scholarly journals Particle acceleration in coalescent and squashed magnetic islands

2020 ◽  
Vol 635 ◽  
pp. A116 ◽  
Author(s):  
Q. Xia ◽  
V. Zharkova
Keyword(s):  
Particle Acceleration ◽  
Current Sheet ◽  
Test Particle ◽  
Pitch Angle ◽  
Magnetic Islands ◽  
Current Sheets ◽  
Aspect Ratios ◽  
Energy Gains ◽  
Particle Approach ◽  
Accelerated Particles

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.

scholarly journals Particle acceleration in coalescent and squashed magnetic islands

2018 ◽  
Vol 620 ◽  
pp. A121 ◽  
Author(s):  
Q. Xia ◽  
V. Zharkova
Keyword(s):  
Particle Acceleration ◽  
Electric Field ◽  
Power Laws ◽  
Particle Energy ◽  
Energy Spectra ◽  
Magnetic Islands ◽  
Current Sheets ◽  
The Difference ◽  
Energy Gains ◽  
Electron Clouds

Aims. Magnetic reconnection in large Harris-type reconnecting current sheets (RCSs) with a single X-nullpoint often leads to the occurrence of magnetic islands with multiple O- and X-nullpoints. Over time these magnetic islands become squashed, or coalescent with two islands merging, as has been observed indirectly during coronal mass ejection and by in-situ observations in the heliosphere and magnetotail. These points emphasise 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 confirms that protons and electrons accelerated in magnetic islands in the presence of a strong guiding field are ejected from a current sheet into the opposite semiplanes with respect to the 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 the 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, which leads to their energy spectra having double maxima that gives rise to fast-growing turbulence. Conclusions. Particles are shown to gain the most energy in multiple X-nullpoints between O-nullpoints (or magnetic islands). This leads to the 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.

Particle acceleration in 3D current sheets with magnetic islands: energy, density and pitch angle distributions

2021 ◽  
Author(s):  
Valentina Zharkova ◽  
Qian Xia
Keyword(s):  
Pitch Angle ◽  
Energetic Electron ◽  
High Energy ◽  
Particle Energy ◽  
Magnetic Islands ◽  
Current Sheets ◽  
Particle In Cell ◽  
Small Areas ◽  
Energy Gains ◽  
Pic Method

<div> <div> <div> <p>We will overview particle motion in 3D Harris-type RCSs without and with magnetic islands using particle-in-cell (PIC) method considering the plasma feedback to electromagnetic fields. We evaluate particle energy gains and pitch angle distributions (PADs) of accelerated particles of both changes in different locations inside current sheets as seen under the different directions by a virtual spacecraft passing through. The RCS parameters are considered comparable to heliosphere and solar wind conditions. </p> <p>The energy gains and the PADs of particles are shown to change depending on a topology of magnetic fields.  We report separation of electrons from ions at acceleeration in current sheets with strong guiding fields  and formation of transit and bounced beams from the particles of the same charge. The  transit particles are shown to form  bi-directional energetic electron beams (strahls), while bounced particles are mainly account from driopout fluxes in the heliosphere. In topologies with weak guding field strahls are mainly present inside the magneticislands and located closely above/below the X-nullpoints in the inflow regions. As the guiding field becomes larger, the regions with bi-directional strahls are compressed towards small areas in the exhausts of current sheets. Mono-directional strahls with PADS along 0 or 180 degrees are found quasi-parallel to the magnetic field lines near the X-nullpoint due to the dominant Fermi-type magnetic curvature drift acceleration. Meanwhile, high-energy electrons confined inside magnetic islands create PADs about 90◦. </p> </div> </div> </div>

Fully kinetic PIC simulations of particle acceleration and non-Maxwellian distribution functions due to current sheets in solar wind turbulence

2021 ◽  
Author(s):  
Patricio A. Munoz ◽  
Jörg Büchner ◽  
Neeraj Jain
Keyword(s):  
Solar Wind ◽  
Particle Acceleration ◽  
Current Sheet ◽  
Plasma Turbulence ◽  
Distribution Functions ◽  
Heat Fluxes ◽  
Magnetic Islands ◽  
Ion Distribution ◽  
Current Sheets ◽  
Particle In Cell

<p>Turbulence is ubiquitous in solar system plasmas like those of the solar wind and Earth's magnetosheath. Current sheets can be formed out of this turbulence, and eventually magnetic reconnection can take place in them, a process that converts magnetic into particle kinetic energy. This interplay between turbulence and current sheet formation has been extensively analyzed with MHD and hybrid-kinetic models. Those models cover all the range between large Alfvénic scales down to ion-kinetic scales. The consequences of current sheet formation in plasma turbulence that includes electron dynamics has, however, received comparatively less attention. For this sake we carry out 2.5D fully kinetic Particle-in-Cell simulations of kinetic plasma turbulence including both ion and electron spectral ranges. In order to further assess the electron kinetic effects, we also compare our results with hybrid-kinetic simulations including electron inertia in the generalized Ohm's law. We analyze and discuss the electron and ion energization processes in the current sheets and magnetic islands formed in the turbulence. We focus on the electron and ion distribution functions formed in and around those current sheets and their stability properties that are relevant for the micro-instabilities feeding back into the turbulence cascade. We also compare pitch angle distributions and non-Maxwellian features such as heat fluxes with recent in-situ solar wind observations, which demonstrated local particle acceleration processes in reconnecting solar wind current sheets [Khabarova et al., ApJ, 2020].</p>

