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.
Well we have that going on in a dramatic way right now, it’s been going on since late yesterday. See the Oulu neutron monitor (a proxy for cosmic rays) graph:
That’s a screencap, you can monitor it live on the WUWT solar page here.
The term Forbush decrease was named after the American physicist Scott E. Forbush, who studied cosmic rays in the 1930s and 1940s.