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 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
, and by instruments onboard Pioneer 10
and Voyager 1
, 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.
The term Forbush decrease
was named after the American physicist Scott E. Forbush
, who studied cosmic rays in the 1930s and 1940s.