Solar wind re-acceleration in local current sheets and their diagnostics from observations

2020 ◽  
Author(s):  
Qian Xia ◽  
Valentina Zharkova
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Cross Sections ◽  
Pitch Angle ◽  
Interplanetary Space ◽  
Angle Distribution ◽  
Magnetic Islands ◽  
Current Sheets ◽  
Particle In Cell ◽  
Accelerated Particles

<p><span>We explore solar wind re-acceleration during their passage through reconnecting current sheets in the interplanetary space using the particle-in-cell approach. We investigate particle acceleration in 3D Harris-type reconnecting current sheets with a single or multiple X-nullpoints taking into account the ambient plasma feedback to the presence of accelerated particles. We also consider coalescent and squashed magnetic islands formed in the current sheets with different magnetic field topologies, thickness, ambient density, and mass ratios. With the PIC approach, we detected distinct populations of two groups of particles, transit and bounced ones, which have very different energy and asymmetric pitch-angle distributions associated with the magnetic field parameters. We present a few cross-sections of the simulated pitch-angle distributions of accelerated particles and compare them with the in-situ observations of solar wind particles. This comparison indicates that locally generated superthermal electrons can account for the counter-streaming ‘strahls’ often observed in pitch-angle distribution spectrograms of the satellites crossing heliospheric current sheets.</span></p>

Current sheets, magnetic islands and associated particle acceleration in the solar wind as observed by Ulysses near the ecliptic plane

2020 ◽  
Author(s):  
Olga Malandraki ◽  
Olga Khabarova ◽  
Roberto Bruno ◽  
Gary Zank ◽  
Gang Li and the ISSI-405 team
Keyword(s):  
Solar Wind ◽  
Particle Acceleration ◽  
Magnetic Reconnection ◽  
Energetic Particle ◽  
Particle Flux ◽  
Energetic Particles ◽  
Magnetic Islands ◽  
Intermittent Turbulence ◽  
Current Sheets ◽  
Energetic Particle Flux

<p>Recent studies of particle acceleration in the heliosphere have revealed a new mechanism that can locally energize particles up to several MeV/nuc. Stream-stream interactions as well as the heliospheric current sheet – stream interactions lead to formation of large magnetic cavities, bordered by strong current sheets (CSs), which in turn produce secondary CSs and dynamical small-scale magnetic islands (SMIs) of ~0.01AU or less owing to magnetic reconnection. It has been shown that particle acceleration or re-acceleration occurs via stochastic magnetic reconnection in dynamical SMIs confined inside magnetic cavities observed at 1 AU. The study links the occurrence of CSs and SMIs with characteristics of intermittent turbulence and observations of energetic particles of keV-MeV/nuc energies at ~5.3 AU. We analyze selected samples of different plasmas observed by Ulysses during a widely discussed event, which was characterized by a series of high-speed streams of various origins that interacted beyond the Earth’s orbit in January 2005. The interactions formed complex conglomerates of merged interplanetary coronal mass ejections, stream/corotating interaction regions and magnetic cavities. We study properties of turbulence and associated structures of various scales. We confirm the importance of intermittent turbulence and magnetic reconnection in modulating solar energetic particle flux and even local particle acceleration. Coherent structures, including CSs and SMIs, play a significant role in the development of secondary stochastic particle acceleration, which changes the observed energetic particle flux time-intensity profiles and increases the final energy level to which energetic particles can be accelerated in the solar wind.</p>

scholarly journals Particle acceleration in three-dimensional reconnection regions: A new test particle approach

1999 ◽  
Vol 6 (11) ◽  
pp. 4318-4327 ◽  
Author(s):  
Rüdiger Schopper ◽  
Guido T. Birk ◽  
Harald Lesch
Keyword(s):  
Particle Acceleration ◽  
Test Particle ◽  
Three Dimensional ◽  
Particle Approach

Current Sheets, Magnetic Islands, and Associated Particle Acceleration in the Solar Wind as Observed by Ulysses near the Ecliptic Plane

2019 ◽  
Vol 881 (2) ◽  
pp. 116 ◽  
Author(s):  
Olga Malandraki ◽  
Olga Khabarova ◽  
Roberto Bruno ◽  
Gary P. Zank ◽  
Gang Li ◽  
...  
Keyword(s):  
Solar Wind ◽  
Particle Acceleration ◽  
Ecliptic Plane ◽  
Magnetic Islands ◽  
Current Sheets
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