Green's formula and variational principles for cosmic-ray transport with application to rotating and shearing flows

G. M. Webb, J. R. Jokipii, G. E. Morfill

Research output: Contribution to journalArticlepeer-review

11 Scopus citations

Abstract

Green's theorem and Green's formula for the diffusive cosmic-ray transport equation in relativistic flows are derived. Green's formula gives the solution of the transport equation in terms of the Green's function of the adjoint transport equation, and in terms of distributed sources throughout the region ℛ of interest, plus terms involving the particle intensity and streaming on the boundary. The adjoint transport equation describes the time-reversed particle transport. An Euler-Lagrange variational principle is then obtained for both the mean scattering frame distribution function f, and its adjoint f†. Variations of the variational functional with respect to f† yield the transport equation, whereas variations of f yield the adjoint transport equation. The variational principle, when combined with Noether's theorem, yields the conservation law associated with Green's theorem. An investigation of the transport equation for steady, azimuthal, rotating flows suggests the introduction of a new independent variable H to replace the comoving frame momentum variable p′. For the case of rigid rotating flows, H is conserved and is shown to be analogous to the Hamiltonian for a bead on a rigidly rotating wire. The variable H corresponds to a balance between the centrifugal force and the particle inertia in the rotating frame. The physical interpretation of H includes a discussion of nonrelativistic and special relativistic rotating flows as well as the cases of azimuthal, differentially rotating flows about Schwarzs-child and Kerr black holes. Green's formula is then applied to the problem of the acceleration of ultra-high-energy cosmic rays by galactic rotation. The model for galactic rotation assumes an angular velocity law Ω = Ω00/ω), where ω denotes radial distance from the axis of rotation. Green's functions for the galactic rotation problem are used to investigate the spectrum of accelerated particles arising from monoenergetic and truncated power-law sources. We conclude that it is possible to accelerate particles beyond the knee by galactic rotation, but not in sufficient number to adequately explain the observed spectrum.

Original languageEnglish (US)
Pages (from-to)158-180
Number of pages23
JournalAstrophysical Journal
Volume424
Issue number1
DOIs
StatePublished - Mar 20 1994

Keywords

  • Cosmic rays
  • Galaxy: kinematics and dynamics
  • Methods: analytical
  • Particle acceleration

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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