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At least 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Controlling the spatio-temporal dose distribution during STEM imaging by subsampled acquisition: In-situ observations of kinetic processes in liquids

Subsampled image acquisition followed by image inpainting in the scanning transmission electron microscope (STEM) is a novel approach to control dose and increase the image frame rate during experiments, thereby allowing independent control of the spatial and temporal dose envelope during image acquisition. Here, subsampled imaging is shown to permit precise in-situ observations of the fundamental kinetic processes behind nucleation and growth of silver (Ag) nanoparticles from an aqueous solution. At high sampling-levels, nanoparticles can be observed with morphologies that are consistent with strong interface interactions, i.e. rafts and pillars, whereas at low sampling-levels, the particles exhibit regular spherical morphologies. The relative numbers of rafts/pillars and regular nanoparticles, their sizes and their incubation times can be attributed to local changes in the molar concentration of the Ag ions in the aqueous solution; higher sampling-levels significantly increase the reactants in the vicinity of the window, leading to rapid supersaturation and the precipitation on the window surface. These precisely controlled kinetics highlight subsampled imaging as a method by which the driving force for nucleation and growth (i.e the electron beam) can be disentangled from the spatial/temporal resolution of the observation in all in-situ experiments, providing a pathway to identify and quantify the importance of individual kinetic factors behind nucleation and growth in a wide variety of complex materialssystems and architectures.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

Comparison of modular analytical wake models to the Lillgrund wind plant

Efficient and accurate wake models are required for wind plant performance modeling and the suite of engineering processes that support wind plant layout, control, and monitoring. Although many analytical and engineering wake models with low computational costs have been proposed, their ability to represent the power production of large wind plants in a wide range of atmospheric conditions is not completely understood. The following validation study reviews the underlying theory for analytical wake models, outlines quality control procedures for observational data, and compares model results with observational data from the Lillgrund Wind Plant. Lillgrund makes a valuable case study for wake modeling because of its regular arrangement and the relatively close spacing of constituent wind turbines, which lead to regular and significant wake interactions within the wind plant and the development of deep array flow conditions. Formulations for the velocity deficit, wake-added turbulence, and wake superposition methods are considered in a modular sense, yielding many possible configurations to represent wind turbine wakes, of which seven are examined in detail. Velocity deficit models that account for flow conditions in the near wake are better able to reproduce power production for wind turbines in the transitional region of the wind plant, where wind turbines experience as many as five wakes from upstream turbines. Finally, in the deep array, where power production reaches asymptotic values and wake statistics become quasi-periodic, wake superposition schemes become the largest driver in error reduction; using the linear or maximum wake superposition methods can reduce the relative root mean square error by as much as 40% in the deep array.

17 WIND ENERGY↗
DOE OSTI2021

Time evolution of ML-MCTDH wavefunctions. II. Application of the projector splitting integrator

The multi-layer multi-configuration time-dependent Hartree (ML-MCTDH) approach can suffer from numerical instabilities whenever the wavefunction is weakly entangled. These instabilities arise from singularities in the equations of motion (EOMs) and necessitate the use of regularization of the EOMs. The Projector Splitting Integrator (PSI) has previously been presented as an approach for evolving ML-MCTDH wavefunctions that is free of singularities. Here, we will discuss the implementation of the multi-layer PSI with a particular focus on how the steps required relate to those required to implement standard ML-MCTDH. We demonstrate the efficiency and stability of the PSI for large ML-MCTDH wavefunctions containing up to hundreds of thousands of nodes by considering a series of spin-boson models with up to 106 bath modes and find that for these problems, the PSI requires roughly 3–4 orders of magnitude fewer Hamiltonian evaluations and 2–3 orders of magnitude fewer Hamiltonian applications than standard ML-MCTDH and 2–3/1–2 orders of magnitude fewer evaluations/applications than approaches that use improved regularization schemes. Finally, we consider a series of significantly more challenging multi-spin-boson models that require much larger numbers of single-particle functions with wavefunctions containing up to ~1.3×10 9 parameters to obtain accurate dynamics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Structures of glasses created by multiple kinetic arrests

