Giant transient decreases of cosmic rays in the outer heliosphere in September 1991

Large transient decreases of >20% were observed in the >70 MeV cosmic ray intensity in September 1991, at the three spacecraft, V1, V2, and P10, in the outer heliosphere between 35 and 53 AU. These decreases appear to be related to the intense solar activity occurring in late May and early June, which was responsible for a series of rapid transient decreases at the Earth that probably reduced the >70-MeV intensity by more than 50% to the lowest level ever recorded. Average transit speeds of ~600 to 800 km/s are deduced for the propagation of these transients between the Earth and the outer heliosphere. The overall picture of the propagation of these transients is consistent with a massive, almost spherical modulating region moving outward in the heliosphere. The smooth almost exponential recovery of intensities at all three spacecraft for over 160 days after the decrease, and the large total modulation beyond P10 at the time of the decrease, suggest that the modulation boundary is well beyond 53 AU and probably beyond 100 AU.