The rigidity dependence of forbush decreases observed at the Earth

The rigidity dependence of the large Forbush decreases occurring on July 23, 1981, July 11, 1982, and February 6, 1986, has been determined using neutron monitor and IMP spacecraft data which cover the energy range from about 50 MeV to 30 GeV. The contribution of solar flare protons to the lower‐energy data from the IMP cosmic ray telescopes was carefully removed. We found that the rigidity dependences of the magnitudes of the July 1981, July 1982, and February 1986 Forbush decreases for P ≥ 2 GV were given by exp (−1/P 0.75 ), exp (−1/P 0.6 ), and exp (−1/P 1.0 ), respectively. For 0.5 ≤ P ≤ 2 GV the magnitude of the Forbush decreases in July 1981 and July 1982 was rigidity independent. The February 1986 event also appeared to be rigidity independent below ∼1 GV. The characteristic recovery time of these Forbush decreases was found to be not strongly rigidity dependent. These results on the rigidity dependence of Forbush decreases for 0.5 < P < 20 GV are discussed in the context of proposed models.