Calibrating GONG Magnetograms Using an End-to-end Magnetograph Model
Abstract
Numerous science and operations projects rely heavily on accurate, consistent magnetic field measurements from the solar surface. These projects are hindered by well-known but poorly understood discrepancies between magnetograms obtained with different solar telescopes and instrumentation. Existing efforts to characterize these discrepancies have been limited to direct comparisons between final data products and have not reached firm conclusions regarding what the correct measurement should be.To solve this problem, we model every step of the measurement and the processing of the data all the way to the final magnetogram. We begin with known MHD (MURaM, Voegler et al. 2005) simulation data for the magnetic field, the 'ground truth'. We compute the radiative transfer for the Stokes (I,Q,U,V) spectra using the Stokes Inversion with Response functions (SIR, Ruiz Cobo et al. 1992) code. We then use the Solar Orbiter Polarimetric and Helioseismic Imager Software siMulator (SOPHISM, Blanco Rodríguez et al. 2018) to simulate the instrument response to emergent spectra. We model every significant process undergone by the solar signal during an observation, including atmospheric seeing, degradation of the signal by instrumental limitations including finite spatial and spectral resolution, polarization cross-talk and modulation, and finally the effects of the data processing software pipelines that transform the solar polarization signal to magnetogram data via calibration, Stokes inversion, etc.We then compare the simulated GONG magnetogram with MHD field data (weighted by the response function). We demonstrate the effect of the calibration on a synoptic magnetogram and a global coronal field model, paying particular attention to a revised estimate of the open magnetic flux. Such end-to-end magnetogram calibration will shed light on well-known but poorly-understood discrepancies between results from different magnetographs, and will become essential as we combine the advantages of multiple magnetogram data sets.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMSH41B..04P
- Keywords:
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- 7511 Coronal holes;
- SOLAR PHYSICS;
- ASTROPHYSICS;
- AND ASTRONOMY;
- 7524 Magnetic fields;
- SOLAR PHYSICS;
- ASTROPHYSICS;
- AND ASTRONOMY;
- 7529 Photosphere;
- SOLAR PHYSICS;
- ASTROPHYSICS;
- AND ASTRONOMY;
- 7594 Instruments and techniques;
- SOLAR PHYSICS;
- ASTROPHYSICS;
- AND ASTRONOMY