The radial variation of solar flare proton anisotropies observed in deep space on Pioneers 10 and 11

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Abstract
The anisotropy of solar flare protons (>3 MeV) evaluated at the time of maximum for each of 24 events measured by the University of Chicago experiments on Pioneers 10 and 11 is observed to decrease gradually between 1 and 4.7 a.u. Since diffusive streaming is related directly to the effective diffusion coefficient parallel to the interplanetary spiral magnetic field (K∥), these observations provide a measure of the radial dependence of interplanetary diffusion parameters. An analysis of these results suggests that K∥ increases gradually with r consistent with a radial dependence given by K∥ (r) = (8±3) × 1021 cm²/sec r0.9±.4 (r in a.u.).