Interpretation of Solar Magnetic Field Strength Observations

TL;DR

This paper investigates how the choice of spectral line and sampling method affects the measurement of solar magnetic fields, proposing a calibration to improve the accuracy of magnetic flux estimates from magnetograms.

Contribution

It introduces a new calibration method for solar magnetic field measurements that accounts for spectral line differences and sampling techniques, enhancing measurement accuracy.

Findings

The bisector method near the line core provides the best estimate of photospheric flux.

A calibration factor of approximately 0.62 improves the accuracy of MWO magnetogram measurements.

The calibration aligns MWO data with more precise measurements from other instruments.

Abstract

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 \lambda5250\AA line of Fe I to the line of Fe I at \lambda5233\AA 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 $\lambda5233$\AA line. We recommend adoption of the field determined with a line bisector method with a sampling point as close as possible to the line core as the best estimate of the emergent photospheric flux. The combination of the line profile measurements and the cross-correlation of fields measured simultaneously with \lambda5250\AA and \lambda5233\AA yields a formula for the scale factor 1/\delta that multiplies the MWO synoptic magnetic fields. 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 deg. Berger and Lites (2003) found this factor to be 0.64+/-0.013 based on a comparison the the Advanced Stokes Polarimeter.