Issue |
A&A
Volume 385, Number 1, April I 2002
|
|
---|---|---|
Page(s) | 273 - 280 | |
Section | Planets and planetary systems | |
DOI | https://doi.org/10.1051/0004-6361:20020142 | |
Published online | 15 April 2002 |
Calcium to hydrogen line ratios in solar prominences
1
Institut d'Astrophysique Spatiale, Univ. Paris XI/CNRS, Bât. 121, 91405 Orsay Cedex, France
2
Astronomical Institute, Academy of Sciences of the Czech Republic, 25165, Ondřejov, Czech Republic e-mail: pheinzel@asu.cas.cz
Corresponding author: P. Gouttebroze, goutte@ias.u-psud.fr
Received:
8
January
2002
Accepted:
18
January
2002
The ratio of Ca II 8542 Å to Hβ line intensities
has been used for a long time to diagnose the gas
pressure in solar prominences. In this paper we
reconsider the theoretical dependence of H
on
the gas pressure, as originally
computed by Heasley & Milkey (1978), and extend this theoretical
correlation to higher pressures. Firstly, we revise the formation of
calcium lines in prominences, using in parallel two independently
developed NLTE radiative transfer codes. Computations consist of two
subsequent steps: (i) the formation of hydrogen spectrum (treated
in a similar way as in Gouttebroze et al. 1993),
and (ii) the formation of
calcium lines, using the electron-density structure obtained in
step (i). The influence of hydrogen Lyman lines on Ca II to Ca III
ionization is found to be very important for the determination
of calcium-to-hydrogen line ratios. In particular, the intensities
obtained for calcium lines at low pressures are significantly
lower than those obtained by Heasley & Milkey (1978), which is
the result of a greater Ca III/Ca II ratio. Our numerical results
have been further checked against an approximate analytical model.
Secondly, we have performed an extended computation using a large grid
of models covering different temperatures, gas pressures, geometrical
thicknesses, microturbulent velocities and prominence altitudes.
For temperatures lower than 10 000 K and pressures lower than 0.1
dyn cm-2, the line ratio
H
undergoes only
small variations, remaining between 0.2 and 0.3. At higher pressures
(0.1 to 1 dyn cm-2), the behaviour of this ratio appears to be
strongly dependent on temperature: rapidly increasing below 6000 K,
moderately increasing between 6000 and 8000 K, and generally
decreasing at higher temperatures. A comparison of the present models
with recent observations of Stellmacher & Wiehr (2000) suggests
the existence of cool prominence structures with temperatures
around 6000 K and gas pressures higher than 0.1 dyn cm-2.
Key words: sun prominences / radiative transfer / line: formation
© ESO, 2002
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