Exact solution of the equation of transfer in a finite exponential atmosphere by the method of laplace transform and linear singular operators

An approximate solution of the transfer equation for coherent scattering in stellar atmospheres with Planck's function as a nonlinear function of optical depth, viz.,

Shear-induced instability and arch filament eruption: A magnetohydrodynamic (MHD) numerical simulation

We investigate, via a two-dimensional (nonplanar) MHD simulation, a situation wherein a bipolar magnetic field embedded in a stratified solar atmosphere (i.e., arch-filament-like structure) undergoes symmetrical shear motion at the footpoints. It was found that the vertical plasma flow velocities grow exponentially leading to a new type of global MHD-instability that could be characterized as a Dynamic Shearing Instability, with a growth rate of about 8{ovV} _A a, where {ovV} _A is the average Alfvén speed and a ^–1 is the characteristic length scale. The growth rate grows almost linearly until it reaches the same order of magnitude as the Alfvén speed. Then a nonlinear MHD instability occurs beyond this point. This simulation indicates the following physical consequences: the central loops are pinched by opposing Lorentz forces, and the outer closed loops stretch upward with the vertically-rising mass flow. This instability may apply to arch filament eruptions (AFE) and coronal mass ejections (CMEs).To illustrate the nonlinear dynamical shearing instability, a numerical example is given for three different values of the plasma beta that span several orders of magnitude. The numerical results were analyzed using a linearized asymptotic approach in which an analytical approximate solution for velocity growth is presented. Finally, this theoretical model is applied to describe the arch filament eruption as well as CMEs.     

Composition-related microstructures in zinc-bearing carbonate assemblages from Broken Hill,New South Wales

Summary Analytical transmission electron microscopy has been used to investigate two zincian dolomites occurring as overgrowths on calcite from the oxidized zone of the ore bodies at Broken Hill, New South Wales. Particular attention has been focussed on the nature of the microstructures, and any correlations between microstructure and local composition, especially zinc content. The possible occurrence of tiny amounts of the zinc dolomite, minrecordite, has also been explored.The most notable features in the zincian dolomite are fine-scale distributions of ribbonlike calcitic second phase, growth bands, and modulated microstructures (strong modulations in diffraction contrast). Their occurrence is correlated with calcium in excess of stoichiometric requirements, but is not directly influenced by zinc content. The associated calcites are low in zinc, but they also exhibit carbonate second phase, which is thought to be dolomitic although giving no detectable order (b-type) reflections. Long-exposure electron diffraction patterns, however, revealed the presence of diffuse c-type superstructure reflections, which are thought to arise from smaller volumes in which Ca and Mg cations are ordered, for which a model is proposed.

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Zusammensetzungsabhängige Mikrogefüge in Zn-haltigen Karbonat-Vergesellschaftungen von Broken Hill, New South Wales

Zusammenfassung Analytische Transmissionselektronenmikroskopie wurde benutzt um zwei Zn-haltige Dolomite zu untersuchen, die als Überzüge von Calcit aus der Oxidationszone der Erzkörper von Broken Hill, New South Wales, auftraten. Besondere Aufmerksamkeit wurde der Art der Mikrogefüge und möglichen Korrelationen zwischen Mikrogefüge und der lokalen Zusammensetzung, vor allem dem Zn-Gehalt, gewidmet. Dem möglichen Auftreten von geringen Mengen des Zn-Dolomit, Minrecordit, wurde ebenfalls nachgegangen.Die bemerkenswertesten Erscheinungen im Zn-haltigen Dolomit sind feine bandartige Verteilung einer calcitischen Phase, Wachstumsbänder und modulierte Strukturen (starke Modulationen im Beugungskontrast sichtbar). Ihr Auftreten ist mit über das stöchiometrische Verhältnis hinaus vorhandenem Ca korreliert; es ist aber vom Zn-Gehalt nicht direkt beeinflußt. Die mitvergesellschafteten Calcite haben einen niedrigen Zn-Gehalt, zeigen aber eine weitere Karbonatphase. Diese wird für dolomitsch gehalten, obwohl keine entsprechenden Ordnungsreflexe (b-Reflexè) nachweisbar waren. Sehr lange belichtete Elektronenbeugungs-Aufnahmen zeigen jedoch diffuse c-Typ-Über-struktur-Reflexe. Diese sind wahrscheinlich auf kleine Bereiche beschränkt, in dener die Ca und Mg Kationen geordnet sind, wofür ein Modell vorgeschlagen wird.

