Magnetic field sensitivity in animals.

Abstract

The earth is, among other things, a large magnet. The field, which measures about 0.5 Gauss (50,000 gamma) in the northeastern United States, is produced primarily in the earth's core. The exact mechanism is a subject of dispute, but most hypotheses invoke a circulation of the earth's molten interior (e.g. 68). The axis of this internal field is only roughly aligned with the planet's axis of rotation; the north magnetic pole is actually near Hudson's Bay in Canada, some 2300 km from the geographic north pole. The magnetic pole is not stable, but instead wanders slowly. Occasionally (every 10,000100,000 years) the field reverses itself. During these reversals the field felt on the surface de­ creases in intensity, but never reaches zero. The duration of these low-field interludes is not known; guesses range from days to centuries. When stable, the earth's magnetic field changes in a more or less regular way from the magnetic equator, where the field lines are horizontal (Le. parallel to the earth's surface) and the field strength is roughly 25,000 gamm a, to the magnetic poles, where the lines are vertical and the strength is 60,000 gamm a. Hence there is a gradient of magnetic intensity and field-line angle. In the northeastern United States the dip angle steepens by about 0.01° km-I to the NNW while the total field strength increases by roughly 3-5 gamma km -I in the same direction. The intensity of the horizontal and vertical components of