The imaging instrumentation for the Dynamics Explorer Mission was designed primarily to obtain global auroral images. The instrument, however, was also used successfully to study marine bioluminescence, the geocorona, and the global distribution of atmospheric ozone. The imager has considerable potential for the study of astronomical sources of ultraviolet radiation as well. The data produced by the imager is used to study the brightness and isotrophy of the diffuse ultraviolet background

Fix, J. D.

English

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INVESTIGATION OF THE DIFFUSE ULTRAVIOLET.
BACKGROUND USING SATELLITE DATA
NAG 5-449
(Dynamics Explorer Guest Investigator Program)
Final Technical Report
INSTITUTION: Department of Physics and Astronomy
The University of Iowa
Iowa City, IA 52242
PRINCIPAL INVESTIGATOR;
John D. Fix
Department of Physics and Astronomy
The University of Iowa
Iowa City, IA 52242
PERIOD COVERED:
15
DATE OF SUBMISSION:
27 June 1986
1984 - 14 january 19gg
{NASA-CR- 17680?) INVESTIGATION OF THE
DIFFUSE ULTRAVIOLET B A C K G R O U N D USING
SATELLITE D A T A : : D I H A M I C S EXPLORES GUEST
INVESTIGATOR PIOGEAff filial Technical •
Report, 15 Jul. 1S84-U Jan, 1966 (Iowa
N86-26739
Onclas
63/46 , 43455
https://ntrs.nasa.gov/search.jsp?R=19860017267 2020-03-20T14:28:36+00:00Z
I. INTRODUCTION
The imaging instrumentation for the Dynamics Explorer Mission
(Frank et al. 1981) was designed primarily to obtain global auroral images.
The instrument, however, has also been used successfully to study marine
bioluminescence, the geocorona, and the global distribution of atmospheric
ozone. The DE imager has considerable potential for the study of astronomical
sources of ultraviolet radiation as well. During the tenure of grant NAG 5-449
I used data produced by the the DE imager to study the brightness and isotropy
of the diffuse ultraviolet background.
II. BACKGROUND
The diffuse ultraviolet background is the cosmic ultraviolet radiation
which is not attributable directly to stars in the galaxy. The nature of the
diffuse ultraviolet background is of interest in both galactic and extragalactic
contexts. The extragalactic component of the diffuse ultraviolet background may
include contributions from quasars, galaxies, and the intergalactic medium
(Paresce and Jakobsen 1980). However, there is a considerable uncertainty
about the magnitude of the contributions of these sources. The extragalactic
component is presumed to be isotropic, but probably would be detectable only in
directions in which the galactic component is very small.
The galactic component of the diffuse ultraviolet background is thought
to be primarily starlight from the galactic plane which has beens scattered by
interstellar dust particles. If so, then the intensity of the galactic compo-
nent at a given wavelength and in a particular direction should depend on the
amount of dust in that direction, the albedo and phase function of the parti-
cles, and the amount of ultraviolet light emerging from the galactic plane.
The galactic component should show a dependence on galactic latitude and may,
moreover, be patchy. Certainly the interstellar gas with which ithe dust is
believed to be mixed varies considerably in column density with direction (e.g.,
Heiles 1975). Also, there are faint, filamentary reflection nebulae at moderate
to high galactic latitudes (Sandage 1976). If the diffuse ultraviolet back-
ground is radiation scattered by the same particles which produce the filament-
ary reflection nebulae, then the ultraviolet background would be spatially vari-
able on a scale of a degree or less. General models of the galactic component
of the diffuse ultraviolet background have been made by several investigators
(e.g., Anderson et al. 1982).
There have already been a number of measurements of the diffuse ultra-
violet background. The extent to which the measurements disagree about both the
magnitude and isotropy of the background is remarkable. Lillie and Witt (1976)
investigated the diffuse ultraviolet background by studying 71 widely distrib-
uted fields with the Wisconsin Experimental Package on OAO-2. They found a
typical intensity at 150 nm of 1000 photons/ cm2 s A sr (or units, following
Anderson et al. 1982) with a strong dependence on galactic latitude. Henry et
al. (1977) observed about half the sky during a rocket flight. After correcting
for the contribution of stars they found, at high galactic latitudes, an inten-
sity of about 1000 to 2000 units at 150 nm. However, they attributed the rad-
iation to airglow. Pitz et al. (1979) measured the ultraviolet background on a
rocket flight and found an intensity of about 2500 units at 180 nm with little
or no spatial variability. Observations with the D2B satellite were used
(Maucherat-Joubert et al. 1979, Joubert et al. 1983) to find an average inten-
