The earth has a magnetic field that originates within its interior and extends thousands of kilometres into space. A magnetic field is a region in which a force acts on a magnet or moving, electrically charged particles. Earth’s magnetic field is thought to be due to circulating electric currents in the iron-rich liquid outer core, some 3,000 kilometres beneath the surface.
What causes a compass to point north?
A magnetic compass needle responds to the magnetic field by aligning itself into a north–south direction, pointing to the place on the earth’s surface known as the magnetic north pole.
Geographic and magnetic poles
The locations of the magnetic and geographic poles are similar, but not exactly the same. The North Pole (also known as the geographic north pole) is the name given to the northern end of earth’s axis of rotation; the South Pole is the southern end.
However, the magnetic poles are related to the magnetic field. For example, in 2002 the magnetic north pole was in the Canadian Arctic Ocean, 950 kilometres south of the geographic north pole.
A further complication is that the magnetic field is not fixed, and the position of the magnetic poles has moved significantly since people started taking measurements. Even more extreme wandering of the magnetic poles has occurred over millions of years, and there are several periods when the magnetic north pole flipped to become the magnetic south pole.
A record of magnetic reversals has been preserved in volcanic rocks that retain the earth’s magnetic field at the time they cooled down. By studying ancient lava flows, scientists have traced the changing path of magnetic poles, which enables them to date the time of magnetic reversals. The last magnetic reversal occurred 780,000 years ago.
When a compass is used for navigation, it points to the magnetic north pole rather than the geographic north pole. The difference in angle between geographic and magnetic north is called the declination, and it varies in different places. For example, in New Zealand in 2005, magnetic north was 18° east of north in Northland, increasing to 25° at Stewart Island.
For careful navigation, measurements made on a magnetic compass need to be corrected, to account for magnetic declination. In New Zealand this correction is normally shown on topographic maps, to assist trampers and others navigating an area.
Magnetic storms and auroras
As well as the longer-term changes in the earth’s magnetic field, there are short-term changes in the upper atmosphere. A continuous flow of charged particles from the sun produces very small daily variations, but when sunspots and solar flares excite the surface of the sun, far more particles are ejected. The interaction of these particles with the earth’s magnetic field produces auroras – undulating bands of light, visible in the night sky. These are part of the phenomena of a magnetic storm.
Intense magnetic storms produce circulating currents in electricity supply. Several major power blackouts have been caused by this overseas. In New Zealand, electricity substations have been damaged by magnetic storms, but without major consequences to the supply network.