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Particle acceleration in relativistic magnetic flux-merging events

2017
78 pages
Published in:
  • J.Plasma Phys. 83 (2017) 066302
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Abstract: (arXiv)
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 σ\sigma. For plasma magnetization σ≤102\sigma \leq 10^2 the spectrum power law index is p>2p> 2; in this case the maximal energy depends linearly on the size of the reconnecting islands. For higher magnetization, σ≥102\sigma \geq 10^2, the spectra are hard, p<2p< 2, yet the maximal energy γmax\gamma_{max} can still exceed the average magnetic energy per particle, ∼σ \sim \sigma, by orders of magnitude (if pp 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.
Note:
  • 78 pages, Invited contribution, Plasma Physics of gamma ray emission from pulsars and their nebulae, Journal of Plasma Physics. arXiv admin note: text overlap with arXiv:1603.05731
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