Full non–LTE spectral line formation

I. Setting the stage

¹ Université de Toulouse, UPS-Observatoire Midi-Pyrénées, Cnrs, Cnes, Irap, Toulouse, France
e-mail: frederic.paletou@univ-tlse3.fr
² Cnrs, Institut de Recherche en Astrophysique et Planétologie, 14 av. E. Belin, 31400 Toulouse, France
³ Université de Toulouse, Faculté des Sciences et de l’Ingénierie, 31062 Toulouse cedex 9, France

Received: 27 November 2020
Accepted: 15 March 2021

Abstract

Radiative transfer out of local thermodynamic equilibrium (NLTE) has been increasingly adressed, mostly numerically, for about six decades now. However, the standard NLTE problem most often refers to the only deviation of the distribution of photons from their equilibrium, that is to say a Planckian distribution. Hereafter we revisit after Oxenius (1986, Kinetic theory of particles and Photons – Theoretical Foundations of non–LTE Plasma Spectroscopy, Springer) the so-called full NLTE problem, which considers coupling and therefore solving self–consistently for deviations from equilibrium distributions of photons as well as for massive particles constituting the atmospheric plasma.