Formation Of Coronal Mhd Shock Waves – I. The Basic Mechanism     

B. Vršnak  S. Lulić 《Solar physics》2000,196(1):157-180

The formation and evolution of a large amplitude MHD perturbation propagating perpendicular to the magnetic field in a perfectly conducting low plasma is studied. The perturbation is generated by an abrupt expansion of the source region. Explicit expressions for the time and the distance needed for the transformation of the perturbation's leading edge into a shock wave are derived. The results are applied to coronal conditions and the dynamic spectra of the radio emission excited by the shock are synthesized, reproducing metric and kilometric type II bursts. The features corresponding to the metric type II burst precursor and the moving type IV burst in the case of kilometric type II bursts are identified. A specific radio signature that is sometimes observed at the onset of a metric type II burst is found to appear immediately before the shock wave formation due to the associated growth of the magnetic field gradient. Time delays and starting frequencies of bursts' onsets are calculated and presented as a function of the impulsiveness of the source-region expansion, using different values of the ambient Alfvén velocity and various time profiles of the expansion velocity. The results are confronted with the observations of metric and kilometric type II solar radio bursts.  相似文献   

Seeding agent dispersion within convective cloud as simulated by a 3-D numerical model     

M. Ćurić  D. Janc  V. Vučković 《Meteorology and Atmospheric Physics》2006,92(3-4):205-216

Summary The three-dimensional mesoscale cloud-resolving model ARPS (Advanced Regional Prediction System) was used to investigate the dispersal of an inert seeding agent within a cumulonimbus (Cb) cloud developing from two different initial states. In this paper, we stress the influence vortices in the cloud have on seeding agent dispersion. If a strong directional ambient wind shear is present in the lowest layer, a vortex pair formed at the flanks of the simulated cloud. Following the velocity field, a considerable amount of the injected seeding agent would be thrown out to the rear of the cloud, where both updrafts associated with vortices and downdrafts occurred. After a short time the agent was present only in the cloud periphery. If the Cb cloud developed under conditions where directional ambient wind shear did not exist, seeding agent dispersion would be quite different. In this case, almost all the seeding agent was transported into the main updraft, while the residence time of the agent within the cloud was longer due to the weaker cloud dynamics. Therefore, we must pay attention to whether or not the cloud contains vortices when we make the decision where to seed. This is necessary in order to minimize the loss of seeding material.  相似文献   

Comparison of three mathematical models for the estimation of 10-day drain discharge     

K. Pandžić  I. Šimunić  F. Tomić  S. Husnjak  T. Likso  D. Petošić 《Theoretical and Applied Climatology》2006,85(1-2):107-115

Summary This paper presents an the evaluation of three mathematical models for the estimation of 10-day drain discharge using weather, pedological and phenological parameters. One model uses modified Palmer’s procedure for the calculation of water balance components and the second two use empirical multiple linear regression. The models have been applied to data of an amelioration experimental field in central Croatia (south-western part of the Pannonian Lowland), where a set of average 10-day air temperature and relative humidity data, average wind speed, cumulative precipitation and drain discharge amounts, insolation and soil moisture data and vegetation indices have been collected. The experiment has been performed during the period 1990–1992. The models have been tested by comparing estimated and observed 10-day drain discharge amounts. All models mainly give acceptable cumulative results although their accuracy depends on the season.  相似文献   

Rapid biotic molecular transformation of fulvic acids in a karst aquifer   总被引：2，自引：0，他引：2  

Florian Einsiedl  Norbert Hertkorn  Moritz Frommberger  Boris P. Koch 《Geochimica et cosmochimica acta》2007,71(22):5474-5482

