Solar BaII 4554 A line as Doppler diagnostics: NLTE analysis in 3D hydrodynamical model

TL;DR

This study evaluates the effectiveness of Dopplergram and lambda-meter techniques using NLTE radiative transfer in 3D solar models to diagnose photospheric velocities with the BaII 4554 A line, considering observational resolution effects.

Contribution

It provides a comprehensive NLTE analysis of the BaII 4554 A line in 3D models, validating Doppler diagnostic techniques and highlighting the importance of NLTE effects.

Findings

Dopplergram velocities match true velocities around 300 km height, except in strong downflows.

Lambda-meter velocities reliably trace true velocities throughout the photosphere.

NLTE effects are significant, especially in intergranular regions, affecting line profile modeling.

Abstract

The aim of this paper is to analyse the validity of the Dopplergram and lambda-meter techniques for the Doppler diagnostics of solar photospheric velocities using the BaII 4554 A line. Both techniques are evaluated by means of NLTE radiative transfer calculations of the BaII 4554 A line in a three-dimensional hydrodynamical model of solar convection. We consider the cases of spatially unsmeared profiles and the profiles smeared to the resolution of ground-based observations. We find that: (i) Speckle-reconstructed Dopplergram velocities reproduce the ``true'' velocities well at heights around 300 km, except for intergranular lanes with strong downflows where the velocity can be overestimated. (ii) The lambda-meter velocities give a good representation of the ``true'' velocities through the whole photosphere, both under the original and reduced spatial resolutions. The velocities derived from the inner wing of smeared BaII 4554 A line profiles are more reliable than those for the outer wing. Only under high spatial resolution does the inner wing velocities calculated in intergranular regions give an underestimate (or even a sign reversal) compared with the model velocities. (iii) NLTE effects should be taken into account in modelling the BaII 4554 A line profiles. Such effects are more pronounced in intergranular regions. Our analysis supports the opinion that the Dopplergram technique applied to the BaII 4554 A line is a valuable tool for the Doppler diagnostics of the middle photosphere around 300 km. The \lambda-meter technique applied to this line gives us a good opportunity to ``trace'' the non-thermal motions along the whole photosphere up to the temperature minimum and lower chromosphere.