The Magnetic-Field Calibration of the Full-Disk Magnetograph Onboard the Advanced Space Based Solar Observatory (ASO-S/FMG)

The full-disk magnetograph is a main scientific payload onboard the Advanced Space based Solar Observatory (ASO-S/FMG) that through Stokes-parameters observation measures the vector magnetic field. The accuracy of magnetic-field values is an important aspect of checking the quality of the FMG magnetic-field measurement. According to the design of the FMG, the linear calibration method under the weak-field approximation is the preferred scheme for magnetic-field calibration. However, the spacecraft orbital velocity can affect the position of observed spectral lines, which then results in a change of the polarization-signal strength. Thus, the magnetic field is modulated by the orbit velocity of the spacecraft. In this article, through cross-calibration between FMG and HMI (Helioseismic and Magnetic Imager onboard the Solar Dynamic Observatory), the effects of spacecraft orbital velocity on the coefficient of magnetic-field calibration are investigated. By comparing the magnetic field of FMG and HMI with spacecraft orbital velocity as an auxiliary reference, revised linear-calibration coefficients that depend on spacecraft orbital velocity are obtained. The magnetic field of FMG corrected by the revised calibration coefficients removing the effect of spacecraft orbital velocity will be more accurate and suitable for scientific research.