The scale size of chondrule formation regions: Constraints imposed by chondrule cooling rates

Abstract— Meteoritic data strongly suggest that most chondrules reached maximum temperatures in a range of 1650–2000 K and cooled at relatively slow rates of 100–1000 K/h, implying a persistence of external energy supply. The presence of fine‐grained rims around chondrules in most unequilibrated chondrites also indicates that a significant quantity of micron‐sized dust was present in chondrule formation regions. Here, we assume that the persistent external energy source needed to explain chondrule cooling rates consists primarily of radiation from surrounding heated chondrules, fine dust, and gas after the formation event. Using an approximate one‐dimensional numerical model for the outward diffusion of thermal radiation from such a system, the scale sizes of formation regions required to yield acceptable cooling rates are determined for a range of possible chondrule, dust, and gas parameters. Results show that the inferred scale sizes depend sensitively on the number densities of micron‐sized dust and on their adopted optical properties. In the absence of dust, scale sizes > 1000 km are required for plausible maximum chondrule number densities and heated gas parameters. In the presence of dust with mass densities comparable to those of the chondrules and with absorptivities and emissivities of ～0.01 calculated for Mie spheres with a pure mineral composition, scale sizes as small as ～100 km are possible. If dust absorptivities and emissivities approach unity (as may occur for particles with more realistic shapes and compositions), then scale sizes as small as ×10 km are possible. Considering all uncertainties in model parameters, it is concluded that small scale sizes (10–100 km) for chondrule formation regions are allowed by the experimentally inferred cooling rates.     

Environmental Models and Public Stakeholders in the Chesapeake Bay Watershed

The Chesapeake Bay is not only North America's largest estuary, but it is also home to one of most comprehensive computational modeling efforts that seeks to study and integrate a very large range of the complex socio-ecological dynamics within its watershed. Known as the Chesapeake Bay Modeling System (CBMS), this suite of models are invaluable to scientists, environmental policymakers, resource managers, and local government and community leaders, all of whom are engaged in efforts to reduce the negative impacts of population growth and development in the watershed on the ecosystems and living resources of the Chesapeake Bay. Until recently, the results of the CBMS were used to guide voluntary efforts to reduce nutrient and sediment runoff into the Chesapeake Bay. However, the results are now being used to guide and evaluate the implementation of Watershed Implementation Plans at the state and county levels, which in turn are based on CBMS estimates of total maximum daily loads (TMDLs) for these sub-watershed regions. The use of the CBMS for these regulatory efforts has also increased the number of public stakeholders who are now able to directly inquire about the how the CBMS works and raise questions about its ability to accurately represent their land use decisions and practices. This "going public" of the CBMS has raised many societal and cultural issues, including the articulation of local expert and scientific knowledge, and modelers, scientists, policymakers, and resource managers are now realizing the need to understand more of the human dimensions arising from the translation and implementation of the CBMS. A multidisciplinary approach is needed to understand these rapidly emerging societal and cultural dimensions of the expanded use of the CBMS. In this paper, we draw upon our collective experience as social and natural scientists, with modeling experience, to describe a range of social, economic, political, and cultural issues that have emerged as a result of the CBMS being used to support mandatory nutrient reduction regulations (TMDL).     

Magnetic and boundary effects on thermal instabilities in solar magnetic fields: Localized modes in a slab geometry

The coupling of thermal and ideal MHD effects in a sheared magnetic field is investigated. A slab geometry is considered so that the Alfvén mode can be decoupled from the system. With the total perturbed pressure approximately zero, the fast mode is eliminated and a system of linearized equations describing magnetic effects on the slow mode and thermal mode is derived. These modes evolve independently on individual fieldlines. One of the main features of this approach is that the influence of the dense photosphere can be included. A variety of different conditions that simulate the photospheric boundary are presented and the different results are discussed. A choice of field geometry and boundary conditions is made which removes mode rational surfaces so that there are no regions in which parallel thermal conduction can be neglected. This provides a stabilizing mechanism for the thermal mode. Growth rates are reduced by 30–40% and there is complete stabilization for sufficiently short fieldlines. The influence of dynamic and thermal boundary conditions on the formation of prominences is discussed.     

