Order Parameter for the Transition from Phase to Amplitude Turbulence

The maximal conserved phase gradient is introduced as an order parameter to characterize the transition from phase to defect turbulence in the complex Ginzburg-Landau equation. It has a finite value in the phase-turbulent regime and decreases to zero when the transition to defect turbulence is approached. Solutions with a nonzero phase gradient are studied via a Lyapunov analysis. The degree of ``chaoticity'' decreases for increasing values of the phase gradient and finally leads to stable traveling wave solutions. A modified Kuramoto-Sivashinsky equation for the phase dynamics is able to reproduce the main features of the stable waves and to explain their origin.