Two-dimensional spectroscopy of a sunspot

II. Penumbral line asymmetries

Kiepenheuer–Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany

Corresponding author: R. Schlichenmaier, schliche@kis.uni-freiburg.de

Received: 2 October 2003
Accepted: 13 November 2003

Abstract

We present, analyse, and interpret line asymmetries from Fe I 557.6 nm of a sunspot penumbra at a heliocentric angle of 23° with high spatial (0.5 arcsec) and spectral () resolution. The data set is described and presented in the first paper of this series [CITE]. Line bisectors are used to quantify the line asymmetries. Our findings are: (1) For averaged limb and center side bisectors the shift increases linearly with the bisector intensity level, but the limb side bisector is more inclined than the center side bisector. (2) Individual bisectors exhibit kinks, such that the bisector at high intensity levels is shifted towards the red for both, limb and center side bisectors. Some of the kinks produce bisector reversals in the outer center side penumbra. The bisector properties and their intriguing differences between center and limb side can be explained if one assumes downflows in deep atmospheric layers (). This is demonstrated by synthetic bisectors. The differences between the two penumbral sides are due to projection effects of non-horizontal flow channels. Our findings also imply that bisectors reversals are not due to elevated channels, but due to the presence of downflows. Along a specific center side flow filament the bisector shift is found to be largest in the line wing, except for the outer end of the filament, where a kink at high bisector intensities toward the red is found. This is consistent with an upflow at the inner footpoint, a deep lying horizontal flow, and, after a spatial distance of 4 arcsec, with a downflow at the end of the flow filament.