X-ray scattering has been used to characterize glassy itraconazole (ITZ) prepared by cooling at different rates. Faster cooling produces ITZ glasses with lower (or zero) smectic order with more sinusoidal density modulation, larger molecular spacing, and shorter lateral correlation between the rod-like molecules. We find that each glass is characterized by not one, but two fictive temperatures T f (the temperature at which a chosen order parameter is frozen in the equilibrium liquid). The higher T f is associated with the regularity of smectic layers and lateral packing, while the lower T f with the molecular spacings between and within smectic layers. This indicates that different structural features are frozen on different timescales. The two timescales for ITZ correspond to its two relaxation modes observed by dielectric spectroscopy: the slower δ mode (end-over-end rotation) is associated with the freezing of the regularity of molecular packing and the faster α mode (rotation about the long axis) with the freezing of the spacing between molecules. Finally, our finding suggests a way to selectively control the structural features of glasses.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Repartitioned Brillouin-Wigner perturbation theory with a size-consistent second-order correlation energy

Second-order Møller-Plesset perturbation theory (MP2) often breaks down catastrophically in small-gap systems, leaving much to be desired in its performance for myriad chemical applications such as noncovalent interactions, thermochemistry, and dative bonding in transition metal complexes. This divergence problem has reignited interest in Brillouin-Wigner perturbation theory (BWPT), which is regular at all orders but lacks size consistency and extensivity, severely limiting its application to chemistry. In this work, we propose an alternative partitioning of the Hamiltonian that leads to a regular BWPT perturbation series that, through the second order, is size-extensive, size-consistent (provided its Hartree–Fock reference is also), and orbital invariant. Here, our second-order size-consistent Brillouin-Wigner (BW-s2) approach can describe the exact dissociation limit of H 2 in a minimal basis set, regardless of the spin polarization of the reference orbitals. More broadly, we find that BW-s2 offers improvements relative to MP2 for covalent bond breaking, noncovalent interaction energies, and metal/organic reaction energies, although rivaling coupled-cluster with single and double substitutions for thermochemical properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Multimode turbulent flow measurements using magnetic resonance imaging- and laser-based techniques and computational fluid dynamics simulations

We studied the flow field characteristics of a turbulent flow over a regularized cube array with a perpendicular injection flow through the floor between the second and third cubical elements, representing the complex flow interactions of a 3D jet and the wake flows behind cubical obstacles. Four different experimental measurements were performed: two magnetic resonance imaging-based measurements for three-dimensional three-component velocity (MRV) and concentration (MRC) and two laser-based techniques, particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF), for two-dimensional two-component velocity and concentration measurement, respectively. The mainstream Reynolds number is Re = 15 000⁠, based on the primary inlet velocity U m and channel height D H ⁠, whereas the injector Reynolds number is Re j = 3400⁠, based on the injector velocity U j and the injector's exit width D j ⁠. Numerical simulations were performed for the studied flow configuration of turbulent flow over a regularized cube array using Reynolds-averaged Navier–Stokes (RANS) and large-eddy simulation (LES) approaches. Results obtained from experimental measurements—including MRV, MRC, PIV, and PLIF—as well as RANS and LES simulations are discussed and compared along several horizontal and vertical planes of the studied configuration. In addition, 3D turbulent flow structures, such as leading-edge vortex, horseshoe vortex, and jet shear-layer vortex, and the isosurfaces of scalar concentration successfully revealed by MRV and MRC techniques were found to be in very good agreement with those 3D features extracted from RANS and LES simulations. In conclusion, the high-resolution experimental and numerical database obtained from this study could be useful for validation and verification of numerical codes.

Computational fluid dynamics↗
DOE OSTI2016

Evaluation of the capacity of communication lines with nonlinear finite memory

We propose a method for calculating the capacity of a finite-memory channel up to the square of the nonlinear memory parameter. A comparison with a regular Gaussian model is performed, in which the Kerr nonlinearity is considered as an additional Gaussian noise. The estimate by the regular Gaussian model is shown to yield a greater capacity as compared with the estimate obtained by the nonlinear memory model. The optimal signal powers, providing the maximum mutual information, are found to be equal. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

CFD Evaluation of Pressure Change Along Coolant Passages in Sodium-Cooled Fast Reactor with Nek5000

