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Proposal ID Andretta_58

Date of Proposal




Mail Code 682.0
Greenbelt, MD 20771


301 286 4584

01 286 1617

In several instances, to off-limb prominences observed at 304 A seem to
correspond relatively dark features in the the shorter wavelength bands.
In some cases (e.g. 18/19 Oct. 1996, filament in the SE sector) filaments
also show a similar effect. Those dark features can be interpreted as due
to absorption by the prominence plasma. The two main absorbers at those
wavelengths are neutral hydrogen and helium (both neutral and ionized). The
ratios of He/H absorption cross-sections at EIT wavelengths other than 304
are the following:

lambda (A): 171 195 284
sigma(He0)/sigma(H): 18.2 19.0 11.4
sigma(He+)/sigma(H): 14.0 13.5 0

The relative ratio H/He is of the order of 10; moreover both He0 and He+
can be effectively depleted by photoionizing coronal radiation even in
cool plasmas. Therefore neutral hydrogen may be a significant contributor
to the observed absorption features.

Observations with CDS/NIS of several prominences show a similar effect.
However, CDS has the capability of observing above the photoionization
threshold of He0 (504 A), where hydrogen is the only absorber. In fact,
absorption by prominences can be observed in lines such as Mg X 625. In
principle, combining CDS (which covers the spectral region where H and He0
absorption can be important) and EIT observations (which extend that range
to cover the He+ range, below 228 A) could allow a determination of
the relative column densities of the three species H, He0 and He+ in

In collaboration with T. A. Kucera, I propose to focus as a first instance
to a prominence observed by T. Kucera with CDS/NIS, on July 19, 1996, on the
NE sector. In that data-set, prominence absorption is clearly observable both
in the Mg X 625 line and in the Mg IX 368 line, and it is detectable in other
lines observed with that study as well. The EIT images available for that data
also show some absorption corresponding to that prominence, in addition, of
course, to the He II 304 emission.

Joseph B. Gurman




Densities of H, He0 and He+ in Prominences

update added on 1998/03/04 at 18:42:21

The analysis started with an off-limb quiescent prominence observed on
July 18, 1996. Unfortunately, the two EIT images (171 and 195 A) and the
CDS spectra were not simultaneous, some of the images being separated by
as much as two hours. The analysis of the individual images, however, was
useful in that it permitted to test some of the methods to obtain precise
quantitative measurements of the absorption of coronal radiation. In
particular, the estimate of the 'background' coronal intensity has proven
itself particularly difficult in some cases, especially in extended absorption

A subsequent analysis of other CDS data (particularly the prominences
observed on June 18 and August 7 1997), has lead to a better understanding of
the nature of the absorption features seen in 'hot' lines (T10^6 K),
showing that at least in some cases the observed absorption can be
convincingly interpreted as due to hydrogen and neutral helium (the CDS/NIS
wavelength coverage does not permit to assess the role of ionized helium).
Some of the results have been presented at the SPD meeting of June 27- July 1
1997 (Bozeman, Montana Kucera, Andretta Poland, BAAS, 29, abstr. 01.12,
1997) a paper on this subject is also in preparation.

The study of the CDS/NIS data highlighted the complementarity of that
detector with EIT: together, the two instruments can cover the range of
wavelengths needed to estimate the column densities of H, He0 and He+
(171 - 630 A). We therefore obtained simultaneous observations of CDS/NIS
and EIT on January 12-16 and 21, 1998. CDS/GIS spectra were also taken, in
order to extend further the long wavelength range (thus covering much of the
Lyman continuum). Kitt Peak (He I 10830) and Sacramento Peak (highest terms
of the Paschen series) provided ground support. On February 20, 1998, we ran
the same CDS+EIT program, this time with SUMER as supporting instrument
(Lyman continuum edge). Analysis of the data is in progress.

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