Slim accretion discs: a model for ADAF–SLE transitions

We numerically construct slim, global, vertically integrated models of optically thin, transonic accretion discs around black holes, assuming a regularity condition at the sonic radius and boundary conditions at the outer radius of the disc and near the black hole. In agreement with several previous studies, we find two branches of shock-free solutions, in which the cooling is dominated either by advection or by local radiation. We also confirm that the part of the accretion flow where advection dominates is in some circumstances limited in size: it does not extend beyond a certain outer limiting radius. New results found in our paper concern the location of the limiting radius and the properties of the flow near to it. In particular, we find that beyond the limiting radius the advective-dominated solutions match on to Shapiro, Lightman &38; Eardley (SLE) discs through a smooth transition region. Therefore, the full global solutions are shock-free and unlimited in size. There is no need to postulate an extra physical effect (e.g. evaporation) for triggering the ADAF–SLE transition. It occurs as a result of standard accretion processes described by the classic slim disc equations.     

Barents Sea upstream events impact the properties of Atlantic water inflow into the Arctic Ocean: Evidence from 2005 to 2006 downstream observations

Inflow of Atlantic water (AW) from Fram Strait and the Barents Sea into the Arctic Ocean conditions the intermediate (100–1000 m) waters of the Arctic Ocean Eurasian margins. While over the Siberian margin the Fram Strait AW branch (FSBW) has exhibited continuous dramatic warming beginning in 2004, the tendency of the Barents Sea AW branch (BSBW) has remained poorly known. Here we document the contrary cooling tendency of the BSBW through the analysis of observational data collected from the icebreaker Kapitan Dranitsyn over the continental slope of the Eurasian Basin in 2005 and 2006. The CTD data from the R.V. Polarstern cruise in 1995 were used as a reference point for evaluating external atmospheric and sea-ice forcing and oxygen isotope analysis. Our data show that in 2006 the BSBW core was saltier (by ～0.037), cooler (by ～0.41 °C), denser (by ～0.04 kg/m³), deeper (by 150–200 m), and relatively better ventilated (by 7–8 μmol/kg of dissolved oxygen, or by 1.1–1.7% of saturation) compared with 2005. We hypothesize that the shift of the meridional wind from off-shore to on-shore direction during the BSBW translation through the Barents and northern Kara seas results in longer surface residence time for the BSBW sampled in 2006 compared with samples from 2005. The cooler, more saline, and better-ventilated BSBW sampled in 2006 may result from longer upstream translation through the Barents and northern Kara seas where the BSBW was modified by sea-ice formation and interaction with atmosphere. The data for stable oxygen isotopes from 1995 and 2006 reveals amplified brine modification of the BSBW core sampled downstream in 2006, which supports the assumption of an increased upstream residence time as indicated by wind patterns and dissolved oxygen values.     

Mercury radar speckle dynamics

Current data reveal that Mercury is a dynamic system with a core which has not yet solidified completely and is at least partially decoupled from the mantle. Radar speckle displacement experiments have demonstrated that the accuracy in spin-dynamics determination for Earth-like planets can approach 10⁻⁵. The extended analysis of space-time correlation properties of radar echoes shows that the behavior of speckles does not prevent estimation of Mercury’s instantaneous spin-vector components to accuracy of a few parts in 10⁷. This limit can be reached with more powerful radar facilities and leads to constraining the interior in more detail from effects of spin dynamics, e.g., from observation of the core-mantle interplay through high precision monitoring of the 88-day spin-variation of Mercury’s crust.     

Mercury’s magnetospheric magnetic field after the first two MESSENGER flybys

The “paraboloid” model of Mercury’s magnetospheric magnetic field is used to determine the best-fit magnetospheric current system and internal dipole parameters from magnetic field measurements taken during the first and second MESSENGER flybys of Mercury on 14 January and 6 October 2008. Together with magnetic field measurements taken during the Mariner 10 flybys on 29 March 1974 and 16 March 1975, there exist three low-latitude traversals separated in longitude and one high-latitude encounter. From our model formulation and fitting procedure a Mercury dipole moment of 196 nT ·  (where R_M is Mercury’s radius) was determined. The dipole is offset from Mercury’s center by 405 km in the northward direction. The dipole inclination to Mercury’s rotation axis is relatively small, ∼4°, with an eastern longitude of 193° for the dipole northern pole. Our model is based on the a priori assumption that the dipole position and the moment orientation and strength do not change in time. The root mean square (rms) deviation between the Mariner 10 and MESSENGER magnetic field measurements and the predictions of our model for all four flybys is 10.7 nT. For each magnetic field component the rms residual is ∼6 nT or about 1.5% of the maximum measured magnetic field, ∼400 nT. This level of agreement is possible only because the magnetospheric current system parameters have been determined separately for each flyby. The magnetospheric stand-off distance, the distance from the planet’s center to the inner edge of the tail current sheet, the tail lobe magnetic flux, and the displacement of the tail current sheet relative to the Mercury solar-magnetospheric equatorial plane have been determined independently for each flyby. The magnetic flux in the tail lobes varied from 3.8 to 5.9 MWb; the subsolar magnetopause stand-off distance from 1.28 to 1.43 R_M; and the distance to the inner edge of the current sheet from 1.23 to 1.32 R_M. The differences in the current systems between the first and second MESSENGER flybys are attributed to the effects of strong magnetic reconnection driven by southward interplanetary magnetic field during the latter flyby.     

