Improved magnetogram calibration of SMFT and its comparison with the HMI

TL;DR

This paper enhances the calibration of SMFT magnetograms by using a six-point nonlinear fitting method, improving accuracy over linear methods, and compares the results with HMI, revealing differences in magnetic field measurements and stray light effects.

Contribution

The paper introduces a six-point nonlinear calibration method for SMFT magnetograms, improving accuracy over traditional linear calibration and providing a detailed comparison with HMI measurements.

Findings

Six-point calibration reduces non-linear discrepancies in magnetic field strength.

SMFT has larger stray light and lower magnetic field strength than HMI.

Magnetic field measurements between SMFT and HMI are roughly in agreement in sunspot regions.

Abstract

In this paper, we try to improve the magnetogram calibration method of the Solar Magnetic Field Telescope (SMFT). The improved calibration process fits the observed full Stokes information, using six points on the profile of Fe {\i} 5324.18 {\AA} line, and the analytical Stokes profiles under the Milne-Eddington atmosphere model, adopting the Levenberg-Marquardt least-square fitting algorithm. In Comparison with the linear calibration methods, which employs one point, there is large difference in the strength of longitudinal field $B_l$ and tranverse field $B_t$, caused by the non-linear relationship, but the discrepancy is little in the case of inclination and azimuth. We conclude that it is better to deal with the non-linear effects in the calibration of $B_l$ and $B_t$ using six points. Moreover, in comparison with SDO/HMI, SMFT has larger stray light and acquires less magnetic field strength. For vector magnetic fields in two sunspot regions, the magnetic field strength, inclination and azimuth angles between SMFT and HMI are roughly in agrement, with the linear fitted slope of 0.73/0.7, 0.95/1.04 and 0.99/1.1. In the case of pores and quiet regions ($B_l$ $<$ 600 G), the fitted slopes of the longitudinal magnetic field strength are 0.78 and 0.87 respectively.