The Fe I 1564.8 nm line and the distribution of solar magnetic fields

TL;DR

This study uses 2D MHD simulations and spectral synthesis of the Fe I 1564.8 nm line to analyze the distribution of magnetic field strengths in the solar photosphere, highlighting the line's effectiveness for fields above 50 mT.

Contribution

It provides a detailed analysis of magnetic field distribution in the solar photosphere using the Fe I 1564.8 nm line and compares its effectiveness with other spectral lines.

Findings

The maximum magnetic field strength distribution peaks around 25-35 mT.

Fe I 1564.8 nm line is most reliable for measuring fields above 50 mT.

The magnetic field distribution aligns with IR polarimetry data and supports the presence of small-scale mixed polarity fields.

Abstract

The distribution of magnetic field strength at different levels in the solar photosphere outside active regions was obtained on the basis of the 2D MHD simulation of magnetogranulation and the synthesis of the V profiles of the Fe I 1564.8 nm line. The shape of the distribution varies essentially with depth in the photosphere. The distribution maximum lies about at 25 mT, but it is found near 35 mT when the spatial averaging of profiles (about 0.5") is taken into account. Our analysis reveals that the use of the Fe I 1564.8 nm line for determination of field strength from V profile splitting is the most efficient and reliable means of measuring fields above 50 mT. The field strength distribution obtained with the 1564.8 nm line in the fields above 50 mT can serve as a standard for testing other techniques and spectral lines. The analysis of the synthesized Fe I 630.2 nm line profiles supports the conclusion that this line is less suitable for studying field strength distributions because of its weak magnetic sensitivity to fields below 120 mT. The derived magnetic field distributions as well as the distributions of asymmetry parameters and V profile zero-crossings are in good agreement with IR polarimetry data. They convincingly confirm the assumption that the structure and strength of photospheric magnetic fields of mixed polarity have continuous spectra down to scales considerably smaller than the resolution threshold. They also suggest that the structure and the scales of magnetic fields are closely related to the granulation structure.