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1.
We have determined the real and imaginary indices of refraction (n and k) for six iron oxide/oxyhydroxide phases—magnetite, maghemite, goethite, lepidocrocite, akaganéite, and ferrihydrite. A single crystal of magnetite was used to derive bulk n and k values from 100-2000 cm−1 (5-100 μm). Synthetic nanocrystalline samples of maghemite, goethite, lepidocrocite, akaganéite, and ferrihydrite were pressed into compact pellets used to determine bulk n and k values from 100-1200 cm−1 (8.33-100 μm). All values of n and k (the optical constants) were determined from specular reflectance spectra acquired at 2 cm−1 spectral sampling using classical Lorentz-Lorenz dispersion theory. In this paper, we present the optical constants of all six minerals and the oscillator parameters with which they were modeled. Use of these optical constants could aid in radiative transfer models of terrestrial dust as well as Mars, the Moon, and airless bodies in the Solar System.  相似文献   

2.
Coral reefs are net sinks for C, principally as CaCO3 accretion. It is possible to predict quite accurately the rate of production, given adequate information about any particular reef environment. The best data set for an extensive region is that for the Great Barrier Reef (GBR). Careful analysis of this region and the incorporation of previously documented present day system calcification rates suggest net production (G) from G = 1 (kg CaCO3 m−2 yr−1) for fringing reefs, to G = 1.9 for planar (infiled platform) reefs, G = 3 for ribbon reefs and lagoonal reefs. The 20,055 km2 of reefs in the GBR are thus estimated to average G = 2.4, resulting in a total production of 50 million tonnes yr−1. In a 50–100 year Greenhouse scenario of rising sealevel, we predict that recolonisation of present day reef flats will be extensive and prolific. Production will increase substantially, and this could be by as much as 40% (ranging from 0% for deep shoals to 180% for fringing reefs) to give 70 million tonnes yr−1 if the rate of sealevel rise reaches or exceeds 6–8 mm yr−1We estimate 115,000 km2 of oceanic atolls worldwide. Drawing on points equivalence from the detailed analysis of the GBR, we estimate the atolls presently produce 160 million tonnes yr−1. We predict that a similar 40% increase could be possible in the next 100 years or so resulting in a production of 220 million tonnes.Accepting an existing estimate of 617,000 km2 for reefs worldwide, drawing from our projections from the GBR and the atolls, and making some assumptions about the remaining reef types (we suggest fringing reefs to dominate) we estimate global reef production at the present time to be 900 million tonnes yr−1. Within the next 100 years or so, we suggest this rate could almost double to 1800 million tonnes. In the long term (several centuries) we predict that the continuing trend of recolonisation, particularly of fringing and planar reefs could result in the production of > 3000 million tonnes yr−1 if rates of sealevel rise approaching or exceeding 6–8 mm yr−1 are achieved. Eventually (> 500 yr), reefs could actually “drown” due to inability to match the rate of sealevel increase if that rate significantly exceeds 6–8 mm yr−1.Thus, coral reefs at present act as a sink for 111 million tonnes C yr−1, the equivalent of 2% of present output of anthropogenic CO2. In the short term Greenhouse scenario (100 yr) we predict this could increase to the equivalent of 4% of the present CO2 output. In the much longer term (several centuries), if all trends continue, this could increase to the equivalent of as much as 9% of the present CO2 output.Unfortunately, we also predict that this considerable sink for C will be most likely of negative value in alleviating Greenhouse because of the immediate effect of CaCO3 precipitation is to raise the PCO2 of the surface oceans — ie, ot encourage CO2 efflux to the atmosphere. We do not attempt to quantify this effect.Other Greenhouse changes such as seawater temperature increase, changes in cloud cover, increased rainfall and runoff, increased storm activity, and changes in dissolved CO2 concentration and surface ocean circulation may complicate the reef response. However, we suggest that sealevel rise will be the dominant influence, at least during the next 100 years or so.  相似文献   

