Realistic Density Research Articles

Variations of Field Line Eigenfrequencies With Ring Current Intensity

AbstractWe present results from the closed magnetosphere (5.9≤L < 9.5 over all magnetic local times) to demonstrate and assess the variations in field line eigenfrequency with geomagnetic activity. Using the time‐of‐flight technique with realistic magnetic field and mass density models, the spatial distributions of field line eigenfrequencies are determined for a range of different geomagnetic activity levels, as defined by the Dst index. The results indicate that during geomagnetically active conditions, the eigenfrequency of a given field line is generally decreased compared to quiet times, in addition to variations in local asymmetries. By comparing the dependence to changes in the magnetic field and mass density distribution, it is established that the inflation and weakening of the geomagnetic field outweighs decreased plasma mass density and is the sole contributor to decreased eigenfrequencies with increased geomagnetic activity. We highlight the importance of considering the magnetic field, mass density, and average ion mass contributions when using observed eigenfrequencies to probe magnetospheric conditions. Furthermore, the estimates significantly improve upon existing time‐of‐flight results, through a consideration of mass density changes with geomagnetic activity. We also provide estimates of eigenfrequencies for a comparatively extended spatial region than available from prior direct observations of field line resonances. The results have clear implications for furthering our understanding of how wave energy propagates throughout the magnetosphere during geomagnetic storms.

Flux Correction and Overturning Stability: Insights from a Dynamical Box Model

AbstractA recent paper suggested that the global climate models used to project future climate changes may significantly overestimate the stability of the Atlantic meridional overturning circulation under anthropogenic global warming. It further asserted that “flux adjusting” the models (adding offsets to heat and freshwater fluxes to reproduce the observed density field) may reduce model stability. However, temperature, salinity, and density fields in climate models likely deviate from observations because of biases in model physics as well as inaccuracies in fluxes. In such cases it is unclear whether adjusting the fluxes to produce a more realistic density field will result in a model with more realistic stability properties, as flux correction may be compensating for other inaccuracies in model formulation. We investigate this question using a simplified dynamical box model, in which we can flux correct one version of the model to match density gradients within another version of the model. We show that flux adjustment can realistically compensate for biases in stability associated with some processes, such as uncertainty in the value of the vertical diffusivity, but not other processes, such as inaccurate simulation of the relationship between density structure and overturning or the isopycnal tracer diffusivity coefficient. An ability to compensate for biases when the overturning shuts off does not imply an ability to compensate for biases in when it reestablishes itself, and vice versa.

Chiral optical response of multifold fermions

Multifold fermions are generalizations of two-fold degenerate Weyl fermions with three-, four-, six- or eight-fold degeneracies protected by crystal symmetries, of which only the last type is necessarily non-chiral. Their low energy degrees of freedom can be described as emergent particles not present in the Standard Model of particle physics. We propose a range of experimental probes for multifold fermions in chiral symmetry groups based on the gyrotropic magnetic effect (GME) and the circular photo-galvanic effect (CPGE). We find that, in contrast to Weyl fermions, multifold fermions can have zero Berry curvature yet a finite GME, leading to an enhanced response. The CPGE is quantized and independent of frequency provided that the frequency region at which it is probed defines closed optically-activated momentum surfaces. We confirm the above properties by calculations in symmetry-restricted tight binding models with realistic density functional theory parameters. We identify a range of previously-unidentified ternary compounds able to exhibit chiral multifold fermions of all types (including a range of materials in the families AsBaPt and Gd$_3$Cl$_3$C), and provide specific predictions for the known multifold material RhSi.

