Propagation of MHD body and surface waves in magnetically structured regions of the solar atmosphere

The fact that magnetically structured regions exist in the solar atmosphere has been known for a number of years. It has been suggested that different kinds of magnetohydrodynamic (MHD) waves can be efficiently damped in these regions and that the dissipated wave energy may be responsible for the observed enhancement in radiative losses. From a theoretical point of view, an important task would be to investigate the propagation and dissipation of MHD waves in these highly structured regions of the solar atmosphere. In this paper, we study the behavior of MHD body and surface waves in a medium with either a single or double (slab) magnetic interface by use of a nonlinear, two-dimensional, time-dependent, ideal MHD numerical model constructed on the basis of a Lagrangean grid and semi-implicit scheme. The processes of wave confinement and wave energy leakage are discussed in detail. It is shown that the obtained results depend strongly on the type of perturbations imposed on the interface or slab and on the plasma parameter, . The relevance of the obtained results to the heating problem of the upper parts of the solar atmosphere is also discussed.     

Surface renewal analysis for sensible and latent heat flux density

High frequency temperature measurements were recorded at five heights and surface renewal (SR) analysis was used to estimate sensible heat flux density (H) over 0.1 m tall grass. Traces of the temperature data showed ramp-like structures, and the mean amplitude and duration of these ramps were used to calculate H using structure functions. Data were compared with H values measured with a sonic anemometer. Latent heat flux density (E) was calculated using an energy balance and the results were compared with E computed from the sonic anemometer data. SR analysis provided good estimates of H for data recorded at all heights but the canopy top and at the highest measurement level, which was above the fully adjusted boundary layer.     

Turbulent flows over mountainous terrain modelled by the Reynolds equations

A numerical model based upon fundamental principles, a standard (k, ) turbulence closure and a finite element integration technique, is applied to separated flows over hills. Predictions are compared to experimental data from a wind tunnel. Although few-equation turbulence closures have been shown to have obvious deficiencies with respect to comparable flows, the model predicts remarkably accurately, without coefficient adjustments of any kind. Even the turbulent intensity is predicted quite realistically.     

About the specification of erosion flux for soft stratified cohesive sediments

The present paper deals with the specification of bed erosion flux that accounts for the effects of sediment-induced stratification in the water column. Owing to difficulties in measuring the bed shear stress _b and the erosive shear strength _s, we suggest a series of methods that combine laboratory and numerical experiments. A simplified turbulent transport model that includes these effects helps to quantify _b and _s. Focusing on soft stratified beds, the present study considers erosion rate formulas of the form =_f exp {[T_b-T_s]} where is a model constant (=1 for Gularte's (1978) formula and =1/2 for Parchure's (1984) formula). First, the bed erosive strength profile _s(Z) is adjusted by forcing the turbulent transport model with measured erosion rates. Second, three procedures are suggested to determine the erosion rate formula coefficients _f and : a global procedure and two different layer-by-layer procedures. Each procedure is applied to an erosion experiment conducted in a rotating annular flume by Villaret and Paulic (1986). The use of the layer-by-layer procedure based on a least squares fitting technique provides a closer fit than the global procedure. The present study points out the complementarity of experimental and numerical approaches and also suggests possible improvements in laboratory test procedures.     

Pb−Sr−Nd−O isotopic constraints on the origin of rhyolites from the Taupo Volcanic Zone of New Zealand: evidence for assimilation followed by fractionation from basalt

A comprehensive Sr–Nd–Pb–O isotopic study is reported for rhyolites from the Maroa Volcanic Centre in the Taupo Volcanic Zone (TVZ) of New Zealand. The Sr–Nd isotopic compositions of the rhyolites (⁸⁷Sr/⁸⁶Sr=0.705236 to 0.705660 and _Nd = 2.0 to 0.2) are intermediate between those of primitive basalts (⁸⁷Sr/⁸⁶Sr=0.70387 and _Nd = 5.3) and the Torlesse basement (⁸⁷Sr/⁸⁶Sr=0.709 and _Nd = -4.5). The relatively low mantle-like oxygen isotopic compositions of ¹⁸ O = 7 ± 0.5 are consistent with the Nd-Sr isotopic constraints in that they can be accounted for by 15% to 25% crustal contamination of a basaltic parent by relatively ¹⁸ O-rich Torlesse metasediment. High precision Pb isotopic analyses of plagioclase separates from the Maroa rhyolites show that they have essentially the same compositions as the Torlesse metasedimentary terrane which is itself distinctive from the Western or Waipapa metasediments. Due to the high concentration of Pb in the Torlesse metasediments (>20 ppm) compared to the basalts (<2 ppm), the Pb isotopic composition of the volcanics may be controlled by relatively small amounts (>10%) of crustal contamination. All these results are shown to be consistent with derivation of the rhyolites by 15% to 25% contamination of relatively primitive basaltic magmas with Torlesse metasedimentary crust, followed by extensive, essentially closed system fractionation of the basalt to a magma of rhyolite composition. It is argued that the processes of assimilation and fractionation are separated in both space and time. The voluminous high silica rhyolites, which make up >97% of the exposed volcanism in the continental margin back-are basin environment of the TVZ, therefore appear to be a product of predominantly new additions to the crust with assimilation-recycling of pre-existing crust being of secondary importance.     

