Toward a Consensus for Multi-Sourced Photospheric Line-of-Sight Magnetic Field Cross-Calibration

Abstract

In this work we compare the full-disk line-of-sight magnetic field measurements of the Helioseismic and Magnetic Imager (HMI) with the magnetograms from multiple sources including the Global Oscillation Network Group (GONG), the Michelson Doppler Imager (MDI), the Synoptic Optical Long-term Investigations of the Sun (SOLIS) instrument, the Mount Wilson Observatory (MWO), and the Wilcox Solar Observatory (WSO). A scaling factor is then derived that matches the magnetograms from these instruments to those of HMI. The scaling factors are \(1.438\pm 0.000\) for GONG, \(0.701\pm 0.001\) for MDI, \(1.038\pm 0.001\) for SOLIS, \(2.795\pm 0.002\) for MWO, and \({3.582\pm 0.006}\) for WSO. This scaling factor varies with the center-to-limb distance and field strength. Distortion maps for these instruments are also determined using the HMI data as a reference.