Evidence for regions of negligible cosmic-ray modulation in the inner heliosphere ( 10 AU)

Gold and Venkatesan report observations of periods during 1974-1976 when extended regions of heliolongitude that emitted lower than average solar wind velocities at 1 AU also exhibited higher than average cosmic ray intensities as measured by the E 35 MeV CPME anti-coincidence scintillator (28 sq cm omnidirectional geometric factor) on IMP-8. Their observations reproduced by a simple model, based on the observed steady solar wind structure, wherein there is little modulation of cosmic rays in the inner heliosphere until they reach the shocked plasma beyond the stream interactions in the outer heliosphere (similar to 5 to 10 AU). Beyond the interaction boundary, the intensity exhibits a constant radial gradient (similar to 2%/AU). The model also offers an explanation for the irregular behavior of the rotation averaged radial gradients observed by inside 10 AU, as well as the significant, but often ephemeral, latitude gradients observed by Voyagers 1 and 2 and IMP-8.