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

Determine the physical mechanism that causes the leftward shift of the peak in the growth-rate-versus-temperature curve for the mixed-modes configuration of an electron current layer (with uniform in-plane magnetic field offset C0 = 0.025 that permits both tearing and surface-preserving modes) in two-dimensional particle-in-cell simulations, relative to the pure tearing configuration (C0 = 0) where the peak occurs at 10 eV. Ascertain whether the shift to approximately 5 eV arises from magnetic field asymmetry across the null-line, the coexistence of the surface-preserving mode, and/or temperature-dependent effects on these modes.

Background

The paper investigates kinetic effects on current gradient-driven instabilities in electron current layers using two-dimensional PIC simulations, focusing on tearing and surface-preserving modes and their dependence on electron temperature and guide fields.

For the pure tearing mode (C0 = 0), the growth rate increases with temperature up to about 10 eV and then decreases; for the surface-preserving mode (C0 = 0.06), the growth rate increases monotonically with temperature.

In the mixed-modes case (C0 = 0.025), both tearing and surface-preserving instabilities coexist, producing asymmetric magnetic island and quadrupole structures that drift toward the weaker magnetic field side. The growth-rate-versus-temperature curve peaks at a lower temperature (around 5 eV) than in the pure tearing case (10 eV), but the reason for this shift is explicitly stated as uncertain.

The authors speculate that magnetic field asymmetry across the null-line, coexistence of modes, and temperature effects might contribute, and indicate that a detailed paper is underway, highlighting a concrete unresolved question.