SUSIM SPACELAB-2 EXPERIMENT
Flown on Space Shuttle Mission 51-F
July 29, 1985 - August 6, 1985
Solar UV radiation in the wavelength range of 120 to 300 nm (1 nm = 10 Å) is absorbed by Earth's upper atmosphere at altitudes between 20 and 120 km (10 and 75 miles). Although this radiation constitutes only a small percentage of the total solar output, it is the main energy source for the upper atmosphere. This radiation plays a major role in the energy balance and chemical composition of the upper atmosphere, which in turn affects the lower atmosphere where we live.
Scientists have already determined that solar UV radiation is more variable than is the total solar radiation. Present estimates of long-term solar UV variations are based on theoretical models and a few observations spaced far apart in time and made with different instruments. Systematic, high-precision measurements have never been carried out over a complete solar cycle. An accurate model of Earth's upper atmosphere must incorporate this variable solar input in its predictions of temperature, density, and chemical composition.
The basic difficulty is that the same solar UV radiation being measured is also responsible for rapid degradation of the instrument making the measurement. As a result, photometers on board a satellite quickly lose their accuracy, and long-term solar effects cannot be distinguished from instrument changes.
The SUSIM overcomes this problem by using new self-check calibration systems to determine whether instrument changes or solar flux variations are being recorded. The shuttle's ability to carry SUSIM into space for a week and then return it allows preflight and postflight calibrations to determine instrument degradation. It also allows the instrument to measure UV radiation at several different times during the 11-year solar cycle.
SUSIM INSTRUMENT DESCRIPTION
The SUSIM instrument is a precision UV spectrometer that incorporates redundant optical paths and an in-flight calibration deuterium (D2) lamp to provide accurate recording of solar spectral irradiance from 120 to 400 nm and the tracking of any UV sensitivity changes. The determination of absolute sensitivity is performed at the National Bureau of Standard's Synchrotron Ultraviolet Radiation Facility (NBS-SURF), both prior to delivery of the instrument for integration and test, and again following the retrieval from the flown payload such as Spacelab 2. The calibration period for Spacelab 2 started in January 1984, with a SURF calibration. It was extended with the internal D2 lamp through the preflight period, continued through the flight in July and August of 1985 with inflight D2 lamp scans, and was completed in January 1986 with the postflight calibration at SURF.
SUSIM CALIBRATION
The SUSIM instrument sensitivity calibration is performed by measuring the spectrometer response to an absolute source of radiation produced by NBS-SURF. The necessity to measure the sensitivity with all possible combinations of elements requires weeks of effort and extensive reduction of the data collected. The instrument has five detectors, two spectrometers - each with two different spectral resolutions - neutral density filters, and second order suppression filters. The instrument also must be measured for angular response within its field of view. To complete these measurements, the response of the instrument to nearly totally polarized light produced by the synchrotron must be measured.
This absolute calibration is then extended by using a D2 lamp which has been calibrated by the
National Physics Laboratory (NPL) and by NBS. The D2 lamp is used primarily to permit the
tracking of sensitivity changes during the integration and test and during flight. The lamp is
carefully selected for aging and operational stability during the calibrations. Vibration tests at
flight levels are performed on the lamps and selection of the lamp with the minimum change is
made.
Fig. 14 - (a) Postflight sensitivity of the spectrometer used for solar observations. Sensitivities
were derived from various calibrations: NBS-SURF (black line), quartz halogen tungsten lamp
(red line), and D2 lamp (green line). Two detectors were used; rubidium telluride 115 to 250 nm
and bi-alkali 250 to 400 nm. (b) Ratios of D2 lamp to SURF (green line) and tungsten to SURF
(red line). SUSIM's 5% calibration accuracy goals were met.
The sensitivity of the instrument in the longer wavelength region (250 to 400 nm) is cross-checked by using a quartz halogen tungsten lamp. These lamps are calibrated by NBS to approximately I % absolute accuracy, and then the instrument is calibrated at NRL. The January 1986 intercomparison with the D2 and NBS-SURF calibrations shown in Fig. 14 was well within the accuracy goals of the SUSIM program.
The calibration of the instrument at NBS-SURF in January 1984, together with the postflight calibration in January 1986, shows an approximately 38% loss in sensitivity at H Lya (121.6 nm) for the solar spectrometer. The calibration spectrometer had a corresponding 20% loss, and both had a declining loss at longer wavelengths (10 % and 7 % at 400 nm). The magnitude of this loss is small considering the long period of time between calibrations. However it is large compared with the anticipated small changes in solar UV output. Models predict a change of 75 % at H Lya, 30% at 160 nm, 10% at 180 nm, and less than 2 % beyond 200 nm. The tracking of the instrument's sensitivity by using the D2 lamp allows it to determine the absolute response during flight to an accuracy of 1 %. The exact relationship to solar and D2 viewing time and the profile with time of these degradations have been developed.
