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Astron. Astrophys. 318, L17-L20 (1997)

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1. Introduction

The asymmetry of chromospheric lines in solar flares has been discovered long ago, but it remains to be a puzzling problem. Most frequently, a red asymmetry is observed during the flare impulsive phase. Usually, such a red asymmetry is interpreted as the result of chromospheric downflows (e.g., Ichimoto & Kurokawa 1984; Wülser & Marti 1989; de la Beaujardière et al. 1992), which are believed to be virtually a downward-propagating chromospheric condensation. Numerical simulations of gas dynamics in the flare loop, when imposed to an impulsive energy deposition, show that the chromospheric condensation is driven by the over-heating in the transition region which, in the mean time, gives rise to the upward chromospheric evaporation (Fisher et al. 1985; Gan et al. 1991).

On the other hand, many evidences showing the existence of a blue asymmetry for chromospheric lines of flares have appeared (Heinzel et al. 1994). The problem is more complicated when blue and red asymmetries are observed simultaneously at different positions in a flaring region (Canfield et al. 1990), or at a same position but different times (Ji et al. 1994). Canfield et al. (1990) have interpreted the blue asymmetry as being caused by an upward motion in the middle chromosphere due to a local pressure excess or some MHD processes. While such possibilities cannot be ruled out, recent line profile calculations show that a downward motion, when confined to the lower transition region or upper chromosphere, can also produce the blue asymmetry of line profiles, under some special circumstances (see e.g., Gan et al. 1993; Heinzel et al. 1994). The physical essence is that when the flare atmosphere is not strongly disturbed (i.e., the coronal mass density and the chromospheric temperature are not too high), the line source function in the upper chromosphere shows a sharp decrease towards upper layers and the effect of a downward motion there is to cause a red-wing absorption. The fact that the observed blue asymmetry more probably exists in those centrally reversed line profiles (vestka 1976) seems to favor such an explanation.

Recently, Fang et al. (1993) studied the flare H [FORMULA] and Ca II K lines with the inclusion of non-thermal excitation and ionization effect of hydrogen and calcium atoms caused by the bombardment of precipitating high-energy electrons, which are considered to be the main flare heating source. Their results show that the non-thermal effect plays an important role in determining the line intensity and width, especially for H [FORMULA]. In order to have a clearer knowledge in line profile diagnostics for solar flares, we need to study in more detail how such a non-thermal effect influences the line asymmetries. In this paper we perform systematic calculations for the H [FORMULA] line profile when both the non-thermal effect and the velocity field are considered, and then check how the line asymmetry varies in response to these factors.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998