Issue |
A&A
Volume 400, Number 2, March III 2003
|
|
---|---|---|
Page(s) | 737 - 752 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20021893 | |
Published online | 03 March 2003 |
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
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.
Key words: radiative transfer / line: formation / Sun: chromosphere / Sun: corona / Sun: UV radiation
© ESO, 2003
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