The axisymmetric antidynamo theorem revisited

The axisymmetric kinematic dynamo problem is reconsidered and a number of open questions are answered. Apart from axisymmetry and smoothness of data and solution we deal with this problem under quite general conditions, i.e. we assume a compressible fluid of variable (in space and time) conductivity moving in an arbitrary (axisymmetric) domain. We prove unconditional, pointwise and exponential decay of magnetic field and electric current to zero. The decay rate of the external (meridional) magnetic field can become very small (compared to free decay) for special flow fields and large magnetic Reynolds numbers. We give an example of that. On the other hand, we show for fluids with weak variation of mass density and conductivity that the meridional and azimuthal decay rates do not drop significantly below those of free decay.Comment: Revised version, 28 pages, 1 figur

Anmerkungen zur ökonomischen Bewertung von Fusionen auf dem Printmedienmarkt

Die aktuelle Diskussion um die Novellierung der deutschen Pressefusionskontrolle macht deutlich, welche Relevanz medienökonomische Fragestellungen haben. Was jedoch bei dieser Diskussion häufig vernachlässigt wird, ist die intensive Betrachtung der ökonomischen Besonderheiten von Medienmärkten, nämlich der Verbundenheit von Anzeigen- und Publikumsmarkt, die Existenz von Kostendegressionen und eine unterschiedliche Marktmacht von Printmedien auf dem Anzeigen- und Publikumsmarkt. Diese Arbeit analysiert diese Charakteristika und deren Wirkungen bei möglichen Unternehmenszusammenschlüssen. Im Ergebnis zeigt sich, dass aufgrund der Heterogenität der einzelnen Märkte eine intensivere Einzelfallbetrachtung und eine stärkere Ökonomisierung der Fusionskontrolle sinnvoll erscheint. Grundlage dieser Ökonomisierung könnten quantitative Analysen sein, die mithilfe vorhandener Marktdaten durchgeführt werden können

Anmerkungen zur ökonomischen Bewertung von Fusionen auf dem Printmedienmarkt

Die aktuelle Diskussion um die Novellierung der deutschen Pressefusionskontrolle macht deutlich, welche Relevanz medienökonomische Fragestellungen haben. Was jedoch bei dieser Diskussion häufig vernachlässigt wird, ist die intensive Betrachtung der Ökonomischen Besonderheiten von Medienmärkten, nämlich der Verbundenheit von Anzeigen- und Publikumsmarkt, die Existenz von Kostendegressionen und eine unterschiedliche Marktmacht von Printmedien auf dem Anzeigen- und Publikumsmarkt. Diese Arbeit analysiert diese Charakteristika und deren Wirkungen bei möglichen Unternehmenszusammenschlüssen. Im Ergebnis zeigt sich, dass aufgrund der Heterogenität der einzelnen Märkte eine intensivere Einzelfallbetrachtung und eine stärkere Ökonomisierung der Fusionskontrolle sinnvoll erscheint. Grundlage dieser Ökonomisierung könnten quantitative Analysen sein, die mithilfe vorhandener Marktdaten durchgeführt werden können. --Printmedien,zweiseitige Märkte,Pressefusionskontrolle,Wettbewerbspolitik

On the electron-induced isotope fractionation in low temperature ³²O₂/³⁶O₂ ices—ozone as a case study

The formation of six ozone isotopomers and isotopologues, 16O16O16O, 18O18O18O, 16O16O18O, 18O18O16O, 16O18O16O, and 18O16O18O, has been studied in electron-irradiated solid oxygen 16O2 and 18O2 (1 : 1) ices at 11 K. Significant isotope effects were found to exist which involved enrichment of 18O-bearing ozone molecules. The heavy 18O18O18O species is formed with a factor of about six higher than the corresponding 16O16O16O isotopologue. Likewise, the heavy 18O18O16O species is formed with abundances of a factor of three higher than the lighter 16O16O18O counterpart. No isotope effect was observed in the production of 16O18O16O versus 18O16O18O. Such studies on the formation of distinct ozone isotopomers and isotopologues involving non-thermal, non-equilibrium chemistry by irradiation of oxygen ices with high energy electrons, as present in the magnetosphere of the giant planets Jupiter and Saturn, may suggest that similar mechanisms may contribute to the 18O enrichment on the icy satellites of Jupiter and Saturn such as Ganymede, Rhea, and Dione. In such a Solar System environment, energetic particles from the magnetospheres of the giant planets may induce non-equilibrium reactions of suprathermal and/or electronically excited atoms under conditions, which are quite distinct from isotopic enrichments found in classical, thermal gas phase reactions

