The first light spectrum - recorded with the high spectral resolution and the
excellent dynamic range available to SUMER - reveals many spectral details and
accommodates many more spectral lines than given in any earlier line list.
Although some of the lines suggested earlier could not be identified.
This near-limb disk spectrum of the quiet Sun in the 
spectral range is dominated
by lines emitted from temperatures typical of the chromosphere and the
transition region. Only the most intense coronal lines are visible in a
quiet Sun spectrum recorded on the disk.
The strength of coronal lines when compared to the transition region lines
should increase in the vicinity of the solar limb. The actual widths of most
of the lines in the list is larger than the instrumental width. In principle,
SUMER can separate lines such as C II at 903.96 and 904.14 Å (Wilhelm et
al. 1997a), which are offset by only
0.18 Å. On the limb, lines are indeed wider and as a result,
two lines that are separated from each other by less than 0.3 Å appear as a
blend. In places where blends are expected we have provided the wavelengths
and the transitions of some of the most prominent lines expected in the blend.
Below we give an account of some interesting line systems presented in the
list.
Hydrogen
The hydrogen Lyman series lines are very prominent in the spectral range covered
by SUMER. The transition of the highest member of the series that can be
distinguished in the spectrum used for this identification is 
.
The higher members of the Lyman series are blended
with the hydrogen free bound continuum. Line shapes of most, if not all, the
hydrogen lines are dominated primarily by opacity effects and to a lesser
degree by the Doppler non-thermal mass motions. In spectra obtained from
regions where opacity effects are reduced several additional members of the
Lyman series can be distinguished, (Wilhelm et al. 1997a).
Helium
The 584.33 Å 
line in second
order is the only He I line visible in this quiet Sun spectrum. He II
multiplets of the type 2k-nk' (Balmer series) where n > 4, are present
in the observed range. Individual lines within each multiplet are not
resolved in the SUMER spectrum. The He II multiplets arising from
transitions with even "n" quantum numbers have wavelengths that are about
0.4 Å shorter than hydrogen Lyman series lines. Therefore, because of the
very large widths of the hydrogen lines in this limb spectrum, the
He II transitions are visible but blended with the blue wings of the
hydrogen lines. The He II Balmer lines belonging to transitions having
odd upper quantum members (i.e., n = 5, 7 and 9) are well observed in our
spectrum.
The excitation energy of the He II Balmer lines is 
while their
formation temperature, assuming collisional excitation, is only equivalent to
about 10 eV.
As a result, the Boltzmann factors that determine the rate of collisional
excitation processes in these lines are very temperature sensitive.
He II line intensities when compared to the intensities of other lines emitted
by plasmas with similar temperatures, are good indicators for the excitation
processes taking place in the quiet Sun atmosphere (Feldman
1995).
Carbon
The series limit of the C I 
is at
1101 Å. Therefore, wavelengths slightly longer than the series limit
are abundant with lines belonging to high members of the CI Rydberg series.
The line list contains many C I lines belonging to transitions between the
upper configurations (where n = 6, 7, ... 13) and the
ground term 
. The 
multiplet of C I at 945 Å is also present in this limb
spectrum.
The ground term of C II is 
and the first excited
configuration 
include, in order of increasing energies, the
, 
, 
, and terms. The lowest
term can be regarded as metastable in electron density conditions typical to
quiet Sun plasmas. Transitions between the excited doublets and the ground
term appear in the SUMER spectrum. The lowest energy transitions are near
1330 Å and the two higher energy transitions are near 1037 and
904 Å. The three sets of C II transitions are among the brightest
emitted by the chromosphere. Three lines belonging to transitions between
the 
and the metastable term that
appear near 1010 Å are also intense. Several more C II lines belonging
to transitions between the ground term and doublet terms from highly excited
configurations are present in the wavelength range. Intensity ratios between
lines belonging to quartet and doublet systems can provide information
regarding the temperature properties of the chromosphere plasmas.
The 977 Å C III 
transition is
extremely bright in the quiet Sun spectrum. It is the strongest line emitted
in the wavelength range discussed in this paper.
Nitrogen
The N I transitions between ground configuration levels (
)
and levels from the excited configurations
fall in the 
wavelength range. There are no N I lines
resulting from transitions between excited configurations visible in this
limb spectrum used to create the line list. The situation with N II is
similar to the situation in N I. Some of the strongest transitions between
the ground configuration and the excited configuration are
present in this spectrum. The N II transitions appear near 671, 746,
915, and 1085 Å. There are no N II transitions arising from levels between
excited configurations among the lines observed in the quiet Sun spectrum.
Resonance lines between the ground and excited configuration and lines between
the first and second excited configurations in N III are observed
near 690, 772, 980 and 990 Å
while the N IV transitions appear near 765, 923, and 955 Å. Intensity
ratios between the two types of lines are temperature sensitive. Intensity
ratios among some of the lines near 923 arising from
transitions between the first and second excited configurations are also
sensitive to electron density conditions present in the quiet Sun.
Oxygen
A well known resonance exists between the 
transition in H I
(1026 Å) and the 
in O I.
As a result the excited term 3d in part is photo-pumped by H I
photons causing transitions between it and lower energy levels
to become unusually bright. Because of the very large widths of all the
Lyman series lines coincidences between many of them and between O I lines
from higher "n" values occur. As a result, H I lines photo-pump many O I
levels which in turn cause many of the O I lines to become very prominent.
