Editing Van Allen radiation belt

{{Short description|Zone of energetic charged particles around the planet Earth}}
[[File:Van Allen Belts.ogv|thumb|upright=1.35|This [[CGI Animation|CGI]] video illustrates changes in the shape and intensity of a cross section of the Van Allen belts.]]
[[File:Van Allen radiation belt.svg|thumb|upright=1.35|A cross section of Van Allen radiation belts]]
{{Heliophysics}}
{{Geophysics|all}}
The '''Van Allen radiation belt''' is a zone of [[energy|energetic]] [[charged particle]]s, most of which originate from the [[solar wind]], that are captured by and held around a planet by that planet's [[magnetosphere]]. [[Earth]] has two such belts, and sometimes others may be temporarily created. The belts are named after [[James Van Allen]], who published an article describing the belts in 1958.<ref>{{cite journal | title =Observation of High Intensity Radiation by Satellites 1958 Alpha and Gamma|author1=
J. A. VAN ALLEN|author2=G. H. LUDWIG|author3=E. C. RAY|author4= C. E. McILWAIN|journal=
Journal of Jet Propulsion |year=1958|volume=28|issue=
9|pages=588–592| doi=10.2514/8.7396 |url= https://digital.lib.uiowa.edu/islandora/object/ui%3Avanallen_369/datastream/OBJ/download/Observations_of_High_Intensity_Radiation_by_Satellites_1958_Alpha_and_Gamma.pdf}}</ref><ref name=dordn>{{cite news |url=https://news.google.com/newspapers?id=kCxQAAAAIBAJ&sjid=HVYDAAAAIBAJ&pg=3307%2C5547467 |work=Victoria Advocate |location=(Texas) |agency=Associated Press |title='Doughnuts' of radiation ring earth in space |date=December 28, 1958 |page=1A}}</ref> 

Earth's two main belts extend from an [[altitude]] of about {{cvt|640|to|58,000|km}}<ref>{{cite news |url=https://www.nasa.gov/content/goddard/van-allen-probes-spot-impenetrable-barrier-in-space |title=Van Allen Probes Spot an Impenetrable Barrier in Space |last=Zell |first=Holly |date=February 12, 2015 |publisher=[[NASA]]/[[Goddard Space Flight Center]] |access-date=2017-06-04 |archive-date=2020-03-06 |archive-url=https://web.archive.org/web/20200306135737/https://www.nasa.gov/content/goddard/van-allen-probes-spot-impenetrable-barrier-in-space |url-status=dead }}</ref> above the surface, in which region [[radiation level]]s vary. The belts are in the inner region of [[Earth's magnetic field]]. They trap energetic [[electron]]s and [[proton]]s. Other nuclei, such as [[alpha particle]]s, are less prevalent. Most of the particles that form the belts are thought to come from the solar wind while others arrive as [[cosmic ray]]s.<ref name="howstuffworks van allen belts">{{cite web |url=http://science.howstuffworks.com/dictionary/astronomy-terms/van-allen-radiation-belts-info.htm |title=Van Allen Radiation Belts |work=[[HowStuffWorks]] |publisher=[[Discovery Communications|Discovery Communications, Inc.]] |location=[[Silver Spring, Maryland]] |access-date=2011-06-05|date=2009-04-23 }}</ref> By trapping the solar wind, the magnetic field deflects those energetic particles and protects [[atmosphere of Earth|the atmosphere]] from destruction.

The belts endanger [[satellite]]s, which must have their sensitive components protected with adequate shielding if they spend significant time near that zone. [[Apollo program|Apollo]] astronauts going through the Van Allen belts received a very low and harmless dose of radiation.<ref name="popsci.com" /><ref name="Woods" />

In 2013, the [[Van Allen Probes]] detected a transient, third radiation belt, which persisted for four weeks.<ref>{{cite web |date=February 28, 2013 |editor-last=Phillips |editor-first=Tony |title=Van Allen Probes Discover a New Radiation Belt |url=https://science.nasa.gov/science-news/science-at-nasa/2013/28feb_thirdbelt/ |access-date=2013-04-05 |work=Science@NASA |publisher=[[NASA]] |archive-date=2019-12-07 |archive-url=https://web.archive.org/web/20191207154753/https://science.nasa.gov/science-news/science-at-nasa/2013/28feb_thirdbelt/ |url-status=dead }}</ref>

==Discovery==
[[Kristian Birkeland]], [[Carl Størmer]], [[Nicholas Christofilos]], and Enrico Medi had investigated the possibility of trapped charged particles in 1895, forming a theoretical basis for the formation of radiation belts.<ref>{{cite web |url=http://www-istp.gsfc.nasa.gov/Education/whtrap1.html |title=Trapped Radiation—History |last1=Stern |first1=David P. |last2=Peredo |first2=Mauricio |website=The Exploration of the Earth's Magnetosphere |publisher=NASA/[[Goddard Space Flight Center|GSFC]] |access-date=2009-04-28}}</ref> The second Soviet satellite [[Sputnik 2]] which had detectors designed by [[Sergei Vernov]],<ref>{{Cite journal |last=Dessler |first=A. J. |date=1984-11-23 |title=The Vernov Radiation Belt (Almost) |url=https://www.science.org/doi/10.1126/science.226.4677.915 |journal=Science |language=en |volume=226 |issue=4677 |pages=915 |doi=10.1126/science.226.4677.915 |pmid=17737332 |bibcode=1984Sci...226..915D |issn=0036-8075}}</ref> followed by the US satellites [[Explorer&nbsp;1]] and [[Explorer&nbsp;3]],<ref>{{cite journal|last1=Li |first1=W.|last2=Hudson|first2=M.K. |title= Earth's Van Allen Radiation Belts: From Discovery to the Van Allen Probes Era| journal = J. Geophys. Res.|date=2019|volume= 124|issue = 11| pages= 8319–8351|doi=10.1029/2018JA025940|bibcode=2019JGRA..124.8319L |s2cid=213666571 |doi-access=free}} </ref> confirmed the existence of the belt in early 1958, later named after [[James Van Allen]] from the [[University of Iowa]].<ref name=dordn/> The trapped radiation was first mapped by [[Explorer&nbsp;4]], [[Pioneer&nbsp;3]], and [[Luna&nbsp;1]].

