Earth's Magnetic Field

Earth's magnetic field is unstable. Magnetic variations over the surface of Earth consist essentially of short-wavelength variations (< 200 km) due to surface rocks, and long-wavelength variations (> 5000 km) due to the main field of Earth.

Over a long period, the average positions of the magnetic poles coincide with the geographic poles, but they wander significantly and the two magnetic poles are not at directly opposite positions on the globe. There is also evidence of global magnetic reversals; at intervals that are usually of hundreds of thousands to millions of years, the North and South magnetic poles interchange.

Calculate your magnetic field(external link) here.

Slow changes of Earth’s magnetic field are caused by changes in the core. Rapid fluctuations are due to the particles and electromagnetic waves from the sun.

Short-period magnetic variations were first observed in the Eighteenth Century, by observing a compass needle with a microscope. It was soon realised that there was a fairly regular daily variation, and also occasional larger irregular variations. These irregular variations were found to coincide with the occurrence of auroras.

By the Nineteenth Century, it was realized that both phenomena followed the occurrence of disturbances on the sun, with a delay of about 18 hours between the observation of a solar flare and the related aurora and magnetic field disturbances. This indicated that something was travelling at a speed greater than 1000 km/s from the Sun to Earth which disturbed the magnetic field as it arrived.

In the early Twentieth Century, it was realized that a cloud of ionized particles would act as a conductor. As such a cloud approached Earth, the interaction between the moving conductor and the static magnetic field would produce circulating currents, producing a variable magnetic field. Our models of these upper atmospheric processes have improved greatly since satellite observations began in 1958.

Learn about Space Weather.

Declination is the difference in horizontal angle between magnetic North and true North. This angle varies with time. Geomagnetic models show the declination around New Zealand. These models include an estimate of how the field is changing with time. Every 5 years a new model is calculated using the latest information (including international geomagnetic models and New Zealand magnetic data).

Actual magnetic field at a point may not agree with these models because:

1. They do not attempt to model the short-wavelength variations (or anomalies), caused by magnetised rocks in the earth's crust.

There are certain areas of New Zealand where there are substantial magnetic variations due to magnetised rocks some kilometres deep, including a region stretching from Eastern Southland through to Westland, and the so-called Mineral Belt of Nelson, which has a northern extension that runs off-shore up the west coast of the North Island. There are more localised magnetic anomalies around Mt Tapuaenuku in Marlborough, and near East Cape.

Most volcanic rocks are magnetised, so places where volcanic rocks are found at or near the surface often have magnetic anomalies. This includes the areas of volcanoes in Whangarei and Auckland, the Taupo Volcanic Zone, Taranaki (including the ironsand beaches which are derived from volcanic rocks) and the old volcanoes of Banks Peninsula and the Otago Peninsula. In any of these areas, one might find a magnetic compass pointing in a different direction to that expected from the large-scale model.

2. As well as the internally produced magnetic field, the earth has a varying magnetic field that results from its interaction with particles and fields coming from the sun.

Large-scale maps of New Zealand (including the 1: 50000 scale Topo50 and 1: 250000 scale Topo250 series) now use the New Zealand Transverse Mercator 2000 (NZTM) projection. In this projection Grid North differs from True North except at a longitude of 173° E, the mid-line of the projection. The difference is called the convergence.

Learn about magnetic declination on the LINZ web-site(external link).