The EUV helium spectrum in the quiet Sun: A by-product of coronal emission?

In this paper we test one of the mechanisms proposed to explain the intensities and other observed properties of the solar helium spectrum, and in particular of its Extreme-Ultraviolet (EUV) resonance lines. The so-called Photoionisation-Recombination (P-R) mechanism involves photoionisation of helium atoms and ions by EUV coronal radiation, followed by recombination cascades. We present calibrated measurements of EUV flux obtained with the two CDS spectrometers on board SOHO, in quiescent solar regions. We were able to obtain an essentially complete estimate of the total photoionising flux in the wavelength range below 504 Angstrom (the photoionisation threshold for He I), as well as simultaneous measurements with the same instruments of the intensities of the strongest EUV He I and He II lines: He I lambda584, He I lambda537, and He II lambda304. We find that there are not enough EUV ionising photons to account for the observed helium line intensities. More specifically, we conclude that He II intensities cannot be explained by the P-R mechanism alone. Our results, however, leave open the possibility that the He I spectrum could be formed by the P-R mechanism, with the He II lambda304 line as a significant photoionisating source.