DETECTION OF SUPERSONIC HORIZONTAL FLOWS IN THE SOLAR GRANULATION
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
... ymmetric 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.