Past plate motions - The Solid Earth - page 47

Figure 3.4. The Earth's
magnetic field far from
the Earth is controlled
largely by the solar wind.
The Van Allen radiation
belts are the cross-
hatched regions either
side of the Earth. (After
Hutton (1976).)

Magnetopause

Solar

Magnetotail

wind

Magnetosheath

exactly a dipole field, these magnetic poles and this equator would be coincident
with the geomagnetic north and south poles and equator. The various poles and
equators are illustrated in Fig. 3.3.
Figure 3.4 shows the magnetic field lines around the Earth. The Sun plays
the major part in the shape of the field far from the Earth and in the short-
term variations of the field. The solar wind ,aconstant stream of ionized parti-
cles emitted by the Sun, confines the Earth's magnetic field to a region known
as the magnetosphere and deforms the field lines so that the magnetosphere
has a long 'tail', the magnetotail ,which extends several million kilometres
away from the Sun. A shock wave, the bow shock ,isproduced where the solar
wind is slowed by interaction with the Earth's magnetic field. A turbulent zone
within the bow shock is known as the magnetosheath , the inner boundary of
which is called the magnetopause . The Earth's magnetic field shields the Earth
from most of the incident radiation, and the atmosphere absorbs much of the
remainder. Major sunspot activity causes changes in the solar wind, which in
turn result in short-term fluctuations in the magnetic field known as magnetic
storms . The Va n Allen radiation belts are zones of charged particles trapped
by the Earth's magnetic field. Any changes in the solar wind, and hence in
the magnetic field, can allow these charged particles to enter the upper atmo-
sphere, where they cause the spectacular auroral displays known as the Northern
and Southern Lights . The active processes and interactions which take place
between the magnetosphere and the solar wind are collectively referred to as
'space weather'. The Sun is also responsible for the diurnal (daily) variation in
the Earth's magnetic field. This variation, which has an amplitude of less than
0.5% of the total field, is the main short-period variation in the Earth's magnetic
field.
The long-term reversals of the magnetic field, mentioned in Section 3.1.1, are
used to date the oceanic lithosphere. However, the geomagnetic field changes