Effect of oxygen fugacity on the H2O storage capacity of forsterite in the carbon-saturated systems

High pressure experiments have been performed in the systems Mg₂SiO₄-C-O-H and Mg₂SiO₄-K₂CO₃-C at 6.3 GPa and 1200 to 1600 °C using a split-sphere multi-anvil apparatus. In the Mg₂SiO₄-C-O-H system the composition of fluid was modeled by adding different amounts of water and stearic acid. The fO₂ was controlled by the Mo-MoO₂ or Fe-FeO oxygen buffers. Several experiments in the Mg₂SiO₄-C-O-H system and all experiments in the Mg₂SiO₄-K₂CO₃-C system have been conducted without buffering the fO₂. Forsterite in the system Mg₂SiO₄-K₂CO₃-C does not reveal OH absorption bands in the IR spectra, while forsterite coexisting with carbon-bearing fluid and silicate melt at logfO₂ from FMQ-2 to FMQ-5 (from 2 to 5 log units below fayalite-magnetite-quartz oxygen buffer) contains 800-1850 wt. ppm H₂O. The maximum concentrations were detected at 1400 °C and FMQ-3.5. We observed an increase in the solidus temperature in the system Mg₂SiO₄-C-O-H from 1200 to above 1600 °C with log fO₂ decreasing from FMQ-2 to FMQ-5. The increase of the solidus temperature and the broadening of the stability field of the H₂O-H₂-CH₄ subsolidus fluid phase at 1400-1600 °C explain the high H₂O storage capacity of forsterite relative to that crystallized from carbon-free, oxidized, hydrous, silicic melt. At temperatures above 1400 °C liquidus forsterite precipitated along with diamond from oxidized (FMQ-1) carbonate-silicate melt and from silicate melt dissolving the moderately reduced C-O-H fluid (from FMQ-2 to FMQ-3.5). Formation of diamond was not detected under ultra-reduced conditions (FMQ-5) at 1200-1600 °C. Olivine co-precipitating with diamond from dry carbonate-silicate or hydrous-silicic fluid/melt can provide information on the H₂O contents and speciation of the diamond-forming media in the mantle. The conditions for minimum post-crystallization alteration of olivine and its hydrogen content are discussed.     

Release the iron: does the infection of magnetotactic bacteria by phages play a role in making iron available in aquatic environments?

Journal of Oceanology and Limnology - Magnetotactic bacteria(MTB) are ubiquitous prokaryotes that orient along magnetic field lines due to magnetosomes’ biomineralization within the cell....     

Transient hysteresis of near-surface permafrost response to external forcing

Estimates of changes in near-surface permafrost (NSP) area S _p relative to change in globally averaged surface air temperature T _g are made by using the global climate model developed at the A.M. Obukhov Institute of Atmospheric Physics RAS (IAP RAS CM). For ensemble of runs forced by scenarios constructed as return-to-preindustrial continuations of the RCP (Representative Concentration Pathways) scenarios family, a possibility of transient hysteresis in dependence of S _p versus T _g is exhibited: in some temperature range which depends on imposed scenario of external forcing, NSP area is larger, at the same global mean surface air temperature, in a warming climate than in a cooling climate. This hysteresis is visible more clearly for scenarios with higher concentration of greenhouse gases in the atmosphere in comparison to those in which this concentration is lower. Hysteresis details are not sensitive to the type of the prescribed continuation path which is used to return the climate to the preindustrial state. The multiple-valued dependence of S _p on T _g arises due to dependence of soil state in the regions of extra-tropical wetlands and near the contemporary NSP boundaries on sign of external climatic forcing. To study the dependence of permafrost hysteresis on amplitude and temporal scale of external forcing, additional model runs are performed. These runs are forced by idealised scenarios of atmospheric CO₂ content varying, depending on run, with periods from 100 to 1,000 year and with different amplitudes. It is shown that the above-mentioned hysteresis is related to the impact of phase transitions of soil water on apparent inertia of the system as well as to the impact of soil state on atmospheric hydrological cycle and radiation transfer in the atmosphere.     

Estimating potential for exploitation of karst aquifer: case example on two Serbian karst aquifer

Rational use of karst water resources depends considerably on the existence of a successful approach to estimating karst groundwater potential for exploitation. The term groundwater potential for exploitation involves the application of regulatory measures affecting natural outflow regime. Generally, the concept of controlling the artificial discharge regime is based on two principles. The first one, the “borrowing” of water from deeper (storage) part of storage, is based mainly on the construction of groundwater wells. The second one, considers increasing dynamic reserves by providing additional artificial water storage thereby controlling the groundwater discharge regime, is based on the construction of underground dams. This paper examines how these two concepts can be used to increase potential groundwater usage from two karst springs in Serbia—St. Petka and Peru?ac. The ability to potentially increase the amount of karst ground water available for future exploitation may facilitate and give important directions to the future research.