Abstract. On December 7, 1995, the NASA Galileo probe provided the first in situ measurements of the helium abundance in the atmosphere of Jupiter. Our Jamin interferometer measured precisely the refractive index of the Jovian atmosphere in the pressure region from 2 to 12 bars. From these measurements, we derive the atmospheric helium mole fraction to be 0.1359 _+ 0.0027. The corresponding helium mass fraction matches closely, but accidentally, the current helium abundance of the atmosphere of the Sun. However, both the Jovian and the solar value fall somewhat below the protosolar value. This suggests that in both Jupiter and the Sun processes are active which separate helium from hydrogen. IntroductionThe planet Jupiter differs in major ways from our planet Earth, e.g., by its enormous size, its hydrogen-rich atmosphere, its huge magnetosphere, and its varied satellite system. For these and many more reasons, Jupiter is an object that many scientists look at with intense curiosity. Thus in 1976, NASA announced its intention to explore the giant planet Jupiter, in particular, its atmosphere, magnetosphere, and complex system of satellites by means of both a space probe entering the atmosphere of Jupiter and an orbiter circling the planet for about 2 years. Subsequently, this mission became known as the NASA Galileo mission. The "probe" was to carry a set of instruments into the atmosphere of Jupiter which were to perform in situ measurements of many atmospheric parameters in the pressure range 0.1-19 bars. Two of these instruments were dedicated to composition measurements: a refractometer for high precision measurements of the abundance ratio of He to H 2 in the Jovian atmosphere and a mass spectrometer. The refractometer was given the project name "helium abundance detector" (HAD). The HAD was included in the probe payload because of the great importance placed on obtaining a precision value for the abundance ratio He/H 2.Hydrogen and helium are by far the most abundant elements in the universe, the stars, the Sun, and the outer planets. If known, the mixing ratios of these two elements in stellar and planetary bodies provide important constraints for theories about their origin and evolution. In 1977, when the instruments for the Galileo probe were chosen, the predominant scientific opinion was that the current helium abundance in the Jovian atmosphere is the same as that created in the Big Bang and later present in the solar nebula from which the Sun and the

We have measured a total of 17 in situ profiles of small-scale density fluctuations (typical resolution: meters) in the lower thermosphere and upper mesosphere, which are used to derive turbulent parameters, such as the turbulent energy dissipation rate e, the turbulent diffusion coefficient K, and the mean turbulent velocity W turb. The accuracy of the absolute numbers is unprecedented thanks to the very high spatial resolution and a recently improved data analysis procedure. Concentrating on the 12 flights which were performed during winter conditions at high latitudes (69øN), we find mean energy dissipation rates of 1-2 mW kg -1 in the lower mesosphere (<75 km) and 10-20 mW kg -• in the upper mesosphere and lower thermosphere (<100 km). The corresponding heating rates are approximately 0. 1 and 1 K d -1 , respectively. These values are at least 1 order of magnitude smaller than most of the previous measurements and are also significantly smaller than typical values assumed in models. Our observations suggest that the heating effect of turbulence is negligible compared to the most prevailing terms of the heat budget. It can be shown by theoretical considerations involving the turbulent energy budget equation that cooling by turbulent heat conduction is also negligible if e is small.

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Temperatures and pressures measured by the Galileo probe during parachute descent into Jupiter's atmosphere essentially followed the dry adiabat between 0.41 and 24 bars, consistent with the absence of a deep water cloud and with the low water content found by the mass spectrometer. From 5 to 15 bars, lapse rates were slightly stable relative to the adiabat calculated for the observed H2/He ratio, which suggests that upward heat transport in that range is not attributable to simple radial convection. In the upper atmosphere, temperatures of >1000 kelvin at the 0.01-microbar level confirmed the hot exosphere that had been inferred from Voyager occultations. The thermal gradient increased sharply to 5 kelvin per kilometer at a reconstructed altitude of 350 kilometers, as was recently predicted. Densities at 1000 kilometers were 100 times those in the pre-encounter engineering model.

Abstract. Correlation studies performed on data from recent mesospheric experiments conducted with the 50-MHz Jicamarca radar in May 2003 and July 2004 are reported. The study is based on signals detected from a combination of vertical and off-vertical beams. The nominal height resolution was 150 m and spectral estimates were obtained after ∼1 min integration. Spectral widths and backscattered power generally show positive correlations at upper mesospheric heights in agreement with earlier findings (e.g., Fukao et al., 1980) that upper mesospheric echoes are dominated by isotropic Bragg scatter. In many instances in the upper mesosphere, a weakening of positive correlation away from layer centers (towards top and bottom boundaries) was observed with the aid of improved height resolution. This finding supports the idea that layer edges are dominated by anisotropic turbulence. The data also suggests that negative correlations observed at lower mesospheric heights are caused by scattering from anisotropic structures rather than reflections from sharp vertical gradients in electron density.