Computersimulationen ferromagnetischer - antiferromagnetischer Mulitlagen

Compound systems consisting of exchange coupled ferromagnetic-antiferromagnetic multilayers are examined within the framework of the domain state model for exchange bias by means of Monte-Carlo simulations. Initially, systems with uniaxial anisotropies as well as systems where the antiferromagnet shows a twinned structure, with two easy axes then perpendicular to each other, are investigated in order to explore the reversal mechanisms during hysteresis. In both cases, a systematic variation of the angle between the field axis and one of the easy axes of the antiferromagnet reveals a rich variety of different reversal modes. For the latter one, a distinctive dependence on the direction of the cooling field is observed as well. For the characterization of the different kinds of reversal modes, the analysis of the in-plane magnetization paths of the ferromagnetic layer plays an important role. Also, the investigation of its corresponding spin structures allows for a deeper insight into the reversal mechanism. It is shown that experimentally observed asymmetries of the reversal modes in corresponding multilayer systems can be explained within the context of the domain state model. Another part of this work deals with a more general aspect of the domain state model itself, where it is shown that a bond-diluted antiferromagnet exchange coupled to a ferromagnetic layer displays the same characteristic features as the model where a site-diluted antiferromagnet is utilized. This supports the idea that it is structural disorder which is crucial for exchange bias to occur in such multilayers and the kind of disorder introduced plays only a secondary role. Finally, an outlook of nanostructured systems is given. The dependence of the exchange bias effect on the variation of the system size is investigated, where in the first setup the lateral dimension of the ferromagnet is varied, while in the second one the size of both the ferromagnet and the antiferromagnet is subject to such a variation. In both cases, a connection between the domain structure of the antiferromagnet and lateral size of the ferromagnet structure seems to be apparent. However, approaching very small systems sizes the superparamagnetic behavior demands for further investigations

Wirkung einer Folie auf den Wärmehaushalt eines Pflanzenbestandes mit und ohne gleichzeitiger Frostschutzberegnung

In der vorliegenden Arbeit wird ein Simulationsmodell zur Bestimmung der Beregnungsmenge vorgestellt, um das Absinken der Temperatur unter 0°C eines mit einer Plastikfolie abgedeckten Bodens möglichst zu verhindern. Es wurden numerische Berechnungen für den nackten Boden und für den mit Folie abgedeckten Boden mit und ohne gleichzeitiger Frostschutzberegnung durchgeführt. Die Ergebnisse zeigen z.B., daß die Bodenoberflächentemperatur bei einer Beregnung von nur 75% der Sollvorgabe relativ stark absinkt. Dieses liegt an der relativ großen Gefrierwärme von Wasser. Ein nur mit Folie abgedeckter Boden schützt nur in den wenigsten Fällen vor Bodenfrost. Die Simulationsergebnisse stimmen gut mit den gemessenen überein.A model to determine the minimum amount of water required to hold the soil temperature above 0°C under a plastic mulch is presented. Numerical experiments with and without consideration of the mulch foil, with and without sprinkling of the mulch foil were performed. The results substantiate that sprinkling par example at 75% of the usual used rate of water is sufficient to avoid that the soil temperature falls below 0°C because of the relative big freezing energy of water and that mulching alone cannot prevent the soil temperature from decreasing below that value. The model results are in good agreement with observations

On the correlation of the mesopause region wind field, the North Atlantic oscillation and the Central Europe winter temperatures

