Observation
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 solar cosmic ray intensity returns to normal. Forbush decreases have also been observed by humans on Mir and the International Space Station (IIS), and 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%. 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 maxima, when CMEs are most prevalent.
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