The Roles of Fluid Compression and Shear in Electron Energization during Magnetic Reconnection

Li, Xiaocan; Guo, Fan; Li, Hui; Birn, J.

doi:10.3847/1538-4357/aaacd5

Cited by 71 publications

(88 citation statements)

References 56 publications

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“…9b and d show that the extrapolated density values at the reconnection site (i.e., n e (d = 0)) vary by at least a factor of two, suggesting a high level of density inhomogeneity of δn e /n e > 100%. One of the possible causes is strong plasma compression at the reconnection site, which may facilitate electron acceleration through a Fermi-type mechanism (Provornikova et al 2016;Li et al 2018). The observation of tens of fast electron beams produced from an extremely compact region within a very short, <50 ms time scale is consistent with the bursty magnetic reconnection scenario (Kliem et al 2000;Drake et al 2006;Aschwanden 2002): The reconnection site is highly inhomogeneous and fragmentary, consisting of many fine dynamic structures such as magnetic islands and fractal currents.…”

Section: Discussionmentioning

confidence: 80%

“…However, here we are able to attribute such the 10km-scale fine structures directly to individual reconnection null points thanks to our ultra-high-cadence radio imaging data. We note that spatial scales at kilometer-level in the corona is now readily accessible by state-of-the-art kinetic or hybrid numerical simulations (Egedal et al 2012;Li et al 2018). Observational constraints derived at such fine spatial scales would undoubtedly shed new light on understanding the detailed reconnection-driven particle acceleration processes.…”

Section: Discussionmentioning

confidence: 92%

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Magnetic Reconnection Null Points as the Origin of Semirelativistic Electron Beams in a Solar Jet

Bin

Battaglia

et al. 2018

ApJ

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Magnetic reconnection, the central engine that powers explosive phenomena throughout the Universe, is also perceived as one of the principal mechanisms for accelerating particles to high energies. Although various signatures of magnetic reconnection have been frequently reported, observational evidence that links particle acceleration directly to the reconnection site has been rare, especially for space plasma environments currently inaccessible to in situ measurements. Here we utilize broadband radio dynamic imaging spectroscopy available from the Karl G. Jansky Very Large Array to observe decimetric type III radio bursts in a solar jet with high angular (∼20 ), spectral (∼1%), and temporal resolution (50 milliseconds). These observations allow us to derive detailed trajectories of semi-relativistic (tens of keV) electron beams in the low solar corona with unprecedentedly high angular precision (< 0 .65). We found that each group of electron beams, which corresponds to a cluster of type III bursts with 1-2-second duration, diverges from an extremely compact region (∼600 km 2 ) in the low solar corona. The beam-diverging sites are located behind the erupting jet spire and above the closed arcades, coinciding with the presumed location of magnetic reconnection in the jet eruption picture supported by extreme ultraviolet/X-ray data and magnetic modeling. We interpret each beam-diverging site as a reconnection null point where multitudes of magnetic flux tubes join and reconnect. Our data suggest that the null points likely consist of a high level of density inhomogeneities possibly down to 10-km scales. These results, at least in the present case, strongly favor a reconnection-driven electron acceleration scenario.

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Section: Discussionmentioning

confidence: 80%

Section: Discussionmentioning

confidence: 92%

Magnetic Reconnection Null Points as the Origin of Semirelativistic Electron Beams in a Solar Jet

Bin

Battaglia

et al. 2018

ApJ

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“…By evaluating relative contributions of the pressure tensor work and the fluid compression (shown in with the contribution from the non-ideal electric field. Li et al (2018) discussed the role of fluid compression and shear in electron energization in detail. They also find that fluid compression is the most important contributor in the pressure tensor term.…”

Section: Discussionmentioning

confidence: 99%

“…This is consistent with our result, though both the simulation setup and analysis methods are different than our study. Whereas Li et al (2018) considered only the perpendicular energization J ⊥ · E ⊥ associated with the perpendicular velocity and electric field, we consider the total energization J · E is considered. The parallel energization that is unaccounted for the pressure work is due to the non-ideal electric field.…”

Section: Discussionmentioning

confidence: 99%

Plasma Energization in Colliding Magnetic Flux Ropes

Guo

Zank

et al. 2018

ApJ

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Magnetic flux ropes are commonly observed throughout the heliosphere, and recent studies suggest that interacting flux ropes are associated with some energetic particle events. In this work, we carry out 2D particle-in-cell (PIC) simulations to study the coalescence of two magnetic flux ropes (or magnetic islands), and the subsequent plasma energization processes. The simulations are initialized with two magnetic islands embedded in a reconnecting current sheet. The two islands collide and eventually merge into a single island. Particles are accelerated during this process as the magnetic energy is released and converted to the plasma energy, including bulk kinetic energy increase by the ideal electric field, and thermal energy increase by the fluid compression and the non-ideal electric field. We find that contributions from these different energization mechanisms are all important and comparable with each other. Fluid shear and a non-gyrotropic pressure tensor also contribute to the energy conversion process. For simulations with different box sizes ranging from L x ∼ 25-100d i and ion-to-electron mass ratios m i /m e = 25, 100 and 400, we find that the general evolution is qualitatively the same for all runs, and the energization depends only weakly on either the system size or the mass ratio. The results may help us understand plasma energization in solar and heliospheric environments.

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“…Particle energisation mechanisms include parallel drift (Gordovskyy & Browning 2012), Fermi reflection of particles from the shortening magnetic field line (Hoshino 2012;Dahlin et al 2014), and betatron acceleration in the perpendicular direction (Fu et al 2011;Wang et al 2016a), etc., under the guiding-centre approximation. They could be linked to fluid compression (Zank 2014;Li et al 2018) and shear effects (le Roux et al 2015;Li et al 2018) in the fluid description. Further studies have shown a remarkable consistency between particle energisation in 2.5D and 3D simulations, as the dominant contribution to the particle acceleration might be the same (Hesse et al 2001;Zharkova et al 2011;Guo et al 2014;Dahlin et al 2017).…”

Section: Introductionmentioning

confidence: 99%

Particle acceleration in coalescent and squashed magnetic islands

Xia

Zharkova

2020

A&A

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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.

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The Roles of Fluid Compression and Shear in Electron Energization during Magnetic Reconnection

Cited by 71 publications

References 56 publications

Magnetic Reconnection Null Points as the Origin of Semirelativistic Electron Beams in a Solar Jet

Magnetic Reconnection Null Points as the Origin of Semirelativistic Electron Beams in a Solar Jet

Plasma Energization in Colliding Magnetic Flux Ropes

Particle acceleration in coalescent and squashed magnetic islands

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