Multiple flows and the fine structure of the transition region around sunspots

The fine structure in the flow field in the transition region above and surrounding a sunspot is determined fromCIV 1548 line profiles, observed with the High Resolution Telescope and Spectrograph (HRTS) during the Spacelab 2 mission. The observed line profiles show one, two, or three distinct velocity components within the resolution element of 1 × 1. Supersonic flows occur in small regions where the line profile has two or three components. The line component that shows supersonic speed often is weaker than the subsonic line component, which may explain why some observers have been unable to detect the supersonic flow component. The broadening of individual line components shows non-thermal velocities close to 20 km s^–1. This suggests that turbulence is less important than usually considered.The presence of multiple flows, which also occurs in quiet solar regions, suggests that the transition region above sunspots has a sub-arc-second fine structure, perhaps consisting of thin fibrils. The Evershed flow in the transition region appears to have a correspondingly complex character, possibly consisting of sub- and supersonic siphon flows along the individual fibrils. Time changes in the flow field over 5 min may correspond to characteristic times of individual fine structures. Possible explanations of the net downward directed mass flux are presented.     

Chemical abundance behavior of type I planetary nebulae

In this work 13 Planetary Nebulae have been classified as Type I according to Peimbert's criteria (Peimbert, 1978). These objects have been added to a previous sample (Maciel and Faúndez-Abans, 1985) and diagrams of O/H versus N/H, S/H, Ne/H and Ar/H, as well as N/H versus S/H, Ne/H and Ar/H have been drawn. All of them exhibit a tendency for linear correlation; moreover, the behavior of O and N versus Ar and S are very similar, with approximately the same slope. When the excitation class parameter was included in the diagrams, no clear tendency can be discerned, for any class.     

Adsorption of fluoride on goethite surfaces-implications on dental epidemiology

Fluoride ion interaction with synthetically prepared goethite has been investigated over a range of pH values (4–9) and F^– concentrations (10^–3–10^–5 M). The amount of F^– retained by goethite suspensions was found to be a function of pH, media ionic strength, F^– concentration, and goethite concentration. The lowest ionic strength (0.001 M KNO₃) gave the highest adsorption medium. Uptake was minimal at pH >7 and increased with decreasing pH. Thermodynamic properties for fluoride adsorption at 298 K and 323 K were investigated. The isosteric heat of adsorption (H _r) was calculated and the heterogeneity and homogeneity of the surface examined for goethite. In view of the importance of fluoride in dental health, the interaction of fluoride on goethite in the physical environment has important implications on dental epidemiology.     

The role of bathymetry and directional wave conditions on observed crescentic bar dynamics

Nearshore sandbars are important features in the surf zone of many beaches because they strongly influence the mean circulation and evolving morphology. Due to variations in wave conditions, sandbars can experience cross-shore migration and vary in shape from alongshore uniform (shore-parallel) to alongshore rhythmic (crescentic). Sandbar dynamics have been studied extensively, but existing observational studies usually do not quantify the processes leading to crescentic bar formation and straightening. This study analyses the dynamics of crescentic bar events at the fetch-limited beach of Castelldefels (northwestern Mediterranean Sea, Spain) using 7.5 years of hourly time-exposure video images and detailed wave conditions. The results show that, despite the generally calm wave conditions, the sandbars were very dynamic in the cross-shore and longshore directions. They often migrated rapidly offshore during storms (up to 70 m in one day) and more slowly onshore during post-storm conditions. Crescentic bars were often present at the study site (48% of the time), but only when the sandbar was at least 10 m from the shoreline. They displayed a large variability in wavelengths (100–700 m), alongshore migration speeds (0–50 m/day) and cross-shore amplitudes (5–20 m). Wavelengths increased for larger bar–shoreline distances and the alongshore migration speeds were strongly correlated with the alongshore component of the radiation stresses. Crescentic patterns typically developed during low–medium energetic waves with limited obliquity ( $\theta \lesssim 20$° at 10 m depth), while bar straightening occurred during medium–high energetic waves with strong oblique angles of incidence ( $\theta \gtrsim 15$°). Overall, this study provides further proof for the important role of wave direction in crescentic bar dynamics and highlights the strong dependence of crescentic bar development on the initial bathymetric configuration.     

