A magnetosphere is the region around an astronomical object, in which phenomena are dominated by its magnetic field. Earth is surrounded by a magnetosphere, as are the magnetized planets Jupiter, Saturn, Uranus and Neptune. Mercury is magnetized, but too weakly to trap plasma. Mars has patchy surface magnetization.
Earth's magnetic field originates in its liquid core, where electric currents are excited by fluid flows (by a so-called dynamo process). The field's intensity is about 6×10-5 tesla at the magnetic poles , located about 10 degrees off the geographic poles. At the surface, the field resembles a dipole with irregular components added, and the field is largely dipole like to distances of 5 to 8 radii of Earth (RE).
The distant field of Earth is greatly modified by the solar wind, a hot outflow from the sun, consisting of solar ions (mainly hydrogen) moving at about 400 km/s (proton energy 1 kiloelectronvolt) with typical density at Earth's orbit of 6 ions/cm³. Earth's field forms an obstacle to the solar wind, which confines its field lines and plasmas into an elongated cavity, known as Earth's magnetosphere. The boundary between the two is called the magnetopause.
Outside the magnetopause is the bow shock, when the velocity of the solar wind abruptly drops as it approaches the magnetosphere. On the sun's side of Earth, the magnetopause distance is approximately 10 Earth radii. Abreast of Earth the distance grows to about 15 earth radii (distances change with solar wind pressure and density; The magnetosphere is made to flap and compress by the solar wind) while on the night side it extends into a long cylindrical magnetotail at least several hundred radii long, gradually turning into a wake.
The magnetosphere contains magnetically trapped plasma (gas of free ions and electrons). One distinguishes the inner radiation belt, a by-product of cosmic radiation discovered in 1958 by James Van Allen using the Explorer 1 and 3 satellites, and the ring current, a large belt of lower energy particles deposited mainly by magnetic storms, source of a widespread magnetic field of its own. The trapped plasma interacts with the low-density conductive plasma of the ionosphere, the upper layer of the atmosphere.
The ionosphere is formed as sunlight, especially ultraviolet, hits the upper atmosphere. It is used to reflect radio waves for communications.
Some scientists believe that without a magnetosphere, Earth would have lost the majority of its water and atmosphere, and resemble Mars or Mercury. However, Venus retains a dense atmosphere even though it lacks any magnetic field.
History of magnetospheric physics
The Earth's magnetosphere was discovered in 1958 by Explorer I during the research performed for the International Geophysical Year. Before this, scientists knew electric currents did flow in space, because solar eruptions sometimes led to "magnetic storm" disturbances. No one knew however where those currents flowed and why, and the solar wind was also unknown.
1γ = 10-5 oersted = Dynamic range of instrumentation
See also Earth's magnetic field, magnetopause, heliopause, interplanetary magnetic field , plasma physics, ring current, Van Allen radiation belt, solar flare, magnetic storm, polar aurora
List of satellites which have provided data on the magnetosphere
For applications to spacecraft propulsion see magnetic sail.
-  Storms from the Sun by M. Carlowicz and R. Lopez "(2002)"
"The Exploration of the Earth's Magnetosphere" home page http://www.phy6.org/Education/Intro.htm (also in Spanish and French)
"The Great Magnet, the Earth" home page http://www.phy6.org/earthmag/demagint.htm (also in Spanish French and German)
Last updated: 09-12-2005 02:39:13