6 Conclusion

In this work we have presented new non-LTE computations of the helium emerging spectrum in a one-dimensional isothermal isobaric prominence. We use a 29+4+1levels model for the helium atom and PRD for the hydrogen Ly$\alpha$, Ly$\beta$, and the first resonance lines of He I and He II. Detailed incident profiles were used for the principal transitions. This approach leads to some differences with the previous calculations carried out by Heasley et al. (HMP; HM3) who used CRD and frequency-independent incident line profiles for the helium transitions, and only 15 bound levels for the neutral helium atom. The theoretical study of Sects. 4 and 5 has shown the different effects of the physical parameters of our computations on the mean populations of several helium states and on the emitted intensities. We particularly noticed the different behaviour of the singlet and of the triplet helium lines which can be useful for the prominence plasma diagnostic. The presence of the resonance lines in the singlet system is of primary importance for the formation of the neutral helium emerging spectrum. We have also shown that, in the range of temperatures and pressures we considered, the He II $\lambda$304 Å line is formed only by the scattering of the incident radiation. The number of levels in the He II ion that we have taken into account allows the study of the formation of this resonance line. In a next paper we will compare our isothermal, isobaric computations for the EUV helium lines with observations obtained with the SUMER instrument onboard of SoHO.

Acknowledgements

We wish to thank J.-C. Vial for reading the manuscript, and T. Holzer for his encouraging comments on this work. Computations are performed on a IBM RS/6000 scalar computer at the Multi-Experiment Data Operation Centre for SoHO (IAS) and at the Institut du Développement et des Ressources en Informatique Scientifique.