Spectro - Polarimetric Observations of Solar Magnetic Fields and the SOHO / MDI Calibration

Comparisons of solar magnetic-field measurements made in different spectral lines are very important, especially in those lines in which observations have a long history or(and) specific diagnostic significance. The spectral lines Fe i 523.3 nm and Fe i 525.0 nm belong to this class. Therefore, this study is devoted to a comprehensive analysis using new high-precision Stokesmeter full-disk observations. The disk-averaged magnetic-field strength ratio R = B(523.3)/B(525.0) equals 1.97 ± 0.02. The center-to-limb variation (CLV) is R = 1.74−2.43μ+3.43μ, where μ is the cosine of the center-to-limb angle. For the disk center, we find R = 2.74, and for near-limb areas with μ = 0.3, R equals 1.32. There is only a small dependence of R on the spatial resolution. Our results are rather close to those published three decades ago, but differ significantly from recent magnetographic observations. An application of our results to the important SOHO/MDI magnetic data calibration issue is discussed. We conclude that the revision of the SOHO/MDI data, based only on the comparison of magnetic field measurements in the line pair Fe i 523.3 nm and Fe i 525.0 nm (increasing by a factor of 1.7 or 1.6 on average according to recent publications) is not obvious and new investigations are urgently needed.