The diffusional evolution of chemical composition in a solar model

We consider the basic physical processes resulting in a differential, microscopic redistribution of stellar matter, generally known as diffusion. The main effect of diffusion in the solar interior is a segregation of light and heavy elements in the gravitational field. As a result, the abundance of helium and heavy elements in the solar envelope is reduced, while it becomes enriched by hydrogen. We present estimates of the degree of settling for a sequence of evolutionary models via numerical solution of the generalized diffusion equation. The effect of the ion charge (in the approximation of full ionization) on the settling rate is studied in detail. Abundance variations are given for the centers of the models, as well as for the convective envelope of the modern Sun. We analyze the effects of the thermal and concentrational diffusion on the evolution of the chemical-composition profile. Quantitatively, the effect of thermal diffusion is not very large, but it leads to the appearance of new features in the hydrogen-abundance profile, namely, a discontinuity at the base of the convective zone. The effect of concentration diffusion is relatively small, and is appreciable only at the model center at late stages of the evolution, and also close to the base of the convective envelope. All the mechanisms studied are necessary components of a modern model for the internal structure and evolution of the Sun.     

Composition of rare earth elements in settling particles collected in the highly productive North Pacific Ocean and Bering Sea: Implications for siliceous-matter dissolution kinetics and formation of two REE-enriched phases

Settling particles were sampled monthly for 1 year using an automated time-series sediment trap positioned at similar depths at two sites of high diatomaceous productivity in the North Pacific Ocean and Bering Sea. The particles were analyzed for rare earth elements (REEs) by inductively coupled plasma mass spectrometry (ICP-MS) with and without chemical treatment of the bulk samples to isolate siliceous fractions. The REE composition of the bulk samples is explained largely by the contribution of two distinct components: (i) carbonate with a higher REE concentration, a negative Ce anomaly and lighter REE (LREE) enrichment; (ii) opal with a lower REE concentration, a weaker negative Ce anomaly and heavier REE (HREE) enrichment.The siliceous fractions of settling particles are characterized by high Si/Al ratios (30-190), reflecting high diatom productivity at the studied sites. The La/Al ratio of the siliceous fraction is close to that of the upper crust, but the Lu/Al and Lu/La ratios are significantly higher than those of the upper crust or airborne particles, indicating the presence of excess HREEs in the siliceous fraction. Diatoms are believed to be important carriers of HREEs.The Ce anomaly, Eu anomaly, slope of the REE pattern, and ΣREE of the siliceous fraction vary exponentially with decreasing total mass flux. They can be well-reproduced according to the differential dissolution kinetics of elements in the order of Ce < lighter REEs (LREEs) < Eu = heavier REEs (HREEs) < Si from settling particles, where the dissolution rate is critically reduced through particle aggregation. This order is consistent with the vertical distribution of dissolved REEs and Si in oceans. The differential dissolution kinetics leads to HREE enrichment of the original diatoms and REE enrichment of dissolved diatoms. The Lu/Si ratio of the siliceous fraction of settling particles recovered from some of the highest diatom fluxes is identical to that of the two elements dissolved in deep seawater, providing further evidence for the dissolution of siliceous matter in deep water.     

Solar and cosmogenic argon in dated lunar impact spherules

We have studied lunar impact spherules from the Apollo 12 and Apollo 14 landing sites, examining the isotopic composition of argon released by stepwise heating. Elsewhere, we reported the formation ages of these spherules, determined by the ⁴⁰Ar/³⁹Ar isochron method. Here, we discuss solar and cosmogenic argon from the same spherules, separating these two components by correlating their partial releases with the releases of calcium-derived ³⁷Ar on a “cosmochron” diagram. We use the abundances of cosmogenic argon to derive a cosmic ray exposure age for each spherule, and demonstrate that single scoops of lunar soil contain spherules which have experienced very different histories of exposure and burial. The solar argon is seen to be separated into isotopically lighter and heavier fractions, which presumably were implanted to different depths in the spherules. The abundance of the isotopically heavy solar argon is too great to explain as a minor constituent of the solar particle flux, such as the suprathermal tail of the solar wind. The fact that the spherules have been individually dated allows us to look for possible variations in the solar wind as a function of time, over the history of the Solar System. However, the isotopic composition and fluence of solar argon preserved in the lunar spherules appear to be independent of formation age. We believe that most of the spherules are saturated with solar argon, having reached a condition in which implantation by the solar wind is offset by losses from solar-wind sputtering and diffusion.     

