Abstract The distribution or wavelength-dependence of the formation regions of frequently used solar lines, Hα, Hβ, Ca II H and Ca II 8542, in quiet Sun, faint and bright flares is explored in the unpolarized case. We stress four aspects characterising the property of line formation process: 1) width of line formation core; 2) line formation region; 3) influence of the temperature minimum region; and 4) wavelength ranges within which one can obtain pure chromospheric and photospheric filtergrams. It is shown that the above four aspects depend strongly on the atmospheric physical condition and the lines used. The formation regions of all the wavelength points within a line may be continuously distributed over one depth domain or discretely distributed because of no contribution coming from the temperature minimum region, an important domain in the solar atmosphere that determines the distribution pattern of escape photons. On the other hand, the formation region of one wavelength point may cover only one height range or spread over two domains which are separated again by the temperature minimum region. Different lines may form in different regions in the quiet Sun. However, these line formation regions become closer in solar flaring regions. Finally, though the stratification of line-of-sight velocity can alter the position of the line formation core within the line band and result in the asymmetry of the line formation core about the shifted line center, it can only lead to negligible changes in the line formation region or the line formation core width. All these results can be instructive to solar filtering observations.

Keywords line: formation --- radiative transfer --- Sun: chromosphere

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