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, as it lacks a stable, global dipole field. The Earth's atmosphere limits the belts' particles to regions above 200–1,000&nbsp;km,<ref name="intro">{{cite book |last=Walt |first=Martin |author-link=Martin Walt |title=Introduction to Geomagnetically Trapped Radiation |orig-year=Originally published 1994 |date=2005 |publisher=[[Cambridge University Press]] |location=Cambridge; New York |isbn=978-0-521-61611-9 |oclc=63270281 |lccn=2006272610}}</ref> (124–620 miles) while the belts do not extend past 8 [[Earth radius|Earth radii]] ''R<sub>E</sub>''.<ref name="intro"/> The belts are confined to a volume which extends about 65[[degree (angle)|°]]<ref name="intro"/> on either side of the [[celestial sphere|celestial equator]].

==Research==
[[File:Jupiter radio.jpg|thumb|Jupiter's variable radiation belts]]
The NASA [[Van Allen Probes]] mission aims at understanding (to the point of predictability) how populations of [[Relativistic electron beam|relativistic electron]]s and ions in space form or change in response to changes in [[solar activity]] and the solar wind.
[[NASA Institute for Advanced Concepts]]–funded studies have proposed magnetic scoops to collect [[antimatter]] that naturally occurs in the Van Allen belts of Earth, although only about 10 micrograms of [[antiproton]]s are estimated to exist in the entire belt.<ref>{{cite web |url=http://www.niac.usra.edu/files/studies/abstracts/1071Bickford.pdf |title=Extraction of Antiparticles Concentrated in Planetary Magnetic Fields |last=Bickford |first=James |publisher=NASA/[[NASA Institute for Advanced Concepts|NIAC]] |access-date=2008-05-24}}</ref>

The Van Allen Probes mission successfully launched on August 30, 2012. The primary mission was scheduled to last two years with expendables expected to last four. The probes were deactivated in 2019 after running out of fuel and are expected to [[deorbit]] during the 2030s.<ref>{{cite web|date=August 30, 2012|editor-last=Zell|editor-first=Holly|title=RBSP Launches Successfully—Twin Probes are Healthy as Mission Begins|url=http://www.nasa.gov/mission_pages/rbsp/news/rbsp-launchnews.html|access-date=2012-09-02|publisher=NASA|archive-date=2019-12-14|archive-url=https://web.archive.org/web/20191214001312/https://www.nasa.gov/mission_pages/rbsp/news/rbsp-launchnews.html|url-status=dead}}</ref> NASA's [[Goddard Space Flight Center]] manages the [[Living With a Star]] program—of which the Van Allen Probes were a project, along with [[Solar Dynamics Observatory]] (SDO). The [[Applied Physics Laboratory]] was responsible for the implementation and instrument management for the Van Allen Probes.<ref>{{cite web |url=http://rbsp.jhuapl.edu/newscenter/intheloop/2010_01.php |archive-url=https://archive.today/20120724194220/http://rbsp.jhuapl.edu/newscenter/intheloop/2010_01.php |url-status=dead |archive-date=2012-07-24 |title=Construction Begins! |date=January 2010 |website=The Van Allen Probes Web Site |publisher=[[Applied Physics Laboratory|The Johns Hopkins University Applied Physics Laboratory]] |access-date=2013-09-27 }}</ref>

