Cause of leftward shift in peak growth-rate temperature for mixed tearing and surface-preserving modes

Determine the physical mechanism responsible for the observed leftward shift in the temperature at which the perturbation-energy growth rate peaks in the mixed-modes configuration of an electron current layer—where both tearing and surface-preserving modes coexist due to a uniform in-plane magnetic offset C0 = 0.025—compared to the pure tearing configuration with C0 = 0, as observed in two-dimensional particle-in-cell simulations of the equilibrium magnetic field B_eq = ŷ[B_00 sech(x/ε) tanh(x/ε) + C0] with B_00 = 0.1 and ε = 1. Ascertain the respective roles of magnetic-field asymmetry about the null line, coexistence of the surface-preserving mode, and temperature-dependent effects on these modes in producing this shift.

Background

The paper studies current-gradient-driven instabilities in electron current layers using 2D PIC simulations for three cases determined by the uniform in-plane magnetic offset C0: pure tearing (C0 = 0), pure surface-preserving (C0 = 0.06), and mixed modes (C0 = 0.025).

For pure tearing, the growth rate versus temperature exhibits a peak around 10 eV before decreasing at higher temperatures. In the mixed-modes case, the authors find a similar nonmonotonic trend, but the peak occurs at a lower temperature (~5 eV), indicating a leftward shift relative to the pure tearing case.

The authors state that the cause of this shift is currently unknown and suggest potential contributing factors, including magnetic-field asymmetry about the null line, the coexistence of the surface-preserving mode, and temperature effects. They indicate that a more detailed paper is underway.