16 Planet Earth Autumn 2010
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
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
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
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