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With 10 Figures     

The Paasivaara PGE reef in the Penikat layered intrusion,northern Finland

Summary Three major PGE-bearing mineralized zones have been found in the layered series of the early Proterozoic Penikat layered intrusion. These are designated as the Sompujärvi (SJ), Ala-Penikka (AP) and Paasivaara (PV) Reefs according to the site of their initial discovery.The uppermost of these, the PV Reef, has the highest Pt/Pd ratio. It is located in the transition zone between the fourth and the fifth megacyclic units. The main host rock is the uppermost anorthosite, disseminated sulphides and associated PGM being concentrated in the interstices of this plagioclase orthocumulate. The Reef has also been encountered in other parts of the transition zone, however, and sometimes even in the lowermost parts of the fifth megacyclic unit. The dominant sulphide paragenesis is chalcopyrite-pyrrhotite-pentlandite, whereas the PGM identified are represented by sperrylite (PtAs₂), kotulskite (PdTe), merenskyite (PdTe₂), isomertieite (Pd₁₁Sb₂As₂), stibiopalladinite (Pd₅Sb₂), cooperite (PtS) and braggite ((Pt, Pd, Ni)S).It is suggested that the PV Reef was formed in the mixing process when the fifth magma pulse intruded into the magma chamber. Mixing of the new magma with the older residual magma in the chamber accounted for the sulphide precipitation. Mixing and convection were probably turbulent at first and the sulphides were thus able to "scavenge" PGE from a large amount of silicate melt. The metal ratios in the mineralization point to a close genetic relationship with the fifth magma pulse.

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Das Paasivaara PGE Reef in der Penikat-Intrusion, Nord-Finnland

Zusammenfassung In den geschichteten Serien der frühproterozoischen Intrusion von Penikat kommen drei grössere PGE-führende Zonen vor. Diese werden als die Sompujärvi (SJ), Ala-Penikka (AP) und Paasivaara (PV) Reefs bezeichnet, entsprechend den Lokalitäten der Entdeckung.Das am höchsten gelegene PV Reef hat die höchsten Pt/Pd Verhältnisse. Es liegt in der Übergangszone zwischen der vierten und der fünften megazyklischen Einheit. Das wichtigste Wirtsgestein ist der oberste Anorthosit, wo disseminierte Sulfide und assoziierte PGM in den Zwischenräumen dieses Plagioklas-Orthokumulates vorkommen. Das Reef wurde auch in anderen Teilen der Überganszone beobachtet und manchmal sogar in den untersten Partien der fünften megazyklischen Einheit. Die dominierende Sulfidparagenese ist Kupferkies-Magnetkies-Pentlandit; PGM sind Sperrylith (PtAs₂), Kotulskit (PdTe), Merenskyit (PdTe₂), Isomertieit (Pd₁₁Sb₂As₂), Stibiopalladinit (Pd₅Sb₂), Cooperite (PtS) und Braggit ((Pt, Pd, Ni)S).Es wird angeregt, dass das PV Reef während der Mischungsvorgänge bei der Intrusion des fünften Magma Pulses in die Magmenkammer entstanden ist. Mischung des neuen Magmas mit dem alten Residual-Magma in der Kammer war für die Ausfällung der Sulfide verantwortlich. Mischung und Konvektion dürften anfangs turbulent gewesen sein, und so konnten die Sulfide die PGE aus einem beträchtlichen Anteil der Silikatschmelze entfernen. Die Metallverhältnisse dieser Vererzung lassen eine enge genetische Verbindung mit dem fünften Magmapuls erkennen.

