Quasi-separatrix layer reconnection for nonlinear line-tied collisionless tearing modes

Abstract

Quasi-separatrix layer (QSL) diagnostics are used to diagnose nonlinear line-tied tearing modes using particle-in-cell code simulation results. For a force-free current sheet, nonlinear tearing mode cases are shown over a wide range of plasma lengths with line-tying boundary conditions at the ends of the plasma. The methods used for performing the field line integrations with noisy PIC data are described. The QSL diagnostics used are the squashing degree Q (Priest and Demoulin J. Geophys. Res. 100 23443) and the ideal magnetohydrodynamic (MHD) scalar potential φi of Richardson and Finn (2012 Commun. Nonlinear Sci. Numer. Simul. 17 2132). We also introduce the non-ideal scalar potential φn, the analog of the resistive MHD scalar potential φr of Richardson and Finn (). These QSL diagnostics are compared with the helical flux contours of the primary modes. They are used in the late linear regime and near mode saturation. For non-overlapping islands, these diagnostics show the QSL associated with the X-lines and their separatrices. These are shown to indicate well field line behavior such as chaotic scattering and separatrix crossing. For overlapping islands, they indicate chaotic scattering behavior either related to Chirikov overlap (Chirikov 1979 Phys. Rep. 52 263) or to separatrix crossing (Tennyson et al 1986 Phys. Rev. Lett. 56 2117).