Calibration of the 6302/6301 Stokes V line ratio in terms of the 5250/5247 ratio

TL;DR

This paper develops a calibration method to translate the contaminated Fe I 6302/6301 line ratio into the cleaner 5250/5247 ratio, enabling more accurate magnetic field measurements in the solar photosphere.

Contribution

It introduces a calibration function linking the red and green line ratios, accounting for thermodynamic effects, based on simultaneous observations with ZIMPOL-3 and STOP telescopes.

Findings

Calibration function enables direct interpretation of the red line ratio in terms of intrinsic magnetic fields.

Line ratios depend on flux density and heliocentric distance (μ value).

Simultaneous green and red line ratio observations improve magnetic diagnostics.

Abstract

Four decades ago the Stokes V line ratio in the Fe I 5247.06 and 5250.22 {\AA} lines was introduced as a powerful means of exploring the intrinsic field strengths at sub-pixel scales, which led to the discovery that most of the photospheric flux is in intermittent kG form. The "green" 5247-5250 line pair is unique because it allows the magnetic-field effects to be isolated from the thermodynamic effects. No other line pair with this property has since been identified. In recent years much of the magnetic-field diagnostics has been based on the "red" Fe I 6301.5 and 6302.5 {\AA} line pair, since it was chosen in the design of the Hinode space observatory. Although thermodynamic effects severely contaminate the magnetic-field signatures for this line ratio, it is still possible to use it to extract information on intrinsic magnetic fields, but only after it has been "renormalized", since otherwise it produces fictitious, superstrong fields everywhere. In the present work we explore the joint behavior of these two line ratios to determine how the "contaminated" red line ratio can be translated into the corresponding green line ratio, which then allows for a direct interpretation in terms of intrinsic magnetic fields. Our observations are mainly based on recordings with the ZIMPOL-3 spectro-polarimeter at IRSOL in Locarno, Switzerland, complemented by data from the STOP telescope at the Sayan solar observatory (Irkutsk, Russia). The IRSOL observations are unique by allowing both the green and red line pairs to be recorded simultaneously on the same CCD sensor. We show how the line ratios depend on both the measured flux densities and on the heliocentric distance (the \mu\ value on the solar disk), and finally derive the calibration function that enables the red line ratio to be translated to the green ratio for each \mu\ value.