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Upper Atmosphere (upper + atmosphere)
Selected AbstractsNiches of the pre-photosynthetic biosphere and geologic preservation of Earth's earliest ecologyGEOBIOLOGY, Issue 2 2007NORMAN H. SLEEP ABSTRACT The tree of terrestrial life probably roots in non-photosynthetic microbes. Chemoautotrophs were the first primary producers, and the globally dominant niches in terms of primary productivity were determined by availability of carbon dioxide and hydrogen for methanogenesis and sulfite reduction. Methanogen niches were most abundant where CO2 -rich ocean water flowed through serpentinite. Black smoker vents from basalt supplied comparable amount of H2. Hydrogen from arc volcanoes supported a significant methanogenic niche at the Earth's surface. SO2 from arc volcanoes reacted with organic matter and hydrogen, providing a significant surface niche. Methane ascended to the upper atmosphere where photolysis produced C-rich haze and CO, and H escaped into space. The CO and C-rich haze supported secondary surface niches. None of these ecologies were bountiful; less than 1% of the CO2 vented by ridge axes, arcs, and metamorphism became organic matter before it was buried in carbonate. In contrast, a photosynthetic biosphere leaves copious amounts of organic carbon, locally concentrated in sediments. Black shales are a classic geologic biosignature for photosynthesis that can survive subduction and high-grade metamorphism. [source] Photoabsorption processes in nitrous oxide and formaldehydeINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2001I. Martín Abstract Absorption oscillator strengths and photoionization cross sections for electronic transitions involving Rydberg states that are relevant to the photochemistry of N2O and H2CO are reported. These compounds have been found to play an important role in the evolution of Earth's upper atmosphere. However, the difficulties encoutered in both laboratory measurements and theoretical calculations on the photoabsorption of these compounds are responsible for the scarcity of data in the literature. The present calculations have been performed with the molecular-adapted quantum defect orbital (MQDO) method, of which the adequacy for this type of studies has recently been assessed. A comparative analysis of the photoabsorption intensities in the molecules and their constituting atoms has enabled us to predict the variation of the extent of atomic character of the molecular Rydberg orbitals with the degree of excitation. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 [source] Laser guide stars for extremely large telescopes: efficient Shack,Hartmann wavefront sensor design using the weighted centre-of-gravity algorithmMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009L. Schreiber ABSTRACT Over the last few years increasing consideration has been given to the study of laser guide stars (LGS) for the measurement of the disturbance introduced by the atmosphere in optical and near-infrared (near-IR) astronomical observations from the ground. A possible method for the generation of a LGS is the excitation of the sodium layer in the upper atmosphere at approximately 90 km of altitude. Since the sodium layer is approximately 10 km thick, the artificial reference source looks elongated, especially when observed from the edge of a large aperture. The spot elongation strongly limits the performance of the most common wavefront sensors. The centroiding accuracy in a Shack,Hartmann wavefront sensor, for instance, decreases proportionally to the elongation (in a photon noise dominated regime). To compensate for this effect, a straightforward solution is to increase the laser power, i.e. to increase the number of detected photons per subaperture. The scope of the work presented in this paper is twofold: an analysis of the performance of the weighted centre of gravity algorithm for centroiding with elongated spots and the determination of the required number of photons to achieve a certain average wavefront error over the telescope aperture. [source] An XMM,Newton observation of Ton S180: constraints on the continuum emission in ultrasoft Seyfert galaxiesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2002S. Vaughan Abstract We present an XMM,Newton observation of the bright, narrow-line, ultrasoft type 1 Seyfert galaxy Ton S180. The 0.3,10 keV X-ray spectrum is steep and curved, showing a steep slope above 2.5 keV (,, 2.3) and a smooth, featureless excess of emission at lower energies. The spectrum can be adequately parametrized using a simple double power-law model. The source is strongly variable over the course of the observation but shows only weak spectral variability, with the fractional variability amplitude remaining approximately constant over more than a decade in energy. The curved continuum shape and weak spectral variability are discussed in terms of various physical models for the soft X-ray excess emission, including reflection off the surface of an ionized accretion disc, inverse Compton scattering of soft disc photons by thermal electrons, and Comptonization by electrons with a hybrid thermal/non-thermal distribution. We emphasize the possibility that the strong soft excess may be produced by dissipation of accretion energy in the hot, upper atmosphere of the putative accretion disc. [source] Living with an active starASTRONOMY & GEOPHYSICS, Issue 3 2001Andy Breen Observations have shown that the Sun is far from being a stable, unchanging object. Rather, we live within the outer atmosphere of a variable star. Andy Breen presents a summary of a meeting that drew together new results from observations and computational modelling of the Sun, interplanetary space and the upper atmosphere of the Earth. [source] | ||||||||||