Small-scale Density Research Articles

Multiple Images and Flux Ratio Anomaly of Fuzzy Gravitational Lenses.

Extremely light bosonic wave dark matter (ψDM) is an emerging dark matter candidate contesting the conventional cold dark matter paradigm and a model subject to intense scrutiny of late. This work for the first time reports testable salient features pertinent to gravitational lenses of ψDM halos. ψDM halos are distinctly filled with large-amplitude, small-scale density fluctuations with δρ/ρ_{halo}∼1 in form of density granules. This halo yields ubiquitous flux ratio anomalies of a few tens of percent, as is typically found for lensed quasars, and may also produce rare hexad and octad images for sources located in well-defined caustic zones. We have found new critical features appearing in the highly demagnified lens center when the halo has sufficiently high surface density near a very compact massive core.

Physical conditions of five O vi absorption systems towards PG 1522+101

ABSTRACT We present the analysis of five O vi absorbers identified across a redshift path of z ∼ (0.6−1.3) towards the background quasar PG 1522+101 with information on five consecutive ionization stages of oxygen from O ii to O vi. The combined HST and Keck spectra cover UV, redshifted extreme-UV, and optical transitions from a multitude of ions spanning ionization energies in the range of ∼(13−300) eV. Low-ionization (C ii, O ii, Si ii, Mg ii) and very high-ionization species (Ne viii, Mg x) are non-detections in all the absorbers. Three of the absorbers have coverage of He i, in one of which it is a >3σ detection. The kinematic structures of these absorbers are extracted from C iv detected in HIRES spectra. The farthest absorber in our sample also contains the detections of Ne v and Ne vi. Assuming co-spatial absorbing components, the ionization models show the medium to be multiphased with small-scale density–temperature inhomogeneities that are sometimes kinematically unresolved. In two of the absorbers, there is an explicit indication of the presence of a warm gas phase (T ≳ 105 K) traced by O vi. In the remaining absorbers, the column densities of the ions are consistent with a non-uniform photoionized medium. The subsolar [C/O] relative abundances inferred for the absorbers point at enrichment from massive Type II supernovae. Despite metal enrichment, the inferred wide range for [O/H] ∼ [−2.1, +0.2] amongst the absorbers along with their anticorrelation with the observed H i suggest poor small-scale mixing of metals with hydrogen in the regions surrounding galaxies and the IGM.

Chemical evolution of the Milky Way: constraints on the formation of the thick and thin discs

ABSTRACT We study the evolution of Milky Way thick and thin discs in the light of the most recent observational data. In particular, we analyse abundance gradients of O, N, Fe, and Mg along the thin disc as well as the [Mg/Fe] versus [Fe/H] relations and the metallicity distribution functions at different Galactocentric distances. We run several models starting from the two-infall paradigm, assuming that the thick and thin discs formed by means of two different infall episodes, and we explore several physical parameters, such as radial gas flows, variable efficiency of star formation, different times for the maximum infall on to the disc, different distributions of the total surface mass density of the thick disc, and enriched gas infall. Our best model suggests that radial gas flows and variable efficiency of star formation should be acting together with the inside-out mechanism for the thin disc formation. The time-scale for maximum infall on to the thin disc, which determines the gap between the formation of the two discs, should be tmax ≃ 3.25 Gyr. The thick disc should have an exponential, small-scale length density profile and gas infall on the inner thin disc should be enriched. We also compute the evolution of Gaia–Enceladus system and study the effects of possible interactions with the thick and thin discs. We conclude that the gas lost by Enceladus or even part of it could have been responsible for the formation of the thick disc but not the thin disc.