A possible mechanism for ionization of the Magellanic stream by disk stars

In our model describing the leakage of ionizing radiation from the Galactic disk into the halo, disk stars can contribute substantially to the ionization of halo objects such as high-velocity clouds and the Magellanic stream. This ionization is produced by a relatively hard radiation field, which can maintain its ionizing effect even at a considerable distance from the plane of the disk.     

A rapid x-ray fluorescence procedure applicable to exploration geochemistry

A rapid X-ray fluorescence method is described for the analysis of all elements heavier than iron. It is based upon the measurement of a peak and background, while primary scattered radiation is used to correct or compensate for matrix effects. About 15 analyses per man hour can be made on individual samples for such elements as Th, Nb, Ta and W, which are normally difficult to determine by other techniques. Lower limits of detection of about 50 ppm are routinely obtained, and even lower limits of detection (e.g., 20 ppm) can be obtained should these be necessary by increasing the counting time (from 20 to 40 seconds).     

Predicting solar radiation fluxes for solar energy system applications

The mean daily global solar radiation flux is influenced by astronomical, climatological, geographical, geometrical, meteorological, and physical parameters. This paper deals with the study of the effects of influencing parameters on the mean daily global solar radiation flux, and also with the computation of the solar radiation flux at the surface of the earth in locations without solar radiation measurements. The reference–real data were borrowed from the Iranian Meteorological Organization. The analysis of data showed that the mean daily solar radiation flux on a horizontal surface is related to parameters such as: mean daily extraterrestrial solar radiation, average daily ratio of sunshine duration, mean daily relative humidity, mean daily maximum air temperature, mean daily maximum dew point temperature, mean daily atmospheric pressure, and sine of the solar declination angle. Multiple regression and correlation analysis were applied to predict the mean daily global solar radiation flux on a horizontal surface. The models were validated when compared with the reference–measured data of global solar radiation flux. The results showed that the models estimate the global solar radiation flux within a narrow relative error band. The values of mean bias errors and root mean square errors were within acceptable margins. The predicted values of global solar radiation flux by this approach can be used for the design and performance estimation in solar applications. The model can be used in areas where meteorological stations do not exist and information on solar radiation flux cannot be obtained experimentally.     

1961-2010年青海玉树地区太阳总辐射的长期变化研究

利用青海玉树国家基本气象观测站1961-2010年的总辐射常规观测资料, 分析了玉树地区总辐射的长期变化趋势. 结果表明: 玉树地区总辐射在1961-2010年期间总体呈现减弱趋势, 其变化经历了增强变"亮"-减弱变"暗"-增强变"亮"-震荡回落4个阶段; 春、夏季的总辐射变化趋势对年际变化趋势有主导作用. 小波分析表明, 玉树地区近50 a的总辐射变化过程中, 存在多时间尺度周期的变化特征; 长期变化趋势和国内其他站点的变化趋势相比均表现出减弱的趋势, 但各站点减弱趋势倾向率不同. 最后, 对比分析了总辐射和同期气象要素、MODIS反演的气溶胶AOD之间的关系.     

青海省太阳辐射强度时空变化特征分析

利用西宁、刚察、格尔木、玉树和玛沁5站的1971-2014年逐日总辐射资料和青海省50个气象台站的温度、降水和日照时数等气象资料,分析了太阳辐射量与气象要素的内在关系,建立了无辐射观测资料地区的太阳辐射推算方法,并根据推算出的各气象站点太阳辐射量,分析了青海省全境太阳辐射量变化规律及分布特征。研究结果表明：青海省年太阳总辐射量在5 668~7 091 MJ·m^-2之间,由西北向东南逐渐递减,全省年太阳总辐射量超过6 000 MJ·m^-2的有42个站点,占全省总站点的84%。太阳总辐射量在春、夏季自东南向西北部逐渐增加,在秋季自西北向东南部逐渐减小,冬季全省各地差异较小。青海省4-8月的太阳辐射量最强,旬太阳辐射量主要集中在3月上旬-10月上旬,周太阳辐射量主要集中在第12周-第43周。青海省太阳日辐射变化趋势均呈抛物线型,早晨和傍晚辐射值较低,日出后开始呈上升趋势,北京时间13：00左右达到最高值后开始下降。日辐射持续时间从3月开始增加,9月开始减少。     

