Search

The three-channel spectrometer (TKS) aboard the Vega 2 spacecraft recorded infrared and visible spectra near the nucleus of comet Halley. Spectra in the range 0.95–1.9 µm at a distance of 300 km from the nucleus reveal the H2O 1.38-µm band and OH bands, the latter excited by dissociation of a parent molecule. Their production rates are 4 × 1029 and 1.7 × 1030 molecules s−1, respectively. The dust particle density is 0.35 g cm−3, typical of fluffy particles of interplanetary dust. Emissions of 10 species are observed in the range 275–715 nm at ∼4,000 km from the nucleus, and 6 of them are analysed in terms of production rates: QOH = 2×1030, QCH = 1029, = 6×1027, = 3×1027, QCN = 1027 and QNH = 2 × 1026, all in molecules s−1. The difference between the OH and H2O production rates is puzzling and needs further analysis. Ionic emissions, not discussed here, are prominent in the tailward part of the comet.

International audience; The global hydrogen Lyman α, helium (584 angstroms), and molecular hydrogen band emissions from Saturn are qualitatively similar to those of Jupiter, but the Saturn observations emphasize that the H2 band excitation mechanism is closely related to the solar flux. Auroras occur near 80° latitude, suggesting Earth-like magnetotail activity, quite different from the dominant Io plasma torus mechanism at Jupiter. No ion emissions have been detected from the magnetosphere of Saturn, but the rings have a hydrogen atmosphere; atomic hydrogen is also present in a torus between 8 and 25 Saturn radii. Nitrogen emission excited by particles has been detected in the Titan dayglow and bright limb scans. Enhancement of the nitrogen emission is observed in the region of interaction between Titan's atmosphere and the corotating plasma in Saturn's plasmasphere. No particle-excited emission has been detected from the dark atmosphere of Titan. The absorption profile of the atmosphere determined by the solar occultation experiment, combined with constraints from the dayglow observations and temperature information, indicate that N2 is the dominant species. A double layer structure has been detected above Titan's limb. One of the layers may be related to visible layers in the images of Titan.

International audience; The Voyager Ultraviolet Spectrometer (UVS) is an objective grating spectrometer covering the wavelength range of 500–1700 Å with 10 Å resolution. Its primary goal is the determination of the composition and structure of the atmospheres of Jupiter, Saturn, Uranus and several of their satellites. The capability for two very different observational modes have been combined in a single instrument. Observations in the airglow mode measure radiation from the atmosphere due to resonant scattering of the solar flux or energetic particle bombardment, and the occultation mode provides measurements of the atmospheric extinction of solar or stellar radiation as the spacecraft enters the shadow zone behind the target. In addition to the primary goal of the solar system atmospheric measurements, the UVS is expected to make valuable contributions to stellar astronomy at wavelengths below 1000 Å.

Mass spectra of cometary dust particles measured by the PIA dust particle analyzer aboard the Giotto spacecraft show some unexpected and striking features. First, small particles below 10 to the -14th g are much more abundant than anticipated by models. Second, most of the particles are rich in light elements such as H, C, N, and O, suggesting the validity of models that describe the cometary dust as including organic material. Third, the light elements specifically seem to have a low ratio of mass to volume. Three examples of original mass spectra showing typical compositions are given; these have been measured, and are compared with a computer-simulated mass spectrum.

International audience; Eight ultra-violet monochromatic pictures at the limb of the sun are analyzed for evidence of granulation in this spectral range (λ = 2000 Å). The spectral window (125 Å) is defined by two interference filters. These high-resolution pictures were obtained with a 20 cm Cassegrain telescope carried at 30 500 m of altitude by a stratospheric balloon. The resolution of the telescope is 1″. The instrument is mounted on a sunpointing control of excellent short time stability (2″). The pictures were taken at intervals of 210 seconds with a 0.25 second exposure. Large brilliant irregularities 10″ to 20″ in diameter are interpreted as chromospheric microfaculae with a half-hour lifetime. On the other hand, the smaller irregularities are interpreted as ultra-violet granulation. Using the correlation and structure functions, the analysis of the density distribution shows that spatial correlation of these irregularities is limited to a 4″ range.; Huit photographies monochromatiques (λ=2000 Å) au bord du soleil sont analysées en vue de mettre en évidence la granulation dans le domaine ultra-violet lointain. La fenêtre spectrale est définie par deux filtres interférentiels qui donnent une bande passante à mi-hauteur de 125 Å. Ces clichés de haute résolution ont été obtenus en ballon stratosphérique à une altitude de 30 500 m en profitant d'une fenêtre de transparence atmosphérique et en utilisant un télescope Cassegrain de 20 cm d'ouverture dont la résolution atteint 1″. Ce télescope était porté par un dispositif pointeur de soleil dont la stabilité à court terme était meilleure que 2″. Les clichés ont été pris à intervalles de 210 secondes avec un temps de pose de 0.25 seconde. On observe d'une part des variations de brillance grossières de largeur comprise entre 10″ et 20″ qui sont interprétées comme microfacules chromosphériques de durée de vie supérieure à 1/2 heure. On observe d'autre part une granulation dont la distribution densitométrique pour laquelle on a calculé les fonctions de corrélation et de structure présente une corrélation limitée à une distance angulaire de 4″.

International audience; Observation of the sodium dayglow in the scattered sunlight observed on the surface of the earth with a magnetic scanning photometer is reported. The intensity (September to November 1960) is unexpectedly strong—30 kilorayleighs toward the zenith—and evidence is that during the period studied the amount of sodium is 4 times as large in the daytime as at twilight. However, a strong correlation is found between the dayglow and twilight intensities. The abundance can be as high as 40×109 atoms/cm2. The possible role of water vapor absorption in the results is discussed.

International audience; The method that has been used in the past to determine the position of superpressure balloons employed in long- term meteorological and technological experiments-projects GHOST (Global Horizontal Sounding Technique), EOLE (French, God of the Winds), SOMEX (Solar Monitoring Experiment), ctc.-was to measure the solar elevation during the daylight hours and then compute the balloon position at local noon: latitude by the noon solar altitude and longitude by the time of balloon noon. This method has disadvantages: it requires a lengthy series of measure ments during the day and has degraded accuracy when the maximum solar angle approaches 90’. Another method was therefore devised in which the solar angle data are complemented by data that indicate the local geomagnetic rigidity; the correlation of the two data yields an improved position determination algorithm. This method was applied to the trajectory determination of the SOMEX balloons with considerable success.

Balmer alpha emissions of intensity larger than three times the nominal value have been recorded during five different nights at geomagnetic latitudes inferior to plus or minus 22 deg . They are correlated with sudden impulses in magnetograms. (auth)

International audience; OBSERVATIONS of the intensity of the zodiacal light at 6530 Å have been made with a photometer on board the satellite D2A ‘Tournesol’. This satellite was launched on April 15, 1971, from Kourou (French Guiana) into an orbit of inclination 46° with a perigee of 456 km and an apogee of 700 km. The spin axis was oriented towards the Sun with an accuracy of ∼15 arc min and a spin period of 60 s (Fig. 1). The line of sight of the photometer is perpendicular to the spin axis and therefore scans the celestial sphere in a plane orthogonal to the ecliptic plane and to the direction of the Sun. Observations in a plane perpendicular to the Earth-Sun line (that is, at elongation ε=90° or ε=−90°) were carried out during the night phase, when the solar depression angle was >25° and the altitude of the lowest observed points was >135 km. Because of this, the signal contained no contribution from the night-glow, but was only due to zodiacal light and starlight. The data cover, without interruption, the period from April 19, 1971, to June 1973.