Solar Filaments and Photospheric Network

The locations of barbs of quiescent solar filaments are compared with the photospheric/chromospheric network, which thereby serves as a proxy of regions with enhanced concentrations of magnetic flux. The study covers quiet regions, where also the photospheric network as represented by flow converging regions, i.e., supergranular cell boundaries, contain largely weak magnetic fields. It is shown that close to 65% of the observed end points of barbs falls within the network boundaries. The remaining fraction points into the inner areas of the network cells. This confirms earlier findings (Lin et al., Solar Physics, 2004) that quiescent filaments are basically connected with weaker magnetic fields in the photosphere below.     

A Generalized Structure-Activity Relationship for the Decomposition of (Substituted) Alkoxy Radicals

A novel and readily applicable Structure-Activity Relationship (SAR) for predicting the barrier height E_b to decomposition by C-C scission of (substituted) alkoxy radicals is presented. Alkoxy radicals are pivotal intermediates in the atmospheric oxidation of (biogenic) volatile organic compounds, and their fate is therefore of crucial importance to the understanding of atmospheric VOC degradation mechanisms. The SAR is based on available theoretical energy barriers and validated against barriers derived from experimental data. The SAR is expressed solely in terms of the number(s) N_i of alkyl-, hydroxy- and/or oxo-substituents on the - and -carbons of the breaking bond: E_b(kcal/mol) =17.5 – 2.1 × N(alk) – 3.1 ×N(alk) – 8.0 × N_,(OH) – 8.0 × N(O=) – 12 × N(O=). For barriers below 7 kcal/mol, an additional, second-order term accounts for the curvature. The SAR reproduces the available experimental and theoretical data within 0.5 to 1 kcal/mol. The SAR generally allows conclusive predictions as to the fate of alkoxy radicals; several examples concerning oxy radicals from prominent atmospheric VOC are presented. Specific limitations of the SAR are also discussed. Using the predicted barrier height E_b, the high-pressure rate coefficient for alkoxy decomposition k _diss (298 K) can be obtained from k _diss (298 K) = L ×1.8 × 10¹³ exp(–E_b/RT) s^–1, with L the reaction path degeneracy.     

Microbial contributions to N-immobilization and organic matter preservation in decaying plant detritus

Microbial contributions to the detritus of two vascular plant tissues, smooth cordgrass (Spartina alterniflora) and black mangrove leaves (Avicennia germinans), were estimated over a 4-year decomposition period under subaqueous marine conditions. During this period, 93-97% of the initial plant tissues was decomposed. Bulk elemental and isotopic compositions of the detritus were measured along with hydrolyzable amino sugars (AS) and amino acids (AA), including the bacterial biomarkers muramic acid and the d-enantiomers of AA. A major enrichment in N relative to C occurred during decomposition. Net increases of AS, AA, and bacterial biomarkers in decaying detritus were observed. Three independent approaches indicated that on average 60-75% of the N and 20-40% of the C in highly decomposed detritus were not from the original plant tissues but were mostly from heterotrophic bacteria. During decomposition hydrolyzable AS + AA yields (∼54% of total N) were strongly correlated with total N in both types of detritus. The uncharacterized N appeared to have the same origin and dynamics as AA, suggesting the contribution of other bacterial biomolecules not measured here. There was little indication of humification or abiotic processes. Instead, N-immobilization appeared primarily bacterially mediated. Although varying dynamics were observed among individual molecules, bacterial detritus exhibited an average reactivity similar to plant detritus. Only a minor fraction of the bacterial detritus escaped rapid biodegradation and the relationship between bacterial activity and N-immobilization is consistent with an enzymatically mediated preservation mechanism. Bacteria and their remains are ubiquitous in all ecosystems and thus could comprise a major fraction of the preserved and uncharacterized organic matter in the environment.     

A Novel Method for Removal of Emitter Noise in SAR Images

Traditionally, interfering emitter signatures have been removed through notched filtering in the range (fast-time) dimension. This works well when a narrowband emitter interferes with a wideband radar pulse; however, when the emitter and radar signal bandwidths are comparable, then this approach fails since the noise is distributed throughout the pulse and the image as well. In cases where the interfering signal is localized in the cross range, joint time-frequency methods can often focus this interference signal, thereby transforming the image. In this transformed image, the interferer is the foreground, and the desired synthetic aperture radar image is blurred and now the background. The focused compressed interferer can be analyzed and censored from the transformed image. Back transformation restores the image with the interference removed. This technique has been fully automated and applied to an Electromagnetics Institute Synthetic Aperture Radar (EMISAR) image contaminated by a nonstationary emitter. The cleansed image is virtually free of the emitter interference     

