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Standard Self-Confinement and Extrinsic Turbulence Models for Cosmic Ray Transport are Fundamentally Incompatible with Observations

Hopkins, Philip F. and Squire, Jonathan and Butsky, Iryna S. and Ji, Suoqing (2021) Standard Self-Confinement and Extrinsic Turbulence Models for Cosmic Ray Transport are Fundamentally Incompatible with Observations. . (Unpublished)

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Models for cosmic ray (CR) dynamics fundamentally depend on the rate of CR scattering from magnetic fluctuations. In the ISM, for CRs with energies ~MeV-TeV, these fluctuations are usually attributed either to 'extrinsic turbulence' (ET) -- a cascade from larger scales -- or 'self-confinement' (SC) -- self-generated fluctuations from CR streaming. Using simple analytic arguments and detailed 'live' numerical CR transport calculations in galaxy simulations, we show that both of these, in standard form, cannot explain even basic qualitative features of observed CR spectra. For ET, any spectrum that obeys critical balance or features realistic anisotropy, or any spectrum that accounts for finite damping below the dissipation scale, predicts qualitatively incorrect spectral shapes and scalings of B/C and other species. Even if somehow one ignored both anisotropy and damping, observationally-required scattering rates disagree with ET predictions by orders-of-magnitude. For SC, the dependence of driving on CR energy density means that it is nearly impossible to recover observed CR spectral shapes and scalings, and again there is an orders-of-magnitude normalization problem. But more severely, SC solutions with super-Alfvenic streaming are unstable. In live simulations, they revert to either arbitrarily-rapid CR escape with zero secondary production, or to bottleneck solutions with far-too-strong CR confinement and secondary production. Resolving these fundamental issues without discarding basic plasma processes requires invoking different drivers for scattering fluctuations. These must act on a broad range of scales with a power spectrum obeying several specific (but plausible) constraints.

Item Type:Report or Paper (Discussion Paper)
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URLURL TypeDescription Paper ItemJournal Article
Hopkins, Philip F.0000-0003-3729-1684
Squire, Jonathan0000-0001-8479-962X
Butsky, Iryna S.0000-0003-1257-5007
Ji, Suoqing0000-0001-9658-0588
Additional Information:Support for PFH was provided by NSF Research Grants 1911233 & 20009234, NSF CAREER grant 1455342, NASA grants 80NSSC18K0562, HST-AR-15800.001-A. Support for JS was provided by Rutherford Discovery Fellowship RDF-U001804 and Marsden Fund grant UOO1727, which are managed through the Royal Society Te Aparangi. Numerical calculations were run on the Caltech compute cluster “Wheeler,” allocations FTA-Hopkins/AST20016 supported by the NSF and TACC, and NASA HEC SMD-16-7592. DATA AVAILABILITY STATEMENT. The data supporting this article are available on reasonable request to the corresponding author.
Group:TAPIR, Astronomy Department
Funding AgencyGrant Number
Royal Society Te ApārangiRDF-U001804
Marsden Fund of the Royal Society of New ZealandUOO1727
Subject Keywords:cosmic rays — plasmas — methods: numerical — MHD — galaxies: evolution — ISM: structure
Record Number:CaltechAUTHORS:20220228-183308775
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:113650
Deposited By: George Porter
Deposited On:28 Feb 2022 23:09
Last Modified:08 Dec 2022 00:53

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