The Earth's Magnetic Field

The Earth's magnetic field varies across the surface of the globe.  Thus, different geographic locations have different magnetic fields associated with them.  An animal with the ability to sense magnetic fields might therefore be able to use magnetic information to determine something about where it is relative to a home area.  To use such a "magnetic map, an animal would need the ability to distinguish among slightly different magnetic fields.  It would also need to learn, as it moves from place to place, something about how the magnetic field varies over the geographic area where it lives and migrates.

To understand how a sea turtle or other migratory animal might use magnetic information in a crude kind of map, let's look at the diagram of the Earth's field below.  To a first approximation, the field resembles that of an enormous bar magnet. The field lines emerge from the southern half of the Earth, wrap around the planet,  and re-enter in the northern half.

Several features of the Earth's field vary in a predictable way across the surface of the globe and might, in principle, be used in assessing geographic position.  For example, the field is strongest near the poles and weakest near the equator.  In addition, the field lines at each location on the Earth intersect the Earth's surface at a specific angle of inclination.  Near the equator, the field lines are parallel to the Earth's surface; the inclination angle there is said to be 0.  As one travels north from the equator, however, the field lines become progressively steeper.  At the northern magnetic pole, the field lines are directed almost straight down into the Earth and the inclination is said to be 90. Thus, inclination angle varies with latitude.

Magnetic isoline maps

To visualize how magnetic fields vary across the Earth's surface, magnetic field features can be plotted on geographic maps.  A common way of representing the field is to draw isolines of various magnetic field parameters on a map.  An isoline is just a line along which a given magnetic parameter is constant. 

The map on the right shows the isolines of magnetic field inclination along the east coast of the U.S.  Along the line labeled 59 degrees, the inclination angle is always 59 degrees; similarly, the inclination angle is 61 degrees along the 61 degree isoline.  Inclination angles increase as one goes north from the magnetic equator.

Along the east coast of the U.S., the isolines of inclination (or "isoclinics", as they are sometimes called) trend east-west, while the coastline runs approximately north-south.  As a consequence, every area of coast along the eastern seaboard of the U.S. is marked by a unique inclination angle.  A  turtle living in a coastal feeding area might therefore be able to use magnetic field information to navigate.  For example, if a turtle learns the inclination angle at a given feeding area along the coast, then returning to the area might be relatively simple.  The turtle might simply compare the inclination angle at its present location to the inclination angle that exists at its home area.  If the angle is too steep, the turtle knows that it is too far north and therefore needs to travel south to reach home.  If the angle is too shallow, then the  turtle knows it is too far south.  A similar strategy could be employed using isolines of intensity (strength), which  have a similar, although not identical, pattern of variation along the U.S. coast. 

Yet another possibility is that turtles detect at least two different magnetic features and use a form of "bicoordinate" magnetic navigation.  Such a system might enable turtles to return to a home area from nearly any oceanic location, not just those that lie north or south of the target.

These ideas, although interesting, have proven very difficult to study in the ocean, where numerous directional and positional cues simultaneously exist and conditions cannot be carefully controlled.  We have recently adopted a new approach to study whether magnetic maps exist in sea turtles.

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last edited 04/28/2004