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861.
Summary Accurate measurements of observed frequencies of solar oscillations are providing a wealth of data on the properties of the solar interior. The frequencies depend on solar structure, and on the properties of the plasma in the Sun. Here we consider in particular the dependence on the thermodynamic state. From an analysis of the equations of stellar structure, and the relevant aspects of the properties of the oscillations, we argue that in the convection zone one can isolate information about the equation of state which is relatively unaffected by other uncertainties in the physics of the solar interior. We review the different treatments that have been used to describe the thermodynamics of stellar plasmas. Through application of several of these to the computation of models of the solar envelope we demonstrate that the sensitivity of the observed frequencies is in fact sufficient to distinguish even quite subtle features of the physics of solar matter. This opens up the possibility of using the Sun as a laboratory for statistical mechanics, under conditions that are out of reach in a terrestrial laboratory. 相似文献
862.
Jehin Emmanuel Boehnhardt Hermann Sekanina Zdenek Bonfils Xavier Schütz Oliver Beuzit Jean-Luc Billeres Malvina Garradd Gordon J. Leisy Pierre Marchis Franck Más Antonio Origlia Livia Scarpa Daniel Thomas Daniel Tozzi Gian Paolo 《Earth, Moon, and Planets》2002,90(1-4):147-151
Comet C/2001 A2 experienced several splitting events duringits 2001 perihelion passage. The first break-upevent was observed
in March 2001 (IAUC 7616).In this paper we report the first results of ourextensive imaging and spectroscopic monitoring campaignwith
ESO telescopes over several weeks before andafter the perihelion passage on May 25 2001. 相似文献
863.
864.
The Source Regions of Impulsive Solar Electron Events 总被引:1,自引:0,他引:1
Benz Arnold O. Lin Robert P. Sheiner Olga A. Krucker Säm Fainberg Joe 《Solar physics》2001,203(1):131-144
Low-energy (2–19 keV) impulsive electron events observed in interplanetary space have been traced back to the Sun, using their interplanetary type III radiation and metric/decimetric radio-spectrograms. For the first time we are able to study the highest frequencies and thus the radio signatures closest to the source region. All the selected impulsive solar electron events have been found to be associated with an interplanetary type III burst. This allows to time the particle events at the 2 MHz plasma level and identify the associated coronal radio emissions. Except for 5 out of 27 cases, the electron events were found to be associated with a coronal type III burst in the metric wavelength range. The start frequency yields a lower limit to the density in the acceleration region. We also search for narrow-band spikes at the start of the type III bursts. In about half of the observed cases we find metric spikes or enhancements of type I bursts associated with the start of the electron event. If interpreted as the plasma emission of the acceleration process, the observed average frequency of spikes suggests a source density of the order of 3×108 cm–3 consistent with the energy cut-off observed. 相似文献
865.
Sui Linhui Holman Gordon D. Dennis Brian R. Krucker Säm Schwartz Richard A. Tolbert Kim 《Solar physics》2002,210(1-2):245-259
We have analyzed a C7.5 limb flare observed by RHESSI on 20 February 2002. The RHESSI images appear to show two footpoints
and a loop-top source. Our goal was to determine if the data are consistent with a simple steady-state model in which high-energy
electrons are continuously injected at the top of a semicircular flare loop. A comparison of the RHESSI images with simulated
images from the model has made it possible for us to identify spurious sources and fluxes in the RHESSI images. We find that
the RHESSI results are in many aspects consistent with the model if a thermal source is included between the loop footpoints,
but there is a problem with the spectral index of the loop-top source. The thermal source between the footpoints is likely
to be a low-lying loop interacting with the northern footpoint of a higher loop containing the loop-top source. 相似文献
866.
