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 solar wind sweeping some of the galactic cosmic rays away from Earth. The term Forbush decrease was named after the American physicist Scott E. Forbush, who studied cosmic rays in the 1930s and 1940s.
The Forbush decrease is usually observable by particle detectors 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, and at Mars with the Mars Science Laboratory rover's Radiation assessment detector and the MAVEN orbiter, as well as in the outer solar system by instruments onboard Pioneer 10 and 11 and Voyager 1 and 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. 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 minima, when the suppressive effect of CMEs is less frequent.
Effects on the atmosphere
A 2009 peer reviewed article 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 until the connection was found in diurnal temperature range, and since confirmed in satellite data.
- Who's Afraid of a Solar Flare? from Science@NASA
- Cosmic Ray Data Applications to Space Weather Forecasting
- "Extreme Space Weather Events". National Geophysical Data Center.
- 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. EDP Sciences. arXiv:2102.12185. doi:10.1051/0004-6361/202039848. ISSN 0004-6361.
- 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". Journal of Geophysical Research: Space Physics. American Geophysical Union (AGU). 123 (1): 39–56. doi:10.1002/2017ja024700. ISSN 2169-9380.
- 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". Astronomy & Astrophysics. EDP Sciences. 611: A79. doi:10.1051/0004-6361/201732087. ISSN 0004-6361.
- Lockwood, J. A.; Webber, W. R.; Debrunner, H. (1991). "The rigidity dependence of forbush decreases observed at the Earth". Journal of Geophysical Research. American Geophysical Union (AGU). 96 (A4): 5447. doi:10.1029/91ja00089. ISSN 0148-0227.
- Svensmark, Henrik; Bondo, Torsten; Svensmark, Jacob (17 June 2009). "Cosmic ray decreases affect atmospheric aerosols and clouds". Geophysical Research Letters. Geophys. Res. Lett. 36 (15): L15101. Bibcode:2009GeoRL..3615101S. CiteSeerX 10.1.1.394.9780. doi:10.1029/2009GL038429.
- Kulmala, M.; Riipinen, I.; Nieminen, T.; Hulkkonen, M.; Sogacheva, L.; Manninen, H. E.; Paasonen, P.; Petäjä, T.; Dal Maso, M.; Aalto, P. P.; Viljanen, A.; Usoskin, I.; Vainio, R.; Mirme, S.; Mirme, A.; Minikin, A.; Petzold, A.; Hõrrak, U.; Plaß-Dülmer, C.; Birmili, W.; Kerminen, V.-M. (2010). "Atmospheric data over a solar cycle: no connection between galactic cosmic rays and new particle formation" (PDF). Atmospheric Chemistry and Physics. 10 (4): 1885–1898. doi:10.5194/acp-10-1885-2010.
- "Sudden Cosmic Ray Decreases. No change of cloud cover" (PDF). 2010. Archived from the original (PDF) on 2010-04-01.
- Dragić, A.; Aničin, I.; Banjanac, R.; Udovičić, V.; Joković, D.; Maletić, D.; Puzović, J. (31 August 2011). "Forbush decreases – clouds relation in the neutron monitor era". Astrophysics and Space Sciences Transactions. 7 (3): 315–318. Bibcode:2011ASTRA...7..315D. doi:10.5194/astra-7-315-2011.
- Svensmark, J; Enghoff, M. B.; Shaviv, N; Svensmark, H (September 2016). "The response of clouds and aerosols to cosmic ray decreases". J. Geophys. Res. Space Phys. 121 (9): 8152–8181. Bibcode:2016JGRA..121.8152S. doi:10.1002/2016JA022689.