The effects of cosmic rays on the formation of Milky Way-mass galaxies in a cosmological context

ABSTRACT We investigate the impact of cosmic rays (CRs) and different modes of CR transport on the properties of Milky Way-mass galaxies in cosmological magnetohydrodynamical simulations in the context of the AURIGA project. We systematically study how advection, anisotropic diffusion, and additional Alfvén-wave cooling affect the galactic disc and the circumgalactic medium (CGM). Global properties such as stellar mass and star formation rate vary little between simulations with and without various CR transport physics, whereas structural properties such as disc sizes, CGM densities, or temperatures can be strongly affected. In our simulations, CRs affect the accretion of gas on to galaxies by modifying the CGM flow structure. This alters the angular momentum distribution that manifests itself as a difference in stellar and gaseous disc size. The strength of this effect depends on the CR transport model: CR advection results in the most compact discs while the Alfvén-wave model resembles more the AURIGA model. The advection and diffusion models exhibit large (r ∼ 50 kpc) CR pressure-dominated gas haloes causing a smoother and partly cooler CGM. The additional CR pressure smoothes small-scale density peaks and compensates for the missing thermal pressure support at lower CGM temperatures. In contrast, the Alfvén-wave model is only CR pressure dominated at the disc–halo interface and only in this model the gamma-ray emission from hadronic interactions agrees with observations. In contrast to previous findings, we conclude that details of CR transport are critical for accurately predicting the impact of CR feedback on galaxy formation.

Understanding scales of density-dependence to improve the use of resources in benthic mussel aquaculture

Shellfish aquaculture is considered a sustainable way to help meet rising protein demands worldwide. In shallow coastal dynamic ecosystems mussels can be cultivated directly on the seabed, however this method has a low return as mussels exposed to natural environments risk dislodgment, high predation rates, sedimentation and competition. The formation of spatial patterns in natural mussel beds, that result in ‘organized patchiness’, is thought to be an adaptive mechanism to reduce population losses. The driver and effects of this patterning need to be disentangled at multiple spatial scales in which patterns are observed. With a field experiment we aimed to understand how small-scale density (actual cover) and patterning (perimeter: area ratio of clumps and number of mussel layers) can be altered by manipulating large scale density (re-laying biomass), that farmers could control during seeding activity. Within this study we considered the interplay between environmental conditions (manipulating flow rate with the use of large mesh cages) and density for pattern development and persistence, and the repercussions of this on mussel productivity (growth and condition). We further investigated local scale processes, such as the role of within-clump biological activity (biodeposition), that may be a predictor for the larger scale observations of losses and persistence relative to density. We found that manipulating density by controlling seeding biomass from boats is not an accurate predictor of actual seabed density and resulting patterning. The growth and condition of the mussels was only influenced by local scale effects, resulting in high ‘within clumps’ variation. Aiming for an intermediate density to avoid both excessive fragmentation and excessive layering may be viewed as an optimal strategy to maximise returns, but we encourage the incorporation of the hierarchy of multiple scales of density in future studies of patterning that will allow the inclusion of these effects in a model of growth and productivity.

Linking Small-Scale Flight Manoeuvers and Density Profiles to the Vertical Movement of Insects in the Nocturnal Stable Boundary Layer

Huge numbers of insects migrate over considerable distances in the stably-stratified night-time atmosphere with great consequences for ecological processes, biodiversity, ecosystem services and pest management. We used a combination of meteorological radar and lidar instrumentation at a site in Oklahoma, USA, to take a new look at the general assistance migrants receive from both vertical and horizontal airstreams during their long-distance flights. Movement in the nocturnal boundary layer (NBL) presents very different challenges for migrants compared to those prevailing in the daytime convective boundary layer, but we found that Lagrangian stochastic modelling is effective at predicting flight manoeuvers in both cases. A key feature for insect transport in the NBL is the frequent formation of a thin layer of fast-moving air – the low-level jet. Modelling suggests that insects can react rapidly to counteract vertical air movements and this mechanism explains how migrants are retained in the jet for long periods (e.g. overnight, and perhaps for several hours early in the morning). This results in movements over much longer distances than are likely in convective conditions, and is particularly significant for the reintroduction of pests to northern regions where they are seasonally absent due to low winter temperatures.

