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1.
Calibrating the GOCE accelerations with star sensor data and a global gravity field model 总被引:1,自引:0,他引:1
A reliable and accurate gradiometer calibration is essential for the scientific return of the gravity field and steady-state
ocean circulation explorer (GOCE) mission. This paper describes a new method for external calibration of the GOCE gradiometer
accelerations. A global gravity field model in combination with star sensor quaternions is used to compute reference differential
accelerations, which may be used to estimate various combinations of gradiometer scale factors, internal gradiometer misalignments
and misalignments between star sensor and gradiometer. In many aspects, the new method is complementary to the GOCE in-flight
calibration. In contrast to the in-flight calibration, which requires a satellite-shaking phase, the new method uses data
from the nominal measurement phases. The results of a simulation study show that gradiometer scale factors can be estimated
on a weekly basis with accuracies better than 2 × 10−3 for the ultrasensitive and 10−2 for the less sensitive axes, which is compatible with the requirements of the gravity gradient error. Based on a 58-day data
set, scale factors are found that can reduce the errors of the in-flight-calibrated measurements. The elements of the complete
inverse calibration matrix, representing both the internal gradiometer misalignments and scale factors, can be estimated with
accuracies in general better than 10−3. 相似文献
2.
Several recent studies have reported quasi-periodicities with a period between 1 and 2 years (to be called here `mid-term
quasi-periodicities') in various heliospheric parameters, like solar wind speed, interplanetary magnetic field, cosmic rays,
and geomagnetic activity. Here we study their long-term occurrence in geomagnetic activity using an extended aa index which
covers the last 15 solar cycles. We confirm their intermittent occurrence and the alternation of their dominant period between
a slightly shorter period of about 1.2–1.4 years and a slightly longer period of about 1.5–1.7 years. We find that the mid-term
quasi-periodicities were strong during two intervals of high solar activity: in the mid-19th century and since 1930. Instead,
contrary to earlier studies, we find that they were consistently weak during low solar activity from 1860s to 1920s. This
implies a long-term connection between the amplitude of mid-term quasi-periodicities and the solar dynamo strength. Since
the rotation speed at the bottom of the solar convection layer (tachocline) has recently been found to vary at a mid-term
periodicity, this suggests that the stronger the solar dynamo is, the more variable the rotation rate of the tachocline is.
We also note that the disappearance of mid-term periodicities may be used as a precursor for long intervals of very weak solar
activity, like great minima. 相似文献
3.
4.
Joana F.M.F. Cardoso Denis Langlet Jos F. Loff Ana R. Martins Johannes IJ. Witte Paulo T. Santos Henk W. van der Veer 《Journal of Sea Research》2007,57(4):303-315
The Pacific oyster Crassostrea gigas was introduced in Europe for commercial purposes in the mid 1960s. It was initially thought that low winter temperatures would restrain this species' reproduction and settlement; however, its present distribution in areas where no introduction has taken place suggests that natural invasion and expansion has occurred. Along the European coast, wild populations of Pacific oysters are already found from northern Germany to southern Portugal. Whether C. gigas will continue to further expand through northern waters will depend on its physiological performance. In this study, the performance of wild oyster populations has been studied in terms of growth and reproduction at three stations: La Rochelle (France; 46°N), Yerseke (Oosterschelde estuary, The Netherlands, 51°N), and Texel (Wadden Sea estuary, The Netherlands, 53°N). The French population had the lowest somatic-shell mass ratio and an increase in maximum shell length, somatic and gonadal mass was observed from France to the Netherlands. In addition, mean oocyte diameter decreased significantly from south to north. The combination of increasing gonadal mass and decreasing oocyte volume suggests an increasing reproductive output in terms of egg numbers from France to The Netherlands. Differences in temperature between locations will at least be partly responsible for the observed patterns; however, other environmental factors (such as food availability, predation pressure, sediment type and/or seston concentration) cannot be excluded. Since smaller eggs (oocytes) are thought to have a longer development time, the environmental conditions along the Dutch coast may result in increased larval dispersal and possibly in further population expansion. 相似文献
5.
Joana F.M.F. Cardoso Johannes IJ. Witte Henk W. van der Veer 《Journal of Sea Research》2007,57(4):316-324
The bivalve Spisula subtruncata is usually abundant in shallow coastal waters along the Dutch coast. However, its biomass has been decreasing since 1995. In order to assess whether reproductive failure may be the cause of the observed decline over the last decades, the energy investment in reproduction of a population of S. subtruncata from central Dutch coastal waters was studied. The population studied consisted of individuals of up to four years old. Shell length reached maximum values of around 32 mm and individual total body, somatic and gonadal ash-free dry mass reached maximum values of about 278 mg AFDM, 252 mg AFDM and 76 mg AFDM, respectively. A clear seasonal cycle in somatic and gonadal mass was observed. Somatic and gonadal mass indices increased in early spring and reached maximum values during summer, followed by a decrease to minimum values at the beginning of the following year. Spawning was in June–July and settlement of spat seems to have occurred in July–August. Mean oocyte diameter was 57.43 ± 0.03 μm, corresponding to a volume of 98972 μm3. These results suggested that reproductive failure was not the cause of the current population decline. Most likely, unsuccessful settlement of spat and/or severe predation during the first months of life were responsible for the observed patterns. 相似文献
6.
