Astronomy & Astrophysics

A&A 477, 273-283 (2008)
DOI: 10.1051/0004-6361:20077820

Temporal evolution of the Evershed flow in sunspots

II. Physical properties and nature of Evershed clouds

D. Cabrera Solana¹, L. R. Bellot Rubio¹, J. M. Borrero², and J. C. del Toro Iniesta¹

¹ Instituto de Astrofísica de Andalucía, CSIC, Apdo. 3004, 18080 Granada, Spain
e-mail: lbellot@iaa.es
² High Altitude Observatory, NCAR, 3080 Center Green Dr. CG-1, 80301 Boulder CO, USA

(Received 8 May 2007 / Accepted 4 September 2007)

Abstract
Context.Evershed clouds (ECs) represent the most conspicuous variation of the Evershed flow in sunspot penumbrae.
Aims.We determine the physical properties of ECs from high spatial and temporal resolution spectropolarimetric measurements. This information is used to investigate the nature of the EC phenomenon.
Methods.The Stokes profiles of four visible and three infrared spectral lines are subject to inversions based on simple one-component models as well as more sophisticated realizations of penumbral flux tubes embedded in a static ambient field (uncombed models).
Results.According to the one-component inversions, the EC phenomenon can be understood as a perturbation of the magnetic and dynamic configuration of the penumbral filaments along which the ECs move. The uncombed inversions, on the other hand, suggest that ECs are the result of enhancements in the visibility of penumbral flux tubes. We conjecture that these enhancements are caused by a perturbation of the thermodynamic properties of the tubes, rather than by changes in the vector magnetic field. This mechanism is investigated performing numerical experiments of thick penumbral tubes in mechanical equilibrium with a background field.
Conclusions.While the one-component inversions confirm many of the properties indicated by a simple line parameter analysis (Paper I of this series), we give more credit to the results of the uncombed inversions because they take into account, at least in an approximate manner, the fine structure of the penumbra.

Key words: sunspots -- Sun: magnetic fields -- Sun: photosphere

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