The Sun at 8.5 mm wavelength — results of observations with high angular resolution     

O. Hachenberg  P. Steffen  W. Harth 《Solar physics》1978,60(1):105-118

High resolution single dish observations of the solar radio emission with the Effelsberg 100-m telescope at 8.5 mm wavelength (antenna beam width is 31) show a good correlation of small active regions, seen in quiet Sun areas, associated with the longitudinal magnetic fields, but not with structures seen in H light.Some characteristics of these 8.5 mm structures, such as diameter and temperature, point to a correlation with supergranular cells. Although the lifetime of these structures is longer than several hours, weak intensity variations within a time scale of minutes could be recorded. From the observational results it was possible to derive roughly an energy dissipation rate for the 8.5 mm structures. This value is a factor of 100 higher than that obtained earlier at 2.8 cm wavelength in a similar manner.A center-to-limb variation at 8.5 mm, especially a limb brightening, could not be detected.  相似文献   

Benefits of receiver clock modeling in code-based GNSS navigation     

Thomas Krawinkel  Steffen Schön 《GPS Solutions》2016,20(4):687-701

Due to the limited frequency stability and poor accuracy of typical quartz oscillators built-in GNSS receivers, an additional receiver clock error has to be estimated in addition to the coordinates. This leads to several drawbacks especially in kinematic applications: At least four satellites in view are needed for navigation, high correlations between the clock estimates and the up-coordinates. This situation can be improved distinctly when connecting atomic clocks to GNSS receivers and modeling their behavior in a physically meaningful way (receiver clock modeling). Recent developments in miniaturizing atomic clocks result in so-called chip-scale atomic clocks and open up the possibility of using stable atomic clocks in GNSS navigation. We present two different methods of receiver clock modeling, namely in an extended Kalman filter and a sequential least-squares adjustment for code-based GNSS navigation using three different miniaturized atomic clocks. Using the data of several kinematic test drives, the benefits of clock modeling for GPS navigation solutions are assessed: decrease in the noise of the up-coordinates by up to 69 % to 20 cm level, decrease in minimal detectable biases by 16 %, and elimination of spikes and subsequently decrease in large position errors (35 %). Hence, a more robust position is obtained. Additionally, artificial partial satellite outages are generated to demonstrate position solutions with only three satellites in view.  相似文献   

School-based environmental education in the context of a research and development project on integrated water resources management: experiences from Mongolia     

Daniel Karthe  Tobias Reeh  Michael Walther  Steffen Niemann  Alexander Siegmund 《Environmental Earth Sciences》2016,75(18):1286

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Seasonal changes in the soil moisture distribution around bare rock outcrops within a karst rocky desertification area (Fuyuan County,Yunnan Province,China)   总被引：3，自引：0，他引：3  

Sheng Li  Steffen Birk  Liang Xue  Huadong Ren  Jun Chang  Xiaohua Yao 《Environmental Earth Sciences》2016,75(23):1482

The spatial and temporal soil moisture distribution is an important control on surface ecological processes in areas with rock outcrops resulting from karst rocky desertification (KRD). To explore the local effects of bare rocks within different seasons, soil moisture was measured in a KRD region (Fuyuan County, Yunnan Province, southwest China) at different depths and in different directions and distances from rock outcrops in both the dry and the wet seasons. The soil moisture north and east of the rocks was higher compared to the other directions and to the control plot. This effect is attributed to the shading by the rocks. The shading effect is evident in all seasons but more pronounced in the dry season. In the wet season, the directional dependency is restricted to the surface layer and the increase in soil moisture with depth is more pronounced around the rocks than at the control plot. This is attributed to precipitation-induced runoff from the rock outcrops infiltrating into the deeper layers at the rock–soil interface. These findings suggest the redistribution of water in the wet season and the spatial variation of evapotranspiration in the dry season are factors controlling the local soil moisture pattern around the rock outcrops.  相似文献   

Reducing climate risk for micro-insurance providers in Africa: A case study of Ethiopia     

Elisabeth Meze-Hausken  Anthony Patt  Steffen Fritz 《Global Environmental Change》2009,19(1):66-73

