A “Forbush Decrease” (also known as Forbush Effect) is the sudden decrease in the intensity of cosmic rays after an increase in solar activity such as a coronal mass ejection (CME).
This process takes place when the solar wind interacts with the magnetosphere and sweeps some of the galactic cosmic rays away from Earth.
Over the following several days, the solar cosmic ray intensity returns to normal.
The main sources of solar cosmic rays are solar flares and coronal mass ejections. They eject huge quantities of material from the Sun at speeds of between 400 – 1,000 km/s.
Particle detectors on our planet observe this decrease process, which appears within a few days after the CME.
The process takes place over the course of a few hours. Forbush decreases have also been observed by crews on Mir and the International Space Station (ISS), and by instruments onboard Pioneer 10 and 11 and Voyager 1 and 2, even past the orbit of Neptune.
American physicist, astronomer and geophysicist, Scott E. Forbush (1904 – 1984), studied cosmic rays in the 1930s and 1940s and discovered the connection between the sudden decrease in their intensity and geomagnetic activity. This event is now called “Forbush Decrease”.
Forbush contributed with many important observations in the field of solar-interplanetary-terrestrial physics, which was at the time still underdeveloped.
The magnitude of this 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%. Even larger decrease of 30 percent or more has been recorded aboard the ISS.
Each Forbush Decrease can warm up the Earth by the same temperature change as the effect of all carbon dioxide emitted by the mankind since the beginning of the Industrial Revolution, according to studies. Major Forbush-Decreases, are also dangerous for people’s health on spacecraft and on the ground; they can increase the rate of myocardial infarctions, brain strokes and car accident road traumas. (source)
Scientists say the overall rate of Forbush decreases tends to follow the 11-year sunspot cycle. As we know, during the cycle, CMEs are most frequent and powerful.
NOTE: The maximum of the current solar cycle was in April, 2014.
It is more difficult to protect astronauts from galactic cosmic rays than from solar wind. Therefore, it is hypothesized that future astronauts might benefit most from radiation shielding during solar maximum.