# Forbush decrease

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Decrease in cosmic ray intensity

A **Forbush decrease** is a rapid decrease in the observed [galactic cosmic ray](/source/Galactic_cosmic_ray) intensity following a [coronal mass ejection](/source/Coronal_mass_ejection) (CME). It occurs due to the [magnetic field](/source/Magnetic_field) of the [plasma](/source/Plasma_(physics)) [solar wind](/source/Solar_wind) sweeping some of the galactic cosmic rays away from [Earth](/source/Earth). The term *Forbush decrease* was named after the [American](/source/United_States) physicist [Scott E. Forbush](/source/Scott_Forbush), who studied [cosmic rays](/source/Cosmic_rays) in the 1930s and 1940s.

## Observation

Forbush Decrease in March 2012.[1]

The Forbush decrease is usually observable by [particle detectors](/source/Particle_detector) 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](/source/Mir)* and the [International Space Station](/source/International_Space_Station) (ISS), at other locations in the inner heliosphere such as the [Solar Orbiter](/source/Solar_Orbiter) spacecraft,[2] and at Mars with the [Mars Science Laboratory](/source/Mars_Science_Laboratory) rover's [Radiation assessment detector](/source/Radiation_assessment_detector)[3] and the [MAVEN](/source/MAVEN) orbiter,[4] as well as in the outer solar system by instruments onboard *[Pioneer 10](/source/Pioneer_10)* and *[11](/source/Pioneer_11)* and *[Voyager 1](/source/Voyager_1)* and *[2](/source/Voyager_2)*, even past the orbit of [Neptune](/source/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.[5] Reductions of 30% or more have been recorded aboard the ISS.

The overall rate of Forbush decreases tends to follow the 11-year [sunspot](/source/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 minima](/source/Solar_minimum), when the suppressive effect of CMEs is less frequent.

## See also

- [Ionizing radiation](/source/Ionizing_radiation)

## References

1. **[^](#cite_ref-1)** ["Extreme Space Weather Events"](https://web.archive.org/web/20120522031032/http://sxi.ngdc.noaa.gov/sxi_greatest.html). [National Geophysical Data Center](/source/National_Geophysical_Data_Center). Archived from [the original](http://sxi.ngdc.noaa.gov/sxi_greatest.html) on 2012-05-22. Retrieved 2012-04-19.

1. **[^](#cite_ref-Freiherr_von_Forstner_Dumbović_Möstl_Guo_p._2-0)** Freiherr von Forstner, J. L.; Dumbović, M.; Möstl, C.; Guo, J.; et al. (2021-03-03). "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". *Astronomy & Astrophysics*. **A1**: 656. [arXiv](/source/ArXiv_(identifier)):[2102.12185](https://arxiv.org/abs/2102.12185). [Bibcode](/source/Bibcode_(identifier)):[2021A&A...656A...1F](https://ui.adsabs.harvard.edu/abs/2021A&A...656A...1F). [doi](/source/Doi_(identifier)):[10.1051/0004-6361/202039848](https://doi.org/10.1051%2F0004-6361%2F202039848). [ISSN](/source/ISSN_(identifier)) [0004-6361](https://search.worldcat.org/issn/0004-6361). [S2CID](/source/S2CID_(identifier)) [232035885](https://api.semanticscholar.org/CorpusID:232035885).

1. **[^](#cite_ref-Freiherr_von_Forstner_Guo_Wimmer‐Schweingruber_Hassler_2018_pp._39–56_3-0)** Freiherr von Forstner, Johan L.; Guo, Jingnan; Wimmer-Schweingruber, Robert F.; Hassler, Donald M.; et al. (2018). ["Using Forbush Decreases to Derive the Transit Time of ICMEs Propagating from 1 AU to Mars"](https://doi.org/10.1002%2F2017ja024700). *Journal of Geophysical Research: Space Physics*. **123** (1). American Geophysical Union (AGU): 39–56. [arXiv](/source/ArXiv_(identifier)):[1712.07301](https://arxiv.org/abs/1712.07301). [Bibcode](/source/Bibcode_(identifier)):[2018JGRA..123...39F](https://ui.adsabs.harvard.edu/abs/2018JGRA..123...39F). [doi](/source/Doi_(identifier)):[10.1002/2017ja024700](https://doi.org/10.1002%2F2017ja024700). [ISSN](/source/ISSN_(identifier)) [2169-9380](https://search.worldcat.org/issn/2169-9380).

1. **[^](#cite_ref-Guo_Lillis_Wimmer-Schweingruber_Zeitlin_2018_p=A79_4-0)** Guo, Jingnan; Lillis, Robert; Wimmer-Schweingruber, Robert F.; Zeitlin, Cary; et al. (2018). ["Measurements of Forbush decreases at Mars: both by MSL on ground and by MAVEN in orbit"](https://doi.org/10.1051%2F0004-6361%2F201732087). *Astronomy & Astrophysics*. **611**: A79. [arXiv](/source/ArXiv_(identifier)):[1712.06885](https://arxiv.org/abs/1712.06885). [Bibcode](/source/Bibcode_(identifier)):[2018A&A...611A..79G](https://ui.adsabs.harvard.edu/abs/2018A&A...611A..79G). [doi](/source/Doi_(identifier)):[10.1051/0004-6361/201732087](https://doi.org/10.1051%2F0004-6361%2F201732087). [ISSN](/source/ISSN_(identifier)) [0004-6361](https://search.worldcat.org/issn/0004-6361).

1. **[^](#cite_ref-Lockwood_Webber_Debrunner_1991_p=5447_5-0)** Lockwood, J. A.; Webber, W. R.; Debrunner, H. (1991). "The rigidity dependence of forbush decreases observed at the Earth". *Journal of Geophysical Research*. **96** (A4). American Geophysical Union (AGU): 5447. [Bibcode](/source/Bibcode_(identifier)):[1991JGR....96.5447L](https://ui.adsabs.harvard.edu/abs/1991JGR....96.5447L). [doi](/source/Doi_(identifier)):[10.1029/91ja00089](https://doi.org/10.1029%2F91ja00089). [ISSN](/source/ISSN_(identifier)) [0148-0227](https://search.worldcat.org/issn/0148-0227).

## External links

- [Who's Afraid of a Solar Flare?](https://web.archive.org/web/20100403062301/http://science.nasa.gov/headlines/y2005/07oct_afraid.htm) from Science@NASA

- [Cosmic Ray Data Applications to Space Weather Forecasting](http://www.ips.gov.au/Geophysical/1/4) [Archived](https://web.archive.org/web/20051016005339/http://www.ips.gov.au/Geophysical/1/4) 2005-10-16 at the [Wayback Machine](/source/Wayback_Machine)

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Adapted from the Wikipedia article [Forbush decrease](https://en.wikipedia.org/wiki/Forbush_decrease) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Forbush_decrease?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.