Solar cycle dependence of the radial gradient of cosmic ray intensity

Abstract

Observation of the interplanetary intensity of cosmic rays (Ep > 80 MeV) by Pioneers 10 and 11 now spans a sixteen‐year time period 1972–1988 and heliocentric radial distances, r10 and r11, out to 43.7 AU for Pioneer 10 and 25.8 AU for Pioneer 11. Solar modulation continues to be present at the current distances of both spacecraft. The radial gradient of intensity is measured continuously over the slowly varying, outward moving radial segment Δr = r10 − r11. The 50‐day mean values of the gradient G vary systematically and cyclically in phase with solar activity as measured by sunspot number, with a maximum value of about 2.1 percent (AU)−1 at sunspot maximum and a minimum value of about 1.2 percent (AU)−1 at sunspot minimum. Thus, the apparent scale size of the heliospheric modulation region as measured by 1/G is about 48 AU at solar max and about 83 AU at solar min—a result that is the inverse of the conjectural inference of Randall and Van Allen [1986] using most of the same body of data but a different analytical point of view. There is persuasive evidence that G is independent of radial distance over the range 2.5 to 34 AU in the mid‐point of the segment Δr. No dependence of G on heliographic latitude is evident, but this result does not lend itself to a quantitative statement.