Radiation belts exist around other planets and moons in the [[Solar System]] that have magnetic fields powerful and stable enough to sustain them. Radiation belts have been detected at [[Jupiter]], [[Saturn]], [[Uranus]] and [[Neptune]] through in-situ observations, such as by the ''[[Galileo (spacecraft)|Galileo]]'' and ''[[Juno (spacecraft)|Juno]]'' spacecraft at Jupiter, ''[[Cassini–Huygens]]'' at Saturn, and fly-bys from the [[Voyager program]] and [[Pioneer program]]. Observations of radio emissions from highly energetic particles that are trapped in a planets magnetic field have also been used to remotely detect radiation belts, including at Jupiter <ref>{{Cite journal |last1=Drake |first1=F. D. |last2=Hvatum |first2=S. |date=1959 |title=Non-thermal microwave radiation from Jupiter. |url=http://adsabs.harvard.edu/cgi-bin/bib_query?1959AJ.....64S.329D |journal=The Astronomical Journal |volume=64 |pages=329 |doi=10.1086/108047|bibcode=1959AJ.....64S.329D }}</ref> and at the ultracool dwarf [[LSR J1835+3259]].<ref>{{Cite journal |last1=Kao |first1=Melodie M. |last2=Mioduszewski |first2=Amy J. |last3=Villadsen |first3=Jackie |last4=Shkolnik |first4=Evgenya L. |date=July 2023 |title=Resolved imaging confirms a radiation belt around an ultracool dwarf |journal=Nature |language=en |volume=619 |issue=7969 |pages=272–275 |doi=10.1038/s41586-023-06138-w |issn=1476-4687 |pmc=10338340 |pmid=37187211|arxiv=2302.12841 |bibcode=2023Natur.619..272K }}</ref> It is possible that [[Mercury (planet)]] may be able to trap charged particles in its magnetic field,<ref>{{Cite journal |last1=Lukashenko |first1=A. T. |last2=Lavrukhin |first2=A. S. |last3=Alexeev |first3=I. I. |last4=Belenkaya |first4=E. S. |date=2020-11-01 |title=Possibility of the Existence of Trapped Radiation near Mercury |url=https://link.springer.com/article/10.1134/S1063773720110043 |journal=Astronomy Letters |language=en |volume=46 |issue=11 |pages=762–773 |doi=10.1134/S1063773720110043 |bibcode=2020AstL...46..762L |issn=1562-6873}}</ref> although its highly dynamic magnetosphere (which varies on the order of minutes <ref>{{Cite journal |last1=Sun |first1=Wei-Jie |last2=Slavin |first2=James A. |last3=Fu |first3=Suiyan |last4=Raines |first4=Jim M. |last5=Zong |first5=Qiu-Gang |last6=Imber |first6=Suzanne M. |last7=Shi |first7=Quanqi |last8=Yao |first8=Zhonghua |last9=Poh |first9=Gangkai |last10=Gershman |first10=Daniel J. |last11=Pu |first11=Zuyin |last12=Sundberg |first12=Torbjörn |last13=Anderson |first13=Brian J. |last14=Korth |first14=Haje |last15=Baker |first15=Daniel N. |date=2015 |title=MESSENGER observations of magnetospheric substorm activity in Mercury's near magnetotail |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2015GL064052 |journal=Geophysical Research Letters |language=en |volume=42 |issue=10 |pages=3692–3699 |doi=10.1002/2015GL064052 |bibcode=2015GeoRL..42.3692S |issn=1944-8007|hdl=2027.42/111983 |hdl-access=free }}</ref>) may not be able to sustain stable radiation belts. [[Venus]] and [[Mars]] do not have radiation belts, as their magnetospheric configurations do not trap energetic charged particles in orbit around the planet.

[[Geomagnetic storm]]s can cause electron density to increase or decrease relatively quickly (i.e., approximately one day or less). Longer-timescale processes determine the overall configuration of the belts. After electron injection increases electron density, electron density is often observed to decay exponentially. Those decay time constants are called "lifetimes." Measurements from the Van Allen Probe B's Magnetic Electron Ion Spectrometer (MagEIS) show long electron lifetimes (i.e., longer than 100 days) in the inner belt; short electron lifetimes of around one or two days are observed in the "slot" between the belts; and energy-dependent electron lifetimes of roughly five to 20 days are found in the outer belt.<ref>{{Cite journal|doi = 10.1029/2019GL086053|title = Empirically Estimated Electron Lifetimes in the Earth's Radiation Belts: Van Allen Probe Observations|year = 2020|last1 = Claudepierre|first1 = S. G.|last2 = Ma|first2 = Q.|last3 = Bortnik|first3 = J.|last4 = O'Brien|first4 = T. P.|last5 = Fennell|first5 = J. F.|last6 = Blake|first6 = J. B.|journal = Geophysical Research Letters|volume = 47|issue = 3|pages = e2019GL086053|pmid = 32713975|pmc = 7375131|bibcode = 2020GeoRL..4786053C}}</ref>

== Inner belt ==
[[File:Rendering of Van Allen radiation belts of Earth 2.jpg|thumb|[[Cutaway drawing]] of two radiation belts around Earth: the inner belt (red) dominated by protons and the outer one (blue) by electrons. Image Credit: NASA]]
The inner Van Allen Belt extends typically from an altitude of 0.2 to 2 Earth radii ([[L-shell|L values]] of 1.2 to 3) or {{Convert|1000|km|abbr=on}} to {{Convert|12000|km|abbr=on}} above the Earth.<ref name="howstuffworks van allen belts" /><ref>{{Cite journal
| author=Ganushkina, N. Yu
| author2=Dandouras, I.
| author3=Shprits, Y. Y.
| author4=Cao, J.
| title=Locations of boundaries of outer and inner radiation belts as observed by Cluster and Double Star
| journal=[[Journal of Geophysical Research]] |volume=116 | issue=A9
| doi=10.1029/2010JA016376
| date=2011
| pages=n/a
| bibcode = 2011JGRA..116.9234G | hdl=2027.42/95464
|url=https://deepblue.lib.umich.edu/bitstream/2027.42/95464/1/jgra21211.pdf| doi-access=free
}}</ref> In certain cases, when solar activity is stronger or in geographical areas such as the [[South Atlantic Anomaly]], the inner boundary may decline to roughly 200 km<ref>{{Cite web |url=http://www.spacewx.com/Docs/ECSS-E-ST-10-04C_15Nov2008.pdf |title=Space Environment Standard ECSS-E-ST-10-04C |date=November 15, 2008 |publisher=ESA Requirements and Standards Division |access-date=2013-09-27 |archive-date=2013-12-09 |archive-url=https://web.archive.org/web/20131209094707/http://spacewx.com/Docs/ECSS-E-ST-10-04C_15Nov2008.pdf |url-status=dead }}</ref> above the Earth's surface. The inner belt contains high concentrations of electrons in the range of hundreds of [[Electronvolt|keV]] and energetic protons with energies exceeding 100 MeV—trapped by the relatively strong magnetic fields in the region (as compared to the outer belt).<ref>{{Cite journal
| author=Gusev, A. A.
| author2=Pugacheva, G. I.
| author3=Jayanthi, U. B.
| author4=Schuch, N.
| title=Modeling of Low-altitude Quasi-trapped Proton Fluxes at the Equatorial Inner Magnetosphere
| journal=Brazilian Journal of Physics | volume= 33 |issue= 4
| date=2003
| pages=775–781
| doi=10.1590/S0103-97332003000400029
| bibcode = 2003BrJPh..33..775G| doi-access=free
}}</ref>

