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Modern Solar Facilities – Advanced Solar Science, 225–228 F. Kneer, K. G. Puschmann, A. D. Wittmann (eds.) c  Universitätsverlag Göttingen 2007 The observational counterpart of the rising flux tube model? J. Jurčák* and M. Sobotka Astronomical Institute of the Academy of Sciences, Ondřejov, Czech Republic * Email: jurcak@asu.cas.cz Abstract. An analysis of Stokes observations of a penumbra in active region NOAA 8990 is presented. The observations were recorded with the La Palma Stokes Polarimeter attached to the 1-m Swedish Solar Telescope. The stratification in the solar atmosphere of different physical parameters is retrieved from these data using the Stokes Inversion based on Response functions (SIR). Our results confirm previous findings, that with increasing distance from the umbra the magnetic field becomes weaker and more horizontal and the line-of-sight velocities increase. The results suggest the existence of unresolved filamentary structure in the spatial distributions of temperature, magnetic field strength and inclination. The maps of temperature and magnetic field strength along the vertical cuts through the penumbra indicate the presence of rising flux tubes, predicted theoretically by Schlichenmaier et al. (1998). 1 Introduction The fine structure of the penumbra was studied, among earlier investigations, by e.g. Beckers & Schröter (1969), who found stronger and more vertical magnetic field in dark filaments than in the bright ones. Recently, many authors agreed on the picture of the uncombed structure of the penumbral magnetic field, which was proposed by Solanki & Montavon (1993) to explain the broadband circular polarisation of sunspots and further developed by Martı́nez Pillet (2000). This uncombed structure is created from horizontal flux tubes (with diameters around 100 km) embedded in a more vertical and stronger background field. Evidences for this empirical model were found in recent analyses of observations (see e.g. Bello González et al. 2005; Bellot Rubio et al. 2006, and references therein). This complex magnetic fine structure of the penumbra can be caused by the convective exchange of the flux tubes. One of the possible scenarios starts with a flux tube which is initially positioned at the penumbra-quiet sun boundary. It is heated by the field-free convection and rises, developing a flow along the tube that points upwards beneath and outwards in the photosphere. The part of the flux tube emerging above the log τ = 0 surface is filled with hot gas and creates a bright grain. At higher layers, the flux tube cools down by radiative losses and becomes more horizontal and the outward flow resembles the observed Evershed effect. Up to this point, the scenario has been confirmed by the two-dimensional simulations made by Schlichenmaier et al. (1998). The theoretical maps of plasma parameters in and around the rising flux tube can be found in that article. In the present work, these theoretical maps are compared with the maps of plasma parameters found in the penumbra using the 226 J. Jurčák and M. Sobotka: The observational counterpart of the flux tube Figure 1. The continuum intensity in the part of the active region observed on May 13, 2000. The arrows are explained in Section 3. observations and data processing described in the following section. 2 Observations and data processing The data were retrieved with the La Palma Stokes Polarimeter (LPSP) attached to the 1-m Swedish Solar Telescope. The resulting spatial resolution is of about 0. 7. The object of the observations was an irregular leading sunspot in the active region NOAA 8990. One of the two fields, which were scanned in magnetically sensitive lines Fe I 630.15 nm (Landé factor g = 1.67) and Fe I 630.25 nm (g = 2.5), is shown in Fig. 1. The field covers a limbward part of a penumbra at heliocentric position 14◦ N and 17◦ W. In each pixel of this field all Stokes profiles of the two mentioned lines were measured and used as an input to the inversion code SIR (Ruiz Cobo & del Toro Iniesta 1992). A one-component model of atmosphere was used with two different settings of numbers of nodes for the inversion. The numbers of nodes, which should correspond to the expected complexity of the stratifications of the appropriate plasma parameters, are listed in Table 1. Other details about the observation, data reduction, and inversion can be found in Jurčák et al. (2006). 3 Results Our results confirm previous findings: The magnetic field becomes weaker and...

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