A radio burst with peculiar polarization behaviour in July 1974

Observing the total solar flux at 17 GHz a radio burst with unusual polarization characteristics (e.g. correlation of intensity maxima with maxima of polarization degree) has been recorded. The discussion shows that there is a possibility for a small optical depth of the source for at least one propagation mode during the burst maximum. Furthermore it could be found that the emission has been caused probably by synchroton radiation with a power law electron energy distribution (N(?)～? ^?γ) with γ≈4.     

On the relationship between hydrographs and chemographs

The spatial representativeness of gauging stations was investigated in two low‐mountainous river basins near the city of Trier, southwest Germany. Longitudinal profiles during low and high flow conditions were sampled in order to identify sources of solutes and to characterize the alteration of flood wave properties during its travel downstream. Numerous hydrographs and chemographs of natural flood events were analysed in detail. Additionally, artificial flood events were investigated to study in‐channel transport processes. During dry weather conditions the gauging station was only representative for a short river segment upstream, owing to discharge and solute concentrations of sources contiguous to the measurement site. During artificial flood events the kinematic wave velocity was considerably faster than the movement of water body and solutes, refuting the idea of a simple mixing process of individual runoff components. Depending on hydrological boundary conditions, the wave at a specific gauge could be entirely composed of old in‐channel water, which notably reduces the spatial representativeness of a sampling site. Natural flood events were characterized by a superimposition of local overland flow, riparian water and the kinematic wave process comprising the downstream conveyance of solutes. Summer floods in particular were marked by a chronological occurrence of distinct individual runoff components originating only from a few contributing areas adjacent to the stream and gauge. Thus, the representativeness of a gauge for processes in the whole basin depends on the distance of the nearest significant source to the station. The consequence of our study is that the assumptions of mixing models are not satisfied in river basins larger than 3 km². Copyright © 2006 John Wiley & Sons, Ltd.     

Method and Excel VBA Algorithm for Modeling Master Recession Curve Using Trigonometry Approach

A new method was developed and implemented into an Excel Visual Basic for Applications (VBAs) algorithm utilizing trigonometry laws in an innovative way to overlap recession segments of time series and create master recession curves (MRCs). Based on a trigonometry approach, the algorithm horizontally translates succeeding recession segments of time series, placing their vertex, that is, the highest recorded value of each recession segment, directly onto the appropriate connection line defined by measurement points of a preceding recession segment. The new method and algorithm continues the development of methods and algorithms for the generation of MRC, where the first published method was based on a multiple linear/nonlinear regression model approach (Posavec et al. 2006). The newly developed trigonometry‐based method was tested on real case study examples and compared with the previously published multiple linear/nonlinear regression model‐based method. The results show that in some cases, that is, for some time series, the trigonometry‐based method creates narrower overlaps of the recession segments, resulting in higher coefficients of determination R², while in other cases the multiple linear/nonlinear regression model‐based method remains superior. The Excel VBA algorithm for modeling MRC using the trigonometry approach is implemented into a spreadsheet tool (MRCTools v3.0 written by and available from Kristijan Posavec, Zagreb, Croatia) containing the previously published VBA algorithms for MRC generation and separation. All algorithms within the MRCTools v3.0 are open access and available free of charge, supporting the idea of running science on available, open, and free of charge software.     

Environmental constraints on lake sediment mineral compositions from the Tibetan Plateau and implications for paleoenvironment reconstruction

