Earth's
magnetic field (and the surface magnetic field) is approximately
a magnetic dipole, with one pole near the geographic north pole
and the other near the geographic south pole. An imaginary line
joining the magnetic poles would be inclined by approximately
11.3° from the planet's axis of rotation. The cause of the
field is probably explained by dynamo theory. The magnetic field
extends several tens of thousands of kilometres into space as
the magnetosphere.
The
field is similar to that of a bar magnet, but this similarity
is superficial. The magnetic field of a bar magnet, or any other
type of permanent magnet, is created by the coordinated motions
of electrons (negatively charged particles) within iron atoms.
The Earth's core, however, is hotter than 1043 K, the Curie point
temperature at which the orientations of electron orbits within
iron become randomized. Such randomization tends to cause the
substance to lose its magnetic field. Therefore the Earth's magnetic
field is caused not by magnetised iron deposits, but mostly by
electric currents in the liquid outer core.
Another
feature that distinguishes the Earth magnetically from a bar magnet
is its magnetosphere. At large distances from the planet, this
dominates the surface magnetic field.
Electric
currents induced in the ionosphere also generate magnetic fields.
Such a field is always generated near where the atmosphere is
closest to the Sun, causing daily alterations which can deflect
surface magnetic fields by as much as one degree.
The
Earth's magnetic field reverses at intervals, ranging from tens
of thousands to many millions of years, with an average interval
of approximately 250,000 years. It is believed that this last
occurred some 780,000 years ago, referred to as the Brunhes-Matuyama
reversal.
The
mechanism responsible for geomagnetic reversals is not well understood.
Some scientists have produced models for the core of the Earth
wherein the magnetic field is only quasi-stable and the poles
can spontaneously migrate from one orientation to the other over
the course of a few hundred to a few thousand years. Other scientists
propose that the geodynamo first turns itself off, either spontaneously
or through some external action like a comet impact, and then
restarts itself with the "North" pole pointing either
North or South. When the "North" reappears in the opposite
direction, we would interpret this as a reversal, whereas turning
off and returning in the same direction is called a geomagnetic
excursion.
At
present, the overall geomagnetic field is becoming weaker at a
rate which would, if it continues, cause the field to disappear,
albeit temporarily, by about 3000-4000 AD. The rapid deterioration
began at least 150 years ago and has accelerated in the past several
years, with a total decrease of 10-15% over these 150 years. This
change is within the normal range of variation, as shown by study
of magnetic fields in rocks, and need not necessarily lead to
a reversal.
