# Particle acceleration in relativistic magnetic flux-merging events

## 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.

- Authors:

- Publication Date:

- Research Org.:
- Purdue Univ., West Lafayette, IN (United States)

- Sponsoring Org.:
- USDOE Office of Science (SC)

- OSTI Identifier:
- 1538924

- DOE Contract Number:
- SC0016369

- Resource Type:
- Journal Article

- Journal Name:
- Journal of Plasma Physics

- Additional Journal Information:
- Journal Volume: 83; Journal Issue: 6; Journal ID: ISSN 0022-3778

- Publisher:
- Cambridge University Press

- Country of Publication:
- United States

- Language:
- English

- Subject:
- Physics

### Citation Formats

```
Lyutikov, Maxim, Sironi, Lorenzo, Komissarov, Serguei S., and Porth, Oliver.
```*Particle acceleration in relativistic magnetic flux-merging events*. United States: N. p., 2017.
Web. doi:10.1017/s002237781700071x.

```
Lyutikov, Maxim, Sironi, Lorenzo, Komissarov, Serguei S., & Porth, Oliver.
```*Particle acceleration in relativistic magnetic flux-merging events*. United States. doi:10.1017/s002237781700071x.

```
Lyutikov, Maxim, Sironi, Lorenzo, Komissarov, Serguei S., and Porth, Oliver. Fri .
"Particle acceleration in relativistic magnetic flux-merging events". United States. doi:10.1017/s002237781700071x.
```

```
@article{osti_1538924,
```

title = {Particle acceleration in relativistic magnetic flux-merging events},

author = {Lyutikov, Maxim and Sironi, Lorenzo and Komissarov, Serguei S. and Porth, Oliver},

abstractNote = {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.},

doi = {10.1017/s002237781700071x},

journal = {Journal of Plasma Physics},

issn = {0022-3778},

number = 6,

volume = 83,

place = {United States},

year = {2017},

month = {12}

}

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