Conversion of the 6302 / 6301 Stokes V Line Ratio to the 5250 / 5247 Ratio for the Diagnostics of Quiet-Sun Magnetic Fields

摘要

Observations in the "green" spectral lines Fe I5247.06 and 5250.22 ? and in the "red" lines Fe I 6301.50 and 6302.50 A are widely used to explore the properties of solar magnetic fields. The green line pair was introduced in 1973 as part of the line- ratio technique, which has been a powerful tool for investigations of intrinsic magnetic field properties at spatially unresolved scales (magnetic flux tubes with kG strengths). The red line pair has recently played a major role for magnetic-field diagnostics due to the large amount of high-quality data provided by the Hinode space observatory. These red lines however differ not only in the values of their Landé factors, but also in their line-formation properties, with the consequence that the magnetic-field information in their line ratio gets tangled up with thermodynamic effects. In contrast, as the green FeI 5247.06 and 5250.22 A lines differ only in their Landé factors, the magnetic field effects become cleanly separated from the thermodynamics, which allows the intrinsic magnetic field parameters to be extracted without ambiguties. The red and green line-ratio values are however statistically correlated. By determining the statistical regression function that relates them, it becomes possible to convert the "contaminated" and ambiguous red line ratio into the green line ratio, with which a reliable direct interpretation in terms of intrinsic field strengths is possible. To determine how the two line ratios are related we have made Stokesmeter observations in these four spectral lines with two solar instruments equipped with high-precision spec- tropolarimeters, ZIMPOL-3 at IRSOL (Locarno, Switzerland), and the STOP telescope at the Sayan Solar Observatory (Irkutsk, Russia). Most of the obtained results are based on IRSOL observations of quiet-sun magnetic fields. In the case of STOP the full-disk magnetograms of large-scale solar magnetic fields are analyzed. A major advantage at IRSOL is the possibility to observe the green and red line pairs simultaneously on the same CCD chip. We have determined how the two line ratios decrease with increasing Stokes V amplitude and increase with increasing distance from disk center. The relation that allows us to convert the observed values for the red line ratio into the corresponding values for the green line ratio has been found and given in analytical form.