Submarine groundwater discharge (SGD) into a coastal lagoon off Perth, Western Australia, contains nitrate and silicate in concentrations two orders of magnitude higher than those of the receiving waters. This discharge delivers enough nitrate to replace that dissolved in the lagoon water mass about every eight days and enough silicate to replace the lagoon silicate in about 48 days. The delivery rate of nitrate nitrogen by SGD is equal to about 48% of that required for observed growth rates of lagoon macrophytes. Surface salinity is lower close to the shore as a result of SGD. During calm conditions a salinity front was observed in the lagoon, with a nearshore pool of nutrient-enriched water floating above the more saline ocean water. 相似文献
7.
Siberian river run-off and Late Quaternary glaciation in the southern Kara Sea, Arctic Ocean: preliminary results 总被引:3,自引:0,他引:3
Ruediger Stein Frank Niessen Klaus Dittmers Michael Levitan Frank Schoster Johannes Simstich Tatjana Steinke Oleg V. Stepanets 《Polar research》2002,21(2):315-322
The extent of the Barents-Kara Ice Sheet during the eastern Last Glacial Maximum (LGM) is not yet fully known. A detailed echo-sounding survey performed during the Boris Petrov Expedition 2001 permitted the detailed mapping of part of it. Based on the profiling results, a southern connection between the LGM Barents-Kara Ice Sheet and a local ice sheet on Taymyr Peninsula appears to be unlikely. Based on sediment core data and profiling results, most of the terrigenous river-derived material accumulated in the estuaries during late Holocene times, whereas during early Holocene times of lowered sea level major amounts were transported further offshore and accumulated on the shelf. During the post-glacial sea level rise, the main depocentre migrated southward, reaching its present position no earlier than about 6 cal. Ky BP (or 5.2 Kya). Future studies of accelerator mass spectrometry (AMS) 14 C-dated sediment cores will allow a detailed reconstruction of the variability of fluvial sediment discharge and the history of glaciation in the Kara Sea during late Quaternary times. 相似文献
8.
M. Krämer Ri. Müller H. Bovensmann J. Burrows J. Brinkmann E. P. Röth J.-U. Grooß Ro. Müller Th. Woyke R. Ruhnke G. Günther J. Hendricks E. Lippert K. S. Carslaw Th. Peter A. Zieger Ch. Brühl B. Steil R. Lehmann D. S. McKenna 《Journal of Atmospheric Chemistry》2003,45(1):51-77
Several stratospheric chemistry modules from box, 2-D or 3-D models, have been intercompared. The intercomparison was focused on the ozone loss and associated reactive species under the conditions found in the cold, wintertime Arctic and Antarctic vortices. Comparisons of both gas phase and heterogeneous chemistry modules show excellent agreement between the models under constrained conditions for photolysis and the microphysics of polar stratospheric clouds. While the mean integral ozone loss ranges from 4–80% for different 30–50 days long air parcel trajectories, the mean scatter of model results around these values is only about ±1.5%. In a case study, where the models employed their standard photolysis and microphysical schemes, the variation around the mean percentage ozone loss increases to about ±7%. This increased scatter of model results is mainly due to the different treatment of the PSC microphysics and heterogeneous chemistry in the models, whereby the most unrealistic assumptions about PSC processes consequently lead to the least representative ozone chemistry. Furthermore, for this case study the model results for the ozone mixing ratios at different altitudes were compared with a measured ozone profile to investigate the extent to which models reproduce the stratospheric ozone losses. It was found that mainly in the height range of strong ozone depletion all models underestimate the ozone loss by about a factor of two. This finding corroborates earlier studies and implies a general deficiency in our understanding of the stratospheric ozone loss chemistry rather than a specific problem related to a particular model simulation. 相似文献
9.
