We analysed sunspot observations obtained during an observing run
in 1991 with the 70 cm Vacuum Tower Telescope (VTT) located at
the Observatorio del Teide, Izaña. A fairly regular medium
sized sunspot (NOAA 6681) was selected with a diameter of about
showing a considerable number of UDs in the
slit-jaw pictures. Observations were made on June 19, when the spot
was located at cos .
The small size of UDs makes them difficult to track although the seeing conditions were quite good. Imperfect positioning of the slit during the exposure can cause a dot to disappear from the spectrogram. The data set we obtained consists of several time series taken first at 846.85 nm and then at 630.25 nm.
Fig. 1 shows two spectrograms of the observed wavelength regions and the spatial variation of the normalized intensity profiles on the left of them. The two scans do not represent the same cut, but they are from the same spot. On the blue side of the 630.25 nm line no data are available.
Since the spectra of the two lines were not recorded simultaneously, it was not possible to compare the umbral features in different wavelengths directly, thus no field gradient could be derived, neither in the UD nor in the inter-dot regions.
2.1. Line characteristics
The two Fe I lines are well suited for magnetic field measurements since both have an effective Landé factor of 2.5 and split into a simple Zeeman triplet. Unlike the 630.25 nm line the 846.85 nm line has an asymmetry in the -components, due to the presence of a Ti I blend whose wavelength and line strength are not exactly known (see Fig. 1). The -component is deformed, indicating that the blend is somewhat shifted to the red part relative to the Fe I line. Thus we cannot exclude that the blend may have an influence on the strength of the -component seen in the 846.85 nm line. The Ti I line has a rather complicated splitting pattern with a of 1.125. The variation of the line profiles along the slit (see Fig. 1) indicates that the blend gets stronger with increasing magnetic field strength and with decreasing temperature.
Although the presence of the blend complicates the analysis of the 846.85 nm line, these observations represent independent measurements of the same sunspot region. We expect to gain information about higher photospheric layers than from the 630.25 nm line. The characteristics of the observed lines and some observational parameters are summarized in Table 1.
Table 1. Some relevant parameters of the observed lines.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998