Carbonatites from Eastern Paraguay and genetic relationships with potassic magmatism: C, O, Sr and Nd Isotopes

Summary Geochemical characteristics were systematically determined for Early Cretaceous samples of carbonatitic rocks from Eastern Paraguay (Rio Apa, Amambay and Central Provinces). The data show that all the occurrences have an enriched isotopic signature and that the carbonatites have negligible or absent crustal signature. A petrogenetic model (parent liquids, fractional crystallization, hydrothermal interactions and weathering) is proposed as a function of incompatible trace element, stable (O-C) and radiogenic (Sr-Nd) isotope variations with the aim to test the significance of carbonatitic complexes as a marker of the metasomatized subcontinental lithospheric mantle. The results indicate that the carbonatites and primary carbonates from eastern Paraguay, and those from the north eastern Paraná Basin (SE Brazil), were affected by metasomatic events distinct in time and composition.

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Karbonatite aus Ost-Paraguay und ihre genetische Beziehung zu Kalium-Magmatismus: C O, Sr und Nd isotope

Zusammenfassung Die geochemischen Charakteristika von frühkretazischen Karbonatitproben aus Ostparaguay (Rio Alpa, Amambay und Zentrale Provinzen) wurden untersucht. Die Daten belegen, daß alle Vorkommen eine isotopische Anreicherungssignatur zeigen und daß ihnen eine entsprechende Krustensignatur fehlt. Ein Petrologisches Modell (Ausgangsschmelze, fraktionierte Kristallisation, hydrothermale Interaktion und Verwitterung) wird auf Grund der Verteilung der inkompatiblen Spurenelemente, der stabilen (C-O) und radiogenen (Sr-Nd) Isotope vorgeschlagen. Es versucht die Bedeutung der Karbonatitkomplexe als Markerhorizonte des metasomatischen subkontinentalen Mantels zu überprüfen. Die Ergebnisse zeigen, daß die Karbonatite und die primären Karbonate in Ostparaguay, und jene aus dem Paraná Becken SüdostBrasiliens durch zeitlich und zusammensetzungsmäßig unterschiedliche metasomatische Prozesse erfaßt wurden.

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With 10 Figures     

Paleomagnetism of the Esterel rocks: a revisit 22 years after the thesis of Hans Zijderveld

For his PhD. thesis, Zijderveld (1975) studied the paleomagnetism of the Permian Esterel rocks (southern France). High-quality thermal and alternating-field demagnetization diagrams were interpreted to determine the direction of the characteristic natural magnetization. For the Esterel volcanics, a mean direction of Dec = 206.5°, Inc = –23°, ₉₅ = 5.7°, k = 112 was found for this magnetization. The dispersion in this mean is remarkably low. Only the declination of the Reyran Rhyolite in the Reyran River quarry clearly deviated from this mean. This deviating direction is not found in our samples, taken at the same site. As many faults occur in this quarry, it is suggested that Zijderveld sampled this rhyolite on a small rotated block. To verify whether the small dispersion in the mean paleomagnetic direction of the Esterel rocks has a geomagnetic or a rock-magnetic origin, two conglomerate tests were carried out. One of these might be interpreted as positive. The results of the other conglomerate test (Agay Formation) are ambiguous: four of the six measured boulders show directions close to the mean paleomagnetic direction of the Esterel rocks. Rock-magnetic measurements show that the remanence is carried by a magnetite and a hematite fraction. The low dispersion in the paleomagnetic directions, the conglomerate tests, and hematite as remanence carrier suggest that the characteristic remanence in the Esterel volcanics was not instantaneously acquired during cooling, but might be affected by remagnetization due to weathering.     

