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

The radial intensity gradients near the ecliptic plane for galactic cosmic ray I-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 Ii-year solar activity cycle but with the 22-year heliomagnetic cycle also having an important role. The data are well ordered out to similar to 55 AU using a radial dependence of the intensity gradient g(r) of the form G(0)r(alpha), where r is the heliocentric distance in astronomical units (AU). With this representation it becomes possible to estimate g, 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 g(r) 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, g(r) increases with increasing r. The radial gradients of the integral rate of ions >70 MeV show a much smaller temporal and spatial variation.