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{{Short description|Decrease in cosmic ray intensity}}
A '''Forbush decrease''' is a rapid decrease in the observed [[galactic cosmic ray]] intensity following a [[coronal mass ejection]] (CME). It occurs due to the [[magnetic field]] of the [[plasma (physics)|plasma]] [[solar wind]] sweeping some of the galactic cosmic rays away from [[Earth]]. The term ''Forbush decrease'' was named after the [[United States|American]] physicist [[Scott Forbush|Scott E. Forbush]], who studied [[cosmic rays]] in the 1930s and 1940s.

==Observation==
[[File:ExtremeEvent 20120304-00h 20120317-24h.jpg|thumb|right|320px|Forbush Decrease in March 2012.<ref>{{cite web | title=Extreme Space Weather Events | publisher=[[National Geophysical Data Center]] | url=http://sxi.ngdc.noaa.gov/sxi_greatest.html | access-date=2012-04-19 | archive-date=2012-05-22 | archive-url=https://web.archive.org/web/20120522031032/http://sxi.ngdc.noaa.gov/sxi_greatest.html | url-status=dead }}</ref>]]

The Forbush decrease is usually observable by [[particle detector]]s on Earth within a few days after the CME, and the decrease takes place over the course of a few hours. Over the following several days, the galactic cosmic ray intensity returns to normal. Forbush decreases have also been observed by humans on ''[[Mir]]'' and the [[International Space Station]] (ISS), at other locations in the inner heliosphere such as the [[Solar Orbiter]] spacecraft,<ref name="Freiherr von Forstner Dumbović Möstl Guo p. ">{{cite journal | last1=Freiherr von Forstner | first1=J. L. | last2=Dumbović | first2=M. | last3=Möstl | first3=C. | last4=Guo | first4=J. | last5=Papaioannou | first5=A. |display-authors=4 | title=Radial evolution of the April 2020 stealth coronal mass ejection between 0.8 and 1 AU. Comparison of Forbush decreases at Solar Orbiter and near the Earth | journal=Astronomy & Astrophysics | date=2021-03-03 | volume=A1 | issn=0004-6361 | doi=10.1051/0004-6361/202039848 | page=656| arxiv=2102.12185 | bibcode=2021A&A...656A...1F | s2cid=232035885 }}</ref> and at Mars with the [[Mars Science Laboratory]] rover's [[Radiation assessment detector]]<ref name="Freiherr von Forstner Guo Wimmer‐Schweingruber Hassler 2018 pp. 39–56">{{cite journal | last1=Freiherr von Forstner | first1=Johan L. | last2=Guo | first2=Jingnan | last3=Wimmer-Schweingruber | first3=Robert F. | last4=Hassler | first4=Donald M. | last5=Temmer | first5=Manuela |display-authors=4 | title=Using Forbush Decreases to Derive the Transit Time of ICMEs Propagating from 1 AU to Mars | journal=Journal of Geophysical Research: Space Physics | publisher=American Geophysical Union (AGU) | volume=123 | issue=1 | year=2018 | issn=2169-9380 | doi=10.1002/2017ja024700 | pages=39–56| arxiv=1712.07301 | bibcode=2018JGRA..123...39F | doi-access=free }}</ref> and the [[MAVEN]] orbiter,<ref name="Guo Lillis Wimmer-Schweingruber Zeitlin 2018 p=A79">{{cite journal | last1=Guo | first1=Jingnan | last2=Lillis | first2=Robert | last3=Wimmer-Schweingruber | first3=Robert F. | last4=Zeitlin | first4=Cary | last5=Simonson | first5=Patrick |display-authors=4 | title=Measurements of Forbush decreases at Mars: both by MSL on ground and by MAVEN in orbit | journal=Astronomy & Astrophysics | volume=611 | year=2018 | issn=0004-6361 | doi=10.1051/0004-6361/201732087 | page=A79| arxiv=1712.06885 | bibcode=2018A&A...611A..79G | doi-access=free }}</ref> as well as in the outer solar system by instruments onboard ''[[Pioneer 10]]'' and ''[[Pioneer 11|11]]'' and ''[[Voyager 1]]'' and ''[[Voyager 2|2]]'', even past the orbit of [[Neptune]].

The magnitude of a Forbush decrease depends on three factors:
* the size of the CME
* the strength of the magnetic fields in the CME
* the proximity of the CME to the Earth

A Forbush decrease is sometimes defined as being a decrease of at least 10% of galactic cosmic rays on Earth, but ranges from about 3% to 20%. The amplitude is also highly dependent on the energy of cosmic rays that is observed by the specific instrument, where lower energies typically show larger decreases.<ref name="Lockwood Webber Debrunner 1991 p=5447">{{cite journal | last1=Lockwood | first1=J. A. | last2=Webber | first2=W. R. | last3=Debrunner | first3=H. | title=The rigidity dependence of forbush decreases observed at the Earth | journal=Journal of Geophysical Research | publisher=American Geophysical Union (AGU) | volume=96 | issue=A4 | year=1991 | issn=0148-0227 | doi=10.1029/91ja00089 | page=5447| bibcode=1991JGR....96.5447L }}</ref> Reductions of 30% or more have been recorded aboard the ISS.

