Weak-Field Magnetogram Calibration using Advanced Stokes Polarimeter Flux-Density Maps – I. Solar Optical Universal Polarimeter Calibration

Abstract

Cotemporal Fe i 630.2 nm magnetograms from the Solar Optical Universal Polarimeter (SOUP) filter and the Advanced Stokes Polarimeter (ASP) are quantitatively compared using observations of active region AR 8218, a large negative polarity sunspot group observed at S20 W22 on 13 May 1998. The SOUP instrument produces Stokes V/I `filter magnetograms' with wide field of view and spatial resolution below 0.5 arc sec in good seeing, but low spectral resolution. In contrast, the ASP uses high spectral resolution to produce very high-precision vector magnetic field maps at spatial resolution values on the order of 1 arc sec in good seeing. We use ASP inversion results to create an ASP `longitudinal magnetic flux-density map' with which to calibrate the less precise SOUP magnetograms. The magnetograms from each instrument are co-aligned with an accuracy of about 1 arc sec. Regions of invalid data, poor field-of-view overlap, and sunspots are masked out in order to calibrate SOUP predominately on the relatively vertical `weak-field' plage magnetic elements. Pixel-to-pixel statistical comparisons are used to determine the SOUP magnetogram linear calibration constant relative to ASP flux-density values. We compare three distinct methods of scaling the ASP and SOUP data to a common reference frame in order to explore filling factor effects. The recommended SOUP calibration constant is 17 000 ± 550 Mx cm−2 per polarization percent in plage regions. We find a distinct polarity asymmetry in SOUP response relative to the ASP, apparently due to a spatial resolution effect in the ASP data: the smaller, less numerous, minority polarity structures in the plage region are preferentially blended with the majority polarity structures. The blending occurs to a lesser degree in the high-resolution SOUP magnetogram thus leading to an apparent increase in SOUP sensitivity to the minority polarity structures relative to the ASP. One implication of this effect is that in mixed polarity regions on the Sun, lower spatial resolution magnetograms may significantly underestimate minority polarity flux levels, thus leading to apparent flux imbalances in the data. *Visiting Astronomer, National Solar Observatory, operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation. †The National Center for Atmospheric Research is sponsored by the National Science Foundation.