Intermittent Behaviour in Axisymmetric Mean-Field Dynamo Models in Spherical Shells     

Andrew Tworkowski  Reza Tavakol  Axel Brandenburg  John M. Brooke  David Moss  & Ilkka Tuominen 《Monthly notices of the Royal Astronomical Society》1998,296(2):287-295

Axisymmetric mean-field dynamo models in spherical shells are shown to be capable of producing temporally intermittent behaviour. This is of potential importance since (i) it is, as far as we are aware, the first time such behaviour has been produced internally by a mean-field dynamo model in a spherical shell, without requiring any additional assumptions or truncations, and (ii) it may be characteristic of the type of behaviour observed in the long-term record of solar activity, such as Maunder minima. We also show that these types of behaviour persist when the functional form of the alpha quenching is altered and also occur over intervals of the shell thickness and the dynamo number.  相似文献   

Centre-To-Limb Variation of the Solar Granulation     

M. Sánchez Cuberes  J. A. Bonet  M. Vázquez  A. D. Wittmann 《Astrophysics and Space Science》1998,263(1-4):343-346

An excellent series of images of solar granulation was taken during the partial solar eclipse of 1994 May 10 at the Swedish Vacuum Solar Telescope at Roque de los Muchachos Observatory. Making use of the lunar limb profile, images at different heliocentric positions were corrected for instrumental and atmospheric effects. The centre-to-limb variation of the granulation contrast was calculated and compared with previous determinations. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

The radial disc structure around a magnetic neutron star: analytic and semi-analytic solutions     

Axel Brandenburg  & Chris G. Campbell 《Monthly notices of the Royal Astronomical Society》1998,298(1):223-230

The radial structure of a thin accretion disc is calculated in the presence of a central dipole magnetic field aligned with the rotation axis. The problem is treated using a modified expression for the turbulent magnetic diffusion, which allows the angular momentum equation to be integrated analytically. The governing algebraic equations are solved iteratively between 1 and 10⁴ stellar radii. An analytic approximation is provided that is valid near the disruption radius at about 100 stellar radii. At that point, which is approximately 60 per cent of the Alfvén radius and typically about 30 per cent of the corotation radius, the disc becomes viscously unstable. This instability results from the fact that both radiation pressure and opacity caused by electron scattering become important. This in turn is a consequence of the magnetic field which leads to an enhanced temperature in the inner parts. This is because the magnetic field gives rise to a strongly enhanced vertically integrated viscosity, so that the viscous torque can balance the magnetic torque.  相似文献   

Observations of line asymmetries with a 2-D diode array     

A. Hanslmeier  A. D. Wittmann  W. Mattig 《Astrophysics and Space Science》1990,170(1-2):47-49

From two-dimensional Reticon spectra obtained at the Gregory Coudé Telescope (Tenerife) two Fei lines were studied. It is shown that the resultant line bisectors could be separated according to their origination in granular-intergranular regions and that the resolved bisectors no longer had a C-shape form.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989 Tenerife, Canary Islands, Spain.  相似文献   

Der untersilurische Brandschiefer von Kuckers (Estland)     

Axel Born 《International Journal of Earth Sciences》1914,5(4):313-317

Ohne Zusammenfassung  相似文献   

Die schwedische Hochgebirgsfrage und die Häufigkeit der Überschiebungen     

Axel Hamberg 《International Journal of Earth Sciences》1912,3(4):219-236

Ohne Zusammenfassung  相似文献   

Der Einfluß einiger aÜßerer Faktoren auf die geschiechtliche Entwicklung und die Segmentbildung bei Anaitides mucosa (Annelida, Polychaeta, Phyllodocidae)     

Axel  Röhrkasten 《Marine Ecology》1984,5(2):135-142

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Controls on erosion patterns and sediment transport in a monsoonal,tectonically quiescent drainage,Song Gianh,central Vietnam          下载免费PDF全文

Tara N. Jonell  Peter D. Clift  Long V. Hoang  Tina Hoang  Andrew Carter  Hella Wittmann  Philipp Böning  Katharina Pahnke  Tammy Rittenour 《Basin Research》2017,29(Z1):659-683

The Song Gianh is a small‐sized (~3500 km²), monsoon‐dominated river in northern central Vietnam that can be used to understand how topography and climate control continental erosion. We present major element concentrations, together with Sr and Nd isotopic compositions, of siliciclastic bulk sediments to define sediment provenance and chemical weathering intensity. These data indicate preferential sediment generation in the steep, wetter upper reaches of the Song Gianh. In contrast, detrital zircon U‐Pb ages argue for significant flux from the drier, northern Rao Tro tributary. We propose that this mismatch represents disequilibrium in basin erosion patterns driven by changing monsoon strength and the onset of agriculture across the region. Detrital apatite fission track and ¹⁰Be data from modern sediment support slowing of regional bedrock exhumation rates through the Cenozoic. If the Song Gianh is representative of coastal Vietnam then the coastal mountains may have produced around 132 000–158 000 km³ of the sediment now preserved in the Song Hong‐Yinggehai Basin (17–21% of the total), the primary depocenter of the Red River. This flux does not negate the need for drainage capture in the Red River to explain the large Cenozoic sediment volumes in that basin but does partly account for the discrepancy between preserved and eroded sediment volumes. OSL ages from terraces cluster in the Early Holocene (7.4–8.5 ka), Pre‐Industrial (550–320 year BP) and in the recent past (ca. 150 year BP). The older terraces reflect high sediment production driven by a strong monsoon, whereas the younger are the product of anthropogenic impact on the landscape caused by farming. Modern river sediment is consistently more weathered than terrace sediment consistent with reworking of old weathered soils by agricultural disruption.  相似文献   

Robustness of truncated α Ω dynamos with a dynamic α     

Covas  Eurico  Tworkowski  Andrew  Tavakol  Reza  Brandenburg  Axel 《Solar physics》1997,172(1-2):3-9

In a recent work (Covas et al., 1996), the behaviour and the robustness of truncated dynamos with a dynamic were studied with respect to a number of changes in the driving term of the dynamic equation, which was considered previously by Schmalz and Stix (1991) to be of the form AB. Here we review and extend our previous work and consider the effect of adding a quadratic quenching term of the form |B|². We find that, as before, such a change can have significant effects on the dynamics of the related truncated systems. We also find intervals of (negative) dynamo numbers, in the system considered by Schmalz and Stix (1991), for which there is sensitivity with respect to small changes in the dynamo number and the initial conditions, similar to what was found in our previous work. This latter behaviour may be of importance in producing the intermittent type of behaviour observed in the Sun.  相似文献   

Comparison of wind and wave measurements and models in the Western Mediterranean Sea   总被引：2，自引：0，他引：2  

Fabrice Ardhuin  Luciana Bertotti  Jean-Raymond Bidlot  Luigi Cavaleri  Valentina Filipetto  Jean-Michel Lefevre  Paul Wittmann 《Ocean Engineering》2007,34(3-4):526-541

We have hindcast the wind and wave conditions in the Mediterranean Sea for two one month periods. Four different meteorological models and three different wave models have been used. The results have been compared with satellite and buoy wind and wave observations.Several conclusions concerning both the instruments and the models have been derived. The quality of both wind and wave results has been assessed. Close to the coasts high resolution, nested wave models are required for sufficient reliability.A wave threshold analysis suggests a sufficient reliability only off the coast, with a substantial decrease for low wave heights.  相似文献