Radial intensity gradients of galactic cosmic rays (1972–1995) in the heliosphere

Abstract

The radial intensity gradients near the ecliptic plane for galactic cosmic ray He (180–450 MeV/nucleon), H (130–220 MeV), and the integral counting rate of ions >70 MeV are studied using the Pioneer 10/11, Voyager 2, and IMP 6, 7, and 8 observing network over the 1972.25–1996.0 time period at heliocentric distances extending to 64 AU. These gradients display complex temporal and spatial changes that are dominated by the 11‐year solar activity cycle but with the 22‐year heliomagnetic cycle also having an important role. The data are well ordered out to ∼55 AU using a radial dependence of the intensity gradient gr of the form G0rα, where r is the heliocentric distance in astronomical units (AU). With this representation it becomes possible to estimate gr as a function of r at a given time or as a function of time at a given radial distance. Over successive solar minima these values of gr decrease rapidly with increasing heliocentric distance, and for He at 50 AU are 3 times larger in 1987 than in 1977, in general agreement with the expected effects of particle gradient and curvature drifts in the large‐scale interplanetary magnetic field. Near solar maximum the gradients increase significantly over their solar minimum values, with a greatly reduced radial dependence for He, while for H, gr increases with increasing r. The radial gradients of the integral rate of ions >70 MeV show a much smaller temporal and spatial variation.