Ultraviolet Albedo Research Articles

Reliability assessment on PV backsheets with and without considering spectral UV albedo effects: A theoretical comparison

Trustworthy evaluation on the long-term reliability of PV backsheets depends largely on the estimation of ultraviolet (UV) damage from the ground reflection; however, current use of total UV irradiance can hardly provide precise estimation due to the spectral UV albedo of grounding materials and spectral sensitivity of backsheet degradation. In this work, effects of spectral UV albedo on the estimation of UV damage were evaluated theoretically through the combinations of 35 types of grounding materials and 4 types of common backsheet materials, including polypropylene (PP), polyethylene terephthalate (PET), polycarbonate (PC) and polyamide (PA). The results showed that, without considering the spectral reflectivity, overestimation of the UV damage for PV backsheets can happen. This work can provide the critical basis for developing more precise lifetime prediction model and designing more reliable accelerated aging tests.

Long-term Variations of Venus’s 365 nm Albedo Observed by Venus Express, Akatsuki, MESSENGER, and the Hubble Space Telescope

Abstract An unknown absorber near the cloud-top level of Venus generates a broad absorption feature from the ultraviolet (UV) to visible, peaking around 360 nm, and therefore plays a critical role in the solar energy absorption. We present a quantitative study of the variability of the cloud albedo at 365 nm and its impact on Venus’s solar heating rates based on an analysis of Venus Express and Akatsuki UV images and Hubble Space Telescope and MESSENGER UV spectral data; in this analysis, the calibration correction factor of the UV images of Venus Express (Venus Monitoring Camera) is updated relative to the Hubble and MESSENGER albedo measurements. Our results indicate that the 365 nm albedo varied by a factor of 2 from 2006 to 2017 over the entire planet, producing a 25%–40% change in the low-latitude solar heating rate according to our radiative transfer calculations. Thus, the cloud-top level atmosphere should have experienced considerable solar heating variations over this period. Our global circulation model calculations show that this variable solar heating rate may explain the observed variations of zonal wind from 2006 to 2017. Overlaps in the timescale of the long-term UV albedo and the solar activity variations make it plausible that solar extreme UV intensity and cosmic-ray variations influenced the observed albedo trends. The albedo variations might also be linked with temporal variations of the upper cloud SO2 gas abundance, which affects the H2SO4–H2O aerosol formation.

Principal components of short-term variability in the ultraviolet albedo of Venus

We explore the dominant modes of variability in the observed albedo at the cloud tops of Venus using Akatsuki UVI 283 nm and 365 nm observations, which are sensitive to SO2 and unknown UV absorber distributions respectively. The observations, taken over the period Dec. 2016 to May 2018, consist of images of the dayside of Venus, most often observed at intervals of two hours, but interspersed with longer gaps. The orbit of the spacecraft does not allow for continuous observation of the full dayside and the unobserved regions cause significant gaps in the datasets. Each dataset is subdivided into three subsets for three observing periods, the unobserved data are interpolated, and each subset is then subjected to a principal component analysis to find six oscillating patterns in the albedo. Principal components in all three periods show similar morphologies at 283 nm, but are much more variable at 365 nm. Some spatial patterns and the timescales of these modes correspond to well-known physical processes in the atmosphere of Venus such as the ~4-day Kelvin wave, 5-day Rossby waves, and the overturning circulation, while others defy a simple explanation. We also a find a hemispheric mode that is not well understood and discuss its implications.

