First operation of bulk micromegas in low pressure negative ion drift gas mixtures for dark matter searches

A bulk micromegas micropattern charge readout device has, for the first time, been operated at room temperature in low pressure carbon disulphide vapour. This is a key step opening prospects for use of micromegas readout for large volume negative ion time projection chambers (TPCs) without magnets, such as proposed for directional dark matter detectors and other rare event applications. The dependence of the gain on the amplification field, pressure and drift field has been evaluated. For the available gap size of 75 μm a maximum gain of 1300 ± 120 was achieved in 40 torr vapour with an energy resolution of 22% for 5.9 keV ⁵⁵Fe X-rays. From a fit to the data, the Townsend coefficient gas parameters A and B have been derived. Operation has also been successfully achieved in xenon:carbon disulphide blends over a range of partial and total pressures. A gain of 890 ± 130 at an energy resolution of 35% has been recorded for a 1:1 blend at a total pressure of 80 torr. Possible improvements are discussed in the context of operation in directional dark matter TPCs as a replacement for multi-wire proportional counters.     

ECHA J0843.3–7905: Discovery of an ''old'' classical T Tauri star in the η Chamaeleontis cluster

Cold, dense clouds of gas have been proposed to explain the dark matter in Galactic haloes, and have also been invoked in the Galactic disc as an explanation for the excess faint submillimetre sources detected by SCUBA. Even if their dust-to-gas ratio is only a small percentage of that in conventional gas clouds, these dense systems would be opaque to visible radiation. We examine the possibility that the data sets of microlensing experiments searching for massive compact halo objects can also be used to search for occultation signatures by such clouds. We compute the rate and time-scale distribution of stellar transits by clouds in the Galactic disc and halo. We find that, for cloud parameters typically advocated by theoretical models, thousands of transit events should already exist within microlensing survey data sets. We examine the seasonal modulation in the rate caused by the Earth's orbital motion and find it provides an excellent probe of whether detected clouds are of disc or halo origin.     

The high hydraulic conductivity of three wooded tropical peat swamps in northeast Peru: measurements and implications for hydrological function

The form and functioning of peatlands depend strongly on their hydrological status, but there are few data available on the hydraulic properties of tropical peatlands. In particular, the saturated hydraulic conductivity (K) has not previously been measured in neotropical peatlands. Piezometer slug tests were used to measure K at two depths (50 and 90 cm) in three contrasting forested peatlands in the Peruvian Amazon: Quistococha, San Jorge and Buena Vista. Measured K at 50 cm depth varies between 0.00032 and 0.11 cm s^?1, and at 90 cm, it varies between 0.00027 and 0.057 cm s^?1. Measurements of K taken from different areas of Quistococha showed that spatial heterogeneity accounts for ~20% of the within‐site variance and that depth is a good predictor of K. However, K did not vary significantly with depth at Buena Vista and San Jorge. Statistical analysis showed that ~18% of the variance in the K data can be explained by between‐site differences. Simulations using a simple hydrological model suggest that the relatively high K values could lead to lowering of the water table by >10 cm within ~48 m of the peatland edge for domed peatlands, if subjected to a drought lasting 30 days. However, under current climatic conditions, even with high K, peatlands would be unable to shed the large amount of water entering the system via rainfall through subsurface flow alone. We conclude that most of the water leaves these peatlands via overland flow and/or evapotranspiration. Copyright © 2013 John Wiley & Sons, Ltd.     

Controls on spatial variability in snow accumulation on glaciers in the Southern Alps,New Zealand; as revealed by crevasse stratigraphy

Net accumulation measurements from two glaciers located on opposite sides of the New Zealand Southern Alps were used to explore processes controlling spatial variability. The degree of variability, as measured by the spatial variogram, differed in each of the three successive years, but the lowest variance occurred on both glaciers in March 2008, after a hot and dry summer. Strong relationships between net accumulation and elevation within the accumulation area were only found on Franz Josef Glacier (FJG), despite this being the primary control used in glacier mass balance modelling. Interaction between wind and topography was found to be important to the distribution of net accumulation on both glaciers. The crevasse stratigraphy method is an ideal way to gain good spatial coverage of net accumulation, and particularly suited to glaciers with high annual precipitation. Copyright © 2010 John Wiley & Sons, Ltd.     

Dewfall features of a tropical station: The case of Onne (Port Hartcourt), Nigeria

Summary This study presents the results of an analysis of the dewfall features of a tropical station. The importance of dewfall cannot be overemphasized, especially during the dry season when plants are under seriously stress. A positive correlation was found between dewfall and rainfall. Dew formation was found to peak on a diurnal basis between 6 a.m. to 8 a.m., suggesting that some steering mechanism is required for dew formation at this station. Dew formation starts later at night during the dry season but much earlier during the rainy season. The August anomaly already highlighted by Ireland showed up in dew formation as there was a marked reduction in dew formation in that month.

