Michael E. Vanhoosier

Validation of the UARS solar ultraviolet irradiances: Comparison with the ATLAS 1 and 2 measurements

The measurements of the solar ultraviolet spectral irradiance made by the two Upper Atmosphere Research Satellite (UARS) solar instruments, Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) and SOLar STellar Irradiance Comparison Experiment (SOLSTICE), are compared with same-day measurements by two solar instruments on the shuttle ATmospheric Laboratory for Applications and Science (ATLAS) missions, ATLAS SUSIM and Shuttle Solar Backscatter UltraViolet (SSBUV) experiment. These measurements from the four instruments agree to within the 2σ uncertainty of any one instrument, which is 5 to 10% for all wavelengths above 160 nm and for strong emission features below 160 nm. Additionally, the long-term relative accuracy of the two UARS data sets is better than the original 2% goal, especially at wavelengths greater than 160 nm. This level of agreement is credited to accurate preflight calibrations coupled with comprehensive inflight calibrations to track instrument degradation. Two solar irradiance spectra, 119 to 410 nm, are presented; the first combines observations from UARS SUSIM and UARS SOLSTICE taken on March 29, 1992, during the ATLAS 1 mission, and the second combines spectra for April 15, 1993, during the ATLAS 2 mission. The ATLAS 1 mission coincided with the initial decline from the maximum of solar cycle 22 when solar activity was relatively high. The ATLAS 2 mission occurred somewhat later during the declining phase of the solar cycle 22 when solar activity was more moderate.

The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) experiment on board the Upper Atmosphere Research Satellite (UARS)

The state of solar ultraviolet irradiance measurements in 1978, when NASA requested proposals for a new generation of solar ultraviolet monitors to be flown on the Upper Atmosphere Research Satellite (UARS), is described. To overcome the radiometric uncertainties that plagued the measurements at this time, the solar ultraviolet spectral irradiance monitor (SUSIM) instrument design included in-flight calibration light sources and multichannel photometers. Both are aimed at achieving a maximum precision of the SUSIM measurements over a long period of time, e.g., one solar cycle. The design of the SUSIM-UARS instrument is compared with the original design specifications for the UARS instruments. Details including optical train, filters, detectors, and contamination precautions are described. Also discussed are the SUSIM-UARS preflight calibration and characterization, as well as the results of the inflight performance of the instrument during the first 3 months of operation. Finally, flight operations, observation strategy, and data reduction schemes are outlined.

Observations of the solar irradiance in the 200–350 nm interval during the ATLAS‐1 Mission: A comparison among three sets of measurements‐SSBUV, SOLSPEC, and SUSIM

The SOLSPEC, SSBUV, and SUSIM spectrometers simultaneously observed the solar spectral irradiance during the ATLAS-1 mission flown on board the Space Shuttle Atlantis in March 1992. The three instruments use different methods and means of absolute calibration and were each calibrated preflight and postflight. The three data sets are reported from 200 to 350 nm at 1.1 nm resolution. The method of comparing the three independent data sets is discussed. The importance of a common, precise wavelength scale is shown when comparing the data in wavelength regions of strong Fraunhofer lines. The agreement among the solar irradiance measurements is better than ±5%. The fact that the calibrations of the three instruments were based on three independent standards provides confidence that the absolute solar spectral irradiance in the range 200–350 nm is now known with an accuracy better than ±5%. The mean ATLAS-1 solar spectrum is compared with simultaneous solar observations from the UARS SOLSTICE and UARS SUSIM instruments. The two mean solar spectra agree to within ±3%.

SUSIM/UARS observations of the 120 to 300 nm flux variations during the maximum of the solar cycle: Inferences for the 11‐year cycle

Since October 1991, the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) on board the Upper Atmosphere Research Satellite (UARS) has been measuring the solar spectral irradiance from 120 to 400 nm with low (5 nm) and moderate (1.1 nm) resolution on a daily basis. By scaling the modulation measured over four solar rotations to proxy indicators of solar activity, we estimate the amplitude of the solar ultraviolet irradiance variation during the 11-year activity cycle, extending the wavelength coverage of existing empirical variability models to 120 nm and distinguishing the variability of the emission lines from that of the underlying solar continuum. These estimates are compared with results from another empirical variability model and from direct measurements.

