33.4 Magnetic Reversals
As mentioned earlier, the Earth’s geographicnorthpole is a magneticSpole. This hasn’t always been the
case, though. The Earth’s magnetic field actually reverses direction at irregular intervals; the last such reversal
was about780;000years ago. Figure 33.3 shows the history of the Earth’s magnetic field reversals going back
to the late Cretaceous period.
Exactly what causes these reversals of the Earth’s magnetic field is unclear, although they have been
reproduced in computer simulations. It appears that the reversal process is quite sudden in geological terms
— it may take a few decades to a century or so for the magnetic field to reverse, after which it typically stays
fairly stable for thousands of years before reversing again. Since these magnetic reversals occur at irregular
intervals, we have no way of knowing when the next one will be. There is occasional speculation that the
polar wandering may indicate that a magnetic reversal may be going on now, but nobody knows for certain.
How do we know when magnetic reversals have occurred in the past? At the mid-Atlantic ridge in the
middle of the Atlantic ocean, the Earth’s crust is spreading apart, and new crust is formed as magma seeps up
into the crack. As it cools to form rock, this magma “locks in” the direction of the magnetic field at the time
it cooled. The result is a set of bands of magnetism on either side of the mid-Atlantic ridge, which records
the past magnetic field direction in very much the same way a tape recorder works (Fig. 33.4).
It is not clear what effect, if any, magnetic reversals have on life on Earth. The fossil record doesn’t show
any correlation between magnetic reversals and mass extinctions, so we can probably infer that any effect on
life is relatively minor.
33.5 The Magnetosphere
Although the Earth’s magnetic field resembles that of a magnetic dipole near the Earth, further away the
dipole becomes distorted due to the presence of thesolar wind, a “wind” of charged particles (mostly protons
and electrons) ejected by the Sun. The solar wind compresses the day side of the Earth’s magnetic field, and
draws the night side out into a longmagnetotail. The presence of the solar wind causes the Earth’s entire
magnetic field to be encapsulated into a structure called themagnetosphere(Fig. 33.5).
The Earth’s magnetic field serves a very important biological role: it deflects potentially dangerous
charged particles from the Sun so that they move harmlessly around the Earth. Without the Earth’s magnetic
field, we would be bombarded by high-energy solar radiation, which could lead to severe health problems
and even death.
The magnetosphere is a fairly complex structure, with various plasmas and electric currents interacting
with the Earth’s magnetic field; these in turn produce magnetic fields of their own, etc. One of the goals of the
field ofspace physicsis to investigate this complex structure of the magnetosphere in detail and to understand
how it all works.
33.6 The Aurora
In far northern latitudes, one may see the “northern lights”, oraurora borealison some nights, especially
during periods of high solar activity (Fig. 33.6). A similar phenomenon is visible in the southern hemisphere,
called theaurora australis.
Auroræ are produced when charged particles from the Sun reach the Earth’s magnetosphere. If the Sun’s
magnetic field lines are pointing southward at the Earth, they meet the Earth’s northward-pointing magnetic
field lines in an event calledmagnetic reconnection. When the Earth’s magnetic field lines reconnect with the
Sun’s magnetic field lines, the Earth’s lines drape back toward the magnetotail, carrying a load of charged
particles with them. A similar reconnection event in the magnetotail causes the magnetic field lines to snap
back like rubber bands, and carry a load of charged particles back toward the Earth, where the enter the polar