Tree diversity improves forest productivity

Forest Ecology Experimental studies in grasslands have shown that the loss of species has negative consequences for ecosystem functioning. Is the same true for forests? Huang et al. report the first results from a large biodiversity experiment in a subtropical forest in China. The study combines many replicates, realistic tree densities, and large plot sizes with a wide range of species richness levels. After 8 years of the experiment, the findings suggest strong positive effects of tree diversity on forest productivity and carbon accumulation. Thus, changing from monocultures to more mixed forests could benefit both restoration of biodiversity and mitigation of climate change. Science , this issue p. [80][1] [1]: /lookup/doi/10.1126/science.aat6405

Impacts of species richness on productivity in a large-scale subtropical forest experiment.

Biodiversity experiments have shown that species loss reduces ecosystem functioning in grassland. To test whether this result can be extrapolated to forests, the main contributors to terrestrial primary productivity, requires large-scale experiments. We manipulated tree species richness by planting more than 150,000 trees in plots with 1 to 16 species. Simulating multiple extinction scenarios, we found that richness strongly increased stand-level productivity. After 8 years, 16-species mixtures had accumulated over twice the amount of carbon found in average monocultures and similar amounts as those of two commercial monocultures. Species richness effects were strongly associated with functional and phylogenetic diversity. A shrub addition treatment reduced tree productivity, but this reduction was smaller at high shrub species richness. Our results encourage multispecies afforestation strategies to restore biodiversity and mitigate climate change.

Investigation of magnetic dipole-dipole interaction using magnetic density on solid oxygen based on first-principles approach

We have developed the computational method to estimate magnetic dipole-dipole interaction energy including magnetic spin density on three-dimensional materials, in the framework of density functional theory. We investigated the stable spin direction in oxygen molecule and α- and δ-phases of solid oxygen. Our method gave the result that the magnetic dipole-dipole interaction of realistic spin density distribution provides the stable magnetic moments perpendicular to the molecular axis, in contrary to the simple expectation from the isolated-spin moment model of the oxygen molecule. The reason has been explained by the spin density distribution of oxygen molecule and magnetic dipole field. Moreover, the result of magnetic anisotropy energy was found to be consistent with the experimental ones.

Broadband numerical simulation of scattering from rough pressure-release and fluid-fluid interfaces

The scattered field from the seafloor is often measured using short, broadband pulses, whereas many models for the mean scattered intensity are in the frequency domain. This As higher resolution seafloor mapping systems, i.e., synthetic aperture sonar, become more common, it is important to understand the effects of high resolution on measurements of both the averaged scattered intensity, and the distribution of the envelope of the scattered field. To address this problem, Monte Carlo simulations of the scattered field due to rough interfaces separating two fluids are perform. The integral equation governing the total pressure, the Helmholtz-Kirchhoff integral equation, is numerically solved using the boundary element method. Simulations are performed both in the limit of pressure release, as well as for more realistic sediment sound speed and density parameters. The von-Karman spectrum is used to generate random rough surfaces. Fourier synthesis is used to to generate time domain signals, which are then analyized in terms of the mean energy and amplitude distribution. The dependence of the scattering cross section and scintillation index on signal bandwidth (resolution) is examined. It is found that there is no observable dependence of the scattering cross section on bandwidth, but that there is significant dependence of the scintillation index.

Effect of elongation on energetic particle-induced geodesic acoustic mode

The effect of the plasma elongation on the energetic-particle-induced geodesic acoustic mode (EGAM) dynamics is studied with numerical simulations performed with the gyrokinetic codes GENE and ORB5 and assessed with analytical models. The former code has been recently extended to support non-Maxwellian distribution functions and for the first time global results are shown here and benchmarked against the code ORB5. Taking advantage of these recent developments in GENE, which allow a joint investigation with ORB5, linear electrostatic simulations with adiabatic electrons have been performed. The impact of the elongation on the EGAM dynamics and excitation mechanism is investigated through the study of the energy exchange terms between the particle and the mode for different plasma geometry. Finally, the results are applied to the study of an ASDEX Upgrade discharge with strongly elongated plasma employing realistic density and temperature profiles.