黑河下游天然胡杨树干液流特征的试验研究

应用热扩散原理,采用ICT2000TE 研究了黑河下游25 a生天然胡杨(Populus euphratica)的树干液流特点及其与环境因子的关系.结果表明:1) 在整个生长季的晴天,胡杨树干液流的日变化呈现明显的单峰宽峰曲线.液流速率在6、7、8、9和10月分别为13.39、12.07、12.69和5.1 L·d-1;2) 胡杨在夜间有微弱上升液流,可能存在根压;3) 在生长季树干液流速率与环境因子逐步回归分析结果表明,树干液流受环境因子的综合影响,影响大小依次是空气温度、土壤含水量、相对湿度、总辐射、土壤温度和风速.     

第84届SEG年会多分量地震技术评述

综合分析2014年度美国SEG年会上多分量地震技术方面的学术论文,不难发现:在多波地震偏移成像研究中,叠前时间偏移仍是实际应用的主要方法;相比于PS折射波初至拾取,径向-道域变换的射线路径一致性静校正方法效果更好;全波形反演在四维地震应用中具有诱人的前景,基于纵波反射系数的流体因子反演方法在稳定性、准确性方面显示出很大的优势,有望产生实际的应用效果。多分量地震技术的发展特点可归纳为"一批亮点,一个重点",即:页岩的岩石物理实验与数值模拟分析、地震波的低频衰减气溶机理、六分量矢量波场特征的研究、多组多尺度裂缝系统的响应特征是该领域研究的亮点;综合利用海洋四分量压制鬼波及径向波、去除海底多次波、提高纵波的信噪比是海洋多分量地震技术发展的重点。      

Timescales determining the degree of kinetic isotope fractionation by evaporation and condensation

A first order characteristic of the relative abundance of the elements in solar system materials ranging in size from inclusions in primitive meteorites to planetary sized objects such as the Earth and the Moon is that they are very much like that of the Sun for the more refractory elements but systematically depleted to varying degrees in the more volatile elements. This is taken as evidence that evaporation and and/or condensation were important processes in determining the distinctive chemical properties of solar system materials. In some instances there is also isotopic evidence suggesting evaporation in that certain materials are found enriched in the heavy isotopes of their more volatile elements. Here model calculations are used to explore how the relative rates of various key processes determine the relationship between elemental and isotopic fractionation during partial evaporation and partial condensation. The natural measure of time for the systems considered here is the evaporation or condensation timescale defined as the time it would take under the prevailing conditions for evaporation or condensation to completely transfer the element of interest between the two phases of the system. The other timescales considered involve the rate of change of temperature, the rate at which gas is removed from further interaction with the condensed phase, and the rates of diffusion in the condensed and gas phases. The results show that a key determinant of whether or not elemental fractionations have associated isotopic effects is the ratio of the partial pressure of a volatile element (P_i) to its saturation vapor pressure (P_i,sat) over the condensed phase. Systems in which the rate of temperature change or of gas removal are slow compared to the evaporation or condensation timescale will be in the limit P_i ∼ P_i,sat and thus will have little or no isotopic fractionation because at the high temperatures considered here there is negligible equilibrium fractionation of isotopes. If on the other hand the temperature changes are relatively fast, then P_i ≠ P_i,sat and there will be both elemental and isotopic fractionation during partial evaporation or partial condensation. Rapid removal of evolved gas results in P_i ? P_i,sat which will produce isotopically heavy evaporation residues. Diffusion-limited regimes, where transports within a phase are not sufficiently fast to maintain chemical and or isotopic homogeneity, will typically produce less isotopic fractionation than had the phases remained well mixed. The model results are used to suggest a likely explanation for the heavy silicon and magnesium isotopic composition of Type B CAIs (as due to rapid partial melting and subsequent cooling at rates of a few °C per hour), for the uniformity of the potassium isotopic composition of chondrules despite large differences in potassium depletions (as due to volatilization of potassium by reheating in regions of large but variable chondrules per unit volume), and that the remarkable uniformity of the potassium isotopic composition of solar system materials is not a measure of the relative importance of evaporation and condensation but rather due to the solar nebula having evolved sufficiently slowly that materials did not significantly depart from chemical equilibrium.     

