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• W3163252144 abstract "School of Physics, Potchefstroom University for CHE, Potchefstroom, South AfricaAbstract. Recent efforts to improve and complete modula-tion theory are described, emphasizing factors that arise inattempts to understand the diffusion tensor based upon tur-bulence theory and the theory of charged particle scattering.Direct numerical solutions of the transport equations are usedto study a new perpendicular diffusion formalism that cansmoothly vary between the field line random walk limit anda recent treatment based upon the Taylor–Green–Kubo equa-tion. Using this new formalism we show that modulation isstrongly influenced by the radial variation of the correlationlength. This variation is still poorly understood, in part ow-ing to the uncertain impact of pickup ion driven turbulencein the outer heliosphere.1 IntroductionCosmic ray modulation is a complex subject relating galac-tic particle populations to heliospheric structure, while in-volving at a detailed level incompletely understood theoreti-cal features of charged particle scattering and plasma turbu-lence. Scattering theory involves turbulence parameters, andthus one needs to understand how plasma turbulence evolvesthroughout the heliosphere. Even in the simplest formula-tions of scattering and turbulence theories this would involvespecification of a turbulence energy density and a similar-ity length scale everywhere in the three dimensional helio-sphere. Boundary data in the inner heliosphere can stand inplace of a fuller understanding of the origins of solar windturbulence, but it is also necessary to understand how tur-bulence is driven within the heliosphere, for example, byshear and excitation of fluctuations by scattering of interstel-lar pickup ions. In more elaborate models of turbulence thefluctuations may be represented as consisting of two or morecomponents, each with a known symmetry (Matthaeus et al.,1990; Bieber et al., 1994). A useful example is the two-component model which includes one dimensional “slab”Correspondence to: S. Parhi (sparhi@bartol.udel.edu)and two dimensional (2D) ingredients. Scattering theory it-self also involves many issues and is incompletely under-stood. Perpendicular diffusion, especially at lower energies(Giacalone and Jokipii, 1999; Mace et al., 2000) is not wellaccounted for theoretically. For these reasons it is obviouswhy there is at present no accepted ab initio modulation the-ory, that is, one in which the diffusion coefficients are deter-mined on the basis of scattering theory and the underlyingmagnetic fluctuation parameters are computed from plasmatheory and known features of heliospheric structure. Re-cently we have been attempting to further develop an ab ini-tio approach to modulation, building on recent efforts alongthese lines (Zank et al, 1998; Burger et al., 2000).?From the start it becomes apparent that modulation isrelatively sensitive to features of turbulence and scatteringwhich themselves are active areas of research. Thus it ap-pears appropriate to view these efforts as investigations ofscattering and turbulence on the same footing as they are in-vestigations of modulation. One such parameter that sensi-tively affects modulation is the parallel correlation scale ofmagnetic fluctuations, and this is the subject of emphasis inthe present short paper. The main point of the present paperis to show the type of effects that can be expected in modula-tion when the assumptions concerning the radial variation ofthe correlation scale are varied in a modulation model.2 Modulation modelThe modulation of galactic cosmic rays is described by Par-ker’s equation. In our model we describe the radial coeffi-cient by " @default.
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• W3163252144 date "2001-08-01" @default.
• W3163252144 modified "2023-09-24" @default.
• W3163252144 title "Challenges for an ab initio Theory of Cosmic Ray Modulation" @default.
• W3163252144 hasPublicationYear "2001" @default.
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