Global Longitudinal Behavior of IRI Bottomside Profile Parameters From FORMOSAT‐3/COSMIC Ionospheric Occultations

AbstractThe bottomside thickness and shape (B0 and B1) are the critical key elements for depicting a realistic electron density profile in the International Reference Ionosphere (IRI) model. We investigated their longitudinal variability using a large database of FORMOSAT‐3/COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) electron density profiles covering the period 2006–2015 from which these parameters are derived using a simple analytic Chapman least squares fitting. The parameters are then compared with the IRI‐2012 submodels, that is, Bil‐2000, Gul‐1987, and ABT‐2009 during different seasons and solar activity. Besides apparent adherence of B0 to the equatorial region, the opposite occurrence pattern of maximum B0 to that of NmF2 is realized, manifesting semiannual distribution and hemispheric asymmetry. Unlike NmF2, the presence of single anomaly crest in B0 in all seasons except March equinox and their leading local time stamps are among the key observations. In spite of regular representation of IRI submodels during equinox, the performance of Gul‐1987 seems to be inferior during solstice with respect to geomagnetic field and equatorial electrodynamics, thereby depicting an erroneous distribution of B0. The striking wave‐like feature in B0 longitudinal variability is also revealed indicating its possible connection with the nonmigrating diurnal/semidiurnal tides. Moreover, the solar activity effect on B0 appears to be relatively higher over the Asian and Pacific longitudes (60–150°E). Although speculations are made over B1 variability, its complete longitudinal characteristics are indistinguishable in our results, except the subordinate magnitude as compared to IRI estimations. The important findings from this study may reinforce the understanding of bottomside variability and future improvements in IRI.

Iterative multi-scale method for estimation of hysteresis losses and current density in large-scale HTS systems

In recent years, commercial high-temperature superconductor (HTS) materials have gained increasing interest for their use in applications involving large-scale superconductor systems. These systems are typically made from hundreds to thousands of turns of conductors. These applications can range from power engineering devices like power transformers, motors and generators, to commercial and scientific magnets. The available analytical models are restricted to the analysis of individual tapes or relatively simple assemblies, therefore it is not possible to apply these models to the study of large-scale systems and other simulation tools are required. Due to the large number of turns, the simulations of a whole system can become prohibitive in terms of computing time and load. Therefore, an efficient strategy which does not compromise the accuracy of calculations is needed. Recently, a method, based on a multi-scale approach, showed that the computational load can be lowered by simulating, in detail, only several significant tapes from the system. The main limitation of this approach is the inaccuracy of the estimation of the background magnetic field, this means the field affecting the significant tapes produced by the rest of the tapes and by external sources. To address this issue, we consider the following two complementary strategies. The first strategy consists of the iterative implementation of the multi-scale method. The multi-scale method itself solves a dynamic problem, the iterative implementation proposed here is the iterative application of the multi-scale method, and a dynamic solution is obtained at each iteration. The second strategy is a new interpolation method for current distributions. With respect to conventional interpolation methods, a more realistic current density distribution is then obtained, which allows for a better estimation of the background magnetic field, and consequently a better estimation of the hysteresis losses. In contrast with previous works, here we do not only focus on the estimation of the hysteresis losses, but also the estimation of the current density distribution is addressed. This new method is flexible enough to simulate different sections of the system with a better level of detail while providing a faster computational speed than other approaches. In order to validate the proposed method, a case study is analyzed via a reference model, which employs the H-formulation of Maxwell’s equations and includes all the system’s tapes. The comparison, between the reference model and the iterative multi-scale model, shows that the computation time and memory demand are greatly reduced. In addition, a very good agreement with respect to the reference model, both at a local and global scale, is achieved.