It is thought that proton energies exceeding 50 MeV in the lower belts at lower altitudes are the result of the [[beta decay]] of [[neutron]]s 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 |last=Tascione |first=Thomas F. |title=Introduction to the Space Environment |edition=2nd |date=2004 |publisher=Krieger Publishing Co.| location=Malabar, FL | isbn=978-0-89464-044-5 |oclc=28926928 |lccn=93036569}}</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]].<ref name="Goddard">{{Cite web |url=http://image.gsfc.nasa.gov/poetry/tour/AAvan.html |title=The Van Allen Belts |publisher=NASA/GSFC |access-date=2011-05-25 |archive-date=2019-12-20 |archive-url=https://web.archive.org/web/20191220163500/https://image.gsfc.nasa.gov/poetry/tour/AAvan.html |url-status=dead }}</ref><ref>{{Cite journal
| author=Underwood, C.
| author2=Brock, D.
| author3=Williams, P.
| author4=Kim, S.
| author5=Dilão, R.
| author6=Ribeiro Santos, P.
| author7=Brito, M.
| author8=Dyer, C.
| author9=Sims, A.
| title=Radiation Environment Measurements with the Cosmic Ray Experiments On-Board the KITSAT-1 and PoSAT-1 Micro-Satellites
| journal=[[IEEE Transactions on Nuclear Science]] | volume=41 |issue=6
| date=December 1994
| pages=2353–2360
| doi=10.1109/23.340587
| bibcode = 1994ITNS...41.2353U}}</ref>

In March 2014, a pattern resembling "zebra stripes" was observed in the radiation belts by the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) onboard [[Van Allen Probes]]. 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.<ref>{{Cite news|title=Twin NASA probes find 'zebra stripes' in Earth's radiation belt|url=http://www.universetoday.com/110482/twin-nasa-probes-find-zebra-stripes-in-earths-radiation-belt|work=Universe Today|access-date=20 March 2014|date=2014-03-19}}</ref> A 2016 study instead concluded that the zebra stripes were an imprint of [[Ionospheric dynamo region|ionospheric winds]] on radiation belts.<ref>{{Cite journal
| author=Lejosne, S.
| author2=Roederer, J.G.
| title=The "zebra stripes": An effect of F region zonal plasma drifts on the longitudinal distribution of radiation belt particles
| journal=[[Journal of Geophysical Research]] |volume=121 | issue=1
| doi=10.1002/2015JA021925
| date=2016
| pages=507–518
| bibcode = 2016JGRA..121..507L| doi-access=free
}}</ref>

==Outer belt==
[[Image:Birkeland-anode-globe-fig259.jpg|thumb|upright=1.15|Laboratory simulation of the Van Allen belt's influence on the Solar Wind; these aurora-like [[Birkeland current]]s were created by the scientist [[Kristian Birkeland]] in his [[terrella]], a magnetized anode globe in an evacuated chamber]]

