Cosmic ray diffusion coefficients in the outer heliosphere at solar maximum

In the heliosphere at solar maximum, cosmic rays undergo a “change-of-state” from a period of several years of predominantly decreasing intensity to the recovery toward the next solar minimum. IMP 8, Voyager and Pioneer cosmic ray observations now cover the solar maximum of cycles 21, 22, and 23 out to a heliocentric distance of 84 AU. For cycles 21 and 23 the depth of modulation for galactic cosmic ray He from solar minimum to solar maximum is essentially the same but it is some 50% larger for cycle 22. In an accompanying paper we introduce a simple procedure for normalizing the data over these three periods, thus obtaining a detailed radial profile of the GCR He and H. A more limited profile of Anomalous Cosmic Rays is available for cycle 23 because of the lower modulation levels in the distant heliosphere. For the combined solar maximum data, diffusion coefficients are calculated using a simple one-dimensional modulation model. It is found that the diffusion coefficient, K, depends on radial distance r as r∼1,2 and on rigidity R as βR∼1.0 respectively in the outer heliosphere beyond 10 AU, where β is particle velocity over light velocity, On the assumption that K is a separable function of r with rγ and R with βRδK is determined as K= (0.31t0.07)βR0.95±0.03r1.21±0.05×1021 cm2sec−1 at heliocentric radial locations from 10 AU to 80 AU, for rigidity from 0.5 GV to 2.0 GV and β from 0.15 to 0.7 (with r in AU and R in GV).