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Zusammenfassung Diese Untersuchung behandelt den Tauabsatz einer tropischen Station. Die Bedeutung des Taus kann nicht überbetont werden, besonders in der Trockenzeit, wenn die Pflanzen unter starkem Wasserstress stehen. Es fand sich ein positiver Zusammenhang zum Niederschlag. Im Tagesgang liegt das Maximum der Taubildung zwischen 6 und 8 Uhr, was einen steuernden Prozeß zugrunde liegen haben dürfte. In der Trockenzeit beginnt die Taubildung später als in der Regenzeit. Die schon von Ireland hervorgehobene Anomalie im August mit deutlich geringeren Werten zeigte sich auch hier.

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With 3 Figures     

Flow visualization experiments on stably stratified flow over ridges and valleys

Summary Flow visualization experiments on stably stratified flow over ridges and valleys formed by a pair of ridges were conducted in a large towing tank filled with stratified salt water. The ambient flow was normal to the axis of the ridges. Three experimental parameters were varied during the study: the steepness of the ridges, the separation distance between the ridges and the Froude number. The flow field in the valley was strongly influenced by flow separation from the lee side of the upstream ridge. For the gentle and intermediate ridges with maximum slopes of 13° and 27°, this separation was controlled by the lee waves when their wavelength was less than or equal to the width of the ridge. The flow field in the valley was similar to that downstream of a single ridge. For the steep ridge with a maximum slope of 40°, separation from the lee side of the upstream ridge and the flow field in the valley were influenced significantly by the presence of the downstream ridge.With 13 FiguresOn assignment from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce.     

Gender,migration and the organisation of work under economic devolution: Ecuador, 1982-90

This study examined the impact of economic deficits due to structural adjustment processes on shifts in the organization of work by gender and migration status in Ecuador. Work is organized according to Lawson's social hierarchy scheme: ownership; authority and control over employees; autonomy in one's own work; and the nature and range of skills used in production. After a brief review of the related empirical literature, the author describes the concepts, categories of, and study area of work and then begins the empirical analysis. Data were obtained from 1,884,816 individual records of economically active persons in 1982 and 2,946,547 persons in 1990, from the censuses of 1982 and 1990 for the entire nation, and from fieldwork observations by Lawson. Structural adjustment policies (SAPs) associated with devolution tend to further aggravate inequities, especially among the disadvantaged. Findings are presented for male and female nonmigrants, migrants, and female migrants. During the 1980s, female migrants experienced primary economic activity, especially as self-employed, family, or low skilled employees; and declines in high skilled public sector employment and service activity, especially in wage labor. The economic impact was greater by gender than by migration status. The shifts only improved the relative position of women in self-employed and ownership jobs. Females lost public-sector employment to males; overall wage declines were more severe in the informal sector. Down-sizing in the public sector and shifts toward capital-intensive production marginalized female migrants. Fieldwork operationalizes losses among females/female migrants.     

Stable Carbon and Nitrogen Isotopic

The perennially ice-covered lakes in the McMurdo Dry Valleys, Antarctica, are part of the coldest and driest ecosystem on earth. To understand lacustrine carbon and nitrogen cycling in this end-member ecosystem, and to define paleolimnological proxies for ice-covered lakes, we measured the stable carbon and nitrogen isotopic composition of particulate organic matter (POM) and benthic organic matter (BOM) within the lakes of Taylor Valley. The ¹³C compositions of seasonally ice-free edges of the lakes (moats) are enriched relative to under-ice organic matter. Thus, the organic carbon isotopic composition of buried sediments may be a proxy for sample position within the lake. In the moats, ¹³C values are governed by limited CO₂diffusion across benthic cyanobacterial cell membranes. During a high glacial melt (2001–2002) season, both ¹³C_POM and ¹³C_BOM in the moats were more depleted than during previous low melt years. We propose that this occurred in response to higher [CO₂]_(aq) and/or reduced growth rates resulting from turbidity-induced light limitation. Though moats and under-ice environments are usually poorly connected, during the 2001–2002 season, the enrichment of the ¹³C_POM values at 6 m depth in the stream-proximal sites relative to deep-profile sites implies enhanced connectivity between these environments. The ¹³C compositions of BOM and POM profiles in Lake Hoare and Lake Fryxell indicate that these lakes are dominated by benthic productivity. In contrast, in Lake Bonney, the similarity of the ¹³C values of BOM and POM indicates the pelagic component dominance in the carbon cycle.