The outer belt consists mainly of high-energy (0.1–10&nbsp;[[Electronvolt|MeV]]) electrons trapped by the Earth's magnetosphere. It is more variable than the inner belt, as it is more easily influenced by solar activity. It is almost [[torus|toroidal]] in shape, beginning at an altitude of 3 Earth radii and extending to 10 Earth radii (''R<sub>E</sub>'')—{{convert|13000|to|60000|km|mi}} above the Earth's surface.{{cn|date=November 2022}} Its greatest intensity is usually around 4 to 5 ''R<sub>E</sub>''. The outer electron radiation belt is mostly produced by inward radial diffusion<ref>{{Cite conference |last1=Elkington |first1=S. R. |last2=Hudson |first2=M. K. |author-link2=Mary Hudson (scientist)|last3=Chan |first3=A. A. |date=May 2001 |title=Enhanced Radial Diffusion of Outer Zone Electrons in an Asymmetric Geomagnetic Field |book-title=Spring Meeting 2001 |publisher=[[American Geophysical Union]] |location=Washington, D.C. |bibcode=2001AGUSM..SM32C04E}}</ref><ref>{{Cite journal |last1=Shprits |first1=Y. Y. |last2=Thorne |first2=R. M. |date=2004 |title=Time dependent radial diffusion modeling of relativistic electrons with realistic loss rates |journal=[[Geophysical Research Letters]] |volume=31 |issue=8 |pages=L08805 |bibcode=2004GeoRL..31.8805S |doi=10.1029/2004GL019591 |doi-access=free}}</ref> and local acceleration<ref name="nature437">{{Cite journal |last1=Horne |first1=Richard B. |last2=Thorne |first2=Richard M. |last3=Shprits |first3=Yuri Y. |display-authors=etal |date=2005 |title=Wave acceleration of electrons in the Van Allen radiation belts |journal=[[Nature (journal)|Nature]] |volume=437 |issue=7056 |pages=227–230 |bibcode=2005Natur.437..227H |doi=10.1038/nature03939 |pmid=16148927|s2cid=1530882 }}</ref> due to transfer of energy from whistler-mode [[Waves in plasmas|plasma waves]] to radiation belt electrons. Radiation belt electrons are also constantly removed by collisions with Earth's atmosphere,<ref name="nature437"/> losses to the [[magnetopause]], and their outward radial diffusion. The [[gyroradius|gyroradii]] of energetic protons would be large enough to bring them into contact with the Earth's atmosphere. Within this belt, the electrons have a high [[flux]] and at the outer edge (close to the magnetopause), where [[Earth's magnetic field|geomagnetic field]] lines open into the [[Magnetosphere#Magnetotail|geomagnetic "tail"]], the flux of energetic electrons can drop to the low interplanetary levels within about {{convert|100|km|mi|abbr=on}}—a decrease by a factor of 1,000.

In 2014, it was discovered that the inner edge of the outer belt is characterized by a very sharp transition, below which highly relativistic electrons (> 5MeV) cannot penetrate.<ref>{{Cite journal |author1=D. N. Baker |author2=A. N. Jaynes |author3=V. C. Hoxie |author4=R. M. Thorne |author5=J. C. Foster |author6=X. Li |author7=J. F. Fennell |author8=J. R. Wygant |author9=S. G. Kanekal |author10=P. J. Erickson |author11=W. Kurth |author12=W. Li |author13=Q. Ma |author14=Q. Schiller |author15=L. Blum |author16=D. M. Malaspina |author17=A. Gerrard |author18=L. J. Lanzerotti |name-list-style=amp |date=27 November 2014 |title=An impenetrable barrier to ultrarelativistic electrons in the Van Allen radiation belts |journal=Nature |volume=515 |issue=7528 |pages=531–534 |bibcode=2014Natur.515..531B |doi=10.1038/nature13956|pmid=25428500 |s2cid=205241480 }}</ref> The reason for this shield-like behavior is not well understood.

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 originate 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 in response to [[geomagnetic storm]]s, 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. Another cause of variability of the outer belt particle populations is the [[Two stream instability#Wave-particle interactions|wave-particle interactions]] with various [[plasma waves]] in a broad range of frequencies.<ref>{{cite journal|last1=Pokhotelov|first1=D.|last2=Lefeuvre|first2=F. |last3=Horne|first3=R.B. |last4=Cornilleau-Wehrlin|first4=N. |title= Survey of ELF-VLF plasma waves in the outer radiation belt observed by Cluster STAFF-SA experiment | journal=Annales Geophysicae |date=2008|volume=26|issue=11|pages=3269–3277|doi=10.5194/angeo-26-3269-2008|bibcode=2008AnGeo..26.3269P |s2cid=122756498 |doi-access=free}}</ref>

On February 28, 2013, a third radiation belt—consisting of high-energy [[Ultrarelativistic limit|ultrarelativistic]] charged particles—was reported to be discovered. In a news conference by NASA's Van Allen Probe team, it was stated that this third belt is a product of [[coronal mass ejection]] from the Sun. It has been represented as a separate creation which splits the Outer Belt, like a knife, on its outer side, and exists separately as a storage container of particles for a month's time, before merging once again with the Outer Belt.<ref>{{YouTube|yLw9a5t-sUs|NASA's Van Allen Probes Discover Third Radiation Belt Around Earth}}</ref>

The unusual stability of this third, transient belt has been explained as due to a 'trapping' by the Earth's magnetic field of ultrarelativistic particles as they are lost from the second, traditional outer belt. While the outer zone, which forms and disappears over a day, is highly variable due to interactions with the atmosphere, the ultrarelativistic particles of the third belt are thought not to scatter into the atmosphere, as they are too energetic to interact with atmospheric waves at low latitudes.<ref name="nphys2760">{{Cite journal |last1=Shprits |first1=Yuri Y. |last2=Subbotin |first2=Dimitriy |last3=Drozdov |first3=Alexander |display-authors=etal |date=2013 |title=Unusual stable trapping of the ultrarelativistic electrons in the Van Allen radiation belts |journal=[[Nature Physics]] |volume=9 |issue=11 |pages=699–703 |bibcode=2013NatPh...9..699S |doi=10.1038/nphys2760 |doi-access=free}}</ref> This absence of scattering and the trapping allows them to persist for a long time, finally only being destroyed by an unusual event, such as the shock wave from the Sun.

== Flux values ==
In the belts, at a given point, the flux of particles of a given energy decreases sharply with energy.