Aqueous alteration of the Bali CV3 chondrite: evidence from mineralogy, mineral chemistry, and oxygen isotopic compositions

A petrographic, geochemical, and oxygen isotopic study of the Bali CV3 carbonaceous chondrite revealed that the meteorite has undergone extensive deformation and aqueous alteration on its parent body. Deformation textures are common and include flattened chondrules, a well-developed foliation, and the presence of distinctive (100) planar defects in olivine. The occurrence of alteration products associated with the planar defects indicates that the deformation features formed prior to the episode of aqueous alteration. The secondary minerals produced during the alteration event include well-crystallized Mg-rich saponite, framboidal magnetite, and Ca-phosphates. The alteration products are not homogeneously distributed throughout the meteorite, but occur in regions adjacent to relatively unaltered material, such as veins of altered material following the foliation. The alteration assemblage formed under oxidizing conditions at relatively low temperatures (<100 degrees C). Altered regions in Bali have higher Na, Ca, and P contents than unaltered regions which suggests that the fluid phase carried significant dissolved solids. Oxygen isotopic compositions for unaltered regions in Bali fall within the field for other CV3 whole-rocks, however, the oxygen isotopic compositions of the heavily altered material lie in the region for the CM and CR chondrites. The heavy-isotope enrichment of the altered regions in Bali suggest alteration conditions similar to those for the petrographic type-2 carbonaceous chondrites.     

Geochemistry and origin of the Proterozoic ultrapotassic rocks of the Churchill Province,Canada

Summary Early Proterozoic ultrapotassic dikes, lava flows, and pyroclastic rocks of the Christopher Island Formation (CIF) erupted throughout an area 600 × 300 km within the Churchill Province of the Canadian Shield at 1.84 Ga. The rocks range from mafic lamprophyres (mg # 60; SiO₂ 47–54%, mean K₂O/Na₂O > 4) with phenocrysts of phlogopite + diopside + apatite ± olivine ± magnetite, to phenocryst-poor felsic rocks and sanidine porphyries (SiO₂55–69%). All samples have high incompatible element contents and display large depletions of high field strength elements relative to K, Rb, Sr, Ba, and Th. The CIF has geochemical and petrographic characteristics of both minettes and lamproites, but overall most closely resembles young Mediterranean lamproites. Felsic rocks of the CIF were produced by crystal fractionation and crustal contamination of mafic ultrapotassic magma, and include both high-silica lamproites strongly enriched in Zr, U, and Th, and weakly potassic to sodic rocks of trachytic composition. Flows and feeder dikes have relatively homogeneous _{Nd, 1840 Ma} (–6 to –11) but highly variable ES., 1840 Ma (–40 to + 100); samples classified as lamproites have higher average _Sr. Dike samples have highly variable present-day Pb isotope compositions, ranging from moderately to strongly nonradiogenic. Geochemical and isotopic data are consistent with contributions from depleted Archean lithospheric mantle, and OIB-type convecting mantle, both metasomatized by subduction-related processes during the Early Proterozoic. The lithospheric mantle probably contained Archean enriched domains as well. Proterozoic enrichment may have accompanied shallow underplating of subducted oceanic lithosphere beneath the Churchill Province during amalgamation of the Laurentian supercontinent. There are strong analogies in isotopic composition, and interpreted source region history, between the CIF and lamproites and minettes of the Wyoming Province and western Greenland, which suggest the existence of a Laurentian ultrapotassic superprovince.

