Kelvin–Helmholtz versus Tearing Instability: What Drives Turbulence in Stochastic Reconnection?

Over the last few years it became clear that turbulent magnetic reconnection and magnetized turbulence are inseparable. It was not only shown that reconnection is responsible for violating the frozen-in condition in turbulence, but also that stochastic reconnection in 3D generates turbulence by itself. The actual mechanism responsible for this driving is still unknown. Processes such as the tearing mode or Kelvin–Helmholtz, among other plasma instabilities, could generate turbulence from irregular current sheets. We address the nature of the driving mechanism for this process and consider the relative role of tearing and Kelvin–Helmholtz instabilities for the process of turbulence generation. In particular, we analyze the conditions for development of these two instabilities within 3D reconnection regions. We show that both instabilities can excite turbulence fluctuations in reconnection regions. However, the tearing mode has a relatively slow growth rate, and at later times it becomes partially suppressed by a component of the magnetic field that runs transversely to the current sheet, which is generated during the growth of turbulent fluctuations. In contrast, the Kelvin–Helmholtz instability quickly establishes itself in the outflow region, and at later times, it dominates the turbulence generation compared to the contribution from the tearing mode. Our results demonstrate that the tearing instability is subdominant to the the Kelvin–Helmholtz instability in terms of generation of turbulence in the 3D reconnection layers, and therefore the self-driven reconnection is turbulent reconnection, and the tearing instability is only important at the initial stage of the reconnection.