Preparing pollen concentrates for AMS dating – a methodological study from a hard-water lake in southern Sweden

Dating pollen concentrated from sediment samples is a way to improve lake-sediment chronology. The predominantly terrestrial origin of pollen assemblages minimizes, for example, the reservoir effect inherent in bulk sediment samples, especially from hard-water lakes. Pollen can be concentrated for dating by a combination of sieving and chemical treatment (Brown et al . 1989). This study illustrates the difficulty in applying a single, standard procedure, and demonstrates the need for flexibility depending on lake sediment characteristics and the particular pollen flora. Samples taken at the Ulmus decline were prepared and AMS-dated following different steps of the pollen concentration procedure. The results showed that both sieving and chemical treatment were needed to obtain an age close to the expected age for the Ulmus decline. The pollen concentrate dated c . 1000 years younger than a bulk date from the same level, but is close to the expected age based on correlation with dates for the Ulmus decline from Sphagnum peat sequences in north-western Europe. A compilation of bulk dates at the Ulmus decline implies that pollen concentrates would be a better material for dating than bulk samples for many lake sediments, not only for those from hard-water lakes.     

The influence of wind and river pulses on an estuarine turbidity maximum: Numerical studies and field observations in Chesapeake Bay

The effect of pulsed events on estuarine turbidity maxima (ETM) was investigated with the Princeton Ocean Model, a three-dimensional hydrodynamic model. The theoretical model was adapted to a straight-channel estuary and enhanced with sediment transport, erosion, deposition, and burial components. Wind and river pulse scenarios from the numerical model were compared to field observations before and after river pulse and wind events in upper Chesapeake Bay. Numerical studies and field observations demonstrated that the salt front and ETM had rapid and nonlinear responses to short-term pulses in river flow and wind. Although increases and decreases in river flow caused down-estuary and up-estuary (respectively) movements of the salt front, the effect of increased river flow was more pronounced than that of decreased river flow. Along-channel wind events also elicited non-linear responses. The salt front moved in the opposite direction of wind stress, shifting up-estuary in response to down-estuary winds and vice-versa. Modeled pulsed events affected suspended sediment distributions by modifying the location of the salt front, near-bottom shear stress, and the location of bottom sediment in relation to stratification within the salt front. Bottom sediment accumulated near the convergent zone at the tip of the salt front, but lagged behind the rapid response of the salt front during wind events. While increases in river flow and along-channel winds resulted in sediment transport down-estuary, only reductions in river flow resulted in consistent up-estuary movement of bottom sediment. Model predictions suggest that wind and river pulse events significantly influence salt front structure and circulation patterns, and have an important role in the transport of sediment in upper estuaries.     

Antipodal effects of lunar basin-forming impacts: Initial 3D simulations and comparisons with observations

3D simulations of basin-scale lunar impacts are carried out to investigate: (a) the origins of strong crustal magnetic fields and unusual terrain observed to occur in regions antipodal to young large basins; and (b) the origin of enhanced magnetic and geochemical anomalies along the northwest periphery of the South Pole-Aitken (SPA) basin. The simulations demonstrate that a basin-forming impact produces a massive, hot, partially ionized cloud of vapor and melt that expands thermally around the Moon, converging near the basin antipode approximately 1 h after the impact for typical impact parameters. In agreement with previous work, analytic calculations of the interaction of this vapor-melt cloud with an initial ambient magnetic field predict a substantial temporary increase in field intensity in the antipodal region. The time of maximum field amplification coincides with a period when impacting ejecta also converge near the antipode. The latter produce antipodal shock stresses within the range of 5-25 GPa where stable shock remanent magnetization (SRM) of lunar soils has been found experimentally to occur. Calculated antipodal ejecta thicknesses are only marginally sufficient to explain the amplitudes of observed magnetic anomalies if mean magnetization intensities are comparable to those produced experimentally. This suggests that pre-existing ejecta materials, which would also contain abundant metallic iron remanence carriers, may be important anomaly sources, a possibility that is consistent with enhanced magnetic anomalies observed peripheral to SPA. The latter anomalies may be produced by amplified secondary ejecta impact shock waves in the thick SPA ejecta mantle occurring near the antipodes of the Imbrium and Serenitatis impacts. Together with converging seismic compressional waves, these antipodal impact shocks may have produced especially deep fracture zones along the northwest edge of SPA near the Imbrium antipode, allowing the ascent of magma with enhanced KREEP concentrations.     

