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Density jump for oblique collisionless shocks in pair plasmas: physical solutions

Published online by Cambridge University Press:  16 April 2024

Antoine 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 Center for Astrophysics — Harvard & Smithsonian, Harvard University, 60 Garden St., Cambridge, MA 02138, USA Black Hole Initiative at Harvard University, 20 Garden St., Cambridge, MA 02138, USA
Colby C. Haggerty
Affiliation:
Institute for Astronomy, University of Hawaii, Manoa, 2680 Woodlawn Dr., Honolulu, HI 96822, USA
Ramesh Narayan
Affiliation:
Center for Astrophysics — Harvard & Smithsonian, Harvard University, 60 Garden St., Cambridge, MA 02138, USA Black Hole Initiative at Harvard University, 20 Garden St., Cambridge, MA 02138, USA
*
Email address for correspondence: antoineclaude.bret@uclm.es

Abstract

Collisionless shocks are frequently analysed using the magnetohydrodynamics (MHD) formalism, even though MHD assumes a small mean free path. Yet, isotropy of pressure, the fruit of binary collisions and assumed in MHD, may not apply in collisionless shocks. This is especially true within a magnetized plasma, where the field can stabilize an anisotropy. In a previous article (Bret & Narayan, J. Plasma Phys., vol. 88, no. 6, 2022b, p. 905880615), a model was presented capable of dealing with the anisotropies that may arise at the front crossing. It was solved for any orientation of the field with respect to the shock front. Yet, for some values of the upstream parameters, several downstream solutions were found. Here, we complete the work started in Bret & Narayan (J. Plasma Phys., vol. 88, no. 6, 2022b, p. 905880615) by showing how to pick the physical solution out of the ones offered by the algebra. This is achieved by 2 means: (i) selecting the solution that has the downstream field obliquity closest to the upstream one. This criterion is exemplified on the parallel case and backed up by particle-in-cell simulations. (ii) Filtering out solutions which do not satisfy a criteria already invoked to trim multiple solutions in MHD: the evolutionarity criterion, that we assume valid in the collisionless case. The end result is a model in which a given upstream configuration results in a unique, or no downstream configuration (as in MHD). The largest departure from MHD is found for the case of a parallel shock.

Type
Research Article
Copyright
Copyright © The Author(s), 2024. Published by Cambridge University Press

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