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Particle acceleration in relativistic magnetic flux-merging events

Part of: Plasma Physics of Gamma Ray Emission Focus on Plasma Astrophysics

Published online by Cambridge University Press:  12 December 2017

Maxim Lyutikov ,
Lorenzo Sironi ,
Serguei S. Komissarov  and
Oliver Porth
Maxim Lyutikov*
Affiliation:
Department of Physics, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907-2036, USA
Lorenzo Sironi
Affiliation:
Department of Astronomy, Columbia University, 550 W 120th St, New York, NY 10027, USA
Serguei S. Komissarov
Affiliation:
Department of Physics, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907-2036, USA School of Mathematics, University of Leeds, LS29JT Leeds, UK
Oliver Porth
Affiliation:
School of Mathematics, University of Leeds, LS29JT Leeds, UK Institut für Theoretische Physik, J. W. Goethe-Universität, D-60438, Frankfurt am Main, Germany
*
†Email address for correspondence: lyutikov@purdue.edu
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Abstract

Using analytical and numerical methods (fluid and particle-in-cell simulations) we study a number of model problems involving merger of magnetic flux tubes in relativistic magnetically dominated plasma. Mergers of current-carrying flux tubes (exemplified by the two-dimensional ‘ABC’ structures) and zero-total-current magnetic flux tubes are considered. In all cases regimes of spontaneous and driven evolution are investigated. We identify two stages of particle acceleration during flux mergers: (i) fast explosive prompt X-point collapse and (ii) ensuing island merger. The fastest acceleration occurs during the initial catastrophic X-point collapse, with the reconnection electric field of the order of the magnetic field. During the X-point collapse, particles are accelerated by charge-starved electric fields, which can reach (and even exceed) values of the local magnetic field. The explosive stage of reconnection produces non-thermal power-law tails with slopes that depend on the average magnetization $\unicode[STIX]{x1D70E}$. For plasma magnetization $\unicode[STIX]{x1D70E}\leqslant 10^{2}$ the spectrum power-law index is $p>2$; in this case the maximal energy depends linearly on the size of the reconnecting islands. For higher magnetization, $\unicode[STIX]{x1D70E}\geqslant 10^{2}$, the spectra are hard, $p<2$, yet the maximal energy $\unicode[STIX]{x1D6FE}_{\text{max}}$ can still exceed the average magnetic energy per particle, ${\sim}\unicode[STIX]{x1D70E}$, by orders of magnitude (if $p$ is not too close to unity). The X-point collapse stage is followed by magnetic island merger that dissipates a large fraction of the initial magnetic energy in a regime of forced magnetic reconnection, further accelerating the particles, but proceeds at a slower reconnection rate.

Keywords

magnetized plasmasplasma nonlinear phenomenaspace plasma physics
Type
Research Article
Information
Journal of Plasma Physics , Volume 83 , Issue 6 , December 2017 , 635830602
Copyright
© Cambridge University Press 2017 

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