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Astron. Astrophys. 321, 643-651 (1997)

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2. Observations

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 [FORMULA] showing a considerable number of UDs in the slit-jaw pictures. Observations were made on June 19, when the spot was located at cos  [FORMULA].
The Echelle-Type spectrograph of the VTT was used to obtain spectrograms in two different spectral regions at [FORMULA]  630.25 nm and [FORMULA]  846.85 nm. A large format (1024 [FORMULA] 1024 pixel) CCD camera with a pixel size of 19 microns squared recorded the data set in 2 [FORMULA] 2 summing mode. With the image scale of the spectrograph in the focal plane of [FORMULA] per mm the spatial FOV was [FORMULA] with a pixel size of [FORMULA]. The spectral FOV is 0.2 nm due to the linear dispersion of the spectrograph of about 0.1 nm/cm in the red part of the visible spectrum. Raster scans of the sunspot were obtained by moving the solar image across the spectrograph. For the [FORMULA]  630.25 nm ([FORMULA]  846.85 nm) line the spot was scanned in 16 (14) steps of 0.5 arcseconds. The slit width was 80 µm corresponding to [FORMULA] on the sun and the exposure time was 0.2 (0.4) seconds. With a step to step time of 5.4 seconds the repetition time for the rasters amounts to 86 (76) seconds.

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 [FORMULA]  846.85 nm and then at [FORMULA]  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 [FORMULA]  630.25 nm line no data are available.

[FIGURE] Fig. 1. Line profiles [FORMULA] along the slit and corresponding spectrogram. The lowermost profile was recorded at a position of 18.7 arcseconds in the corresponding spectrum, the following ones in steps of 1.87 arcseconds. Left: [FORMULA]  846.85 nm, CUD1 sequence; right: [FORMULA]  630.25 nm, CUD2 sequence. In both spectra the umbra is inserted in a different intensity scale. The position of the extracted line profiles is marked in the corresponding spectrogram. The thick lines are UD line profiles and their location in the spectrogram is marked with an arrow.

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 [FORMULA]  630.25 nm line the [FORMULA]  846.85 nm line has an asymmetry in the [FORMULA] -components, due to the presence of a Ti I blend whose wavelength and line strength are not exactly known (see Fig. 1). The [FORMULA] -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 [FORMULA] -component seen in the [FORMULA]  846.85 nm line. The Ti I line has a rather complicated splitting pattern with a [FORMULA] 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 [FORMULA]  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 [FORMULA]  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.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998