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Magnetic field

by  Tony Hurst

Perhaps the most spectacular signs of the earth’s magnetic field in New Zealand are the streaming lights of the aurora australis in the night sky. Less visible phenomena have also been carefully documented in the region – from Captain Cook’s measurements in 1770, to the unusual rocks noted around Nelson in the 1850s, to magnetic storms monitored at the Eyrewell observatory today.

The magnetic field and its direction

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.

Flipping poles

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.

Magnetic declination

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.

Magnetic measurements in New Zealand

Earliest magnetic measurements

The earliest magnetic measurements in New Zealand were made at Ship Cove, Marlborough Sounds, during Captain James Cook’s first visit in 1770. Cook determined the magnetic declination, but at that time the technology was not available for measuring the strength of a magnetic field.

Between 1848 and 1854 the British Admiralty conducted an extensive survey of New Zealand, using the ships Acheron and Pandora. This included measurements of the magnetic declination in many harbours to detect any local magnetic features.

National magnetic surveys

A more extensive magnetic survey was conducted in the first decade of the 20th century to identify any magnetically disturbed regions. Another complete magnetic survey of New Zealand was undertaken from 1941 to 1948.

Magnetic observatories provide records of events like magnetic storms for scientific analysis. They also provide baseline values for other magnetic measurements. In 1901 continuous measurements started at a Magnetic Observatory located in the Botanic Gardens, Christchurch, and one of the original buildings still survives. The observatory was shifted to Amberley, 50 kilometres north of Christchurch, in 1928 because of interference from the DC supply to the electric trams. In 1978 it was relocated at the current Eyrewell site, because of increasing development at Amberley.

New Zealand has been responsible for continuous magnetic measurements at Scott Base in the Antarctic, since the base was built during the 1957–58 International Geophysical Year. Both the Eyrewell and Scott Base stations (and a station at Apia in Samoa, now operated by the Samoan government) provide near real-time magnetic data to International Data Centres as part of the global magnetic observatory network INTERMAGNET.

GNS Science now has responsibility for maintaining the observatories and undertaking regular magnetic measurements in the New Zealand region.

Local variation in magnetic rocks

In some areas, magnetised rocks alter the magnetic field – for example, the magnetised black sand along the west coast of the North Island, or magnetised volcanic rocks. Aeromagnetic surveying can identify magnetic rocks, and is widely used in mineral exploration. Most of New Zealand is covered by reconnaissance aeromagnetic maps at a 1:250,000 scale, and maps of the oceans are also available.

Because rocks lose their magnetism as they are heated, magnetic techniques are also used to locate thermal regions, which have lower than normal magnetisation.

Stokes Magnetic Anomaly

A linear belt of deep-seated magnetic anomalies that extends along the western side of New Zealand has been named the Stokes Magnetic Anomaly after John Lort Stokes, the captain of HMS Acheron, who first recognised unusual magnetic features near Nelson during surveys in 1849–51. They are thought to be caused by a belt of volcanic and intrusive rocks, and are one of the features that has been offset about 480 kilometres along the Alpine Fault.

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How to cite this page: Tony Hurst, 'Magnetic field', Te Ara - the Encyclopedia of New Zealand, (accessed 19 November 2018)

Story by Tony Hurst, published 12 Jun 2006