The stratospheric and mesospheric wind field in winter is dominated by the stratospheric polar vortex, which reaches out up into the mesopause region and leads to strong westerlies there in winter. On the other hand, the tropospheric mean winter wind field is also connected with the polar vortex which thus can be considered as being extended from the surface up to the lower thermosphere. lt is found that the winter mesopause region zonal winds, as measured at the Collm Observatory of the University of Leipzig, are closely connected with the North Atlantic Oscillation (NAO) being an integrated measure for the northem hemispheric mean circulation. The NAO itself is found to be a measure for Central Europe winter temperatures. Thus also the mesopause region winds are closely correlated to the Central European winter surface temperatures.Das stratosphärische und mesosphärische Windfeld wird im Winter wesentlich vom stratosphärischen Polarwirbel bestimmt, der von der Stratosphäre bis in die Mesopausenregion reicht und dort zu starken Westwinden führt. Auf der anderen Seite ist die mittlere troposphärische Zirkulation ebenfalls mit dem stratosphärischen Wirbel korreliert, so daß letzterer als von der Erdoberfläche bis in die untere Thermosphäre reichend betrachtet werden kann. Aus diesem Grund sind die mittleren Zonalwinde, gemessen am Observatorium Collm der Universität Leipzig, mit der Nordatlantikoszillation (NAO) korreliert. Andererseits hat die NAO einen starken Einfluß auf mitteleuropäische Wintertemperaturen, und damit ist der Wind im Mesopausenbereich ebenfalls mit den Wintertemperaturen korreliert

Developing and enhancing biodiversity monitoring programmes: a collaborative assessment of priorities

1.Biodiversity is changing at unprecedented rates, and it is increasingly important that these changes are quantified through monitoring programmes. Previous recommendations for developing or enhancing these programmes focus either on the end goals, that is the intended use of the data, or on how these goals are achieved, for example through volunteer involvement in citizen science, but not both. These recommendations are rarely prioritized. 2.We used a collaborative approach, involving 52 experts in biodiversity monitoring in the UK, to develop a list of attributes of relevance to any biodiversity monitoring programme and to order these attributes by their priority. We also ranked the attributes according to their importance in monitoring biodiversity in the UK. Experts involved included data users, funders, programme organizers and participants in data collection. They covered expertise in a wide range of taxa. 3.We developed a final list of 25 attributes of biodiversity monitoring schemes, ordered from the most elemental (those essential for monitoring schemes; e.g. articulate the objectives and gain sufficient participants) to the most aspirational (e.g. electronic data capture in the field, reporting change annually). This ordered list is a practical framework which can be used to support the development of monitoring programmes. 4.People's ranking of attributes revealed a difference between those who considered attributes with benefits to end users to be most important (e.g. people from governmental organizations) and those who considered attributes with greatest benefit to participants to be most important (e.g. people involved with volunteer biological recording schemes). This reveals a distinction between focussing on aims and the pragmatism in achieving those aims. 5.Synthesis and applications. The ordered list of attributes developed in this study will assist in prioritizing resources to develop biodiversity monitoring programmes (including citizen science). The potential conflict between end users of data and participants in data collection that we discovered should be addressed by involving the diversity of stakeholders at all stages of programme development. This will maximize the chance of successfully achieving the goals of biodiversity monitoring programmes

A national-scale assessment of climate change impacts on species: assessing the balance of risks and opportunities for multiple taxa

It is important for conservationists to be able to assess the risks that climate change poses to species, in order to inform decision making. Using standardised and repeatable methods, we present a national-scale assessment of the risks of range loss and opportunities for range expansion, that climate change could pose for over 3,000 plants and animals that occur in England. A basic risk assessment that compared projected future changes in potential range with recently observed changes classified 21% of species as being at high risk and 6% at medium risk of range loss under a B1 climate change scenario. A greater number of species were classified as having a medium (16%) or high (38%) opportunity to potentially expand their distribution. A more comprehensive assessment, incorporating additional ecological information, including potentially confounding and exacerbating factors, was applied to 402 species, of which 35 % were at risk of range loss and 42 % may expand their range extent. This study covers a temperate region with a significant proportion of species at their poleward range limit. The balance of risks and opportunities from climate change may be different elsewhere. The outcome of both risk assessments varied between taxonomic groups, with bryophytes and vascular plants containing the greatest proportion of species at risk from climate change. Upland habitats contained more species at risk than other habitats. Whilst the overall pattern was clear, confidence was generally low for individual assessments, with the exception of well-studied taxa such as birds. In response to climate change, nature conservation needs to plan for changing species distributions and increasing uncertainty of the future