To support the design efforts of advanced sodium-cooled fast reactors (SFRs), a series of computational fluid dynamics (CFD) simulations are performed to investigate the pressure change along various flow passages in the proposed SFR system. The simulations are carried out with the state-of-the-art spectral element flow solver, Nek5000. Two specific case studies are presented in this paper: the flow exiting the axial neutron reflector channels and the flow entering the fuel pin bundle. Due to the high Reynolds numbers expected, a Reynolds-averaged Navier-Stokes (RANS) approach is necessary to model the turbulence. A newly developed regularized k-omega RANS model is adopted in the related CFD calculations. The first case study explores the effect of Reynolds number on the pressure change when flow exits the reflector channels. The pressure change in this case has two major contributors: the change due to wall friction and the Bernoulli effect. It is noted that the nondimensional pressure loss follows a log-linear trend up to Re = 10 5 , and then the trend is flattened. In the second case study, the advanced NekNek coupling capability is tested where an integral domain can be divided into multiple subdomains with coupling interfaces, which would greatly ease the meshing process of complex engineering geometries and potentially save computational resources. The preliminary results obtained so far confirm the consistency between the NekNek results and those produced by regular Nek5000 simulation. The presented work demonstrates the readiness and flexibility of the related CFD techniques, which is part of the broader effort to leverage cutting-edge CFD to inform the advanced nuclear reactor designs.

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2022

Numerical characterization of support recovery in sparse regression with correlated design

Sparse regression is employed in diverse scientific settings as a feature selection method. A pervasive aspect of scientific data is the presence of correlations between predictive features. These correlations hamper both feature selection and estimation and jeopardize conclusions drawn from estimated models. On the other hand, theoretical results on sparsity-inducing regularized regression have largely addressed conditions for selection consistency via asymptotics, and disregard the problem of model selection, whereby regularization parameters are chosen. In this numerical study, we address these issues through exhaustive characterization of the performance of several regression estimators, coupled with a range of model selection strategies. These estimators and selection criteria were examined across correlated regression problems with varying degrees of signal to noise, distributions of non-zero model coefficients, and model sparsity. Our results reveal a fundamental tradeoff between false positive and false negative control in all regression estimators and model selection criteria examined. Additionally, we numerically explore a transition point modulated by the signal-to-noise ratio and spectral properties of the design covariance matrix at which the selection accuracy of all considered algorithms degrades. Overall, we find that SCAD coupled with BIC or empirical Bayes model selection performs the best feature selection across the regression problems considered.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2013

EXTENDED NEUTRAL HYDROGEN IN THE ALIGNED SHELL GALAXIES Arp 230 AND MCG -5-7-1: FORMATION OF DISKS IN MERGING GALAXIES?

As part of an ongoing study of the neutral hydrogen (H I) morphology and kinematics of 'shell' elliptical galaxies, we present Very Large Array observations of two shell galaxies with aligned shells, Arp 230 and MCG -5-7-1. Our data provide the first H I images of Arp 230 and deeper images of MCG -5-7-1 than previously reported. Optical images of Arp 230 reveal a bright, aligned, interleaved shell system, making it an ideal candidate for 'phase-wrapped' shell formation following a radial encounter with a smaller companion. The fainter, non-interleaved shells of MCG -5-7-1 do not clearly favor a particular formation scenario. The H I we detect in both galaxies extends to nearly the same projected distance as the optical shells. In Arp 230 this gas appears to be anti-correlated with the aligned shells, consistent with our expectations for phase-wrapped shells produced in a radial encounter. In MCG -5-7-1, we observe gas associated with the shells making a 'spatial wrapping' or looping scenario more plausible. Although the extended gas component in both galaxies is unevenly distributed, the gas kinematics are surprisingly regular, looking almost like complete disks in rotation. We use the H I kinematics and optical data to determine mass-to-light ratios M/L{sub B} of 2.4{sup +3.0}{sub -0.5} (at 13.5 kpc, 4.5 R{sub e} ) for Arp 230 and M/L{sub B} of 30 {+-} 7 (at 40 kpc, 7 R{sub e} ) in MCG -5-7-1. In both systems we find that this ratio changes as a function of radius, indicating the presence of a dark halo. By comparing orbital and precession timescales, we conclude that the potentials are slightly flattened. We infer a 5%-10% flattening for Arp 230 and less flattening in the case of MCG -5-7-1. Finally, we present images of the H I associated with the inner disk or (polar) ring of each galaxy and discuss possible explanations for their different present-day star formation rates. We detect total H I masses of 1.1 Multiplication-Sign 10{sup 9} M{sub Sun} in Arp 230 and 3.7 Multiplication-Sign 10{sup 9} M{sub Sun} in MCG -5-7-1. Both systems have H I masses, scale sizes, and regular kinematics similar to those of non-aligned shell galaxies we have previously studied (Cen A and NGC 2865). Furthermore, we (re-)emphasize in this paper that shell galaxies such as MCG -5-7-1, along with previously studied galaxies NGC 5128 (Cen A) and NGC 2865, are unique in that they provide evidence of recent accretion with gas and collisionless stars showing clear association, though the displacement suggests the presence of significant gas-dynamical interaction.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