3.
Reconstruction of Mediterranean sea level fields for the period 1945–2000   总被引：1，自引：1，他引：0  
The distribution of sea level in the Mediterranean Sea is recovered for the period 1945–2000 by using a reduced space optimal interpolation analysis. The method involves estimating empirical orthogonal functions from satellite altimeter data spanning the period 1993–2005 that are then combined with tide gauge data to recover sea level fields over the period 1945–2000. The reconstruction technique is discussed and its robustness is checked through different tests. For the altimetric period (1993–2000) the prediction skill is quantified over the whole domain by comparing the reconstructed fields with satellite altimeter observations. For past times the skill can only be tested locally, by validating the reconstruction against independent tide gauge records. The reconstructed distribution of sea level trends for the period 1945–2000 shows a positive peak in the Ionian Sea (up to 1.5 mm yr− 1) and a negative peak of − 0.5 mm yr− 1 in a small area to the south-east of Crete. Positive trends are found nearly everywhere, being larger in the western Mediterranean (between 0.5 and 1 mm yr− 1) than in the eastern Mediterranean (between 0 and 0.5 mm yr− 1). The estimated rate of mean sea level rise for the period 1945–2000 is 0.7 ± 0.2 mm yr− 1, i.e. about a half of the rate estimated for global mean sea level. These overall results do not appear to be very sensitive to the distribution of tide gauges. The poorest results are obtained in open-sea regions with intense mesoscale variability not correlated with any tide gauge station, such as the Algerian Basin.  相似文献   

4.
Towards the high-latitude cloud MBM 40, we identify 3 dense molecular cores of M0.2–0.5 M, and sizes of 0.2 pc in diameter embedded in the H I cloud of 8 M which is observed to be extended along the northeast–southwest direction. The molecular cloud is located almost perpendicularly to the H I emission. We confirm the previous result of Magnani et al. that MBM 40 is not a site for new star formations. We found a very poor correlation between the H I and the IRAS 100 μm emissions, but the CO (1–0) and 100 μm emissions show a better correlation of WCO/I100=1±0.2 K km s−1 (MJy sr−1)−1. This ratio is larger by a factor of ≥5 than in dense dark clouds, which may indicate that the CO is less depleted in MBM 40 than in dense dark clouds.  相似文献   

5.
This paper presents data concerning recent (1990–2007) surface morphological and ice-dynamical changes on the Tasman Glacier, New Zealand. We use remote-sensing data to derive rates of lake growth, glacier velocities and rates of glacier surface lowering. Between 1990 and 2007, the glacier terminus receded ~ 3.5 km and a large ice-contact proglacial lake developed behind the outwash head. By 2007 the lake area was ~ 6 km2 and had replaced the majority of the lowermost 4 km of the glacier tongue. There is evidence that lake growth is proceeding at increasing rates — the lake area doubled between 2000 and 2007 alone. Measured horizontal glacier velocities decline from 150 m a− 1 in the upper glacier catchment to almost zero at the glacier terminus and there is a consequent down-glacier increase in surface debris cover. Surface debris mapping shows that a large catastrophic rockfall onto the glacier surface in 1991 is still evident as a series of arcuate debris ridges below the Hochstetter icefall. Calculated glacier surface lowering is most clearly pronounced around the terminal area of the glacier tongue, with down-wasting rates of 4.2 ± 1.4 m a− 1 in areas adjacent to the lateral moraine ridges outside of the current lake extent. Surface lowering rates of approximately 1.9 ± 1.4 m a− 1 are common in the upper areas of the glacier. Calculations of future lake expansion are dependent on accurate bathymetric and bed topography surveys, but published data indicate that a further 8–10 km of the glacier is susceptible to calving and further lake development in the future.  相似文献   

6.
The paper presents simulations of the energetic neutral atom (ENA) production in the Mercury magnetosphere and the obtained ENA images for the equatorial and polar vantage points. The ENA fluxes are found to be 102–103 (cm2 srskeV)−1 and up to 104–105 (cm2 srskeV)−1 in the energy range 10–50 keV. Due to the small size of the magnetosphere, the particles injected in the tail can fill up the entire dayside magnetosphere making possible ENA imaging of the magnetospheric shape. The high variability of the Hermean magnetosphere gives rise to pulsating ENA emissions (ENA “flashes”) which can be used to study the global dynamics. The ENA instrument requirements, 10°×10° angular resolution and 20 s accumulation time, can be easily met by modern ENA instrumentation. Therefore, ENA imaging of the Mercury magnetosphere is feasible.  相似文献   

7.
We report on observations, with sub-parsec resolution, of neutral hydrogen seen in absorption in the λ=21 cm line against the nucleus of the active spiral galaxy NGC 5793. The absorption line consists of three components separated in both location as well as velocity. We derive HI column densities of 2×1022 cm−2 assuming a gas spin temperature of 100 K. For the first time we are able to reliably estimate the HI cloud sizes (≈15 pc) and atomic gas densities (≈200 cm−3). Our results suggest that the HI gas is not associated with the <10 pc region which presumably contains the H2O masers, but it is more distant from the nucleus, and is probably associated with the r1 kpc gas seen in CO.  相似文献   

