The tendency of a magnet to align itself in a north-south direction, so giving a magnetic compass, was discovered by the Chinese about 2000 years ago. Some hundreds of years later, they discovered that magnetic North, to which a compass points, did not necessarily coincide with true North. The horizontal angle between them is known as the declination, and the Chinese also discovered that this angle varied with time. (Smith & Needham, 1967) The magnetic compass arrived in Europe during the twelfth century, and proved a valuable aid to ocean navigation. By the sixteenth century the declination was being measured at various places so compass directions could be corrected for more accurate navigation. Also in this century, Georg Hartmann and Robert Norman independently discovered magnetic inclination, the angle between the magnetic field and the horizontal. Then in 1600 William Gilbert published De Magnete, in which he concluded that the earth behaved as a giant magnet. It has taken nearly four hundred years since then to produce a convincing theory of how this magnetic field is produced. It gradually became apparent that the obvious theory, that the earth is composed of magnetic rock, was incorrect, as rocks lose their magnetism at the temperatures found at any significant depth within the earth. Fourier analysis shows that magnetic variations over the surface of the 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 the earth. Larmor suggested in 1919 that a self-exciting dynamo could explain the magnetic field of the earth, as well as that of the sun and other stars, but it was Elsasser and Bullard in the 1940s who showed how motion in the liquid core of the earth might produce a self-sustaining magnetic field. By this time seismology and other studies had given a clearer picture of the earth, as having a solid inner core, a liquid outer core, both with a composition more of metal (mainly iron) than rock, and a rocky mantle, all below a thin crust that is all we can directly see. Energy from radioactivity travels outwards as heat, producing thermal convection in the core. It seems that this convection is the cause of the earth's magnetic field, although our knowledge of the core and its dynamics is sketchy. Our knowledge is limited to saying that flow regimes somewhat like those that may be occurring in the core can potentially produce self-sustaining dynamos. The earth's dynamo is unstable, as is shown by magnetic reversals,
when the polarity of the whole magnetic field changes over. These
have been a continuing feature of the earths history, with the
last about 500,000 years ago. In fact, some magnetic field changes
seen at the earth's surface with a timescale of a year or two
(magnetic jerks) may be produced by changes in the dynamo, although
this is still being argued. The importance of this is that we
cannot exactly predict magnetic values. We can describe the current
field, from observatory and satellite measurements, and how it
has changed from the previous field, which is calculated internationally
on a 5-year basis, but in perhaps 5 or 10 years there may be changes
which we can't foresee. Thus our predictions are somewhat unreliable.
Declination around New Zealand 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. Secondly, 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, which is the topic of the next section. The declination, or the angle between magnetic north and true
north, across New Zealand as at December 2004, is shown in Figure
1. Large-scale maps of New Zealand (including Topomaps
of scales 1 : 50000 and 1 : 250,000) currently use the New Zealand
Map Grid (NZMG), but will soon be changing to the New Zealand
Transverse mercator (NZTM). In both of these map projections 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. Figure 2
and Figure 3 show the
difference between Magnetic North and Grid North in the North
Island and South Island respectively for these maps. |
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Figure 1. Larger image here. |
Figure 2.Larger image here. |
 Figure 3. Larger image here. |
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The level of activity of the sun follows the well-known 11-year sunspot cycle. Currently activity is declining, with the lowest level of activity expected in late 2006. This means that magnetic storms are less likely to occur during the next few years.
Magnetic Observatories Another major use of magnetic observatories is the recording of short-period magnetic variations. Eyrewell is one of two Southern Hemisphere stations which contributes to the planetary index Kp, which is an indication of the strength of the magnetic variations in every 3-hour interval. Magnetic data from these observatories is sent to INTERMAGNET, an international organisation that archives and disseminates magnetic observatory data. In previous years, summarised data has been distributed in an Annual Report, which more recently was in microfiche format. We are now moving towards making a greater range of information available on the web, and future Annual Reports will be .pdf documents. Link to Annual Reports and Data archive >>
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