16 Planet Earth Autumn 2010 T o find your way using a magnetic compass with a map, you need to know the difference between magnetic north and map north. This difference is called ‘grid magnetic angle’, and in the UK it is derived from a model of the Earth’s magnetic field, which is updated every year. The variation To go north, you just follow your compass towards magnetic north, right? Not quite. Geophysicists have to work hard so we can continue to navigate with map and compass. Susan Macmillan and Tom Shanahan describe how the UK magnetic repeat station network helps. in grid magnetic angle reflects changes in the Earth’s magnetic field arising from sources in the Earth’s fluid outer core. We don’t yet understand these changes well enough to make good forecasts, so we need to monitor them continuously. Some of the data we need has been provided by an important, UK-wide network of magnetic survey stations that has been operating since the early 20th century. At these ‘repeat stations’, very accurate measurements are made of the magnetic field strength and direction over a whole day, every few years, at exactly the same place. The readings are influenced by different sources of magnetism (see explanations to the right) and all these need to be carefully considered when making and processing magnetic field observations. For example, in the UK the horizontal direction of the main field is currently changing by about 0.2° each year. But we can also see this much variation between sites just a few metres apart because of variations in the crustal fields. Taking repeated measurements at exactly the same spot lets us measure the core magnetic field signal without the risk of distortions from changes in the crustal field. Likewise, variations in the magnetosphere surrounding the Earth cause the overall magnetic field to fluctuate by about 0.2° each day in the UK, and by considerably more during a magnetic storm. During a storm in October 2003 the magnetic field direction was observed in the UK to change by over 5° in six minutes. Fortunately these variations are short-lived compared to those from the core. We measure them at the three UK magnetic observatories, and can then subtract them from the repeat station data. Having processed and modelled the data, Where is North? The Earth’s magnetic field. The magnetic poles are shown as red lines. Magnetic field lines (orange) can be seen emerging from the south magnetic pole and converging at the north magnetic pole, which is offset from the geographic north pole (blue lines) by eleven degrees. Mark Garlick/Science Photo Libarary Pasieka/Science Photo Libary