Astron. Astrophys. 360, 702-706 (2000)

1. Introduction

It is well known that in the impulsive phase of solar flares, the Balmer lines are very broad, while the metal lines remain extremely narrow (Svestka 1976). However, in limb flare spectra the already rather broad profiles of the hydrogen lines still broaden (e.g. Graeter & Kucera 1992; Ding et al. 1999). Moreover, metal lines lose their sharp and narrow characters and, in some cases, their width becomes comparable to that of the high Balmer lines (Svestka 1976). In this case, if one compares the widths of lines from two elements of very different atomic weights, unacceptable high values of the temperature and of the micro-turbulence velocity are obtained (e.g. Jefferies & Orral 1961; Svestka 1965). Recently an unusually broad profile of the HeI 10830 line was observed by You et al. (1998) in a large limb flare. This behaviour could be interpreted neither by a Doppler broadening mechanism, nor by a pure Stark effect, that would imply an unacceptable high electron density (You & Oertel 1992). Here we give a new example (Ding et al. 1999). Fig. 1 shows an H image of a limb flare that occurred on 11 November, 1998. The flare is a small one (SF/C3.2) that was observed with an imaging spectrograph at the solar tower of Nanjing University (Huang et al. 1995). Fig. 2 gives the line profiles of the H and CaII 8542 lines, which were observed simultaneously before the H maximum. It can be seen that, even for this small flare, the line profiles are extremely broad. The widths at half-peak intensity of the H and CaII 8542 lines are about 6 Å  and 1.6 Å, respectively. By using these values and assuming a Doppler broadening, a temperature as high as 1.6 10⁶ K and a micro-turbulence velocity of 21 km s^-1 would be obtained. Obviously, this is not reasonable. Thus, how the extreme broadening of the line profiles observed in limb flare spectra can be explained is an interesting problem worth studying in detail.

Fig. 1. Monochromatic image at H line center of a limb flare at 02:14:38 UT. The field of view is . North is up, east is to the left

Fig. 2. Line profiles of the H (solid lines) and Ca II 8542 (dashed lines) lines at 02:14:18 UT. Each panel has a one to one correspondence to each small square shown in Fig. 1

By considering the effect of non-thermal collisional excitation and ionization of hydrogen and CaII, by an electron beam, we have shown that the hydrogen lines in the spectra of disk flares are greatly strengthened and broadened, while the resulting broadening is weaker for the CaII K line (Fang et al. 1993; Hénoux et al. 1995, hereafter referred to as Paper I and Paper II, respectively).

It is worth computing for limb flares the hydrogen and CaII line spectra, including the effect of non-thermal collisional excitation and ionization, in order to learn how these processes contribute to the line broadening observed in limb flares spectra. We follow the theory and the method given in Paper I and Paper II, and show non-thermal line profiles, computed for a limb flare atmosphere bombarded by an electron beam. The methodology is given in Sect. 2. The main results are shown in Sect. 3 and followed by a discussion and conclusion in Sect. 4.

© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000
helpdesk.link@springer.de