Experimental simulation of Titan's atmosphere: Detection of ammonia and ethylene oxide

For several years now, an experimental simulation of Titan's atmosphere has been on going at LISA. A cold plasma is established in a gas mixture representative of the atmosphere of the satellite. In these experiments, more than 70 organic compounds have been identified, including the first identification in this type of experimental simulation of C₄N₂ already detected in its solid form on Titan, which suggests that the setup correctly mimics the chemistry of Titan's atmosphere.We have carried out the first experimental simulation including O-containing compounds in order to study the influence of the presence of CO on the chemical behavior of Titan's atmosphere. With the help of gas chromatography–mass spectrometry (GC–MS) and infrared spectroscopy (IRS) we can thus determine which minor species still undetected in Titan's atmosphere are likely to be present and understand the complex chemistry of the atmosphere of this satellite. Surprisingly we have identified unpredicted O-containing gaseous compounds, mainly ethylene oxide (also named oxirane, C₂H₄O). This molecule has been observed in the interstellar medium by observation in the millimeter region (Astrophys. J. 489 (1997) 553; Astron. Astrophys. 337 (1998) 275). On the contrary, the predicted O-compounds (formaldehyde and methanol) have not been identified in this experiment. Furthermore, we have identified NH₃ in the gaseous products with an initial mixture of N₂ (98%) and CH₄ (2%).The paper describes the experimental device used in this work, in particular the IRS and GC–MS techniques. We also comment the results related to the detection of the O-containing compounds and NH₃ and their implications on our knowledge of the chemistry of Titan's atmosphere and on the retrieval of the future Titan data expected from Cassini-Huygens.     

Simulating methane emission from a Chinese rice field as influenced by fertilizer and water level

Natural and agricultural wetlands are considered to be the major sources of global atmospheric methane (CH₄). A one‐dimensional model was developed to simulate methane emission and used to examine the influence of various physical processes on the rate of methane emission. Three processes involved in the methane emission are implemented in the model: production, reoxidation and transport. Three transport pathways were considered: diffusion across water–air or soil–air interfaces, ebullition and diffusion through plants. These pathways are influenced by soil properties, plant growth, water‐table conditions, temperature and external inputs (e.g. fertilizer). The model was used to examine the seasonal variation of the methane emission at a rice field in Hunan, China, which was observed during a field experiment for consecutive (early and late) rice seasons in 1992. The observed seasonal variations of methane emission, and role of plants in transporting methane to the atmosphere, are captured by the model simulation. Further model applications were conducted to simulate effects of fertilizer and water‐level condition on the methane emission. The results indicate that unfermented organic fertilizer produces a higher methane emission rate than mineral fertilizer. The simulations with treatments of a deep‐water covering and constant moisture reduced the methane emission. The rice field study provides a framework for further development of the model towards simulations based on spatially distributed variables (e.g. water table, soil temperature and vegetation) at a regional scale. Copyright © 2003 John Wiley & Sons, Ltd.     

Physical parameters of the atmosphere of Venus from Venera 15 and 16 missions

The following physical parameters have been computed for the atmosphere of Venus between 65 and 90 km, by intervals of 1 km. (1) Pressure, (2) Density, (3) Speed of sound, (4) Number density, (5) Density scale, (6) Pressure scale, (7) Collisional frequency, (8) Mean particle velocity (9) Mean free path, (10) Columnar mass, (11) Viscosity. For these calculations we have used the temperature altitude measurements of Venera 15 and 16 at 52 °N and 72 °N latitudes, the night and 70 °N and 72 °N latitudes the day.     

Frequency distributions and correlations of solar X-ray flare parameters

We have determined frequency distributions of flare parameters from over 12000 solar flares recorded with the Hard X-Ray Burst Spectrometer (HXRBS) on the Solar Maximum Mission (SMM) satellite. These parameters include the flare duration, the peak counting rate, the peak hard X-ray flux, the total energy in electrons, and the peak energy flux in electrons (the latter two computed assuming a thick-target flare model). The energies were computed above a threshold energy between 25 and 50 keV. All of the distributions can be represented by power laws above the HXRBS sensitivity threshold. Correlations among these parameters are determined from linear regression fits as well as from the slopes of the frequency distributions. Variations of the frequency distributions were investigated with respect to the solar activity cycle.Theoretical models for the frequency distribution of flare parameters depend on the probability of flaring and the temporal evolution of the flare energy build-up. Our results are consistent with stochastic flaring and exponential energy build-up, with an average build-up time constant that is 0.5 times the mean time between flares. The measured distributions of flares are also consistent with predicted distributions of flares from computer simulations of avalanche models that are governed by the principle of self-organized criticality.     

Interplanetary dust particles and solar radiation

The problem of the action of the solar radiation on the motion of interplanetary dust particle is discussed. Differences between the action of electromagnetic solar radiation and that of the solar wind are explained not only from the point of view of the physical nature of these phenomena but also from the point of view of dust particle's orbital evolution. As for the electromagnetic solar radiation, general equation of motion for the particle is written and the most important consequences are: (i) the process of inspiralling toward the Sun is not the only possible motion - even spiralling from the Sun is also possible, and, (ii) the orbital plane of the particle (its inclination) may change in time. As for the solar wind, the effect corresponding to the fact that solar wind particles spread out from the Sun in nonradial direction causes that the process of inspiralling toward the Sun is in more than 50% less effective than for radial spread out; in the region of the asteroid belt (long period orbits) the process of inspiralling is changed into offspiralling. Also shift in the perihelion of dust particle's orbit exists.     

A 20 cm2 CCD mosaic camera for a dark matter search part I : Mechanics,optics and cryogeny

A 20cm ² CCD mosaic camera has been especially built to search for dark galactic halo objects by the gravitational microlensing effect. The sensitive area is made of 16 edge-buttable CCDs developped by Thomson-CTS, with 23×23 µm ² pixels. The 35 kg camera housing and mechanical equipment is presented. The associated electronics and data acquisition system are described in a separate paper. The camera resides at the focal plane of a 40 cm, f/10, Ferson reflector. The instrument has been in operation since December 1991 at the La Silla Observatory (ESO).     

The estimation of the shape of an array using a hidden Markov model

A hidden Markov model (HMM) technique for the estimation of the shape of a towed array is presented. It is assumed that there is a far-field source radiating sound containing possibly weak spectral lines. The technique uses either the Fourier coefficients at a given frequency computed from a single time block or the maximal eigenvector of a sample spectral covariance matrix. The technique is illustrated using several simulations. The results of these simulations indicate that the HMM technique yields shape and bearing estimates more accurate than those provided by a maximum-likelihood array shape estimation technique     

Coronal heating

The prospects for observational tests of coronal heating theories are examined. Three heating mechanisms (Alfvén waves, D.C. electric currents, and MHD turbulence) are reviewed from this point of view. Some suggestions are offered for future tests but none, so far, would give unambiguous critical results.Operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.