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Inferring vector magnetic fields from HMI observations of a solar plage region: one-component versus two-component Milne-Eddington inversions

Presentation #125.17 in the session General Topics: Solar — Poster Session.

Published onOct 20, 2022
Inferring vector magnetic fields from HMI observations of a solar plage region: one-component versus two-component Milne-Eddington inversions

The Very Fast Inversion of the Stokes Vector (VFISV) is a Milne-Eddington (ME) inversion code that was specifically designed to operate in the HMI data processing pipeline. To speed up the inversion process, the pipeline VFISV code (hereafter, VFISV1C) employs one component of an ME model atmosphere with the magnetic filling factor of unity for all HMI pixels. Recently, VFISV1C was modified to enable inversion of observed HMI Stokes profiles with two ME components, allowing the filling factor as a free parameter. Here we test whether the updated inversion code (hereafter, VFISV2C) can improve the quality of HMI vector magnetic field data, in particular, for decaying active regions with intermediate field strength. This test is done with a long-lived plage of which the magnetic field is supposed to slowly evolve over a couple of solar rotations. The plage can be therefore considered as a quasi-steady target for comparison of HMI magnetic field observations at different times. For a large portion of the plage pixels, the inverted vector magnetic field with VFISV1C shows a systematic overestimation in the transverse field, which causes a sign reversal of both the zonal (Bp, east-west) and meridional (Bt, north-south) components across the central meridian of the solar disk. We find that the east-west hemispheric sign reversal is substantially mitigated with VFISV2C for most pixels in the plage with magnetic flux density greater than 300 Mx/cm2. We attribute this improvement to the fact that the vector magnetic field inverted with VFISV2C is determined in favor of having a smaller inclination angle from the line of sight and consequently a weaker transverse component, when compared to the one with VFISV1C. The updated VFISV2C can help us to more reliably infer magnetic fields from HMI observations of plages and use them for simulations.

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