Comparison of magnetic data products from Solar Orbiter SO/PHI-FDT and SDO/HMI

Abstract

Context. The Polarimetric and Helioseismic Imager (SO/PHI), on board the Solar Orbiter mission, is the first photospheric magnetograph and tachograph to observe the Sun from outside the Sun-Earth line. The Full Disc Telescope (FDT) of SO/PHI, images the whole solar disk with a spatial resolution that varies with the distance between the Sun and the spacecraft. Aims. We check for consistency between the magnetic field strength (B), the field inclination (γ), the line-of-sight (LoS) magnetic component (BLoS) and the field azimuth (ϕ), inferred by SO/PHI-FDT and the Helioseismic and Magnetic Imager (HMI), on board Solar Dynamics Observatory (SDO), and obtain linear correlation coefficients among them. Methods. We use data from both instruments obtained on 8 March 2022, when the angle between SDO and Solar Orbiter was 3.4° and the solar disk showed four developed active regions. Before comparing the magnetic field products of both instruments we perform a precise alignment of the data, including a matching of the plate scale. Further, in order to improve the homogeneity of the compared data products, the SDO/HMI data were convolved with the SO/PHI-FDT point spread function (PSF). The linear correlation coefficients are obtained through a linear regression of SDO/HMI to SO/PHI-FDT. Results. The two instruments yield comparable magnetic field data products. The slope coefficients for a linear fit are 1.37 for B, 1.11 for γ, 1.35 for BLoS and 1 for the azimuth. The corresponding fit offsets are −94 G, −9.8°, 5.2 G and 0.1°, respectively. The agreement between both instruments is significantly better when we take into account the different spatial resolution of both instruments. The fitting results vary slightly depending on the analyzed active region except for one of the four active regions, which shows larger differences and has been excluded from the comparison. The comparison of the LoS magnetic field products from SDO/HMI at 45 s and 720 s with SO/PHI-FDT shows a slope value of 1.17, with the offset less than 6 G, in both cases.