Editing Cosmic ray (section)

===Cosmic-ray flux===
[[File:SpaceEnvironmentOverview From 19830101.jpg|thumb|right|400px|An overview of the space environment shows the relationship between the solar activity and galactic cosmic rays.<ref>{{cite web|title=Extreme Space Weather Events|publisher=[[National Geophysical Data Center]]|url=http://sxi.ngdc.noaa.gov/sxi_greatest.html|access-date=19 April 2012|archive-date=22 May 2012|archive-url=https://web.archive.org/web/20120522031032/http://sxi.ngdc.noaa.gov/sxi_greatest.html|url-status=dead}}</ref>]]
The flux of incoming cosmic rays at the upper atmosphere is dependent on the [[solar wind]], the [[Earth's magnetic field]], and the energy of the cosmic rays. At distances of ≈94&nbsp;[[astronomical unit|AU]] from the Sun, the solar wind undergoes a transition, called the [[termination shock]], from supersonic to subsonic speeds. The region between the termination shock and the [[heliopause (astronomy)|heliopause]] acts as a barrier to cosmic rays, decreasing the flux at lower energies (≤&nbsp;1&nbsp;GeV) by about&nbsp;90%. However, the strength of the solar wind is not constant, and hence it has been observed that cosmic ray flux is correlated with solar activity.

In addition, the Earth's magnetic field acts to deflect cosmic rays from its surface, giving rise to the observation that the flux is apparently dependent on [[latitude]], [[longitude]], and [[azimuth|azimuth angle]].

The combined effects of all of the factors mentioned contribute to the flux of cosmic rays at Earth's surface. The following table of participial frequencies reach the planet<ref>{{cite web|url=http://www.auger.org/cosmic_rays/faq.html#how_many|title=How many?|series=Cosmic rays|publisher=Pierre Auger Observatory|website=Auger.org|access-date=17 August 2012|url-status=dead|archive-url=https://web.archive.org/web/20121012090629/http://www.auger.org/cosmic_rays/faq.html#how_many|archive-date=12 October 2012}}</ref> and are inferred from lower-energy radiation reaching the ground.<ref>{{cite web|url=https://www.auger.org/index.php/cosmic-rays/cosmic-ray-mystery|title=The mystery of high-energy cosmic rays|publisher=Pierre Auger Observatory|website=Auger.org|access-date=15 July 2015|archive-date=8 March 2021|archive-url=https://web.archive.org/web/20210308042139/https://www.auger.org/index.php/cosmic-rays/cosmic-ray-mystery|url-status=dead}}</ref>

::{| class="wikitable"
|+Relative particle energies and rates of cosmic rays
!scope="col"| Particle energy ([[Electronvolt|eV]])
!scope="col"| Particle rate (m{{sup|−2}}s{{sup|−1}})
|-
!scope="row"| {{val|1|e=9}} ([[GeV]])
| {{val|1|e=4}}
|-
!scope="row"| {{val|1|e=12}} ([[TeV]])
| 1 
|-
!scope="row"| {{val|1|e=16}} (10&nbsp;[[PeV]])
| {{val|1|e=-7}}
|-
!scope="row"| {{val|1|e=20}} (100&nbsp;[[EeV]])
| {{val|1|e=-15}}
|}

In the past, it was believed that the cosmic ray flux remained fairly constant over time. However, recent research suggests one-and-a-half- to two-fold millennium-timescale changes in the cosmic ray flux in the past forty thousand years.<ref>{{cite journal|first1=D.|last1=Lal|first2=A.J.T.|last2=Jull|first3=D.|last3=Pollard|first4=L.|last4=Vacher|year=2005|title=Evidence for large century time-scale changes in solar activity in the past 32&nbsp;Kyr, based on in-situ cosmogenic <sup>14</sup>C in ice at Summit, Greenland|journal=[[Earth and Planetary Science Letters]]|volume=234|issue=3–4|pages=335–349|doi=10.1016/j.epsl.2005.02.011|bibcode=2005E&PSL.234..335L}}</ref>

The magnitude of the energy of cosmic ray flux in interstellar space is very comparable to that of other deep space energies: cosmic ray energy density averages about one electron-volt per cubic centimetre of interstellar space, or ≈1&nbsp;eV/cm<sup>3</sup>, which is comparable to the energy density of visible starlight at 0.3&nbsp;eV/cm<sup>3</sup>, the [[galactic magnetic fields|galactic magnetic field]] energy density (assumed 3&nbsp;microgauss) which is ≈0.25&nbsp;eV/cm<sup>3</sup>, or the [[cosmic microwave background]] (CMB) radiation energy density at ≈0.25&nbsp;eV/cm<sup>3</sup>.<ref>{{cite book|author1=Castellina, Antonella|author2=Donato, Fiorenza|author2-link=Fiorenza Donato|year=2012|title=Planets, Stars, and Stellar Systems|chapter=Astrophysics of Galactic charged cosmic rays|editor1=Oswalt, T.D.|editor2=McLean, I.S.|editor3=Bond, H.E.|editor4=French, L.|editor5=Kalas, P.|editor6=Barstow, M.|editor7=Gilmore, G.F.|editor8=Keel, W.|publisher=Springer|isbn=978-90-481-8817-8|edition=1}}</ref>