Emission lines of H2 in the extreme-ultraviolet solar spectrum

We report the identification of emission lines of H2 in the solar EUV spectrum. The observations were made during the first rocket flight of a new high-resolution telescope combined with a new stigmatic EUV spectrograph. The present paper concerns mainly the sunspot spectrum which shows approx.200 lines not previously reported in solar spectra. About 65 of the new lines have been identified as due to transitions in the Lyman bands of H2. The identification of 29 of these has already been reported by the present authors.The strongest set of transitions is excited by fluorescence from H Lα. Photons emitted in the red wing of H Lα down toward the photosphere are absorbed by molecular hydrogen. The strong excitation takes place from the third vibrational state (ν''=2) of the ground state 1Σ+/sub g/ to the second vibrational states (ν'=1) of the excited state 1Σ+/sub u/. The observed lines then result from spontaneous emission over the progression ν'=1→ν''=2--9. Thus the strong lines represent only one line in the P and R branches of each band, giving rise to a characteristic fluorescence spectrum.Analysis of the line intensities now shows that other fluorescence routes are also effective, and give rise to moderately strong progressions in the 0--ν'' bands (0<ν''<7. Other weaker fluorescence also occurs.At present the limiting factor in the interpretation of the line intensities is the lack of knowledge of the line profile of H Lα where it reaches the H2 molecules. However, three contributing factors to the enhancement of H2 over the sunspot are identified. The absolute and relative intensities are discussed as far as is possible at present. The conditions under which polarization of the lines might be observed are discussed briefly. Finally, the implications of the H2 observations for other areas of astrophysics are mentioned