Effect of pick-up ions on the magnetic field correlation length: Implications for solar modulation of galactic cosmic rays Original Research Article

In this study the solar modulation of cosmic rays is modeled by direct numerical simulation of Parkerʹs transport equation. We use our ab initio modulation model where the diffusion tensor is specified by particle scattering theory based on observed properties of the turbulence, and the spatial variation is determined by turbulence transport models that specify how turbulence properties vary throughout the heliosphere. We discuss the effect of pick-up ions on the spatial dependence of the magnetic field correlation length which is not well understood. If the effect is negligible, turbulence models suggest that the correlation length should increase almost monotonically with radial distance. If not, it should at some point (determined by the size of the ionization cavity) start to decrease. We model such a two-stage behavior by assuming two power laws for the radial dependence of the correlation length. We show that varying the position of the breakpoint between the two power laws by as little as 10 AU has large effects on the radial intensity profiles and provides better agreement with observation than when a single power law is used (Parhi et al. 2002, 2003).