Role of electron physics in the development of turbulent magnetic reconnection in collisionless plasmas

Abstract

Magnetic reconnection releases energy explosively as field lines break and reconnect in plasmas ranging from the Earth's magnetosphere to solar eruptions and astrophysical applications. Collisionless kinetic simulations have shown that this process involves both ion and electron kinetic-scale features, with electron current layers forming nonlinearly during the onset phase and playing an important role in enabling field lines to break 1-4 . In larger two-dimensional studies, these electron current layers become highly extended, which can trigger the formation of secondary magnetic islands 5-10 , but the influence of realistic three-dimensional dynamics remains poorly understood. Here we show that, for the most common type of reconnection layer with a finite guide field, the three-dimensional evolution is dominated by the formation and interaction of helical magnetic structures known as flux ropes. In contrast to previous theories 11 , the majority of flux ropes are produced by secondary instabilities within the electron layers. New flux ropes spontaneously appear within these layers, leading to a turbulent evolution where electron physics plays a central role.Thin current layers are the preferred locations for magnetic reconnection to develop. The most common configuration in nature is guide-field geometry, where the rotation of magnetic field across the layer is less than 180 • . Present theoretical ideas of how reconnection proceeds in these configurations are deeply rooted in early analytical work 11 that, if correct, would imply a direct transition to three-dimensional (3D) turbulence due to a broad spectrum of interacting tearing instabilities. At the core of this idea is the notion that a spectrum of tearing instabilities develops across the initial current sheet for perturbations satisfying the local resonance condition. As these modes grow, the resulting magnetic islands would overlap, leading to stochastic magnetic-field lines and a turbulent evolution. Recently, this type of scenario was proposed as a mechanism for accelerating energetic particles during reconnection 12 . Similar ideas for generating turbulence have been studied in fusion plasmas 13 using resistive magnetohydrodynamics (MHD) and two-fluid 14 models. Alternatively, other researchers have imposed turbulent fluctuations within MHD models in an attempt to understand the consequences 15 . In either case, these results are not applicable to the highly collisionless environment of the magnetosphere, where reconnection is initiated within kinetic ion-scale current layers. The ability to study the self-consistent generation of turbulence during magnetic reconnection with first-principles 3D simulations has only become feasible in the past year.1 Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA, 2 University of California San Diego, La Jolla, California 92093, USA. *e-mail: daughton@lanl.gov. tearing instability gives rise to flux ropes as illustrated by an isosurface of the particle density coloured by the magnitude of the curr...

Review on Current Sheets in CME Development: Theories and Observations

“…Particle-in-cell (PIC) simulations of small-scale reconnection by Daughton et al (2011) show that plasmoids and magnetic islands in 2D correspond to highly structured flux ropes in 3D. In the future, it will be important to couple these small-scale, fully kinetic simulations to larger scales where the resistive MHD approximation is more appropriate.…”

“…Pontin (2014a, 2014b) present simulations of null point reconnection that show the formation of flux ropes which are the 3D analogs of 2D magnetic islands. Fully kinetic 3D simulations by Daughton et al (2011) show that the current sheet develops into a tangled web of interconnected flux ropes, which is likely to also occur in 3D resistive MHD simulations. Over all, developing 3D model is the inevitable tendency, it is worth investing more effort in this issue in the future.…”

“…6.1 and 6.3 of Priest and Forbes 2000, which described possible modes of instabilities that may develop in the CS, and more detailed features of a torn CS, respectively). In three dimensions, the current sheet develops flux ropes which are the counterpart of twodimensional magnetic islands (compare to Daughton et al 2011). The turbulence developing from the tearing mode enhances diffusion of the large-scale magnetic field.…”

Three-dimensional magnetic reconnection and its application to solar flares

“…This is because in three dimensions, plasmoids are not bounded by flux surfaces in the 2-D system (see e.g. Daughton et al 2011). Still, this remains promising for further studies to understand the behaviour of the current volume and link it to observational consequences of reconnection (see § 4.3).…”

“…They are represented in two dimensions by nested magnetic field lines (see figure 17), while in three dimensions it is much more difficult to define what a plasmoid actually is. Indeed, in three dimensions, what would look like a coherent entanglement of field lines in one location may not be actually so different from the surrounding magnetic field in other locations in the 3-D volume, as discussed in § 3.3 (see for example the magnetic field rendering of the 3-D simulation in Daughton et al 2011;Nishida et al 2013). Recent numerical simulations such as by Baalrud, Bhattacharjee & Huang (2012), Wyper & Pontin (2014) provide invaluable tools to comprehend the link between 2-D and 3-D plasmoid dynamics.…”