CHANG-ES. III. UGC 10288—An edge-on galaxy with a background double-lobed radio source

This third paper in the Continuum Halos in Nearby Galaxies—an EVLA Survey (CHANG-ES) series shows the first results from our regular data taken with the Karl G. Jansky Very Large Array. The edge-on galaxy, UGC 10288, has been observed in the B, C, and D configurations at L band (1.5 GHz) and in the C and D configurations at C band (6 GHz) in all polarization products. We show the first spatially resolved images of this galaxy in these bands, the first polarization images, and the first composed image at an intermediate frequency (4.1 GHz) which has been formed from a combination of all data sets. A surprising new result is the presence of a strong, polarized, double-lobed extragalactic radio source (CHANG-ES A) almost immediately behind the galaxy and perpendicular to its disk. The core of CHANG-ES A has an optical counterpart (SDSS J161423.28–001211.8) at a photometric redshift of z {sub phot} = 0.39; the southern radio lobe is behind the disk of UGC 10288 and the northern lobe is behind the halo region. This background ''probe'' has allowed us to do a preliminary Faraday rotation analysis of the foreground galaxy, putting limits on the regular magnetic field and electron density in the halo of UGC 10288 in regions in which there is no direct detection of a radio continuum halo. We have revised the flux densities of the two sources individually as well as the star formation rate (SFR) for UGC 10288. The SFR is low (0.4-0.5 M {sub ☉} yr{sup –1}) and the galaxy has a high thermal fraction (44% at 6 GHz), as estimated using both the thermal and non-thermal SFR calibrations of Murphy et al. UGC 10288 would have fallen well below the CHANG-ES flux density cutoff, had it been considered without the brighter contribution of the background source. UGC 10288 shows discrete high-latitude radio continuum features, but it does not have a global radio continuum halo (exponential scale heights are typically ≈1 kpc averaged over regions with and without extensions). One prominent feature appears to form a large arc to the north of the galaxy on its east side, extending to 3.5 kpc above the plane. The total minimum magnetic field strength at a sample position in the arc is ∼10 μG. Thus, this galaxy still appears to be able to form substantial high latitude, localized features in spite of its relatively low SFR.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

THE RADIO CONTINUUM STRUCTURE OF CENTAURUS A AT 1.4 GHz

A 45 deg{sup 2} radio continuum imaging campaign of the nearest radio galaxy, Centaurus A, is reported. Using the Australia Telescope Compact Array and the Parkes 64 m radio telescope at 1.4 GHz, the spatial resolution of the resultant image is {approx}600 pc ({approx}50''), resolving the {approx}>500 kpc giant radio lobes with approximately five times better physical resolution compared to any previous image, and making this the most detailed radio continuum image of any radio galaxy to date. In this paper, we present these new data and discuss briefly some of the most interesting morphological features that we have discovered in the images. The two giant outer lobes are highly structured and considerably distinct. The southern part of the giant northern lobe naturally extends out from the northern middle lobe with uniformly north-streaming emission. The well known northern loop is resolved into a series of semi-regular shells with a spacing of approximately 25 kpc. The northern part of the giant northern lobe also contains identifiable filaments and partial ring structures. As seen in previous single-dish images at lower angular resolution, the giant southern lobe is not physically connected to the core at radio wavelengths. Almost the entirety of the giant southern lobe is resolved into a largely chaotic and mottled structure which appears considerably different (morphologically) to the diffuse regularity of the northern lobe. We report the discovery of a vertex and a vortex near the western boundary of the southern lobe, two striking, high surface brightness features that are named based on their morphology and not their dynamics (which are presently unknown). The vortex and vertex are modeled as reaccelerated lobe emission due to shocks from the active galactic nucleus itself or from the passage of a dwarf elliptical galaxy through the lobe. Preliminary polarimetric and spectral index studies support a plasma reacceleration model and could explain the origin of the Faraday rotation structure detected in the southern lobe. In addition, there are a series of low surface brightness wisps detected around the edges of both the giant lobes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ORDER AND CHAOS IN A THREE-DIMENSIONAL BINARY SYSTEM OF INTERACTING GALAXIES

