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Departure from MHD prescriptions in shock formation over a guiding magnetic field

Published online by Cambridge University Press:  09 August 2017

A. Bret*
Affiliation:
ETSI Industriales, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain Instituto de Investigaciones Energéticas y Aplicaciones Industriales, Campus Universitario de Ciudad Real, 13071 Ciudad Real, Spain
A. Pe'er
Affiliation:
Physics Department, University College Cork, Cork, Ireland
L. Sironi
Affiliation:
Department of Astronomy, Columbia University, New York, NY 10027, USA
M.E. Dieckmann
Affiliation:
Department of Science and Technology, Linköping University, SE-60174 Norrköping, Sweden
R. Narayan
Affiliation:
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-51 Cambridge, MA 02138, USA
*
Address correspondence and reprint requests to: A. Bret, ETSI Industriales, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain and Instituto de Investigaciones Energéticas y Aplicaciones Industriales, Campus Universitario de Ciudad Real, 13071 Ciudad Real, Spain. E-mail: [email protected]

Abstract

In plasmas where the mean-free-path is much larger than the size of the system, shock waves can arise with a front much shorter than the mean-free-path. These so-called “collisionless shocks” are mediated by collective plasma interactions. Studies conducted so far on these shocks found that although binary collisions are absent, the distribution functions are thermalized downstream by scattering on the fields, so that magnetohydrodynamics prescriptions may apply. Here we show a clear departure from this pattern in the case of Weibel shocks forming over a flow-aligned magnetic field. A micro-physical analysis of the particle motion in the Weibel filaments shows how they become unable to trap the flow in the presence of too strong a field, inhibiting the mechanism of shock formation. Particle-in-cell simulations confirm these results.

Type
Research Article
Copyright
Copyright © Cambridge University Press 2017 

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