Paper detail

On the velocity field of sunspot penumbrae - I. A global view

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.