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41.
Michael Mueller Willem Jellema Bertrand Delforge David Teyssier Martin Eggens Robert Huisman Geert Keizer 《Experimental Astronomy》2014,37(2):369-379
HIFI is a heterodyne spectrometer aboard the Herschel Space Observatory, providing high-spectral-resolution capabilities. Of its seven frequency bands, four (bands 3, 4, 6, and 7) employ Martin-Puplett diplexers to combine sky signal and local oscillator at the two linear polarizations H and V, prior to feeding them into the mixers (receivers). The optical path difference in each of these 8 diplexers must be tuned to the observed frequency. The required actuator currents were determined in flight before the start of routine science observations. We here report on regular (roughly quarterly) engineering test observations to validate the repeatability of the HIFI diplexers during the routine phase of Herschel operations. We find the optical path difference to be stable to within 0.4 % of the relevant wavelength, typically at the sub-micron level. We conclude that the repeatability and precision of the diplexer tuning mechanism are so high that science data are in no way negatively affected. With the diplexer calibration established and validated, this line of reasoning can be reversed, and the diplexers can be used as relative spectrometers to measure the local-oscillator frequency, i.e., to check the spectral purity of the local oscillator across the diplexer bands. This was done from before launch out to the last months of cryogenic operations in space. 相似文献
42.
Ronan Higgins David Teyssier Colin Borys Jonathan Braine Claudia Comito Bertrand Delforge Frank Helmich Michael Olberg Volker Ossenkopf John Pearson Russell Shipman 《Experimental Astronomy》2014,37(2):433-452
The Heterodyne Instrument for the Far Infrared (HIFI) on board the Herschel Space Observatory is composed of a set of fourteen double sideband mixers. We discuss the general problem of the sideband ratio (SBR) determination and the impact of an imbalanced sideband ratio on the line calibration in double sideband heterodyne receivers. The HIFI SBR is determined from a combination of data taken during pre-launch gas cell tests and in-flight. The results and some of the calibration artefacts discovered in the gas cell test data are presented here along with some examples of how these effects appear in science data taken in orbit. 相似文献
43.
Oxfordian reefal episodes of Lorraine and Burgundy have a long time been considered as contemporaneous. Biostratigraphic data and sequential evolutions peculiar to each region indicate their structural autonomy during Oxfordian times. A north‐south‐oriented well‐logging transect shows that, during the Middle Oxfordian, a shallow reefal platform developed in Lorraine while thin deeper deposits occurred in Burgundy. In spite of their different ages, reefal episodes of Middle Oxfordian in Lorraine and Upper Oxfordian in Burgundy exhibit a broadly similar vertical evolution of coral communities. During the Late Oxfordian, the contemporaneous occurrence of a diversified assemblage in the Burgundy region, a colder coral assemblage characterized by eurytopic genera and the decrease in seawater isotopic temperatures in Lorraine can be explained by a shift in trophic conditions, a climatic change related to structural rearrangements in this strategic place and a modification of oceanic circulations between the arctic and the Tethyan regions. 相似文献
44.
Characterization of potential zones of dust generation at eleven stations in the southern Sahara 总被引:2,自引:0,他引:2
Summary Synoptic wind data for multi-decadal periods at eleven stations located in the southern Sahara region (Agadez, Atar, Bilma, Dori, Gao, Kayes, Nema, Niamey, Nouadhibou, Ouagadougou and Tessalit) are used to study the monthly dust deflation power over the region. We found that, regardless of the conditions of the soil, the deflation power (or wind efficiency) is not sufficient to generate significant amounts of aerosols south of 15°N. North of this latitude, the deflation power is much larger, with potential zones of either very strong deflation (Nouadhibou and Bilma) or severe deflation (Gao, Tessalit, Nema, Atar, Agadez). Stations in the Sahel region such as Gao, Agadez and Tessalit are characterized by a gradual reinforcement of the deflation power between 1970 and 1984 in correspondence of increasing desertification over the region. During this same period, Bilma, a well know region of dust source, experienced a major reduction in deflation power due to shifts in large scale wind patterns. 相似文献
45.
The transfer of reactive bromine into the atmosphere was recently observed by Hönninger et al. [Hönninger, G., Bobrowski, N., Palenque, E.R., Torrez, R., Platt, U., 2004. Reactive bromine and sulfur emission at salar de Uyuni, Bolivia. Geophys. Res. Lett.31, doi:10.1029/2003GL018818] in a large salt pan of the Bolivian Altiplano: the salar de Uyuni. However, bromide is considered to be an excellent conservative tracer, which leads to the questioning of its actual conservation in surficial geochemical processes. The relation between bromide and lithium, thought to be a conservative component in waters and brines of the Central Andes, points to a depletion of Br relative to Li in Uyuni brines of about 50-300 kg/day, a flux close to that measured by Hönninger et al. (2004): ?200 kg/day. Such values are very low in regard to the size of the salar (5-30 g/km2/day). Salt efflorescences have a much higher surface area than a flat salt crust, which should enhance the release of bromine. Leach solutions of salt efflorescences in closed basins of northern Chile are compared to their parent waters. Conservative components should have the same concentration ratios in both solutions. Actually, a strong depletion in Br is observed in the leach solutions, which could suggest a significant release of Br from the salt into the atmosphere. During the rainy season, efflorescences are leached and their dissolved components brought in saline lakes and salars. Evaporative profiles show a slight but noticeable fractionation between Li and Br which could be due to the contribution of Br-depleted leach solutions of salt efflorescences. Therefore, bromide does not behave conservatively in surface waters and brines of the Central Andes. If the loss of Br is really due to its transfer into the atmosphere, then the flux would be much higher than that estimated for the salar de Uyuni alone. Numerous salt pans, saline lakes, and widespread efflorescences covering large land surfaces would contribute significantly more reactive bromine into the atmosphere. 相似文献
46.
