  Hydrodynamic simulations of granular convection predict the existence of
supersonic flows covering ~3-4% of the solar surface at any time, but these
flows have not been detected unambigously as yet. Using data from the
spectropolarimeter aboard the Hinode satellite, I present direct evidence of
fast horizontal plasma motions in quiet Sun granules. Their visibility
increases toward the limb due to more favorable viewing conditions. At the
resolution of Hinode, the horizontal flows give rise to asymmetric intensity
profiles with very inclined blue wings and even line satellites located
blueward of the main absorption feature. Doppler shifts of up to 9 km/s are
observed at the edges of bright granules, demonstrating that the flows reach
supersonic speeds. The strongest velocities occur in patches of 0.5 arcsec or
less. They tend to be associated with enhanced continuum intensities, line
widths, and equivalent widths, but large values of these parameters do not
necessarily imply the existence of supersonic flows. Time series of
spectropolarimetric measurements in regions away from disk center show the
transient nature of the strong horizontal motions, which last only for a
fraction of the granule lifetime. Supersonic flows are expected to produce
shocks at the boundaries between granules and intergranular lanes, and may also
play a role in the emergence of small-scale magnetic fields in quiet Sun
internetwork regions.
