The solar BaII 4554 angstrom line as a Doppler diagnostic: NLTE analysis in 3D hydrodynamical model

Aims. The aim of this paper is to analyse the validity of the Dopplergram and λ-meter techniques for the Doppler diagnostics of solar photospheric velocities using the Ba ii 4554 Å line. Methods. Both techniques are evaluated by means of NLTE radiative transfer calculations of the Ba ii 4554 Å line in a threedimensional hydrodynamical model of solar convection.We consider the cases of spatially unsmeared profiles and the profiles smeared to the resolution of ground-based observations. Results. 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 λ-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 Ba ii 4554 Å 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 Ba ii 4554 Å line profiles. Such effects are more pronounced in intergranular regions. Conclusions. Our analysis supports the opinion that the Dopplergram technique applied to the Ba ii 4554 Å line is a valuable tool for the Doppler diagnostics of the middle photosphere around 300 km. The λ-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.status: publishe