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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. Copyright 1993 by the American Geophysical Union.

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Geophysical Research Letters

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