Dynamics of the convective turbulence in the solar granulation studied by the spectral line broadening and asymmetry

In the quiet region on the solar surface, turbulent convective motions of the granulation are the most important driver creating small-scale magnetic structures. The turbulent nature can be studied using spectral line profiles, especially line width, that contains information on flow field smaller than the spatial resolution of an instrument. Spectral line broadening can be caused not only by laterally unresolved flow field but also by a line-of-sight (LOS) gradient of Doppler velocities, but it has not been well understood how the line broadening and the velocity gradient are coupled. In this study, we perform bisector analyses using spectral profiles of the Fe I 630.2 nm lines obtained with the Spectro-Polarimeter of Hinode Solar Optical Telescope, and study how the spectral line widths and the bisector velocities are related with granulation flows. Our result indicates that the line widths have positive correlation to the LOS gradient of Doppler velocities. There exist two types of the velocity gradients; faster downward or upward flows in the lower height. The former ones are preferentially seen in intergranular lanes, and some of which are associated with a strong concentration of magnetic fields. The latter ones are often seen in the course of decaying granules, and there is no clear association with magnetic fields. We also find an excess of the spectral line widths in the decaying granules, which cannot be explained only by the LOS velocity gradient. If we attribute the excessive line broadening to small-scale turbulent motions, the average turbulent velocity is estimated at 1.6 km/s.