Effects of concentration-dependent settling velocity on non-equilibrium transport of suspended sediment

In this study, non-equilibrium transport of suspended sediment from one equilibrium state to another is investigated. Based on a convective-diffusion equation, a numerical model for flow with suspended sediment is developed by considering the effect of concentration-dependent settling velocity. The numerical model is validated by comparing analytical solutions and experimental results. The concentration profiles, mean concentrations and distance necessary to reach a new equilibrium state are examined by comparing them with the results of constant settling velocity. For a high concentration flow, the results indicate that evident differences between the above three indicators can be determined with and without concentration-dependent settling velocity. Additionally, the effects of concentration-dependent settling velocity are sensitive to the sediment mobility parameter (or Rouse number), although they are nearly independent of the diffusion Reynolds number.     

Apatite (U-Th)/He thermochronometry using a radiation damage accumulation and annealing model

Helium diffusion from apatite is a sensitive function of the volume fraction of radiation damage to the crystal, a quantity that varies over the lifetime of the apatite. Using recently published laboratory data we develop and investigate a new kinetic model, the radiation damage accumulation and annealing model (RDAAM), that adopts the effective fission-track density as a proxy for accumulated radiation damage. This proxy incorporates creation of crystal damage proportional to α-production from U and Th decay, and the elimination of that damage governed by the kinetics of fission-track annealing. The RDAAM is a version of the helium trapping model (HeTM; Shuster D. L., Flowers R. M. and Farley K. A. (2006) The influence of natural radiation damage on helium diffusion kinetics in apatite. Earth Planet. Sci. Lett.249, 148-161), calibrated by helium diffusion data in natural and partially annealed apatites. The chief limitation of the HeTM, now addressed by RDAAM, is its use of He concentration as the radiation damage proxy for circumstances in which radiation damage and He are not accumulated and lost proportionately from the crystal.By incorporating the RDAAM into the HeFTy computer program, we explore its implications for apatite (U-Th)/He thermochronometry. We show how (U-Th)/He dates predicted from the model are sensitive to both effective U concentration (eU) and details of the temperature history. The RDAAM predicts an effective He closure temperature of 62 °C for a 28 ppm eU apatite of 60 μm radius that experienced a 10 °C/Ma monotonic cooling rate; this is 8 °C lower than the 70 °C effective closure temperature predicted using commonly assumed Durango diffusion kinetics. Use of the RDAAM is most important for accurate interpretation of (U-Th)/He data for apatite suites that experienced moderate to slow monotonic cooling (1-0.1 °C/Ma), prolonged residence in the helium partial retention zone, or a duration at temperatures appropriate for radiation damage accumulation followed by reheating and partial helium loss. Under common circumstances the RDAAM predicts (U-Th)/He dates that are older, sometimes much older, than corresponding fission-track dates. Nonlinear positive correlations between apatite (U-Th)/He date and eU in apatites subjected to the same temperature history are a diagnostic signature of the RDAAM for many but not all thermal histories.Observed date-eU correlations in four different localities can be explained with the RDAAM using geologically reasonable thermal histories consistent with independent fission-track datasets. The existence of date-eU correlations not only supports a radiation damage based kinetic model, but can significantly limit the range of acceptable time-temperature paths that account for the data. In contrast, these datasets are inexplicable using the Durango diffusion model. The RDAAM helps reconcile enigmatic data in which apatite (U-Th)/He dates are older than expected using the Durango model when compared with thermal histories based on apatite fission-track data or other geological constraints. It also has the potential to explain at least some cases in which (U-Th)/He dates are actually older than the corresponding fission-track dates.     