Wavelet analysis of density fluctuation in supersonic turbulent boundary layer

In order to obtain the time-varying information and dynamic characteristics of density fluctuation in compressible turbulence, the wavelet method is used to analyze the flow density field of zero-pressure-gradient flat plate turbulent boundary layer at <i>Ma</i> = 3.0, which is measured based on Nano-tracer plane laser scattering technique. Utilizing Taylor’s frozen hypothesis, the spatial signal of density field converts into the temporal signal. The one-dimensional orthogonal wavelet multi-resolution analysis is used to reveal multi-scale turbulent structures, and the results suggest that large-scale structures play a leading role in the density fluctuation of turbulent boundary layer while the small-scale structures make the probability density function (PDF) of density fluctuation manifested as an “M” distribution. The density fluctuation scalar PDF deviates from Gaussian distribution. The Hilbert transformation is used to analyze amplitude modulation effects between large- and small-scale structure, and the results suggest that positive (negative) large scale density excursion in the outer layer induces local enhancement (suppression) of the small scale density fluctuation in the inner layer near the wall. The time-varying spectral density estimation method based on the wavelet transform is used to analyze the density fluctuation at different heights of turbulent boundary layer after proving its viability in time and frequency domain. The results suggest a wide range of frequencies throughout the turbulent boundary layer, mainly distributed within 1 MHz. The density fluctuation in the dominant frequency band is intermittent, most of which transits from high frequency to low frequency while the spectral density first increases and then decreases. Near the wall, the time-frequency distributions of density fluctuation in the logarithmic layer are similar. In the middle part of the turbulent boundary layer, the frequency distribution and spectral density of the density fluctuation each reach a peak. Near the mainstream region, the spectral density decreases obviously, which is mainly distributed near the structure formed by the interaction between the boundary layer and mainstream. The wall constraint, viscous dissipation, and uniform mainstream make the fluctuation nearby the region relatively weak. As a result, the spectrum amplitude of density fluctuation first increases and then decreases from the wall to the mainstream.

Research on small-scale structures of ice particle density and electron density in the mesopause region

Abstract. The formation of ice particle density irregularities with a meter scale in the mesopause region is explored in this paper by developing a growth and motion model of ice particles based on the motion equation of a variable mass object. The growth of particles by water vapor adsorption and the action of gravity and the neutral drag force on particles are considered in the model. The evolution of the radius, velocity, and number density of ice particles is then investigated by solving the growth and motion model numerically. For certain nucleus radii, it is found that the velocity of particles can be reversed at a particular height, leading to a local gathering of particles near the boundary layer, which then forms small-scale ice particle density structures. The spatial scale of the density structures can be affected by vertical wind speed, water vapor density, and altitude, and it remains stable as long as these environmental parameters do not change. The influence of the stable small-scale structures on electron and ion density is further calculated by a charging model, which considers the production, loss, and transport of electrons and ions, along with dynamic particle charging processes. Results show that the electron density is anti-correlated to the charged ice particle density and ion density for particles with radii of 11 nm or less due to plasma attachment by particles and plasma diffusion. This finding is in accordance with most rocket observations. The small-scale electron density structures created by small-scale ice particle density irregularities can produce the polar mesosphere summer echo (PMSE) phenomenon.

Cosmological constraints on the velocity-dependent baryon-dark matter coupling

We present the cosmological constraints on the cross section of baryon-dark matter interactions for the dark matter mass below the MeV scale from the Planck CMB (cosmic microwave background) and SDSS (Sloan Digital Sky Survey) Lyman-α forest data. To explore the dark matter mass mχ≲ 1 MeV for which the dark matter's free-streaming effect can suppress the observable small scale density fluctuations, in addition to the acoustic oscillation damping in existence of the baryon-dark matter coupling, we apply the approximated treatment of dark matter free-streaming analogous to that of the conventional warm dark matter. We demonstrate the mass dependence of the baryon-dark matter cross section bounds (for the dark matter mass down to mχ ∼ 5 keV), in contrast to the dark matter mass independence of the cross section constraints for the light dark matter below the MeV scale claimed in the previous literature. We also investigate the degeneracy between the baryon-dark matter coupling and the running spectral index in the scalar primordial spectrum. We find that this degeneracy is weak and the constraint on the baryon-dark matter coupling is not affected by the inclusion of the running spectral index in our MCMC analysis.

