The Inner Heliosphere — Outer Heliosphere Comparison for Cosmic Ray Modulation

Abstract

This paper summarizes cosmic ray data on both galactic and anomalous particles in the inner and outer heliosphere near the sunspot minimum in 1995 and 1996 at the end of solar cycle 22. These data come from the IMP spacecraft in the inner heliosphere and the Voyager and Pioneer spacecraft in the outer heliosphere. In the inner heliosphere, the cosmic ray intensities at all energies in 1996 have recovered to almost the same maximum values they had at the last sunspot minimum in 1987 and the intensities are an even closer match to those observed two 11-year cycles earlier in 1976. In the outer heliosphere beyond 40 AU the intensity recovery is very slow and the intensities at all energies and for all species are almost constant in 1995–96 indicating that little further recovery can be expected in this cycle. The intensity of galactic cosmic rays in 1996 is only ~.0.3–0.5 of that observed at the same radius of 42 AU in 1987 and for anomalous cosmic rays this ratio is only 0.1–0.2. This suggests a dramatically different entry of particles into the heliosphere in the two cycles for both types of particles as well as significantly different particle flow characteristics in the outer heliosphere. The net result of these different characteristics is that near the Earth only a relatively small intensity difference is observed between successive II-year solar cycles whereas in the outer heliosphere the differences between cycles become very large and may even dominate the overall modulation.