The asymmetries of chromospheric line profiles (Balmer series of hydrogen,
CaII lines, ...) as well as soft X-ray lines (Ca XIX, Fe XXV, ...) are
related to mass motions in the flare atmosphere.
The most extended observational data are available for the 
line
that is regarded as the most important tool for analysis. The observations
reveal -line asymmetries of various kind. While the red asymmetry
is present in most flares, the blue one was detected only in some of them
and only at the very onset of the flare (Švestka 1976;
Wülser 1987). The red asymmetry of appears
either as a shift of the peak to longer wavelength (Ichimoto &
Kurokawa 1984) or as a stronger red wing of the line (Švestka 1976). It is observed throughout the impulsive phase of the
flare, reaches its maximum before the maximum of the line intensity and is
temporally correlated with X-ray radiation (Ichimoto & Kurokawa
1984; Canfield et al. 1990). The blue asymmetry of is almost every time connected with central reversal
(Canfield et al. 1990; Heinzel et al. 1994).
The velocities of the chromospheric plasma responsible for observed
asymmetries are usually obtained using bisector technique.
So Ichimoto & Kurokawa (1984) found downward motion
velocities between 
and they report drop from 100 to
within 30 s. Wülser & Marti (1989) get
drop from 90 to 
within 12 s. Although the bisector
technique is widely used, because of its straightforwardness, from the
radiative transfer point of view it cannot be generally applied
and as was shown by Heinzel et al. (1994) it may lead to
misleading results.
The numerical simulations show that the heating of the chromosphere
during a flare produces upward as well as downward motions of the
chromospheric plasma (Somov et al. 1982; Fisher et al.
1985; Karlický & Hénoux 1992). These simulations predict an
explosive evaporation with upward velocities up to 
,
accompanied by a chromospheric condensation - a thin dense and cool
layer propagating downward with velocity up to 
.
Various observations based on bisector method seem to confirm this result
(Zarro et al. 1988; Zarro & Canfield 1989;
Wülser & Marti 1989; Canfield et al. 1990;
Wülser et al. 1993).
Falchi et al. (1992) observed a flare in the , 
,
, 
, CaII K and 
lines and their
results show that the chromospheric condensation probably has a velocity
gradient. From the observation of two flares in the , Ca II K,
He 
, Na 
and other metallic lines Shoji &
Kurokawa (1995) conclude that flare emission comes from two layers - one
thin, fast-downward-moving and very turbulent, and another stationary and
optically thick.
Fang et al. (1992) analysed red asymmetry of CaII K line for 12 solar flares
using semi-empirical NLTE calculations. They found the observed asymmetry can
be explained by downward motion above temperature minimum region with
velocity 
. Using profiles for two flares
observed by Wülser & Marti (1989) and Graeter (1990)
Gan et al. (1993) constructed semi-empirical NLTE models with
chromospheric condensation. They showed that the chromospheric condensation
is responsible not only for red asymmetry of but also for blue
asymmetry with central reversal.
A realistic determination of the velocity structure in the flaring atmosphere is extremely important for better understanding of the flare energetic processes. It is highly desirable to replace the simple bisector method by the NLTE dynamic model calculations and compare the synthetic profiles of a number of various lines with the observed ones.
In this paper we would like to present NLTE simulations of the flare
atmosphere with prescribed run of the temperature and the velocity field,
in order to show the influence of velocities on emergent hydrogen
-line intensities. The results are compared with the bisector
technique.
The NLTE method we used is described in Sect. 2 (click here), the models we performed the calculations for are described in Sect. 3 (click here) and in Sect. 4 (click here) we discuss the results.