Magnetic reconnection as an element of turbulence

Abstract

Abstract. In this work, recent advances on the study of reconnection in turbulence are reviewed. Using direct numerical simulations of decaying incompressible two-dimensional magnetohydrodynamics (MHD), it was found that in fully developed turbulence complex processes of reconnection locally occur (Servidio et al., , 2010a. In this complex scenario, reconnection is spontaneous but locally driven by the fields, with the boundary conditions provided by the turbulence. Matching classical turbulence analysis with a generalized Sweet-Parker theory, the statistical features of these multiple-reconnection events have been identified. A discussion on the accuracy of our algorithms is provided, highlighting the necessity of adequate spatial resolution. Applications to the study of solar wind discontinuities are reviewed, comparing simulations to spacecraft observations. New results are shown, studying the time evolution of these local reconnection events. A preliminary study on the comparison between MHD and Hall MHD is reported. Our new approach to the study of reconnection as an element of turbulence has broad applications to space plasmas, shedding a new light on the study of magnetic reconnection in nature.

Statistical and Scaling Features of Fluctuations in the Dissipation Range During a Reconnection Event

“…A value of the connectivity index d > 0 (d = 0for a compact non-fractal set) indicates that there are topological obstacles for the current flow, so that a highly structured irregular pattern for the current is formed. These obstacles can be associated with the formation of coherent magnetic structures as observed in numerical simulations (Wu & Chang 2000;Servidio et al 2011;Donato et al 2012). We underline that the above scenario is in agreement with a coarse-grained nature of plasma media due to the formation of multiscale coherent structure as suggested by Chang (1999).…”

“…This finding supports the idea of a notspace-filling structure of the current in the dissipation region. We note that in numerous MHD and Hall-MHD simulations (see, e.g., Wu & Chang 2000;Servidio et al 2011;Donato et al 2012) there is a wide evidence for the formation of filamentary current structures from MHD scales down to scales below the ion inertial length. Furthermore, making use of the Alexander-Orbach conjecture (Alexander & Orbach 1982), which relates the fractal dimension d s of a percolating network to the connectivity exponent d˜via the fracton dimension = D 4 3, i.e.,…”

Nonthermal particles and photons in starburst regions and superbubbles

“…In the scenario considered by Servidio et al (2011) spontaneous magnetic reconnection is locally driven by the dynamical forces under boundary conditions provided by the turbulence.…”

Identification of Intermittent Multifractal Turbulence in Fully Kinetic Simulations of Magnetic Reconnection

“…Depending on the application, the turbulence may arise from a spectrum of instabilities within the reconnection layer or from pre-existing magnetic fluctuations in the ambient plasma. Within the magnetohydrodynamic (MHD) model, there has been progress on both ideas -either by starting with a laminar current sheet to explore instabilities [3][4][5][6] or by directly driving turbulence [7][8][9][10][11].…”

“…Depending on the application, the turbulence may arise from a spectrum of instabilities within the reconnection layer or from pre-existing magnetic fluctuations in the ambient plasma. Within the magnetohydrodynamic (MHD) model, there has been progress on both ideas -either by starting with a laminar current sheet to explore instabilities [3][4][5][6] or by directly driving turbulence [7][8][9][10][11].Moving beyond the MHD model into kinetic regimes, most research has focused on initially laminar current sheets within a variety of descriptions [12] including two fluid, hybrid and fully kinetic simulations, which allow a complete description of the electron physics responsible for breaking the frozen-flux condition in collisionless parameter regimes [13,14]. As larger kinetic simulations became possible, one surprising result was that the nonlinear evolution of reconnection produced extended electron-scale current sheets, with half-thickness on the order of the electron inertial length and lengths that can extend beyond the ion inertial scale [15][16][17][18][19].…”