Van Allen radiation belt
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225653700
2008-07-14T19:19:26Z
Leitha Fea
7034319
/* References in popular culture */
[[Image:Van Allen radiation belt.svg|thumb|350px|Van Allen radiation belts]]
The '''Van Allen Radiation Belt''' is a [[torus]] of [[energy|energetic]] [[charged particle]]s ([[Plasma (physics)|plasma]]) around [[Earth]], held in place by Earth's [[magnetic field]]. The earth's geomagnetic field is not uniformly distributed around its surface. On the sun side, it is compressed because of the [[solar wind]] and on the other side, it is elongated to around 3 earth radii. This creates a cavity called the Chapman Ferraro Cavity, in which the Van Allen radiation belts reside. The Van Allen belts are closely related to the [[polar aurora]] where particles strike the upper [[Earth's atmosphere|atmosphere]] and [[fluorescence|fluoresce]].
The possibility of trapped charged particles had previously been investigated by [[Kristian Birkeland]], [[Carl Størmer]], and [[Nicholas Christofilos]]<ref>See "[http://www-istp.gsfc.nasa.gov/Education/whtrap1.html Trapped Radiation -- History]" by Dr. David P. Stern and Dr. Mauricio Peredo</ref> prior to the [[Space Age]]. The existence of the belt was confirmed by the [[Explorer 1]] and [[Explorer 3]] missions in early 1958, under Dr. [[James Van Allen]] at the [[University of Iowa]]. The trapped radiation was first mapped out by [[Sputnik 3]], [[Explorer 4]], [[Pioneer 3]] and [[Luna 1]].
Energetic electrons form two distinct radiation belts, while protons form a single belt. Within these belts are particles capable of penetrating about 1 [[gram|g]]/[[centimetre|cm]]<sup>2</sup> <ref>[[The Radiation Belt and Magnetosphere]] by [[Wilmot N. Hess|Wilmot Hess]] ([[1968]])</ref> of shielding (e.g., 1 [[millimetre]] of [[lead]]).
The term ''Van Allen Belts'' refers specifically to the radiation belts surrounding [[Earth]]; however, similar radiation belts have been discovered around other [[planet]]s. The [[Sun]] does not support long-term radiation belts. The Earth's atmosphere limits the belts' particles to regions above 200-1,000 [[kilometer|km]],<ref name="intro">''Introduction to Geomagnetically Trapped Radiation'' by [[Martin Walt]] ([[1994]])</ref> while the belts do not extend past 7 [[Earth radius|Earth radii]] ''R<sub>E</sub>''.<ref name="intro"/> The belts are confined to an area which extends about 65[[degree (angle)|°]]<ref name="intro"/> from the [[celestial sphere|celestial equator]].
An upcoming NASA mission, [[Radiation Belt Storm Probes]] will go further and gain scientific understanding (to the point of predictability) of how populations of [[relativistic electron]]s and ions in space form or change in response to changes in solar activity and the solar wind.
==Outer belt==
[[Image:Birkeland-anode-globe-fig259.jpg|thumb|300px|Laboratory simulation of the Van Allen belts' influence on the Solar Wind; these auroral-like [[Birkeland current]]s were created by the scientist [[Kristian Birkeland]] in his [[terrella]], a magnetized anode globe in an evacuated chamber.]]
The large outer radiation belt extends from an altitude of about (3 to 10 Earth radii) above the Earth's surface, and its greatest intensity is usually around 4-5 ''R<sub>E</sub>''. Outer electron radiation belt is mostly produced by the inward radial diffusion [e.g. Elkinkington et al., 2001; Shprits and Thorne, 2004] and local acceleration [Horne et al., 2005; Shprits et al., 2006] due to transfer of energy from whistler mode plasma waves to radiation belt electrons. Radiation belt electrons are also constantly removed by collisions with atmospheric neutrals[Thorne et al., 2005], losses to magnetopause, and the outward radial diffusion[Shprits et al., 2006]. The outer belt consists mainly of high energy(0.1–10 MeV) electrons trapped by the Earth's [[magnetosphere]]. The [[gyroradius|gyroradii]] for energetic protons would be large enough to bring them into contact with the Earth's atmosphere. The electrons here have a high [[flux]] and at the outer edge (close to the [[magnetopause]]), where geomagnetic field lines open into the geomagnetic "tail", fluxes of energetic electrons can drop to the low interplanetary levels within about 100 km (a decrease by a factor of 1,000).
The trapped particle population of the outer belt is varied, containing electrons and various ions. Most of the ions are in the form of energetic protons, but a certain percentage are [[alpha particles]] and O<sup>+</sup> oxygen ions, similar to those in the [[ionosphere]] but much more energetic. This mixture of ions suggests that [[ring current]] particles probably come from more than one source.
The outer belt is larger than the inner belt, and its particle population fluctuates widely. Energetic (radiation) particle fluxes can increase and decrease dramatically as a consequence of [[geomagnetic storms]], which are themselves triggered by magnetic field and plasma disturbances produced by the Sun. The increases are due to storm-related injections and acceleration of particles from the tail of the magnetosphere.
