Forbush decrease
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161441836
2007-10-01T01:41:25Z
219.90.221.229
[[WP:UNDO|Undid]] revision 161130004 by [[Special:Contributions/81.110.123.73|81.110.123.73]] ([[User talk:81.110.123.73|talk]])
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.
==Observation==
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 solar cosmic ray intensity returns to normal. Forbush decreases have also been observed by humans on [[Mir]] and the [[International Space Station]], and 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%. 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. Because it is more difficult to shield astronauts from galactic cosmic rays than from solar wind, it is hypothesized that future astronauts might benefit most from radiation shielding during [[solar maximum|solar maxima]], when CMEs are most prevalent.
==History==
The term ''Forbush decrease'' was named after the American physicist [[Scott E. Forbush]], who studied cosmic rays in the 1930s and 1940s.
==See also==
*[[Ionizing radiation]]
*[[Solar cosmic ray]]
*[[Coronal mass ejection]]
==External links==
*[http://science.slashdot.org/article.pl?sid=05/10/12/236219 Slashdot article on the Forbush decrease]
*[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]
[[Category: Cosmic rays]]
[[Category: Solar phenomena]]
[[de:Forbush-Ereignis]]