Michel Hersé

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.

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%.

Waves in the OH emissive layer: photogrammetry and topography

The waves in the OH emissive layer, which appear on photographs of the sky in the near IR taken with large aperture cameras, are distorted by atmospheric refraction and by a perspective effect. Two methods have been developed to allow restitution of the topography of the waves in a simple way. In the first method a grid is computed, which is superimposed on an enlargement of the original 24 x 36-mm negative frame. In the second method the image is projected on an aspherical surface that is tilted with respect to the enlarger beam. The topography of a wave display, photographed from the Pic du Midi Observatory during the night of 19-20 November 1976, is obtained using both methods. The photometric aspect of the photographs may be interpreted under the simple assumption that the emissive layer has a constant thickness and is ruffled like the wavy surface of the sea.

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.

PICARD: Simulataneous measurements of the solar diameter, differential rotation, solar constant and their variations

Abstract PICARD is a CNES micro-satellite mission due for flight by the end of 2002, named after the name of a French astronomer who first observed with consistency the solar diameter changes during the Maunder minimum in the 16th century. It consists of two instruments measuring (i) the solar diameter and differential rotation, and (ii) the total solar irradiance. These quantities are fundamental for the understanding of the solar-Terrestrial relations, e.g. the influence of the Sun on the Earths climate, and of the internal structure of the Sun. The continuous — or nearly continuous — viewing of the Sun from an appropriate orbit, the 5 minutes sampling rate and the very low noise measurements, will allow g-modes detection and precise diameter measurements besides accurately establishing the relationship between irradiance and diameter changes. Providing an absolute measure of the solar diameter to 1 milliarcsecond, PICARD is the first step towards instruments capable of accurate and perennial measurements, for the centuries to come, of the solar-terrestrial influence. The objectives of the mission, instrument capabilities, observing modes and performances are described.

Co-ordinated EISCAT-MICADO interferometer measurements of neutral winds and temperatures in E- and F-regions

Abstract The MICADO instrument has been built to measure temperature and wind in the E - and F -regions. It employs a thermally stable field-compensated Michelson interferometer to allow wind measurements. During the winter of 1988–1989, the MICADO instrument was operated at Sodankyla (67°22′N, Finland). Measurements were made by observing the O 1 S (low thermosphere) and the O 1 D lines (high thermosphere) emission. Two co-ordinated campaigns were organized with the EISCAT radar, which operated in special modes. Neutral wind and temperature are derived from EISCAT data. Results of the two instruments are shown. The differences between the two sets of results are discussed and show that most of the discrepancy is due to the presence of vertical winds during the observations where the magnetic activity was high.

In-Flight Calibration of the Wind Imaging Interferometer (WINDII) on Board the Upper Atmosphere Research Satellite.

The Wind Imaging Interferometer (WINDII) was launched on the Upper Atmosphere Research Satellite to measure the Earths upper-atmospheric wind and temperature. It is a remote-sensing instrument that employs a field-compensated Michelson interferometer to measure the Doppler shift, line width, and emission rate of naturally occurring airglow emission lines. The data analysis uses calibration data that were obtained in the laboratory prior to launch. A calibration package to monitor instrument parameters was built and placed in the instrument. This package consists of a He-Ne laser, spectral lamps, and a tungsten lamp. These sources and their performance during six years of operation in orbit are described. It is shown that the WINDII principle of wind measurement can be assessed fully by the use of in-flight calibration data and that the preflight and in-flight phase calibrations can be related to each other with a precision of the order of 1 ms(-1).

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.

Spectrophotometry of comet P/Halley at wavelengths 275—710 nm from Vega 2

Spectrophotometric observations of the inner coma of comet Halley in the range 275-710 nm were conducted from Vega 2 on March 8, 9, 10, 11, 1986. During the encounter session, when the impact parameter was minimum, p = 120 km, the gaseous emissions were still present, superimposed on an intense continuum. In the inner coma, C2 and C3 emissions steadily increase when the impact parameter decreases. The emissions of OH at 309 nm and CN in its band head at 388 nm show a plateau between 1500 and 600 km. The decrease in OH emission when the impact parameter is less than 500 km is interpreted in terms of high optical thickness of water vapor at Lyman-alpha. Pixel-to-pixel ratios of spectra show that the color of dust continuum presents small fluctuations which might be produced by a discontinuous emission of icy grains.