Automatic detection and correction algorithms for magnetic saturation in the SMFT/HSOS longitudinal magnetograms

Abstract

A longitudinal magnetic field often suffers the saturation effect in a strong magnetic field region when the measurement is performed at a single-wavelength point and linear calibration is adopted. In this study, we develop a method that can judge the threshold of saturation in Stokes V/I observed by the Solar Magnetic Field Telescope (SMFT) and correct it automatically. The procedure is to first perform the second-order polynomial fit to the Stokes V/I vs. I/Im (Im is the maximum value of Stokes I) curve to estimate the threshold of saturation, then reconstruct Stokes V/I in a strong field region to correct for saturation. The algorithm is demonstrated to be effective by comparing with the magnetograms obtained by the Helioseismic and Magnetic Imager (HMI). The accuracy rate of detection and correction for saturation is ∼99.4% and ∼88% respectively among 175 active regions. The advantages and disadvantages of the algorithm are discussed.