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Astron. Astrophys. 334, 1136-1144 (1998)

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5. Conclusion

From accurate calculations, we have obtained all the cross sections that characterize excitation of hydrogen atoms by a proton beam in the 1-100 keV range. All these cross sections agree well with the available experimental data.

Excitation with capture is as important as direct excitation at the lowest energies and the capture process characterizes 1-50 keV proton beams. The subsequent Doppler shifted emission yields chromospheric lines wings' enhancement (Fang et al. 1995) and can be used as a diagnostic method.

The directly excited atoms recoil velocity is of the same order of magnitude, or may be larger, than the thermal velocity, and may produce a macroscopic broadening of the emitted line. This nonthermal broadening process is quite general when excitation by a proton beam is dominant and, for example, may explain the H [FORMULA] line profile in YZ Canis Minoris flare star (Aboudarham & Abada-Simon 1997 - Personal communication).

Comparison between the observed and the calculated H [FORMULA] polarization fraction allows us to conclude that, at the chromospheric level, the proton beam may have a significant population of protons in the 1-50 keV energy range. A quantitative comparison between the calculated and the observed polarization needs to consider all the polarizing and depolarizing processes in the statistical populations equilibrium, and indeed such work is in progress. More observations are needed, namely for the frequency dependence of the polarization percentage, that will be available in the near future with the solar telescope THEMIS.

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© European Southern Observatory (ESO) 1998

Online publication: June 2, 1998