Two-dimensional spectroscopy of a sunspot. III Thermal and kinematic structure of the penumbra at 0.5"

Abstract

We investigate the thermal and kinematic configuration of a sunspot penumbra using very high spectral and spatial resolution intensity profiles of the non-magnetic Fe I 557.6 nm line. The dataset 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. We confirm the existence of a thermal asymmetry between the center and limb-side penumbra, the former being hotter by 100-150 K on average. We also investigate the nature of the bright ring that appears in the inner penumbra when sunspots are observed in the wing of spectral lines. The line-of-sight velocities retrieved from the inversion are used to determine the flow speed and flow angle 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_{500} < -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 0.5" resolution are not individual flow channels, but a collection of them.

15 pages with 18 Postscript figures. Submitted to Astronomy and Astrophysics (December 2005)

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