Inorganic minerals form a major component of lacustrine sediments and have the potential to reveal detailed information on previous climatic and hydrological conditions. The ability to extract such information however, has been restricted by a limited understanding of the relationships between minerals and the environment. In an attempt to fill in this gap in our knowledge, 146 surface sediment samples have been investigated from 146 lakes on the Tibetan Plateau. The mineral compositions derived from these samples by X-Ray Diffraction (XRD) were used to examine the relationships between mineral compositions and the environmental variables determined for each site. Statistical techniques including Multivariate regression trees (MRT) and Redundancy Analysis (RDA), based on the mineral spectra and environmental variables, reveal that the electrical conductivity (EC) and Mg/Ca ratios of lake water are the most important controls on the composition of endogenic minerals. No endogenic minerals precipitate under hyper-fresh water conditions (EC lower than 0.13 mS/cm), with calcite commonly forming in water with EC values above 0.13 mS/cm. Between EC values of 0.13 and 26 mS/cm the mineral composition of lake sediments can be explained in terms of variations in the Mg/Ca ratio: calcite dominates at Mg/Ca ratios of less than 33, whereas aragonite commonly forms when the ratio is greater than 33. Where EC values are between 26 and 39 mS/cm, monohydrocalcite precipitates together with calcite and aragonite; above 39 mS/cm, gypsum and halite commonly form. Information on the local geological strata indicates that allogenic (detrital) mineral compositions are primarily influenced by the bedrock compositions within the catchment area. By applying these relationships to the late glacial and Holocene mineral record from Chaka Salt Lake, five lake stages have been identified and their associated EC conditions inferred. The lake evolved from a freshwater lake during the late glacial (before 11.4 cal. ka BP) represented by the lowest EC values (<0.13 mS/cm), to a saline lake with EC values slightly higher than 39 mS/cm during the early and mid Holocene (ca. 11.4–5.3 cal. ka BP), and finally to a salt lake (after 5.3 cal. ka BP). These results illustrate the utility of our mineral-environmental model for the quantitative reconstruction of past environmental conditions from lake sediment records.     

Post-kinematic recovery of fractured cordierite in the lower amphibolite facies: the significance of intra-crystal cation diffusion for retrograde partial mineral breakdown

Brittle intra-crystal fracturing during a micro-seismic event under amphibolite facies P–T conditions (ca. 4.5 kbar, 555°C) formed micro-shear zones and brittle fragments in cordierite from migmatites of the Ordovician Sierras Pampeanas (NW Argentina). During post-seismic static recovery and coarsening of crystal fragments, primary cordierite (X_Mg?=?0.65) underwent partial breakdown within the fractured and subsequently recovered and recrystallized part of primary cordierite. Post-kinematic partial primary cordierite breakdown results in the formation of secondary sillimanite, magnetite, staurolite, quartz, and secondary cordierite (X_Mg?=?0.70–0.80) within a ca. 300–500 μm thick alteration zone. Secondary cordierite, volumetrically the most important secondary phase, essentially forms by iron and magnesium diffusion and substitution. All other secondary phases form by nucleation. The retrograde breakdown occurred under near-isochemical conditions with only limited influx of hydrothermal fluids containing minor zinc, potassium, silica, and excess oxygen. The observed modal composition of the alteration zone is similar to results of thermodynamic modeling of primary cordierite breakdown, using multiphase equilibria software. This indicates that the observed reaction reflects conditions of, or close to, thermodynamic equilibrium. Although fluid was present during, and may have been an important factor contributing to, partial mineral breakdown, we consider post-kinematic recovery and recrystallization, effected by a combination of cation diffusion and elimination of dislocations, as the key process facilitating retrograde metamorphism and the generation of the secondary mineral assemblage in (near-) equilibrium modes.     

Velocity and Intensity Power and Cross Spectra in Numerical Simulations of Solar Convection

Fitting observed power and cross spectra of medium-degree p modes in velocity (V) and intensity (I) has been widely used for getting information about the p-mode excitation process and, in particular, for trying to determine the type and location of the acoustic sources. Numerical simulations of solar convection allow one to “observe” velocity and temperature (T, used as proxy for I) fluctuations in different reference frames. Sampling the oscillations on planes of constant optical depth (τ-frame) closely corresponds to the observer’s point of view, whereas sampling the oscillations at constant geometrical height (z-frame) is more appropriate for comparison with predictions from theoretical models based on Eulerian hydrodynamics. The results of the analysis in the two frames show significant differences. Considering the effects introduced on oscillations by the steep temperature gradient of the photosphere and by the temperature- and pressure-dependent continuum opacity, we develop a new model for fitting the simulated V and T power and cross spectra both in the τ- and z-frames and discuss its merits and limitations.