5 Some examples

After discussing the problems and solutions provided for this first light data set in the MTR mode of THEMIS, some examples of this data should be shown. No physical interpretation has been given to the present data. They have been used to guess what is happening in the telescope and not in the sun. They are just the first step on the way to obtaining physically meaningful data in future observations.

The data have been selected from observations on August 22nd at 8:37 AM at the referred spot (see Fig. 10). The two plates in Figs. 11 and 12 show 3 slit positions for both the 6301 Å  and the 6149 Å  regions. For each one of the slit positions the 4 Stokes parameters are presented. As a first illustration, we have extracted from this data a point (Fig. 13) which shows an inversion in Stokes V in the center of the line probably due to anomalous dispersion. The signal level is of the same order of the measured noise, we therefore conclude that, at least at first order and without further analysis, the errors introduced by the analyser and any instrumental polarization are at most comparable with the noise.

The most apparent aspect of those profiles is the presence of polarization signal for the two O₂ telluric lines. This is the result of the different equivalent widths of the lines in the two orthogonally polarized paths. As a normalization has been imposed to the continuum signals, this difference translates into a difference in the depth of lines. After comparison of the images, it becomes an absorption-like signal. This phenomenon can also be seen in the Fe I line at 6149 Å, which due to its atomic level configuration, lacks linear polarization signal. However, for this line the equivalent width problem and the other errors were less prominent (probably due to its larger equivalent width compared to the telluric lines) and only a small residual can be found.

Figure 16: Charts of longitudinal magnetic field as measured by the center of gravity method on the Fe I lines at 6301 Å  (left), 6302 Å  (center) and 6151 Å (right)

Figures 14 and 15 show two sets of Stokes profiles extracted from the same image (the first slit in plate 12). The asymmetries in the Stokes U profiles are evident but different in sign, indicating a clear and exclusive solar origin. Stokes V has also very pronounced asymmetries which deform completely the usual antisymmetries of this profile. Comparison with the V profiles at other wavelengths (as the one shown in Fig. 13, taken in a near solar point and simultaneously with the two referred profiles) excludes crosstalk between Stokes parameters as the primary origin of these strongly asymmetric V profiles. Finally, in Fig. 16 we present three charts of longitudinal magnetic field as measured by the center of gravity method on the 6301 Å, 6302 Å  and 6151 Å  Fe I lines. No attempt at calibration has been made and the grey scale is in terms of pixel length displacements. They must be seen in a qualitative way: the distribution of magnetic field corresponds to what can be expected and there is a general coherence between the images measured on three different lines. The differences between these images show the importance of observing and analysing several spectral lines simultaneously in order to obtain a more precise diagnostic of the distribution of magnetic field and its gradients in the solar photosphere.