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Transmission and damping of hydromagnetic waves behind a strong shock front: implications for cosmic ray acceleration

Achterberg, A. and Blandford, R. D. (1986) Transmission and damping of hydromagnetic waves behind a strong shock front: implications for cosmic ray acceleration. Monthly Notices of the Royal Astronomical Society, 218 (3). pp. 551-575. ISSN 0035-8711. doi:10.1093/mnras/218.3.551.

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The transmission of hydromagnetic wave modes by a strong shock front is analyzed. The character and damping rates of the low frequency modes in the high beta plasma behind the shock are described. These results are applied to the theory of Fermi acceleration by shock waves. It is shown that for a sufficiently large angle between the shock normal and the incident magnetic field, wave modes in the post-shock region are subject to linear transit time damping thus impairing the particle acceleration. Short-wavelength modes propagating along the field are subject to non-linear transit time damping (taking account of the saturation associated with particle trapping). Long-wavelength modes, responsible for scattering extremely relativistic ions, should not be damped and should thus mediate particle acceleration. If low-energy cosmic rays are to be accelerated by the Fermi process at a shock front, then either the wave turbulence ahead of the shock must have large amplitude (possibly invalidating the perturbation expansion upon which the calculations described in this paper are based) or hydromagnetic turbulence must be generated behind the shock front. The role of firehose instabilities in generating such post-shock turbulence is briefly discussed.

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Blandford, R. D.0000-0002-1854-5506
Additional Information:© 1986 Royal Astronomical Society. Provided by the NASA Astrophysics Data System. Accepted 1985 August 19. Received 1985 August 12; in original form 1985 June 25. We thank R. Kulsrud for drawing our attention to this problem (in 1979) and H.. Volk for helpful discussion. Discussions with participants in the 1983 College in Plasma Physics held at the International Centre for Theoretical Physics, Trieste, Italy, further clarified our view of the issues involved. RB thanks the director of the Institute of Astronomy, Cambridge for hospitality during summer 1983. Financial support by the National Science Foundation under grant AST-8213001, AST-84-75355 (RB, AA}, and grant AST-82-15456 (AA) is gratefully acknowledged. This work was supported in part by the Netherlands Organization for the Advancement of Pure Research (ZWO) through a Huijgens Fellowship (AA).
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Nederlandse Organisatie voor Zuiver-Wetenschappelijk Onderzoek (ZWO)UNSPECIFIED
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Official Citation:A. Achterberg, R. D. Blandford, Transmission and damping of hydromagnetic waves behind a strong shock front: implications for cosmic ray acceleration, Monthly Notices of the Royal Astronomical Society, Volume 218, Issue 3, 1 February 1986, Pages 551–575,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:95661
Deposited By: George Porter
Deposited On:23 May 2019 17:50
Last Modified:16 Nov 2021 17:14

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