SPRG Seminars
September 30, 2008:
"Fourier phase, coherent structures and intermittency in space plasmas: diagnosis using the surrogate data technique"
Fouad Sahraoui, NASA/GSFC, CNRS/CETP
Several observations in space plasmas have reported the presence of coherent structures at different plasma scales. Coherent structures result from nonlinear interactions between the plasma modes, which depend strongly on their phase synchronization. Despite this important role of the phases in turbulence, very limited work has been however devoted to study the phases as potential tracers of nonlinearities in comparison with the wealth of literature on power spectra of turbulence where phases are totally missed.
Even though the Cluster data have made it possible to reveal new properties on the nature of the ULF turbulence (e.g., k-spectra, anisotropies, scaling), I will show the limitations of the existing techniques, based only on power spectra, in answering fundamental questions of turbulence such as the presence (or not) of turbulent energy cascade when power law spectra are observed or the actual nature of intermittency. I will then introduce the Surrogate data technique allowing one to fill this gap by considering the information of the Fourier phases in turbulent signals. I will show the capabilities of this new method in detecting coherent structures and estimating their scales. I will show in particular the role of the Fourier phase in energy cascade and in creating intermittency. Further implications on theoretical modelling of turbulence will be discussed.