  The properties of the edge ion-scale turbulence are studied using the beam
emission spectroscopy (BES) diagnostic on MAST. Evidence of the formation of
large-scale high-amplitude coherent structures, filamentary density blobs and
holes, 2$-$4 cm inside the plasma separatrix is presented. Measurements of
radial velocity and skewness of the density fluctuations indicate that density
holes propagate radially inwards, with the skewness profile peaking at 7$-$10
cm inside the separatrix. Poloidal velocities of the density fluctuations
measured using cross-correlation time delay estimation (CCTDE) are found to
exhibit an intermittent behaviour. Zonal-flow analysis reveals the presence of
poloidally symmetric coherent oscillations $-$ low-frequency (LF) zonal flows
and geodesic acoustic modes (GAM). Shearing rates of the observed zonal flows
are found to be comparable to the turbulence decorrelation rate. The observed
bursts in density-fluctuation power are followed by quiescent periods with a
transient increase in the power of sheared flows. Three-wave interactions
between broadband turbulence and a GAM are illustrated using the autobispectral
technique. It is shown that the zonal flows and the density-fluctuation field
are nonlinearly coupled and LF zonal flows mediate the energy transfer from
high- to low-frequency density fluctuations.
