On apparent differences in magnetic field strengths measured from Zeeman splittings of molecular lines and Feiλ 5250.2 Å

✍ Scribed by Jürgen Staude

• Publisher: Springer
• Year: 1971
• Tongue: English
• Weight: 95 KB
• Volume: 17
• Category: Article
• ISSN: 0038-0938
• DOI: 10.1007/bf00150036

✦ Synopsis

In two recent papers W6hl (1969) and Mallia (1970) reported on observations of Zeeman splittings of molecular lines within sunspots. Particularly splittings of the MgH line at 5190 2k were measured and the resulting magnetic field strengths H were compared with those from the line FeI 25250.2 ]~ which has been observed at the same positions of the sunspot. The differences in the H values being opposite in sign for both observers seem to exceed the uncertainties of the splitting factors. Moreover Mallia discussed other influences on the observations like stray light, magnetic field gradients, differences in time and position between the observations of both lines, but all appear unable to result in discrepancies like the observed ones. Therefore in the following we propose another explanation.

Provided that the MgH line yields the correct 11, the differences may be explained by saturation effects of the line FeI 2 5250.2 ~. Observations (Wiehr, 1969) as well as calculations of line contours for different umbral models (Staude, 1970a, b) have shown that the Fe~ line is strengthened and broadened within sunspots. The resulting saturation effects may cause errors in the measurements of Zeeman splittings (see also Maltby and Kjeldseth Moe, 1968).

Using the computer programs and data of the earlier work (Staude, 1970a) line contours were obtained for the spot model of H6noux (1969); compared with observations older spot models with steep temperature gradients produce too large line depressions. The calculated r t contours of the FeI line (observation without polarization optics) show maximum depressions the distance of which is smaller than that of the real position of the a-components (Figure 1). Thus a smaller H may be simulated as it has been measured by Mallia; Figure 1 shows that this is valid also for the large value H= 3150 G which has been measured by him from the MgH line. The calculated effect is even larger than the observed one, the difference could be reduced, however, by assuming a similar but smaller influence on the weaker molecular line contour.

Often the splitting is not measured from the distance of the maximum depressions but from the width of the composed line. If in this case the strengthening of the line in sunspots compared with the photospheric contour (Figure 1) is neglected, the saturation of the FeI line may simulate larger values H than the real one, just as it