A simulation study of the formation of a bipolar magnetic structure

Some recent solar observations show that a bipolar magnetic flux in an active region tends to disappear in situ, in less than one solar rotation, without evidence of spreading. This feature is difficult to explain if it is assumed that magnetic buoyancy is the dominant force in controlling dynamics of a magnetic flux tube, since the assumption implies no other force to submerge the tube. These observations may be explained by assuming that a convective motion is the major cause for the formation of a $\text{Ω}$-shaped geometry of the magnetic flux tube, but that the flux tube thus arisen is submerged by the counteracting Lorentz force as the convective motion decays. A two-dimensional MHD simulation method is used to demonstrate this possibility.     

Interaction of chemical and physical processes during deformation at fluid-present conditions: a case study from an anorthosite–leucogabbro deformed at amphibolite facies conditions

We present microstructural and chemical analyses of chemically zoned and recrystallized plagioclase grains in variably strained samples of a naturally deformed anorthosite–leucogabbro, southern West Greenland. The recorded microstructures formed in the presence of fluids at mid-crustal conditions (620–640 °C, 7.4–8.6 kbar). Recrystallized plagioclase grains (average grain size 342 μm) with a random crystallographic orientation are volumetrically dominant in high-strain areas. They are characterized by asymmetric chemical zoning (An₈₀ cores and An₆₄ rims) that are directly associated with areas exhibiting high amphibole content and phase mixing. Analyses of zoning indicate anisotropic behaviour of bytownite plagioclase with a preferred replacement in the $ \left\langle {0 10} \right\rangle $ direction and along the (001) plane. In areas of high finite strain, recrystallization of plagioclase dominantly occurred by bulging recrystallization and is intimately linked to the chemical zoning. The lack of CPO as well as the developed asymmetric zoning can be explained by the activity of grain boundary sliding accommodated by dissolution and precipitation creep (DPC). In low-strain domains, grain size is on average larger and the rim distribution is not related to the inferred stress axes indicating chemically induced grain replacement instead of stress-related DPC. We suggest that during deformation, in high-strain areas, pre-existing phase mixture and stress induced DPC-caused grain rotations that allowed a deformation-enhanced heterogeneous fluid influx. This resulted in local plagioclase replacement through interface-coupled dissolution and precipitation and chemically induced grain boundary migration, accompanied by bulging recrystallization, along with neocrystallization of other phases. This study illustrates a strong interaction and feedback between physical and chemical processes where the amount of stress and fluids dictates the dominant active process. The interaction is a cause of deformation and external fluid infiltration with a result of strain localization and chemical re-equilibration at amphibolite facies conditions.     

A petrologic, geochemical and Sr–Nd isotopic study on contact metamorphism and degassing of Devonian evaporites in the Norilsk aureoles, Siberia

Devonian evaporites and associated sedimentary rocks in the Norilsk region were contact metamorphosed during emplacement of mafic sills that form part of the end-Permian (~252 Ma) Siberian Traps. We present mineralogical, geochemical and Sr–Nd isotopic data on sedimentary rocks unaffected by metamorphism, and meta-sedimentary rocks from selected contact aureoles at Norilsk, to examine the mechanisms responsible for magma-evaporite interaction and its relation to the end-Permian environmental crisis. The sedimentary rocks include massive anhydrite, rock salt, dolostone, calcareous siltstones and shale, and the meta-sedimentary rocks comprise calcareous hornfels, siliceous hornfels and minor meta-anhydrite and meta-sandstone. Contact metamorphism took place at low pressure and at maximum temperatures corresponding to the phlogopite-diopside stability field. Calcareous hornfels have high CaO, MgO, CΟ₂, SΟ₃, low SiO₂ and initial Sr isotopic ratios of 0.7079–0.7092, features indicative of calcareous siltstone protoliths. Siliceous hornfels, in contrast, have high SiO₂, Al₂O₃, Na₂O, low in other major element oxides and initial Sr isotopic ratios of 0.7083–0.7152, consistent with pelitic or shaley protoliths. Loss of CO₂ in a subset of calcareous hornfels can be explained by decarbonation reactions during metamorphism, but release of SO₂ from evaporites cannot be accounted for by a similar mechanism. Occurrences of wollastonite and a variety of hydrous minerals in the calcareous hornfels are consistent with equilibration with hydrous fluid, which was capable of leaching large quantities of anhydrite in the presence of dissolved NaCl. In this way, substantial sediment-derived sulfur could have been mobilized, incorporated into the magmatic system and released to the atmosphere. The release of CO₂ and SO₂ from Siberian evaporites added to the variety of toxic gases generated during metamorphism of organic matter, coal and rock salt, contributing to the end-Permian environmental crisis.     

