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 non-zero 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 travelling 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.