Improvement of the Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Inversion Code

Ana Belén Griñón-Marín,Aimee Norton,J Todd Hoeksema,Yang Liu,Adur Pastor Yabar

doi:10.3847/1538-4357/ac2aa8

Ana Belén Griñón-Marín, Aimee Norton + Show 3 more

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https://doi.org/10.3847/1538-4357/ac2aa8

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Abstract

Abstract A spectral line inversion code, Very Fast Inversion of the Stokes Vector (VFISV), has been used since 2010 May to infer the solar atmospheric parameters from the spectropolarimetric observations taken by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory. The magnetic filling factor, the fraction of the surface with a resolution element occupied by magnetic field, is set to have a constant value of 1 in the current version of VFISV. This report describes an improved inversion strategy for the spectropolarimetric data observed with HMI for magnetic field strengths of intermediate values in areas spatially not fully resolved. The VFISV inversion code has been modified to enable inversion of the Stokes profiles with two different components: one magnetic and one nonmagnetic. In this scheme, both components share the atmospheric components except for the magnetic field vector. In order to determine whether the new strategy is useful, we evaluate the inferred parameters inverted with one magnetic component (the original version of the HMI inversion) and with two components (the improved version) using a Bayesian analysis. In pixels with intermediate magnetic field strengths (e.g., plages), the new version provides statistically significant values of filling fraction and magnetic field vector. Not only does the fitting of the Stokes profile improve, but also the inference of the magnetic parameters and line-of-sight velocity are obtained uniquely. The new strategy is also proven to be effective for mitigating the anomalous hemispheric bias in the east–west magnetic field component in moderate field regions.

Highlights

A variety of magnetic structures can be observed at the solar surface, from the most impressive ones such as sunspots and filaments, to the smallest ones such as individual elements of the network or faculae
A spectral line inversion code, Very Fast Inversion of the Stokes Vector (VFISV), has been used since May 2010 to infer the solar atmospheric parameters from the spectropolarimetric observations taken by the Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO)
In order to determine whether the new strategy is useful, we evaluate the inferred parameters inverted with one magnetic component and with two components using a Bayesian analysis

Summary

Introduction

A variety of magnetic structures can be observed at the solar surface, from the most impressive ones such as sunspots and filaments, to the smallest ones such as individual elements of the network or faculae. Since the early 1970s various approaches have been used to solve the radiative transfer equation to find a model that closely matches an observed quantity by optimizing a merit function. One of the simplest approximations for the solution of the radiative transfer equation that keeps a rigorous enough treatment of the magnetic field in the Zeeman regime is the Milne-Eddington (ME) approach (Landolfi & Landi Degl’Innocenti 1982). The ME model assumes that the physical parameters in the atmosphere are independent of the optical depth except for the source function, which is assumed to vary linearly. While this is an oversimplified model, it has been widely used and leads to reasonable results in regions where field gradients are not too great

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