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

¹ Istituto Nazionale di Astrofisica/Osservatorio Astronomico di Capodimonte, Salita Moiariello, 16, 80131 Napoli, Italy
² Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge, UK
³ NASA's Goddard Space Flight Center, Laboratory for Astronomy and Solar Physics, Code 682, Greenbelt, 20771 MD, USA

Corresponding author: V. Andretta, andretta@na.astro.it

Received: 12 April 2002
Accepted: 23 December 2002

Abstract

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 Å (the photoionisation threshold for ), as well as simultaneous measurements with the same instruments of the intensities of the strongest EUV and lines: , , and . We find that there are not enough EUV ionising photons to account for the observed helium line intensities. More specifically, we conclude that intensities cannot be explained by the P-R mechanism alone. Our results, however, leave open the possibility that the spectrum could be formed by the P-R mechanism, with the line as a significant photoionisating source.