Balloon observations of galactic cosmic ray helium before and during a Forbush decrease

The energy spectrum of galactic cosmic ray Helium was measured in two different balloon experiments launched four days apart from Canada: SMILI-I on 1-Sept-1989 and MASS on 5-Sept-1989. A slow Forbush decrease began on 4-Sept-1989 and had not reached its maximum at the time of the MASS flight. Comparison of the balloon measurements shows a fractional decrease of 0.37 to 0.15 in the Helium flux between 200 and 450 MeV/nucleon (1.2-2.0 GV). The rigidity dependence is analyzed in two models and found to be steeper than previous observations. Interplanetary particle data and ground-based Neutron Monitor results are consistent with the balloon observations. Probable sources for this Forbush decrease are discussed.