## 2. The H absorption profileThe starting point of our theory is to model the photospheric incident radiation around the H line by a continuum from which we substract a gaussian profile to fit the atlas data by Delbouille et al. (1973 ). Such a profile is written as (see Fig. 1) where the constants are fixed by the data available in the atlas quoted (see Fig. 1).
The result of the fit brings to and to corresponding to a line width at half maximum of 1.41 Å . ## 2.1. The density matrices of the problemTaking into account the particular form of the profile, we use Eq. (1) of Sahal-Bréchot et al. (1992 ) to obtain the density matrix of the incident photons. We recall that an atom having an individual velocity absorbs, in its atomic frame, at the frequency such that where is the unit vector of the ray of light
whose direction is specified by the angles ()
(see Fig. 2.1), and is the atomic
frequency of the unperturbed transition corresponding to an upper
level
where is the limb darkening coefficient, and
where the index Let The scalar product , depending on (), leads to the appearance of coherences , , in As we shall see later on, the Stokes parameters
of the H line are obtained
once the density matrix of the reemitted
photons is specified in the observer's frame In order to obtain the frequency dependence of the total scattered radiation we take into account the contributions of all atoms contained in a unit of volume of radiating matter. Due to the Doppler effect, each frequency of the scattered radiation corresponds to an atom having a velocity such that We need to perform the average of the density matrix over the atomic distribution of velocities projected onto a plane perpendicular to the line of sight, i.e., over and , and to multiply by the density of the H atoms (Sahal-Bréchot et al. 1998 ). The distribution of the velocities is (Sahal-Bréchot et al. 1992 , Sahal-Bréchot et al. 1998 ) where or is the
velocity vector of the stream of radiating H atoms, and where
and We write the velocity distribution in units of frequency using the following transformations also and thus A temperature K (corresponding to the value
s We define, as usual, the polarization degree and the angle of rotation of the polarization direction by the formulae where the positive ## 2.2. The equations of statistical equilibriumWe use the theory of atom-radiation interaction in the density matrix formalism given by Bommier (1977), and in particular Eq. (III-39) of that paper (see also Eq. (36) of Bommier & Sahal-Bréchot 1978 ). Neglecting coherences between level and (with ) and neglecting the possible presence of a magnetic field, the equations assume the form given in 7, where is the Einstein coefficient for spontaneous de-excitation between the levels and where is given by Eq. (3). We set looking for solutions of a stationary system. In these equations we take into account that the radiation field in the Ly and Ly is isotropic, because these lines are opticaly thick in the solar chromosphere, so that Our further assumption is to neglect any velocity effect on the profile of the Ly and Ly lines for the radiating atom. If, for instance, we consider a velocity of 60 km/s for the atom, the shift in wavelength is of the order of 0.2 Å for both lines. At this distance from line center the profiles are nearly constant (Gouttebroze et al. 1978 ). We also neglect the central dip of the profiles that would affect the radiator for smaller velocities. © European Southern Observatory (ESO) 1998 Online publication: April 28, 1998 |