AForbush decrease is a rapid decrease in the observedgalactic cosmic ray intensity following acoronal mass ejection (CME). It occurs due to themagnetic field of theplasmasolar wind sweeping some of the galactic cosmic rays away fromEarth. The termForbush decrease was named after theAmerican physicistScott E. Forbush, who studiedcosmic rays in the 1930s and 1940s.
The Forbush decrease is usually observable byparticle 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 onMir and theInternational Space Station (ISS), at other locations in the inner heliosphere such as theSolar Orbiter spacecraft,[2] and at Mars with theMars Science Laboratory rover'sRadiation assessment detector[3] and theMAVEN orbiter,[4] as well as in the outer solar system by instruments onboardPioneer 10 and11 andVoyager 1 and2, even past the orbit ofNeptune.
The magnitude of a Forbush decrease depends on three factors:
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-yearsunspot 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 duringsolar minima, when the suppressive effect of CMEs is less frequent.