In order to simulate the plume produced by large natural draft cooling towers, a semi-spectral warm cloud parameterization has been implemented in an anelastic and non-hydrostatic 3D micro-scale meteorological code. The model results are compared to observations from a detailed field experiment carried out in 1980 at Bugey (location of an electrical nuclear power plant in the Rhône valley in East Central France) including airborne dynamical and microphysical measurements. Although we observe a slight overestimation of the liquid-water content, the results are satisfactory for all the 15 different cases simulated, which include different meteorological conditions ranging from low wind speed and convective conditions in clear sky to high wind and very cloudy. Such parameterization, which includes semi-spectral determination for droplet spectra, seems to be promising to describe plume interaction with atmosphere especially for aerosols and cloud droplets. 相似文献
47.
Sugata Kaviraj Sebastien Peirani Sadegh Khochfar Joseph Silk Scott Kay 《Monthly notices of the Royal Astronomical Society》2009,394(4):1713-1720
We demonstrate that the large scatter in the ultraviolet (UV) colours of intermediate-mass early-type galaxies in the local Universe and the inferred low-level recent star formation (RSF) in these objects can be reproduced by minor mergers in the standard Λ cold dark matter (ΛCDM) cosmology. Numerical simulations of mergers with mass ratios ≤1:4, with reasonable assumptions for the ages, metallicities and dust properties of the merger progenitors, produce good agreement with the observed UV colours of the early-type population, if the infalling satellites are assumed to have (cold) gas fractions ≥20 per cent. Early-types that satisfy ( NUV − r ) ≲ 3.8 are likely to have experienced mergers with mass ratios between 1:4 and 1:6 within the last ∼1.5 Gyr, while those that satisfy 3.8 < ( NUV − r ) < 5.5 are consistent with either recent mergers with mass ratios ≤1:6 or mergers with higher mass ratios that occurred more than ∼1.5 Gyr in the past. We demonstrate that the early-type colour–magnitude relations and colour distributions, in both the UV and optical spectral ranges, are consistent with the expected frequency of minor merging activity in the standard ΛCDM cosmology at low redshift. We present a strong plausibility argument for minor mergers to be the principal mechanism behind the large UV scatter and associated low-level RSF observed in early-type galaxies in the nearby Universe. 相似文献
48.
49.
Simon Mardle Sean Pascoe Jean Boncoeur Bertrand Le Gallic Juan J. García-Hoyo Ins Herrero Ramon Jimenez-Toribio Concepcin Cortes Nuria Padilla Jesper Raakjaer Nielsen Christoph Mathiesen 《Marine Policy》2002,26(6)
The main objectives of fisheries management are generally similar throughout the world. These are often stated in policy documents such as the Common Fisheries Policy and the Magnuson–Stevens Fishery Conservation and Management Act. However, at the local level often the key objectives of management are more detailed, characterised by both the overriding management structure and the status and type of fishery concerned. In this paper, we consider case study fisheries from the UK, France, Spain and Denmark to compare some of the various types of fisheries and fisheries management systems that exist in the European Union. From this, we define the key objectives for each management system. 相似文献
50.
Metamorphic CK carbonaceous chondrites display matrix textures that are best explained by a transient thermal event with temperatures in the 550–950 K range and durations in the order of days to years, longer than what is commonly admitted for shock events but shorter than what is required for nuclide decay. We propose that radiative heating of small carbonaceous meteoroids with perihelia close to the Sun could account for the petrological features observed in CK chondrites. Numerical thermal modeling, using favorable known NEOs orbital parameters (perihelion distances between 0.07 and 0.15 AU) and physical properties of CV and CK chondrites (albedo in the range 0.01–0.1, 25% porosity, thermal diffusivity of 0.5–1.5 W m?1 K?1), shows that radiative heating can heat carbonaceous meteoroids in the meter size range to core temperatures up to 1050 K, consistent with the metamorphic temperatures estimated for CK chondrites. Sizes of known CV and CK chondrites indicate that all these objects were small meteoroids (radii from a few cm to 2.5 m) prior to their atmospheric entry. Simulations of dynamic orbits for NEO objects suggest that there are numerous such bodies with suitable orbits and properties, even if they are only a small percentage of all NEOs. Radiative heating would be a secondary process (superimposed on parent-body processes) affecting meteoroids formed by the disruption of an initially homogeneous CV3-type parent body. Different petrologic types can be accounted for depending on the sizes and heliocentric distances of the objects in such a swarm. 相似文献