8.
X-radiation may result from active plasma phenomena in the interactions of comets with the solar wind. We have carried out a limited but sensitive search for soft X-radiation from Comet Bradfield (1979 ), on 1980 Feb. 5. No X-radiation was detected at a level (3σ) of 1.7 × 10−13 erg(cm2sec keV)−1 in the 0.2 – 4.0 kev range. This corresponds to a limit on the power dissipated in the comet by non-thermal electrons of approximately 1019 ergs sec−1, averaged over the 2568-sec exposure to the comet. This energy deposition is near the magnitude suggested by simple theoretical ideas, and further searches of appropriate comets both in soft X-radiation and at radio wavelengths seem warranted.  相似文献   

9.
We describe an imaging telescope for observations of celestial sources in the energy range between 30 keV and 1.8 MeV onboard stratospheric balloons. The detector is a 41 cm diameter, 5 cm thick NaI(Tl) crystal coupled to 19 photomultipliers in an Anger camera configuration. It is surrounded by a plastic scintillator 15 cm thick on the sides, 0.2 cm thick at the top and 20 cm thick at the bottom. The imaging device is based upon a 19 × 19 element square MURA (Modified Uniformly Redundant Array) coded mask mounted in an one-piece mask-antimask configuration. The detector's spatial resolution is about 10 mm at 100 keV. This is the first experiment to use such a mask pattern and configuration for astrophysical purposes. The expected 3 sensitivity for an on-axis source observed for 104 s at a residual atmosphere of 3.5 g cm–2 is 1.44 × 10–5 photons cm–2 s–1 keV–1 at 100 keV and 1.00 × 10–6 photons cm–2 s–1 keV–1 at 1 MeV. The angular resolution is approximately 14 arcminutes over a 13°field of view. The instrument is mounted in an automatic platform with a capability for pointing and stabilization in both azimuth and elevation axis with 2 arcmin accuracy.Presented at the 2nd UN/ESA Workshop, held in Bogotá, Colombia, 9-13 November, 1992.  相似文献   

10.
Three years of regular weekly/biweekly monitoring of seasonal changes in temperature, transparency, chlorophyll a (CHL) and bacteria [erythrosine-stained microscopic counts and cultivable colony forming units (CFUs)] at the vertical profile in the South basin of Lake Baikal (51°54′195″N, 105°04′235″E, depth 800 m) were evaluated. In more detail, the structure and function of phytoplankton and the microbial loop in the euphotic layer at the same site were investigated during the late-winter–early-spring period under the ice. The depth of euphotic zone (up to 1% of surface irradiation) was 35 to 40 m. Primary production was measured three times a week with the 14C method in 2, 10, 20, 30 and 40 m. Maximum production was found in 10 m, with lower values towards the surface (light inhibition) and towards the lower layers. The total production in cells larger than 1 μm in the column (0–40 m) was 204–240 mg C d−1 m−2, 30–40% of it being in cells 1–3 μm (mostly picocyanobacteria), which represented roughly 9% of the total chlorophyll a (estimated from pigment analyses). A major part of phytoplankton biomass was formed by diatoms (Synedra acus Hust., Asterionella formosa Hass. and Stephanodiscus meyerii Genkal & Popovskaya). Total production (including extracellular, dissolved organic matter) was 235–387 mg C day−1 m−2, and the exudates were readily used by bacteria (particles 0.2–1 μm). This part amounted to 1–5% of cellular production in 2 to 20 m and 11–77% of cellular production in 20–40 m, i.e., in light-limited layers. From 0 to 30 m, chlorophyll a concentration was 0.8 to 1.3 μg l−1, wherefrom it decreased rapidly to 0.1 μg l−1 towards the depth of 40 m. Bacteria (DAPI-stained microscopic counts) reached 0.5–1.4×106 ml−1; their cell volumes measured via image analysis were small (average 0.05 μm−3), often not well countable when erythrosine stain was used. Bacterial biomasses were in the range of 6–21 μg C l−1. Numbers of colony forming units (CFUs) on nutrient fish-agar were c. 3–4 orders lower than DAPI counts. The amounts of heterotrophic protists were low, whereby flagellates reached 6 to 87 ml−1 and ciliates, 0.2–1.2 ml−1 (mostly Oligotrichida). Bacterial production was measured in the same depths as primary production using 3H-thymidine (Thy) and 14C-leucine (Leu) uptake. Consistently, bacterial abundances, biomasses, thymidine and leucine production were higher by 30–50% in layers 2, 10 and 20 m compared with that in the deeper 30 and 40 m, where cellular primary production was negligible. Leucine uptake in the deeper layers was even three times lower than in the upper ones. From the comparison of primary and bacterial production, bacteria roughly use 20–40% of primary production during 24 h in the layers 2 to 20 m.  相似文献   

