On the effect of directional medium-scale interplanetary variations on the diffusion of galactic cosmic rays and their solar cycle variation

It is argued that the interplanetary magnetic field cannot be simply considered as consisting of small short-scale resonant fluctuations superposed on the long-term average spiral field if one wants to calculate the average transport properties of galactic cosmic rays. The interest is focused on pitch angle diffusion, where the relevant average field varies strongly on a medium scale exceeding the magnetic correlation length. This leads to an average Fokker-Planck equation for the average galactic intensity. Direct reflections and large-scale drift effects that may play a role on the same scale are not considered. When the interplanetary medium is assumed to be statistically axisymmetric, an average spatial diffusion coefficient is derived from the above Fokker-Planck equation and compared with the Pioneer 10 and 11 radial intensity gradients as obtained by the Chicago group. The medium-scale variations also introduce a solar cycle variation of the diffusion coefficient from which a corresponding variation of the galactic intensity is estimated. Quantitatively, the resulting solar cycle variation appears to be only of marginally sufficient strength. This may not be too surprising, since changes in magnetic field topology, drift effects, and direct particle reflections have not been considered.