Intensity variations of >70-Mev cosmic rays measured by Pioneer 10, Voyager 1 & 2 and IMP in the heliosphere during the recovery period from 1992-1995
Using data from the >70-MeV channel on the Pioneer 10 and Voyager 1 & 2 spacecraft we have examined the gradients and intensities of cosmic rays in the outer heliosphere between 1992 and 1995. During this time there is a slow increase of intensity towards the next maximum in ∼1997 and the radial gradient is small-decreasing to ∼0.8%/AU at the end of this period. The latitude gradient has also decreased to only ∼0.1-0.2%/AU but remains negative as it was in the previous 11-year cycle. These gradients are similar to those for >70-MeV particles reported recently in the inner heliosphere but quite different from the positive gradients observed at this time throughout the heliosphere by the Ulysses and Voyager spacecraft for anomalous cosmic rays. Overall the cosmic ray intensity is almost uniform in radius, azimuth and latitude at this time in the outer heliosphere-and far lower than the estimated interstellar intensity. The intensities are, in fact, much lower than would be expected based on data from the previous cycle. This, along with the very slow increase that is observed, suggests that, at the same radial distance, the intensity in 1977 will be much less than in the previous cycle in 1987-resulting in a large 22-year modulation wave. The data show some features which suggest that large scale drift patterns, which change when the solar magnetic polarity changes every 11 years, play a important role in the overall intensity distribution in the outer heliosphere-but specific details, particularly in the present positive polarity cycle beginning in 1990 are not well predicted by current models.