Solar activity expressed in a modern varve thickness sequence

Annually laminated sediments (varves) form in particular depositional settings, e.g., where seasonal climate produces fluctuations in runoff volume; variations in runoff affect the amount and type of sediment delivered to a catchment. Prior studies confirm that variations in selected varve traits correlate with inter-annual climate signals. In some locations, solar activity also appears to be expressed in varve characteristics, either through a direct effect or indirectly via influence of solar activity on climate. Evidence from proglacial Iceberg Lake, Alaska, indicates that solar activity may have directly contributed to varve deposition. A varve thickness sequence is compared to sunspot observations from 1610–1995 CE. Maunder and Dalton minima are clearly expressed in a varve power spectrogram; varve signal amplification beginning ca. 1950s CE coincides with increasing activity evident in a sunspot spectrogram, features that are only vaguely discernible in the raw time-series plots. Spectral relationships at sunspot periodicities are consistent with direct solar forcing of varve thickness, independent of any effect solar activity might otherwise have on climate. Simulations based on a meltwater model indicate that direct forcing could result from amplified ultraviolet (UV) emission during solar maxima, combined with lower UV albedo of glacial ice. The plausible forcing mechanism bolsters epistemology for concluding a cause–effect relationship: solar variability likely contributed directly to inter-decadal patterns in Iceberg Lake varve thicknesses. The putative effect could be enhanced at higher latitudes, where Earth’s atmosphere absorbs less of the UV energy emitted by the Sun; periods of lowered ozone concentration near the poles would exacerbate the natural abetting UV phenomena, potentially linking human activity to recent and accelerated polar ice cap melting.

Ultraviolet Radiation Albedo and Reflectance in Review: The Influence to Ultraviolet Exposure in Occupational Settings.

Ultraviolet (UV) albedo and UV reflectance are defined, compared and contrasted, to explain their roles and place in studies focusing on UV radiation and exposure measurements, in the context of localised albedo measurement and human UV exposure studies. This review recommends that the term UV albedo be used when investigating natural horizontal surfaces when the albedo is not known to change significantly over time. The term UV reflectance should be mostly used for non-natural surfaces and non-horizontal measurements and will change with respect to the geometry of the irradiances reflected and received, and due to the intrinsic nature of the surface itself. UV albedo measurements made in the literature have been compiled, in both broadband and spectral UV albedo measurements. Broadband measurements have been tabulated and spectral UV measurements have been displayed visually. The methodology of measurements is briefly discussed. Finally, studies that consider how high albedo or reflectance sites influence UV exposure are reviewed. It was concluded that there is currently no known relationship between the albedo or reflectance of a surface and the resulting influence it has on individual UV exposure. This presents an opportunity for researchers to continue exploring the influence of reflective UV surfaces.

Icy Saturnian satellites: Disk-integrated UV-IR characteristics and links to exogenic processes

Combined Cassini observations obtained at similar observing geometries in the ultraviolet through infrared spectral range, along with additional ultraviolet (UV) data from Hubble Space Telescope where available, are used to study system-wide trends in spectral albedos of the inner icy Saturnian satellites (Mimas, Enceladus, Tethys, Dione, Rhea). We derive UV and visible geometric albedos and UV absorption strengths of the leading and trailing hemispheres and compare with E ring grain flux and charged particle intensities (electrons and ions of varying energies) to those hemispheres. We find that the UV absorption strength on the leading and trailing hemispheres is anti-correlated with E ring grain flux. On the trailing hemispheres, the UV absorption strength is correlated with intensity of electrons in the tens of keV range. We suggest that these relationships could imply links with the organic component of the E ring. Radiolytic processing of organics causes the products to become spectrally redder, increasing the UV absorption strength. Such processing occurs while organic-rich grains are in the E ring, and increases with exposure time in the E ring, such that grains interacting with Rhea are redder (more processed) than those impacting moons closer to Enceladus. Further processing (and associated darkening/reddening) occurs on the trailing hemispheres of the satellites, via radiolysis by electrons in the tens of keV range. Silicates and salts also redden with weathering; however because organics are present in the E ring in significantly greater abundance than salts or silicates, we suggest here that weathering of organics dominates the coloring of the inner Saturnian moons.

Impact of soil moisture and winter wheat height from the Loess Plateau in Northwest China on surface spectral albedo