The study of molecular transformation processes of dissolved organic carbon (DOC) in the environment significantly contributes to a better understanding of the global biogeochemical organic matter cycle. In an oxic karst groundwater system, in which the most powerful abiotic DOC degradative reactions, photodegradation and metal-mediated redox chemistry, are at best marginal contributors, a near complete turnover of fulvic acids (FAs) has been observed within decades (∼60 years). Depletion of oxygen for a very extensive range of aliphatic and aromatic carbon chemical environments has been confirmed as well as the formation of novel classes of compounds, suggesting a major contribution from biotic processes. From these results we infer that FAs must be perceived as a rather active participant in the global carbon cycle. Molecular-level alterations of such magnitude and rapidity on such short-time scales ought to be considered as widespread in the processing of “refractory” DOC in the environment.  相似文献   

Probabilistic elasto-plasticity: solution and verification in 1D     

Kallol Sett  Boris Jeremić  M. Levent Kavvas 《Acta Geotechnica》2007,2(3):211-220

In this paper, a solution is presented for evolution of probability density function (PDF) of elastic–plastic stress–strain relationship for material models with uncertain parameters. Developments in this paper are based on already derived general formulation presented in the companion paper. The solution presented is then specialized to a specific Drucker–Prager elastic–plastic material model. Three numerical problems are used to illustrate the developed solution. The stress–strain response (1D) is given as a PDF of stress as a function of strain. The presentation of the stress–strain response through the PDF differs significantly from the traditional presentation of such results, which are represented by a single, unique curve in stress–strain space. In addition to that the numerical solutions are verified against closed form solutions where available (elastic). In cases where the closed form solution does not exist (elastic–plastic), Monte Carlo simulations are used for verification.  相似文献   

Three-dimensional sedimentary architecture of Quaternary sand and gravel resources: a case study of economic sedimentology (SW Germany)     

Boris Kostic  M. Peter Süss  Thomas Aigner 《International Journal of Earth Sciences》2007,96(4):743-767

Quaternary sands and gravels form important, yet often highly heterogeneous economic deposits. Detailed 3-D analysis of the sedimentary structure and stratigraphy of these deposits allows for an accurate estimation of exploitable material. This paper presents a case study in SW Germany reconstructing the 3-D distribution of glacial sediments based on a high-resolution, process-orientated sedimentary facies classification and lithostratigraphy integrated within the geo-modelling package gOcad. Situated along the maximal ice-extent of the Rhine glacier during the last glaciation, the study area is characterised by a morphologically prominent terminal moraine and its associated sandur, which form the stratigraphically youngest sediments of a glacial basin, partially exposed in two gravel pits. These outcrops helped to reconstruct the complex sedimentary architecture of the northern part of the glacial basin. The regional analysis is based on core data, as well as geoelectric and geomagnetic surveys. All data were integrated into km-scale, high-resolution geological 3-D models, depicting the large-scale dynamics of multiple glacier advances and retreats during the Early to Late Pleistocene. These models allow quantifying the thickness and volume distribution of exploitable sandy gravel and help to evaluate the occurrence of non-exploitable interbedded diamicton horizons. The high quality and quantity of gravels particularly north of the terminal moraine classifies the investigated area as very prospective for raw materials exploitation in the future.  相似文献   

Spectral analysis of the “Koshava” wind     

M. Unkašević  I. Tošić  M. Obradović 《Theoretical and Applied Climatology》2007,89(3-4):239-244

Summary “Koshava” is a gusty wind of changeable intensity, blowing from a south-easterly direction, over Serbia, Romania and Bulgaria. It is caused by the interaction between the synoptic circulation and the orography of the Carpathian and the Balkan mountains. This paper analyzes wind data measured at the Belgrade-Observatory during the longest period of consecutive days of “Koshava” which occurred from 14 January to 13 February 1972. Mean hourly wind speed data has been examined using spectral analysis. The power spectra are calculated using autocorrelation spectral analysis, the multi-taper method and wavelet transform. The maximum of which is about 122 h (5 days) corresponds to the time span of synoptic processes.  相似文献   

Quasi-isotropic approximation of ray theory for anisotropic media     

Matthias Zillmer  Boris Kashtan  & Dirk Gajewski 《Geophysical Journal International》1998,132(3):643-653

