Capabilities of bisector analysis of the Si I 10827 A line for estimating line-of-sight velocities in the quiet Sun

TL;DR

This paper evaluates a fast, simple bisector and line core fitting method for accurately inferring line-of-sight velocities in the quiet Sun using the Si I 10827 A line, enabling efficient atmospheric velocity profiling.

Contribution

It demonstrates that bisector analysis and line core fitting of Si I 10827 A reliably estimate LOS velocities across different atmospheric layers, improving quick-look solar observations.

Findings

Bisector velocities correlate well with simulation data at various depths.

Line core fitting provides reliable information about higher atmospheric layers.

Combined methods enable seamless velocity profiling from deep photosphere to higher layers.

Abstract

We examine the capabilities of a fast and simple method to infer line-of-sight (LOS) velocities from observations of the photospheric Si I 10827 A line. This spectral line is routinely observed together with the chromospheric He I 10830 A triplet as it helps to constrain the atmospheric parameters. We study the accuracy of bisector analysis and a line core fit of Si I 10827 A. We employ synthetic profiles starting from the Bifrost enhanced network simulation. The profiles are computed solving the radiative transfer equation, including non-local thermodynamic equilibrium effects on the determination of the atomic level populations of Si I. We found a good correlation between the inferred velocities from bisectors taken at different line profile intensities and the original simulation velocity at given optical depths. This good correlation means that we can associate bisectors taken at different line-profile percentages with atmospheric layers that linearly increase as we scan lower spectral line intensities. We also determined that a fit to the line-core intensity is robust and reliable, providing information about atmospheric layers that are above those accessible through bisectors. Therefore, by combining both methods on the Si I 10827 A line, we can seamlessly trace the quiet-Sun LOS velocity stratification from the deep photosphere to higher layers until around $\log \tau = -3.5$ in a fast and straightforward way. This method is ideal for generating quick-look reference images for future missions like the Daniel K. Inoue Solar Telescope and the European Solar Telescope, for example.