The understanding of surface spectral radiation and reflected radiation characteristics of different surfaces in different climate zones aids in the interpretation of regional surface energy transfers and the development of land surface models. This study analysed surface spectral radiation variations and corresponding surface albedo characteristics at different wavelengths as well as the relationship between 5-cm soil moisture and surface albedo on typical sunny days during the winter wheat growth period. The analysis was conducted using observational Loess Plateau winter wheat data from 2015. The results show that the ratio of atmospheric downward radiation to global radiation on typical sunny days is highest for near-infrared wavelengths, followed by visible wavelengths and ultraviolet wavelengths, with values of 57.3, 38.7 and 4.0%, respectively. The ratio of reflected spectral radiation to global radiation varies based on land surface type. The visible radiation reflected by vegetated surfaces is far less than that reflected by bare ground, with surface albedos of 0.045 and 0.27, respectively. Thus, vegetated surfaces absorb more visible radiation than bare ground. The atmospheric downward spectral radiation to global radiation diurnal variation ratios vary for near-infrared wavelengths versus visible and ultraviolet wavelengths on typical sunny days. The near-infrared wavelengths ratio is higher in the morning and evening and lower at noon. The visible and ultraviolet wavelengths ratios are lower in the morning and evening and higher at noon. Visible and ultraviolet wavelength surface albedo is affected by 5-cm soil moisture, demonstrating a significant negative correlation. Excluding near-infrared wavelengths, correlations between surface albedo and 5-cm soil moisture pass the 99% confidence test at each wavelength. The correlation with 5-cm soil moisture is more significant at shorter wavelengths. However, this study obtained surface spectral radiation characteristics that were affected by land surface vegetation coverage as well as by soil physical properties.

Evidence for exposed water ice in the Moon’s south polar regions from Lunar Reconnaissance Orbiter ultraviolet albedo and temperature measurements

We utilize surface temperature measurements and ultraviolet albedo spectra from the Lunar Reconnaissance Orbiter to test the hypothesis that exposed water frost exists within the Moon’s shadowed polar craters, and that temperature controls its concentration and spatial distribution. For locations with annual maximum temperatures Tmax greater than the H2O sublimation temperature of ∼110K, we find no evidence for exposed water frost, based on the LAMP UV spectra. However, we observe a strong change in spectral behavior at locations perennially below ∼110K, consistent with cold-trapped ice on the surface. In addition to the temperature association, spectral evidence for water frost comes from the following spectral features: (a) decreasing Lyman-α albedo, (b) decreasing “on-band” (129.57–155.57nm) albedo, and (c) increasing “off-band” (155.57–189.57nm) albedo. All of these features are consistent with the UV spectrum of water ice, and are expected for water ice layers >∼100nm in thickness. High regolith porosity, which would darken the surface at all wavelengths, cannot alone explain the observed spectral changes at low temperatures. Given the observed LAMP off-band/on-band albedo ratios at a spatial scale of 250m, the range of water ice concentrations within the cold traps with Tmax<110K is ∼0.1–2.0% by mass, if the ice is intimately mixed with dry regolith. If pure water ice is exposed instead, then up to ∼10% of the surface area on the 250-m scale of the measurements may be ice-covered. The observed distribution of exposed water ice is highly heterogeneous, with some cold traps <110K having little to no apparent water frost, and others with a significant amount of water frost. As noted by Gladstone et al. (Gladstone, G.R. et al. [2012]. J. Geophys. Res.: Planets 117(E12)), this heterogeneity may be a consequence of the fact that the net supply rate of H2O molecules to the lunar poles is very similar to the net destruction rate within the cold traps. However, an observed increase in apparent H2O abundance with decreasing temperature from ∼110K to 65K suggests that destruction of surface frosts by impact gardening and space weathering is spatially heterogeneous. We find a loosely bimodal distribution of apparent ice concentrations with temperature, possibly due to competition between vertical mixing by impact gardening and resupply of H2O by vapor diffusion at sites ∼110K. Finally, we cannot rule out the possibility that the colder population of ice deposits is in fact primarily carbon dioxide ice, although peak temperatures of ∼65K are slightly higher than the usual CO2 sublimation temperature of ∼60K.

UV Irradiance and Albedo at Union Glacier Camp (Antarctica): A Case Study

We report on the first spectral measurements of ultraviolet (UV) irradiance and the albedo at a Camp located in the southern Ellsworth Mountains on the broad expanse of Union Glacier (700 m altitude, 79° 46′ S; 82° 52′W); about 1,000 km from the South Pole. The measurements were carried out by using a double monochromator-based spectroradiometer during a campaign (in December 2012) meant to weight up the effect of the local albedo on the UV irradiance. We found that the albedo measured at noon was about 0.95 in the UV and the visible part of the spectrum. This high surface reflectivity led to enhancements in the UV index under cloudless conditions of about 50% in comparison with snow free surfaces. Spectral measurements carried out elsewhere as well as estimates retrieved from the Ozone Monitoring Instrument (OMI) were used for further comparisons.