Recurrent climate hazards challenge subsistence farmers in developing countries. Reliance on various diversification strategies and traditional risk sharing among kin and families has serious limitations, such as the problem of covariate risk within such networks. Index-based crop insurance could help to reduce people's climate-related risk, but raising the necessary capital to make insurance schemes financially secure is difficult for micro-insurance providers. We examine the extent to which spatial pooling of micro-insurance schemes could reduce these capital requirements. We simulate a hypothetical insurance market operating in Ethiopia, using rainfall data and yield estimates for 15 stations. By performing a Monte Carlo analysis, risk capital required to keep the probability of financial ruin below a threshold value is identified. We investigate the marginal benefits of pooling increasing numbers of sites, as well as the relationship between the benefits of pooling and the spatial covariance of rainfall. We find spatial diversification to offer considerable savings in required capitalization with as few as three sites pooled, as well as a weak but significant relationship between rainfall covariance and those benefits. The results suggest that spatial pooling may be an attractive option for micro-insurers, worthy of a detailed case-by-case analysis when designing index-insurance schemes.  相似文献   

The pace of Holocene vegetation change – testing for synchronous developments     

Thomas Giesecke  K.D. Bennett  H. John B. Birks  Anne E. Bjune  Elisaveta Bozilova  Angelica Feurdean  Walter Finsinger  Cynthia Froyd  Petr Pokorný  Manfred Rösch  Heikki Seppä  Spasimir Tonkov  Verushka Valsecchi  Steffen Wolters 《Quaternary Science Reviews》2011,30(19-20):2805-2814

Mid to high latitude forest ecosystems have undergone several major compositional changes during the Holocene. The temporal and spatial patterns of these vegetation changes hold potential information to their causes and triggers. Here we test the hypothesis that the timing of vegetation change was synchronous on a sub-continental scale, which implies a common trigger or a step-like change in climate parameters. Pollen diagrams from selected European regions were statistically divided into assemblage zones and the temporal pattern of the zone boundaries analysed. The results show that the temporal pattern of vegetation change was significantly different from random. Times of change cluster around 8.2, 4.8, 3.7, and 1.2 ka, while times of higher than average stability were found around 2.1 and 5.1 ka. Compositional changes linked to the expansion of Corylus avellana and Alnus glutinosa centre around 10.6 and 9.5 ka, respectively. A climatic trigger initiating these changes may have occurred 0.5 to 1 ka earlier, respectively. The synchronous expansion of C. avellana and A. glutinosa exemplify that dispersal is not necessarily followed by population expansion. The partly synchronous, partly random expansion of A. glutinosa in adjacent European regions exemplifies that sudden synchronous population expansions are not species specific traits but vary regionally.  相似文献   

Rock Maker: an MS Excel? spreadsheet for the calculation of rock compositions from proportional whole rock analyses, mineral compositions, and modal abundance     

Steffen H. Büttner 《Mineralogy and Petrology》2012,104(1-2):129-135

Rock Maker is a simple software tool that computes bulk rock compositions resulting from mixing or unmixing of rocks or minerals. The calculations describe the chemical expression of processes such as magma mixing, fractional crystallization, assimilation, residual melt extraction, or formation of solid solutions. Rock Maker can also be used for the elimination of thermodynamically inactive or unwanted chemical components from the whole rock composition, such as cores of porphyroblasts that are considered not to be in equilibrium with the matrix. The calculation of the resulting rock composition is essentially based on modal proportions and compositions of different components in rocks, which may include specific portions of the rock or individual mineral phases. Compositional data, obtained using XRF, ICPMS, EDS, or EPMA, may include major and trace element concentrations. Depending upon the nature of the problem to be solved, the concentrations of oxidic and elemental components can be added to, or subtracted from, each other, producing the calculated normalized whole rock composition after completion of the investigated process (mixing, unmixing, depletion, enrichment, etc.). Furthermore, the software allows the calculation of whole rock compositions from ideal mineral compositions, for which modal proportions can be chosen from pre-defined mineral compositions. The data set includes the most common rock forming minerals and allows the addition of further phases. This section can be used to calculate the approximate whole rock compositions from petrographic modal analysis. This part of Rock Maker is specifically suitable as a teaching tool that illustrates the interrelationship between mineral compositions, modes, and the corresponding whole rock compositions.  相似文献   

Comparative study to evaluate three ground-based optical remote sensing techniques under field conditions by a gas tracer experiment     

Nils Reiche  Tanja Westerkamp  Steffen Lau  Helko Borsdorf  Peter Dietrich  Claudia Schütze 《Environmental Earth Sciences》2014,72(5):1435-1441

