Particle acceleration in relativistic magnetic flux-merging events
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.
- Research Organization:
- Purdue Univ., West Lafayette, IN (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- SC0016369
- OSTI ID:
- 1538924
- Journal Information:
- Journal of Plasma Physics, Vol. 83, Issue 6; ISSN 0022-3778
- Publisher:
- Cambridge University Press
- Country of Publication:
- United States
- Language:
- English
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