Ultraviolet and visible photometry of asteroid (21) Lutetia using the Hubble Space Telescope

The asteroid (21) Lutetia is the target of a planned close encounter by the Rosetta spacecraft in July 2010. To prepare for that flyby, Lutetia has been extensively observed by a variety of astronomical facilities. We used the Hubble Space Telescope (HST) to determine the albedo of Lutetia over a wide wavelength range, extending from ~150 nm to ~700 nm. Using data from a variety of HST filters and a ground-based visible light spectrum, we employed synthetic photometry techniques to derive absolute fluxes for Lutetia. New results from ground-based measurements of Lutetia's size and shape were used to convert the absolute fluxes into albedos. We present our best model for the spectral energy distribution of Lutetia over the wavelength range 120-800 nm. There appears to be a steep drop in the albedo (by a factor of ~2) for wavelengths shorter than ~300 nm. Nevertheless, the far ultraviolet albedo of Lutetia (~10%) is considerably larger than that of typical C-chondrite material (~4%). The geometric albedo at 550 nm is 16.5 +/- 1%. Lutetia's reflectivity is not consistent with a metal-dominated surface at infrared or radar wavelengths, and its albedo at all wavelengths (UV-visibile-IR-radar) is larger than observed for typical primitive, chondritic material. We derive a relatively high FUV albedo of ~10%, a result that will be tested by observations with the Alice spectrograph during the Rosetta flyby of Lutetia in July 2010.

Diurnal variations in the UV albedo of arctic snow

Abstract. The relevance of snow for climate studies is based on its physical properties, such as high surface reflectivity. Surface ultraviolet (UV) albedo is an essential parameter for various applications based on radiative transfer modeling. Here, new continuous measurements of the local UV albedo of natural Arctic snow were made at Sodankylä (67°22'N, 26°39'E, 179 m a.s.l.) during the spring of 2007. The data were logged at 1-min intervals. The accumulation of snow was up to 68 cm. The surface layer thickness varied from 0.5 to 35 cm with the snow grain size between 0.2 and 2.5 mm. The midday erythemally weighted UV albedo ranged from 0.6 to 0.8 in the accumulation period, and from 0.5 to 0.7 during melting. During the snow melt period, under cases of an almost clear sky and variable cloudiness, an unexpected diurnal decrease of 0.05 in albedo soon after midday, and recovery thereafter, was detected. This diurnal decrease in albedo was found to be asymmetric with respect to solar midday, thus indicating a change in the properties of the snow. Independent UV albedo results with two different types of instruments confirm these findings. The measured temperature of the snow surface was below 0°C on the following mornings. Hence, the reversible diurnal change, evident for ~1–2 h, could be explained by the daily metamorphosis of the surface of the snowpack, in which the temperature of the surface increases, melting some of the snow to liquid water, after which the surface freezes again.