Today, ground-based optical remote sensing (ORS) has become an intensively used method for quantifying pollutant or greenhouse gas emissions from point or area sources, and for the validation of airborne or satellite remote sensing data. In this study, we present the results of a release experiment using acetylene (C₂H₂) as a tracer gas, where three ORS techniques were simultaneously tested for two main purposes: (1) the detection of emission sources and (2) the quantification of release rates. Therefore, passive and active open-path Fourier transform infrared spectroscopy (OP-FTIR) and open-path tunable diode laser absorption spectroscopy (TDLAS) were applied and evaluated. The concentration results of the active ORS methods are compared to those estimated by a Lagrangian stochastic atmospheric dispersion model. Our results reveal that passive OP-FTIR is a valuable tool for the rapid detection and imaging of emission sources and the spatial tracer gas distribution; while with active OP-FTIR and TDLAS, C₂H₂ concentrations in the sub-ppm range could be quantified that correlated well with the concentration data obtained by our modeling approach.  相似文献   

Comparison of local grid refinement methods for MODFLOW     

Mehl S  Hill MC  Leake SA 《Ground water》2006,44(6):792-796

Many ground water modeling efforts use a finite-difference method to solve the ground water flow equation, and many of these models require a relatively fine-grid discretization to accurately represent the selected process in limited areas of interest. Use of a fine grid over the entire domain can be computationally prohibitive; using a variably spaced grid can lead to cells with a large aspect ratio and refinement in areas where detail is not needed. One solution is to use local-grid refinement (LGR) whereby the grid is only refined in the area of interest. This work reviews some LGR methods and identifies advantages and drawbacks in test cases using MODFLOW-2000. The first test case is two dimensional and heterogeneous; the second is three dimensional and includes interaction with a meandering river. Results include simulations using a uniform fine grid, a variably spaced grid, a traditional method of LGR without feedback, and a new shared node method with feedback. Discrepancies from the solution obtained with the uniform fine grid are investigated. For the models tested, the traditional one-way coupled approaches produced discrepancies in head up to 6.8% and discrepancies in cell-to-cell fluxes up to 7.1%, while the new method has head and cell-to-cell flux discrepancies of 0.089% and 0.14%, respectively. Additional results highlight the accuracy, flexibility, and CPU time trade-off of these methods and demonstrate how the new method can be successfully implemented to model surface water-ground water interactions.  相似文献   

A hydrodynamical study of multiple‐shell planetary nebulae     

D. Schnberner  R. Jacob  H. Lehmann  G. Hildebrandt  M. Steffen  A. Zwanzig  C. Sandin  R.L.M. Corradi 《Astronomische Nachrichten》2014,335(4):378-408

We present the result of a study on the expansion properties and internal kinematics of round/elliptical planetary nebulae of the Milky Way disk, the halo, and of the globular cluster M 15. The purpose of this study is to considerably enlarge the small sample of nebulae with precisely determined expansion properties (Schönberner et al. 2005b). To this aim, we selected a representative sample of objects with different evolutionary stages and metallicities and conducted highresolution ´echelle spectroscopy. In most cases we succeeded in detecting the weak signals from the outer nebular shell which are attached to the main line emission from the bright nebular rim. Next to the measurement of the motion of the rim gas by decomposition of the main line components into Gaussians, we were able to measure separately, for most objects for the first time, the gas velocity immediately behind the leading shock of the shell, i.e. the post‐shock velocity. We more than doubled the number of objects for which the velocities of both rim and shell are known and confirm that the overall expansion of planetary nebulae is accelerating with time. There are, however, differences between the expansion behaviour of the shell and the rim: The post‐shock velocity is starting at values as low as around 20 km s^–1 for the youngest nebulae, just above the AGB wind velocity of ∼ 10–15 km s^–1, and is reaching values of about 40 km s^–1 for the nebulae around hotter central stars. Contrarily, the rim matter is at first decelerated below the typical AGB‐wind velocity and remains at about 5–10 km s^–1 for a while until finally a typical flow velocity of up to 30 km s^–1 is reached. This observed distinct velocity evolution of both rim and shell is explained by radiation‐hydrodynamics simulations, at least qualitatively: It is due to the ever changing stellar radiation field and wind‐wind interaction together with the varying density profile ahead of the leading shock during the progress of evolution. The wind‐wind interaction works on the rim dynamics while the radiation field and upstream density gradient is responsible for the shell dynamics. Because of these time‐dependent boundary conditions, a planetary nebula will never evolve into a simple self‐similar expansion. Also the metal‐poor objects behave as theory predicts: The post‐shock velocities are higher and the rim flow velocities are equal or even lower compared to disk objects at similar evolutionary stage. The old nebula around low‐luminosity central stars contained in our sample expand still fast and are dominated by reionisation. We detected, for the first time, in some objects an asymmetric expansion behaviour: The relative expansions between rim and shell appear to be different for the receding and approaching parts of the nebular envelope. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献