  The solar photosphere is the visible surface of the Sun, where many bright
granules, surrounded by narrow dark intergranular lanes, are observed
everywhere. The granular pattern is a manifestation of convective motion at the
photospheric level, but its velocity structure in the height direction is
poorly understood observationally. Applying bisector analysis to a photospheric
spectral line recorded by the Hinode Solar Optical Telescope, we derived the
velocity structure of the convective motion in granular regions and
intergranular lanes separately. The amplitude of motion of the convective
material decreases from 0.65 to 0.40 km/s as the material rises in granules,
whereas the amplitude of motion increases from 0.30 to 0.50 km/s as it descends
in intergranular lanes. These values are significantly larger than those
obtained in previous studies using bisector analysis. The acceleration of
descending materials with depth is not predicted from the convectively stable
condition in a stratified atmosphere. Such convective instability can be
developed more efficiently by radiative cooling and/or a gas pressure gradient,
which can control the dynamical behavior of convective material in
intergranular lanes. Our analysis demonstrated that bisector analysis is a
useful method for investigating the long-term dynamic behavior of convective
material when a large number of pixels is available. In addition, one example
is the temporal evolution of granular fragmentation, in which downflowing
material develops gradually from a higher layer downward.
