The solar Ba${\sf II}$ 4554 Å line as a Doppler diagnostic: NLTE analysis in 3D hydrodynamical model

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.