Particle acceleration in coalescent and squashed magnetic islands II. Particle-In-cell approach
Valentina Zharkova Submitted: 2020-01-27 09:04
Aims. Particles are known to have efficient acceleration in reconnecting current sheets with multiple magnetic islands,
formed during a reconnection process. Using 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 consider the ambient
plasma feedback to the presence of accelerated particles, which aects 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 3D Harris-type reconnecting current sheets with coalescent (merging) and squashed (contracting) magnetic islands
with different magnetic field topologies, ambient densities ranging 108 - 1012 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, 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 gain 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.
Authors: Xia, Q. and Zharkova V.V.
Publication Status: Astronomy and Astrophysics, accepted
Last Modified: 2020-01-27 14:53