Interpretation of Solar Magnetic Field Strength Observations

This study based on longitudinal Zeeman effect magnetograms and spectral line scans investigates the dependence of solar surface magnetic fields on the spectral line used and the way the line is sampled in order to estimate the magnetic flux emerging above the solar atmosphere and penetrating to the corona from magnetograms of the Mt. Wilson 150-foot tower synoptic program (MWO). We have compared the synoptic program λ5250Å line of Fe I to the line of Fe I at λ5233Å since this latter line has a broad shape with a profile that is nearly linear over a large portion of its wings. The present study uses five pairs of sampling points on the λ5233Å line. Line profile observations show that the determination of the field strength from the Stokes V parameter or from line bisectors in the circularly polarized line profiles lead to similar dependencies on the spectral sampling of the lines with the bisector method being the less sensitive. We recommend adoption of the field determined with the line bisector method as the best estimate of the emergent photospheric flux and further recommend the use of a sampling point as close to the line core as is practical. The combination of the line profile measurements and the cross-correlation of fields measured simultaneously with λ5250Å and λ5233Å yields a formula for the scale factor δ that multiplies the MWO synoptic magnetic fields. Using ρ as the center-to-limb angle (CLA), a fit to this scale factor is δ = 4.15− 2.82 sin(ρ). Previously δ = 4.5−2.5 sin(ρ) had been used. The new calibration shows that magnetic fields measured by the MDI system on the SOHO spacecraft are equal to 0.619 ± 0.018 times the true value at a center-to-limb position 30. Berger and Lites (2003) found this factor to be 0.64± 0.013 based on a comparison the the Advanced Stokes Polarimeter.