Recently, H6noux (1969)
, , Yun (1971), and have analysed line profiles in sunspot umbra spectra, and derived temperature distributions in a wide range of optical depths (-7<logr0<l).
An empirical determination of the corresponding gas pressure gives a distribution corresponding to hydrostatic equilibrium, but, as has pointed out, the derived values may be uncertain by as much as a factor of two. The hydrostatic scale height, defined by P(h)=Po exp(-h/Ho) is given by Ho=RT/pg=75 kin.
A second characteristic quantity is the optical scale height ho=Ah/A logr o. From the recently derived umbral models with hydrostatic equilibrium pressure, we obtain h0 = 110 to 120 km. This value is somewhat larger (h0 ~ 1.5 Ho) than the pressure scale height.
From observations of medium strong Fraunhofer lines in sunspot spectra taken near the limb, Mattig (1969) deduced a value for h0= 100 kin, which corresponds to a pressure distribution in hydrostatic equilibrium. A new discussion of these observations by however resulted in values of h0~300 kin, which is remarkably larger.
The same sunspot had been also studied by Wittmann and Schr6ter (1969). From the center-to-limb variation of the Wilson effect, they deduced 250 km< h0 < 650 km. This means that the pressure distribution in this sunspot is subhydrostatic by a factor as large as 2-3. Measuring height differences between the line core and the continuum of Balmer lines in sunspot spectra, detected scatter of these values from different spots, which later has been clearly established by for the Ha line. According to these measurements, the height differences may differ by a factor of up to three from one spot to another, implying geometrical structure.
In order to further investigate this fact, we have analysed spectra of umbrae close to the limb, taken with the domeless Coud6 refractor in Anacapri in 1969. The shift of the position of the spot measured in the line core and in the adjacent continuum are summarized in Table (for details of the observing procedure and definition of the shift see ). Comparison of the results for the umbrae Rome No. 5559/1 and 5559/2 reveals at once a difference in Ah of a factor 5.
Knowing the difference in optical depth A logT0 between the line core and the continuum, we can derive the optical scale height ho. Therefore, contribution func-* Mitteilungen aus dem Fraunhofer Institut, Nr. 127.