earth.

A tornado stretching and twisting the air around it. It is visible due to the water and dust that it sucks up with it. Source: BBC America. This twisting tube is called a vortex (plural: vortices), and is an important feature in fluid flows. When the fluid is a good electrical conductor or metallic, the vortices can twist and stretch the magnetic field lines!

Although computational simulations have been successful in replicating a dynamo, it cannot reproduce the exact dynamics on Earth because of the limited range of scales that computers can capture: in the outer core, the features of the fluid flow can range from hundreds of miles all the way down to the tiny molecular level! 

To truly understand and better replicate the outer core's characteristics, we need a physical model, or a laboratory analogue. At the University of Maryland, one such experiment exists: the Three Meter.

A peek into our experiment: our former grad student stands on top of the experiment's house: a metal box. Directly below him sits the Three Meter. You are looking at the exterior, which looks like it has a tube wrapped around it. This tube is filled with hot oil that keeps the sodium inside melted!

 

As its name suggests, the experiment is 3 meters (or roughly 3 yards) large and inside has a shell that is filled with liquid metal. In our case this metal is not iron, but sodium, as it melts at regular cooking temperatures, is lightweight, and a very good conductor (the Geodynamo needs a good electrical conductor to work). We can spin the shell to bring on fluid flow in ways that may occur in the outer core, due to the spin of Earth and the natural circulation in the outer core. By taking magnetic measurements of the experiment while it is spinning, we can determine what sorts of conditions are required for there to be a growth in the magnetic field. We may be able to translate this into the conditions that Earth was under to achieve a growth in the magnetic field.