sity of about 1000 units at 170 nm at high galactic latitude and a correlation
with neutral hydrogen column density. Anderson et al. (1982) found the spectrum
of the diffuse ultraviolet background between 118 nm and 168 nm using data from
the Apollo 17 mission. They found, after correction for internally scattered
light and starlight, that there was no significant residual intensity to a level
of about 300 units. Voyager ultraviolet spectrometer data were used by Sandel
et al. (1979) to measure an average intensity of about 5000 units at 150 nm.
Finally, Faresce et al. (1980) used data from the Apollo-Soyuz mission and found
that the diffuse ultraviolet background between 135 nm and 155 nm has a typical
value of 800 to 1000 units with definite spatial variability. They found that
intensity correlates well with neutral hydrogen column density except near the
galactic poles. They also found that the minimum measured intensity was 300
units, which they suggested was attributable to the isotropic extragalactic
component of the diffuse ultraviolet background.
Virtually all of the published measurements have been criticized for
instrumental problems, calibration deficiencies, or contamination by local
sources of ultraviolet radiation, the measurements span a factor of at least
ten in the average intensity of the diffuse ultraviolet background. There is
also considerable disagreement about its spatial variability. I have used DE
imaging data to contribute to the resolution of these problems.
III. DE OBSERVATIONS OF THE DIFFUSE ULTRAVIOLET BACKGROUND
The DE imager scans a strip .of sky 0.29 degrees wide during each space-
craft rotation (6 seconds), obtaining data for 1550 pixels at intervals of 0.23
degrees. Normally, the field of view is rotated by 0.25 degrees by means of a
stepping mirror after each spacecraft rotation in order to build up an image.
In order to build up adequate signal to noise, however, I have used repeated
measurements of the same strip of sky in investigating the diffuse ultraviolet
background. The data with which I worked were obtained on four days during
July 1984 during times when DE was near apogee. The observations used filter
136V, which results in a photometric response which peaks at 150 nm and has
a FWHM of 41 nm. The response at Lyman alpha is essentially zero. After the
elimination of data for which the imager was pointed at earth, data dropouts
occurred, or DE began to enter the trapped particle zones, 9.5 hours of usable
data (22 seconds per pixel) remained. A portion of the data, showing the strip
of sky at 10.6 hours right ascension from the north celestial pole to the equa-
tor is shown in Figure 1. The few identifiable stars are seen as sharp peaks
about 3 pixels in width. The standard errors of the counting rates are shown
below the counting rates and demonstrate that the diffuse ultraviolet background
apparently has been detected at approximately the ten sigma level.
Stars in the scan line were identified using the SAO catalog. For the
brighter identified stars, flux distributions from the IUE Ultraviolet Spectral
Atlas and the normalized instrumental response function were used to calibrate
the photometric system. The calibration has a standard error of about 10%.
Pixels containing identified stars were then removed from the data. This proce-
dure eliminated contamination of the data by stars with spectral type later than
AO. 0 and B stars fainter than the limiting magnitude of the SAO catalog may
still contribute to the data. These, however, are strongly confined to the
plane of the galaxy and are a possible problem only for small galactic lati-
tudes. After eliminating pixels contaminated by identified stars, 1350 pixels
remained for further analysis.
The intensity of the ultraviolet background shows a definite dependence
on galactic latitude, the precise nature of the relationship depending on gal-
actic longitude. The ultraviolet intensity is shown against galactic latitude
for galactic plane crossing near longitudes of 100 degrees and 280 degrees in
Figures 2 and 3. Pixels containing stars are shown as having zero intensity.
The dependence on galactic latitude is similar to that found by Lille and Witt
(1976). At high galactic latitude, the ultraviolet intensity has a mean value
of about 1500 units.
I have also investigated the relationship between ultraviolet "intensity
and neutral hydrogen column density. Figure 4 shows the relationship between
the two quantities for the region with galactic latitude less than -30 degrees
and between longitudes of 24 and 88 degrees. A least squares fit to the
data has a slope which is nearly the same as that found by Joubert et al.
(1983), who used averages over 30 square degree regions, and about 30 percent
smaller than the average slope found by Paresce et al. (1980) for averages over
regions of about 6 square degrees. The dispersion of points about the least