11.
An ASCA observation of the Jovian impact of the comet Shoemaker-Levy 9 is reported. Four impacts of H, L, Q1 and R were observed and four impacts of B, C, G, and Q2 were observed within 60 minutes after their impacts. No significant flaring of X-ray emission was observed. Upper limit X-ray fluxes of 90 % confidence level, averaged 5 minutes just after the impacts, were 2.4 × 10–13 erg sec–1 cm–2, 3.5 × 10–13 erg sec–1 cm–2, 1.6 × 10–13 erg sec–1 cm–2 and 2.9 × 10–13 erg sec–1 cm–2 for the impacts of H, L, Q1 and R, respectively, in the 0.5(0.7 for H and Q1)–10 keV energy range. However, a hint of X-ray enhancement around Jupiter from July 17 to July 19 was detected with about 2 6 × 10–14 erg sec–1 cm–2 in the 0.5–10 keV energy range.  相似文献   

12.
Colliding comets in the Solar System may be an important source of gamma ray bursts. The spherical gamma ray comet cloud required by the results of the Venera Satellites (Mazets and Golenetskii, 1987) and the BATSE detector on the Compton Satellite (Meeganet al., 1992a, b) is neither the Oort Cloud nor the Kuiper Belt. To satisfy observations ofN(>P max) vsP max for the maximum gamma ray fluxes,P max > 10–5 erg cm–2 s–1 (about 30 bursts yr–1), the comet density,n, should increase asn a 1 from about 40 to 100 AU wherea is the comet heliocentric distance. The turnover above 100 AU requiresn a –1/2 to 200 AU to fit the Venera results andn a 1/4 to 400 AU to fit the BATSE data. Then the masses of comets in the 3 regions are from: 40–100 AU, about 9 earth masses,m E; 100–200 AU about 25m E; and 100–400 AU, about 900m E. The flux of 10–5 erg cm–2 s–1 corresponds to a luminosity at 100 AU of 3 × 1026 erg s–1. Two colliding spherical comets at a distance of 100 AU, each with nucleus of radiusR of 5 km, density of 0.5 g cm–3 and Keplerian velocity 3 km s–1 have a combined kinetic energy of 3 × 1028 erg, a factor of about 100 greater than required by the burst maximum fluxes that last for one second. Betatron acceleration in the compressed magnetic fields between the colliding comets could accelerate electrons to energies sufficient to produce the observed high energy gamma rays. Many of the additional observed features of gamma ray bursts can be explained by the solar comet collision source.  相似文献   

13.
Measurements of electron concentrations in the ionosphere, between 100 and 250 km altitude, were used to compute the increase in solar ionizing radiation during two flares on 21 and 23 May 1967. Since the altitude of maximum absorption of the solar energy (approximately unit optical depth) depends on the wavelength of the radiation, it is possible to estimate separately the energy enhancement in different portions of the spectrum. An ionizing energy flux increase of nearly 5 erg cm–2 sec–1 was observed on 21 May, while on the 23rd, the increase was over 7 erg cm–2 sec–1. In both flares, most of the absolute increase occurred in the 20–205 Å region of the spectrum, although the relative increase was much larger at the shorter wavelengths.  相似文献   

14.
We quantify the level of polarization of the atmosphere due to Zeeman splitting of oxygen in the Earth’s magnetic field and compare it to the level of polarization expected from the polarization of the cosmic microwave background radiation. The analysis focuses on the effect at mid-latitudes and at large angular scales. We find that from stratospheric balloon borne platforms and for observations near 100 GHz the atmospheric linear and circular polarized intensities are about 10−12 and 100 × 10−9 K, respectively, making the atmosphere a negligible source of foreground. From the ground the linear and circular polarized intensities are about 10−9 and 100 × 10−6 K, making the atmosphere a potential source of foreground for the CMB E (B) mode signal if there is even a 1% (0.01%) conversion of circular to linear polarization in the instrument.  相似文献   

