Using SDO/HMI magnetograms as a source of the solar mean magnetic field data

TL;DR

This study compares the solar mean magnetic field data from WSO and SDO/HMI, finding high correlation and discussing measurement artifacts like orbital-induced oscillations and their relation to sunspots.

Contribution

It provides a cross-instrument comparison of SMMF data and analyzes artifacts affecting magnetic field measurements, enhancing data reliability.

Findings

High correlation (0.86) between WSO and HMI SMMF data.

Artificial 12 and 24-hour oscillations linked to spacecraft orbit.

Opposite phase oscillations observed in sunspots of opposite polarities.

Abstract

The solar mean magnetic field (SMMF) provided by the Wilcox Solar Observatory (WSO) is compared with the SMMF acquired by the \textit{Helioseismic and Magnetic Imager} (HMI) onboard the \textit{Solar Dynamic 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 SMMF and in sunspots magnetic fields that might be caused by orbital motions of the spacecraft. The artificial harmonics of SMMF reveal significant changes in amplitude and the nearly stable phase. The connection between the 24-hour harmonic amplitude of SMMF and the presence of sunspots is examined. We also found that opposite phase artificial 12 and/or 24-hour oscillations exist in sunspots of opposite polarities.