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 tearing mode or Kelvin-Helmholtz, among other plasma instabilities, could generate turbulence from irregular current sheets. We address the nature of driving mechanism for this process and consider a 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 three-dimensional reconnection regions. We show that both instabilities can excite turbulence fluctuations in reconnection regions. However, tearing mode has relatively slow growth rate, and at later times it becomes partially suppressed by transverse to the current sheet component of magnetic field, generated during the growth of turbulent fluctuations. On the contrary, the Kelvin-Helmholtz instability establishes quickly in the outflow region, and at later times it dominates the turbulence generation comparing to the contribution from tearing mode. Our results demonstrate that the tearing instability is subdominant compared 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 with tearing instability being important only at the initial stage of the reconnection.