SUSIM RESULTS
The goal of SUSIM is to measure the solar irradiance from 120 to 400 nm with an accuracy of 6% to 10%. A number of factors must be taken into account to meet this goal.
One of these factors includes the effects of offpointing-when the Sun is not centered in the field of view of the instrument. The instrument shows a small change in sensitivity until it is pointed greater than 16 arc min off the Sun's center.
A second effect that must be taken into account is the amount of instrument degradation that
occurs during the flight. Analysis of pre- and postflight calibration data indicates that the
spectrometer that viewed the Sun decreased in sensitivity by 18 % at 120 m-n during flight.
Fig. 15 - (a) Hydrogen Lya spectrum (121.6 nm) taken at four different times: A-Day 214, 8 h
(black); B-Day 214, 15 h (red); C-Day 215, 8 h (green); and D-Day 217, 9 h (blue). (b) Plot of degradation for these spectra shows a 21% decrease in the H Lya intensity over a 3-day period.
Figure 15 shows four high-resolution spectra of H Lya at 121.6 nm that were taken on
different days during the Spacelab 2 mission. A 21 % decrease in H Lya can be seen, which is in
agreement with an 18% change of the instrument sensitivity as determined from pre- and
postflight calibrations. The H Lya scans are placed side-by-side for the convenience of display.
Fig. 16 - Two solar spectra-one taken during Day 215, 6 h (red), the other taken on Day 217, 7 h
(blue)-are shown in an area of overlap (160 to 180 nm). The measured intensity decrease in
these regions is 5% or less in 2 days.
Similarly, spectra from 160 to 180 nm can be used to study the degradation in this region.
Figure 16 shows spectra taken on Day 215, 6 hours and Day 217, 7 hours. Almost no noticeable
depredation of the instrument in this wavelength region occurs during flight.
Fig. 17 - Solar spectral irradiance measured by the SUSIM at 5 nm bandpass (red) and 0.15 nm
bandpass (blue). The absolute accuracy of these spectra is 3.5%.
The final spectral irradiance produced by the SUSIM experiment is shown in Fig. 17. The
error analysis for all sources of errors and corrections provides an estimate of absolute accuracy
to be 3.5 %.
Fig. 18 - (a) SUSIM spectra of Spacelab 2 (thin black line) and Spacelab I of Labs et al. [4]
(thick red line), both with 10 nm resolution. (b) Ratios of Spacelab I/Spacelab 2 with 0.1 nm
averaging (black line); with 10 nm averaging: Spacelab I/Spacelab 2 (red line), Mount and
Rottman [5]/Spacelab 2 (green line), Heath [6]/Spacelab 2 (blue line), and Mentall et al.
[7]/Spacelab 2 (orange line).
Figure 18 shows a comparison between the SUSIM Spacelab 2 spectrum 200 nm < < 350 nm (August 1985) and a solar UV spectrum obtained from a European group [4] during the Spacelab I mission in December of 1983. There is a better than 2 % agreement of the intensity values that were integrated over 10 nm intervals. Ratios of other previous measurements to the Spacelab 2 spectrum are also plotted in Fig. 18; these ratios show much larger disagreement.
For the first time in the history of solar UV flux measurements, two independently calibrated spectra exist. These spectra agree with each other on the 3 % level in the wavelength range of 200 nm < < 350 nm. If these measurements can be repeated frequently over a solar cycle, they will answer the still outstanding questions of the Sun's variability in the UV. Recently, long-term backscatter measurements of the Earth's atmosphere indicate that there may be a depletion of ozone. The cause of this alarming effect is unknown. It could be related to the solar cycle, to long-term dynamics in the Earth's atmosphere, or it could be caused by the release of man-made carbon fluorides. A precise knowledge of the Sun's UV variability is needed to separate these effects. If the alleged ozone depletion is real and if it is caused by man-made substances, it could be catastrophic for the continuation of life on Earth because the small effects observed over a 4-year period would continue over 40 to 60 years to come and would result in a 60% ozone depletion. As a result, solar UV radiation at the 300 nm wavelength would increase considerably, which could have a disastrous impact on the life of microorganisms, even on the DNA composition of more highly developed forms of life.
REFERENCES
1. J.W. Cook, G.E. Brueckner, and J.-D.F. Bartoe, Ap. J. (Letters) 270, L89 (1983).
2. G.E. Brueckner and J.-D.F. Bartoe, Ap. J. 272, 329 (1983).
3. J.W. Cook, G.E. Brueckner, J.-D.F. Bartoe, and D.G. Socker, Adv. Space Res. 4, 59 (1984).
4. D. Labs, H. Neckel, P.C. Simon, and G.Thuillier, Solar Phys., 107, 203 (1987).
5. G.H. Mount and G.J. Rottman, J. Geophys. Res. 90, 13-31 (1985).
6. D. F. Heath, Proc. of Int. Conf. on Sun and Climate, Centre National D'Etudes Spatiales, 63 (1980).
7. J. E. Mentall et al., J. Geophys. Res., 86, 9981 (1981).