The beginning of dehydration melting in the tonalite system (biotite-plagioclase-quartz) is investigated in the pressure
range of 2–12 kbar. A special method consisting of surrounding a crystal of natural plagioclase (An45) with a biotite-quartz mixture, and observing reactions at the plagioclase margin was employed for precise determination
of the solidus for dehydration melting. The beginning of dehydration melting was worked out at 5 kbar for a range of compositions
of biotite varying from iron-free phlogopite to iron-rich Ann70, with and without titanium, fluorine and extra aluminium in the biotite. The dehydration melting of phlogopite + plagioclase
(An45) + quartz begins between 750 and 770°C at pressures of 2 and 5 kbar, at approximately 740°C at 8 kbar and between 700 and
730°C at 10 kbar. At 12 kbar, the first melts are observed at temperatures as low as 700°C. The data indicate an almost vertical
dehydration melting solidus curve at low pressures which bends backward to lower temperatures at higher pressures (> 5 kbar).
The new phases observed at pressures ≤ 10 kbar are melt + enstatite + clinopyroxene + potassium feldspar ± amphibole. In addition
to these, zoisite was also observed at 12 kbar. With increasing temperature, phlogopite becomes enriched in aluminium and
deficient in potassium. Substitution of octahedral magnesium by aluminium and titanium in the phlogopite, as well as substitution
of hydroxyl by fluorine, have little effect on the beginning of dehydration melting temperatures in this system. The dehydration
melting of biotite (Ann50) + plagioclase (An45) + quartz begins 50°C below that of phlogopite bearing starting composition. Solid reaction products are orthopyroxene +
clinopyroxene + potassium feldspar ± amphibole. Epidote was also observed above 8 kbar, and garnet at 12 kbar (750°C). The
experiments on the iron-bearing system performed at ≤ 5 kbar were buffered with NiNiO. The f
O
2 in high pressure runs lies close to CoCoO. With the substitution of octahedral magnesium and iron by aluminium and titanium,
and replacement of hydroxyl by fluorine in biotite, the beginning of dehydration melting temperatures in this system increase
up to 780°C at 5 kbar, which is 70°C above the beginning of dehydration melting of the assemblage containing biotite (Ann50) of ideal composition. The dehydration melting at 5 kbar in the more iron-rich Ann70-bearing starting composition begins at 730°C, and in the Ann25-bearing assemblage at 710°C. This indicates that quartz-biotite-plagioclase assemblages with intermediate compositions of
biotite (Ann25 and Ann50) melt at lower temperatures as compared to those containing Fe-richer or Mg-richer biotites. This study shows that the dehydration
melting of tonalites may begin at considerably lower temperatures than previously thought, especially at high pressures (>5
kbar).
Received: 27 December 1995 / Accepted: 7 May 1996 相似文献
10.
Dehydration melting of tonalitic compositions (phlogopite or biotite-plagioclase-quartz assemblages) is investigated within
a temperature range of 700–1000°C and pressure range of 2–15 kbar. The solid reaction products in the case of the phlogopite-plagioclase(An45)-quartz starting material are enstatite, clinopyroxene and potassium feldspar, with amphiboles occurring occasionally. At
12 kbar, zoisite is observed below 800°C, and garnet at 900°C. The reaction products of dehydration melting of the biotite
(Ann50)-plagioclase (An45)-quartz assemblage are melt, orthopyroxene, clinopyroxene, amphibole and potassium feldspar. At pressures > 8 kbar and temperatures
below 800°C, epidote is also formed. Almandine-rich garnet appears above 10 kbar at temperatures ≥ 750°C. The composition
of melts is granitic to granodioritic, hence showing the importance of dehydration melting of tonalites for the formation
of granitic melts and granulitic restites at pressure-temperature conditions within the continental crust. The melt compositions
plot close to the cotectic line dividing the liquidus surfaces between quartz and potassium feldspar in the haplogranite system
at 5 kbar and a
H
2O = 1. The composition of the melts changes with the composition of the starting material, temperature and pressure. With increasing
temperature, the melt becomes enriched in Al2O3 and FeO+MgO. Potash in the melt is highest just when biotite disappears. The amount of CaO decreases up to 900°C at 5 kbar
whereas at higher temperatures it increases as amphibole, clinopyroxene and more An-component dissolve in the melt. The Na2O content of the melt increases slightly with increase in temperature. The composition of the melt at temperatures > 900°C
approaches that of the starting assemblage. The melt fraction varies with composition and proportion of hydrous phases in
the starting composition as well as temperature and pressure. With increasing modal biotite from 20 to 30 wt%, the melt proportion
increases from 19.8 to 22.3 vol.% (850°C and 5 kbar). With increasing temperature from 800 to 950°C (at 5 kbar), the increase
in melt fraction is from 11 to 25.8 vol.%. The effect of pressure on the melt fraction is observed to be relatively small
and the melt proportion in the same assemblage decreases at 850°C from 19.8 vol.% at 5 kbar to 15.3 vol.% at 15 kbar. Selected
experiments were reversed at 2 and 5 kbar to demonstrate that near equilibrium compositions were obtained in runs of longer
duration.
Received: 27 December 1995 / Accepted: 7 May 1996 相似文献