15.
Twenty-eight of the thirty-nine diffuse interstellar bands identified by Herbig (1975) are shown to constitute three vibronic systems with origins at 14 321, 15 153, and 15 343 cm–1 (vac). Structure within these three systems arises from the excitation of vibrational modes withv 1=275 cm–1,v 2=445.5 cm–1, andv 3=793 cm–1. The electronic origins at 14 321 and 15 343 cm–1 correspond to narrow lines observed in the spectrum of Cr3+ ions at cubic sites in MgO solids while the 15 153 cm–1 origin arises in Mn4+ : MgO. Hence, many of the diffuse bands in the visible likely are due to small MgO particles containing these ions. This observation is compatible with recent experimental data showing broad bands at 160 nm and 220 nm from finely divided MgO solids that match features in the interstellar extinction curve.  相似文献   

16.
Two Mediterranean sea level distributions spanning the last decades are examined. The first one is a reconstruction of sea level obtained by a reduced-space optimal interpolation applied to tide gauge and altimetry data. The second distribution is obtained from a 3D (baroclinic) regional circulation model. None of the two representations includes the mechanical atmospheric forcing. Results are presented for two different periods: 1993–2000 (for which altimetry data are available) and 1961–2000 (the longest period common to both distributions).The first period is examined as a test period for the model, since the reconstruction is very similar to altimetry observations. The modelled sea level is in fair agreement with the reconstruction in the Western Mediterranean and in the Aegean Sea (except in the early nineties), but in the Ionian Sea the model departs from observations. For the whole period 1961–2000 the main feature is a marked positive trend in the Ionian Sea (up to 1.8 mm yr− 1), observed both in the reconstruction and in the model. Also the distribution of positive trends in the Western Mediterranean (mean value of 1.1 mm yr− 1) and the smaller trends in the Aegean Sea (0.5 mm yr− 1) are similar in the reconstruction and in the model, despite the first implicitly accounts for sea level variations due to remote sources such as ice melting and the second does not. The interannual sea level variability associated with key regional events such as the Eastern Mediterranean Transient is apparently captured by the reconstruction but not by the model (at least in its present configuration). Hence, the reconstruction can be envisaged as a useful tool to validate further long-term numerical simulations in the region.  相似文献   

17.
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18.
On the basis of Sobolev's method, the population of 30 levels of hydrogen atom is determined allowing for the radiative and collision processes of the heating and ionization of the medium with velocity gradient gradv=10–9–10–11s–1, electron temperatureT e=104 K-2×104 K and electron densityN e=1010 cm–3–1011 cm–3. The central source radiation is characterized by a power spectrum with spectral indices varying from 0 to 2. A region of possible physical conditions is found where the thermal diffuse radiation of the envelope exceeds the emission in the Balmer H line.  相似文献   

19.
Bearing load vs penetration curves have been measured on a 1.3 g sample of lunar soil from the scoop of the Surveyor 3 soil mechanics surface sampler, using a circular indentor 2 mm in diameter. Measurements were made in an Earth laboratory, in air. This sample provided a unique opportunity to evaluate earlier, remotely controlled, in-situ measurements of lunar surface bearing properties. Bearing capacity, measured at a penetration equal to the indentor diameter, varied from 0.02–0.04 N cm–2 at bulk densities of 1.15 g cm–3 to 30-100 N cm–2 at 1.9 g cm–3. Deformation was by compression directly below the indentor at bulk densities below 1.61 g cm–3, by outward displacement at bulk densities over 1.62 g cm–3. Preliminary comparison of in-situ remote measurements with those on returned material indicates good agreement if the lunar regolith at Surveyor 3 has a bulk density of 1.6 g cm–3 at 2.5 cm. depth; definitive comparison awaits both better data on bulk density of the undisturbed lunar soil and additional mechanical-property measurements on returned material.  相似文献   

20.
We have sought interstellar ethyl-cyanide via its 202–101 transition towards two cold, dark clouds and report upper limits of the total column densities of 3×1012cm–2 and 2×1012cm–2 for TMC-1 and L134N, respectively. We also observed the 202–101 transition of vinyl cyanide previously identified in TMC-1 by Matthews and Sears (1983b). The detection of vinyl cyanide and the non-detection of ethyl cyanide in TMC-1 are consistent with gas phase ion-molecule chemical models, and there is thus no necessity of invoking grain surface synthesis for vinyl cyanide in cold clouds.  相似文献   

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