POLARIMETRY AND SPECTROSCOPY OF A SIMPLE SUNSPOT .3. VELOCITY AND MAGNETIC-FIELD OF SUNSPOT UMBRAL DOTS

Velocities and magnetic fields in sunspot umbrae and in umbral dots ha ve been determined from high resolution Stokes-I spectra obtained at t he VTT on Tenerife. Except for oscillatory motions the umbra is at res t and flow velocities in umbral dots, measured at an optical depth of about 0.01, are smaller than 25 m/s. The magnetic field strength is we akened in the dots by 10 - 20 % for a central umbral dot and by 5 - 10 % for a peripheral umbral dot. The line of sight decrease of the magn etic field in a peripheral dot shows no difference to the surrounding umbra and amounts to 2.5 - 3 G/km.