Past plate motions - The Solid Earth - page 45

Figure 3.2. A magnetic dipole (e.g., a bar magnet) at the centre of the Earth has a
magnetic field that is a good first approximation to the Earth's magnetic field. Note
that the geomagnetic North Pole is so called because the north end of the compass
needle points towards it (it is therefore actually a south magnetic pole). Such a
dipole field would also be produced by a uniformly magnetized Earth, a uniformly
magnetized core or particular current systems within the core (Fig. 8.23). The origin
of the field and models of the core and possible reasons for the reversals of the
Earth's magnetic field are discussed further in Section 8.3.2.

magnitude of the IGRF for 1995. To a first approximation the Earth's magnetic
field is a dipole field . 2 This means that the Earth's magnetic field can be repre-
sented by a magnetic dipole situated at the centre of the Earth (imagine a bar
magnet at the centre of the Earth, Fig. 3.2). This fact was first pointed out in 1600
by Sir William Gilbert. 3 The difference between the Earth's magnetic field and
the best dipole field is termed the non-dipole field .Figure 3.1(b) shows the mag-
nitude of the non-dipole field. At the Earth's surface the non-dipole field is small
compared with the dipole field, though this is not the case for the field at the
core-mantle boundary.
Today, the best-fitting dipole is aligned at about 11.5 ◦ to the Earth's geographic
north-south axis (spin axis). The geomagnetic poles are the two points where the
axis of this best-fitting dipole intersects the Earth's surface. Now at 79 ◦ N, 71 ◦ W
and 79 ◦ S, 109 ◦ E, they are called the geomagnetic north and g eomagnetic south
poles , respectively. The geomagnetic equator is the equator of the best-fitting
dipole axis. The two points on the Earth's surface at which the magnetic field
is vertical and has no horizontal component are called the magnetic poles ,or
dip poles . The present north magnetic pole is at 76 ◦ N, 101 ◦ W, and the south
magnetic pole is at 66 ◦ S, 141 ◦ E. The magnetic equator is the line along which
the magnetic field is horizontal and has no vertical component. If the field were

2

The magnetic field produced by a bar magnet is a dipole field.

3

William Gilbert was a distinguished physician. He was President of the College of Physicians and
physician to Queen Elizabeth I and King James I. His topic De Magnete is in print in translation.