POLARIMETRY AND SPECTROSCOPY OF A SIMPLE SUNSPOT .4. UMBRAL STRUCTURES OBSERVED IN FE-I-1027-NM

Magnetic fields and velocities in bright umbral structures have been d etermined from long time sequences of high resolution Stokes-I spectra obtained with the near infrared Fe I 1026.52 nm line. This weak line is unblended in the umbra and has a triplet-like Zeeman pattern simila r to a g(eff)=2.5 line. The umbral bright structures show velocity osc illations identical to those of the surrounding umbra. A bright struct ure with a size of 2 arcsec close to the center of the umbra shows a s lightly lower magnetic field strength than its surroundings. This diff erence decreases whereas the structure becomes brighter during several hours of observations.