Analysis of the positive ionospheric response to a moderate geomagnetic storm using a global numerical model   总被引：1，自引：0，他引：1  

A. A. Namgaladze  M. Förster  R. Y. Yurik 《Annales Geophysicae》2000,18(4):461-477

Current theories of F-layer storms are discussed using numerical simulations with the Upper Atmosphere Model, a global self-consistent, time dependent numerical model of the thermosphere-ionosphere-plasmasphere-magnetosphere system including electrodynamical coupling effects. A case study of a moderate geomagnetic storm at low solar activity during the northern winter solstice exemplifies the complex storm phenomena. The study focuses on positive ionospheric storm effects in relation to thermospheric disturbances in general and thermospheric composition changes in particular. It investigates the dynamical effects of both neutral meridional winds and electric fields caused by the disturbance dynamo effect. The penetration of short-time electric fields of magnetospheric origin during storm intensification phases is shown for the first time in this model study. Comparisons of the calculated thermospheric composition changes with satellite observations of AE-C and ESRO-4 during storm time show a good agreement. The empirical MSISE90 model, however, is less consistent with the simulations. It does not show the equatorward propagation of the disturbances and predicts that they have a gentler latitudinal gradient. Both theoretical and experimental data reveal that although the ratio of [O]/[N₂] at high latitudes decreases significantly during the magnetic storm compared with the quiet time level, at mid to low latitudes it does not increase (at fixed altitudes) above the quiet reference level. Meanwhile, the ionospheric storm is positive there. We conclude that the positive phase of the ionospheric storm is mainly due to uplifting of ionospheric F2-region plasma at mid latitudes and its equatorward movement at low latitudes along geomagnetic field lines caused by large-scale neutral wind circulation and the passage of travelling atmospheric disturbances (TADs). The calculated zonal electric field disturbances also help to create the positive ionospheric disturbances both at middle and low latitudes. Minor contributions arise from the general density enhancement of all constituents during geomagnetic storms, which favours ion production processes above ion losses at fixed height under day-light conditions.  相似文献   

Morphology of NmF2 nighttime increases in the Eurasian sector     

A. V. Mikhailov  T. Yu. Leschinskaya  M. Förster 《Annales Geophysicae》2000,18(6):618-628

A statistical analysis of two peaks (pre-midnight and post-midnight) occurrence in NmF2 daily variations was made on a latitudinal chain of four ionosonde stations in the Eurasian longitudinal sector. Overall 6182 cases of the first and 5600 cases of the second peak occurrence were analyzed using all available foF2 observations for the years of solar maximum and minimum. Well-pronounced and systematic variations with season and solar activity were revealed in the occurrence probability of the peaks, their amplitude and timing. The pattern of both peaks occurrence is similar during winter and equinoxes for midlatitude stations implying one and the same mechanism of their formation. The pre-midnight summer peak shows specific variations in particular during solar maximum pointing to a different mechanism controlling its appearance. Possible mechanisms of both peaks formation are discussed.  相似文献   

On the way to a better state? The role of NGOs in the planning and implementation of protected areas in Brazil     

Monika Röper 《GeoJournal》2000,52(1):61-69

Protected areas are considered some of the most versatile as well as important instruments of nature conservation and environmental policies. The `classic' model of nature conservation aims at the isolation of large areas in order to preserve their `pristine' nature from human interference. However, the transfer of this model to developing countries led to serious conflicts with local people. From a socio-geographical viewpoint, protected areas can be understood as regulative tools for the shaping and controlling of space. This approach helps to recognise the influence of distinct modes of appropriation of space and nature on the emergence and course of conflicts. In the present article it is used to analyse the leading role played by Brazilian NGOs in the transformation of the classic model of protected area management at the beginning of the nineties. Two case studies – dealing with the implementation of the Amazonian Mamirauá Reserve and with the reform of the Brazilian protected area legislation – are used to illustrate not only the success of the new approaches, but also the expectations and contradictions which surround the future development of protected areas in Brazil.  相似文献   

