Particle acceleration in coalescent and squashed magnetic islands

Abstract

Aims. Particles are known to have efficient 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 3D Harris-type reconnecting current sheets with coalescent (merging) and squashed (contracting) magnetic islands with different magnetic field topologies, ambient densities ranging between 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, 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.

Investigating Particle Acceleration by Dynamic Small-scale Flux Ropes behind Interplanetary Shocks in the Inner Heliosphere

“…These features can explain why the flux enhancement in Zhao et al (2018) is much larger than the one for this event. Smaller SMFRs with stronger magnetic fields are more active and thus are better at accelerating particles than SMFRs that are larger with weaker magnetic fields (Xia & Zharkova 2020). As a thought experiment, consider two SMFR regions: one of them is not as effective at accelerating particles as the other, and both of them are observed by the same spacecraft over a specific energy range.…”

Division of Magnetic Flux Rope via Magnetic Reconnection Observed in the Magnetotail

“…FRs are always observed around the diffusion region of magnetic reconnection (Russell & Elphic, 1978; Slavin et al., 2003; Teh et al., 2017; R. S. Wang, Lu, Du, & Wang, 2010), and are supposed to be generated by magnetic reconnection (Eastwood et al., 2005; Hasegawa et al., 2010; Lee & Fu, 1985; M. Øieroset et al., 2011; Russell & Elphic, 1978; Scholer, 1988). Meanwhile, the FRs play an important role in the process of magnetic reconnection, such as accelerating electrons (L. J. Chen et al., 2008; Drake et al., 2006; Fu et al., 2006; R. S. Wang, Lu, Li, et al., 2010; Xia & Zharkova, 2018; Z. H. Zhong et al., 2020), accomplishing fast reconnection (Bhattacharjee et al., 2009; Daughton et al., 2009; Loureiro et al., 2007; Samtaney et al., 2009), and dominating the reconnection evolution (Daughton et al., 2011; R. S. Wang et al., 2016).…”

Automated Identification of Current Sheets—A New Tool to Study Turbulence and Intermittency in the Solar Wind

“…The last decade brought a lot of discoveries about properties of CSs in space plasmas, mainly owing to the fast development of the numerical analysis methods such as magnetohydrodynamic (MHD) numerical simulations of fluids that allow studying CS formation in turbulent plasmas (e.g., http://www.mhdturbulence.com/; Burkhart et al., 2020), the Test Particle and Particle‐In‐Cell (PIC) coding on supercomputers (e.g., Hesse et al., 2001; Muñoz & Büchner, 2018; Pritchett, 2003; Xia & Zharkova, 2018, 2020), also implementing General‐Purpose Graphics Processing Units (e.g., https://gyires.inf.unideb.hu/KMITT/a53/ch05.html; Dokken et al., 2007; Mingalev et al., 2019, 2020; Stantchev et al., 2009). Theoretical studies of processes associated with CSs, including numerical simulations employing hybrid models, PIC and MHD/Hall‐MHD simulations, show that CSs effectively contribute to the solar wind dynamics and energy transformation occurring at different scales, from MHD energy‐containing scales to kinetic (see Donato et al., 2012; le Roux et al., 2018, 2020; Papini et al., 2019; Pezzi et al., 2021; Servidio et al., 2010; Wan et al., 2015; Zhdankin et al., 2013 and references therein).…”