Radial diffusion coefficients and the distance to the modulation boundary for galactic and anomalous cosmic rays Original Research Article

When cosmic ray streaming in the heliosphere is negligible, the basic transport equation gives a simple approximation for the diffusion coefficient of cosmic ray particles, Krr = CVSW/gr, where C is the Compton-Getting factor, VSW is the solar wind velocity and gr is the radial intensity gradients. Using this equation we calculated the particle diffusion coefficients from 1974 to 1995 out to heliocentric distances of ∼70 AU, using the measured gr for galactic 180–450 MeV/n He, 130–220 MeV H and anomalous 30–57 MeV/n He and 10–20 MeV/n He. Further we made first order estimates of the distance to the modulation boundary, using these diffusion coefficients and the modulation function for galactic 180–450 MeV/n He. It was shown that the distance is from about 65 AU to 110 AU varying inversely proportional to the solar activity for the period from 1978 to 1990. After 1990 these estimation were not significant because of the very small gradients in the outer heliosphere.