Spectral measurements of PMCs from SBUV/2 instruments

The SBUV/2 (Solar Backscattered Ultraviolet, model 2) instrument is designed to monitor ozone stratospheric profile and total column ozone using measurements of the Earth's backscattered ultraviolet albedo. We have previously demonstrated that the normal radiance measurements from SBUV/2 instruments, which sample 12 discrete wavelengths between 252 and 340 nm during each scan, can be used to identify polar mesospheric clouds (PMCs). Some SBUV/2 instruments also periodically view the earth in continuous scan mode, covering the wavelength range 160–400 nm with 0.15 nm sampling. Analysis of these data show PMC occurrence rates similar to the normal discrete scan results, although the observation technique reduces the number of daily measurements by a factor of six. PMC observed by SBUV/2 instruments show a monotonic variation in the residual spectral albedo over the wavelength range 250–300 nm, with maximum enhancements of 10–15% at 250 nm. This result is consistent with microphysical model predictions from Jensen [1989.A numerical model of polar mesospheric cloud formation and evolution, Ph. D. Thesis, University of Colorado]. We find no evidence for a systematic localized increase in PMC residual albedo for wavelengths near 260 nm, in contrast to the recently reported results from the MSX UVISI instrument [Carbary J.F., et al., 2004. Evidence for bimodal particle distribution from the spectra of polar mesospheric clouds. Geophysics Research. Letters 31, L13108]. This result is observed for three different SBUV/2 instruments in both Northern and Southern Hemisphere data over a 13-year span. Our Mie scattering calculations show that the location and magnitude of the 260 nm “hump” feature is dependent upon the specific scattering angles appropriate to the MSX measurements. Although it explains the MSX spectrum, the bimodal size distribution proposed by Carbary et al. (2004), cannot explain the lack of scattering angle dependence of the SBUV/2 spectral shapes. The spectral signature of the SBUV/2 continuous scan PMC data is thus inconsistent with the bimodal particle size distribution suggested by Carbary et al. (2004).

Spectral ultraviolet albedo of roofing surfaces and human facial exposure

Spectral field measurements were used to quantify the ultraviolet (UV) spectral albedos of four different metallic roofing surfaces. The effect of the albedos of two of these surfaces on erythemal exposure to human facial anatomical sites was quantified by UV dosimetry. The albedos of all roofing surfaces were greater than the albedo of grass. Little SZA dependence was observed for any of the surfaces. The albedos of the coloured metallic corrugated surfaces were strongly dependent on wavelength in the UVA, increasing from 3 to 12%. Facial erythemal measurements showed significant exposure enhancements over the galvanised corrugated surface compared to grass. The undersides of the chin and nose received exposure enhancements over the galvanised corrugated surface of about 1290 and 190%, respectively, of the exposure of these sites over grass. It is concluded that the albedo of the galvanised surfaces are higher than those of the coloured surfaces by at least 20%, and higher than grass by at least 27%. Consequently, normally shaded facial anatomical sites receive substantially higher UV exposures over these galvanised surfaces compared to grass.

A method to generate surface UV radiation maps over Europe using GOME, Meteosat, and ancillary geophysical data

This paper presents a method for generating surface ultraviolet (UV) radiation maps over Europe, with a spatial resolution of 0.05°, and potentially on a half‐hour basis. The UV irradiance is obtained by interpolation in a look‐up table (LUT), the entries of which are solar zenith angle, total column ozone amount, cloud liquid water thickness, near‐surface horizontal visibility, surface elevation, and UV albedo. Both satellite (Meteosat, GOME) and nonsatellite (synoptic observations, meteorological model results, digital elevation model) data are exploited to assign values to the influencing factors. With the help of another LUT simulating the visible signal, Meteosat data are processed to retrieve the cloud liquid water thickness. The radiative transfer calculations are performed with the UVspec code. A preliminary step consists in generating an effective surface Meteosat albedo map from a series of 10 consecutive days. In this process the well‐known difficulty of distinguishing clouds from snow‐covered surfaces is encountered. An attempt is made to partially resolve the ambiguity by using the Meteosat infrared channel and modeled snow cover data. After additional empirical cloud filtering, the effective albedo map is used as a baseline to estimate the cloud liquid water thickness. The UV surface albedo is assigned uniform values for land and sea/ocean, except in the presence of snow. In this case it is given a value proportional to the Meteosat effective albedo. The total column ozone is extracted from the level 3 GOME products. The aerosol optical thickness is mapped by gridding the daily measurements performed by ∼1000 ground stations. The digital elevation model is the GTOPO30 data set from the U.S. Geological Survey. European wide UV dose rate maps are presented for one day in April 1997, and the influence of the various factors is illustrated. A daily integrated dose map was also generated using 27 Meteosat acquisitions at half‐hour intervals on the same day. The dose map produced in this way takes into account the evolution of the cloud field and is thought to be more accurate than if it were estimated from one data take, in particular at the relatively high spatial resolution of the product. Finally, a preliminary comparison of modeled dose rate and daily dose with measurements performed with a ground instrument is discussed.