squares line is also about the same as those of previous studies. However,
where Joubert et al. and Paresce et al. found intensities of about 700 units
and 200 units for small hydrogen column densities, the intercept of the line
fit to my data is about 1100 units. Since the residual intensity in the
absence of ultraviolet radiation from the galactic component is presumed to
be the extragalactic component of the diffuse radiation, I have tried to be
as certain as possible about the validity of the relatively large intensities
which I have found. (Although a number of the published values of the average
intensity are still larger.)
I have investigated the level at which the data may be contaminated
by other contributions to the counting rate of the photometer. I am virtually
certain that .there is no zodiacal contribution, since the data show no correl-
ation with ecliptic latitude. No contribution from trapped particles is pres-
ent, since the data show no dependence on invariant latitude for the range of
latitudes over which data were retained. The counting rates for times when
DE was well within the trapped particle zones show that the expected rates for
the greatly reduced particle fluxes at high latitudes should be negligible.
Contamination by geocoronal Lyman alpha or auroral radiation is not present,
as the data show no dependence on zenith angle. The design of the DE imager
ensures that scattered light cannot be a problem. A remaining concern, how-
ever, is the possible influence of galactic cosmic rays on the counting rate.
I have estimated the counting rate due to Cherenkov radiation produced by
cosmic rays in various optical elements of the photometer and conclude that
the effect is probably small enough to be ignored. However, the possible
contribution due to Cherenkov photons as well as the contribution due to
direct cosmic ray hits on the photomultiplier tube requires experimental
investigation. I have proposed for (and will recieve) another DE Guest
Investigator Grant to study this problem. This will be done by measuring
the counting rates using filter 136W (which was used for the data which have
already been taken) and filter 136N, for which the wavelength of peak response
is the same as that of 136W, but which has a sensitivity of only about 50%
that of 136W. The precise value for the relative instrumental sensitivities
of the two filters will be found using data for bright stars. By comparing
the minimum counting rates using the two filters it will be possible to
distinguish between the contribution to the background which is due to cosmic
ultraviolet background (and would be reduced by the same ratio as the intensity
of bright stars when using the narrower filter) and any other source of counts,
which would be unchanged. The data for this study have already been acquired
(in November 1985 and March 1986). Analysis will be carried out as soon as
the data (and orbital information) are available from the DE imaging team.
When this is finished the calibration of the data on the diffuse ultraviolet
background can be completed and the study can be published.
BIBLIOGRAPHY
Anderson, R.C., Henry, R.C., and Fastle, W.G. 1982, Astrophys. J., 229, 573.
Frank, L.A., Craven, J.D., Ackerson, K.L., English, M.R., Bather, R.H., and
Carovillano, R.L. 1981, Space Sci. Instr., 5, 369.
Heiles, C. 1975, Astron. Astrophys. Suppl. 20, 37.
Henry, R.C, Swandic, J.R., Shulman, S.D., and Fritz, G. 1977, Astrophys. J.,
212, 707.
Joubert, M., Masnou, J.L., Lequeux, J., Deharveng, J.M., and Cruvellier, P.
1983, Astron. Astrophys., 128, 114.
Lillie, C.F., and Witt, A.N. 1976, Astrophys. J., 208, 64.
i
Maucherat-Joubert, M. Cruvellier, P., and Deharveng, J.M. 1979, Astron.
Astrophys., 70, 467.
Paresce, F., and Jakobsen, P. 1980, Nature, 288, 119.
Paresce, F., McKee, C.F., and Bowyer, S. 1980, Astrophys. J., 240, 387.
Pitz, E., Leinert, C. , Schulz, A., and Link, H. 1979, Astron. Astrophys.,
72, 92.
Sandage., A. 1976, Astron. J., 81, 954.
Sandel, B.R., Shemansky, D.E., and Broadfoot, A.L. 1979, Astrophys J.,
227, 808.
TOTAL.DAT
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ascension between the north celestial pole and the celestial
equator.
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Figure 2. Ultraviolet intensity versus galactic latitude
near galactic longitude 100 degrees.
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Figure 3. Ultraviolet intensity versus galactic latitude
for near galactic longitude 280 degrees.
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Figure 4. Ultraviolet intensity versus neutral hydrogen column
density at high galactic latitude. The neutral hydrogen column
density is shown in units of 2.23 x 10**18 per square centimeter.


Investigation of the diffuse ultraviolet background using satellite data: Dynamics explorer guest investigator program

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