Nuclear surface diffuseness revealed in nucleon-nucleus diffraction

Nuclear surface provides useful information on nuclear radius, nuclear structure as well as properties of nuclear matter. We discuss the relationship between the nuclear surface diffuseness and elastic scattering differential cross section at the first diffraction peak of high-energy nucleon-nucleus scattering as an efficient tool in order to extract the nuclear surface information from limited experimental data involving short-lived unstable nuclei. The high-energy reaction is described by a reliable microscopic reaction theory, the Glauber model. Extending the idea of the black sphere model, we find one-to-one correspondence between the nuclear bulk structure information and proton elastic scattering diffraction peak. This implies that we can extract both the nuclear radius and diffuseness simultaneously, using the position of the first diffraction peak and its magnitude of the elastic scattering differential cross section. We confirm the reliability of this approach by using realistic density distributions obtained by a mean-field model.

Equilibrating high-molecular-weight symmetric and miscible polymer blends with hierarchical back-mapping

Understanding properties of polymer alloys with computer simulations frequently requires equilibration of samples comprised of microscopically described long molecules. We present the extension of an efficient hierarchical backmapping strategy, initially developed for homopolymer melts, to equilibrate high-molecular-weight binary blends. These mixtures present significant interest for practical applications and fundamental polymer physics. In our approach, the blend is coarse-grained into models representing polymers as chains of soft blobs. Each blob stands for a subchain with Nb microscopic monomers. A hierarchy of blob-based models with different resolution is obtained by varying Nb. First the model with the largest Nb is used to obtain an equilibrated blend. This configuration is sequentially fine-grained, reinserting at each step the degrees of freedom of the next in the hierarchy blob-based model. Once the blob-based description is sufficiently detailed, the microscopic monomers are reinserted. The hard excluded volume is recovered through a push-off procedure and the sample is re-equilibrated with molecular dynamics (MD), requiring relaxation on the order of the entanglement time. For the initial method development we focus on miscible blends described on microscopic level through a generic bead-spring model, which reproduces hard excluded volume, strong covalent bonds, and realistic liquid density. The blended homopolymers are symmetric with respect to molecular architecture and liquid structure. To parameterize the blob-based models and validate equilibration of backmapped samples, we obtain reference data from independent hybrid simulations combining MD and identity exchange Monte Carlo moves, taking advantage of the symmetry of the blends. The potential of the backmapping strategy is demonstrated by equilibrating blend samples with different degree of miscibility, containing 500 chains with 1000 monomers each. Equilibration is verified by comparing chain conformations and liquid structure in backmapped blends with the reference data. Possible directions for further methodological developments are discussed.

Ridges, Seamounts, Troughs, and Bowls: Topographic Control of the Dianeutral Circulation in the Abyssal Ocean

AbstractIn situ observations obtained over the last several decades have shown that the intensity of turbulent mixing in the abyssal ocean is enhanced toward the seafloor. Consequently, a new paradigm has emerged whereby dianeutral downwelling dominates in the ocean interior and dianeutral upwelling only occurs within thin bottom boundary layers. This study shows that when mixing is bottom intensified the net abyssal dianeutral transports and the stratification can depend on subtle features of the seafloor geometry. Under an assumption of depth-independent net dianeutral upwelling, small changes in the curvature of the seafloor can result in interior stratification that is bottom intensified, uniform, or surface intensified. Further, when the net dianeutral transport is allowed to vary in the vertical, changes in the seafloor slope and bathymetric contour length with height can drive lateral exchange between the boundary layer and interior, with particularly strong lateral outflows predicted at the crests of midocean ridges. Finally, using a realistic neutral density climatology the authors suggest that the increase in the perimeter of abyssal neutral density surfaces with height drives much of the dianeutral upwelling at depths greater than 4 km, while the increase in the slope of the seafloor at shallower depths acts to oppose upwelling. These results add to a growing body of literature highlighting the key control of seafloor geometry on the abyssal overturning circulation.

Cell wall microstructure, pore size distribution and absolute density of hemp shiv.