Biogeomorphology has been expanding as a discipline, due to increased recognition of the role that biology can play in geomorphic processes, as well as due to our increasing capacity to measure and quantify feedback between biological and geomorphological systems. Here, we provide an overview of the growth and status of biogeomorphology. This overview also provides the context for introducing this special issue on biogeomorphology, and specifically examines the thematic domains of biogeomorphological research, methods used, open questions and conundrums, problems encountered, future research directions, and practical applications in management and policy (e.g. nature-based solutions). We find that whilst biogeomorphological studies have a long history, there remain many new and surprising biogeomorphic processes and feedbacks that are only now being identified and quantified. Based on the current state of knowledge, we suggest that linking ecological and geomorphic processes across different spatio-temporal scales emerges as the main research challenge in biogeomorphology, as well as the translation of biogeomorphic knowledge into management approaches to environmental systems. We recommend that future biogeomorphic studies should help to contextualize environmental feedbacks by including the spatio-temporal scales relevant to the organism(s) under investigation, using knowledge of their ecology and size (or metabolic rate). Furthermore, in order to sufficiently understand the ‘engineering’ capacity of organisms, we recommend studying at least the time period bounded by two disturbance events, and recommend to also investigate the geomorphic work done during disturbance events, in order to put estimates of engineering capacity of biota into a wider perspective. Finally, the future seems bright, as increasingly inter-disciplinary and longer-term monitoring are coming to fruition, and we can expect important advances in process understanding across scales and better-informed modelling efforts. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd 相似文献
867.
Hanna Hadler Andreas Vött Timo Willershäuser Dennis Wilken Ruth Blankenfeldt Claus von Carnap-Bornheim Kurt Emde Peter Fischer Ulf Ickerodt Stefanie Klooß Bente Majchczack Lea Obrocki Wolfgang Rabbel 《地球表面变化过程与地形》2021,46(15):3228-3251
In ad 1362, a major storm surge drowned wide areas of cultivated medieval marshland along the north-western coast of Germany and turned them into tidal flats. This study presents a new methodological approach for the reconstruction of changing coastal landscapes developed from a study site in the Wadden Sea of North Frisia. Initially, we deciphered long-term as well as event-related short-term geomorphological changes, using a geoscientific standard approach of vibracoring, analyses of sedimentary, geochemical and microfaunal palaeoenvironmental parameters and radiocarbon dating. In a next step, Direct Push (DP)-based Cone Penetration Testing (CPT) and the Hydraulic Profiling Tool (HPT) were applied at vibracore locations to obtain in situ high-resolution stratigraphic data. In a last step, multivariate linear discriminant analysis (LDA) was successfully applied to efficiently identify different sedimentary facies (e.g., fossil marsh or tidal flat deposits) from the CPT and HPT test dataset, to map the facies' lateral distribution, also in comparison to reflection seismic measurements and test their potential to interpolate the borehole and CPT/HPT data. The training dataset acquired for the key site from coring and DP sensing finally allows an automated facies classification of CPT/HPT data obtained elsewhere within the study area. The new methodological approach allowed a detailed reconstruction of the local coastal landscape development in the interplay of natural marsh formation, medieval land reclamation and storm surge-related land losses. 相似文献
868.
Gerald Herrling Marius Becker Alice Lefebvre Anna Zorndt Knut Krämer Christian Winter 《地球表面变化过程与地形》2021,46(11):2211-2228
The bed of estuaries is often characterized by ripples and dunes of varying size. Whereas smaller bedforms adapt their morphological shape to the oscillating tidal currents, large compound dunes (here: asymmetric tidal dunes) remain stable for periods longer than a tidal cycle. Bedforms constitute a form roughness, that is, hydraulic flow resistance, which has a large-scale effect on tidal asymmetry and, hence, on hydrodynamics, sediment transport, and morphodynamics of estuaries and coastal seas. Flow separation behind the dune crest and recirculation on the steep downstream side result in turbulence and energy loss. Since the energy dissipation can be related to the dune lee slope angle, asymmetric dune shapes induce variable flow resistance during ebb and flood phases. Here, a noncalibrated numerical model has been applied to analyze the large-scale effect of symmetric and asymmetric dune shapes on estuarine tidal asymmetry evaluated by residual bed load sediment transport at the Weser estuary, Germany. Scenario simulations were performed with parameterized bed roughness of symmetric and asymmetric dune shapes and without dune roughness. The spatiotemporal interaction of distinct dune shapes with the main drivers of estuarine sediment and morphodynamics, that is, river discharge and tidal energy, is shown to be complex but substantial. The contrasting effects of flood- and ebb-oriented asymmetric dunes on residual bed load transport rates and directions are estimated to be of a similar importance as the controls of seasonal changes of discharge on these net sediment fluxes at the Lower Weser estuary. This corroborates the need to consider dune-induced directional bed roughness in numerical models of estuarine and tidal environments. 相似文献
869.