There is debate as to whether the outer belt was discovered by the [[United States|US]] [[Explorer 4]] or the [[USSR]] [[Sputnik 2]]/[[Sputnik 3|3]].
==Inner belt==
The inner Van Allen Belt extends from an altitude of 700–10,000 km (0.1 to 1.5 Earth radii) above the Earth's surface, and contains high concentrations of energetic protons with energies exceeding 100 MeV and electrons in the range of hundreds of kiloelectronvolts, trapped by the strong (relative to the outer belts) magnetic fields in the region.
It is believed that protons of energies exceeding 50 MeV in the lower belts at lower altitudes are the result of the [[beta decay]] of [[neutrons]] created by cosmic ray collisions with nuclei of the upper atmosphere. The source of lower energy protons is believed to be proton diffusion due to changes in the magnetic field during geomagnetic storms. <ref>{{cite book | first=Thomas F. | last=Tascione | title=Introduction to the Space Environment, 2nd. Ed. | publisher=Kreiger Publishing CO.| location=Malabar, Florida USA | year=1994 | id=ISBN 0-89464-044-5}}</ref>
Due to the slight offset of the belts from Earth's geometric center, the inner Van Allen belt makes its closest approach to the surface at the [[South Atlantic Anomaly]].
==Impact on space travel==
[[Solar cell]]s, [[integrated circuit]]s, and [[sensor]]s can be damaged by [[radiation]]. In [[1962]], the Van Allen belts were temporarily amplified by a high-altitude [[nuclear weapon|nuclear explosion]] (the [[Starfish Prime]] test) and several [[satellite]]s ceased operation.{{Fact|date=June 2008}} [[Geomagnetic storm]]s occasionally damage electronic components on spacecraft. Miniaturization and [[Digital electronics|digitization]] of [[electronics]] and [[logic gate|logic circuit]]s have made satellites more vulnerable to radiation, as incoming ions may be as large as the circuit's [[electric charge|charge]]. Electronics on satellites must be [[Radiation hardening|hardened]] against radiation to operate reliably. The [[Hubble Space Telescope]], among other satellites, often has its sensors turned off when passing through regions of intense radiation.
During [[Project Apollo]], astronauts traveled through the Van Allen belts on both the outbound and return trips to the moon. The crews spent only limited time in transit in the region, and consequently the radiation exposure was limited. The [[Apollo 14]] crew recorded the highest Van Allen belt exposures during their February 1971 mission, but the crew's short-term exposure was still within acceptable levels. Future manned missions beyond earth orbit must also transit the Van Allen belts, but these missions will be shielded and hardened for much longer-duration exposure to [[cosmic rays]] and [[solar wind]].
An object satellite shielded by 3 [[millimeter|mm]] of [[aluminium]] in an elliptic orbit passing through the radiation belt will receive about 2,500 [[Röntgen equivalent man|rem]] (25 [[sievert|Sv]]) per [[year]].<ref>{{cite web | author=Ptak, Andy| year=1997| title=Ask an Astrophysicist | publisher = NASA GSFC |
url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970228a.html | accessdate=2006-06-11}}</ref>
==Causes==
[[Image:simulated-van-allen-belts.jpg|thumb|350px|Simulated Van Allen Belts generated by a [[plasma thruster]] in tank #5 Electric Propulsion Laboratory at the then-called [[Lewis Research Center]], Cleveland Ohio,]]
It is generally understood that the inner and outer Van Allen belts result from different processes. The inner belt, consisting mainly of energetic protons, is the product of the decay of albedo neutrons which are themselves the result of cosmic ray collisions in the upper atmosphere. The outer belt consists mainly of electrons. They are injected from the geomagnetic tail following geomagnetic storms, and are subsequently energized though [[Two stream instability#Wave-particle interactions|wave-particle interactions]]. Particles are trapped in the Earth's magnetic field because it is basically a [[magnetic mirror]]. Particles gyrate around field lines and also move along field lines. As particles encounter regions of stronger magnetic field where field lines converge, their "longitudinal" velocity is slowed and can be reversed, reflecting the particle. This causes the particle to bounce back and forth between the earth's poles, where the magnetic field increases.
A gap between the inner and outer Van Allen belts, sometimes called safe zone or safe slot, is caused by the very low frequency (VLF) waves which scatter particles in [[pitch angle]] which results in the loss of particles to the atmosphere. Solar outbursts can pump particles into the gap but they drain again in a matter of days. The radio waves were originally thought to be generated by turbulence in the radiation belts, but recent work by [[James Green]] of the NASA [[Goddard Space Flight Center]] comparing maps of lightning activity collected by the [[Micro Lab 1]] spacecraft with data on radio waves in the radiation-belt gap from the [[IMAGE]] spacecraft suggests that they're actually generated by lightning within Earth's atmosphere. The radio waves they generate strike the ionosphere at the right angle to pass through it only at high latitudes, where the lower ends of the gap approach the upper atmosphere. These results are still under scientific debate.