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Geochemie und Entstehung der Proterozoischen ultrapotassischen Gesteine der Churchill Provinz, Kanada

Zusammenfassung Altproterozoische, ultrapotassische Gänge, Lavaströme und pyroklastische Gesteine der Christopher Island Formation (CIF), eruptierten in einem Gebiet von 600 × 300 km in der Churchill Provinz des Kanadischen Schildes vor 1.84 Ga. Die Zusammensetzung dieser Gesteine variiert von mafischen Lamprophyren (mg > 60; SiO₂ = 47–54%, durchschnittliches K₂O/Na₂O > 4) mit Phänokristallent von Phlogopit + Diopsid + Apatit + Olivin + Magnetit, bis zu phänokristallarmen felsischen Gesteinen und Sanidinporphyren (SiO₂ = 55–69%). Alle Proben zeigen hohe Gehalte an inkompatiblen Elementen und zeigen beträchtliche Verarmung an high field strength Elementen relativ zu K, Rb, Sr, Ba und Th. Die CIF hat geochemische und petrographische Eigenschaften sowohl von Minetten wie von Lamproiten, aber im allgemeinen ähnelt sie am stärksten jungen mediterranen Lamproiten. Felsische Gesteine der CIF wurden durch Fraktionierung und Krustenkontamination aus mafischen ultrapotassischen Magmen gebildet. Letztere umfassen sowohl siliziumreiche Lamproite, die deutlich an Zr, U und Th angereichert sind und schwach potassische bis sodische Gesteine von trachytischer Zusammensetzung. Lavenergüsse und zufuhrgänge zeigent ein relativ homogenes _{Nd, 1840 Ma} (–6 bis –11) aber ein sehr variables _{Sr, 1840 Ma} (-40 bis + 100); Proben die als Lamproite klassifiziert wurden, zeigent höhere durchschnittliche _Sr-Werte. Proben von Gängen haben sehr variable Bleiisotopen-Zusammensetzungen, die von mäßig bis stark nichtradiogen variieren. Geochemische und Isotopendaten weisen auf Beiträge aus verarmtem archaischen lithosphärischen Mantel und aus konvektierendem OIB-Typ Mantel hin, die beide während des Alproterozoikums durch Subduktions-Vorgänge metasomatisiert wurden. Der lithosphärische Mantel enthielt wahrscheinlich auch angereicherte archaische Domänen. Proterozoische Anreicherungsvorgänge dürften seichtes Underplating subduzierter ozeanischer Lithosphäre unter der Churchill Provinz während der Amalgamation des laurentischen Superkontinentes begleitet haben. Es gibt starke Analogien in der Isotopenzusammensetzung und in der interpretierten Geschichte der Ursprungsregion, zwischen den CIF und Lamproiten und Minetten der Wyoming Provinz, und des westlichen Grönland. Diese weisen auf die Existenz einer laurentischen ultrapotassischen Superprovinz hin.

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With 7 Figures     

Rosat wide field camera data and the temperature of the interstellar medium

The shadowing effects of the molecular clouds in the nearby interstellar medium on the soft x-ray background has been investigated, using ROSAT WFC data in conjunction with previous rocket B and C band surveys. Shadowing over a 5° extent occurs only for a few percent of the sky, but the mixed model of the ISM is supported.Detailed modelling of the Draco shadowing region shows little evidence for a multi-temperature, hot ISM component.     

A photochemical model of the martian atmosphere

The factors governing the amounts of CO, O2, and O3 in the martian atmosphere are investigated using a minimally constrained, one-dimensional photochemical model. We find that the incorporation of temperature-dependent CO2 absorption cross sections leads to an enhancement in the water photolysis rate, increasing the abundance of OH radicals to the point where the model CO abundance is smaller than observed. Good agreement between models and observations of CO, O2, O3, and the escape flux of atomic hydrogen can be achieved, using only gas-phase chemistry, by varying the recommended rate constants for the reactions CO + OH and OH + HO2 within their specified uncertainties. Similar revisions have been suggested to resolve discrepancies between models and observations of the terrestrial mesosphere. The oxygen escape flux plays a key role in the oxygen budget on Mars; as inferred from the observed atomic hydrogen escape, it is much larger than recent calculations of the exospheric escape rate for oxygen. Weathering of the surface may account for the imbalance. Quantification of the escape rates of oxygen and hydrogen from Mars is a worthwhile objective for an upcoming martian upper atmospheric mission. We also consider the possibility that HOx radicals may be catalytically destroyed on dust grains suspended in the atmosphere. Good agreement with the observed CO mixing ratio can be achieved via this mechanism, but the resulting ozone column is much higher than the observed quantity. We feel that there is no need at this time to invoke heterogeneous processes to reconcile models and observations.     

Near-infrared spectroscopy of dark asteroids

Near-infrared (J, H and K bands) spectra of nine dark asteroids (chosen among a sample of supposed primitive objects between C and D classes) have been obtained at the Mauna Kea Observatory (Hawaii) with the 2.2-m telescope using KSPEC as spectrograph. The aim of this work was to search for evidence of the presence of organic materials in these objects as found in other planetary bodies as 5145 Pholus, and in some cometary nuclei. A careful analysis of the data has revealed flat or slightly redder spectra than the solar one for all observed asteroids. No evidence of distinct absorption features was found.