On the Properties of the Galactic Dust Layer within 700 pc of the Sun

We compare the spatial stellar color variations with our 3D model of the spatial dust distribution to refine the properties of the dust layer in Galactic solar neighborhoods. We use a complete sample of 93992 clump giants with an admixture of branch giants from the Gaia DR2 catalogue in a spatial cylinder with a radius of 700 pc around the Sun extending to |Z|=1800 pc. Accurate data of these stars in the Gaia DR2 GRP and WISE W3 bands have allowed the spatial GRP-W3 color variations to be used to calculate the model parameters and two characteristics of the sample, the mode of the dereddened color (GRP-W3)0 of the giant clump and the linear change of this mode with coordinate |Z|. As a result, an improved version of the three-dimensional model first proposed by Gontcharov (2009b) has been obtained. As in the previous version, the model suggests two dust layers, along the Galactic equator and in the Gould Belt, that intersect near the Sun at an angle of 18\pm2 degs. In contrast to the previous version of the model with a midplane of the Gould Belt dust layer in the form of a circle with the center at the Sun, in the new version this midplane is an ellipse decentered relative to the Sun. A scale height of 170\pm40 pc has been found for both dust layers. The modes of the absolute magnitude MW3=-1.70\pm0.02 and the dereddened color (GRP-W3)0=(1.43\pm0.01)-(0.020\pm0.007)|Z|, where Z is expressed in kpc, have been calculated for the giant clump near the Sun. The dispersions of the quantities under consideration have allowed the natural small-scale density fluctuations of the dust medium relative to the mean reddening calculated from the model to be characterized. These fluctuations make a major contribution to the uncertainty in the reddening.

Beyond subhalos: Probing the collective effect of the Universe’s small-scale structure with gravitational lensing

Gravitational lensing has emerged as a powerful probe of the matter distribution on subgalactic scales, which itself may contain important clues about the fundamental origins and properties of dark matter. Broadly speaking, two different approaches have been taken in the literature to map the small-scale structure of the Universe using strong lensing, with one focused on measuring the position and mass of a small number of discrete massive subhalos appearing close in projection to lensed images, and the other focused on detecting the collective effect of all the small-scale structure between the lensed source and the observer. In this paper, we follow the latter approach and perform a detailed study of the sensitivity of galaxy-scale gravitational lenses to the ensemble properties of small-scale structure. As in some previous studies, we adopt the language of the substructure power spectrum to characterize the statistical properties of the small-scale density field. We present a comprehensive theory that treats lenses with extended sources as well as those with time-dependent compact sources (such as quasars) in a unified framework for the first time. Our approach uses mode functions to provide both computational advantages and insights about couplings between the lens and source. The goal of this paper is to develop the theory and gain the intuition necessary to understand how the sensitivity to the substructure power spectrum depends on the source and lens properties, with the eventual aim of identifying the most promising targets for such studies.

Stringent Limit on Primordial Magnetic Fields from the Cosmic Microwave Background Radiation.

Primordial magnetic fields (PMFs), being present before the epoch of cosmic recombination, induce small-scale baryonic density fluctuations. These inhomogeneities lead to an inhomogeneous recombination process that alters the peaks and heights of the large-scale anisotropies of the cosmic microwave background (CMB) radiation. Utilizing numerical compressible MHD calculations and a MonteCarlo Markov chain analysis, which compares calculated CMB anisotropies with those observed by the WMAP and Planck satellites, we derive limits on the magnitude of putative PMFs. We find that the total remaining present day field, integrated over all scales, cannot exceed 47pG for scale-invariant PMFs and 8.9pG for PMFs with a violet Batchelor spectrum at 95%confidence level. These limits are more than one order of magnitude more stringent than any prior stated limits on PMFs from the CMB, which have not accounted for this effect.

