Calibrating GONG Magnetograms with End-to-End Instrument Simulation III: Comparison, Calibration, and Results
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Calibrating GONG Magnetograms with End-to-End Instrument Simulation III: Comparison, Calibration, and Results Joseph E. Plowman1
· Thomas E. Berger2
Received: 6 February 2020 / Accepted: 5 August 2020 / Published online: 28 October 2020 © Springer Nature B.V. 2020
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 ∼ 2 near disk center, but that factor decreases to ∼ 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. Keywords Instrumental effects · Magnetic fields, interplanetary · Magnetic fields, models · Magnetic fields, photosphere
B J.E. Plowman
[email protected] T.E. Berger [email protected]
1
National Solar Observatory, 3665 Discovery Drive, Boulder, CO 80303, USA
2
University of Colorado at Boulder, Space Weather Technology, Research, and Education Center, 3775 Discovery Drive, Boulder, CO 80309, USA
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J.E. Plowman, T.E. Berger
1. Introduction Measurement of the solar photospheric magnetic field is well-established and long-standing: routine observations have been now been made for over 75 years (see Babcock, 1953; Howard and Babcock, 1960; Howard et al., 1983). A variety of instruments currently make regular measurements of the photospheric magnetic field; this work concentrates on the ‘synoptic’ measurements made by the GONG instruments, which are designed to make regularly recurring observations of the entire solar hemisphere visible from Earth. A primary use of these ‘synoptic magnetograms’ is as the ‘boundary condition’ for models of the coronal ma
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