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).     

Coronal Holes and Icosahedral Symmetry

The study of the expansion of the solar wind out of a system of coronal holes is continued. To this end, we consider the numerical integration of partial differential equations for problems with icosahedral symmetry, in general. First, employing Weyl theory, orbifold coordinates are introduced. Second, the icosahedral coordinates are discussed in detail. Third, following an analysis of the properties of these coordinates and the derivation of a few expressions useful for grid construction, various alternatives for the distribution of lattice points required for numerical integration are considered. A comparison of these numerical grids motivates the choice of a specific grid optimized for the numerical integration carried out in the accompanying paper by Kalish et al.(2002). This revised version was published online in July 2006 with corrections to the Cover Date.     

Vertical variations of the chlorophyll maximum during a red tide in a shallow lagoon

The results of eight sets of repeated observations on the vertical variations of the chlorophyll maximum layer in a shallow lagoon during a red tide show that these were more frequently hydrologically induced, rather than due to active vertical migrations of the red tide-forming organism. These results are discussed and compared to those existing in the literature, with special regard to the role of light and nitrogen in conditioning vertical migrations in red tide-forming dinoflagellates.     

An overview and general results of the Lopez island OBS experiment

The purpose of the experiment was to determine the effects of coupling and bottom currents on ocean bottom seismometers. Twelve operational OBSs, three specially designed three-component systems, and a hydrophone were compared with each other. Unlike seismometers placed on hard rock at land stations, ocean bottom seismometers can be affected by soft sediments (which act as lossy mechanical springs) and by buoyancy. Coupling through soft sediments can modify the response to ground motion much as a low pass filter does, and high buoyancy tends to counteract this effect. These effects are observed in the Lopez data, which consist of signals from mechanical transient tests, cap shots, airgun pulses, and general background noise. The modification of response is pronounced for some instruments and barely noticeable in others. Instruments that stand high in the water relative to their base width tend to be susceptible to rocking motion that shows up as a mechanical cross coupling between horizontal and vertical motion. Correlation of Lopez results with coupling theory suggests that it is possible to design ocean bottom seismometers that will couple well to any sediment. Current levels at the Lopez site (<5 cm s^-1) were too small to produce noticeable effect on any of the instruments; however, the same design criteria that will minimize coupling problems will also lessen problems caused by ocean currents.Hawaii Institute of Geophysics Contribution No. 1171.     

Water vapor adsorption by sodium montmorillonite at −5°C

A large amount of interest has recently been expressed pertaining to the quantity of physically adsorbed water by the Martian regolith. Thermodynamic calculations based on experimentally determined adsorption and desorption isotherms and extrapolated to subzero temperatures indicate that physical adsorption of more than one or two monomolecular layers is highly unlikely under Martian conditions. Any additional water would find ice to be the state of lowest energy and therefore the most stable form. To test the validity of the thermodynamic calculations we have measured adsorption and desorption isotherms of sodium montmorillonite at ?5°C. To a first approximation it was found to be valid.     

Evolution of the fogwater composition in Strasbourg (France) from 1990 to 1999

The chemical composition of fogwater has been studied in the city of Strasbourg (France) from 1990 to 1999. During these years, fogwater samples have been collected and analysed for major ions and trace metals. This paper reports on the analysis of the collected dataset. The analysis revealed a significant decrease in acidity of approximately one pH unit over the course of the study. This decrease in acidity appears to be linked to a decrease in SO_2(g) and the resulting SO₄²⁻. Trace metal concentrations have also strongly decreased over the 10-year period. Pb concentrations, following the elimination of leaded gasoline, decreased by more than one order of magnitude.     

Direct observations of aluminosilicate weathering in the hyporheic zone of an Antarctic Dry Valley stream

This study focused on chemical weathering and bacterial ecology in the hyporheic zone of Green Creek, a McMurdo Dry Valley (Antarctica) stream. An in situ microcosm approach was used to observe dissolution features on the basal-plane surface of muscovite mica. Four mica chips were buried in December 1999 and dug up 39 d later. Atomic force microscopy (AFM) of the basal-plane surfaces revealed small, anhedral ∼10-Å-deep etch pits covering ∼4% of the surfaces, from which an approximate basal-plane dissolution rate of 8.3 × 10⁻¹⁸ mol muscovite cm⁻² s⁻¹ was calculated (on the basis of the geometric surface area) for the study period. This is an integrated initial dissolution rate on a fresh surface exposed for a relatively brief period over the austral summer and should not be compared directly to other long-term field rates. The observation of weathering features on mica agrees with previous stream- and watershed-scale studies in the Dry Valleys, which have demonstrated that weathering occurs where liquid water is present, despite the cold temperatures.AFM imaging of mica surfaces revealed biofilms including numerous small (<1 μm long), rounded, oblong bacteria. The AFM observations agreed well with X-ray photoelectron microscopy results showing increased organic C and N. Bacteriologic analysis of the hyporheic zone sediments also revealed <1-μm-long bacteria. α-Proteobacteria were observed, consistent with the oligotrophic conditions of the hyporheic zone. Nitrate-reducing bacteria were found, in agreement with a previous tracer test at Green Creek that suggested nitrate reduction occurs in the hyporheic zone. The results of this study thus provide direct evidence of dynamic geochemical and microbial processes in the hyporheic zone of a Dry Valley stream despite the extreme conditions; such processes were inferred previously from stream-scale hydrogeochemical studies.     

Integrated Two-Dimensional Modeling of Fluid Flow and Compaction in a Sedimentary Basin

In this paper we employ mixed finite elements and numerically study an integrated two-dimensional model of fluid flow and compaction in a sedimentary basin. This model describes a single phase incompressible flow in a two-dimensional section of a sedimentary basin with vertical compaction. At each time step, an iterative algorithm is used to solve this model. The determination of the grid movement is based on the mass conservation and movement of sediments in the basin, while the mixed method is utilized to solve the fluid flow over the moving grid. Numerical experiments are presented to verify this iterative algorithm and show representative solutions for the model under consideration.     

A field comparison of indicators of sublethal stress in the salt-marsh grassSpartina patens

There is a need for research into bioindicators of stress in threatened plant communities such as coastal wetlands. Land subsidence, diversion of sediment, and salt-water intrusion produce stresses associated with waterlogging, elevated salinity, and nutrient depletion. Temporal and spatial environmental variation (soil redox potential, interstitial water salinity, pH, ammonium and phosphorus, and cation and trace metal concentrations) was analyzed near Lake de Cade, Louisiana, in a brackish marsh which is a mosaic of healthy plant communities interspersed with areas where wetland loss is occurring. Environmental variation was related to indicators of stress inSpartina patens, which included variables derived from the adenine nucleotide levels in plants, leaf spectral reflectance, leaf proline concentrations, and shoot elongation. In a comparison of burned and unburned sites, streamside and inland marsh, and along a salinity gradient, among-site differences were found in spectral reflectance and adenine-nucleotide-related indicators. Although it was difficult to relate a single causal environmental variable to the response of a specific indicator, spectral reflectance in the visible light range responded to salinity or to elements borne in seawater, and adenine-nucleotide indices were sensitive to nutrient availability. The ability of indicators to detect plant responses changed during the growing season, suggesting that they were responding to the changing importance of different environmental factors. In addition, some reflectance indicator responses occurred along salinity gradients when salinity differences were less than those that were found to have ecologically meaningful effects in greenhouse experiments. A multivariate numerical approach was used to relate environmental variation with indicator responses. We concluded that factors which in combination cause the degradation and loss of Louisiana wetlands produce environmental conditions that are only subtly different from those in vigorously growing marsh communities.