A
magnetosphere is the area of
space near an
astronomical object in which
charged particles are controlled by that object's
magnetic field.
[1][2] Near the surface of the object, the
magnetic field lines resemble those of a
magnetic dipole.
Farther away from the surface, the field lines are significantly
distorted by electric currents flowing in the plasma (e.g. in
ionosphere or
solar wind).
[3][4] When speaking about Earth,
magnetosphere is typically used to refer to the outer layer of the
ionosphere,
[3] although some sources consider the ionosphere and magnetosphere to be separate.
[2]
History
Study of Earth's magnetosphere began in 1600, when
William Gilbert discovered that the magnetic field on the surface of Earth resembled that on 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. Through the use of
magnetometers,
scientists were able to study the variations in Earth's magnetic field
as functions of both time and latitude and longitude. Beginning in the
late 1940s, rockets were used to study
cosmic rays. In 1958,
Explorer 1, the first of the
Explorer series
of space missions, was launched to study the intensity of cosmic rays
above the atmosphere and measure the fluctuations in this activity. This
mission observed the existence of the
Van Allen radiation belt (located in the inner region of Earth's magnetosphere), with the
Explorer 3 mission later that year definitively proving its existence. Also in 1958,
Eugene Parker proposed the idea of the
solar wind. In 1959, the term magnetosphere was proposed by
Thomas Gold.
The Explorer 12 mission in 1961 led to the observation by Cahill and
Amazeen in 1963 of a sudden decrease in the strength of the magnetic
field near the noon meridian, later named the
magnetopause. In 1983, the
International Cometary Explorer observed the magnetotail, or the distant magnetic field.
[4]
Types of magnetospheres
The structure and behavior of magnetospheres is dependent on several
variables: the type of astronomical object, the nature of sources of
plasma and momentum, the
period
of the object's spin, the nature of the axis whereabout the object
spins, the axis of the magnetic dipole, and the magnitude and direction
of the
velocity of the flow of solar wind.
The distance at which a planet can withstand the solar wind pressure
is called the Chapman–Ferraro distance. This is modeled by a formula
wherein
represents the radius of the planet,
represents the magnetic field on the surface of the planet at the equator, and
represents the velocity of the solar wind.
A magnetosphere is classified as "intrinsic" when
, or when the primary opposition to the flow of solar wind is the magnetic field of the object.
Mercury, Earth,
Jupiter,
Ganymede,
Saturn,
Uranus, and
Neptune exhibit intrinsic magnetospheres. A magnetosphere is classified as "induced" when
,
or when the solar wind is not opposed by the object's magnetic field.
In this case, the solar wind interacts with the atmosphere or ionosphere
of the planet (or surface of the planet, if the planet has no
atmosphere).
Venus has an induced magnetic field. What this means is that because Venus appears to have no
internal dynamo effect, the only magnetic field present is that formed by the solar wind's wrapping around the physical obstacle of Venus (see also
Venus' Induced Magnetosphere). When
, the planet itself and its magnetic field both contribute. It is possible that
Mars is of this type.
[5]
Structure
An artist's rendering of the structure of a magnetosphere. 1) Bow shock.
2) Magnetosheath. 3) Magnetopause. 4) Magnetosphere. 5) Northern tail
lobe. 6) Southern tail lobe. 7) Plasmasphere.
Bow shock
The bow shock forms the outermost layer of the magnetosphere: the
boundary between the magnetosphere and the ambient medium. For stars,
this is usually the boundary between the
stellar wind and
interstellar medium; for planets, the speed of the solar wind there plummets as it approaches the magnetopause.
[6]
Magnetosheath
Main article:
Magnetosheath
The magnetosheath is the region of the magnetosphere between the bow
shock and the magnetopause. It is formed mainly from shocked solar wind,
though it contains a small amount of
plasma from the magnetosphere.
[7] It is an area exhibiting high particle
energy flux,
where the direction and magnitude of the magnetic field varies
erratically. This is caused by the collection of solar wind gas that has
effectively undergone
thermalization.
It acts as a cushion that transmits the pressure from the flow of the
solar wind and the barrier of the magnetic field from the object.
[4]
Magnetopause
Main article:
Magnetopause
The magnetopause is the area of the magnetosphere wherein the
pressure from the planetary magnetic field is balanced with the pressure
from the solar wind.
[3]
It is the convergence of the shocked solar wind from the magnetosheath
with the magnetic field of the object and plasma from the magnetosphere.
Because both sides of this convergence contain magnetized plasma, the
interactions between them are very complex. The structure of the
magnetopause depends upon the
Mach number and
beta of the plasma, as well as the magnetic field.
[8] The magnetopause changes size and shape as the pressure from the solar wind fluctuates.