Approaching Earth’s core conditions in high-resolution geodynamo simulations

SUMMARY The geodynamo features a broad separation between the large scale at which Earth’s magnetic field is sustained against ohmic dissipation and the small scales of the turbulent and electrically conducting underlying fluid flow in the outer core. Here, the properties of this scale separation are analysed using high-resolution numerical simulations that approach closer to Earth’s core conditions than earlier models. The new simulations are obtained by increasing the resolution and gradually relaxing the hyperdiffusive approximation of previously published low-resolution cases. This upsizing process does not perturb the previously obtained large-scale, leading-order quasi-geostrophic (QG) and first-order magneto-Archimedes-Coriolis (MAC) force balances. As a result, upsizing causes only weak transients typically lasting a fraction of a convective overturn time, thereby demonstrating the efficiency of this approach to reach extreme conditions at reduced computational cost. As Earth’s core conditions are approached in the upsized simulations, Ohmic losses dissipate up to 97 per cent of the injected convective power. Kinetic energy spectra feature a gradually broadening self-similar, power-law spectral range extending over more than a decade in length scale. In this range, the spectral energy density profile of vorticity is shown to be approximately flat between the large scale at which the magnetic field draws its energy from convection through the QG-MAC force balance and the small scale at which this energy is dissipated. The resulting velocity and density anomaly planforms in the physical space consist in large-scale columnar sheets and plumes, respectively, co-existing with small-scale vorticity filaments and density anomaly ramifications. In contrast, magnetic field planforms keep their large-scale structure after upsizing. The small-scale vorticity filaments are aligned with the large-scale magnetic field lines, thereby minimizing the dynamical influence of the Lorentz force. The diagnostic outputs of the upsized simulations are more consistent with the asymptotic QG-MAC theory than those of the low-resolution cases that they originate from, but still feature small residual deviations that may call for further theoretical refinements to account for the structuring constraints of the magnetic field on the flow.

Large-scale velocity dispersion and the cosmic web

Gravitational collapse in cosmological context produces an intricate cosmic web of voids, walls, filaments and nodes. The anisotropic nature of collisionless collapse leads to the emergence of an anisotropic velocity dispersion, or stress, that absorbs most of the kinetic energy after shell-crossing. In this paper, we measure this large-scale velocity dispersion tensor $\sigma^2_{ij}$ in $N$-body simulations using the phase-space interpolation technique. We study the environmental dependence of the amplitude and anisotropy of the velocity dispersion tensor field, and measure its spatial correlation and alignment. The anisotropy of $\sigma^2_{ij}$ naturally encodes the collapse history and thus leads to a parameter-free identification of the four dynamically distinct cosmic web components. We find this purely dynamical classification to be in good agreement with some of the existing classification methods. In particular, we demonstrate that $\sigma^2_{ij}$ is well aligned with the large-scale tidal field. We further investigate the influence of small scale density fluctuations on the large scale velocity dispersion, and find that the measured amplitude and alignments are dominated by the largest perturbations and thus remain largely unaffected. We anticipate that these results will give important new insight into the anisotropic nature of gravitational collapse on large scales, and the emergence of anisotropic stress in the cosmic web.

First detection of frequency-dependent, time-variable dispersion measures

Context. High-precision pulsar-timing experiments are affected by temporal variations of the dispersion measure (DM), which are related to spatial variations in the interstellar electron content and the varying line of sight to the source. Correcting for DM variations relies on the cold-plasma dispersion law which states that the dispersive delay varies with the squared inverse of the observing frequency. This may, however, give incorrect measurements if the probed electron content (and therefore the DM) varies with observing frequency, as is predicted theoretically due to the different refraction angles at different frequencies. Aims. We study small-scale density variations in the ionised interstellar medium. These structures may lead to frequency-dependent DMs in pulsar signals. Such an effect could inhibit the use of lower-frequency pulsar observations as tools to correct time-variable interstellar dispersion in higher-frequency pulsar-timing data. Methods. We used high-cadence, low-frequency observations with three stations from the German LOng-Wavelength (GLOW) consortium, which are part of the LOw-Frequency ARray (LOFAR). Specifically, 3.5 yr of weekly observations of PSR J2219+4754 are presented. Results. We present the first detection of frequency-dependent DMs towards any interstellar object and a precise multi-year time-series of the time- and frequency-dependence of the measured DMs. The observed DM variability is significant and may be caused by extreme scattering events. Potential causes for frequency-dependent DMs are quantified and evaluated. Conclusions. We conclude that frequency dependence of DMs has been reliably detected and is indeed caused by small-scale (up to tens of AUs) but steep density variations in the interstellar electron content. We find that long-term trends in DM variability equally affect DMs measured at both ends of our frequency band and hence the negative impact on long-term high-precision timing projects is expected to be limited.

