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Sunspots and Geomagnetic Disturbances
For information on sunspots and sunspot numbers,
please see the Background Information page in the activity Exploring
The magnetic field of the Earth, or the
geomagnetic field, as measured at ground based observatories, is a
combination of contributions from the main field internal to the planet,
currents flowing within the Earth's crust and its liquid core, as well as
electric current systems surrounding the Earth.
The main field creates a cavity in interplanetary space called the
magnetosphere is shaped somewhat like a comet in response to the dynamic
pressure of the solar wind. The
magnetosphere is compressed on the side toward the sun to about 10 Earth
radii and is extended tail-like on the side away from the sun to more than
100 Earth radii.
The strength of geomagnetic activity is measured
by a number of indices, some of which have been available for many years.
The longest running series of any index is that of the AA index (a
3-hourly measure of the equivalent amplitude antipodal index).
This index is based on observations at two nearly antipodal
stations (Canberra in Australia and Hartland in the United Kingdom).
This means that the index is a global index, representing
world-wide magnetic activity rather than local features.
Australian observatories are particularly important in a global
context and have a considerable history. They are among the few
observatories in the southern hemisphere, and have achieved long records
of continuous observations. The observatory set up in Hobart in 1840 by
the polar explorer James Clark Ross, at the behest of the British
Government, was one of the first magnetic observatories to be established
anywhere in the world.
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Last modified on July 27, 2001.