The velocity field of sunspot penumbrae

I. A global view

Kiepenheuer Institut für Sonnenphysik, Schöneckstraße 6, 79104 Freiburg, Germany e-mail: morten@kis.uni-freiburg.de

Received: 6 August 2009
Accepted: 24 September 2009

Abstract

Aims. We investigated the vertical penumbral plasma flow on small spatial scales using data recorded by the spectropolarimeter of the solar optical telescope onboard Hinode.

Methods. We computed maps of apparent Doppler velocities by comparing the spectral positions 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. The small heliocentric angle () of our data sets means that the horizontal component of the Evershed flow (EF) does not contribute significantly to the line shift.

Results. We found that in the quiet Sun (QS), the area showing upflows is always larger than the one exhibiting downflows. In the penumbra, upflows dominate only at low velocities  km s^-1, while at higher velocities  km s^-1 downflows prevail. Additionally, the maximal upflow velocity in penumbrae is lower, while the maximal downflow velocity is larger with respect to the QS velocities. Furthermore, on a spatial average, the penumbra shows a redshift, corresponding to a downflow of more than 0.1 km s^-1. Upflows are elongated and appear predominately in the inner penumbra. Strong downflows with velocities of up to 9 km s^-1 are concentrated at the penumbra-QS boundary. They are magnetized and are rather round. The inner penumbra shows an average upflow, which turns into a mean downflow in the outer penumbra. The upflow patches in the inner penumbra and the downflow 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.