Statistics of small length scale density fluctuations in supercooled viscous liquids.

Many successful theories of liquids near the melting temperature assume that small length scale density fluctuations follow Gaussian statistics. This paper presents a numerical investigation of density fluctuations in the supercooled viscous regime using an enhanced sampling method. Five model systems are investigated: the single component Lennard-Jones liquid, the Kob-Andersen binary mixture, the Wahnström binary mixture, the Lewis-Wahnström model of ortho-terphenyl, and the TIP4P/Ice model of water. The results show that the Gaussian approximation persists to a good degree into the supercooled viscous regime; however, it is less accurate at low temperatures. The analysis suggests that non-Gaussian fluctuations are related to crystalline configurations. Implications for theories of the glass transition are discussed.

Effects of baryonic and dark matter substructure on the Pal 5 stream

Gravitational encounters between small-scale dark matter substructure and cold stellar streams in the Milky Way halo lead to density perturbations in the latter, making streams an effective probe for detecting dark matter substructure. The Pal 5 stream is one such system for which we have some of the best data. However, Pal 5 orbits close to the center of the Milky Way and has passed through the Galactic disk many times, where its structure can be perturbed by baryonic structures such as the Galactic bar and giant molecular clouds (GMCs). In order to understand how these baryonic structures affect Pal 5's density, we present a detailed study of the effects of the Galactic bar, spiral structure, GMCs, and globular clusters on the Pal 5 stream. We estimate the effect of each perturber on the stream density by computing its power spectrum and comparing it to the power induced by a CDM-like population of dark matter subhalos. We find that the bar and GMCs can each individually create power that is comparable to the observed power on large scales, leaving little room for dark matter substructure, while spirals are subdominant on all scales. On degree scales, the power induced by the bar is small, but GMCs create small-scale density variations that are similar in amplitude to the dark-matter induced variations but otherwise indistinguishable from it. These results demonstrate that Pal 5 is a poor system for constraining the dark matter substructure fraction and that observing streams further out in the halo will be necessary to confidently detect dark matter subhalos.

