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A&A 453, 1117-1127 (2006)
DOI: 10.1051/0004-6361:20054768

Two-dimensional spectroscopy of a sunspot

III. Thermal and kinematic structure of the penumbra at 0$\farcs$5 resolution
L. R. Bellot Rubio1, 2, R. Schlichenmaier2 and A. Tritschler3, 2

1  Instituto de Astrofísica de Andalucía (CSIC), Apdo. 3004, 18080 Granada, Spain
    e-mail: lbellot@iaa.es
2  Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany
3  National Solar Observatory/Sacramento Peak, PO Box 62, Sunspot, NM 88349, USA

(Received 23 December 2005 / Accepted 28 March 2006 )

Abstract
We investigate the thermal and kinematic configuration of a sunspot penumbra using high spectral and spatial resolution intensity profiles of the non-magnetic $\ion{i}$ 557.6 nm line. The data set was acquired with the 2D solar spectrometer TESOS. The profiles are inverted using a one-component model atmosphere with gradients of the physical quantities. From this inversion we obtain the stratification with depth of temperature, line-of-sight velocity, and microturbulence across the penumbra. Our results suggest that the physical mechanism(s) responsible for the penumbral filaments operate preferentially in the lower photosphere. The spot, located at an heliocentric angle of 23°, exhibits larger continuum intensities in the center-side penumbra as compared with the limb side, which translates into an average temperature difference of 100-150 K at $\log \tau_ = 0$. We investigate the nature of the bright ring that appears in the inner penumbra when sunspots are observed in the wing of spectral lines. It is suggested that the bright ring does not reflect a temperature enhancement in the mid photospheric layers. The line-of-sight velocities retrieved from the inversion are used to determine the flow geometry at different heights in the photosphere. Both the flow speed and flow angle increase with optical depth and radial distance. Downflows are detected in the mid and outer penumbra, but only in deep layers ( $\log \tau_ \geq -1.4$). We demonstrate that the velocity stratifications retrieved from the inversion are consistent with the idea of penumbral flux tubes channeling the Evershed flow. Finally, we show that larger Evershed flows are associated with brighter continuum intensities in the inner center-side penumbra. Dark structures, however, are also associated with significant Evershed flows. This leads us to suggest that the bright and dark filaments seen at 0$\farcs$5 resolution are not individual flow channels, but a collection of them. Our analysis highlights the importance of very high spatial resolution spectroscopic and spectropolarimetric measurements for a better understanding of sunspot penumbrae.


Key words: line: profiles -- Sun: photosphere -- sunspots



© ESO 2006