Annu. Rev. Astron. Astrophys. 1991. 29:
89-127 Copyright © 1991 by Annual Reviews. All rights reserved |
5.8 UVX
We consider now in some detail the UVX experimental results and their bearing on the previous results of the Berkeley group where bright patches of thousands of units over the sky are reported and those of the Johns Hopkins group where ~ 400 units are reported for |b| > 30°. The dual experiment involved co-aligned spectrometers, although unfortunately the targets were only eight locations in the sky.
5.8.1 BERKELEY UVX BACKGROUND INTENSITIES
How reliable are the Berkeley intensities? Their experiment
is of excellent design, the best that has ever been flown for the
study of diffuse ultraviolet background radiation. Control of dark
current is excellent, and an imager was used. Preliminary
publication of the Berkeley data
(49) showed six
observations, at
hydrogen column densities < 6 x 1020 cm-2
(presumably these are the
six targets that have |b| > 30°), with background
intensities at 1800 Å
that are all less than 300 units. Asked to account for the difference
at the 1989 Heidelberg IAU Symposium #139 on background
radiation, Berkeley workers indicated ``overenthusiastic stellar
subtraction'' as the culprit and that the Berkeley workers later
decided that their higher flux levels were real. One must accept the
finally published intensities as most authoritative.
So from UVX, the Berkeley group finds no trace of the intense
bright patches that were reported earlier from Apollo-Soyuz and
that are shown in Figure 15. The
Berkeley UVX data do show a
somewhat higher cosmic background at middle latitudes (~ 750 units
at b ~ 40°) than the Johns Hopkins group has claimed.
5.8.2 JOHNS HOPKINS UVX BACKGROUND INTENSITES
The final conclusion is that neither experiment saw the ``bright
patches'' of many thousands of units that had been previously
reported by Berkeley (90,
91).
Of particular interest is the target
named ERIDANUS of Murthy et al
(81,
82).
Paresce et al (90)
reported background intensities in Eridanus, from Apollo-Soyuz, of
~ 6000 units. The location of the UVX ERIDANUS target was selected
by Berkeley, and turns out to be rather removed from the Paresce et
al (90) location.
At ERIDANUS, Murthy et al
(81) find 200 ± 200
units in their short-wavelength spectrometer, while Murthy et al
(82)
find 650 ± 200 units in their long-wavelength spectrometer. By
some detective work from the published Berkeley UVX data one can
deduce that the correct number from the Berkeley experiment is
about 750 units. However, the UVX observations were made at a
different location than where Paresce et al
(90) had previously
reported a background of 6000 units; hence these three observations
do not decisively rule out that earlier extraordinary result.
5.8.3 MOLECULAR HYDROGEN FLUORESCENCE
From the amount of molecular hydrogen emission that is
deduced by Martin et al, one can estimate the amount of diffuse
background that would be seen by Voyager. Sternberg
(120) shows
that about two thirds of the fluorescent radiation will fall in the
region below L, where
Voyager is most sensitive, and only one third
will occur at longer wavelengths where Martin et al claim a
detection. However, Voyager should have seen hundreds of units of
diffuse background if the Martin et al result is correct but it has not
(Figure 12). In particular, Murthy,
Henry & Holberg (83)
have taken
observations with Voyager near the beginning and the end of the
GRADIENT UVX target, which is where Berkeley reports the strongest
molecular hydrogen emission. They obtain only the usual Voyager
upper limits of ~ 100 units.
5.8.4 LINE EMISSION FROM HOT GAS
The Feldman et al observation was statistically marginal, but
it has in its favor the fact that the data set was very well-behaved
and free of contamination. The Berkeley data should be of much
higher quality, but they have not yet been published in detail. The
complete spectrum has been published for only a single target.
5.8.5 SCATTERING FROM DUST