喀斯特常绿落叶阔叶混交林粗木质残体的空间分布格局

PDF HTML阅读 XML下载 导出引用 引用提醒 喀斯特常绿落叶阔叶混交林粗木质残体的空间分布格局 DOI: 10.5846/stxb201805201107 作者: 作者单位: 贵州大学林学院,贵州大学林学院,贵州茂兰国家级自然保护区管理局,贵州茂兰国家级自然保护区管理局 作者简介: 通讯作者: 中图分类号: 基金项目: 贵州省科技计划项目（黔科合LH字[2016]7446号）；国家自然科学基金青年科学基金项目（31700385）；贵州省科技计划项目（黔科合平台人才[2017]5788） Spatial point pattern analysis for coarse woody debris in karst mixed evergreen and deciduous broadleaved forest Author: Affiliation: College of forestry,Guizhou university,College of forestry,Guizhou university,Maolan National Nature Reserve Management Bureau in Guizhou,Maolan National Nature Reserve Management Bureau in Guizhou Fund Project: Provincial and department research program fund, The National Science Fund for Young Scholars 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:粗木质残体（coarse woody debris，CWD）是森林生态系统中重要的结构性和功能性组成要素，是维护系统完整性和稳定性的关键。对CWD空间格局的研究将有助于深入探索种群格局的形成和森林生态系统的维持机制。采用g（r）函数对茂兰喀斯特常绿落叶阔叶混交林1.28 hm2固定样地内不同径级、不同腐烂等级、不同存在形式的CWD的空间分布格局及空间关联性进行了研究。结果表明：1）在40 m的空间尺度内，CWD总体在0-12 m尺度上表现为集群分布，随着尺度的增加格局强度降低，趋于随机分布，剔除生境异质性后，格局尺度降低至7 m。2）CWD径级格局表现为：小径级 > 中径级 > 大径级。拔根倒和干中折断在整个研究尺度上为随机分布，其他不同径级、不同腐烂等级、不同存在形式的CWD均在小规模尺度（2-8 m）表现为集群分布，随着尺度的增加聚集强度急剧变小，趋于随机分布或均匀分布。3）除了干中折断与树段之间、大径级与小径级之间的CWD在空间上相互独立，其他不同径级、不同腐烂等级或不同存在形式的CWD之间均在小规模尺度（2-8 m）上表现为显著的正相关，随着尺度的增加空间关联性降低。喀斯特常绿落叶阔叶混交林CWD的分布格局可能是在小尺度内由密度制约、在大尺度内由生境过滤和个体自然衰老等生态学过程所形成，大径级对临近的小径级、先死亡对后死亡、站杆对倒下的个体具有一定的正向影响作用，在一定程度上揭示了该林型天然更新的作用和机制。 Abstract:Coarse woody debris (CWD) is one of the most important components in the structure and function of forest ecosystems and it is also key for maintaining system integrity and stability. Therefore, studying spatial patterns of CWD can help us understand the formation of population patterns and the maintenance mechanisms of forest ecosystems. Karst evergreen broadleaved mixed forest is one of the best preserved primordial subtropical forests. To date, there have been few studies of spatial patterns and correlations in this forest, especially research on CWD and its spatial distribution. The pair correlation function g(r) was used to study the spatial distribution patterns and correlations of CWD of different forms, decomposition levels, and diameter classes in a 1.28 hm2 fixed plot in Maolan karst mixed evergreen and deciduous broadleaf forest. The results showed that 1) at a spatial scale of 40 m, overall CWD showed a clustered distribution at spatial scales ≤12 m. With an increase in scale, CWD tended to be randomly distributed. After removing habitat heterogeneity, the pattern scale was reduced to 7 m. 2) The distribution pattern of CWD diameter classes was: small tree > medium tree > large tree. CWD, in the form of root inversion and breakage at trunk, was randomly distributed throughout the study area and other types of CWD of different diameters, decay levels, and forms were clustered at a small scale (2-8 m). With an increase in scale, the intensity of aggregation decreased sharply and CWD tended to be randomly or regularly distributed. 3) Aside from the spatial independence of CWD between breakage at trunk and segment, and large and small diameter classes, there were significant positive correlations between different diameter classes, decay levels, and forms at a small scale of 2-8 m. With an increase in scale, the spatial correlation decreased. The distribution patterns of CWD in this Karst forest may be formed by ecological processes such as small-scale density restrictions, large-scale endogenous habitat filtration, and individual natural senescence. The large diameter, earlier mortality, and standing individuals had a positive effect on nearby small diameter, later mortality, and fallen individuals, respectively, which revealed the mechanism of natural regeneration of this forest type. 参考文献 相似文献 引证文献

Controlling the Self-Injection Threshold in Laser Wakefield Accelerators

Controlling the parameters of a laser plasma accelerated electron beam is a topic of intense research with a particular focus placed on controlling the injection phase of electrons into the accelerating structure from the background plasma. An essential prerequisite for high-quality beams is dark-current free acceleration (i.e., no electrons accelerated beyond those deliberately injected). We show that small-scale density ripples in the background plasma are sufficient to cause the uncontrolled (self-)injection of electrons. Such ripples can be as short as ∼50 μm and can therefore not be resolved by standard interferometry. Background free injection with substantially improved beam characteristics (divergence and pointing) is demonstrated in a gas cell designed for a controlled gas flow. The results are supported by an analytical theory as well as 3D particle in cell simulations.

A comparison of refinement indicators for p-adaptive simulations of steady and unsteady flows using discontinuous Galerkin methods

This paper presents an analysis of refinement indicators for the simulation of steady and unsteady flows by means of p-adaptive algorithms for discontinuous Galerkin (DG) methods. Residual-error, discretization-error and feature-based indicators are compared by studying their effect on convergence history, computational gain and refinement regions selected by the adaptive algorithm. The analysis is initially carried out on steady flow configurations. Static p-adaptive simulations of the periodic flow past a cylinder at Re=100 are then performed. Compared to uniform p-refinement, a reduction between 50% and 75% in the number of degrees of freedom is obtained for all test cases considered. The accuracy and efficiency observed for the small-scale energy density [1] and spectral decay [2] indicators demonstrate their great potential for the adaptive simulation of unsteady flows.