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The magnetic field is generated by a feedback loop: current loops generate magnetic fields (Ampère's circuital law); a changing magnetic field generates an electric field (Faraday's law); and the electric and magnetic fields exert a force on the charges that are flowing in currents (the Lorentz force). [58]
Study of Earth's magnetosphere began in 1600, when William Gilbert discovered that the magnetic field on the surface of Earth resembled that of a terrella, a small, magnetized sphere. In the 1940s, Walter M. Elsasser proposed the model of dynamo theory, which attributes Earth's magnetic field to the motion of Earth's iron outer core.
The magnetic field of a magnetic dipole has an inverse cubic dependence in distance, so its order of magnitude at the earth surface can be approximated by multiplying the above result with (R outer core ⁄ R Earth) 3 = (2890 ⁄ 6370) 3 = 0.093 , giving 2.5×10 −5 Tesla, not far from the measured value of 3×10 −5 Tesla at the equator.
Like the North Magnetic Pole, the North Geomagnetic Pole attracts the north pole of a bar magnet and so is in a physical sense actually a magnetic south pole. It is the center of the 'open' magnetic field lines which connect to the interplanetary magnetic field and provide a direct route for the solar wind to reach the ionosphere.
The condition governing this position is that the dynamic ram pressure from the solar wind is equal to the magnetic pressure from the Earth's magnetic field: [note 1] (()) where and are the density and velocity of the solar wind, and B(r) is the magnetic field strength of the planet in SI units (B in T, μ 0 in H/m).
The initial theory proposed in 2014 was that—due to the tilt in Earth's magnetic field axis—the planet's rotation generated an oscillating, weak electric field that permeates through the entire inner radiation belt. [26] A 2016 study instead concluded that the zebra stripes were an imprint of ionospheric winds on radiation belts. [27]
Subsequent studies have demonstrated that loggerhead and green turtles can also use the magnetic field of the earth as a map, because different parameters of the Earth's magnetic field vary with geographic location. The map in sea turtles was the first ever described though similar abilities have now been reported in lobsters, fish, and birds. [48]
The magnetic field of larger magnets can be obtained by modeling them as a collection of a large number of small magnets called dipoles each having their own m. The magnetic field produced by the magnet then is the net magnetic field of these dipoles; any net force on the magnet is a result of adding up the forces on the individual dipoles.