Problems with the interpretation of the 220 nm interstellar feature     

C. Friedemann  J. Gürtler 《Astrophysics and Space Science》1986,128(1):71-81

The bump in the ultraviolet part of the interstellar extinction curve provides a great challenge in the modelling of interstellar dust. Its shape can be well approximated by a classical dispersion profile with a total halfwidth of 48 nm centred at 217 nm. Apart from few slightly deviating cases the parameters of the band seem to be surprisingly constant in the solar neighbourhood.The equivalent widthW of the 217 nm band shows a very tight correlation with the colour excessE(B-V). Studies of correlations with the strength of diffuse interstellar bands gave no conclusive results as to the nature of the band.The most common interpretation of the 217 nm feature as originating from small graphite grians meets several difficulties. No final decision on the carrier can be made at present.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986.  相似文献   

Solar wind helium, neon, and argon isotopic and elemental composition: Data from the metallic glass flown on NASA’s Genesis mission     

Ansgar Grimberg  Heinrich Baur  Peter Bochsler 《Geochimica et cosmochimica acta》2008,72(2):626-645

Solar wind (SW) helium, neon, and argon trapped in a bulk metallic glass (BMG) target flown on NASA’s Genesis mission were analyzed for their bulk composition and depth-dependent distribution. The bulk isotopic and elemental composition for all three elements is in good agreement with the mean values observed in the Apollo Solar Wind Composition (SWC) experiment. Conversely, the He fluence derived from the BMG is up to 30% lower than values reported from other Genesis bulk targets or in-situ measurements during the exposure period. SRIM implantation simulations using a uniform isotopic composition and the observed bulk velocity histogram during exposure reproduces the Ne and Ar isotopic variations with depth within the BMG in a way which is generally consistent with observations. The similarity of the BMG release patterns with the depth-dependent distributions of trapped solar He, Ne, and Ar found in lunar and asteroidal regolith samples shows that also the solar noble gas record of extraterrestrial samples can be explained by mass separation of implanted SW ions with depth. Consequently, we conclude that a second solar noble gas component in lunar samples, referred to as the “SEP” component, is not needed. On the other hand, a small fraction of the total solar gas in the BMG released from shallow depths is markedly enriched in the light isotopes relative to predictions from implantation simulations with a uniform isotopic composition. Contributions from a neutral solar or interstellar component are too small to explain this shallow sited gas. We tentatively attribute this superficially implanted gas to low-speed, current-sheet related SW, which was fractionated in the corona due to inefficient Coulomb drag. This fractionation process could also explain relatively high Ne/Ar elemental ratios in the same initial gas fraction.  相似文献   

Methodology and use of tensor invariants for satellite gravity gradiometry   总被引：2，自引：1，他引：1  

Oliver Baur  Nico Sneeuw  Erik W. Grafarend 《Journal of Geodesy》2008,82(4-5):279-293

Although its use is widespread in several other scientific disciplines, the theory of tensor invariants is only marginally adopted in gravity field modeling. We aim to close this gap by developing and applying the invariants approach for geopotential recovery. Gravitational tensor invariants are deduced from products of second-order derivatives of the gravitational potential. The benefit of the method presented arises from its independence of the gradiometer instrument’s orientation in space. Thus, we refrain from the classical methods for satellite gravity gradiometry analysis, i.e., in terms of individual gravity gradients, in favor of the alternative invariants approach. The invariants approach requires a tailored processing strategy. Firstly, the non-linear functionals with regard to the potential series expansion in spherical harmonics necessitates the linearization and iterative solution of the resulting least-squares problem. From the computational point of view, efficient linearization by means of perturbation theory has been adopted. It only requires the computation of reference gravity gradients. Secondly, the deduced pseudo-observations are composed of all the gravitational tensor elements, all of which require a comparable level of accuracy. Additionally, implementation of the invariants method for large data sets is a challenging task. We show the fundamentals of tensor invariants theory adapted to satellite gradiometry. With regard to the GOCE (Gravity field and steady-state Ocean Circulation Explorer) satellite gradiometry mission, we demonstrate that the iterative parameter estimation process converges within only two iterations. Additionally, for the GOCE configuration, we show the invariants approach to be insensitive to the synthesis of unobserved gravity gradients.  相似文献   

Efficient GOCE satellite gravity field recovery based on least-squares using QR decomposition   总被引：3，自引：0，他引：3  

Oliver Baur  Gerrit Austen  Jürgen Kusche 《Journal of Geodesy》2008,82(4-5):207-221