Europa: Disk-Resolved Ultraviolet Measurements Using the Galileo Ultraviolet Spectrometer

The Galileo ultraviolet spectrometer has measured the first-ever disk-resolved spectral ultraviolet albedos of regions on Europa's trailing and antijovian hemispheres at spatial resolutions approaching 55 km. We find that the ultraviolet albedo (2400–3200 Å) increases with distance from the trailing hemisphere apex (270° W). Regions close to 270° W display an absorption feature centered near 2800 Å, which generally decreases with distance from the trailing hemisphere apex. These results substantiate the idea that magnetospheric bombardment darkens the surface of the entire trailing hemisphere and creates an absorber in the central portion of the trailing hemisphere. The absorption is related to sulfur, but is much darker than any laboratory spectra, likely due to the large grain size on Europa's trailing hemisphere. The disk-resolved observations, in combination with three disk-integrated observations, are used to derive the ultraviolet particle scattering function for Europa's trailing hemisphere.

Anterior eye focusing of peripheral ultraviolet and visible radiation albedo

Concern about short‐ and long‐term ultraviolet radiation (particularly UVB) damage to the eye has led to increased research in this area. Numerous studies have confirmed the pathogenic enhancing roles of reflected ultraviolet (UV) and visible radiation in our environment. There is concern that conventional sunglasses do not protect the eye adequately from reflected rays (albedo), especially on the lateral aspect, from behind and from below. Using eye models and computer ray tracing methods, the pathways of oblique rays incident at the temporal peripheral cornea have been plotted by Maloof, Ho and Coroneo.1 These rays are refracted and focused and theoretically can result in up to 20 times the concentration of incident irradiance at the nasal anterior chamber angle and nasal equatorial cortex of the crystalline lens. The purpose of this study was to determine the limits of angular subtense of the incident peripheral light which is refracted in this manner in human subjects and to investigate the relation between corneal shape and certain ocular parameters to the limits. A statistically significant positive correlation was found between temporal entrance angle and anterior chamber depth (r = 0.70, P< 0.0006). The entrance angle ranged from 15 degrees to 30 degrees and was located 10 degrees to 45 degrees posterior to the coronal plane. Our results support Maloof and colleagues' predictions for the implication of focused peripheral UV and high intensity visible radiation in the pathogenesis of pterygium and cortical cataract and emphasise the need for lateral eye protection in conditions of high ultraviolet albedo.

Mars ozone measurements near the 1995 aphelion: Hubble space telescope ultraviolet spectroscopy with the faint object spectrograph

Ultraviolet (225–330 nm) spectral scans of Mars were obtained with the Hubble space telescope (HST) faint object spectrograph (FOS) in February of 1995. These spectra yield ozone column abundances, cloud opacities (0.2–0.4 at low latitudes), and polar seasonal ice albedos from southern midlatitudes to northern high latitudes on Mars. At the time of these measurements, Mars was at a solar longitude (Ls) of 63.5°, corresponding to the late northern spring season on Mars, and very near to Mars aphelion. The most important result of these observations is the measurement of low‐latitude ozone abundances (3.1−0.5+2.1), which are significantly (≥100%) elevated relative to the northern fall (Ls = 208°, pre‐perihelion) IR ozone measurements of Espenak et al. [1991] in 1988. The implied perihelion‐to‐aphelion increase in the global Mars ozone column (from 1.5−1.0+0.4 to 3.1−0.5+2.1 μm atm) is quantitatively consistent with photochemical modeling analysis of Clancy and Nair [this issue], which predicts large annual variations in Mars photochemistry due to orbital forcing of the altitude of global water vapor saturation on Mars [Clancy et al., 1996]. However, the HST FOS observations are not diagnostic of the altitudes at which Mars ozone densities vary with Ls, which is a key aspect of the Clancy and Nair model prediction. Furthermore, it is the uncertain ozone density profile which leads to the large asymmetric uncertainties in the derived FOS and IR ozone columns. An ozone column of 7.3 ± 2.5 μm atm is retrieved for a high northern latitude region (71–75°N). The derived ultraviolet albedo of the north polar seasonal CO2 cap is 0.18 ± 0.07, which is roughly 10 times the ultraviolet albedo of the silicate surface of Mars, but only one quarter the visible albedo of the seasonal CO2 ice cap.

