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

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4. Conclusions

We have performed line profile calculations for a flare atmosphere including the effect of non-thermal excitation and ionization of the hydrogen atoms caused by precipitating high-energy electrons, and the effect of possible chromospheric downflows. The results confirm the earlier finding that a downflow, if confined to the upper chromosphere, can sometimes produce a blue asymmetry of the H [FORMULA] line. In addition, we find that the existence of such non-thermal effects tends to enhance the blue-asymmetry magnitude. In particular, the larger the energy flux, or the harder the energy spectrum for the beam electrons, the easier it will be to produce a blue-asymmetric H [FORMULA] profile, which is simultaneously intensified and broadened. As we have made some simplifications in our model, further computations which can treat the radiative transfer and gas dynamics in detail are needed to check these results.

It should also be noted that the computations are model-dependent. However, we find that the general conclusions above are valid for a wide variety of model atmospheres. Besides the temperature structure and the velocity field, other important parameters include the column mass density of the corona ([FORMULA]) and that at the source of high-energy electrons ([FORMULA]). For the latter, a more appropriate parameter is [FORMULA], i.e., the column mass that non-thermal electrons have to penetrate before reaching the chromosphere. In this work, we have adopted [FORMULA] 10-4 g cm-2 given by the F1 model and [FORMULA], assuming that the electron source is located far above the transition region.

If we set a lower located source site of high-energy electrons (smaller [FORMULA]), the non-thermal excitation and ionization effect will become more obvious, especially in the upper chromosphere. Therefore, this can lead to a relative increase of the line source function there. In most cases, it makes the computed line profile slightly less possible to show a blue asymmetry, if other parameters remain unchanged. On the other hand, a higher coronal mass density (larger [FORMULA]) tends to enhance the thermal collisional transition rates in the upper chromosphere. Its effect on the line asymmetries will depend on the relative importances of thermal and non-thermal collisional rates.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998