Polycyclic aromatic hydrocarbons (PAHs) in Antarctic Martian meteorites, carbonaceous chondrites, and polar ice

Recent analyses of the carbonate globules present in the Martian meteorite ALH84001 have detected polycyclic aromatic hydrocarbons (PAHs) at the ppm level (McKay et al., 1996). The distribution of PAHs observed in ALH84001 was interpreted as being inconsistent with a terrestrial origin and were claimed to be indigenous to the meteorite, perhaps derived from an ancient martian biota. We have examined PAHs in the Antarctic shergottite EETA79001, which is also considered to be from Mars, as well as several Antarctic carbonaceous chondrites. We have found that many of the same PAHs detected in the ALH84001 carbonate globules are present in Antarctic carbonaceous chondrites and in both the matrix and carbonate (druse) component of EETA79001. We also investigated PAHs in polar ice and found that carbonate is an effective scavenger of PAHs in ice meltwater. Moreover, the distribution of PAHs in the carbonate extract of Antarctic Allan Hills ice is remarkably similar to that found in both EETA79001 and ALH84001. The reported presence of L-amino acids of apparent terrestrial origin in the EETA79001 druse material (McDonald and Bada, 1995) suggests that this meteorite is contaminated with terrestrial organics probably derived from Antarctic ice meltwater that had percolated through the meteorite. Our data suggests that the PAHs observed in both ALH84001 and EETA79001 are derived from either the exogenous delivery of organics to Mars or extraterrestrial and terrestrial PAHs present in the ice meltwater or, more likely, from a mixture of these sources. It would appear that PAHs are not useful biomarkers in the search for extinct or extant life on Mars.     

The Ms = 6.2, June 15, 1995 Aigion earthquake (Greece): evidence for low angle normal faulting in the Corinth rift

We present the results of a multidisciplinary study of the M_s = 6.2, 1995, June 15, Aigion earthquake (Gulf of Corinth, Greece). In order to constrain the rupture geometry, we used all available data from seismology (local, regional and teleseismic records of the mainshock and of aftershocks), geodesy (GPS and SAR interferometry), and tectonics. Part of these data were obtained during a postseismic field study consisting of the surveying of 24 GPS points, the temporary installation of 20 digital seismometers, and a detailed field investigation for surface fault break. The Aigion fault was the only fault onland which showed detectable breaks (< 4 cm). We relocated the mainshock hypocenter at 10 km in depth, 38 ° 21.7 N, 22 ° 12.0 E, about 15 km NNE to the damaged city of Aigion. The modeling of teleseismic P and SH waves provides a seismic moment M_o = 3.4 10¹⁸ N.m, a well constrained focal mechanism (strike 277 °, dip 33 °, rake – 77°), at a centroidal depth of 7.2 km, consistent with the NEIC and the revised Harvard determinations. It thus involved almost pure normal faulting in agreement with the tectonics of the Gulf. The horizontal GPS displacements corrected for the opening of the gulf (1.5 cm/year) show a well-resolved 7 cm northward motion above the hypocenter, which eliminates the possibility of a steep, south-dipping fault plane. Fitting the S-wave polarization at SERG, 10 km from the epicenter, with a 33° northward dipping plane implies a hypocentral depth greater than 10 km. The north dipping fault plane provides a poor fit to the GPS data at the southern points when a homogeneous elastic half-space is considered: the best fit geodetic model is obtained for a fault shallower by 2 km, assuming the same dip. We show with a two-dimensional model that this depth difference is probably due to the distorting effect of the shallow, low-rigidity sediments of the gulf and of its edges. The best-fit fault model, with dimensions 9 km E–W and 15 km along dip, and a 0.87 m uniform slip, fits InSAR data covering the time of the earthquake. The fault is located about 10 km east-northeast to the Aigion fault, whose surface breaks thus appears as secondary features. The rupture lasted 4 to 5 s, propagating southward and upward on a fault probably outcropping offshore, near the southern edge of the gulf. In the shallowest 4 km, the slip – if any – has not exceeded about 30 cm. This geometry implies a large directivity effect in Aigion, in agreement with the accelerogram aig which shows a short duration (2 s) and a large amplitude (0.5 g) of the direct S acceleration. This unusual low-angle normal faulting may have been favoured by a low-friction, high pore pressure fault zone, or by a rotation of the stress directions due to the possible dip towards the south of the brittle-ductile transition zone. This fault cannot be responsible for the long term topography of the rift, which is controlled by larger normal faults with larger dip angles, implying either a seldom, or a more recently started activity of such low angle faults in the central part of the rift.     