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With 8 Figures     

Les gisements d'émeraude de la Cordillère Orientale de la Colombie: Nouvelles données métallogéniques

Emerald deposits of Colombia are confined to lower Cretaceous shales of the Eastern Cordillera. The tectonic pattern of the deposits is related to deep reverse and large regional fault systems. Hydrofracturing is the main factor controlling emerald mineralization. It permitted to the hydrothermal solutions to permeate through fractures but also along stratification planes forming in this case stratabound mineralizations. Emerald occurs in calcite veins, veinlets, pockets and brecciated zones associated mainly with pyrite, quartz, parisite, codazzite and fluorite. Emerald mineralization belongs to an epigenetic hydrothermal process. The alternance of arenite-shale formations in the Cretaceous probably played an important role in the accumulation of solutions and in the propagation of the hydrothermal channels. The origin of emerald involves chemical elements mobilized by the fluids in the Cr-V-Fe-Al-Si-bearing black shales. The source of beryllium remains a problem and is discussed in the paper.     

Dynamical instability in accreting white dwarfs

We study the evolution of solid, CO white dwarfs after explosive carbon ignition at central densities around 10¹⁰ g cm^–3 triggered by steady accretion in a close binary system, in order to elucidate whether these stars can collapse to form a neutron star. We show that as long as the velocity of the burning front remains below a critical value of 0.006c _s (60 km s^–1), gravitational collapse is the final fate. These calculations support the accretion-induced collapse (AIC) scenario for the origin of a fraction of low-mass X-ray binaries.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

The GRECOS operating system of the GCT

The computer operating system of the Gregory Coudé Telescope (GCT) at Observatorio del Teide is described. The GRECOS command language provides the user with a flexible means of operating both telescope and spectrograph, to execute complex measuring sequences, and to handle and display his observational data.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Photoionization models of QSO's intervening absorption clouds

A new sufficiently compact code LINESPEC is described which is designed to determine the kinetic (collisional+photo) ionization equilibrium in a slab of a hot (T10⁴K) rarefied gas, and to simulate synthetic absorption-line spectra formed due to passing continuous radiation (from a quasar) through the slab. Eighty-six resonant absorption lines of ions and atoms of H, He, C, N, O, Ne, Mg, Al, Si, S, Ca, Fe are included with wavelengths in the range 9125000 Å. the behaviour and observability of various lines are analysed as a function of intensity of the ionizing radiation and kinetic plasma temperature (for a power-law spectrum of ionizing photons,N _ph (E) E ^– with =1.5;E is photon energy). For the purely collisional ionization (N _ph =0), the spectrum contains various combinations of absorption lines of hydrogen and/or of atoms and ions of other elements, the relative intensities of the lines being strongly temperaturedependent. If the ionizing radiation is intense enough, the L line dominates. In a wide parameter range L may be the only line visible in the UV spectral range.     

The nonlinear solar dynamo and differential rotation: A Taylor number puzzle?

We consider dynamically consistent mean-field dynamos in a spherical shell of incompressible fluid. The generation of magnetic field and differential rotation is parameterized by the - and -effects, respectively. Extending previous investigations, we include now the cases of moderate and rapid rotation in the sense that the inverse Rossby number can approach or exceed unity: This can lead to disk-shaped -contours, which are in better accordance with recent results of helioseismology than cylindrical -contours. On the other hand, in order to obtain -dynamo cycles the Taylor number has to be so large, that eventually cylindrical -contours become unavoidable (cf. Taylor-Proudman theorem). We discuss the different possibilities in a state diagram, where the inverse Rossby number and the relative correlation length are taken as the elementary parameters for mean-field dynamos.     

The formation of spicules in the course of the chromospheric network magnetic field reconnection

We investigate the physical processes occurring in the supergranule boundary cylinder layer (SBCL). Taking into account the Coriolis force, we obtain an expression for the component of the magnetic field and velocity in the SBCL. Within the framework of linear MHD, we consider the formation and coalescence of magnetic tubes, i.e. spicules, in the course of the reconnection of the SBCL magnetic field. The estimated number of spicules appearing on each supergranule cell is in agreement with observations. This number depends on the solar latitude : (1) if the normal component of the magnetic fieldB _z is assumed to be independent of , then the maximum number of spicules should be at = 71°; (2) ifB _z is assumed to be the component of the dipolar fieldB _z sin , then the maximum number should be at the pole: = 90°. The timescale of the formation and the coalescence of the magnetic tubes is 10–20 min, which is of the order of the observed lifetime of the spicules.