Vertical flows and mass flux balance of sunspot umbral dots

Astronomy & Astrophysics A&A 554, A53 (2013) DOI: 10.1051/0004-6361/201321075 c ESO 2013  Vertical flows and mass flux balance of sunspot umbral dots T. L. Riethmüller1,2 , S. K. Solanki1,3 , M. van Noort1 , and S. K. Tiwari1 1 Max-Planck-Institut für Sonnensystemforschung (MPS), Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany e-mail: [riethmueller;solanki;vannoort;tiwari]@mps.mpg.de 2 Technische Universität Braunschweig, Institut für Geophysik und Extraterrestrische Physik, Mendelssohnstr. 3, 38106 Braunschweig, Germany 3 School of Space Research, Kyung Hee University, Yongin, 446-701 Gyeonggi, Republic of Korea Received 10 January 2013 / Accepted 30 April 2013 ABSTRACT A new Stokes inversion technique that greatly reduces the effect of the spatial point spread function of the telescope is used to constrain the physical properties of umbral dots (UDs). The depth-dependent inversion of the Stokes parameters from a sunspot umbra recorded with Hinode SOT/SP revealed significant temperature enhancements and magnetic field weakenings in the core of the UDs in deep photospheric layers. Additionally, we found upflows of around 960 m/s in peripheral UDs (i.e., UDs close to the penumbra) and ≈600 m/s in central UDs. For the first time, we also detected systematic downflows for distances larger than 200 km from the UD center that balance the upflowing mass flux. In the upper photosphere, we found almost no difference between the UDs and their diffuse umbral background. Key words. Sun: photosphere – sunspots – techniques: polarimetric 1. Introduction Umbral dots (UDs) are small brightness enhancements in sunspot umbrae or pores and were first detected by Chevalier (1916). The strong vertical magnetic field in umbrae suppresses the energy transport by convection (Biermann 1941), but some form of remaining heat transport is needed to explain the observed umbral brightness (Adjabshirzadeh & Koutchmy 1983). Magnetoconvection in umbral fine structure, such as UDs and light bridges, is thought to be the main contributor to the energy transport in the umbra (Weiss 2002), see reviews by Solanki (2003), Sobotka (2006), and Borrero & Ichimoto (2011). Progress in the physical understanding of umbral dots was made with numerical simulations of 3D radiative magnetoconvection (Schüssler & Vögler 2006; Bharti et al. 2010). Most of the simulated UDs have a horizontally elongated shape and show a central dark lane in their bolometric intensity images. In the deepest photospheric layers, the inner parts of UDs exhibit magnetic-field weakenings and upflow velocities. The simulated UDs are surrounded by downflows that are often concentrated in narrow downflow channels at the endpoints of the dark lanes (Schüssler & Vögler 2006). Higher up in the photosphere, the UDs in the simulations do not differ significantly from the diffuse background. Considerable efforts on the observational side were made to test these theoretical predictions. Dark lanes inside UDs were found in the observations of Bharti et al. (2007) with the 50-cm Hinode telescope and by Rimmele (2008), who observed with the 76-cm Dunn Solar Telescope. However, Louis et al. (2012) analyzed straylight-corrected Hinode/BFI data and did not find dark lanes in their observed UDs, which leaves room for doubt whether the observed phenomena are really identical with the synthetic ones. The UDs described in Bharti et al. (2007) differ from those reported in Schüssler & Vögler (2006) in that the area of the observed features is an order of magnitude larger; possibly they are the remains of a decayed light bridge. More important than the dark lanes are the flows, since they are central to the convective nature of the UDs. Riethmüller et al. (2008a), using inversions of Hinode/SP data, discovered upflows in the deep layers of peripheral UDs (PUDs) but not in central UDs (CUDs), while downflows were not detected. Subsequently, Ortiz et al. (2010) studied a small pore recorded with the CRISP instrument of the 1-m Swedish Solar Telescope and found irregular and diffuse downflows in the range 500–1000 m/s for a small set of five UDs. In contrast, in their recent study, Watanabe et al. (2012) analyzed a larger set of 339 UDs, also observed with CRISP, and found significant UD upflows, but no systematic downflow signals. Thus, the existence of downflows in or around UDs remains uncertain, so that the fate of the material flowing up in UDs is unclear. The depth-dependent inversions of full Stokes profiles derived in Socas-Navarro et al. (2004) and later at higher resolution in Riethmüller et al. (2008a) revealed a temperature enhancement and a field weakening for the UDs compared to the nearby umbral background, which both were strongest in the deepest observed layers. Since the observational picture is inhomogeneous, there is a need for a more detailed UD study for which high spatial and spectral resolution is of utmost importance. In this work, the improved Stokes inversion method of van Noort (2012) is applied to Hinode/SP data (see van Noort et al. 2013). This so-called 2D inversion method allows the depth-dependent structure to be obtained basically as it would be in the absence of the telescope’s point spread function (PSF). 2. Observation, data reduction, and analysis The data we analyzed in this study were recorded from 12:43 to 13:00 UT on 2007 January 5 with the spectropolarimeter Article published by EDP Sciences A53, page 1 of 5 A&A 554, A53 (2013) Fig. 1. Stokes I continuum intensity of the Hinode/SP map of a sunspot umbra of NOAA AR 10933. The original data are plotted in the top panel. The Stokes I continuum resulting from the 2D inversion is shown in the bottom panel. The intensity is normalized to the mean quiet-Sun intensity IQS . The outer contour line in the bottom panel indicates the edge of the umbra as retrieved from the magnetic field inclination map (see main text), the inner contour line separates central from peripheral umbral dots (UDs). Four typical UDs are marked by circles and letters. (SP, Lites et al. 2001) of the Solar Optical Telescope (SOT, Tsuneta et al. 2008) on the Hinode spacecraft (Kosugi et al. 2007). The SP was operated in its normal map mode, i.e., the integration time per slit position was 4.8 s, resulting in a noise level of 10−3 (in units of the continuum intensity). The sampling along the slit, the slit width, and the scanning step size were 0. 16, the spectral sampling in the considered range from 6300.89 to 6303.26 Å was 21 mÅ pixel−1 . The center of the observed umbra was located very close to the disk center, at a heliocentric angle of 2.6◦ . The full Stokes profiles were corrected for dark current as well as flat-field effects and calibrated with the sp_prep routine of the SolarSoft package. Part of the calibrated Stokes I continuum intensity map obtained with Hinode SP is shown in the upper panel of Fig. 1. The original field of view is much larger and contains quiet-Sun regions that are u (...truncated)