Particle acceleration in coalescent and squashed magnetic islands: II. Particle-in-cell approach

Qian Xia, Valentina Zharkova

Research output: Contribution to journalArticlepeer-review

4 Citations (Scopus)
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Abstract

Aims. Particles are known to have effcient acceleration in reconnecting current sheets with multiple magnetic islands that are formed during a reconnection process. Using the test-particle approach, the recent investigation of particle dynamics in 3D magnetic islands, or current sheets with multiple X- and O-null points revealed that the particle energy gains are higher in squashed magnetic islands than in coalescent ones. However, this approach did not factor in the ambient plasma feedback to the presence of accelerated particles, which affects their distributions within the acceleration region.
Methods. In the current paper, we use the particle-in-cell (PIC) approach to investigate further particle acceleration in 3 D Harris-type reconnecting current sheets with coalescent (merging) and squashed (contracting) magnetic islands
with different magnetic field topologies, ambient densities ranging between 1e8 − 1e12/m^3 , proton-to-electron mass ratios, and island aspect ratios.
Results. In current sheets with single or multiple X-nullpoints, accelerated particles of opposite charges are separated and ejected into the opposite semiplanes from the current sheet midplane, generating a strong polarisation electric field across a current sheet. Particles of the same charge form two populations: transit and bounced particles, each with very different energy and asymmetric pitch-angle distributions, which can be distinguished from observations. In some cases, the difference in energy gains by transit and bounced particles leads to turbulence generated by Buneman instability. In
magnetic island topology, the different reconnection electric fields in squashed and coalescent islands impose different particle drift motions. This makes particle acceleration more efficient in squashed magnetic islands than in coalescent ones. The spectral indices of electron energy spectra are ∼ −4.2 for coalescent and ∼ −4.0 for squashed islands, which are lower than reported from the test-particle approach. The particles accelerated in magnetic islands are found trapped in the midplane of squashed islands, and shifted as clouds towards the X-nullpoints in coalescent ones.
Conclusions. In reconnecting current sheets with multiple X- and O-nullpoints, particles are found accelerated on a much shorter spatial scale and gaining higher energies than near a single X-nullpoint. The distinct density and pitch-
angle distributions of particles with high and low energy detected with the PIC approach can help to distinguish the observational features of accelerated particles.
Original languageEnglish
Article numberA116
Number of pages12
JournalAstronomy & Astrophysics
Volume635
Early online date18 Mar 2020
DOIs
Publication statusPublished - Mar 2020

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