MadSci Network: Earth Sciences

Re: Why do lines of zero declination in Earth's magnetic field drift westward?

Date: Mon Oct 23 07:11:30 2006
Posted By: David McMillan, Post-doc/Fellow, Earth and Space Science, York University
Area of science: Earth Sciences
ID: 1160156816.Es

As a general rule, lines of zero magnetic declination don't drift to the west, but this question reveals interconnections that encourage us to take a brief tour of physics, history and Earth's core. Click here for some good introductory reading.

The geomagnetic field is a vector, which means that we can measure a direction as well as an "intensity" or magnitude. Some vector fields can be thought of as lines of force that emanate from a source and terminate at a sink; a special case has no sources or sinks. The vector direction is tangent to a field line and its magnitude is proportional to the density of field lines in space. When we talk about the direction of the geomagnetic field, we are describing the orientation of these lines (and their tangents) in space. If we were to suspend a small magnetized needle by a thread attached at its centre of mass, it would align along a field line, thereby approximating the direction of the magnetic vector at that location (see Tauxe, for example).

Declination is a partial measure of geomagnetic direction; it is the angle between compass direction and geographic north, which, strictly speaking, is measured in a horizontal plane. Geographic north is well approximated by the position of Polaris, the North Star. Declination varies globally, with typical values of +/- 15 degrees around the equator to mid latitudes and much larger nearer the poles. Since declination means that a compass generally doesn't point to geographic north, we need to know local declination when we use a compass for navigation. A line of zero magnetic declination is the collection of points along which the compass points directly to the geographic north pole and separates regions of positive (easterly) and negative (westerly) declinations.

Inclination, which is the angle the magnetic vector makes relative to the horizontal, is also a partial measure of geomagnetic field direction. Zero inclination means there is no vertical component of the magnetic field; the magnetic field line is exactly horizontal. A line of zero inclination forms a boundary between areas where lines of magnetic force are directed outward and areas where they are directed inward. Measurements of declination and inclination give the total direction of the magnetic field and are easily determined with simple mechanical devices (like the needle and thread above).

In addition to global variations in geomagnetic direction and intensity, there are also changes with time that are collectively called secular variation. "Westward drift" is a phrase that refers to the apparent motion over time of directional features such as Halley's line of zero declination. It is a kind of historical artifact from the era of European exploration to the west, when the first measurements of declination and inclination were made over the Atlantic ocean. Since these quantities vary across the globe, they were useful to explorers and sailors as navigation tools. For this reason navigators continued to take and upgrade observations of magnetic field direction. Geomagnetic intensity also varies with geographic location, but is much more difficult to measure, especially with the technological limitations of the day. The first contour map of magnetic declination in the Atlantic ocean was created in 1700 by Sir Edmund Halley, who has a comet named after him.

Over the next 400 years or so a lot of these measurements were made over the Atlantic region, which augmented land measurements in Europe but still left virtually no observations from the Pacific Ocean and the Americas. It is primarily this biased data coverage from which westward drift derives. For several decades following world war II, there was extensive air surveying of magnetic direction and intensity over much of the planet, especially over North America and the Pacific Ocean. In the past 25 years, we have been making observations of the magnetic vector from satellites. From this modern perspective, we know that in the Atlantic regions some magnetic observations seem to drift to the west, but it is not an all-encompassing feature; for example, there are areas over the Pacific where there is very little variation at all. So ultimately westward drift is really just one aspect of a much more complicated global secular variation, as seen in this animation. This secular variation demonstrates a source of similar variations in Earth's interior.

This brings us to Earth's fluid outer core, where the magnetic field sustained by the reinforcing effects of dynamo action in churning liquid iron. The changes in direction and intensity that we measure on Earth's surface are probably related to the movement of the liquid iron near the top of the core (near the core-mantle boundary). Although the situation is complicated by things such as the thickness of the mantle (3500 km) and uncertainty about the physical conditions down there, our knowledge of secular variation is one of the few direct windows to Earth's core and to understanding how the geodynamo works. In fact, it is now thought that the growth of "reverse flux patches" (regions in which magnetic field lines are reversed), which are bounded by lines of zero inclination, are related to the onset of geomagnetic field reversals.

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