On the fine structure of sunspot penumbrae

I. A quantitative comparison of two semiempirical models with implications for the Evershed effect

¹ Max-Planck Institut für Aeronomie, 37191 Katlenburg-Lindau, Germany e-mail: borrero@linmpi.mpg.de
² Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany
³ Instituto de Astrofísica de Canarias, vía Láctea s/n, 38200 La Laguna, Tenerife, Spain

Received: 1 April 2004
Accepted: 26 April 2004

Abstract

Sunspot penumbrae exhibit prominent fine structure. Different interpretations of spectropolarimetric observations suggest different, sometimes contradictory, properties of this fine structure. In this paper we show that the results of inversions of penumbral infrared profiles based on one-component models with gradients of the atmospheric parameters and two-component models without gradients are compatible with each other. Our analysis reconciles the results of previous investigations and provides further support for the picture that sunspot penumbrae are composed of penumbral flux tubes embedded in a magnetic background. The magnetic field in the tubes is more horizontal and weaker than that of the background atmosphere. While the tubes carry most of the Evershed flow, the background is essentially at rest. We notice also that the magnetic field strength in the flux tubes drops much more slowly with radial distance than the background field. This finding is discussed as a possible driver for the Evershed flow.