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.