We present a galactic gravitational model of three degrees of freedom in order to investigate and reveal the behavior of orbits in a binary quasar system. The two quasars are hosted in a pair of interacting disk galaxies. We study in detail the regular or chaotic character of motion in two different cases: the time-independent model in both two-dimensional (2D) and 3D dynamical systems, and the evolving 3D model. Our numerical calculations indicate that a large fraction of orbits in the 2D system are chaotic in the time-independent case. A careful analysis suggests that several Lindblad resonances are also responsible for the chaotic motion of stars in both host galaxies. In the time-dependent system, we follow the evolution of 3D orbits in our dynamical model, as the two interacting host galaxies develop dense and massive quasars in their cores by mass transportation from the disks to their nuclei. In this interesting case, there are orbits that change their orbital character from regular to chaotic and vice versa; there are also orbits that maintain their characters during the galactic evolution. These results strongly indicate that the ordered or chaotic nature of 3D orbits depends not only on the galactic interaction but also on the presence of quasars in the galactic cores of the host galaxies. The outcomes derived from our dynamical model are compared with observational data. Some theoretical arguments to support the numerically derived outcomes are presented, both in 2D and 3D systems, and a comparison with similar earlier work is also made.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

THE PERIOD-LUMINOSITY RELATION OF RED SUPERGIANT STARS IN THE SMALL MAGELLANIC CLOUD

The characteristics of light variation of red supergiant (RSG) stars in the Small Magellanic Cloud (SMC) are analyzed based on the nearly 8-10 year data collected by the ASAS and MACHO projects. The 126 identified RSGs are classified into five categories accordingly: 20 with poor photometry, 55 with no reliable period, 6 with semi-regular variation, 15 with a long secondary period (LSP) and distinguishable short period, and 30 with only an LSP. For the semi-regular variables and the LSP variables with distinguishable short period, the K{sub S} -band period-luminosity (P-L) relation is analyzed and compared with that of the Galaxy, the Large Magellanic Cloud, and M33. It is found that the RSGs in these galaxies obey a similar P-L relation except for those in the Galaxy. In addition, the P-L relations in the infrared bands, namely, the 2MASS JHK{sub S} , Spitzer/IRAC, and Spitzer/MIPS 24 {mu}m bands, are derived with high reliability. The best P-L relation occurs in the Spitzer/IRAC [3.6] and [4.5] bands. Based on the comparison with the theoretical calculation of the P-L relation, the mode of pulsation of RSGs in the SMC is suggested to be the first-overtone radial mode.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

RELATIONSHIP BETWEEN LOW AND HIGH FREQUENCIES IN {delta} SCUTI STARS: PHOTOMETRIC KEPLER AND SPECTROSCOPIC ANALYSES OF THE RAPID ROTATOR KIC 8054146

Two years of Kepler data of KIC 8054146 ({delta} Sct/{gamma} Dor hybrid) revealed 349 statistically significant frequencies between 0.54 and 191.36 cycles day{sup -1} (6.3 {mu}Hz to 2.21 mHz). The 117 low frequencies cluster in specific frequency bands, but do not show the equidistant period spacings predicted for gravity modes of successive radial order, n, and reported for at least one other hybrid pulsator. The four dominant low frequencies in the 2.8-3.0 cycles day{sup -1} (32-35 {mu}Hz) range show strong amplitude variability with timescales of months and years. These four low frequencies also determine the spacing of the higher frequencies in and beyond the {delta} Sct pressure-mode frequency domain. In fact, most of the higher frequencies belong to one of three families with spacings linked to a specific dominant low frequency. In the Fourier spectrum, these family regularities show up as triplets, high-frequency sequences with absolutely equidistant frequency spacings, side lobes (amplitude modulations), and other regularities in frequency spacings. Furthermore, within two families the amplitude variations between the low and high frequencies are related. We conclude that the low frequencies (gravity modes, rotation) and observed high frequencies (mostly pressure modes) are physically connected. This unusual behavior may be related to the very rapid rotation of the star: from a combination of high- and low-resolution spectroscopy we determined that KIC 8054146 is a very fast rotator ({upsilon} sin i = 300 {+-} 20 km s{sup -1}) with an effective temperature of 7600 {+-} 200 K and a surface gravity log g of 3.9 {+-} 0.3. Several astrophysical ideas explaining the origin of the relationship between the low and high frequencies are explored.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