We develop and apply an efficient strategy for Earth gravity field recovery from satellite gravity gradiometry data. Our approach is based upon the Paige-Saunders iterative least-squares method using QR decomposition (LSQR). We modify the original algorithm for space-geodetic applications: firstly, we investigate how convergence can be accelerated by means of both subspace and block-diagonal preconditioning. The efficiency of the latter dominates if the design matrix exhibits block-dominant structure. Secondly, we address Tikhonov-Phillips regularization in general. Thirdly, we demonstrate an effective implementation of the algorithm in a high-performance computing environment. In this context, an important issue is to avoid the twofold computation of the design matrix in each iteration. The computational platform is a 64-processor shared-memory supercomputer. The runtime results prove the successful parallelization of the LSQR solver. The numerical examples are chosen in view of the forthcoming satellite mission GOCE (Gravity field and steady-state Ocean Circulation Explorer). The closed-loop scenario covers 1 month of simulated data with 5 s sampling. We focus exclusively on the analysis of radial components of satellite accelerations and gravity gradients. Our extensions to the basic algorithm enable the method to be competitive with well-established inversion strategies in satellite geodesy, such as conjugate gradient methods or the brute-force approach. In its current development stage, the LSQR method appears ready to deal with real-data applications.  相似文献   

Integration of stress testing with graph theory to assess the resilience of urban road networks under seismic hazards     

Nazli Yonca Aydin  H. Sebnem Duzgun  Friedemann Wenzel  Hans Rudolf Heinimann 《Natural Hazards》2018,91(1):37-68

Transportation networks daily provide accessibility and crucial services to societies. However, they must also maintain an acceptable level of service to critical infrastructures in the case of disruptions, especially during natural disasters. We have developed a method for assessing the resilience of transportation network topology when exposed to environmental hazards. This approach integrates graph theory with stress testing methodology and involves five basic steps: (1) establishment of a scenario set that covers a range of seismic damage potential in the network, (2) assessment of resilience using various graph-based metrics, (3) topology-based simulations, (4) evaluation of changes in graph-based metrics, and (5) examination of resilience in terms of spatial distribution of critical nodes and the entire network topology. Our case study was from the city of Kathmandu in Nepal, where the earthquake on April 25, 2015, followed by a major aftershock on May 12, 2015, led to numerous casualties and caused significant damage. Therefore, it is a good example for demonstrating and validating the developed methodology. The results presented here indicate that the proposed approach is quite efficient and accurate in assisting stakeholders when evaluating the resilience of transportation networks based on their topology.  相似文献   

Natrolite,part I: Refinement of high-order data,separation of internal and external vibrational amplitudes from displacement parameters     

E. Stuckenschmidt  W. Joswig  W. H. Baur 《Physics and Chemistry of Minerals》1993,19(8):562-570

A single crystal of natrolite, Na₂Al₂Si₃O₁₀·2H₂O, was studied by X-ray diffraction methods at room temperature. The intensities were measured with MoKα radiation (λ = 0.7107 Å) in a complete sphere of reflection up to sin θ/λ = 0.903 Å^?1. The structure was refined in the orthorhombic space group Fdd2 with a = 18.2929 (7) Å, b = 18.6407(9) Å, c = 6.5871(6) Å, V = 2246 ų, Z = 8. A refinement of high-order diffraction data yielded reliability factors of R(F) = 0.9%, R _w(F) = 0.8%, GoF = 1.40 for 1856 high-angle reflections (0.7 ?in θ/λ <0.903 Å^?1) and R(F) = 1.0%, R _W(F) = 1.2%, GoF = 3.07 for all 3471 independent reflections in the complete sphere of reflection. The T-O distances as well as the T-O-T angles were found to be strongly influenced by the different bond strengths received by the individual oxygen atoms. The T O distances calculated using Baur's extended valence rule agree on average within 0.003 Å with the observed values. An analysis of the mean square displacement amplitudes allowed a separation of the external and internal vibrational amplitudes along the T-O bonds as well as along the Na O and H₂O-O bond directions and the calculation of force constants. The internal vibrational amplitudes (ΔU) of the T-O vibrations are in the range of 5 to 11 × 10^-4 Ų, that is about one order of magnitude smaller than the mean square displacement amplitudes of the external vibrations. The corresponding force constants are F = 354 to 824 Nm^?1. The values of the force constants of the motion of the Na-ion and the water molecule against the framework oxygen atoms lie in the range between F = 57 and 293 Nm^?1. This is the first instance where displacement amplitudes from a zeolite structure refinement could be apportioned between contributions from internal and external vibrations for individual bonds.  相似文献   

A new solid state diffusion model applied to inverse zoning and diffusion rims in minerals     

