Factors affecting longitudinal dispersion in estuaries of different scale

Traditionally, the overall diluting capacity of an estuary is characterized using a coefficient of longitudinal dispersion, K_xe, which is given by the ratio of the dispersive flux of a dissolved substance to its tidally averaged longitudinal gradient. A steady-state model, which assumes a balance between the dispersive and non-dispersive fluxes and an exponential increase in estuary cross-sectional area towards the sea, has been used to derive expressions for the axial salinity distribution and the dispersive flux of salt. The model was set up assuming either a constant dispersion coefficient along the estuary or one that increased with distance towards the sea. By comparing salinity predictions with data from five UK estuaries, estimates were made of the maximum dispersive salt flux and the corresponding maximum salinity gradient for each system. The results indicated that there was an approximately linear relation between the fluxes and gradients, and the slope of a line plotted through the origin provided an estimate of a common K_xe for all five estuaries. The magnitude of K_xe was found to be about 90 m² s^–1 with a standard deviation of approximately ±32 m² s^–1. It is concluded that a representative value of 100 m² s^–1 for K_xe is a reasonable first choice when setting up a cross-sectionally averaged estuary model. The results also showed that larger systems, such as the Thames, had lower salinity gradients and lower dispersive salt fluxes, whilst smaller estuaries displayed the opposite characteristics. The model was used to predict the variation in the non-advective flux of salt along an estuary. The distribution was found to be similar to the corresponding flux distribution estimated from observations at the seaward end of the Tees estuary, despite appreciable spatial variations in the individual flux components. Allowing for a small decrease in freshwater flow, the model indicated that there was a decrease in the maximum dispersive flux between neap and spring tides. It is argued that such a reduction in flux can result in a seaward shift in the salinity distribution to a region of greater cross-section, where the freshwater transport per unit area again balances the reduced upstream dispersive flux, as found in the neap to spring response in the Tees estuary.Responsible Editor: Hans Burchard     

Algorithms for GPS operation indoors and downtown

The proliferation of mobile devices and the emergence of wireless location-based services has generated consumer demand for availability of GPS in urban and indoor environments. This demand calls for enhanced GPS algorithms that accommodate high degrees of signal attenuation and multipath effects unique to the "urban channel." This paper overviews the market for location-based services and discusses algorithmic innovations that address challenges posed by urban environments. Electronic Publication     

Trace-element concentrations in single circumstellar silicon carbide grains from the Murchison meteorite

Abstract— Concentrations of the trace elements Mg, Al, Ca, Ti, V, Fe, Sr, Y, Zr, Ba and Ce were determined by ion microprobe mass spectrometry in 60 individual silicon carbide (SiC) grains (in addition, Nb and Nd were determined in 20 of them), from separate KJH (size range 3.4–5.9 μm) of the Murchison carbonaceous meteorite, whose C-, N- and Si-isotopic compositions have been measured before (Hoppe et al., 1994) and provide evidence that these grains are of stellar origin. The selected SiC grains represent all previously recognized subgroups: mainstream (20 < ¹²C/¹³C < 120; 200 < ¹⁴N/¹⁵N; Si isotopes on slope 1.34 line), grains A (¹²C/¹³C < 3.5), grains B (3.5 < ¹²C/¹³C < 10), grains X (¹⁵N excesses, large ²⁸Si excesses) and grains Y (150 < ¹²C/¹³C < 260; Si isotopes on slope 0.35 line). Data on these grains are compared with measurements on fine-grained SiC fractions. Trace-element patterns reflect both the condensation behavior of individual elements and the source composition of the stellar atmospheres. A detailed discussion of the condensation of trace elements in SiC from C-rich stellar atmospheres is given in a companion paper by Lodders and Fegley (1995). Elements such as Mg, Al, Ca, Fe and Sr are depleted because their compounds are more volatile than SiC. Elements whose compounds are believed to be more refractory than SiC can also be depleted due to condensation and removal prior to SiC condensation. Among the refractory elements, however, the heavy elements from Y to Ce (and Nd) are systematically enriched relative to Ti and V, indicating enrichments by up to a factor of 14 of the s-process elements relative to elements lighter than Fe. Such enrichments are expected if N-type carbon stars (thermally pulsing AGB stars) are the main source of circumstellar SiC grains. Large grains are less enriched than small grains, possibly because they are from different AGB stars. The trace-element patterns of subgroups such as groups A and B and grains X can at least qualitatively be understood if grains A and B come from J-type carbon stars (known to be lacking in s-process enhancements shown by N-type carbon stars) or carbon stars that had not experienced much dredge-up of He-shell material and if grains X come from supernovae. However, a remaining puzzle is how stars become carbon stars without much accompanying dredge-up of s-process elements.     