The overall rate of Forbush decreases tends to follow the 11-year [[sunspot]] cycle. It is more difficult to shield astronauts from galactic cosmic rays than from solar wind, so future astronauts might benefit most from radiation shielding during [[Solar minimum|solar minima]], when the suppressive effect of CMEs is less frequent.

==Effects on the atmosphere==
A 2009 peer-reviewed article<ref>{{cite journal | title=Cosmic ray decreases affect atmospheric aerosols and clouds | journal=Geophysical Research Letters | volume=36 | issue=15 | pages=L15101 | publisher=[[Geophys. Res. Lett.]] | date=17 June 2009 | url=http://www.agu.org/pubs/crossref/2009/2009GL038429.shtml | bibcode=2009GeoRL..3615101S | last1=Svensmark | first1=Henrik | last2=Bondo | first2=Torsten | last3=Svensmark | first3=Jacob | doi=10.1029/2009GL038429 | citeseerx=10.1.1.394.9780 | s2cid=15963013 | access-date=18 November 2009 | archive-date=15 December 2009 | archive-url=https://web.archive.org/web/20091215005041/http://www.agu.org/pubs/crossref/2009/2009GL038429.shtml | url-status=dead }}</ref> found that low clouds contain less liquid water following Forbush decreases, and for the most influential events the liquid water in the oceanic atmosphere can diminish by as much as 7%. Further peer-reviewed work found no connection between Forbush decreases and cloud properties<ref>{{cite journal |title=Atmospheric data over a solar cycle: no connection between galactic cosmic rays and new particle formation| journal=Atmospheric Chemistry and Physics| volume=10| issue=4| pages=1885–1898| doi=10.5194/acp-10-1885-2010| year=2010| last1=Kulmala| first1=M.| last2=Riipinen| first2=I.| last3=Nieminen| first3=T.| last4=Hulkkonen| first4=M.| last5=Sogacheva| first5=L.| last6=Manninen| first6=H. E.| last7=Paasonen| first7=P.| last8=Petäjä| first8=T.| last9=Dal Maso| first9=M.| last10=Aalto| first10=P. P.| last11=Viljanen| first11=A.| last12=Usoskin| first12=I.| last13=Vainio| first13=R.| last14=Mirme| first14=S.| last15=Mirme| first15=A.| last16=Minikin| first16=A.| last17=Petzold| first17=A.| last18=Hõrrak| first18=U.| last19=Plaß-Dülmer| first19=C.| last20=Birmili| first20=W.| last21=Kerminen| first21=V.-M.| url=https://elib.dlr.de/61509/1/acp-10-1885-2010.pdf| doi-access=free}}</ref><ref>{{cite web |title= Sudden Cosmic Ray Decreases. No change of cloud cover|url= http://www.eawag.ch/organisation/abteilungen/surf/publikationen/2010_calogovic.pdf|archive-url= https://web.archive.org/web/20100401033338/http://www.eawag.ch/organisation/abteilungen/surf/publikationen/2010_calogovic.pdf|url-status= dead|archive-date= 2010-04-01| year=2010}}</ref> until the connection was found in diurnal temperature range,<ref>{{cite journal | title=Forbush decreases – clouds relation in the neutron monitor era | journal=Astrophysics and Space Sciences Transactions | volume=7 | issue=3 | pages=315–318 | date=31 August 2011 | doi=10.5194/astra-7-315-2011 | last1=Dragić | first1=A. | last2=Aničin | first2=I. | last3=Banjanac | first3=R. | last4=Udovičić | first4=V. | last5=Joković | first5=D. | last6=Maletić | first6=D. | last7=Puzović | first7=J. | bibcode=2011ASTRA...7..315D | doi-access=free }}</ref> and since confirmed in satellite data.<ref>{{cite journal|last1=Svensmark|first1=J|last2=Enghoff|first2=M. B.|last3=Shaviv|first3=N|last4=Svensmark|first4=H|title=The response of clouds and aerosols to cosmic ray decreases|journal=J. Geophys. Res. Space Phys.|date=September 2016|volume=121|issue=9|pages=8152–8181|doi=10.1002/2016JA022689|bibcode=2016JGRA..121.8152S|doi-access=free}}</ref>

==See also==
*[[Ionizing radiation]]

==References==
{{Reflist}}

==External links==
*[https://web.archive.org/web/20100403062301/http://science.nasa.gov/headlines/y2005/07oct_afraid.htm Who's Afraid of a Solar Flare?] from Science@NASA
*[http://www.ips.gov.au/Geophysical/1/4 Cosmic Ray Data Applications to Space Weather Forecasting] {{Webarchive|url=https://web.archive.org/web/20051016005339/http://www.ips.gov.au/Geophysical/1/4 |date=2005-10-16 }}

{{DEFAULTSORT:Forbush Decrease}}
[[Category:Cosmic rays]]
[[Category:Solar phenomena]]

Forbush decrease - Revision history

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