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

3. Results and Discussions

3.1. Non-thermal Effect on Line Asymmetries

Ding & Fang (1996) have shown that a downward velocity field can be responsible for both the blue asymmetry and the red asymmetry of the H line, provided that the velocity field is confined to different heights in the upper chromosphere. The Ca II K line, however, mostly shows a red asymmetry. Here we study in more detail the influence of various electron beams on the asymmetry property of the H line.

We adopt five atmospheric models with a same temperature structure (F1 model of Machado et al. 1980), but different moving regions, which are located at heights of , 1318, 1279, 1241, and 1202 km, with nearly the same widths of about 23 km. (The height at the top chromosphere in the F1 model is km). The velocity value is assumed to be 30 km s^-1 and uniform within the moving region. For the electron beam, we assume that the electrons have a power law energy distribution and a low cut-off energy of keV. Five energy fluxes , 3 10¹⁰, 10¹¹, 3 10¹¹, and 10¹² ergs cm^-2 s^-1 with power indices , 4, and 5 are considered. Figure 1 displays the H line profiles calculated at the disk center for all the above circumstances. A Gaussian macro-turbulent velocity of 20 km s^-1 has been used to convolve these profiles.

Fig. 1. Asymmetric H line profiles computed from the F1 model with five moving regions confined to different atmospheric heights (from left to right in each row), and with the non-thermal effect of electron beams with five fluxes (from top to bottom in each column). The fluxes are in units of ergs cm-2 s-1. Different cases for power indices are indicated by line types (: solid lines; : dashed lines; : dotted lines). Notice the different intensity scales for different rows

Figure 1 reveals some interesting phenomena. First, as has already been pointed out by Fang et al. (1993), the non-thermal effect of electron beams causes the H line profile to be more intensified and broadened, compared to the case without including the non-thermal effect. Second, downward mass motions can make the profile either blue-asymmetric or red-asymmetric, depending on the vertical location of the velocity field (Ding & Fang 1996). In addition to these, we can notice another fact that, the line asymmetry also depends on the parameters of the electron beam: an intense (large ) or hard (small ) beam makes the profile more probable to have a blue asymmetry. In other words, many examples can be found that for the same distribution of temperature and velocity field (i.e., the same column in Fig. 1), the generated profile can vary from red-asymmetric to blue-asymmetric as increases or decreases.

3.2. A Brief Interpretation

The reason for the above results is the following: the height distribution of the line source function depends sensitively on the parameters of and of the electron beams (see Hénoux et al. 1993), and one can anticipate that the same moving region may have different absorption (emission) properties relative to the underlying specific intensity in different and cases. To show this point clearly, we plot in Fig. 2 the H line source functions () with a fixed but various values, and a fixed but various values. The velocity region is confined to the height of 1356 km, which can be distinguished by a slight dip in the curves. Also plotted in the figure are the heights of optical depth unity at , 2.0, and 3.0 Å.

Fig. 2. H line source functions computed from the F1 model with non-thermal effects included for a ergs cm-2 s-1 while , 4, and 5, and b while , 1011, and 1012 ergs cm-2 s-1. From left to right, the three short vertical bars on each curve represent heights of optical depth unity at , 2.0, and 3.0 Å, respectively

Figure 2a shows that the effect of decreasing is mainly to raise the source function in the middle chromosphere, while the line-formation height is not changed significantly, due to the large opacity in the red wing produced by the moving region. These two factors help to produce a more intense wing emission. Correspondingly, the moving region in the upper chromosphere will absorb more photons in the red wing and make the emergent profile less red-asymmetric or more blue-asymmetric. For the case of increasing , the source function is enhanced as well but in a slightly different way. However, the effect on the line asymmetries can be expected to be similar.

Although we have obtained many examples of H line profiles with blue asymmetry (see Fig. 1), this does not imply there is a large probability in detecting the blue asymmetry in observations. First, one should be aware that the production of blue-asymmetric profile needs some special conditions including the existence of an intense and hard electron beam, and a moving region confined to higher layers. Such a circumstance can only appear in the early impulsive phase and at the foot point where the electron beam bombardment occurs. Thus it is a very short-lived and spatially restricted phenomenon. Another fact comes from our simplification in the flare dynamics. A real flare is much more complex, involving differentially distributed and rapidly changing velocity fields, as well as other atmospheric parameters, which can smooth out the source function distribution to some extent and reduce the appearance of blue asymmetry. These reasons account for why blue asymmetries are less popular than red asymmetries in flare spectral observations.

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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