At the [[Magnetic dip|magnetic equator]], electrons of energies exceeding 5000&nbsp;keV (resp. 5&nbsp;MeV) have omnidirectional fluxes ranging from 1.2×10<sup>6</sup> (resp. 3.7×10<sup>4</sup>) up to 9.4×10<sup>9</sup> (resp. 2×10<sup>7</sup>) particles per square centimeter per second.

The proton belts contain protons with kinetic energies ranging from about 100&nbsp;keV, which can penetrate 0.6&nbsp;μm of [[lead]],  to over 400&nbsp;MeV, which can penetrate 143&nbsp;mm of lead.<ref>{{cite book |last=Hess |first=Wilmot N. |author-link=Wilmot N. Hess |title=The Radiation Belt and Magnetosphere |date=1968 |publisher=Blaisdell Pub. Co. |location=Waltham, MA |oclc=712421 |lccn=67019536|title-link=The Radiation Belt and Magnetosphere }}</ref>

Most published flux values for the inner and outer belts may not show the maximum probable flux densities that are possible in the belts. There is a reason for this discrepancy: the flux density and the location of the peak flux is variable, depending primarily on solar activity, and the number of spacecraft with instruments observing the belt in real time has been limited. The Earth has not yet experienced a solar storm of [[Solar storm of 1859|Carrington event]] intensity while spacecraft with the proper instruments have been available to observe the event.

Radiation levels in the belts would be dangerous to humans if they were exposed for an extended period of time. The Apollo missions minimised hazards for astronauts by sending spacecraft at high speeds through the thinner areas of the upper belts, bypassing inner belts completely, except for the Apollo 14 mission where the spacecraft traveled through the heart of the trapped radiation belts.<ref name="Goddard" /><ref>{{cite conference |title=Radiation Plan for the Apollo Lunar Mission |last1=Modisette |first1=Jerry L. |last2=Lopez |first2=Manuel D. |last3=Snyder |first3=Joseph W. |date=January 20–22, 1969 |conference=AIAA 7th Aerospace Sciences Meeting |location=New York |id=AIAA Paper No. 69-19 |doi=10.2514/6.1969-19 }}</ref><ref name="popsci.com">{{cite web|url=http://www.popsci.com/blog-network/vintage-space/apollo-rocketed-through-van-allen-belts|title=Apollo Rocketed Through the Van Allen Belts|date=7 January 2019}}</ref><ref>{{Cite web|url=https://www.hq.nasa.gov/alsj/a14/a14mr10.htm|title=Apollo 14 Mission Report, Chapter 10|website=www.hq.nasa.gov|access-date=2019-08-07}}</ref>

<gallery caption="Flux values, normal solar conditions" class="center" widths="300px" heights="240px">
File:Ap8-omni-0.100MeV.png|AP8 MIN omnidirectional proton flux ≥ 100 keV
File:Ap8-omni-1.000MeV.png|AP8 MIN omnidirectional proton flux ≥ 1 MeV
File:Ap8-omni-400.0MeV.png|AP8 MIN omnidirectional proton flux ≥ 400 MeV
</gallery>

==Antimatter confinement==
In 2011, a study confirmed earlier speculation that the Van Allen belt could confine antiparticles. The [[PAMELA detector|Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics]] (PAMELA) experiment detected levels of [[antiproton]]s orders of magnitude higher than are expected from normal [[particle decay]]s while passing through the [[South Atlantic Anomaly]]. This suggests the Van Allen belts confine a significant flux of antiprotons produced by the interaction of the Earth's upper atmosphere with cosmic rays.<ref>{{Cite journal |last1=Adriani |first1=O. |last2=Barbarino |first2=G. C. |last3=Bazilevskaya |first3=G. A. |last4=Bellotti |first4=R. |last5=Boezio |first5=M. |last6=Bogomolov |first6=E. A. |last7=Bongi |first7=M. |last8=Bonvicini |first8=V. |last9=Borisov |first9=S. |display-authors=3 |date=2011 |title=The Discovery of Geomagnetically Trapped Cosmic-Ray Antiprotons |journal=[[The Astrophysical Journal|The Astrophysical Journal Letters]] |volume=737 |issue=2 |pages=L29 |arxiv=1107.4882 |bibcode=2011ApJ...737L..29A |doi=10.1088/2041-8205/737/2/L29}}</ref> The energy of the antiprotons has been measured in the range from 60 to 750 MeV.

The very high energy released in antimatter annihilation has led to proposals to harness these antiprotons for spacecraft propulsion. The concept relies on the development of antimatter collectors and containers.<ref>James Bickford, ''[https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=0e3f755cb0b3dcb80432d14750e2d37717f49bd2 Extraction of Antiparticles Concentrated in Planetary Magnetic Fields]'', [[NASA Institute for Advanced Concepts]] phase II report, [[Draper Laboratory]], August 2007.</ref>

