  We report an experimental investigation of the statistical time dynamic of
spatial Fourier modes in a fully developped turbulent jet flow. Measurements
rely on an original acoustic scattering technique, allowing the direct and
continuous probing in a time of spatial Fourier modes of the turbulent
vorticity field at well defined spatial wavevectors. From the recordings of the
amplitude of the spatial Fourier modes corresponding to different wavevectors
in the inertial range, we compute the time correlation functions of the modal
amplitude. The modal vorticity dynamics exhibits two well separated time
scales: a short one which is scale dependant and related to the well known
sweeping effect (random advection), and a much longer one, of order the
integral time scale, which is not scale dependent. By computing the
cross-correlation of the amplitude of two different wavevectors, we evidence a
clear statistical dependance between scales, whatever their separation, at
variance with the original Kolmogorov 41 theory. We ascribe our experimental
results to a manifestation of a new type of statistical intermittency. We call
this new statistical feature a large scale time intermittency effect, in
contrast with the usual small scale spatial intermittency reported in Eulerian
velocity measurements. We propose to attribute this time intermittency, to the
fluctuations in time of the injected energy at large scale, in the spirit of
the objection formulated by Landau against the K41 theory.
