Calibrating GONG Magnetograms with End-to-end Instrument Simulation III: Comparison, Calibration, and Results

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 over 2 near disk center, but that factor decreases to about 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.

arXiv commentSubmitted to Solar Physics, third of three papers (the other two are arXiv submissions 3034511 and 3034641)

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• Calibrating GONG Magnetograms with End-to-end Instrument Simulation III: Comparison, Calibration, and Results
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Cited by in corpus (6)

• Magnetograph Saturation and the Open Flux Problem
• Calibrating GONG Magnetograms with End-to-end Instrument Simulation III: Comparison, Calibration, and Results
• Magnetograms underestimate even unipolar magnetic flux nearly everywhere on the solar disk
• Limitations and biases in the retrieval of the polar magnetic field I: the role of the magnetic filling factor in Milne-Eddington inversions of simulated Hinode/SP data
• Calibrating GONG Magnetograms with End-to-end Instrument Simulation I: Background, the GONG Instrument, and End-to-end Simulation
• Calibrating GONG Magnetograms with End-to-end Instrument Simulation II: Theory of Calibration