Volume 506, Number 3, November II 2009
|Page(s)||1393 - 1404|
|Published online||11 August 2009|
The solar Ba 4554 Å line as a Doppler diagnostic: NLTE analysis in 3D hydrodynamical model *
Main Astronomical Observatory, NAS, 03680 Kyiv, Zabolotnogo str. 27, Ukraine
2 Instituto de Astrofísica de Canarias, 38205, C/ vía Láctea, s/n, Tenerife, Spain e-mail: firstname.lastname@example.org
Accepted: 25 June 2009
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 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.
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.
Key words: Sun: photosphere / Sun: granulation / line: formation / techniques: spectroscopic / hydrodynamics / radiative transfer
© ESO, 2009
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