There have been nuclear tests in space that have caused artificial radiation belts. [[Starfish Prime]], a high altitude nuclear test created an artificial radiation belt that damaged or destroyed as many as one third of the satellites in low earth orbit at the time. [[Thomas Gold]] has argued that the outer belt is left over from the [[polar aurora|aurora]] while [[Alex Dessler]] has argued that the belt is a result of [[volcano|volcanic activity]].
In another view, the belts could be considered a flow of electric current that is fed by the solar wind. With the protons being positive and the electrons being negative, the area between the belts is sometimes subjected to a current flow, which "drains" away. The belts are also thought to drive auroras, lightning and many other electrical effects.
== Removal==
The belts are a hazard for artificial satellites and moderately dangerous for human beings, difficult and expensive to shield against.
There is a proposal by the late [[Robert Forward|Robert L. Forward]] called [[HiVolt]] which may be a way to drain at least the inner belt to 1% of its natural level within a year. The proposal involves deploying highly electrically charged tethers in orbit. The idea is that the electrons would be deflected by the large electrostatic fields and intersect the atmosphere and harmlessly dissipate.
<ref>[http://www.space.com/scienceastronomy/radiation_belts_020916.html Proposal: Removing Earth's Radiation Belts]</ref>
==References in popular culture==
* In a Halloween special of ''[[The Simpsons]]'' (The "Attack of the 50-foot Eyesores" section of "[[Treehouse of Horror VI]]"), a radio broadcast warns Homer that, "Astronomers from Tacoma to Vladivostok have just reported an ionic disturbance in the vicinity of the Van Allen Belt. Scientists are recommending that necessary precautions be taken." These cause large advertising logos to come to life and cause havoc and destruction across Springfield.
* In Episodes 44 and 45, "The Sky is Falling Part One and Part Two" of ''[[Battle of The Planets]]'', The Van Allen Radiation Belt is the source of Spectra's newest weapon, The X-3 Plan, which launches rockets into the Belt, lowering it disintegrating it causing the ionization of the Earth's atmosphere.
* In the 1961 movie ''[[Voyage to the Bottom of the Sea]]'', the Van Allen radiation belt catches fire, threatening Earth.
* In the cartoon ''[[The Adventures of Jimmy Neutron]]'', an episode titled "The 'N' Men" (spoof of the [[X-Men]]) had Jimmy and his friends careening out of control through the Van Allen radiation belt, giving them all superpowers based on what each was doing at the moment they went through the radiation belt. This situation parodies the origins of the [[Fantastic Four]].
* In [[Osamu Tezuka]]'s manga series "[[Astro Boy]]", Astro Boy uses the Van Allen Belt to destroy parasites from outer space.
* In [[GONZO]]'s OVA [[Blue Submarine no. 6]], the antagonist Zorndyke has shifted the poles, resulting in the destruction of the van Allen radiation belt. Therefore, melting the poles and increasingly heating the air, the countries are flooded, resulting in genocide.
* In the 1970 [[Doctor Who]] episode "[[Doctor Who and the Silurians]]", the Silurians plan to attack the Van Allen Belt to cause the Earth to warm, killing all the humans.
* The Van Allen Belt is mentioned in the 1979 [[B-52s]] song, "There's a Moon in the Sky (Called the Moon)."
* Some additional information and discussion about the Van Allen Belt may be accessed in "[[A Funny Thing Happened on the Way to the Moon]]", a 2001 documentary written, produced, and directed by [[Bart Winfield Sibrel]].
* The Van Allen Belt is also mentioned in the song "Poet Laureate II" by rap artist [[Canibus]] on his 2003 CD ''Rip The Jacker''.
==References==
<div class="references-small">
<references />
# Holmes-Siedle, A. G. and Adams, L (2002). Handbook of Radiation Effects (Oxford University Press, England 2002). ISBN 0-19-850733-X
# Adams, L., Harboe Sorensen, R., Holmes Siedle, A. G., Ward, A. K. and Bull, R. (1991). Measurement of SEU and total dose in geostationary orbit under normal and solar flare conditions. IEEE Transactions on Nuclear Science. NS 38 (6) 1686-92 (Dec 1991)
</div>
==See also==
*[[L-shell]]
*[[List of artificial radiation belts]]
*[[Space weather]]
== External links ==
*[http://www.phy6.org/Education/Iradbelt.html An explanation of the belts]
* <http://www.spenvis.oma.be/spenvis/help/background/traprad/traprad.html > Introduction to the Trapped Radiation Belts.
* < http://www.spenvis.oma.be/spenvis/forum/index.php> Gateway to the SPENVIS orbital dose calculation software.
* {{cite web | author = D. P. Stern, M. Peredo | date= 2004-09-28 | url = http://www-istp.gsfc.nasa.gov/Education/Intro.html | title = The Exploration of the Earth's Magnetosphere | publisher = NASA | language = English | accessdate = 2006-08-22 }}
*[http://rbsp.jhuapl.edu NASA Radiation Belt Storm Probe Mission]
{{Magnetosphere}}
[[Category:Earth]]
[[Category:Space plasmas]]
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