A numerical model of the partitioning of trace chemical solutes during drop freezing

Partitioning of volatile chemicals among the gas, liquid, and solid phases during freezing of liquid water in clouds can impact trace chemical distributions in the troposphere and in precipitation. We describe here a numerical model of this partitioning during the freezing of a supercooled liquid drop. Our model includes the time-dependent calculation of the coupled processes of crystallization kinetics, heat transport, and solute mass transport, for a freezing hydrometeor particle. We demonstrate the model for tracer partitioning during the freezing of a 1000 μm radius drop on a 100 μm ice substrate, under a few ambient condition scenarios. The model effectively simulates particle freezing and solute transport, yielding results that are qualitatively and quantitatively consistent with previous experimental and theoretical work. Results suggest that the ice shell formation time is governed by heat loss to air and not by dendrite propagation, and that the location of ice nucleation is not important to freezing times or the effective partitioning of chemical solutes. Even for the case of nucleation at the center of the drop, we found that dendrites propagated rapidly to form surface ice. Freezing then proceeded from the outside in. Results also indicate that the solid-liquid interfacial surface area is not important to freezing times or the effective partitioning of chemical solutes, and that the rate aspects of trapping are more important than equilibrium solid-liquid partitioning to the effective partitioning resulting from freezing.     

From the Sun to the Earth: The 13 May 2005 Coronal Mass Ejection

We report the results of a multi-instrument, multi-technique, coordinated study of the solar eruptive event of 13 May 2005. We discuss the resultant Earth-directed (halo) coronal mass ejection (CME), and the effects on the terrestrial space environment and upper Earth atmosphere. The interplanetary CME (ICME) impacted the Earth’s magnetosphere and caused the most-intense geomagnetic storm of 2005 with a Disturbed Storm Time (Dst) index reaching ?263 nT at its peak. The terrestrial environment responded to the storm on a global scale. We have combined observations and measurements from coronal and interplanetary remote-sensing instruments, interplanetary and near-Earth in-situ measurements, remote-sensing observations and in-situ measurements of the terrestrial magnetosphere and ionosphere, along with coronal and heliospheric modelling. These analyses are used to trace the origin, development, propagation, terrestrial impact, and subsequent consequences of this event to obtain the most comprehensive view of a geo-effective solar eruption to date. This particular event is also part of a NASA-sponsored Living With a Star (LWS) study and an on-going US NSF-sponsored Solar, Heliospheric, and INterplanetary Environment (SHINE) community investigation.     

Detection and extraction of signals from the epoch of reionization using higher-order one-point statistics

Detecting redshifted 21-cm emission from neutral hydrogen in the early Universe promises to give direct constraints on the epoch of reionization (EoR). It will, though, be very challenging to extract the cosmological signal (CS) from foregrounds and noise which are orders of magnitude larger. Fortunately, the signal has some characteristics which differentiate it from the foregrounds and noise, and we suggest that using the correct statistics may tease out signatures of reionization. We generate mock data cubes simulating the output of the Low Frequency Array (LOFAR) EoR experiment. These cubes combine realistic models for Galactic and extragalactic foregrounds and the noise with three different simulations of the CS. We fit out the foregrounds, which are smooth in the frequency direction, to produce residual images in each frequency band. We denoise these images and study the skewness of the one-point distribution in the images as a function of frequency. We find that, under sufficiently optimistic assumptions, we can recover the main features of the redshift evolution of the skewness in the 21-cm signal. We argue that some of these features – such as a dip at the onset of reionization, followed by a rise towards its later stages – may be generic, and give us a promising route to a statistical detection of reionization.     

Structure of the Long-Living Elements of Solar Granulation

Spatial and temporal variations in thermodynamic and kinematic parameters of structural elements of solar granulation are investigated by solving the inverse nonequilibrium radiative transfer problem using the observational data from the Vacuum Tower Telescope (duration of observations 2.6 h). In the lower photosphere, we have detected long-living (with lifetime up to 1.5 h) structures—trees of fragmenting granules. They occur as a result of the division of an ascending granular flow into several fragments, which can be repeated multiple times. We have found that approximately 67% of the regions with the highest positive variations of pressure correspond to the time and place of fragmentation of granular flows; approximately 12% of the regions correspond to the approach of adjacent structures.     