Solar coronal heating by magnetic cancellation – III. Thermodynamics

We present two-dimensional numerical magnetohydrodynamics simulations of a coronal X-ray bright point (XBP) caused by a cancelling magnetic feature (CMF). Cancellation is driven by converging motions of two magnetic bipolar sources. These sources are initially disconnected from each other so that both, the CMF and the associated reconnection/heating event (i.e. the XBP), are modelled in a self-consistent way. In the initial state, there is no X-point but two separatrices are present. Hence, the reconnection/heating and the cancellation phases have not yet started. Our numerical experiments end shortly after the converging magnetic bipole has fully cancelled. By this time, reconnection in the inner domain has ceased and occurs only at the base. Solving the energy equation with various heating and cooling terms included, and considering different bottom boundary conditions, reveals that the unrealistically high temperatures produced by Ohmic heating are reduced to more moderate temperatures of 1.5–2 MK consistent with observations of XBPs, if thermal conduction is included and density and temperature are fixed at the base.     

3D MHD Coronal Oscillations about a Magnetic Null Point: Application of WKB Theory

This paper is a demonstration of how the WKB approximation can be used to help solve the linearised 3D MHD equations. Using Charpit’s method and a Runge?–?Kutta numerical scheme, we have demonstrated this technique for a potential 3D magnetic null point, B=[x,ε y,?(ε+1)z]. Under our cold-plasma assumption, we have considered two types of wave propagation: fast magnetoacoustic and Alfvén waves. We find that the fast magnetoacoustic wave experiences refraction towards the magnetic null point and that the effect of this refraction depends upon the Alfvén speed profile. The wave and thus the wave energy accumulate at the null point. We have found that current buildup is exponential and the exponent is dependent upon ε. Thus, for the fast wave there is preferential heating at the null point. For the Alfvén wave, we find that the wave propagates along the field lines. For an Alfvén wave generated along the fan plane, the wave accumulates along the spine. For an Alfvén wave generated across the spine, the value of ε determines where the wave accumulation will occur: fan plane (ε=1), along the x-axis (0<ε<1) or along the y-axis (ε>1). We have shown analytically that currents build up exponentially, leading to preferential heating in these areas. The work described here highlights the importance of understanding the magnetic topology of the coronal magnetic field for the location of wave heating.     

Modeling the Effects of Oyster Reefs and Breakwaters on Seagrass Growth

Seagrass beds have declined in Chesapeake Bay, USA as well as worldwide over the past century. Increased seston concentrations, which decrease light penetration, are likely one of the main causes of the decline in Chesapeake Bay. It has been hypothesized that dense populations of suspension-feeding bivalves, such as eastern oysters (Crassostrea virginica), may filter sufficient seston from the water to reduce light attenuation and enhance seagrass growth. Furthermore, eastern oyster populations can form large three-dimensional reef-like structures that may act like breakwaters by attenuating waves, thus decreasing sediment resuspension. We developed a quasi-three-dimensional Seagrass-Waves-Oysters-Light-Seston (SWOLS) model to investigate whether oyster reefs and breakwaters could improve seagrass growth by reducing seston concentrations. Seagrass growth potential (SGP), a parameter controlled by resuspension-induced turbidity, was calculated in simulations in which wave height, oyster abundance, and reef/breakwater configuration were varied. Wave height was the dominant factor influencing SGP, with higher waves increasing sediment resuspension and decreasing SGP. Submerged breakwaters parallel with the shoreline improved SGP in the presence of 0.2 and 0.4 m waves when sediment resuspension was dominated by wave action, while submerged groins perpendicular to the shoreline improved SGP under lower wave heights (0.05 and 0.1 m) when resuspension was dominated by along-shore tidal currents. Oyster-feeding activity did not affect SGP, due to the oysters’ distance from the seagrass bed and reduced oyster filtration rates under either low or high sediment concentrations. Although the current implementation of the SWOLS model has simplified geometry, the model does demonstrate that the interaction between oyster filtration and along-shore circulation, and between man-made structures and wave heights, should be considered when managing seagrass habitats, planning seagrass restoration projects, and choosing the most suitable methods to protect shorelines from erosion.