The farmer,the worker and the MP

The territorial dimension of the digital divide is usually considered as a phenomenon that penalizes the peripheral regions, especially in terms of regional economic development. Taking into account the territorial networking of ICT (Information & Communication Technologies) infrastructures—particularly high-speed networks—provides what is probably the principal reason for such a perception. This is particularly true considering that the most-peripheral regions and those with the smallest population densities are also the poorest in terms of ICT infrastructures. In Western countries, however, the digital divide is no longer the result of network-related problems. Nowadays, the issue of the skills required to adequately exploit the potential of ICT is at the forefront. Yet this evolution is likely to lead to an inversion of the inequalities between the centre and the periphery, as populations without such skills—recent immigrants, the unemployed, the illiterate, people with little education or on low incomes and other socially marginalized people—are generally concentrated in urban centres. Consequently, the priority for reducing inequalities of access to ICT resources is no longer the provision of high-performance ICT infrastructures for peripheral regions, but rather the implementation of continuing education and social action policies within the urban centres.     

Anion exclusion effects in compacted bentonites: Towards a better understanding of anion diffusion

Diffusion of ³⁶Cl⁻ in compacted bentonite was studied using through-diffusion, out-diffusion and profile analysis techniques. Both the bulk dry density of the bentonite and the composition of the external solution were varied. Increasing the bulk dry density of the bentonite resulted in a decrease of both the effective diffusion coefficient and the Cl-accessible porosity. Increasing the ionic strength of the external solutions resulted in an increase of both the effective diffusion coefficient and the Cl-accessible porosity. This can be explained by anion exclusion effects (Donnan exclusion). At high ionic strength values (I ? 1 M NaCl) the Cl-accessible porosity approaches the interparticle porosity. This interparticle porosity is the difference between the total and interlayer porosity of the bentonite. The interlayer porosity was found to depend on the degree of compaction. Up to a bulk dry density of 1300 kg m⁻³ the interlayer is built up of 3 water layers. Between 1300 and 1800 kg m⁻³ the interlayer water is reduced from 3 to 2 layers of water. Above 1800 kg m⁻³ evidence for a further decrease to 1 layer of water was found. These findings are in agreement with X-ray data found in the literature showing a decrease of the basal spacing of montmorillonite (the main clay mineral in bentonite) with increasing degree of compaction. The relationship between the effective diffusion coefficient of Cl⁻ and the diffusion-accessible porosity can be described by an empirical relationship analogous to Archie’s law. To predict the effective diffusion coefficient of Cl⁻ in compacted bentonite, the diffusion coefficient of Cl⁻ in water, the bulk dry density and the ionic strength of the pore water have to be known.     

Short note: Applications of third-order statistics for the automatic time picking of seismic events

We present a new automatic time-picking method based on third-order statistics, namely bicoherence correlation. Contrary to conventional methods, which are based on second-order statistics (i.e. cross-correlation or neural-network trainings), our method is less sensitive to coloured noise as well as the bandwidth of the signal. Bicoherence correlation can also be used for autotracking events in seismic data for an interpretation.     

Propagation algorithms for attitude-related ambiguity resolution

A new approach for ambiguity resolution is presented and tested by means of Monte-Carlo simulations. This approach applies to both single and double phase differences sampled at a given instant, and to any number of baselines and lines of sight, provided the number of lines of sight exceeds a certain threshold. The algorithms are based on the combined global geometry of the baselines and lines of sight and thereby consider all geometric constraints that are inherent in the problem. The algorithms have been programmed and applied to a large number of different configurations of GPS sight lines, and limited to baselines of approximately 1 m in length and the GPS L1 frequency only. The cause of the occurrence of multiple solutions has been investigated. To check the adequacy and limitations of the new algorithms for practical application, it was assessed in which proportion the algorithms either failed to provide a result, provided a false result, or provided a good result as a function of number of baselines, lines of sight and accuracy of the ambiguous phase difference measurements. Electronic Publication     

Les minéralisations Cu(NiBiUAuAg) d'Ifri (district du Haut Seksaoua,Maroc) : apport de l'étude texturale au débat syngenèse versus épigenèse

The Cu ore of Ifri is a chalcopyrite stockwork hosted by Cambrian formations and was until now interpreted as a syngenetic massive sulphide deposit. Textural studies highlight two generations of pyrite early (Py I) and late (Py II) with respect to the regional deformation. The chalcopyrite stockwork overprinted Py II, outlining the epigenetic nature of the Cu mineralization. Regarding the origin of Cu-depositing fluids, the presence in the stockwork paragenesis of an U, W, Sn association and preliminary Pb/Pb dating of a brannerite belonging to this association suggest a contribution of the Tichka granite. To cite this article: L. Barbanson et al., C. R. Geoscience 335 (2003).