Solar ultraviolet spectral-irradiance observations from the SUSIM-UARS experiment

The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) instrument on board the Upper Atmosphere Research Satellite (UARS) has measured solar ultraviolet spectral-irradiance values since October 1991. The calibration and tracking methods are described. The intensity values have a precision of 1% at wavelengths longer than 200 nm. Short-term ultraviolet (UV) variability is correlated with other chromospheric indices at wavelengths shorter than 280 nm. At longer wavelengths, this correlation changes gradually into an anticorrelation. There exists a long-term solar-cycle component at all wavelengths, which seems to be independent of the rotational modulation. The variability in the integrated UV from 110 nm to 300 nm amounts to 33% of the variability in the total solar irradiance.

A high precision Solar Ultraviolet Spectral Irradiance Monitor for the wavelength region 120–400 nm

There exists a growing need to improve the accuracy of measurement of the absolute solar flux within the wavelength range 120–400 nm. Although full-disk solar fluxes and variations thereof in the 120–400 nm region are required to model the solar atmosphere, current increased interest in the measurements arises from their importance in modeling the terrestrial atmosphere. We describe the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) experiment under development at the Naval Research Laboratory (NRL) for flight aboard the Space Shuttle and the Upper Atmospheric Research Satellite (UARS). SUSIM will monitor the solar flux in the 120–400 nm region with high precision, using an in-flight calibration system to reduce absolute error to < 10%, and error relative to the 400 nm continuum to < 1%.

Experience with Schumann-type XUV film on Skylab

The Naval Research Laboratory XUV solar spectrograph (S082A) and spectroheliograph (S082B) in the Apollo Telescope Mount of Skylabrequired Schumann-type photographic film in quantities greater than had ever before been needed. The procurement, testing, handling, and processing of this film are described. Eastman Kodak type 104 and a small quantity of type 101 were used. All problems that were anticipated were met satisfactorily, and excellent results were obtained. Two new problems arose; fog associated with the stainless steel carriers and a large reduction of contrast and maximum density for the flight film that was exposed to the space vacuum.

Calibration of the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) on ATLAS-2

Uncertainty in the calibration of the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) produces an uncertainty in the measurement of solar irradiance. The three primary sources of uncertainty in the irradiance during the ATLAS-2 mission are from wavelength determination, irradiance standard, and the in-flight ageing correction. The wavelength determination leads to an uncertainty in the irradiance in both the calibration data and the solar observations. The first effect is important only at short wavelengths. The second effect is important wherever the solar spectrum is steep but can be eliminated by summing over spectral features. The uncertainty in the irradiance standard is about 2%. The ageing correction produces a wavelength-dependent uncertainty which is typically 1% to 2%. The total 1σ uncertainty in the ATLAS-2 SUSIM irradiance measurement (130 nm to 410 nm) is 2% to 4%. For the ATLAS-3 mission, the performance of the instrument has been enhanced by an improved wavelength determination, a new D2 lamp and power supply, and the addition of a filter wheel in place of a fixed entrance filter.

Irradiance Observations from the UARS/SUSIM and ATLAS/SUSIM Experiments

The SUSIM (Solar Ultraviolet Spectral Irradiance Monitor) on board the UARS (Upper Atmosphere Research Satellite) has measured the solar UV output from 120 nm to 400 nm on a daily basis since October 1991. A reference channel records a solar spectrum semi-annually only to reduce the instrument degradation of this channel and to provide long-term stability marks. Four deuterium lamps are used at monthly, semi-annual and annual intervals to provide long term calibration of the instrument. A preliminary analysis of the long term stability of SUSIM-UARS indicates that the precision of the instrument should be better than a few percent. The repeatability of two scans is better than 0.2%. A simplified SUSIM instrument is flying on NASA’s ATLAS Spacelab missions anually to provide calibration points for the SUSIM-UARS.