ARE COSMIC RAYS MODULATED BEYOND THE HELIOPAUSE?

• Published in: The Astrophysical journal 2014-02, Vol.782 (1), p.1-6
• Main Authors: Kota, J, Jokipii, J R
• Format: Article
• Language: English
• 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
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/782/1/24

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.