Kinetic turbulence in fast three-dimensional collisionless guide-field magnetic reconnection

Abstract

Although turbulence has been conjectured to be important for magnetic reconnection, still very little is known about its role in collisionless plasmas. Previous attempts to quantify the effect of turbulence on reconnection usually prescribed Alfvénic or other low-frequency fluctuations or investigated collisionless kinetic effects in just two-dimensional configurations and antiparallel magnetic fields. In view of this, we analyzed the kinetic turbulence self-generated by three-dimensional guidefield reconnection through force-free current sheets in frequency and wavenumber spaces, utilizing 3D particle-in-cell code numerical simulations. Our investigations reveal reconnection rates and kinetic turbulence with features similar to those obtained by current in-situ spacecraft observations of MMS as well as in the laboratory reconnection experiments MRX, VTF and Vineta-II. In particular we found that the kinetic turbulence developing in the course of 3D guide-field reconnection exhibits a broadband power-law spectrum extending beyond the lower-hybrid frequency and up to the electron frequencies. In the frequency space the spectral index of the turbulence appeared to be close to -2.8 at the reconnection X-line. In the wavenumber space it also becomes -2.8 as soon as the normalized reconnection rate reaches 0.1. The broadband kinetic turbulence is mainly due to current-streaming and electron-flow-shear instabilities excited in the sufficiently thin current sheets of kinetic reconnection. The growth of the kinetic turbulence corresponds to high reconnection rates which exceed those of fast laminar, non-turbulent reconnection.

Reconnection from a turbulence perspective

“…On the other hand, the role of turbulence associated with reconnection has received significant scrutiny. Reconnection is known to generate several secondary instabilities (e.g., 7,12,25,49 and references therein) which themselves have been shown to generate energy spectra with power laws consistent with a turbulent cascade 26,31,37 . Turbulent fluctuations in the plasma flowing into the reconnection region have been found to affect the process of reconnection 30 , and even amplify the reconnection rate 23 .…”

Electron Heating by Debye-Scale Turbulence in Guide-Field Reconnection

“…Numerical simulations suggest that in such situation streaming instabilities may be satisfied at the X-line, and these instabilities can lead to electron heating and anomalous resistivity [11,12]. Kinetic turbulence over a broad frequency range reaching up to the electron plasma frequency develops in guide field reconnection [13]. Buneman and two-streaming instabilities are responsible for the highfrequency (above the lower-hybrid frequency) kinetic turbulence.…”

Particle acceleration in coalescent and squashed magnetic islands

“…Magnetic reconnection is a fundamental physical process driving the primary release of magnetic field energy in many events in the Sun (Priest & Forbes 2000;Somov 2000;Benz 2017), Earth magnetosphere (Angelopoulos et al 2008;Chen et al 2008), and heliosphere (Gosling 2007;Manchester et al 2014). The magnetic reconnection process converts magnetic energy into the energy of energetic particles (Lin et al 2003;Zharkova et al 2011;Zharkova & Khabarova 2012;Muñoz & Büchner 2018), emission generated by particles, and associated plasma flows. Observations of hard X-ray (HXR; Holman et al 2011) and γ-ray (Vilmer et al 2011) emission in solar flares support large-extent magnetic reconnection as the main source of energetic particles, from which they gain energy while passing through a diffusion region of magnetic reconnection (see, for example Zharkova et al 2011;Egedal et al 2013, and references therein).…”

“…During a 3D magnetic reconnection with a guiding magnetic field, particles are firstly accelerated within a very short sub-millisecond timescale by a reconnection electric field (Litvinenko 1996;Zenitani & Hoshino 2001;Zharkova & Gordovskyy 2004;Bessho & Bhattacharjee 2012;Melzani et al 2014;Sironi & Spitkovsky 2014). Later the particles can be further accelerated by turbulence (Siversky & Zharkova 2009;Kowal et al 2012;Lazarian et al 2012;Muñoz & Büchner 2018;Zhdankin et al 2019). 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.…”

“…It is evident that the transit particles would gain much more energy than the bounced ones (Siversky & Zharkova 2009;Zharkova & Khabarova 2012). Besides, the difference between the energy gains by transit and bounced particle beams would introduce the bump-on-tail distribution, which leads to Buneman instabilities (Buneman 1958) and generates turbulence (see for example Siversky & Zharkova 2009;Drake et al 2010;Muñoz & Büchner 2018). The second-order Fermi acceleration by stochastic turbulence is also suggested as one of important particle acceleration mechanisms by a number of authors (see, e.g., Petrosian 2012;Lazarian et al 2012, and references therein).…”