The spectrum of the dayglow of Venus between 1250 and 1430 A was measured in high resolution with the International Ultraviolet Explorer. Seven exposures which were made with the short wavelength camera in the high dispersion mode using the large aperture were combined to give a total exposure time of 309 min. The atomic oxygen lines at 1302.2, 1304.9, 1306.0, and 1355.6 A are present. In addition, the (14,3) and (14,4) bands of the carbon monoxide fourth positive system at 1317 and 1354 A respectively are identified. These bands are compared with synthetic spectra, showing the excitation mechanism to be fluorescent scattering of solar Lyman alpha radiation

Barth, C. A.

Conway, R. R.

Durrance, S. T.

Lane, A. L.

English

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HIGH-RESOLUTION OBSERVATION OF THE VENUS
DAYGLOW SPECTRUM 1250-1430
Samuel T. Durrance, Robert R. Conway, Charles A. Barth
Department of Astro-Geophysics and
Laboratory for Atmospheric and Space Physics
University of Colorado, Boulder, Colorado 80309
and
A. L. Lane
Jet Propulsion Laboratory _
California Institute of Technology
Pasadena, California 91103
ABSTRACT
The spectrum of the dayglow of Venus between 1250 and 1430 _ has been
measured in high-resolution with the International Ultraviolet Explorer.
Seven exposures which were made with the short wavelength camera in the
high-disperslon mode using the large aperture were combined to give a
total exposure time of 309 min. The atomic oxygen lines at 1302.2, 1304.9,
1306.0, and 1355.6 _ are present. In addition, the (14,3) and (14,4)_
bands of the carbon monoxide fourth positive system at 1317 and 1354
respectively are identified. These bands are compared with synthetic
spectra, showing the excitation mechanism to be fluorescent scattering of
solar Lyman alpha radiation.
INTRODUCTION
From April to August of 1979 the Pioneer Venus Orbiter Ultraviolet
pectrometer obtained spectra of the Venus dayglow at a resolution of 13
• Figure I shows a portion of a sum of 480 of these spectra which were
obtained at illumination, emission, and phase angles less than 60° . The
data are shown as the solid curve. The identification in these data of
carbon monoxide fourth positive bands (C04+) in the (14,v") progression
resolved some long-standing uncertainties in the interpretation of the
Venus dayglow (Durrance, et al., 1980). A synthetic spectrum of this
progression is shown as the dotted curve. A prominent unblendedfeature
in this spectrum was identified as the C04+ (14,5) band at 1392 _. The
(14,3) and (14,4) bands at 1317 and 1354 _ are blended with atomic oxygen
(OI) 1304 and 1356 _ lines, and were identified through inference.
We present here a high-resolution spectrum of the Venus dayglow in
which the atomic oxygen and carbon monoxide features are resolved. The
spectrum was obtained from observations made with the International Ultra-
violet Explorer satellite (IUE) at a resolution of about 0.4 _. It is
also shown that the theory of the excitation mechanism as scattering of
solar Lyman alpha is consistent with the shape of the observed C04+ bands.
27
https://ntrs.nasa.gov/search.jsp?R=19810017360 2020-03-21T12:29:22+00:00Z
OBSERVATIONS
Venus is a particularly difficult object to observe with IUE. It is
outside the restricted region around the sun only near its greatest
elongation when its visual magnitude is about -4. This extreme brightness
saturates the Fine Error Sensor (FES) tracking system so that it is not
possible to automatically track during an observation or to point precisely
at a location on the disk.
Tracking is accomplished by calculating the drift rate of the planet
with respect to the stars in spacecraft-centered coordinates and converting
these to spacecraft gyro rates. The precise location of the planet is
then determined by minimizing the figure of scattered light in the FES
image after the crescent of Venus is placed over the large entrance aper-
ture of the spectrometer. The image of Venus is only slightly larger than
the aperture so a substantial reduction in the reflected light for the FES
camera is seen. It is estimated that the center-of-light can be found
with this technique to an accuracy of about g2 arc sec.
On April 12, 1980, during the time of Eastern elongation, a series of
7 exposures were made with the short wavelength camera in the high-disper-
sion mode using the large aperture. Exposure times varied from 20 to 60
min. with a total exposure time of 309 min. To minimize the effects of
drift during each observation, the exposure was stopped periodically while
the scattered light figure in the FES image was recentered and then the
exposure was continued. Also for some of these observations the large
aperture was offset toward the bright limb in order to enhance the airglow
intensity relative to that of the disk.
A large background is present in these spectra as can be seen in
Figure 2 which shows a portion of the sum of the gross spectra for all 7
observations. Altogether 7 overlapping echelle orders are shown which
cover the region from 1280 to 1380 _. The data shown here have been
smoothed with a 9-point running average and are overplotted with the