Convective mixing influenced by the capillary transition zone

Convective mixing of dissolved carbon dioxide (CO₂) with formation brine has been shown to be a significant factor for the rate of dissolution of CO₂ and thus for determining the viability of geological CO₂ storage sites. In most previous convection investigations, a no-flow boundary condition was used to represent the interface between an upper region with CO₂ and brine and the single-phase brine region beneath. However, due to interfacial tension between the phases, the water phase is partly mobile in the upper region and advection may occur. Based on linear stability analysis and numerical simulations, we show that advection across the interface leads to considerable destabilization of the system. In particular, the time of onset of instability is reduced by a factor of two and the rate of dissolution is enhanced by a factor of two for three of four formations we consider, and by 40 % for the fourth formation. It is found that exponential decay of the relative permeability away from the interface provides a useful approximation to the real system. In addition, the exponential decay also simplifies the linear stability analysis. Interestingly, formations with large absolute permeability and small porosity have the largest impact from the transition zone, despite the fact that the relative permeability decays quickly above the interface in these formations. This is because the length-scale of instability is smallest in these formations.     

Cassini VIMS observations of latitudinal and hemispheric variations in Saturn’s infrared auroral intensity

The intensity of Saturn’s infrared aurora is investigated using Cassini VIMS images acquired during October 2006–February 2009. Polar and main oval auroral regions were defined in both hemispheres, which extend between 0–10° and 10–25° co-latitude, respectively. Average intensities were computed for these regions and compared. While the northern and southern main oval regions covered a similar range of intensities, the southern main oval was on average more intense by a factor of ∼1.3. The emission from the southern polar region was usually less intense than the main oval emissions, while this was only the case for approximately half of the northern hemisphere images. The northern hemisphere polar region displayed intensities more than twice as high as those in the south and the difference between the two hemispheres was most pronounced on the dayside. In general, more intense polar emissions were accompanied by more intense main oval emissions. Possible explanations for the hemispheric and latitudinal differences are discussed in terms of particle energies and fluxes, ionospheric conductivity, temperature and magnetic field strength.     

Ørkenspredningen i Sudan-Sahel regionen—et af tidens alvorlige problemer/Et spændende og markant 100 års minde—om et helt fantastisk menneske!/De fynske landsbyers historie

Erland Porsmose: De fynske landsbyers historie: i dyrkningsfællesskabets tid. Odense University studies in history and social sciences, Vol. 109.     

Risk and vulnerability analysis in practice: evaluation of analyses conducted in Swedish municipalities

Risk and vulnerability analysis (RVA) can benefit the process of preventing and preparing for disasters, both by generating a basis for making decisions and by enhancing risk awareness, safety culture and response capacity through the RVA process itself. In studying and understanding the practices related to RVA, insights can be gained regarding ways in which the RVA can be improved in society, as well as into how methods for RVA can be designed to suit the particular context. However, studies of this sort are rare. This paper presents an evaluation of RVA performed by Swedish municipalities, which are important actors in the Swedish emergency management system. This is done by employing a systematic, design science approach outlined in the paper. Document studies and interviews were used to collect data on the analyses performed by the municipalities, and the evaluation shows that there is room for improvement. The results can be especially relevant for municipalities developing their RVA practices, as well as for other actors performing similar types of analyses.     

Incorporating multiple observations for distributed hydrologic model calibration: An approach using a multi-objective evolutionary algorithm and clustering

The use of distributed data for model calibration is becoming more popular in the advent of the availability of spatially distributed observations. Hydrological model calibration has traditionally been carried out using single objective optimisation and only recently has been extended to a multi-objective optimisation domain. By formulating the calibration problem with several objectives, each objective relating to a set of observations, the parameter sets can be constrained more effectively. However, many previous multi-objective calibration studies do not consider individual observations or catchment responses separately, but instead utilises some form of aggregation of objectives. This paper proposes a multi-objective calibration approach that can efficiently handle many objectives using both clustering and preference ordered ranking. The algorithm is applied to calibrate the MIKE SHE distributed hydrologic model and tested on the Karup catchment in Denmark. The results indicate that the preferred solutions selected using the proposed algorithm are good compromise solutions and the parameter values are well defined. Clustering with Kohonen mapping was able to reduce the number of objective functions from 18 to 5. Calibration using the standard deviation of groundwater level residuals enabled us to identify a group of wells that may not be simulated properly, thus highlighting potential problems with the model parameterisation.