Editing Van Allen radiation belt (section)

==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.