Chapter 33
Geomagnetism
33.1 Earth’s Magnetic Dipole
The Earth generates its own internal magnetic field, which is thought to be due to a westward-moving electric
current inside the Earth’s molten outer core. The resulting field approximates that of a magnetic dipole, with
the “poles” of the dipole near (but notat) the Earth’s geographic poles.
There is a bit of confusing nomenclature to be aware of. If you suspend a bar magnet by a string so that is
free to rotate horizontally, it will rotate to align itself with the Earth’s magnetic field, with theNpole pointing
toward geographic north. (That’s actually why the poles of a magnet are labeledNandS: theNpole is the
“north-seeking” pole and theSpole is the “south-seeking pole”.) But since unlike poles attract, the pole near
the Earth’s geographicnorthpole must be a magneticSpole, and vice versa (Fig. 33.1).
Figure 33.1: Schematic represenation of the Earth’s internal magnetic field. This should not be taken literally;
there is no bar magnet at the Earth’s center. This figure is just meant to illustrate that the Earth’s geographic
northpole is a magnetic dipoleSpole, and vice versa. Note that the Earth’s dipole is tilted with respect to
the geographic axis, which is vertical in this illustration. (©GNU-FDL, Wikimedia Commons [11].)
The Earth’s magnetic poles are not located at the geographic poles, but are some distance away; this is