Ultraviolet Albedo of the Moon with the Hopkins Ultraviolet Telescope

During the 1995 March flight of the Hopkins Ultraviolet Telescope, as part of the Astro-2 Space Shuttle mission, we observed the Moon over the wavelength range 820-1840 A, with a resolution of 3-4 A, finding the ultraviolet albedo of the Moon to be 0.038 ± 0.0038 at 1700 A. We also find a gentle increase in the albedo toward shorter wavelengths. Ultraviolet albedo measurements may represent a useful tool for selenographic exploration.

Calibration and Radiometric Stability of the Shuttle Solar Backscatter Ultraviolet (SSBUV) Experiment

The SSBUV is part of the National Plan for monitoring the stratosphere. Ozone monitoring from space employs backscatter ultraviolet-type instruments on NOAA operational, NASA research and foreign environmental satellites. The SSBUV provides calibration data for these instruments using nearly coincident observations of the Earth's ultraviolet albedo from a series of Space Shuttle flights. The SSBUV also measures the middle ultraviolet solar irradiance as part of the ozone measurement. A major requirement for the SSBUV is that its calibration be known to about 1% from one flight to the next. To achieve this, the SSBUV conducts a rigorous calibration program involving multiple standard sources and detectors. The SSBUV has now flown four times, over a thirty-month period beginning in October 1989. This paper deals with the calibration and radiometric stability before and after each of four Shuttle flights and is an update to the results reported after the first SSBUV flight. For the more recent flights, laboratory calibration precision continues to be better than 1%. Instrument sensitivity continued to degrade by 1% to 3% after each flight, and is wavelength dependent. Measurements of the solar irradiance in the wavelength region 200 nm to 400 nm have a precision of 1% to 2% from one flight to the next. Comparison of the SSBUV and the NOAA-11 SBUV-2 solar irradiance measurements indicates that the SBUV-2 instrument is degrading less than 1% per year at wavelengths longer than 300 nm and by as much as 6% per year at wavelengths shorter than 250 nm.

Rotationally resolved midultraviolet studies of Triton and the Pluto/Charon system I: IUE results

We present a uniform analysis of the full set of usable International Ultraviolet Explorer (IUE) long wavelength spectra of Pluto+Charon and Triton. Our objective is to study the variation in UV photometric properties of these objects with rotational phase. The data presented here contain useful information in the range 2550–3200 Å. In the case of the Pluto+Charon system, there are eight spectra spanning about 30% of the rotational lightcurve. Six of these spectra were selected for analysis on the basis of S/ N; each good quality spectrum required an integration of 6–13 hr to obtain. Owing to IUE's limited angular resolution, the data represent the combined spectra of Pluto and Charon. Based on analysis of these data, we report the first evidence of UV lightcurve variation in the Pluto system, finding that the ultraviolet albedo decreases substantially between 0.45 and 0.7 rotational phase; this is in contrast to the visible lightcurve, which reaches a maximum near the 0.65 rotational phase. This implies the visible-UV color difference increases as Pluto reaches its maximum bolometric brightness. Based on the IUE data, we suggest that such behavior could be caused by a UV surface absorption feature on Pluto or Charon. In the case of Triton, seven spectra were obtained, and three 12- to 13- hr integrations were selected for analysis. All seven measurements were made near greatest E or W elongation from Neptune. We find typical UV albedos on Triton in accord with Voyager photopolarimeter results, and a hemispheric lightcurve asymmetry in accord with the sense of Voyager visible imaging measurements. The UV hemispheric asymmetry is ∼3× higher than the Voyager-derived visible-light hemisphere asymmetry. Over the 1-year time-base 1988–1989, we found no significant change in Triton's UV reflectance properties. Comparing Pluto to Triton, we found that Pluto's UV albedo is much lower than Triton's, but that the amplitude of Pluto's UV lightcurve is greater than Triton's. The lower UV albedo of Pluto's surface indicates that fresh frost may have been more recently deposited on Triton than on Pluto.