By Chops

Some research came out of RSES last week regarding the rotation of the inner core, and how it speeds up and slows down. This research, made by Hrvoje Tkalcic and others, has got a little bit of publicity (http://rses.anu.edu.au/news-events/news/earth039s-centre-out-sync) and also was published in Nature Geoscience last week. In this post, I want to explore a bit of background on how we know what’s happening deep down inside our planet, how this particular research was performed and finally, what sort of significance this sort of work has.

Cutaway of the earth's interior
(Image from iStockphoto/Baris Simsek)

As many of our readers will be aware, the Earth is divided into a series of layers. From the surface down, we have the crust, the mantle, and then the inner and the outer core. The crust is the hard, brittle stuff we stand on; the mantle the silly-putty like stuff the plates ride upon; the outer core a ball of molten iron and nickel, and the inner core a solid version of the same.

Cutaway view of earth's internal structure
(Image from http://en.wikipedia.org/wiki/File:Earth-crust-cutaway-english.svg)

Without resorting to the fantasy of getting ourselves down into the depths of the planet a la The Core, how, may you ask, do we understand what these layers are made up of, and where they are? The answer, believe it or not, is sound. Not just any sounds, though, the sonic energy produced by earthquakes. In many cases, this is energy that we can’t hear (without trickery, at least), but we can feel, and it can be measured by seismometers. This energy travels as waves, and different types of these waves travel differently within the earth.

Relating to our topic at hand, the core and how we can understand it, let’s focus for a moment on the outer core. This is understood to be liquid; the reason being that it doesn’t support a type of wave called a shear wave. These waves can’t travel in a liquid, and energy that has travelled as shear waves can’t be detected in certain parts of the earth after an earthquake. Now, onto this particular research and how it was performed.

This research on the inner core has been conducted by looking at special types of earthquakes, called earthquake doublets. These are earthquakes that practically sound the same, have occurred at almost exactly the same place but not at the same time. Some doublets occur only weeks after the first earthquake; others many years later. By building up a catalogue of doublets, the group at RSES were able to use the varying time differences to look for doublets and recording stations that mean the earthquake energy has penetrated the inner core. By looking at the differences between how waves from the doublets occur at very distant seismometers, the researchers are then able to piece together the rotation history of the inner core.

The results themselves are a little surprising: previously, it was thought the inner core rotated at much the same rate as the rest of the inner workings of earth. The discovery that the inner core speeds up and slows down, almost like it’s taking short steps then big strides, has implications for not just how earthquakes travel through the earth. It also has implications for earth’s magnetic field.

One of the features of earth is its strong planetary magnetic field. Since the ancients, this magnetic field has been one of our primary methods of navigation. The magnetic field also serves to shield us from cosmic rays, and in the process of doing so it also produces the beautiful northern and southern lights (visible occasionally in the far southern parts of Australia, such as Tasmania).

solar_wind_and_mag_field
The interaction of the solar wind and earth’s magnetic field produces the beautiful auroras australis and borealis, and also protects us from harmful radiation from the sun and interstellar space.

The earth’s magnetic field is ever changing. It changes slowly within a human lifespan, but it is ever moving.

If anyone has a paper topographic map printed nearby, they usually note something about ‘magnetic declination’ and how it varies year to year. Magnetic declination is how different magnetic north is from true north (direction to the north pole) and also grid north (direction of north on the map).  This is valid for a certain period of time; as the earth’s magnetic field moves and changes, this value changes and it needs to be taken into account for navigation. It has an influence on the markings on runway for commercial aviation: periodically they need to be re-marked with the new magnetic headings so that pilots land at the correct runway.

The earth’s magnetic field can also change more dramatically than these more gradual changes. Periodically, the earth’s magnetic field reverses. This means north becomes south and south becomes north, and we’d really have to be careful about navigation. These reversals occur every 100 000 to 1 million years or so; the last one about 800 000 years ago. The geomagneticists, who study the earth’s magnetic field, aren’t sure how rapidly the field switches, and what happens when it does. Knowing how it changes within these reversals is important, especially to modern society. Not a lot of our computers or power infrastructure are tolerant of solar radiation, such as occur in solar radiation storms. Predictions can be made from models of the earth’s core, but these predictions are only as good as the models that generated them.

If we assume certain properties of the dynamics, such as the rate of rotation of the inner core is constant and consistent with the rest of the core, then our models of how the earth’s magnetic field is generated could be wrong. This means any predictions drawn from these models may also be invalid. Having a better understanding of the dynamics of the core can only allow us to better understand this very important dynamo that generates our planet’s main sunscreen.