A Solution of Einstein's Field Equations fora Tachyonic Gas: Possible Astrophysical Applications

In this paper we show that a change in the signs of some of the metric components of the solution of the field equations for the classical cosmic string results in a solution which we interpret as a time-dependent wall composed of tachyons. We show that the walls have the property of focusing the paths of particles which pass through them. As an illustration of this focusing, we demonstrate the results of a simple simulation of the interaction between one such tachyon wall and a rotating disk of point masses. This interaction leads to the temporary formation of spiral structures. These spiral structures exist for a time on the order of one galactic rotation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Palaeolimnological records of shallow lake biodiversity change: exploring the merits of single versus multi-proxy approaches

Shallow lakes are among the most threatened ecosystems in the world and many contemporary studies have demonstrated declines in biodiversity due to anthropogenic forcing. Mostly, however, these studies have not covered the full period of human-induced diversity change in lakes which is typically over decades-centuries. Here we provide two examples of palaeoecological studies focussed on reconstructing biodiversity changes in contrasting shallow lake environments that demonstrate the efficacy of the approach—a shallow UK lake and a suite of floodplain lakes (called billabongs) in the Murray-Darling basin, Australia. In the Murray-Darling billabongs, complex sedimentary processes operate, sediment chronologies are less certain and replication of sites is needed to confirm patterns. The combination of sediment records from 10 billabongs showed that diatom diversity changes pre- and post-European (>1850) disturbance were inconsistent; however, reductions in diversity were more common and appear to reflect reductions in macrophyte abundance. At Felbrigg Lake, a multi-proxy study with strong chronological control demonstrated divergent responses of macrophyte, diatom, cladoceran and chironomid richness and diversity to a century of eutrophication. Eutrophication of the site was qualitatively inferred from changes in the macrophyte community in turn reconstructed from plant macrofossils. Benthic cladocerans showed a consistent decline in richness through the record, reflecting the gradual reduction in their macrophyte associated habitat over the past century. Diatom richness and diversity responses were complex, with increases in diversity and richness linked to both increases and decreases in macrophyte species richness and abundance. Chironomid richness and diversity patterns were less consistently linked to eutrophication. The loss of the dominant zooplanktivore (perch) in the 1970s was reflected in the richness and diversity profiles for all groups. Our study reveals clear potential for using sediment cores to infer biodiversity change in shallow lakes and shallow lake regions. However, given the contrasting patterns of diversity change for the different biological groups both in Felbrigg Lake and between Felbrigg and the billabongs, caution is required when interpreting whole-ecosystem biodiversity changes (or the absence of change) based on single as opposed to multi-proxy studies.     

Internal structure of Europa and Callisto

Models of the internal structure of completely differentiated Europa and partially differentiated Callisto have been constructed on the basis of Galileo gravity measurements, geochemical constraints on composition of ordinary and carbonaceous chondrites, and thermodynamic data on the equations of state of water, high-pressure ices, and meteoritic material. We assume thermal and mechanical equilibrium for the interiors of the satellites. A geophysically and geochemically permissible thickness of Europa's outer water-ice shell lies between 105 and 160 km (6.2-9.2% of total mass). Our results show that the bulk composition of the rock-iron core of Europa may be described by material approaching the L/LL-type chondrites in composition, but cannot be correlated either with the material of CI chondrites or H chondrites. For Europa's L/LL-chondritic models, core radii are estimated to be 470-640 km (5.3-12.5% of total mass). The allowed thickness of Europa's H₂O layer ranges from 115±10 km for a differentiated L/LL-type chondritic mantle with a crust to 135±10 km for an undifferentiated mantle. We show that Callisto must only be partially differentiated into an outer ice-I layer, a water ocean, a rock-ice mantle, and a rock-iron core (mixture of anhydrous silicates and/or hydrous silicates + FeFeS alloy). We accept that the composition of the rock-iron material of Callisto is similar to the bulk composition of L/LL-type chondritic material containing up to 10-15% of iron and iron sulfide. Assuming conductive heat transfer through the ice-I crust [Ruiz, 2001. The stability against freezing of an internal liquid-water ocean on Gallisto. Nature, 412, 409-411], heat flows were estimated and the possibility of the existence of a water ocean in Callisto was evaluated. The liquid phase is stable (not freezing) beneath the ice crust, if the heat flow is between 3.3 and 3.7 mW m⁻², which corresponds to the heat flow from radiogenic sources. The thickness of the ice-I crust is 135-150 km, and that of the underlying water layer, 120-180 km. The results of modeling support the hypothesis that Callisto may have an internal liquid-water ocean. The allowed total (maximum) thickness of the outer water-ice shell is up to 270-315 km. Rock-iron core radii, depending on the presence or absence of hydrous silicates, do not exceed 500-700 km, the thickness of an intermediate ice-rock mantle is not less than 1400 km, and its density is in the range of 1960-2500 kg m⁻³. The surface temperature of Callisto is expected to be 100-112 K. The total amount of H₂O in Callisto is found to be 49-55 wt%. The correspondence between the density and moment of inertia values for bulk ice-free Io, rock-iron core of ice-poor Europa, and rock-iron cores of Ganymede and Callisto shows that their bulk compositions may be, in general, similar and may be described by the composition close to a material of the L/LL-type chondrites with the (Fe_tot/Si) weight ratios ranging from 0.9 to 1.3. Planetesimals composed of these types of ordinary chondrites could be considered as analogues of building material for the rock-iron cores of the Galilean satellites. Similarity of bulk composition of the rock-iron cores of the inner and outer satellites implies the absence of iron-silicon fractionation in the protojovian nebula.