Autonomous optimization of UWB link access

We present a novel approach for interference management in low data rate IR-UWB networks that enables concurrent transmissions at full power while allows each source to independently adapt its pulse rate (transmitted pulses per second) to mitigate multi-user interference. The work is motivated by the fact that the distributed adaptation of IR-UWB pulse rate has not been sufficiently addressed in the literature before. Existing approaches rely on the presence of a central authority or assume a definite receiver technique. Our approach enables users to share the communication medium in an efficient way compatible with individual QoS requirements and it is independent of any particular modulation scheme or receiver technique

Cosmic-ray-mediated Formation of Benzene on the Surface of Saturn's Moon Titan

The aromatic benzene molecule (C_6H_6)—a central building block of polycyclic aromatic hydrocarbon molecules—is of crucial importance for the understanding of the organic chemistry of Saturn's largest moon, Titan. Here, we show via laboratory experiments and electronic structure calculations that the benzene molecule can be formed on Titan's surface in situ via non-equilibrium chemistry by cosmic-ray processing of low-temperature acetylene (C_2H_2) ices. The actual yield of benzene depends strongly on the surface coverage. We suggest that the cosmic-ray-mediated chemistry on Titan's surface could be the dominant source of benzene, i.e., a factor of at least two orders of magnitude higher compared to previously modeled precipitation rates, in those regions of the surface which have a high surface coverage of acetylene

Novel muon imaging techniques

Owing to the high penetrating power of high-energy cosmic ray muons, muon imaging techniques can be used to image large bulky objects, especially objects with heavy shielding. Muon imaging systems work just like CT scanners in the medical imaging field—that is, they can reveal information inside of a target. There are two forms of muon imaging techniques: muon absorption imaging and muon multiple scattering imaging. The former is based on the flux attenuation of muons, and the latter is based on the multiple scattering of muons in matter. The muon absorption imaging technique is capable of imaging very large objects such as volcanoes and large buildings, and also smaller objects like spent fuel casks; the muon multiple scattering imaging technique is best suited to inspect smaller objects such as nuclear waste containers. Muon imaging techniques can be applied in a broad variety of fields, i.e. from measuring the magma thickness of volcanoes to searching for secret cavities in pyramids, and from monitoring the borders of countries checking for special nuclear materials to monitoring the spent fuel casks for nuclear safeguards applications. In this paper, the principles of muon imaging are reviewed. Image reconstruction algorithms such as Filtered Back Projection and Maximum Likelihood Expectation Maximization are discussed. The capability of muon imaging techniques is demonstrated through a Geant4 simulation study for imaging a nuclear spent fuel cask

Abnormal connectional fingerprint in schizophrenia: a novel network analysis of diffusion tensor imaging data

The graph theoretical analysis of structural magnetic resonance imaging (MRI) data has received a great deal of interest in recent years to characterize the organizational principles of brain networks and their alterations in psychiatric disorders, such as schizophrenia. However, the characterization of networks in clinical populations can be challenging, since the comparison of connectivity between groups is influenced by several factors, such as the overall number of connections and the structural abnormalities of the seed regions. To overcome these limitations, the current study employed the whole-brain analysis of connectional fingerprints in diffusion tensor imaging data obtained at 3 T of chronic schizophrenia patients (n = 16) and healthy, age-matched control participants (n = 17). Probabilistic tractography was performed to quantify the connectivity of 110 brain areas. The connectional fingerprint of a brain area represents the set of relative connection probabilities to all its target areas and is, hence, less affected by overall white and gray matter changes than absolute connectivity measures. After detecting brain regions with abnormal connectional fingerprints through similarity measures, we tested each of its relative connection probability between groups. We found altered connectional fingerprints in schizophrenia patients consistent with a dysconnectivity syndrome. While the medial frontal gyrus showed only reduced connectivity, the connectional fingerprints of the inferior frontal gyrus and the putamen mainly contained relatively increased connection probabilities to areas in the frontal, limbic, and subcortical areas. These findings are in line with previous studies that reported abnormalities in striatal–frontal circuits in the pathophysiology of schizophrenia, highlighting the potential utility of connectional fingerprints for the analysis of anatomical networks in the disorder