  We investigated the vertical penumbral plasma flow on small spatial scales
using data recorded by the spectropolarimeter of the solar optical telescope
onboard Hinode. To this end we computed maps of apparent Doppler velocities by
comparing the spectral position of the Fe I 630.15 nm & Fe I 630.25 nm lines
with the averaged line profiles of the quiet Sun. To visualize the flow pattern
in the low photosphere, we used a bisector of the wing of the absorption lines.
Due to the small heliocentric angle (3 < Theta < 9) of our data sets, the
horizontal component of the Evershed flow (EF) does not contribute
significantly to the line shift. We found that in the quiet Sun (QS), the area
showing up-flows is always larger than the one exhibiting down-flows. In the
penumbra, up-flows dominate only at low velocities |v_dop| < 0.4 km/s while at
larger velocities |v_dop| > 0.6 km/s down-flows prevail. Additionally, the
maximal up-flow velocity in penumbrae is smaller, while the maximal down-flow
velocity is larger with respect to the QS velocities. Furthermore, on a spatial
average, the penumbra shows a red-shift, corresponding to a down-flow of more
than 0.1 km/s. Up-flows are elongated and appear predominately in the inner
penumbra. Strong down-flows with velocities of up to 9 km/s are concentrated at
the penumbra-QS boundary. They are magnetized and are rather round in shape.
The inner penumbra shows an average up-flow, which turns into a mean down-flow
in the outer penumbra. The up-flow patches in the inner penumbra and the
down-flow locations in the outer penumbra could be interpreted as the sources
and the sinks of the EF. We did not find any indication of roll-type convection
within penumbral filaments.
