Abstract
This is the last of three papers describing an ‘absolute’ calibration of the GONG magnetograph using and end-to-end simulation of its measurement process. The simulation begins with a MURaM 3D MHD datacube and ends with a ‘synthetic magnetogram’ of the corresponding magnetic field values as they would be observed by GONG. We determine a calibration by comparing the synthetic magnetic field measurements with the MURaM magnetic field values that produced them. The previous two papers have described the GONG measurement process (both instrument and data processing), our simulation of it, and the theory of magnetogram comparison and calibration. In this paper, we address some final points on calibration, combine all of this work into a set of calibration curves, and consider the results. We also review the results of the previous two papers for locality of reference. Our calibration indicates that GONG magnetograms underestimate weak flux by a factor of \(\sim 2\) near disk center, but that factor decreases to \(\sim 1\) as the line-of-sight approaches the limb. A preliminary investigation of the generalizability of these results suggests other instruments will be affected in a similar way. We also find that some differences in previous magnetograph comparisons are artifacts of instrumental resolution which do not reflect an intrinsic calibration difference, and the measurements are more similar than sometimes thought. These results are directly applicable to question of solar wind prediction model accuracies, particularly in the search for the cause of the common discrepancy between predicted solar wind magnetic flux at 1 AU and values measured in situ by current satellite missions.
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Acknowledgements
This work was funded in part by the NASA Heliophysics Space Weather Operations-to-Research program, grant number 80NSSC19K0005, and by a University of Colorado at Boulder Chancellor’s Office Grand Challenge grant for the Space Weather Technology, Research, and Education Center (SWx TREC).
We acknowledge contributions, discussion, information, and insight from a variety of sources: Gordon Petrie, Jack Harvey, Valentin Martínez Pillet, Sanjay Gosain, and Frank Hill, among others.
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Plowman, J.E., Berger, T.E. Calibrating GONG Magnetograms with End-to-End Instrument Simulation III: Comparison, Calibration, and Results. Sol Phys 295, 144 (2020). https://doi.org/10.1007/s11207-020-01683-3
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DOI: https://doi.org/10.1007/s11207-020-01683-3