Dietrich Labs

THE SOLAR SPECTRAL IRRADIANCE FROM 200 TO 2400 nm AS MEASURED BY THE SOLSPEC SPECTROMETER FROM THE ATLAS AND EURECA MISSIONS

The SOLar SPECtrum (SOLSPEC) and the SOlar SPectrum (SOSP) spectrometers are two twin instruments built to carry out solar spectral irradiance measurements. They are made of three spectrometers dedicated to observations in the ultraviolet, visible and infrared domains. SOLSPEC flew with the ATmospheric Laboratory for Applications and Science (ATLAS) while SOSP flew on the EUropean Retrieval CArrier (EURECA) missions. ATLASxa01 and 2 data being already published, this paper is mostly dedicated to the ATLASxa03 and EURECA data in the IR domain. Comparisons between the ATLAS data sets and the Upper Atmosphere Research Satellite (UARS) results are made. EURECA IR data are shown and compared with previous results. Our best UV, visible and IR spectra are finally merged into a single absolute solar irradiance spectrum covering the 200 to 2400xa0nm domain.

Solar Irradiance Reference Spectra

The solar spectrum is a key input for the study of the planetary atmospheres. It allows the understanding through theoretical modeling of the atmospheric properties (e.g., composition and variability). Furthermore, a reference model is useful for the preparation of instruments and platforms to be operated in space. New composite solar irradiance spectra are formed from 0.1 to 2400 nm using recent measurements for two distinct time periods during solar cycle 22. These two time periods correspond to the activity levels encountered during the ATmospheric Laboratory for Applications and Science (ATLAS) Space Shuttle missions which were moderately high (ATLAS 1, March 1992) and low (ATLAS 3, November 1994). The two reference times span approximately half of the total solar cycle amplitude in terms of the Mg II and F10.7 indices. The accuracy of the two presented spectra varies from 40% in the X-ray range to a mean of 3% in the UV, visible, and near IR ranges. After integration over all wavelengths, a comparison with the total solar irradiance measured at the same time shows an agreement of the order of 1%.

The Visible Solar Spectral Irradiance from 350 to 850 nm as Measured by the Solspec Spectrometer During the Atlas I Mission

The SOLSPEC instrument has been built to carry out solar spectral irradiance measurements from 200 to 3000 nm. It consists of three spectrometers designed to measure the solar spectral irradiance in ultraviolet, visible, and infrared domains. It flew with the ATLAS I mission in March 1992. This paper is dedicated to the visible part of the solar spectrum. Comparisons with recent data are shown and differences below 450 nm are discussed.

Ultraviolet solar irradiance measurement from 200 to 358 nm during spacelab 1 mission

The paper presents the results obtained from the UV-spectrometer of the ‘Solar Spectrum Experiment’ during the Spacelab 1 mission in December 1983. The irradiance data concern 492 passbands, which are located between 200 and 358 nm at almost equidistant wavelengths separated by about 0.3 nm. The passbands have a well-defined, bell-shaped profile with a full width at half maximum of about 1.3 nm. The data, which have an error budget between 4 and 5%, agree closely with the spectral distributions observed by Heath (1980) and Mentall et al. (1981) and confirm that the solar irradiance and the fluxes of Sun-like stars show about the same spectral distribution down to at least 240 nm.

Observation of the solar spectral irradiance from 200 nm to 870 nm during the ATLAS 1 and ATLAS 2 missions by the SOLSPEC spectrometer

The SOLSPEC instrument was built to measure the solar spectral irradiance from 200 nm to 3000 nm. The instrument participated in the three ATLAS missions. It consists of three spectrometers, which measure the solar spectral irradiance in the ultraviolet, visible and infrared domains. The ultraviolet and visible solar spectra obtained during the ATLAS 1 mission have been presented elsewhere. Measurements during the second flight are presented in this paper. Most of the ATLAS 1 results are confirmed by the ATLAS 2 measurements, but some improvements are obtained in certain wavelength domains owing to better calibration procedures and selection of measurements. The data gathered during the first two flights by the SSBUV, SUSIM and SOLSPEC spectrometers are compared and discussed.

OBSERVATION OF THE UV SOLAR SPECTRAL IRRADIANCE BETWEEN 200 AND 350 nm DURING THE ATLAS I MISSION BY THE SOLSPEC SPECTROMETER

The SOLSPEC instrument has been built to carry out solar spectral irradiance measurements from space. It consists of three spectrometers designed to measure the solar spectral irradiance from 180 to 3000 nm. It flew for the first time in December 1983 with the SpaceLab 1 mission (SL1) and later with the ATLAS missions after significant improvement of the instrument optics and calibration procedures. For the ATLAS 1 mission in March 1992, the thermal conditions encountered during the measurements were better than those of SL1, leading to better data quality. Furthermore, other Sun spectrometers, two on the same platform and two others on board the Upper Atmosphere Research Satellite, have also carried out UV absolute spectral measurements at the same time. These opportunities allowed comparisons of solar irradiance determinations. The UV part of the measurements made during that mission is presented here as well as its calibration and accuracy analysis.

An instrument to measure the solar spectrum from 170 to 3200 nm on board Spacelab

This instrument, at the present time in development, will fly on board Spacelab I in May 1983. Other flights are foreseen during the following missions. This instrument is composed by three double monochromators covering the range 170 to 3200 nm. The spectrometers have band-passes of 1 nm up to 900 nm and 20 nm from 850 to 3200 nm with an accuracy 10−2 nm. Calibration lamps are included in the instrument to monitor any change of its sensitivity and wavelength scale.

Calibration of the SOLSPEC spectrometer to measure the solar irradiance from space

The SOLSPEC instrument has been built to carry out absolute solar spectral irradiance measurements from space. It was first flown in December 1983 on mission STS 9 (Spacelab 1), from August 1992 to June 1993 on board the free-flying EURECA platform with mission STS 46 (deployment) and STS 57 (retrieval), then flown three times more with the ATLAS missions STS 45, 56 and 66. Further applications are foreseen in combination with the International Space Station Alpha (ISSA). SOLSPEC covers a wavelength range from 180 nm to 3000 nm split into three separate spectrometer channels (ultraviolet, visible and infrared) and has been calibrated pre- and post-flight using a black body at the Landessternwarte Heidelberg with temperatures up to 3300 K. In this paper the calibration concept is presented, together with a short instrument description and suggestions for improvements in order that higher black-body temperatures can be reached.

Irradiance Observations in Near-UV, Visible and Near Infrared Spectral Bands from Measurements Carried out during ATLAS-1 and EURECA-1 Missions

For the ATLAS and EURECA missions, we have used two identical instruments to measure the absolute solar spectral irradiance from 180 to 3200 nm. These instruments are calibrated by use of a blackbody and a set of lamp standards. The measurements are carried out with 1 nm bandpass up to 800 nm and 20 nm above. The instruments and calibration procedures are described by Thuillier et al . (1981). The platform capability of instruments retrieval after measurements allows a post-flight calibration which is essential for accurate measurements. The main results obtained up to now are: - In the UV, the ATLAS-1 and EURECA-1 solar spectral irradiance are consistent with the SpaceLab 1 data obtained in 1983 (Labs et al . 1987). Figure 1 shows the ATLAS 1 and SL 1 spectra. The origin of the existing differences is presently under investigation. - In the visible domain, our measurements agree with the solar spectrum from Neckel & Labs (1984) within a few percent difference at certain wavelength. - In the IR domain, the preliminary processing shows a spectrum close to the one obtained by Thekaekara (1974).