Using SDO/HMI Magnetograms as a Source of the Solar Mean Magnetic Field Data

The solar mean magnetic field (SMMF) provided by the Wilcox Solar Observatory (WSO) is compared with the SMMF acquired by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). We found that despite the different spectral lines and measurement techniques used in both instruments, the Pearson correlation coefficient between these two datasets equals 0.86, while the conversion factor is very close to unity: B HMI = 0.99 ( 2 ) B WSO . We also discuss artifacts of the SDO/HMI magnetic field measurements, namely the 12- and 24-hour oscillations in the SMMF and in sunspot magnetic fields that are thought to be caused by orbital motions of the spacecraft. The artificial harmonics of the SMMF reveal significant changes in amplitude and the nearly stable phase. The connection between the 24-hour harmonic amplitude of the SMMF and the presence of sunspots is examined. We also found that opposite-phase artificial 12- and/or 24-hour oscillations exist in magnetic field strength of sunspots of opposite polarities.