==Implications for space travel==
{{Comparison_satellite_navigation_orbits}}
Spacecraft travelling beyond [[low Earth orbit]] enter the zone of radiation of the Van Allen belts. Beyond the belts, they face additional hazards from cosmic rays and [[solar particle event]]s. A region between the inner and outer Van Allen belts lies at 2 to 4 Earth radii and is sometimes referred to as the "safe zone".<ref>{{cite web
 | title=Earth's Radiation Belts with Safe Zone Orbit
 | date=15 December 2004
 | publisher=NASA/GSFC
 | url=http://svs.gsfc.nasa.gov/vis/a000000/a003000/a003052/index.html
 | access-date=2009-04-27
 | archive-date=2016-01-13
 | archive-url=https://web.archive.org/web/20160113122436/http://svs.gsfc.nasa.gov/vis/a000000/a003000/a003052/index.html
 | url-status=dead
 }}</ref><ref>{{cite web | first=Rachel A. | last=Weintraub | title=Earth's Safe Zone Became Hot Zone During Legendary Solar Storms | date=December 15, 2004 | publisher=NASA/GSFC | url=http://www.nasa.gov/vision/universe/solarsystem/safe_zone.html | access-date=2009-04-27 | archive-date=2016-05-07 | archive-url=https://web.archive.org/web/20160507111835/http://www.nasa.gov/vision/universe/solarsystem/safe_zone.html | url-status=dead }}</ref>

[[Solar cell]]s, [[integrated circuit]]s, and [[sensor]]s can be damaged by radiation. Geomagnetic storms occasionally damage [[electronics|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 the total [[electric charge]] in these circuits is now small enough so as to be comparable with the charge of incoming ions. Electronics on satellites must be [[Radiation hardening|hardened]] against radiation to operate reliably. The [[Chandra Space Telescope]], has its sensors turned off when passing through the Van Allen belts.<ref>{{Cite web |title=Chandra Observatory Launch Lights Up the Night Sky |url=https://imagine.gsfc.nasa.gov/news/23jul99.html |archive-url=https://web.archive.org/web/20241216065723/https://imagine.gsfc.nasa.gov/news/23jul99.html |archive-date=2024-12-16 |access-date=2025-03-23 |website=imagine.gsfc.nasa.gov}}</ref> The [[INTEGRAL]] space telescope was placed in an orbit designed to avoid time within the belts.<ref>{{Cite web |title=Imagine the Universe News - 17 October 2002 |url=https://imagine.gsfc.nasa.gov/news/17oct02.html |access-date=2025-03-23 |website=imagine.gsfc.nasa.gov}}</ref>

The [[List of Apollo missions|Apollo missions]] marked the first event where humans traveled through the Van Allen belts, which was one of several radiation hazards known by mission planners.<ref name=Vernon>{{cite web|last=Bailey|first=J. Vernon|title=Radiation Protection and Instrumentation|url=https://history.nasa.gov/SP-368/s2ch3.htm|work=Biomedical Results of Apollo|date=January 1975 |volume=NASA-SP-368 |access-date=2011-06-13}}</ref> The astronauts had low exposure in the Van Allen belts due to the short period of time spent flying through them.<ref name="popsci.com"/><ref name=Woods>{{Cite book| last=Woods| first=W. David| title=How Apollo Flew to the Moon| date=2008| publisher=[[Springer Science+Business Media|Springer-Verlag]]| location=New York| isbn=978-0-387-71675-6| page=[https://archive.org/details/howapolloflewtom0000wood/page/109 109]| url=https://archive.org/details/howapolloflewtom0000wood/page/109}}</ref>

==Causes==
It is generally understood that the inner and outer Van Allen belts result from different processes. The inner belt is mainly composed of energetic protons produced from the decay of [[neutrons]], which are themselves the result of [[cosmic ray]] collisions in the upper atmosphere. The outer Van Allen belt consists mainly of electrons. They are injected from the geomagnetic tail following geomagnetic storms, and are subsequently energized through [[Two stream instability#Wave-particle interactions|wave-particle interactions]].

In the inner belt, particles that originate from the Sun are trapped in the Earth's magnetic field. Particles spiral along the magnetic lines of flux as they move "latitudinally" along those lines. As particles move toward the poles, the magnetic field line density increases, and their "latitudinal" velocity is slowed and can be reversed, deflecting the particles back towards the equatorial region, causing them to bounce back and forth between the Earth's poles.<ref>{{cite web |last1=Stern |first1=David P. |last2=Peredo |first2=Mauricio |title=The Exploration of the Earth's Magnetosphere |website=The Exploration of the Earth's Magnetosphere |publisher=[[NASA]] / [[Goddard Space Flight Center]] |url=http://www-spof.gsfc.nasa.gov/Education/Intro.html |access-date=2013-09-27 |archive-date=2013-08-15 |archive-url=https://web.archive.org/web/20130815210747/http://www-spof.gsfc.nasa.gov/Education/Intro.html |url-status=dead }}</ref> In addition to both spiralling around and moving along the flux lines, the electrons drift slowly in an eastward direction, while the protons drift westward.

The gap between the inner and outer Van Allen belts is sometimes called the "safe zone" or "safe slot", and is the location of [[medium Earth orbit]]s. The gap is caused by the [[Very low frequency|VLF radio waves]], which scatter particles in [[pitch angle (particle motion)|pitch angle]], which adds new ions to the atmosphere. Solar outbursts can also dump particles into the gap, but those drain out in a matter of days. The VLF radio waves were previously thought to be generated by turbulence in the radiation belts, but recent work by [[James L. Green|J.L. Green]] of the [[Goddard Space Flight Center]]{{citation needed|date=February 2023}} compared maps of lightning activity collected by the [[Microlab 1]] spacecraft with data on radio waves in the radiation-belt gap from the [[IMAGE (spacecraft)|IMAGE]] spacecraft; the results suggest that the radio waves are actually generated by lightning within Earth's atmosphere. The generated radio waves strike the ionosphere at the correct angle to pass through only at high latitudes, where the lower ends of the gap approach the upper atmosphere. These results are still being debated in the scientific community.

