Spectropolarimetery of umbral fine structures from Hinode: evidence for magnetoconvection

L. Bharti at al. 0 Present address: Max-Planck Institute for Solar System Research , 37191 Katlenburg-Lindau, Germany 1 Physical Research Laboratory, Department of Space , Government of India, Navrangpura, Ahmedabad 380 009, India 2 Department of Physics, University College of Science, Mohanlal Sukhadia University , Udaipur 313 001, India A B S T R A C T We present spectropolarimetric analysis of umbral dots and a light bridge fragment that show dark lanes in G-band images. Umbral dots show upflow as well as associated positive Stokes V area asymmetry in their central parts. Larger umbral dots show downflow patches in their surrounding parts that are associated with negative Stokes V area asymmetry. Umbral dots show weaker magnetic field in central part and higher magnetic field in peripheral area. Umbral fine structures are much better visible in total circularly polarized light than in continuum intensity. Umbral dots show a temperature deficit above dark lanes. The magnetic field inclination shows a cusp structure above umbral dots and a light bridge fragment. We compare our observational findings with 3D magnetohydrodynamic simulations. 1 I N T R O D U C T I O N Two models of the umbral dots (UDs) are under discussion these days. The first is the cluster model (Parker 1976; Choudhuri 1986) that suggests that UDs are the top of the intrusion of the field free material between the flux tubes beneath the sunspot. The second model is known as the monolithic model (Weiss 2002, and reference therein) and suggests that UDs show up because of magnetoconvection in monolithic flux tube. Recent simulations by Schussler & Vogler (2006) with grey radiative transfer show UDs appearing due to magnetoconvection in strong background magnetic field. The knowledge of the nature of UDs is essential to understand the energy transport from below the sunspot (see reviews from Solanki 2003; Thomas & Weiss 2004, and reference therein on the subject). Bharti, Joshi & Jaaffrey (2007a) analysed Dopplergrams obtained from filtergraph data, and found a correlation between intensity and velocity in UDs, which suggests a magnetoconvective origin. Using high-quality G-band images from Hinode, Bharti, Jain & Jaaffrey (2007b) reported on dark lanes in UDs. These separate observational findings are compatible with some aspects of simulations by Schussler & Vogler (2006). Socas-Navarro et al. (2004) analysed peripheral UDs in detail from spectropolarimetric data and found higher temperature (1 kK), weaker field (500 G), small upflow (100 m1) and more inclined field (10) in UDs. In this article, we present spectropolarimetric analysis of dark laned umbral fine structure from Hinode spectropolarimetric data. Figure 1. A G-band image as observed in the broad-band filter on Solar Optical Telescope (SOT) at 04:15:32 UT. The image is byte scaled and shows the fine structure of the sunspot around the time that our spectropolarimetric maps were taken. physical parameters are computed at only a few grid points called nodes instead of computing at all optical depths of the model. For rest of depths, they are approximately computed by the cubic-spline interpolation between the equidistantly distributed grid points. We perform the SIR inversion with only one magnetic component, for which we allow five nodes in T( ), three for B( ), Vlos( ) and ( ). G-band time series obtained in the broad-band filter were used to follow the evolution of the sunspot fine structure as seen in the spectropolarimetric maps (see Bharti et al. 2007b). Wiener filtering was applied to the G-band images for the point spread function correction of telescope. Understanding of the evolution of the umbral fine structure is necessary as they may have common physical origin (Bharti et al. 2007c; Katsukawa et al. 2007; Rimmele 2008). Here, we would like to mention that it is our aim to investigate dark lane in UDs as reported by Bharti et al. (2007b) and the spectropolarimetric fast scan at 0.6 arcsec spatial resolution cover similar features. The calibrated Stokes profiles were used to create maps of total circular polarization and Stokes V area asymmetry (Bellot Rubio et al. 2007). 3 R E S U LT S We have chosen G-band images, whose timing was close to the spectropolarimetric scans. Fig. 1 shows one of the G-band images taken close to the fast spectropolarimetric scan time at 04:15:32 UT that covers emerging sunspot in the spectropolarimetric map. The G-band time series shows that the UD a has been formed from a peripheral UD that fragmented in two UDs. The UD b emerges from a bright bands (Bharti et al. 2007b). The bright band fragments and a UD forms, it grows gradually and shows a threefold dark lane. At 04:15:32 UT, it shows a central bright structure surrounded by a dark ring and five fragments separated by dark lanes. The time series show that this UD fragments and again converts into a bright band. A larger UD c forms from a bright band that shows complex shapes during its evolution. In Fig. 1, it shows clearly threefold dark lanes. The light bridge that develops from the dark cored penumbral filament shows central dark lanes and its fragments show dark lanes. At the head of the light bridge, a triangular-shaped bright structure d is seen that forms from the light bridge fragments in upper part and conglomeration of a UD that appears from the diffuse Figure 2. Enlarged view of an area of Fig. 1. It clearly shows dark lanes inside the umbral fine structure. Figure 3. Maps showing continuum intensity (a) and total circular polarization (b) for the emerging sunspot that rises below a developed sunspot in active region 10 930. Marked in white in panel (a) are the locations where stratifications of various physical parameters along cut were measured. All maps are up scaled two times using cubic-spline interpolation. The outer rectangle shows the field of view that was subjected to the SIR inversion and the inner rectangle shows the region that was subjected to area asymmetry measurement. background. This UD is marked by e. Individual UDs are seen. However, their boundary is not clearly visible. Close to the vertex of the triangular structure d, a light bridge fragment is seen that shows dark lane. Dark lanes in UDs and light bridge fragments are visible but only UD c shows more clearly dark lane in the G-band image. Fig. 2 shows enlarged view of a part of Fig. 1 that shows dark lanes in umbral fine structures very clearly. Fig. 3 illustrates map of the emerging sunspot taken from the spectropolarimetric fast scan. Panel (a) of Fig. 3 shows the continuum intensity map; the fine structure is very similar to that of the G-band image. Comparison with the G-band image in Fig. 1 illustrates that it is reasonable to analyse these fine structures using the spectropolarimetric fast scan at 0.6 arcsec resolution. However, the UD marked by an a in Fig. 1 shows clear structure in this figure, but not at the lower resolution in Fig. 3. The (...truncated)