The equatorial Atlantic oscillation and its response to ENSO   总被引：6，自引：3，他引：3  

M. Latif  A. Grötzner 《Climate Dynamics》2000,16(2-3):213-218

An internal equatorial Atlantic oscillation has been identified by analyzing sea surface temperature (SST) observations. The equatorial Atlantic oscillation can be viewed as the Atlantic analogue of the El Niño/Southern Oscillation (ENSO) phenomenon in the equatorial Pacific, but it is much less vigorous. The equatorial Atlantic oscillation is strongly influenced by the Pacific ENSO with the equatorial Atlantic sea surface temperature lagging by about six months. This lag can be explained by the dynamical adjustment time of the equatorial Atlantic to low-frequency wind stress variations and the seasonally varying background state, which favours strongest growth of perturbations in summer. Results of an extended-range simulation with a coupled ocean-atmosphere GCM support this picture.  相似文献   

The shoshonite porphyry Cu-Au association at Bajo de la Alumbrera, Catamarca Province, Argentina: a reply     

D. Müller  P. Forrestal 《Mineralogy and Petrology》2000,68(4):305-308

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Reservoir sediments – a witness of mining and industrial development (Malter Reservoir, eastern Erzgebirge, Germany)     

J. Müller  H. Ruppert  Y. Muramatsu  J. Schneider 《Environmental Geology》2000,39(12):1341-1351

The Malter Reservoir is situated about 30 km south of Dresden (eastern Germany) in a historical mining area of the eastern Erzgebirge. It was built in 1913 for the protection from floodwaters, droughts and for generating electricity. The river Rote Wei?eritz is the main source of clastic input into the lake. Geochemical and sedimentological data of gravity-and piston-cores, recovered from the deepest point of the lake, document the environmental history of the drainage area since 1963. ¹³⁷Cs dating gives an average sedimentation rate of ∼2.9 cm/year. Within the whole core, heavy metals are strongly enriched (parentheses refer to enrichment factors as compared with average shale): cadmium (290), silver (140), bismuth (90), antimony (25), lead (21), zinc (14), tin (13), uranium (9), tungsten (9), molybdenum (5), copper (4), thallium (3) and chromium (2). Enrichments are detectable for the whole registered time-period of 81 years. Peaks of up to 27 mg/kg silver, 37 mg/kg bismuth, 91 mg/kg cadmium, 410 mg/kg chromium, 240 mg/kg copper, 20 mg/kg molybdenum, 14000 mg/kg phosphorus, 740 mg/kg lead, 6,5 mg/kg antimony, 74 mg/kg tin, 52 mg/kg tungsten and 1900 mg/kg zinc reflect local events caused by human impact. Inputs from different pollution sources at different times are represented by highly variable elemental concentrations and ratios within the core. High pH values within the water and the sediment column, the large adsorption capacity of the fine-grained C_org.-rich sediment, and the presence of low Eh-values and sulphide ions in the sediment prevent the remobilisation of the toxic elements. Erosion of these contaminated sediments during floods, channel flows or resuspension during removal of the sediments may lead to a downstream transfer of pollutants. Contents of P and C_org., as well as diatom abundance, indicate a change from oligotrophic to eutrophic conditions in the lake during ∼1940–1950. This was mainly caused by high agricultural activity in the drainage area. Reduced contents of Cu, Zn, Cd and Cr since the reunification of East and West Germany are obviously caused by increasing environmental protection measures, such as wastewater purification and especially the closing of contaminating industries. Revision received: 23 September 1999 · Accepted: 17 December 1999  相似文献   

Style and evolution of salt pillows and related structures in the northern part of the Northeast German Basin     

Dirk Kossow  Charlotte Krawczyk  Tommy McCann  Manfred Strecker  Jörg F.W. Negendank 《International Journal of Earth Sciences》2000,89(3):652-664

The northern part of the Northeast German Basin contains a large number of Late Permian (Zechstein) salt pillows, whereas diapiric structures are almost completely absent. This lack of diapirs facilitated the study of early stages of salt movement in the basin. Salt pillows and related structures were investigated in terms of distribution, geometry and time of initiation of salt flow within the regional geological context. The primary Zechstein thickness in the study area was reconstructed to gain more insight into the relationship between the geometry of the salt layer and the style of the salt-related structures. In this study, no clear spatial relationship between the salt structures and basement faults has been found and the location of the salt structures in this area appears to be highly independent of the underlying structural grain. The overburden is affected by minor faulting. We propose that buckling of the overburden due to regional compression significantly contributed to the initiation of the Late Jurassic to Early Cretaceous salt structures in the basin. Reverse faulting of the Gardelegen and Haldensleben Faults is related to inversion tectonics and exerted a compression on the basin fill. During the deformation, the Late Permian salt layer acted as an efficient detachment and led to a marked decoupling of the Mesozoic overburden from the underlying pre-Zechstein rocks.  相似文献   