GOCE orbit predictions for SLR tracking 总被引:1,自引:0,他引:1
After a descent phase of about half a year, the Gravity field and steady-state Ocean Circulation Explorer (GOCE) reached the
final orbital altitude of the first measurement and operational phase (MOP-1) in September 2009. Due to this very low orbital
altitude and the inactive drag compensation during descent, the generation of reliable predictions of the GOCE trajectory
turned out to be a major challenge even for short prediction intervals. As predictions of good quality are a prerequisite
for frequent ranging from the tracking network of the International Laser Ranging Service (ILRS), Satellite Laser Ranging
(SLR) data of GOCE was very sparse at mission start and made it difficult to independently calibrate and optimize the orbit
determination based on data of the Global Positioning System (GPS). In addition to the GOCE orbit predictions provided by
the European Space Agency (ESA), the Astronomical Institute of the University of Bern (AIUB) started providing predictions
on July 22, 2009, as part of the Level 1b to Level 2 data processing performed at AIUB. The predictions based on the 12-h
ultra-rapid products of the International GNSS Service (IGS) were originally intended to primarily serve the daylight passes
in the early evening hours over Europe. The corresponding along-track prediction errors were often kept below 50 m during
the descent phase and allowed for the first successful SLR tracking of GOCE over Europe on July 29, 2009, by the Zimmerwald
observatory. Additional predictions based on the IGS 18-h ultra-rapid products are provided by AIUB since September 20, 2009,
to further optimize the GOCE SLR tracking. In this article, the development of the GOCE prediction service at AIUB is presented,
and the quality of the orbit predictions is assessed for periods with and without active drag compensation. The prediction
quality is discussed as a function of the prediction interval, the quality of the input products for the GPS satellite orbits
and clocks, and the availability of the GOCE GPS data. From the methodological point of view, different approaches for the
treatment of the non-gravitational accelerations acting on the GOCE satellite are discussed and their impact on the prediction
quality is assessed, in particular during the descent phase. Eventually, an outlook is given on the significance of GOCE SLR
tracking to identify systematic errors in the GPS-based orbit determination, e.g., cross-track errors induced by mismodeled
GOCE GPS phase center variations (PCVs). 相似文献
870.
Ulf B��ntgen Rudolf Br��zdil Petr Dobrovolny Mirek Trnka Tom��? Kyncl 《Theoretical and Applied Climatology》2011,105(1-2):167-180
Past, present, and projected fluctuations of the hydrological cycle, associated to anthropogenic climate change, describe a pending challenge for natural ecosystems and human civilizations. Here, we compile and analyze long meteorological records from Brno, Czech Republic and nearby tree-ring measurements of living and historic firs from Southern Moravia. This unique paleoclimatic compilation together with innovative reconstruction methods and error estimates allows regional-scale May?CJune drought variability to be estimated back to ad 1500. Driest and wettest conditions occurred in 1653 and 1713, respectively. The ten wettest decades are evenly distributed throughout time, whereas the driest episodes occurred in the seventeenth century and from the 1840s onward. Discussion emphasizes agreement between the new reconstruction and documentary evidence, and stresses possible sources of reconstruction uncertainty including station inhomogeneity, limited frequency preservation, reduced climate sensitivity, and large-scale constraints. 相似文献