A rapid decrease in the observed galactic cosmic ray intensity following the passage of an
outwardly convecting interplanetary magnetic field disturbance, such as those associated with
large CME's, that sweep some galactic cosmic rays away from Earth.
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.
