# The potential of many-line inversions of photospheric spectropolarimetric data in the visible and near UV

## TL;DR

This study demonstrates that many-line spectropolarimetric inversions in the visible and near UV provide significantly more detailed and reliable information about the solar atmosphere than traditional few-line methods, especially at short wavelengths.

## Contribution

It shows that many-line inversions outperform few-line approaches in the visible and near UV, particularly for the upper photosphere, and supports future use of many-line spectropolarimetry.

## Key findings

- Many-line inversions yield more atmospheric information than few-line methods.
- Short wavelengths provide more reliable data on the upper photosphere.
- Temperature determination improves by a factor of three with many-line inversions.

## Abstract

Our knowledge of the lower solar atmosphere is mainly obtained from spectropolarimetric observations, which are often carried out in the red or infrared spectral range and almost always cover only a single or a few spectral lines. Here we compare the quality of Stokes inversions of only a few spectral lines with many-line inversions. We investigate the feasibility of spectropolarimetry in the short-wavelength range, 3000 \AA{} - 4300 \AA{}, where the line density but also the photon noise are considerably higher than in the red, so that many-line inversions could be particularly attractive in that wavelength range. This is also timely because this wavelength range will be the focus of a new spectropolarimeter in the third science flight of the balloon-borne solar observatory SUNRISE. For an ensemble of MHD atmospheres we synthesize exemplarily spectral regions around 3140 \AA{}, 4080 \AA{}, and 6302 \AA{}. The spectral coverage is chosen such that at a spectral resolving power of 150000 the spectra can be recorded by a 2K detector. The synthetic Stokes profiles are degraded with a typical photon noise and afterwards inverted. The atmospheric parameters of the inversion of noisy profiles are compared with the inversion of noise-free spectra. We find that significantly more information can be obtained from many-line inversions than from a traditionally used inversion of only a few spectral lines. We further find that information on the upper photosphere can be significantly more reliably obtained at short wavelengths. In the mid and lower photosphere, the many-line approach at 4080 \AA{} provides equally good results as the many-line approach at 6302 \AA{} for the magnetic field strength and the LOS velocity, while the temperature determination is even more precise by a factor of three. We conclude that many-line spectropolarimetry should be the preferred option in the future.