Aims. Magnetic reconnection in large Harris-type reconnecting current sheets (RCSs) with a single X-nullpoint often leads to occurrence of magnetic islands with multiple O- and X-nullpoints. Over time these magnetic islands become
squashed, or coalescent with two islands merging, as it has been observed indirectly during coronal mass ejection and
by in-situ observations in the heliosphere and magnetotail. These points emphasize the importance of understanding the basic energising processes of ambient particles dragged into current sheets with magnetic islands of different configuration.
Methods. Trajectories of protons and electrons accelerated by a reconnection electric field are investigated using a
test particle approach in RCSs with different 3D magnetic field topologies defined analytically for multiple X- and
O-nullpoints. Trajectories, densities and energy distributions are explored for 106 thermal particles dragged into the current sheets from different sides and distances.
Results. This study confrms that protons and electrons accelerated in magnetic islands in a presence of strong guiding field are ejected from a current sheet into the opposite semiplanes with respect to its midplane. Particles are found to escape O-nullpoints only through the neighbouring X-nullpoints along (not across) the midplane following the separation
law for electrons and protons in a given magnetic topology. Particles gain energy either inside O-nullpoints or in a vicinity
of X-nullpoints that often leads to electron clouds formed about the X-nullpoint between the O-nullpoints. Electrons
are shown to be able to gain sub-relativistic energies in a single magnetic island. Energy spectra of accelerated particles are close to power laws with spectral indices varying from 1.1 to 2.4. The more squashed the islands the larger the difference between the energy gains by transit and bounced particles. Their energy spectra are often with double maxima leading to fast growing turbulence.
Conclusions. Particles are shown to gain most energy in multiple X-nullpoints between O-nullpoints (or magnetic islands).
This leads to a formation of electron clouds between magnetic islands. Particle energy gains are much larger in squashed islands than in coalescent ones. In summary, particle acceleration by a reconnection electric field in magnetic islands is much more effective than in an RCS with a single X-nullpoint.
Authors: Q.Xia and V. Zharkova
Publication Status: Astronomy and Astrophysics, accepted
Last Modified: 2018-10-10 13:24