THE RADIAL AND LATITUDINAL DEPENDENCE OF THE COSMIC-RAY DIFFUSION TENSOR IN THE HELIOSPHERE

The radial and latitudinal dependence of the cosmic ray diffusion tens or is investigated on the basis of a recently developed model of magne tohydrodynamic turbulence in the expanding solar wind [Zank et al., 19 96a,b; Matthaews et al., 1996]. In the ecliptic plane, decaying magnet ohydrodynamic turbulence is assumed to be replenished in situ by turbu lence generated through the interaction of streams (both shear and com pressional effects) and by the creation of pickup ions. In the polar r egion, at least during solar minimum, stream interaction driven turbul ence is neglected and only pickup ion driven turbulence is included. T o model the perpendicular and drift elements of the cosmic ray diffusi on tensor, we employ both a quasi-linear theory (QLT) and a newly deve loped nonperturbative theory (NPT) to describe the field line wanderin g which drives perpendicular transport. A resonant quasi-linear descri ption is applied to the parallel component. For the QLT approach, we f ind that in the solar wind ecliptic plane (1) the radial diffusive len gth scale or mean free path (mfp) is very nearly constant until some 1 0 AU, after which it experiences some variation with increasing helioc entric distance; (2) the radial mfp is dominated at all radial distanc es by the component parallel to the mean magnetic field and the perpen dicular component is completely unimportant; (3) the length scale asso ciated with the drift component of the cosmic ray diffusion tensor is only comparable to the radial mfp beyond similar to 10 AU; and (4) the rigidity P dependence of the radial mfp within 10 - 20 AU is weak and proportional to P-1/3, but in the far outer heliosphere it is proport ional to P-2. For the QLT model in the polar region of the solar wind, we find that the radial cosmic ray mfp is much greater than the corre sponding mfp in the ecliptic region, consistent with observed mfps for pickup ions reported by Gloeckler et al. [1995]. The polar models are , however, preliminary and assume vanishing cross-helicity. The polar radial mfp is dominated by the parallel component, and drift length sc ales are never comparable to the radial mfp in the high polar latitude s. By using instead a nonperturbative model for the perpendicular and drift components of the cosmic ray diffusion tensor, it was found that the mfps for these coefficients could be significantly larger than th eir QLT counterparts. The increased perpendicular mfp was found to be important in the radial mfp only beyond similar to 20 AU, which remain s dominated by the parallel diffusion within this distance. Within the ecliptic, the nonperturbative model yields a radial mfp for cosmic ra ys that is almost constant with heliocentric distance. Similar order o f magnitude differences between the radial mfps in the ecliptic and po lar regions of the solar wind are found with the nonperturbative model s.