GPR study of pore water content and salinity in sand   总被引：5，自引：0，他引：5  

al Hagrey  & Müller 《Geophysical Prospecting》2000,48(1):63-85

High‐resolution studies of hydrological problems of the near‐surface zone can be better accomplished by applying ground‐probing radar (GPR) and geoelectrical techniques. We report on GPR measurements (500 and 900 MHz antennae) which were carried out on a sorted, clean sand, both in the laboratory and at outdoor experimental sites. The outdoor sites include a full‐scale model measuring 5 × 3 × 2.4 m³ and a salinity site measuring 7.0 × 1.0 × 0.9 m³ with three buried sand bodies saturated with water of various salinities. Our studies investigate the capability of GPR to determine the pore water content and to estimate the salinity. These parameters are important for quantifying and evaluating the water quality of vadose zones and aquifers. The radar technique is increasingly applied in quantifying soil moisture but is still rarely used in studying the problems of water salinity and quality. The reflection coefficient at interfaces is obtained from the amplitude spectrum in the frequency and time domains and is confirmed by 1D wavelet modelling. In addition, the GPR velocity to a target at a known depth is determined using techniques of two‐way traveltime, CMP semblance analysis and fitting an asymptotic diffraction curve. The results demonstrate that the reflection coefficient increases with increasing salinity of the moisture. These results may open up a new approach for applications in environmental problems and groundwater prospecting, e.g. mapping and monitoring of contamination and evaluating of aquifer salinity, especially in coastal areas with a time‐varying fresh‐water lens. In addition, the relationship between GPR velocity and water content is established for the sand. Using this relationship, a subsurface velocity distribution for a full‐scale model of this sand is deduced and applied for migrated radargrams. Well‐focused diffractions separate single small targets (diameter of 2–3 cm, at a depth of 20–180 cm and a vertical interval of 20 cm). The results underscore the high potential of GPR for determining moisture content and its variation, flow processes and water quality, and even very small bodies inside the sand or soil.  相似文献   

Radiation Induced Dynamics of the Galactic Halo     

A. Schröer  Y. Shchekinov 《Astrophysics and Space Science》2000,272(1-3):99-103

We show results of numerical simulations of a three component plasma consisting of electrons, ions and dust with external gravitation and radiation fields. We perform simulation runs, starting from an analytic halo equilibrium, balancing pressure, gravitational, and radiative forces. Within these the equilibrium is perturbed by the radiation of a typical OB-star association. The perturbation has a total energy input of 10⁷ L _⊙ and a duration of 30 Myrs. After switching off the perturbation, the simulations are continued to further investigate the dynamics induced. We start with a self consistent one-fluid MHD model without background magnetic field and show for an asymmetric case that the system approaches a new equilibrium after switching on the perturbation. Later it relaxes into the starting configuration again, when the additional radiation is turned off. We then show, first by including a disk-parallel magnetic field and then by redoing the simulations with a full three-fluid code, the influence of magnetic fields and species separation on the plasma dynamics. With our computations we demonstrate that these features can be important for the explanation of the structures of galactic halos and large scale mass flows. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Shaping of Planetary Nebulae by Dust Driven AGB Winds     

C. Reimers  E.A. Dorfi  S. Höfner 《Astrophysics and Space Science》2000,272(1-3):205-212

We have investigated the interacting winds model (IWM) in which the shapes of elliptical Planetary Nebulae (PNe) can be explained by the asymmetric mass loss produced by a rotating AGB star. The mass loss mechanism is based on a dust driven wind calculated for stationary situations. Already for small rotation rates of the AGB star we get a significantly angle-dependent mass loss which is concentrated towards the equatorial plane. This pole to equator density variation in the space surrounding the star influences the shape of the later developed PN. We compare these theoretical shapes with observed PNe and for some objects with well known quantities our model can fit the observations quite well. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献