==Proposed removal==
Draining the charged particles from the Van Allen belts would open up new orbits for satellites and make travel safer for astronauts.<ref name="choi" >
Charles Q. Choi.
[https://spectrum.ieee.org/hacking-the-van-allen-belts "Hacking the Van Allen Belts"].
2014.
</ref> Draining radiation belts around other planets has also been proposed, for example, before exploring [[Europa (moon)|Europa]], which orbits within [[Jupiter]]'s radiation belt.<ref>
[https://www.nasa.gov/home/hqnews/2005/mar/HQ_05070_radiation_belt.html "NASA Finds Lightning Clears Safe Zone in Earth's Radiation Belt"]. NASA, 2005.
</ref> Since the radiation belts are part of a complex system, it is unknown if there could be [[unintended consequences]] to removing these radiation belts.<ref name="choi" />

One concept proposed to drain and remove the radiation fields of the Van Allen radiation belts is known as
High Voltage Orbiting Long Tether, or HiVOLT, a concept proposed by Russian physicist [[Valentin Danilov|V. V. Danilov]] and further refined by [[Robert P. Hoyt]] and [[Robert L. Forward]].<ref>{{cite web |url=http://radbelts.gsfc.nasa.gov/outreach/RadNews.html |title=NASA outreach: RadNews |archive-url=https://web.archive.org/web/20130613193849/http://radbelts.gsfc.nasa.gov/outreach/RadNews.html |archive-date=2013-06-13 |url-status=dead |access-date=2013-09-27}}</ref> that surround the Earth.<ref name=mirnov1996>{{cite journal
| last1 = Mirnov
| first1 = Vladimir
| last2 = Üçer
| first2 = Defne
| last3 = Danilov
| first3 = Valentin
| author-link3 = Valentin Danilov
| date = November 10–15, 1996
| title = High-Voltage Tethers For Enhanced Particle Scattering In Van Allen Belts
| journal = APS Division of Plasma Physics Meeting Abstracts
| volume = 38
| id = Abstract #7E.06
| pages = 7
| oclc = 205379064
| bibcode = 1996APS..DPP..7E06M
}}</ref>
Another proposal for draining the Van Allen belts involves beaming very-low-frequency (VLF) radio waves from the ground into the Van Allen belts.<ref>
Saswato R. Das.
[https://spectrum.ieee.org/military-experiments-target-the-van-allen-belts "Military Experiments Target the Van Allen Belts"].
2007.
</ref>

==See also==
* [[Dipole model of the Earth's magnetic field]]
* [[L-shell]]
* [[List of artificial radiation belts]]
* [[Space weather]]
* [[Paramagnetism]]

== Explanatory notes ==
{{notelist}}

== Citations ==
{{Reflist}}

==Additional sources==
* {{cite journal |last1=Adams |first1=L. |last2=Daly |first2=E. J. |last3=Harboe-Sorensen |first3=R. |last4=Holmes-Siedle |first4=A. G. |last5=Ward |first5=A. K. |last6=Bull |first6=R. A. |date=December 1991 |title=Measurement of SEU and total dose in geostationary orbit under normal and solar flare conditions |journal=IEEE Transactions on Nuclear Science |volume=38 |issue=6 |pages=1686–1692 |oclc=4632198117 |doi=10.1109/23.124163|bibcode = 1991ITNS...38.1686A }}
* {{cite book |last1=Holmes-Siedle |first1=Andrew |last2=Adams |first2=Len |title=Handbook of Radiation Effects |edition=2nd |date=2002 |publisher=Oxford University Press |location=Oxford; New York |isbn=978-0-19-850733-8 |oclc=47930537 |lccn=2001053096}}
* {{cite journal |last1=Shprits |first1=Yuri Y. |last2=Elkington |first2=Scott R. |last3=Meredith |first3=Nigel P. |last4=Subbotin |first4=Dmitriy A. |date=November 2008 |title=Review of modeling of losses and sources of relativistic electrons in the outer radiation belt |journal=Journal of Atmospheric and Solar-Terrestrial Physics |volume=70 |issue=14}} Part I: Radial transport, pp.&nbsp;1679–1693, {{doi|10.1016/j.jastp.2008.06.008}}; Part II: Local acceleration and loss, pp.&nbsp;1694–1713, {{doi|10.1016/j.jastp.2008.06.014}}.

== External links ==
{{Commons category|Van Allen radiation belts}}
* [http://www.phy6.org/Education/Iradbelt.html An explanation of the belts] by David P. Stern and Mauricio Peredo
* [http://www.spenvis.oma.be/help/background/traprad/traprad.html Background: Trapped particle radiation models]—Introduction to the trapped radiation belts by [[Space Environment Information System|SPENVIS]]
* [http://www.spenvis.oma.be/ SPENVIS—Space Environment, Effects, and Education System]—Gateway to the SPENVIS orbital dose calculation software
*[http://vanallenprobes.jhuapl.edu The Van Allen Probes Web Site] Johns Hopkins University Applied Physics Laboratory
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