青藏铁路旱桥桥面遮阳对桥下及周边冻土太阳辐射影响

通过分析太阳辐射强度和旱桥桥面影子轨迹随时间的变化规律,建立时间、太阳辐射强度、太阳位置和影子轨迹关系的数学模型,并利用该模型对旱桥桥面遮阳效应进行研究。提出直射率概念表示周边冻土获得的太阳直接辐射能量的比例。研究发现旱桥的高度、走向、桥面宽度等因素对桥下及周边范围冻土的太阳直射率影响很大。随着旱桥高度的增加,桥面遮阳影响范围增大,遮阳中心直射率增大;随着旱桥宽度的增加,桥下及周边冻土的直射率降低;东西走向的旱桥桥下及周边冻土表面直射率非对称性最为明显。旱桥桥面的遮阳可以有效减少桥下及周边冻土的太阳辐射热量,同时也会引起桩基周边冻土表面非均匀太阳辐射受热,这种太阳辐射的非均匀性不可忽视,在旱桥长期稳定性分析中应予以考虑。     

Stability of toroidal magnetic fields in the radiation zone of a star

The stability of a toroidal magnetic field in the rotating radiation zone of a star is analyzed to estimate the maximum possible magnitude of relic fields. Equations for small perturbations are obtained taking into account the finite diffusivity and the stabilizing effect of the subadiabatic stratification. The numerical solution of the eigenvalue problem indicates that the threshold field strength for the onset of instability in the radiation zone of the Sun is about 600 G. This figure sets an upper bound for the strength of the relic field. The assumption that magnetic instabilities are present in the solar radiation zone disagrees with the observed abundance of lithium. Our analysis of joint stability of toroidal field and nonuniform rotation shows that two-dimensional MHD solutions for the solar tachocline are stable against three-dimensional perturbations.     

Thermal stability of wind near a hot star

We present computations of the ionization and thermal balance in the wind from WR and OB stars. The star’s radiation field is modeled using a Planck function with a temperature from 2 to 6 eV. We assume that the principal contribution from gas heating and cooling is from ions and atoms of hydrogen, carbon, nitrogen, oxygen, and neon. We take into account photoionization from the ground state and excited states, ionization by electron collisions, radiative and two-electron recombination, and excitation and de-excitation of discrete levels by the star’s dilute radiation and electron collisions. It is demonstrated that the thermal regime of gas in the conditions typical of the winds near WR and OB stars is stable.     

Circumstellar nebulae of WR and OB stars with strong winds: Simultaneous excitation by stellar radiation and shocks

We use a two-phase model for the structure of the circumstellar nebulae of hot stars to analyze the radiative cooling of a dense, compact cloud behind the shock produced by the compression of the cloud by hot gas from the stellar wind, taking into account ionization and heating by radiation from the central star. We can distinguish three stages of the evolution of the cloud during its compression. In the first stage, relevant for the entire cloud before compression and the gas ahead of the shock front, the state of the gas is determined purely by ionization by the stellar radiation. The next stage is characterized by the simultaneous action of two gas excitation mechanisms—photoionization by the stellar radiation and shock heating. In this stage, the gas intensively radiates thermal energy received at the shock front. After radiative cooling, in the final stage, ionization and heating of the gas are again determined mainly by the star. To compute the spectrum of the cloud radiation, we solved for the propagation of a plane-parallel, homogeneous flux through the shock front in the radiation field of the hot star. The computations show that a combination of two excitation mechanisms considerably enriches the theoretical spectrum. The relative intensities of emission lines of a single cloud may resemble either those for an HII region or of a supernova remnant.     

Why did life develop on the surface of the Earth in the Cambrian?

Life was limited for most of Earth's history, remaining at a primitive stage and mostly marine until about 0.55 Ga. In the Paleozoic, life eventually exploded and colonized the continental realm. Why had there been such a long period of delayed evolution of life? Early life was dominated by Archaea and Bacteria,which can survive ionizing radiation better than other organisms. The magnetic field preserves the atmosphere, which is the main shield of UV radiation. We explore the hypothesis that the Cambrian explosion of life could have been enabled by the increase of the magnetic field dipole intensity due to the solidification of the inner core, caused by the cooling of the Earth, and the concomitant decrease with time of the high-energy solar flux since the birth of the solar system. Therefore, the two phenomena could be responsible for the growth and thickening of the atmosphere and the development of land surface life.