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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 magnetosphere
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 [1], 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 kilo-electron-volt) 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.
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
INCOMPLETE
Related topics
See also Earth's magnetic field, magnetopause, heliopause,
interplanetary magnetic field, plasma
physics, ring current, Van Allen radiation belt, solar flare,
magnetic storm, northern lights
List of satellites which have provided data on the
magnetosphere
For applications to spacecraft propulsion see magnetic sail.
References
"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)
External links
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