Analysis of unresolved photospheric magnetic field structure using Fe I 6301 and 6302 lines

TL;DR

This study investigates how unresolved small-scale magnetic structures in the solar photosphere affect observed Stokes profiles, proposing efficient methods for estimating magnetic filling factors and field gradients from low-resolution data.

Contribution

It introduces and evaluates three methods for rapid estimation of magnetic filling factors and line-of-sight magnetic field gradients using 3D MHD models and synthetic spectral profiles.

Findings

Statistical and Stokes V width methods reliably estimate magnetic filling factors.

Stokes I ± V bisector splitting gradient effectively estimates line-of-sight magnetic field gradients.

Proposed methods are suitable for large observational data sets with low spatial resolution.

Abstract

Early magnetographic observations indicated that magnetic field in the solar photosphere has unresolved small-scale structure. Near-infrared and optical data with extremely high spatial resolution show that these structures have scales of few tens of kilometres, which are not resolved in the majority of solar observations. The goal of this study is to establish the effect of unresolved photospheric magnetic field structure on Stokes profiles observed with relatively low spatial resolution. Ultimately, we aim to develop methods for fast estimation of the photospheric magnetic filling factor and line-of-sight gradient of the photospheric magnetic field, which can be applied to large observational data sets. We exploit 3D MHD models of magneto-convection developed using MURAM code. Corresponding profiles of Fe I 6301.5 and 6302.5 $\mathrm{\AA}$ spectral lines are calculated using NICOLE radiative transfer code. The resulting I and V Stokes [x,y,$\lambda$] cubes with reduced spatial resolution of 150 km are used to calculate magnetic field values as they would be obtained in observations with Hinode/SOT or SDO/HMI. Three different methods of the magnetic filling factor estimation are considered: the magnetic line ratio method, Stokes V width method and a simple statistical method. We find that the statistical method and the Stokes V width method are sufficiently reliable for fast filling factor estimations. Furthermore, we find that Stokes $I\pm V$ bisector splitting gradient can be used for fast estimation of line-of-sight gradient of the photospheric magnetic field.