Wave propagation in weakly anisotropic inhomogeneous media is studied by the quasi-isotropic approximation of ray theory. The approach is based on the ray-tracing and dynamic ray-tracing differential equations for an isotropic background medium. In addition, it requires the integration of a system of two complex coupled differential equations along the isotropic ray.
The interference of the qS waves is described by traveltime and polarization corrections of interacting isotropic S waves. For qP waves the approach leads to a correction of the traveltime of the P wave in the isotropic background medium.
Seismograms and particle-motion diagrams obtained from numerical computations are presented for models with different strengths of anisotropy.
The equivalence of the quasi-isotropic approximation and the quasi-shear-wave coupling theory is demonstrated. The quasi-isotropic approximation allows for a consideration of the limit from weak anisotropy to isotropy, especially in the case of qS waves, where the usual ray theory for anisotropic media fails.  相似文献   

Magnetostratigraphy of the lower Triassic volcanics from deep drill SG6 in western Siberia: evidence for long-lasting Permo–Triassic volcanic activity     

Michel Westphal  Evgueni L. Gurevitch  Boris V. Samsonov  Hugues Feinberg  & Jean Pierre Pozzi 《Geophysical Journal International》1998,134(1):254-266

The deep drill hole SG6 in western Siberia (66°N, 78.5°E) penetrated 1.1  km of lower Triassic basalts, which are possibly an extension of the central Siberian Permo– Triassic flood basalt province. About 300 samples of these basalts were progressively demagnetized and measured. Principal component analysis often shows multiple magnetizations carried by haematite and magnetite. The corrected mean inclinations are +77° and −77° for the haematite component. A magnetostratigraphic scale was derived and showed a N–R–N–R–N succession. This is quite different from the Noril'sk and Taimyr typical polarity scale, R–N.
  The basalts found in the SG6 deep drill hole are slightly younger than those of central Siberia and Taimyr. They correspond to middle–upper Induan age, whereas the Noril'sk and Taimyr sections correspond to an uppermost Permian and lower Induan age. Altogether they indicate that, after a high output rate of volcanic material near the Permo–Triassic boundary, this activity slowed down drastically on the Siberian platform and Taimyr, but persisted for several million years in western Siberia.  相似文献   

A Photometric Investigation of the Eclipsing System OO Aql     

G. Djurašević  S. Erkapić 《Astrophysics and Space Science》1998,262(3):305-313

The paper is concerning with the light curve analysis of an active binary system of OO Aql. Basic orbital and physical parameters of this system were estimated by analysing photometric observations by Lafta and Grainger (1985) and Essam et al. (1992). Both groups of observations gave quite different light curves with a conspicuous asymmetry arising from unequal height of successive maxima. A slightly changed programme by Djuraevi (1992a) was applied. The programme was based on the Roche model and it included an option of overcontact configuration and active (bright or dark) spotted regions on the system's components. An inverse-problem method by Djuraevi (1992b) was used to estimate the parameters of the system and active regions. The light-curve analysis suggested a significant change in the system. For Lafta and Grainger's light curves our analysis showed the presence of a dark spotted area near the polar region of the primary component. This dark area covered about 22% of the primary's hemisphere. Essam et al.'s light curves had very complex shape. In this case the mechanism of the mass transfer and the exchange of the thermal energy between the components could produce a bright-spot (bs) area on the secondary, near the Lagrange equilibrium point L₁ around the neck region between the stars. To explain a sharp increase of the system's brightness in the B filter immediately after the secondary minimum, we had to introduce also a hot-spot (hs) area of a high temperature contrast on the secondary. With the increase of the brightness in the V-filter being of much lower intensity, this hot spot region could be taken as a local eruption. The behaviour of the B–V colour index with the orbital phase is in favour of this hypothesis also. The obtained results show that the light-curve changes are due to development and moving of active spot regions on the components.  相似文献