[9]
Magnetotail
Opposite the compressed magnetic field is the magnetotail, where the
magnetosphere extends far beyond the astronomical object. It contains
two lobes, referred to as the northern and southern tail lobes. The
northern tail lobe points towards the object and the southern tail lobe
points away. The tail lobes are almost empty, with very few charged
particles opposing the flow of the solar wind. The two lobes are
separated by a plasma sheet, an area where the magnetic field is weaker
and the density of charged particles is higher.
[10]
Earth's magnetosphere
Artist's rendition of Earth's magnetosphere
Diagram of Earth's magnetosphere
Over Earth's
equator,
the magnetic field lines become almost horizontal, then return to
connect back again at high latitudes. However, at high altitudes, the
magnetic field is significantly distorted by the solar wind and its
solar magnetic field. On the dayside of Earth, the magnetic field is
significantly compressed by the solar wind to a distance of
approximately 65,000 kilometers (40,000 mi). Earth's bow shock is about
17 kilometers (11 mi) thick
[11] and located about 90,000 kilometers (56,000 mi) from Earth.
[12]
The magnetopause exists at a distance of several hundred kilometers off
earth's surface. Earth's magnetopause has been compared to a
sieve because it allows solar wind particles to enter.
Kelvin–Helmholtz instabilities
occur when large swirls of plasma travel along the edge of the
magnetosphere at a different velocity from the magnetosphere, causing
the plasma to slip past. This results in
magnetic reconnection, and as the magnetic field lines break and reconnect, solar wind particles are able to enter the magnetosphere.
[13]
On Earth's nightside, the magnetic field extends in the magnetotail,
which lengthwise exceeds 6,300,000 kilometers (3,900,000 mi).
[3] Earth's magnetotail is the primary source of the
polar aurora.
[10]
Also, NASA scientists have suggested or "speculated" that Earth's
magnetotail can cause "dust storms" on the Moon by creating a potential
difference between the day side and the night side.
[14]
Other objects
The
magnetosphere of Jupiter
is the largest planetary magnetosphere in the Solar System, extending
up to 7,000,000 kilometers (4,300,000 mi) on the dayside and almost to
the orbit of
Saturn on the nightside.
[15] Jupiter's magnetosphere is stronger than Earth's by an
order of magnitude, and its
magnetic moment is approximately 18,000 times larger.
[16]
See also
Geomagnetism –
Wikipedia book
References
- "Magnetospheres". NASA Science. NASA.
- Ratcliffe, John Ashworth (1972). An Introduction to the Ionosphere and Magnetosphere. CUP Archive. ISBN 9780521083416.
- "Ionosphere and magnetosphere". Encyclopedia Britannica. Encyclopedia Britannica, Inc. 2012.
- Van Allen, James Alfred (2004). Origins of Magnetospheric Physics. Iowa City, Iowa, USA: University of Iowa Press. ISBN 9780877459217. OCLC 646887856.
- Blanc, M.; Kallenbach, R.; Erkaev, N.V. (2005). "Solar System Magnetospheres". Space Science Reviews (116): 227–298. Bibcode:2005SSRv..116..227B. doi:10.1007/s11214-005-1958-y.
- Sparavigna, A.C.; Marazzato, R. (10 May 2010). "Observing stellar bow shocks" (PDF).
- Paschmann, G.; Schwartz, S.J.; Escoubet, C.P. et al., eds. (2005). "Outer Magnetospheric Boundaries: Cluster Results". Space Science Reviews (Dordrecht, The Netherlands: Springer) 118 (1-4). ISBN 1-4020-3488-1.
- Russell, C.T. (1990). "The Magnetopause". Physics of Magnetic Flux Ropes (Washington, D.C., USA: American Geophysical Union): 439–453.
- "The Magnetopause". NASA.
- "The Tail of the Magnetosphere". NASA.
- "Cluster reveals Earth's bow shock is remarkably thin". European Space Agency. 16 November 2011.
- "Cluster reveals the reformation of Earth's bow shock". European Space Agency. 11 May 2011.
- "Cluster observes a 'porous' magnetopause". European Space Agency. 24 October 2012.
- http://www.nasa.gov/topics/moonmars/features/magnetotail_080416.html NASA, The Moon and the Magnetotail
- Khurana, K.K.; Kivelson, M.G. et al. (2004). "The configuration of Jupiter's magnetosphere" (PDF). In Bagenal, F.; Dowling, T.E.; McKinnon, W.B. Jupiter: The Planet, Satellites and Magnetosphere. Cambridge University Press. ISBN 0-521-81808-7.
- Russell, C.T. (1993). "Planetary Magnetospheres" (PDF). Reports on Progress in Physics 56 (6): 687–732. Bibcode:1993RPPh...56..687R. doi:10.1088/0034-4885/56/6/001.
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