estimated background which was determined as follows: Since the spectrum
is expected to have no continuum, or at most a very weak one, the spectra
are assumed to consist only of emission lines plus a large background.
Order 107 was assumed to consist of background only which was determined
as a 25-point smooth of the data, after the removal of any reseaux. This
background was then used to mask out the emission lines in order 106; that
is, the background for order 106 was then determined as a 25-point smooth
of the data excluding any points which were higher than the background of
order 107 by an amountthat was determined visually. This process was
then repeated for successive orders, each time using the background of the
adjacent order to form a mask.
THEORY AND DISCUSSION
Since carbon monoxide is present in the Venus upper atmosphere
(Niemann et al., 1979), fluorescent scattering of solar radiation is a
plausible excitation mechanism to explain its airglow emissions (Barth,
1969). In particular Kassal (1976) has shown that the scattering of solar
Lyman alpha by the C04+ (14,0) band is comparable to or greater than the
scattering by the rest of that system for CO column densities _i017cm-2.
28
A high-resolution synthetic spectrum of the C04+ (14,v") progression which
assumes fluorescent scattering of solar Lyman alpha as the excitation
mechanism has been produced. The details of this spectral synthesis are
given in an earlier paper (Durrance etal., 1980) and will not be included
here.
The data minus the fitted background are shown in Figure 3. The Ol
1302.2, 1304.9, 1306.0, and 1355.6 lines are resolved and easily visible
in this spectrum. A reseau at the position of the 1302.2 line has been
removed so the relative intensity of this line is not accurately indicated
here. A synthetic spectrum of the C04+ (14,S) and (14,4) bands is shown
_lotted below the data and is offset for clarity. They are both resolved
although the (14,4) band is not completely separated from the Ol 1355.6
llne. The agreement in position, shape, and relative intensity is quite
good.
Agreement between theory and data is shown with more detail for the
(14,4) band in Figure 4. The high-resolutlon synthetic spectrum has been
degraded to 0.4 _ resolution using a rectangular slit function to agree
with the data. The atomic llne, uslng the same procedure for degrading
the resolution, is also included for comparison. In this figure the data
have been filtered with a low pass filter and the synthetic spectrum is
normalized to the data. It can be seen here that the agreement between
theory and data is quite good although there does appear to be a small
discrepancy in the intensities of the satellite branches.
CONCLUSIONS
With IUE it has been possible to obtain high-resolution spectra of
the Venus dayglow in the 1250-1430 _ range which resolve the C04+ (14,3)
and (14,4) bands from the Ol 1304 and 1356 _ lines. Comparison of these
CO bands with synthetic spectra confirms the conclusion that they arise as
a result of the fluorescent scattering of solar Lyman alpha.
REFERENCES
Barth, C.A., "Planetary Ultraviolet Spectroscopy", Appl. Opt., 8, 1295-
1304, 1969.
Durrance, S.T., C.A. Barth, and A.I.F. Stewart, "Pioneer Venus Obser-
vations of the Venus Dayglow Spectrum 1250-1430 _", J. Geophys. Res.,
_, 222-224, 1980.
Kassal, T.T., "Scattering of Solar Lyman'Alpha by the (14,0) Band of CO",
J. Geophys. Res., 81, 1411-1412, 1976.
Neimann, H.B., R.E. Hartle, W.T. Kasprzuk, N.W. Spencer, D.M. Hunten and
G.R. Carlgan, "Venus Upper Atmosphere Neutral Composition: Prelimin-
ary Results from the Pioneer Venus Orbiter", Science, 203, 770-772,
1979.
29
IO
_" _" _-co_n-x'z°)
• ," 'T T T
0.8 - _ _ oz
T T
r_ o.6
_ 0.4
o.o .... "".,.!i"...
1200 1300 I,_X) 1500
WAVELENGTH (_,)
Figure I. Comparison of the Pioneer Venus observations with a synthetic
spectrum of the C04+ (14,v") progression. The solid curve is the observa-
tions, which were obtained at i11umlnatlon and emission angles less than
60° , and the dotted curve is the synthetic spectrum degraded to 13
resolution.
ORDER NUMBER
O
T T T T T T T
30000
20000
 ,oooo0
0 ,,, I , I ..... I '1 I
1280 1300 1320 1340 1380 1380
WAVELENGTH(_)
Figure2. IUE spectrum of the Venus dayglow 1280-1380 _. This is a sum
of 7 different observations for a total exposure time of 309 mln., using
the short wavelength camera in the high-dlspersion mode with the large
aperture. The fitted background is shown as the smooth curve.
30
10000 - _ _o ©
ooo _ _T
8000- _; T
>.6000 -
Z_W4000
2000
0
-2000 I I l,
1280 1300 1320 1340 1360 1380
WAVELENGTH (_)
Figure 3. Comparison of the CO4+ (14,v") progression with the observations.
The Jagged curve is the data minus background. The solid curve is the
synthetic spectrum degraded to 0.4 _ resolution and offset for clarity.
c
6000
>.
4000
2000
o
-2oooi I I I I
1350 1352 1354 1366 1368 1360
WAVELENGTH(_)
Figure 4. Comparison of the synthetic spectrum of the C04+ (14,4) band
and the Ol 1355.6 line with the observations. The solld curve is the data
and the dotted curve is the theory.
31


High-resolution observation of the Venus dayglow spectrum 1250-1430 angstroms

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