Attenuation of small-amplitude oscillations in a prominence–corona model with a transverse magnetic field

Observations show that small-amplitude prominence oscillations are usually damped after a few periods. This phenomenon has been theoretically investigated in terms of non-ideal magnetoacoustic waves, non-adiabatic effects being the best candidates to explain the damping in the case of slow modes. We study the attenuation of non-adiabatic magnetoacoustic waves in a slab prominence embedded in the coronal medium. We assume an equilibrium configuration with a transverse magnetic field to the slab axis and investigate wave damping by thermal conduction and radiative losses. The magnetohydrodynamic equations are considered in their linearised form and terms representing thermal conduction, radiation and heating are included in the energy equation. The differential equations that govern linear slow and fast modes are numerically solved to obtain the complex oscillatory frequency and the corresponding eigenfunctions. We find that coronal thermal conduction and radiative losses from the prominence plasma reveal as the most relevant damping mechanisms. Both mechanisms govern together the attenuation of hybrid modes, whereas prominence radiation is responsible for the damping of internal modes and coronal conduction essentially dominates the attenuation of external modes. In addition, the energy transfer between the prominence and the corona caused by thermal conduction has a noticeable effect on the wave stability, radiative losses from the prominence plasma being of paramount importance for the thermal stability of fast modes. We conclude that slow modes are efficiently damped, with damping times compatible with observations. On the contrary, fast modes are less attenuated by non-adiabatic effects and their damping times are several orders of magnitude larger than those observed. The presence of the corona causes a decrease of the damping times with respect to those of an isolated prominence slab, but its effect is still insufficient to obtain damping times of the order of the period in the case of fast modes.     

The north-south asymmetry of sunspot areas during solar cycle 22

We have analyzed the asymmetry of sunspot areas during the current solar cycle 22, finding that it has been statistically significant and that the shape of the underlying trend within the full asymmetry time series (1874–1993) indicates that the dominance of solar activity has started to shift, during the current cycle, from the northern hemisphere to the southern one.     

Sources and geochemical constraints of polycyclic aromatic hydrocarbons (PAHs) in sediments and mussels of two Northern Irish Sea-loughs.

Polycyclic aromatic hydrocarbons (PAHs) were sampled from 30 sediment and 8 mussel (Mytilus edulis) stations in two Northern Irish Sea-toughs. Analysis was performed by gas chromatography coupled to mass spectrometry (GC-MS). Sedimentary organic carbon, % silt/clay and mean grain size were analysed in order to assess the role of geochemistry on PAH distribution. With the exception of two sites in Larne Lough representing localized regions of high contamination, sum(PAHs) in sediments ranged between 83 and 2300 ng g(-1). Regression analysis indicated that particle size and organic C were dominant factors in controlling the distribution of PAHs throughout the sediments. Sources of PAHs in both sea-loughs were dominated by pyrogenic inputs suggesting that diffuse sources such as atmospheric deposition may be a major source to both water bodies. The sum of PAHs in mussels ranged between 95 and 184 ng g(-1). Variations in concentrations may be explained by differences in the condition between mussel populations. Mussels in both sea-loughs exhibit similar metabolic activities towards the PAH compounds which were predominantly bioaccumulated from sediments.     

Evidence of Dry and Cold Climatic Conditions at Glacial Times in Tropical Southeastern Brazil

Last-glacial paleoenvironments have been reconstructed from a pollen and charcoal record analyzed in organic sediments and dated between ca. 18,000 and >48,000¹⁴C yr B.P. The site is located near the village Catas Altas in the lower highland region of southeastern Brazil. The last-glacial landscape was covered by extensive areas of subtropical grasslands and small areas of gallery forests along the rivers, where tropical semideciduous forests and cerrado ecosystems exist today. The subtropical gallery forests were composed ofAraucariaforest trees such asAraucaria angustifolia, Podocarpus, Drimys, Ilex,andSymplocos.Paleofires were frequent. The record indicates that subtropical grassland vegetation, which today is found in patches on the highlands in southern Brazil (especially in the state of Santa Catarina), expanded from southern Brazil to southeastern Brazil, over a distance of more than 750 km, from latitudes of about 28° S to at least 20° S. The completely different last-glacial environment, in comparison to the present-day environment, reflects a dry and cold climate with strong frosts during the winter months. Temperatures of 5°–7°C below those of the present are inferred for the last glaciation.     

The windows of SETI--frequency and time in the search for extraterrestrial intelligence

On Earth intelligent life evolved as a natural consequence of the events set in motion when the planet formed over 4 billion years ago. Since chemical evolution and solar-system formation appear to be occurring throughout the universe, we theorize that our universe may be rich with planets populated by intelligent beings who, like us, can search for evidence of other technological civilizations. Terrestrial civilization now has this capability. But if we do not begin the search soon, we'll lose the opportunity to do it from Earth as interfering signals of Earthly origin rapidly close the microwave window.     

Nontronite synthesis at low temperatures

The low-temperature synthesis of clay minerals is possible through the aging of freshly prepared hydroxide—silica precipitates. The rapid synthesis of nontronite is only possible at surface temperatures under reducing conditions. Under oxidizing conditions, pure Fe(III)- or pure Al-smectite minerals could not be synthesized at low temperatures. It is only from Fe(II)-containing solutions that nontronite and lembergite, the di-[Fe(III)] and tri-[Fe(II)] octahedral three-layer silicates, are built up in several days at low temperatures. The presence of Fe(II) enables an octahedral layer of the brucite—gibbsite type to be formed. These are necessary for the bidimensional orientation of SiO₄ tetrahedrons, leading to clay-mineral formation. The Fe²⁺ and/or Mg²⁺ ions are necessary for the formation of the Al³⁺- and Fe³⁺-containing three-layer silicate minerals.Under reducing diagenetic conditions, the Fe contents in recent sediments are sufficient to build up Al-rich three-layer minerals under both fresh-water and salt-water conditions.     

Interconversion of iron(III) hydroxy complexes in seawater

Interconversion rates of the mononuclear ferric iron species Fe(OH)₃⁰ and Fe(OH)₂⁺ are derived and their implications for the behavior of these species in seawater are examined. The previously reported formation constant for Fe(OH)₃⁰ and its claimed extreme adsorptive reactivity in seawater are shown to be mutually inconsistent. Although Fe(OH)₃⁰ is probably a stoichiometrically minor dissolved iron species, its rapid formation from Fe(OH)₂⁺ could substantially enhance the rates of heterogeneous reaction rates of the [Fe(OH)₂⁺ + Fe(OH)₃⁰] pool if the latter species is very reactive.