Bücher- und Zeitschriftenschau

Ohne Zusammenfassung     

The helicity constraint in spherical shell dynamos

The motivation for considering distributed large scale dynamos in the solar context is reviewed in connection with the magnetic helicity constraint. Preliminary accounts of 3-dimensional direct numerical simulations (in spherical shell segments) and simulations of 2-dimensional mean field models (in spherical shells) are presented. Interesting similarities as well as some differences are noted. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of the radiative background flux in convection

Numerical simulations of turbulent stratified convection are used to study models with approximately the same convective flux, but different radiative fluxes. As the radiative flux is decreased, for constant convective flux: the entropy jump at the top of the convection zone becomes steeper, the temperature fluctuations increase and the velocity fluctuations decrease in magnitude, and the distance that low entropy fluid from the surface can penetrate increases. Velocity and temperature fluctuations follow mixing length scaling laws. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A Late Cretaceous theropod caudal vertebra from the Sultanate of Oman

A caudal vertebra collected from conglomerates of the Al-Khod Formation (Late Cretaceous) in the Al-Khod area, Sultanate of Oman, is assigned to a medium-sized theropod dinosaur. The Al-Khod discovery represents one of the very few dinosaur records from the Middle East.     

The role of chilean mountain areas in time of drought and energy crisis: New pressures and challenges for vulnerable ecosystems

In the summer of 2010/2011 Chile suffered the third most severe energy and water supply crisis in only one decade.This may be surprising as the Andes which accompany the country along 4,200 km are a water tower and could provide more electricity and water than needed.On top of that,it has to be mentioned that Chile also counts with sunny and windy climates and with excellent geothermal energy resources and thus has a huge potential as far as renewable energies are concerned.After analyzing the existing natural conditions,the infrastructure and the present challenges of climate change,it has to be asked in which way Chile can make use of this potential in the near future,considering the legal and political situation and the technological opportunities.It seems that the resources are still hidden in the Andes,and only the key is missing to realize the country’s natural potential.This research has been based on the analysis of existing literature,media,quantitative data from government institutions and other antecedents obtained by the authors in field work done in the South of Chile in 2010.The added value of this compilation lies in the comprehensive perspective,linking the knowledge of climatologists,hydrologists,mountain researchers and energy experts in order to support a sustainable energy and water supply for the future in Chile.From this present research,it can be concluded that Chile necessarilyhas to adopt new energy-related strategies,particularly those aimed at diminishing the strong dependence on traditional sources of energy and establishing new techniques and technologies for generating electricity and utilizing the vast potential that the country is ready to offer,such as that provided by the Andes along 4,200 kilometers.     

Modeling a Maunder minimum

We introduce on/off intermittency into a mean field dynamo model by imposing stochastic fluctuations in either the alpha effect or through the inclusion of a fluctuating electromotive force. Sufficiently strong small scale fluctuations with time scales of the order of 0.3–3 years can produce long term variations in the system on time scales of the order of hundreds of years. However, global suppression of magnetic activity in both hemispheres at once was not observed. The variation of the magnetic field does not resemble that of the sunspot number, but is more reminiscent of the ¹⁰Be record. The interpretation of our results focuses attention on the connection between the level of magnetic activity and the sunspot number, an issue that must be elucidated if long term solar effects are to be well understood. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The dual role of shear in large‐scale dynamos

The role of shear in alleviating catastrophic quenching by shedding small‐scale magnetic helicity through fluxes along contours of constant shear is discussed. The level of quenching of the dynamo effect depends on the quenched value of the turbulent magnetic diffusivity. Earlier estimates that might have suffered from the force‐free degeneracy of Beltrami fields are now confirmed for shear flows where this degeneracy is lifted. For a dynamo that is saturated near equipartition field strength those estimates result in a 5‐fold decrease of the magnetic diffusivity as the magnetic Reynolds number based on the wavenumber of the energy‐carrying eddies is increased from 2 to 600. Finally, the role of shear in driving turbulence and large‐scale fields by the magneto‐rotational instability is emphasized. New simulations are presented and the 3π /4 phase shift between poloidal and toroidal fields is confirmed. It is suggested that this phase shift might be a useful diagnostic tool in identifying mean‐field dynamo action in simulations and to distinguish this from other scenarios invoking magnetic buoyancy as a means to explain migration away from the midplane. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Morphodynamique d'une dune sous-marine du détroit du pas de CalaisMorphodynamics of an undersea sandwave of the Dover Straits

This paper investigates the dynamics of sandwaves in relationship with tidal currents and weather conditions. The studied sandwaves are located in the Dover Straits and are covered by megaripples. These megaripples have migration speeds of $\text{1 }\text{m}\text{h}{}^{\mathrm{?1}}$ and slopes of 34°, suggesting the potential for avalanches to occur along the flanks of the sandwave. Tidal cycles without reversing currents were observed during stormy weather. Wind-induced currents lead to a unidirectional migration of megaripples and sandwaves. Well-defined areas without megaripples were observed and correlated with an increase in sandwave height. We propose interpretations for understanding sandwave saturation and migration. To cite this article: D. Idier et al., C. R. Geoscience 334 (2002) 1079–1085.     

Constraints on the Martian cratering rate based on the SNC meteorites and implications for Mars climatic history

Two constraints placed upon the cratering flux at Mars by the SNC meteorites are examined: crystallization ages as a constraint on surface ages and cosmic ray exposure ages and number of impacts as a constraint on absolute rates. The crystallization ages of the SNC meteorites appear to constrain the Martian cratering rate to be 4xLunar or more if the parent lavas are in the north of Mars and the number of SNC ejecting impacts are small. If the SNCs result from a single impact that formed the Lyot basin then the cratering rate must be at least 7xLunar or higher to produce a basin age less than the SNC crystallization age because the basin ages are themselves determined by crater counting. Assuming multiple uncorrelated impacts for SNC ejection from Mars over 10 million years a cratering rate of approximately 4xLunar is also found for ejecting impacts that form craters over 12km in diameter. Therefore, both crystallization ages and ejection ages and number of impacts appear consistent with a 4xLunar cratering rate at Mars. The effect on Martian chronologies of such a high cratering rate is to place the SNC crystallization ages partly within the epoch of channel formation on Mars and to extend this liquid water epoch over much of Mars history.