THE HEIGHT VARIATION OF SUNSPOT UMBRAL DOTS

The visibility of sunspot umbral dots is measured in different spectra l lines that are insensitive to Zeeman splitting. At the formation hei ght of the Fe 5576 line core, the dot to inter-dot brightness ratio am ounts to about 30% of that at the continuum level. It further drops to less than 15% at the formation height of the Fe 5434 line core, that is expected to correspond to the temperature minimum. A contrast of 50 % occurs at about 130 km height where the inner wings of Fe 5576 (DELT Alambda = 45 mangstrom) and the outer wings of Fe 5434 (DELTAlambda = 75 mangstrom) are formed. Simultaneously observed Ni and Si lines from very deep layers show the same brightness excess as the adjacent cont inuum. Neither systematic Doppler shifts nor line asymmetries are foun d in umbral dots with respect to their inter-dot neighbourhood.