A note on the self-consistency of the EIH equations of motion

The self-consistency of the Einstein Infeld and Hoffman (EIH) equations of motion is critically examined in the limiting case of a threebody problem where two bodies are very close to each other and a third quite far removed from them     

Vereinfachte Berechnung der Radioaktivität und ihrer Streuung im Wasser für ein festgelegtes Arbeitsschema

Ohne Zusammenfassung Herrn Prof. Dr. O. Faag zu seinem 60. Geburtstag in Verebrung gewidmet     

Interstellar graphite in meteorites: Isotopic compositions and structural properties of single graphite grains from Murchison

Abstract— One hundred forty-three carbon grains, ranging in size from 2 to 8 μm, from two chemical and physical separates from the Murchison CM2 chondrite, were analyzed by ion microprobe mass spectrometry for their C- and N-isotopic compositions. Both separates are enriched in the exotic noble gas component Ne-E(L). Ninety grains were also analyzed for their H and O contents and 118, for Si. Thirteen grains were analyzed by micro-sampling laser Raman spectroscopy. Round grains have large C-isotopic anomalies with ¹²C/¹³C ratios ranging from 7 to 4500 (terrestrial ratio = 89). Nitrogen in these grains is also anomalous but shows much smaller deviations from the terrestrial composition, ¹⁴N/¹⁵N ratios ranging from 193 to 680 (terrestrial ratio = 272). Spherulitic aggregates and non-round compact grains have normal C-isotopic ratios but ¹⁵N excesses (up to 35%). Raman spectra of the analyzed grains indicate varying degrees of crystalline disorder of graphite with estimated in-plane crystallite dimensions varying from 18 Å (highly disordered, similar to terrestrial kerogen) to ～750 Å (well-crystallized graphite). Element contents of H, O, and Si are correlated with one another, and H and O are probably present in the form of organic molecules. On the basis of morphology, the round grains fall into two groups: grains with smooth, shell-like surfaces (“onions”) and grains that appear to be dense aggregates of small scales (“cauliflowers”). “Onions” tend to have lower trace element contents, isotopically light C (¹²C/¹³C > 89) and a high degree of crystalline order, whereas “cauliflowers” have a larger spread in trace element contents and C-isotopic ratios (they range from isotopically light to heavy) but tend to have a low degree of crystalline order. However, these differences exist only on average, and no clear distinction can be made for individual grains. A few limited conclusions can be drawn about the astrophysical origin of the carbon grains of this study. The ¹⁵N excesses in spherulitic aggregates and non-round grains can be explained as the result of ion-molecule reactions in molecular clouds. The round grains, on the other hand, must have formed in stellar atmospheres (circumstellar grains). Grains with isotopically light C must have formed in stellar environments characterized by He-burning, either in the atmosphere of Wolf-Rayet stars during the WC phase or in the He-burning, ¹²C-rich zone of a massive star, ejected by a supernova explosion. Isotopically heavy C is produced by H-burning in the CNO cycle. Possible sources for grains with heavy C are carbon stars (AGB stars during the thermally pulsing phase) or novae, but the detailed distribution of ¹²C/¹³C ratios agree neither with the distribution observed in carbon stars nor with theoretical predictions for these two types of stellar sources.