ARE COSMIC RAYS MODULATED BEYOND THE HELIOPAUSE?

We discuss the possible spatial variation of Galactic and anomalous cosmic rays (GCRs and ACRs) at and beyond the heliopause (HP). Remaining within the framework of the Parker transport equation and assuming incompressible plasma in the heliosheath, we consider highly idealized simple-flow models an...

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Bibliographic Details
Published inThe Astrophysical journal Vol. 782; no. 1; pp. 1 - 6
Main Authors Kota, J, Jokipii, J R
Format Journal Article
LanguageEnglish
Published United States 10.02.2014
Subjects
Anomalous cosmic rays
ASTROPHYSICS, COSMOLOGY AND ASTRONOMY
COMPARATIVE EVALUATIONS
COSMIC RADIATION
Cosmic rays
DIFFUSION
Diffusion coefficient
Diffusion rate
FLOW MODELS
Heliopause
HELIOSPHERE
MAGNETIC FIELDS
MODULATION
PLASMA
SPHERICAL MODEL
SUN
TRANSPORT THEORY
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Summary:We discuss the possible spatial variation of Galactic and anomalous cosmic rays (GCRs and ACRs) at and beyond the heliopause (HP). Remaining within the framework of the Parker transport equation and assuming incompressible plasma in the heliosheath, we consider highly idealized simple-flow models and compare our GCR results with recent publications of Scherer et al. and Strauss et al. First, we discuss an order-of-magnitude estimate and a simple spherical model to demonstrate that the modulation of GCRs beyond the HP must be quite small if the diffusion coefficient beyond the HP is greater than approximately 10 super(26) cm super(2) s super(-1), a value that is two orders of magnitude smaller than the value of 10 super(28) cm super(2) s super(-1) determined from observations of GCR composition. Second, we construct a non-spherical model, which allows lateral deflection of the flow and uses different diffusion coefficients parallel and perpendicular to the magnetic field. We find that modulation of GCRs beyond the HP remains small even if the perpendicular diffusion coefficient beyond the HP is quite small ( approximately 10 super(22) cm super(2) s super(-1)) as long as the parallel diffusion is sufficiently fast. We also consider the case when the parallel diffusion beyond the HP is fast, but the perpendicular diffusion is as small as approximately 10 super(20) cm super(2) s super(-1); this results in a sharp, almost step-like increase of GCR flux (and decrease of ACRs) at the HP. Possible implications are briefly discussed. We further suggest the possibility that the observed sharp gradient of OCRs at the HP might push the HP closer to the Sun than previously thought.
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ISSN:0004-637X
1538-4357
DOI:10.1088/0004-637X/782/1/24