Story: Magnetic field
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.
Full story by Tony Hurst
Main image: Aurora australis, Bluff Harbour
The Short Story
A quick, easy summaryRead the Full Story
What is a magnetic field?
When you place some paper over a magnet, and then sprinkle some iron filings on it, the filings gather in lines. These are the lines of the magnetic field, the regions where the magnet exerts a force. Every magnet has two points, or poles – a north pole and a south pole.
What is the earth’s magnetic field?
Like a giant magnet, the earth has north and south poles. It also has a magnetic field that reaches thousands of kilometres into space. You can find where north is by using a compass, which has a magnetic needle. This responds to the earth’s magnetic field, and points in a north–south direction.
The magnetic field is not fixed. Over time, it moves, and so do the magnetic poles.
Geographic and magnetic poles
The geographic poles are the North Pole (in the Arctic Ocean) and the South Pole (in Antarctica). They are fixed, and in a different place from the magnetic poles, which move. In 2002 the magnetic north pole was north of Canada – 950 kilometres away from the North Pole. So geographic north (shown on a map) and magnetic north (shown by a compass) are slightly different.
An aurora is a display of streaming lights in the night sky. This occurs in the upper atmosphere, where electrically charged particles from the sun are guided along the earth's magnetic lines of force. New Zealanders are most likely to see an aurora from the southern South Island.
Magnetic measurements in New Zealand
When the magnetic field changes it affects compasses, so it is important to measure the field regularly. When Captain James Cook visited in 1770, scientists on board made the first measurements comparing geographic north and magnetic north in New Zealand.
In the 1850s, a ship’s captain noticed that the magnetic field near Nelson was unusual. This is now known to be due to highly magnetic rocks. The black sand of the North Island’s west coast is also magnetic. Like the rocks, it can alter the magnetic field.
The magnetic south pole was first located in 1909, in Antarctica. By 2000 it lay in the Southern Ocean.