Several allowed transitions between the ground configuration and the first
excited configuration in O II and O III are prominent in this SUMER spectrum.
Two O IV lines belonging to transitions between
appear at 787 and 790 Å. Additional O IV
lines belonging to 
appear at
779 Å. The relative intensities between the two sets of lines can be
used as electron temperature indicators for the O IV formation region.
The 2s2p 
transitions of O V appear near 760 Å.
The intensities of lines originating from the 
level
are sensitive to quiet Sun type densities when compared to the intensity
from lines originating from the 
or from the
levels. The O VI resonance lines at 1031.924 and
1037.614 Å are among the brightest emitted by upper transition region
plasmas.
Neon
Lines emitted by neutral neon, Ne I, and by several neon ions are present
in this limb spectrum. The two Ne I resonance lines at 735.896 and
743.720 Å,
which belong to transitions between the first excited configuration
and the ground configuration 
are present
in the spectrum though they are quite faint. No Ne II, Ne III or Ne IV lines
are visible in the 
range SUMER disk spectrum. The
Ne V transition at
1136.51 Å and the 
transition at 1145.61 Å which is expected to be about three times
brighter can be identified.
The Ne VI lines belonging to the
transitions near 1000 Å appear with medium brightness. Several of the
Ne VI transitions are blended with nearby lines. Lines belonging to the same
transitions in N III and O IV are used as electron density indicators in
quiet and active region plasmas. The Ne VI lines are sensitive to fairly
high electron densities (larger than 
) they
will be good electron density indicators in unusually dense active region
plasmas or in flares. The two 
Ne VIII resonance lines in the
SUMER spectral range represent the upper transition region type plasmas.
Magnesium
Lines from magnesium ions, at best, are poorly represented in this SUMER
limb spectrum. Only the 
946.70 Å Mg II
line may be present in the spectrum but cannot be identified without
ambiguity.
Silicon
Many lines from Si III and Si IV are found in this spectrum. The Si III lines
near 995 and 1108 Å belong to transitions
between the first excited configuration 3s3p and the excited configurations
3s3d and 3s4s. Assuming that the opacity of Si III 1206.510 Å
(
3s3p ) resonance line, also in the
SUMER range, can be determined, the relative intensities between these lines
and the resonance line, could be used as density indicators. A number of
Si IV lines near 1122 and 816 Å belonging to transitions between the
first excited configuration 3p and the excited configurations 3d and 4s, and
the 1066 Å line between the second excited configuration 3d and the 4f
configuration are also present in the range. As in the
Si III case the relative intensities between these transitions and the Si IV
resonance lines 
near 1400 Å which is also in the SUMER range
can be used for electron temperature determination.
Sulfur
S I to S VI lines are numerous in the quiet Sun spectrum. The S I lines which
appear in the 
range are among the narrowest lines
observed in the SUMER wavelength range. Lines of S II, S III and S IV
between the ground configuration and several excited configurations are
quite prominent. A number of S II lines appear in the 
range. S III lines appear near 680, 700, 725, 1015, 1021 and 1077 Å and
S IV lines appear near 750, 810 and 1070 Å. No transitions between
excited configurations of S II, S III or S IV could be identified. The
S V and S VI resonance lines and lines between the first and second excited
configurations are also visible. Line ratios among some of these lines can be
used for temperature diagnostic purposes. The S V lines appear near 696 and
780 Å and the S VI lines appear near 712, and very dominantly at 933
and 944 Å.
Argon
The abundance of argon in the solar atmosphere is more than 30 times lower
than the abundance of neon. As a result lines emitted by argon ions are
expected to be about an order of magnitude less intense when compared to lines
of similar transitions emitted by neon ions. The Ar II 
transitions at 919.781 and 932.053 Å are
expected to be present in this limb spectrum, however the 919.781 is blended
with the red wing of the 919.351 Å H I line and with the 919.658 Å
O I line. The 932.653 Å line similarly is blended with H I Ly 6.
The 
transitions near 720 Å
range are the only Ar V lines visible in this spectrum. Because of the low
argon abundance the lines are fairly faint. Two Ar VI lines at 767.06 and
1000.16 Å, belonging to transitions between 
levels may be associated with lines in our spectrum. The Ar VIII
lines at 700.245 and 713.812 Å, although fairly faint, probably are also
present. The 700.245 Å line is blended with a nearby S III line. The
Ar VIII lines are emitted from plasmas with temperatures of approximately of
placing them among the hottest transition region lines
in the list.
No transitions between the first and second excited configuration in argon ions
are found in our spectrum. Thus, argon lines do not appear to be
useful for temperature determinations or density diagnostics. However since
argon is a high FIP (first ionisation potential) element, lines from argon ions can be used in conjunction
with ions from low FIP elements to determine variations of elemental abundances
in the upper solar atmosphere.
Iron
A large number of Fe III lines have been identified at
wavelengths longer than 860 Å. The Fe III lines belong to transitions
between levels of the first excited configuration 
and levels belonging to the 
ground configuration. It
is expected that many additional Fe III lines at wavelengths short of
860 Å should be present in the spectrum, but are not detected, because
of the brightness of the Lyman continuum. No Fe II lines, which are quite
plentiful in the longer wavelengths of the disk spectrum are undoubtedly
identified in the wavelength range shorter than 1150 Å.