Vertical flows and mass flux balance of sunspot umbral dots

¹ Max-Planck-Institut für Sonnensystemforschung (MPS), Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
e-mail: riethmueller@mps.mpg.de; solanki@mps.mpg.de; vannoort@mps.mpg.de, 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.