This paper, for the first time, fully characterizes the intrinsic physical parameters of hemp shiv including cell wall microstructure, pore size distribution and absolute density. Scanning electron microscopy revealed microstructural features similar to hardwoods. Confocal microscopy revealed three major layers in the cell wall: middle lamella, primary cell wall and secondary cell wall. Computed tomography improved the visualization of pore shape and pore connectivity in three dimensions. Mercury intrusion porosimetry (MIP) showed that the average accessible porosity was 76.67 ± 2.03% and pore size classes could be distinguished into micropores (3–10 nm) and macropores (0.1–1 µm and 20–80 µm). The absolute density was evaluated by helium pycnometry, MIP and Archimedes' methods. The results show that these methods can lead to misinterpretation of absolute density. The MIP method showed a realistic absolute density (1.45 g cm−3) consistent with the density of the known constituents, including lignin, cellulose and hemi-cellulose. However, helium pycnometry and Archimedes’ methods gave falsely low values owing to 10% of the volume being inaccessible pores, which require sample pretreatment in order to be filled by liquid or gas. This indicates that the determination of the cell wall density is strongly dependent on sample geometry and preparation.

Propagation of GeV neutrinos through Earth

We have studied the Earth matter effect on the oscillation of upward going GeV neutrinos by taking into account the three active neutrino flavors. For neutrino energy in the range 3 to 12 GeV we observed three distinct resonant peaks for the oscillation process νe↔νμ,τ in three distinct densities. However, according to the most realistic density profile of the Earth, the second peak at neutrino energy 6.18 GeV corresponding to the density 6.6 g/cm3 does not exist. So the resonance at this energy can not be of MSW-type. For the calculation of observed flux of these GeV neutrinos on Earth, we considered two different flux ratios at the source, the standard scenario with the flux ratio 1:2:0 and the muon damped scenario with 0:1:0. It is observed that at the detector while the standard scenario gives the observed flux ratio 1:1:1, the muon damped scenario has a different ratio. For muon damped case with Eν<20 GeV, we always get observed neutrino fluxes as Φνe<Φνμ≃Φντ and for Eν>20 GeV, we get the average Φνe∼0 and Φνμ≃Φντ≃0.45. The upcoming PINGU will be able to shed more light on the nature of the resonance in these GeV neutrinos and hopefully will also be able to discriminate among different processes of neutrino production at the source in GeV energy range.

Effect of density of states peculiarities on Hund's metal behavior

We investigate a possibility of Hund's metal behavior in the Hubbard model with asymmetric density of states having peak(s). Specifically, we consider the degenerate two-band model and compare its results to the five-band model with realistic density of states of iron and nickel, showing that the obtained results are more general, provided that the hybridization between states of different symmetry is sufficiently small. We find that quasiparticle damping and the formation of local magnetic moments due to Hund's exchange interaction are enhanced by both, the density of states asymmetry, which yields stronger correlated electron or hole excitations, and the larger density of states at the Fermi level, increasing the number of virtual electron-hole excitations. For realistic densities of states these two factors are often interrelated because the Fermi level is attracted towards peaks of the density of states. We discuss the implication of the obtained results to various substances and compounds, such as transition metals, iron pnictides, and cuprates.

Observation and Numerical Simulation of Cavity Mode Oscillations Excited by an Interplanetary Shock

AbstractCavity mode oscillations (CMOs) are basic magnetohydrodynamic eigenmodes in the magnetosphere predicted by theory and are expected to occur following the arrival of an interplanetary shock. However, observational studies of shock‐induced CMOs have been sparse. We present a case study of a dayside ultralow‐frequency wave event that exhibited CMO properties. The event occurred immediately following the arrival of an interplanetary shock at 0829 UT on 15 August 2015. The shock was observed in the solar wind by the Time History of Events and Macroscale Interactions during Substorms‐B and ‐C spacecraft, and magnetospheric ultralow‐frequency waves were observed by multiple spacecraft including the Van Allen Probe‐A and Van Allen Probe‐B spacecraft, which were located in the dayside plasmasphere at L ∼1.4 and L ∼ 2.4, respectively. Both Van Allen Probes spacecraft detected compressional poloidal mode oscillations at ∼13 mHz (fundamental) and ∼26 mHz (second harmonic). At both frequencies, the azimuthal component of the electric field (Eϕ) lagged behind the compressional component of the magnetic field (Bμ) by ∼90°. The frequencies and the Eϕ‐Bμ relative phase are in good agreement with the CMOs generated in a dipole magnetohydrodynamic simulation that incorporates a realistic plasma mass density distribution and ionospheric boundary condition. The oscillations were also detected on the ground by the European quasi‐Meridional Magnetometer Array, which was located near the magnetic field footprints of the Van Allen Probes spacecraft.

