3. Numerical results
Fig. 3 shows the H line profiles for models F1 and F2, with (lower panel) and without (upper panel) non-thermal hydrogen collisional excitation and ionization included. Fig. 4 shows the Ly and Ly line profiles for the model F1. The profiles obtained by adding or not the collisional excitation and ionization rates to the thermal ones are shown respectively in the lower and upper panels. It can be seen that non-thermal processes greatly influence the intensity and the broadening of the hydrogen line profiles of limb flares. This effect is especially obvious in H line, while the Ly and Ly line profiles are relatively less affected, though in the non-thermal case the intensities of both lines are increased by more than one order of magnitude and they are broader than in the thermal case.
Fig. 5 shows the CaII K and IR 8542 Å line profiles for the model F2, with (lower panel) and without (upper panel) non-thermal effects included. It can be seen that the CaII lines of a flare at the limb are broader than at the center of the solar disk, though the effect of non-thermal broadening for the CaII lines is less obvious than for the H line. This is not surprising, because non-thermal effects for the CaII lines are weaker than for the hydrogen lines, as was indicated in our Paper I.
To clearly understand the non-thermal broadening effect, the column mass distributions of the source function and of the optical depth at the center and at +1.5Å of the H line for models F1 and F2, respectively, are plotted in Fig. 6. The figure shows that the source function in the non-thermal case is much larger than that in the thermal case, as already indicated in Paper II. However, its value changes with height: generally it decreases with height, except for the F1 model in the non-thermal case, where the source function attains its maximum around M=0.01. At the line center, except for the upper layers of flaring atmosphere, the optical depth ; while at the line wings, through all the layers. When a flare is observed on the solar disk, the line wing intensity is some weighted average of the source function through all the atmosphere. For limb flares, if the height of observation just corresponds to the place where the source function attains its maximum, then the intensities at line wings are greatly increased; so that the width of the line profile is much enlarged. In the F1 model, this happens around M=0.01, where M is the column mass, corresponding to a height of around 750-850 km; while in the F2 model, it is in the very upper layers of the flaring atmosphere. This is illustrated in Fig. 3 and Fig. 4. As concerns the CaII lines, as shown in Fig. 7, the situation is similar to the one for hydrogen lines. However, there is no obvious difference between the thermal and the non-thermal source functions, except at the very upper layers of the F1 model; so the line widths of limb flares are similar for both cases, though in the non-thermal case, the lines are somewhat broader than those in the thermal case.
© European Southern Observatory (ESO) 2000
Online publication: August 17, 2000