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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.