Monitoring polariton dynamics in the LHCII photosynthetic antenna in a microcavity by two-photon coincidence counting

The relaxation dynamics of light-harvesting complex II in an optical cavity is explored theoretically by multidimensional photon coincidence counting spectroscopy. This technique reveals the dynamics in both single (e) and double (f) excitation bands. We study how the polariton dynamics are affected by coupling to photon modes and molecular vibrations described by a realistic spectral density at 77 K. Without the cavity, the e- and f-band energy transfer pathways are not clearly resolved due to the line broadening caused by fast exciton dephasing. The strong coupling to cavity photons results in well-resolved polariton modes. The hybrid nature of polaritons slows down their energy transfer rates.

Dissipative dark matter halos: The steady state solution

Dissipative dark matter, where dark matter particle properties closely resemble familiar baryonic matter, is considered. Mirror dark matter, which arises from an isomorphic hidden sector, is a specific and theoretically constrained scenario. Other possibilities include models with more generic hidden sectors that contain massless dark photons (unbroken $U(1)$ gauge interactions). Such dark matter not only features dissipative cooling processes, but is also assumed to have nontrivial heating sourced by ordinary supernovae (facilitated by the kinetic mixing interaction). The dynamics of dissipative dark matter halos around rotationally supported galaxies, influenced by heating as well as cooling processes, can be modelled by fluid equations. For a sufficiently isolated galaxy with stable star formation rate, the dissipative dark matter halos are expected to evolve to a steady state configuration which is in hydrostatic equilibrium and where heating and cooling rates locally balance. Here, we take into account the major cooling and heating processes, and numerically solve for the steady state solution under the assumptions of spherical symmetry, negligible dark magnetic fields, and that supernova sourced energy is transported to the halo via dark radiation. For the parameters considered, and assumptions made, we were unable to find a physically realistic solution for the constrained case of mirror dark matter halos. Halo cooling generally exceeds heating at realistic halo mass densities. This problem can be rectified in more generic dissipative dark matter models, and we discuss a specific example in some detail.

Intrinsic electron trapping in amorphous oxide

We demonstrate that electron trapping at intrinsic precursor sites is endemic in non-glass-forming amorphous oxide films. The energy distributions of trapped electron states in ultra-pure prototype amorphous (a)-HfO2 insulator obtained from exhaustive photo-depopulation experiments demonstrate electron states in the energy range of 2–3 eV below the oxide conduction band. These energy distributions are compared to the results of density functional calculations of a-HfO2 models of realistic density. The experimental results can be explained by the presence of intrinsic charge trapping sites formed by under-coordinated Hf cations and elongated Hf–O bonds in a-HfO2. These charge trapping states can capture up to two electrons, forming polarons and bi-polarons. The corresponding trapping sites are different from the dangling-bond type defects responsible for trapping in glass-forming oxides, such as SiO2, in that the traps are formed without bonds being broken. Furthermore, introduction of hydrogen causes formation of somewhat energetically deeper electron traps when a proton is immobilized next to the trapped electron bi-polaron. The proposed novel mechanism of intrinsic charge trapping in a-HfO2 represents a new paradigm for charge trapping in a broad class of non-glass-forming amorphous insulators.