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Title: Impact of compressibility and a guide field on Fermi acceleration during magnetic island coalescence

Authors:
 [1];  [2];  [2]; ORCiD logo [2]
  1. Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
  2. Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA
Publication Date:
Sponsoring Org.:
USDOE
OSTI Identifier:
1421078
Grant/Contract Number:
FG02-97ER25308
Resource Type:
Journal Article: Publisher's Accepted Manuscript
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 24; Journal Issue: 6; Related Information: CHORUS Timestamp: 2018-02-14 21:16:24; Journal ID: ISSN 1070-664X
Publisher:
American Institute of Physics
Country of Publication:
United States
Language:
English

Citation Formats

Montag, P., Egedal, J., Lichko, E., and Wetherton, B.. Impact of compressibility and a guide field on Fermi acceleration during magnetic island coalescence. United States: N. p., 2017. Web. doi:10.1063/1.4985302.
Montag, P., Egedal, J., Lichko, E., & Wetherton, B.. Impact of compressibility and a guide field on Fermi acceleration during magnetic island coalescence. United States. doi:10.1063/1.4985302.
Montag, P., Egedal, J., Lichko, E., and Wetherton, B.. Thu . "Impact of compressibility and a guide field on Fermi acceleration during magnetic island coalescence". United States. doi:10.1063/1.4985302.
@article{osti_1421078,
title = {Impact of compressibility and a guide field on Fermi acceleration during magnetic island coalescence},
author = {Montag, P. and Egedal, J. and Lichko, E. and Wetherton, B.},
abstractNote = {},
doi = {10.1063/1.4985302},
journal = {Physics of Plasmas},
number = 6,
volume = 24,
place = {United States},
year = {Thu Jun 01 00:00:00 EDT 2017},
month = {Thu Jun 01 00:00:00 EDT 2017}
}

Journal Article:
Free Publicly Available Full Text
This content will become publicly available on June 22, 2018
Publisher's Accepted Manuscript

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  • A number of studies have considered how the rate of magnetic reconnection scales in large and weakly collisional systems by the modelling of long reconnecting current sheets. However, this set-up neglects both the formation of the current sheet and the coupling between the diffusion region and a larger system that supplies the magnetic flux. Recent studies of magnetic island merging, which naturally include these features, have found that ion kinetic physics is crucial to describe the reconnection rate and global evolution of such systems. In this paper, the effect of a guide field on reconnection during island merging is considered.more » In contrast to the earlier current sheet studies, we identify a limited range of guide fields for which the reconnection rate, outflow velocity, and pile-up magnetic field increase in magnitude as the guide field increases. The Hall-MHD fluid model is found to reproduce kinetic reconnection rates only for a sufficiently strong guide field, for which ion inertia breaks the frozen-in condition and the outflow becomes Alfvénic in the kinetic system. The merging of large islands occurs on a longer timescale in the zero guide field limit, which may in part be due to a mirror-like instability that occurs upstream of the reconnection region.« less
  • Cited by 1
  • The mechanisms for the production of relativistic electrons associated with the coalescence/reconnection of multiple magnetic islands are investigated using two-dimensional particle-in-cell simulations for the case where the initial island half width L is comparable to the ion inertia length. Configurations without and with a uniform magnetic guide field are considered. Significant energization occurs only when the number of islands is reduced to 2 or 3 with wavelength satisfying k{sub x}L(less-or-similar sign)0.2. The energization proceeds in two distinct stages. In the first stage, a small number of electrons are accelerated to relativistic energies at the X-line by the inductive electric field,more » corresponding to perpendicular acceleration in the absence of the guide field and parallel/anti-parallel acceleration with a guide field. The second stage is associated with the final coalescence into one large island and produces a considerably larger number of relativistic electrons. With a guide field, this stage is dominated by the formation of elongated density cavities along one pair of separatrices and continued direct acceleration at the X-line. Without the guide field, the direct X-line acceleration becomes unimportant, and the acceleration is localized in the flux pile-up regions and results from the curvature drift interacting with the localized inductive electric field. Typically, some 15%-20% of the decrease in magnetic field energy is transferred to the electrons, with a few percent appearing in relativistic (E/m{sub e}c{sup 2}>0.3) electrons.« less
  • The system size dependence of electron acceleration during large-scale magnetic island coalescence is studied via a two-dimensional particle-in-cell simulation. Using a simulation box that is larger than those used in previous studies, injection by merging line acceleration and subsequent reacceleration inside a merged island are found to be the mechanisms for producing the most energetic electrons. This finding and knowledge of the reacceleration process enable us to predict that the high energy end of the electron energy spectrum continues to expand as the merged island size increases. Both the merging line acceleration and the reacceleration within a merged island requiremore » the island coalescence process to be so dynamic as to involve fast in-flow toward the center of a merged island. Once this condition is met in an early stage of the coalescence, it is likely to stay in the subsequent phase. In other words, if the thin elongated current sheet is initially able to host the dynamic magnetic island coalescence process, it will be a site where repeated upgrades in the maximum energy of electrons occur in a systematic manner.« less
  • Radio emission from colliding coronal mass ejection flux ropes in the interplanetary medium suggested the local generation of superthermal electrons. Inspired by those observations, a fully kinetic particle-in-cell simulation of magnetic island coalescence models the magnetic reconnection between islands as a source of energetic electrons. When the islands merge, stored magnetic energy is converted into electron kinetic energy. The simulation demonstrates that a mechanism for electron energization originally applied to open field line reconnection geometries also operates near the reconnection site of merging magnetic islands. The electron heating is highly anisotropic, and it results mainly from an electric field surroundingmore » the reconnection site that accelerates electrons parallel to the magnetic field. A detailed theory predicts the maximum electron energies and how they depend on the plasma parameters. In addition, the global motion of the magnetic islands launches low-frequency waves in the surrounding plasma, which induce large-amplitude, anisotropic fluctuations in the electron temperature.« less