We demonstrate that in plane Couette turbulence a separation of the
velocity field in large and small scales according to a streamwise
Fourier decomposition allows us to identify an active subspace
comprising a small number of the gravest streamwise components of the
flow that can synchronize all the remaining streamwise flow components.
The critical streamwise wavelength, l(xc), that separates the active
from the synchronized passive subspace is identified as the streamwise
wavelength at which perturbations to the time-dependent turbulent flow
with streamwise wavelengths l(x) < l(xc) have negative characteristic
Lyapunov exponents. The critical wavelength is found to be approximately
130 wall units and obeys viscous scaling at these Reynolds numbers
