Magnetic Structure of Umbral Dots with SOT SP

High resolution and seeing-free spectroscopic observations of a decaying sunspot were done with the Solar Optical Telescope aboard Hinode satellite. We report the magnetic structure and Doppler velocity fields around umbral dots (UDs), based on the Milne-Eddington inversion of the two iron absorption lines at 6302 Å.

The histograms of magnetic field strength (B), inclination angle (i), and Doppler velocity (v) of UDs showed a center-to-limb variation. Observed at the disk center, UDs had (i) slightly smaller field strength (ΔB = -17 Gauss) and (ii) relative blue shifts (Δv =28 m s^-1) compared to their surroundings. When the sunspot approached to the limb, UDs and their surroundings showed almost no difference in the magnetic and Doppler values. This center-to-limb variation can be understood by the formation height difference in a cusp-shaped magnetized atmosphere around UDs, due to the weakly magnetized hot gas intrusion. In addition, some UDs showed oscillatory light curves with multiple peaks around 10 min, which may indicate the presence of the oscillatory convection.