MASS ESTIMATES OF RAPIDLY MOVING PROMINENCE MATERIAL FROM HIGH-CADENCE EUV IMAGES

We present a new method for determining the column density of erupting filament material using state-of-the-art multi-wavelength imaging data. Much of the prior work on filament/prominence structure can be divided between studies that use a polychromatic approach with targeted campaign observations and those that use synoptic observations, frequently in only one or two wavelengths. The superior time resolution, sensitivity, and near-synchronicity of data from the Solar Dynamics Observatory's Advanced Imaging Assembly allow us to combine these two techniques using photoionization continuum opacity to determine the spatial distribution of hydrogen in filament material. We apply the combined techniques to SDO/AIA observations of a filament that erupted during the spectacular coronal mass ejection on 2011 June 7. The resulting 'polychromatic opacity imaging' method offers a powerful way to track partially ionized gas as it erupts through the solar atmosphere on a regular basis, without the need for coordinated observations, thereby readily offering regular, realistic mass-distribution estimates for models of these erupting structures.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

JOINT ANALYSIS OF CLUSTER OBSERVATIONS. II. CHANDRA/XMM-NEWTON X-RAY AND WEAK LENSING SCALING RELATIONS FOR A SAMPLE OF 50 RICH CLUSTERS OF GALAXIES

We present a study of multiwavelength X-ray and weak lensing scaling relations for a sample of 50 clusters of galaxies. Our analysis combines Chandra and XMM-Newton data using an energy-dependent cross-calibration. After considering a number of scaling relations, we find that gas mass is the most robust estimator of weak lensing mass, yielding 15% {+-} 6% intrinsic scatter at r{sub 500}{sup WL} (the pseudo-pressure Y{sub X} yields a consistent scatter of 22% {+-} 5%). The scatter does not change when measured within a fixed physical radius of 1 Mpc. Clusters with small brightest cluster galaxy (BCG) to X-ray peak offsets constitute a very regular population whose members have the same gas mass fractions and whose even smaller (<10%) deviations from regularity can be ascribed to line of sight geometrical effects alone. Cool-core clusters, while a somewhat different population, also show the same (<10%) scatter in the gas mass-lensing mass relation. There is a good correlation and a hint of bimodality in the plane defined by BCG offset and central entropy (or central cooling time). The pseudo-pressure Y{sub X} does not discriminate between the more relaxed and less relaxed populations, making it perhaps the more even-handed mass proxy for surveys. Overall, hydrostatic masses underestimate weak lensing masses by 10% on the average at r{sub 500}{sup WL}; but cool-core clusters are consistent with no bias, while non-cool-core clusters have a large and constant 15%-20% bias between r{sub 2500}{sup WL} and r{sub 500}{sup WL}, in agreement with N-body simulations incorporating unthermalized gas. For non-cool-core clusters, the bias correlates well with BCG ellipticity. We also examine centroid shift variance and power ratios to quantify substructure; these quantities do not correlate with residuals in the scaling relations. Individual clusters have for the most part forgotten the source of their departures from self-similarity.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

DETERMINATION OF STOCHASTIC ACCELERATION MODEL CHARACTERISTICS IN SOLAR FLARES

Following our recent paper, we have developed an inversion method to determine the basic characteristics of the particle acceleration mechanism directly and non-parametrically from observations under the leaky box framework. Earlier, we demonstrated this method for obtaining the energy dependences of the escape time and pitch angle scattering time. Here, by converting the Fokker-Planck equation to its integral form, we derive the energy dependences of the energy diffusion coefficient and direct acceleration rate for stochastic acceleration in terms of the accelerated and escaping particle spectra. Combining the regularized inversion method of Piana et al. and our procedure, we relate the acceleration characteristics in solar flares directly to the count visibility data from RHESSI. We determine the timescales for electron escape, pitch angle scattering, energy diffusion, and direct acceleration at the loop top acceleration region for two intense solar flares based on the regularized electron flux spectral images. The X3.9 class event shows dramatically different energy dependences for the acceleration and scattering timescales, while the M2.1 class event shows a milder difference. The discrepancy between the M2.1 class event and the stochastic acceleration model could be alleviated by a turbulence spectrum that is much steeper than the Kolmogorov-type spectrum. A likely explanation of the X3.9 class event could be that the escape of electrons from the acceleration region is not governed by a random walk process, but instead is affected by magnetic mirroring, in which the scattering time is proportional to the escape time and has an energy dependence similar to the energy diffusion time.

79 ASTRONOMY AND ASTROPHYSICS↗