Test-electron analysis of the magnetic reconnection topology

Abstract

Three-dimensional (3D) investigations of the magnetic reconnection field topology in space and laboratory plasmas have identified the abidance of magnetic coherent structures in the stochastic region, which develop during the nonlinear stage of the reconnection process. Further analytical and numerical analyses highlighted the efficacy of some of these structures in limiting the magnetic transport. The question then arises as to what is the possible role played by these patterns in the dynamics of the plasma particles populating the chaotic region. In order to explore this aspect, we provide a detailed description of the nonlinear 3D magnetic field topology in a collisionless magnetic reconnection event with a strong guide field. In parallel, we study the evolution of a population of test electrons in the guiding-center approximation all along the reconnection process. In particular, we focus on the nonlinear spatial redistribution of the initially thermal electrons and show how the electron dynamics in the stochastic region depends on the sign and on the value of their velocities. While the particles with the highest positive speed populate the coherent current structures that survive in the chaotic sea, the presence of the manifolds calculated in the stochastic region defines the confinement area for the electrons with the largest negative velocity. These results stress the link between the magnetic topology and the electron motion and contribute to the overall picture of a non-stationary fluid magnetic reconnection description in a geometry proper to physical systems where the effects of the curvature can be neglected.

A reduced Landau-gyrofluid model for magnetic reconnection driven by electron inertia

“…In this limit, the model yields a fluid reduction, with electron temperature anisotropy, of the hybrid kinetic-fluid model of Zocco & Schekochihin (2011). Neglecting temperature fluctuations, one retrieves (up to the normalization) the two-field model derived in Schep et al (1994) and investigated in Borgogno et al (2005).…”

“…In terms of particle moments, the model (4.1)-(4.6) can then be reformulated as In the absence of temperature fluctuations (i.e. t e = t ⊥e = 0), the model corresponds, up to the normalization, to the two-field model investigated in Borgogno et al (2005) and Grasso et al (2007).…”

“…(2.42). The new code has been benchmarked in the non-dissipative isothermal limit against previous codes solving the equations described in Grasso et al (2007) and Borgogno et al (2005).…”

“…In order to reduce the computational time, while preserving the accuracy in the spatial discretization, we adopted an alternative approach valid for single helicity problems. As described in Borgogno et al (2005), it consists in the solution of an equivalent twodimensional problem in the new coordinates (x, y * ), where y * = y + (k z /k y )z, with a shifted equilibrium magnetic flux function A * eq (x) = A eq (x) − (k z /k y )x. Figure 3 shows the evolution of characteristic properties of the magnetic island with and without the Landau term versus the time normalized with respect to the corresponding growth rate.…”

“…In our context, reduced fluid models are those which apply to plasmas in the presence of a magnetic field with an intense component along one direction (strong guide field assumption) and are characterized by quadratic nonlinearities and strong scale anisotropy. Over the years, a large number of such models were derived, for instance for describing drift wave turbulence (Hasegawa & Mima 1978;Hasegawa & Wakatani 1983;Camargo et al 1996), magnetic reconnection (Schep et al 1994;Grasso et al 2001;Borgogno et al 2005;Del Sarto et al 2006;Muraglia et al 2009;Grasso & Tassi 2015), fundamental aspects of nonlinear plasma dynamics in a tokamakrelevant ordering (Strauss 1976;Hazeltine et al 1985Hazeltine et al , 1987, turbulence accounting for magnetic fluctuations parallel to the guide field direction, as of interest for the solar wind (Boldyrev et al 2013;Schekochihin et al 2009;Tassi et al 2016), and ion temperature gradient driven modes (Kim et al 1993), only to mention a very few examples. A number of reduced gyrofluid models, obtained from the evolution equations of moments of gyrokinetic distribution functions, were also derived in order to describe low-frequency plasma phenomena (see, e.g.…”