The annual cycle and the predictability of the tropical coupled ocean-atmosphere system

Summary Using large-scale circulation statistics from the Pacific Ocean basin, predictability of the coupled ocean-atmosphere system on interannual time scales is found both to be limited in extent and to possess a strong annual cycle. Irrespective of when lagged correlations are commenced, correlations decrease rapidly through the boreal spring, indicating an inherent predictability limitation for large scale coupled oceanicatmospheric processes such as El Niño. Long term prediction experiments using numerical coupled-models show that the models are excellent facsimiles of the real system. They, too, encounter the predictability barrier and exhibit a substantial decrease in observation-prediction correlation across the boreal spring. Thus, a predictive system based solely on the interactive physics of the Pacific Basin appears limited to a maximum of less than one year and a minimum of only one or two months.Two hypotheses are made to explain the existence of the predictability barrier. First, it is argued that the tropical coupled system is at its frailest state during the boreal spring and that the signal-to-noise ratio is weakest. In such a system, maximum random error growth may occur as the atmosphere and the ocean become temporally detached and wander onto different climate trajectories. A series of 144 preliminary Monte Carlo experiments were conducted with a coupled ocean-atmosphere model to test the hypothesis. Irrespective of when the experiments were commenced, error growth was maximized at the same time of the year. The second hypothesis suggests that the near-equatorial circulation is perturbed at the time of its weakest state by external influences such as the monsoon and that the climate wanderings are nudged deterministically. There is observational and theoretical evidence to support the hypothesis. Observations suggest that anomalous monsoons impart basin-wide coherent alterations of the wind stress field in the Pacific Ocean. Experiments with a coupled ocean-atmosphere model show that the period of an ENSO event is altered substantially by an anomalous monsoon. Given that there appear to be precursors to anomalous monsoons, it is suggested that there may be ways to avoid the predictability barrier and thus extend prediction of the entire system.Finally, noting that the two hypotheses are not mutually exclusive, they are combined to form a unified theory. As the asymmetric monsoonal and the symmetric near-equatorial heating are in approximate quadrature, it is argued that the monsoons influence the Walker circulation during the boreal spring. However, during the boreal fall and early winter the near-equatorial heating variability dominates the winter monsoon.With 18 Figures     

Biomass burning in the tropical savannas of Ivory Coast: An overview of the field experiment Fire of Savannas (FOS/DECAFE 91)

FOS/DECAFE 91 (Fire of Savannas/Dynamique et Chimie Atmosphérique en Forêt Equatoriale) was the first multidisciplinary experiment organized in Africa to determine gas and aerosol emissions by prescribed savanna fires. The humid savanna of Lamto in Ivory Coast was chosen for its ecological characteristics representative of savannas with a high biomass density (900 g m^–2 dry matter). Moreover the vegetation and the climate of Lamto have been studied for more than twenty years. The emission ratios (X/CO₂) of the carbon compounds (CO₂, CO, NMHC, CH₄, PAH, organic acids and aerosols), nitrogen compounds (NO_x, N₂O, NH₃ and soluble aerosols) and sulfur compounds (SO₂, COS and aerosols) were experimentally determined by ground and aircraft measurements. To perform this experiment, 4 small plots (100×100 m) and 2 large areas (10×10 km) were prepared and burnt in January 1991 during the period of maximum occurrence of fires in this type of savanna. The detailed ecological study shows that the carbon content of the vegetation is constant within 1% (42 g C for 100 g of vegetal dry matter), the nitrogen content (0.29 g N for 100 g of dry matter) may vary by 10% and the sulfur content (0.05 g S/100 d.m.) by 20%. These variations of the biomass chemical content do not constitute an important factor in the variation of the gas and particle emission levels. With the emission ratios characteristic of humid savanna and flaming conditions (CO/CO₂ of 6.1% at the ground and 8% for airborne measurements), we propose a set of new emission factors, taking into account the burning efficiency which is about 80%: 74.4% of the carbon content of the savanna biomass is released to the atmosphere in the form of CO₂, 4.6% as CO, 0.2% as CH₄, 0.5% as NMHC and 0.7% as aerosols. 17.2% of the nitrogen content of the biomass is released as NO_x, 3.5% as N₂O, 0.6% as NH₃ and 0.5% as soluble aerosols.