Friedemann Freund  Bruce V. King 《Physics and Chemistry of Minerals》1984,11(3):113-124

Any oxide and silicate mineral which is nominally anhydrous but crystallized in the presence of H₂O incorporates traces of H₂O in solid solution. In the case of MgO it can be shown that OH^? pairs convert into H₂+O ₂ ^2? . If the H₂ molecules are lost, the O ₂ ^2? remain in the lattice as excess oxygen stabilized by excess cation vacancies. When the O ₂ ^2? anions decay either thermally or by decompression unbound O^? states (positive holes) are generated which lead to surface charges and subsurface space charge layers. Calculated space charge profiles are presented. O^? concentrations as small as 10–20 ppm suffice to create electric surface fields of the order of 4·10⁷ V·m^?1. The diffusion mechanism which derives from these premises incorporates novel features: the cation diffusion is coupled to the counterdiffusion of unbound and vacancy-bound O^? states. The cation diffusion is predicted to be very fast because first, it is field-enhanced (electrochemically driven) and second, it is not rate-limited by the intrinsic cation vacancy concentration nor by the counter-diffusion of other cations. The model may apply to cases of inverse zoning and diffusion rim formation in minerals under certain P-T conditions.  相似文献   

A deuterium-hydrogen fractionation mechanism in magnesium oxide     

Friedemann Freund  Heinz Wengeler  Reinhard Martens 《Geochimica et cosmochimica acta》1982,46(10):1821-1829

Water dissolved in a nominally anhydrous oxide like MgO forms not only OH^? ions, as is usually assumed, but also molecular H₂ and peroxy anions O₂^2?. The specific lattice site where this reaction occurs has been identified for the model-type MgO structure: it is the fully OH^? compensated cation vacancy. In partially deuterated synthetic MgO, molecular D₂ forms preferentially to H₂, by a factor of the order of 4. Three methods have been used to study this very large D/H fractionation experimentally: mass spectrometry, electron spin resonance and infrared spectroscopy. All give consistent results and confirm that internal D/H fractionation occurs and is a function of the temperature. Theoretically the D/H fractionation is explained by the lower zeropoint energy of D₂ with respect to H₂, amplified by secondary processes through which the molecular hydrogen species become mobilized and eventually lost via diffusion. The possible consequences of such internal D/H fractionation processes are indicated for the interpretation of water and hydrogen release patterns, with varying D/H ratios, from terrestrial and extraterrestrial samples.  相似文献   

Bemerkungen zu W. Dammann „Untersuchung über Schwankungen der Winterniederschläge in Deutschland”     

Franz Baur 《Theoretical and Applied Climatology》1960,10(3):422-424

Zusammenfassung An der Methode wird die physikalische Unwirklichkeit der Zusammenfassung der Monate Januar, Februar, März und November und Dezember des gleichen Kalenderjahres als Winter und die Unterdrückung wesentlicher Zusammenhänge mit dem Sonnenfleckenzyklus durch Bildung übergreifender Fünfjahresmittel bemängelt. Hinsichtlich des Ergebnisses wird die behauptete Ähnlichkeit der erhaltenen Kurven mit der Sonnenfleckenwelle widerlegt.

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Summary It is shown that the practice to define the months of January, February, March and November, December of the same calendar year as winter is not justifiable, from a physical point of view. By using running 5-yearly means important relations to the cycle of sun-spots are suppressed. A proposed similarity of the curves obtained byDammann with the solar cycle is disproved.

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Résumé L'auteur montre que l'hiver défini par la période de janvier à mars et de novembre à decembre de la même année est une irréalité physique et que l'emploi de la moyenne mobile de cinq ans masque le cycle des taches solaires; les courbes obtenues parDammann ne correspondent pas rythme des taches.

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Über kristallstrukturelle Beziehungen zwischen Amblygonit,Kieserit und Titanit     

Dr. Werner H. Baur 《Contributions to Mineralogy and Petrology》1959,6(6):399-404

Zusammenfassung Der Vergleich der Gitterkonstanten und der Atomparameter von Amblygonit LiAI[PO₄](OH,F), Kieserit Mg[SO₄](H₂O) und Titanit CaTi[SiO₄](O) zeigt, daß diese Strukturen analog gebaut sind. Das Geriist von tetraedrischen und oktaedrischen Koordinationspolyedern ist in allen 3 Fallen gleich. Die Grundstruktur ist die des Kieserits. Die Amblygonitstruktur entsteht aus ihr durch Verzerrung und Auffüllung mit einem weiteren Bauelement, dem Li, die Titanitstruktur nur durch Auffullung ohne Symmetrieverminderung.  相似文献