Comparison of the magnetic fields in solar active regions derived with different instruments

Abstract

The paper presents a comparison of the magnetic fluxes in active regions (ARs) measured with SOHO/MDI and SDO/HMI. The transition coefficients from HMI to MDI data are as follows: 1.46 for ARs observed within 10° of the central meridian and 1.29 for ARs observed at longitudes [−60°; −10°] and [10°; 60°]. The maximum magnetic field values in sunspots obtained in 2014 from SDO/HMI and with the ground-based telescope BST-2 at the Crimean Astrophysical Observatory of the Russian Academy of Sciences were compared using the orthogonal regression method. It is shown that the best agreement between data from the two instruments is achieved when comparing Crimean data with HMI data on the full field vector (correlation coefficient 0.71) rather than on its  longitudinal component (correlation coefficient 0.66). This conclusion indicates that the field measurements by the spectral line shift method (applied at BST-2) yield a value of the full field vector with the sign of its longitudinal component. The comparison results evidence for the possibility of using Crimean sunspot magnetic field data, which is particularly valuable for studying solar cycles prior to the space age.