Physics of Solar Prominences: I - Spectral Diagnostics and Non-LTE Modelling

This review paper outlines background information and covers recent advances made via the analysis of spectra and images of prominence plasma and the increased sophistication of non-LTE (ie when there is a departure from Local Thermodynamic Equilibrium) radiative transfer models. We first describe the spectral inversion techniques that have been used to infer the plasma parameters important for the general properties of the prominence plasma in both its cool core and the hotter prominence-corona transition region. We also review studies devoted to the observation of bulk motions of the prominence plasma and to the determination of prominence mass. However, a simple inversion of spectroscopic data usually fails when the lines become optically thick at certain wavelengths. Therefore, complex non-LTE models become necessary. We thus present the basics of non-LTE radiative transfer theory and the associated multi-level radiative transfer problems. The main results of one- and two-dimensional models of the prominences and their fine-structures are presented. We then discuss the energy balance in various prominence models. Finally, we outline the outstanding observational and theoretical questions, and the directions for future progress in our understanding of solar prominences.Comment: 96 pages, 37 figures, Space Science Reviews. Some figures may have a better resolution in the published version. New version reflects minor changes brought after proof editin

Methane thermometry in deep-sea hydrothermal systems: evidence for re-ordering of doubly-substituted isotopologues during fluid cooling

Deep-sea hydrothermal fluids are often enriched in carbon dioxide, methane, and hydrogen. Methane effuses from metal-rich black smokers such as the Rainbow hydrothermal field, at temperatures higher than 200 °C. At the Lost City field, CH4 emanates from alkaline fluids at < 100 °C. The abundance of the rare, mass-18 CH4 isotopologues, 13CH3D and 12CH2D2, can mitigate degeneracies in the conventional isotopic signatures of methane. We studied the isotopologue compositions of methane from the Rainbow, Lucky Strike, Von Damm, and Lost City hydrothermal fields. At Rainbow, where the vented fluids are at ∼360°C, our coupled Δ12CH2D2 - Δ13CH3D data establish that methane is in internal equilibrium at °C. This may track the formation temperature of abiotic methane, or it may be the result of equilibration of methane isotopologues within the carrier fluid. Lucky Strike and Von Damm have fluid temperatures < 300°C and although Δ13CH3D values are indistinguishable from those at Rainbow, 12CH2D2 abundances are marginally higher. At Lost City, Δ13CH3D data show a range of values, which at face value correspond to apparent temperatures of between °C and °C, far hotter than fluid temperatures. A unique aspect of the Lost City data is the range of large 12CH2D2 excesses. The Δ12CH2D2 data correspond to temperatures of between °C and °C, showing a near-perfect match with fluid temperatures. We find that mixing scenarios involving microbial methane may not account for all of the isotope data. We suggest that Δ12CH2D2 values, unlike Δ13CH3D values, are prone to near-complete re-equilibration at host fluid temperatures. We suggest that 13CH3D isotopologue data are consistent with abiotic methane being synthesized at ∼350 °C. On the other hand, 12CH2D2 isotopologue ordering records post formation residence temperatures. We explore a possible mechanism decoupling the re-equilibration systematics of the doubly-substituted isotopologues

Multi-criteria analyses of two solvent and one low-temperature concepts for acid gas removal from natural gas

This paper evaluates three acid gas removal concepts studied in the project “A Green Sea”. Two solvent concepts (aMDEA/MDEA and Selexol) and a low-temperature concept are modelled and assessed, taking different raw natural gases and natural gas product requirements into consideration. The analyses and comparisons of the concepts and cases consider nine criteria in order to include both energy efficiencies and compactness. The assessment shows that acid gas removal using aMDEA/MDEA technology seems to perform well in terms of energy efficiency, volume and weight for low CO2 removal. However, for high CO2 content or strong polishing requirements, the chemical solvent technology loses its efficiency in terms of weight and volume. The assessment shows that the Selexol concept is an inefficient option in terms of energy efficiency, volume and weight, especially when large quantities of CO2 have to be removed from the gas stream. The assessment also shows that the low-temperature technology can be a compact and energy-efficient option, both in the case of strong polishing requirements and high bulk removal of CO2. However, the higher the amount of CO2 to be removed, the less energy efficient is the low-temperature technology. The case evaluation underlines the fact that the aMDEA/MDEA solvent concept exhibits the best or close to the best key performance indicators (KPIs) for all parameters for the RNG1Pipe case (raw natural gas specification 1 to pipeline quality specification) and therefore appears to be the best technology option. For this case, the two other technologies are slightly less energy efficient than the aMDEA/MDEA, but both are significantly less compact. For the RNG1 LNG (raw natural gas specification 1 to LNG quality specification) case, the aMDEA/MDEA and low-temperature concepts have similar KPIs. The chemical solvent technology, however, is slightly more energy efficient and compact and would therefore be preferred for the RNG1 LNG case. Finally, the RNG2 Pipe (raw natural gas specification 2 to pipeline quality specification) case shows that the low-temperature technology can be a compact option for acid gas removal, which is a critical factor in the case of offshore applications for both the equipment costs and the weight constraints on the platform. Despite its lower energy efficiency, it is therefore likely that the low-temperature technology will be selected in the RNG2 Pipe case. This choice is strengthened by some regulations which recommend that solvents such as MDEA and aMDEA should be phased out for offshore applications, as is seen, e.g. in Norway. In addition, if stricter regulations are also enforced for onshore applications, this might also argue in favour of the low-temperature technology or other chemical solvents that are otherwise less efficient than aMDEA/MDEA. Finally, the potential of hybrid concepts is discussed and suggested for future works, in order to combine the advantages of the different technologies, such as the energy-efficient performances of the aMDEA/MDEA concept and the compactness of the low-temperature concept.acceptedVersio