Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “sub scale”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,711 records · Page 95
All sourcesNASA NTRSDOE OSTI
NASA NTRS2002

X-Ray Properties of Lyman Break Galaxies in the Hubble Deep Field North Region

We describe the X-ray properties of a large sample of z approximately 3 Lyman Break Galaxies (LBGs) in the region of the Hubble Deep Field North, derived from the 1 Ms public Chandra observation. Of our sample of 148 LBGs, four are detected individually. This immediately gives a measure of the bright AGN (active galactic nuclei) fraction in these galaxies of approximately 3 per cent, which is in agreement with that derived from the UV (ultraviolet) spectra. The X-ray color of the detected sources indicates that they are probably moderately obscured. Stacking of the remainder shows a significant detection (6 sigma) with an average luminosity of 3.5 x 10(exp 41) erg/s per galaxy in the rest frame 2-10 keV band. We have also studied a comparison sample of 95 z approximately 1 "Balmer Break" galaxies. Eight of these are detected directly, with at least two clear AGN based on their high X-ray luminosity and very hard X-ray spectra respectively. The remainder are of relatively low luminosity (< 10(exp 42) erg/s, and the X-rays could arise from either AGN or rapid star-formation. The X-ray colors and evidence from other wavebands favor the latter interpretation. Excluding the clear AGN, we deduce a mean X-ray luminosity of 6.6 x 10(exp 40) erg/s, a factor approximately 5 lower than the LBGs. The average ratio of the UV and X-ray luminosities of these star forming galaxies L(sub UV)/L (sub X), however, is approximately the same at z = 1 as it is at z = 3. This scaling implies that the X-ray emission follows the current star formation rate, as measured by the UV luminosity. We use our results to constrain the star formation rate at z approximately 3 from an X-ray perspective. Assuming the locally established correlation between X-ray and far-IR (infrared) luminosity, the average inferred star formation rate in each Lyman break galaxy is found to be approximately 60 solar mass/yr, in excellent agreement with the extinction-corrected UV estimates. This provides an external check on the UV estimates of the star formation rates, and on the use of X-ray luminosities to infer these rates in rapidly starforming galaxies at high redshift.

Nandra, K.↗
NASA NTRS2006

The Evolution of the Cluster X-ray Scaling Relations in the Wide Angle ROSAT Pointed Survey Sample at 0.6 < z < 1.0

The X-ray properties of a sample of 11 high-redshift (0.6 < z < 1 .O) clusters observed with Chardm and/or XMM-Newton are used to investigate the evolution of the cluster scaling relations. The observed evolution in the normalization of the L-T, M-T, M(sub 2)-T and M-L relations is consistent with simple self-similar predictions, in which the properties of clusters reflect the properties of the Universe at their redshift of observation. Under the assumption that the model of self-similar evolution is correct and that the local systems formed via a single spherical collapse, the high-redshift L-T relation is consistent with the high-z clusters having virialized at a significantly higher redshift than the local systems. The data are also consistent with the more realistic scenario of clusters forming via the continuous accretion of material. The slope of the L-T relation at high redshift (B = 3.32 +/- 0.37) is consistent with the local relation, and significantly steeper than the self-similar prediction of B = 2. This suggests that the same non-gravitational processes are responsible for steepening the local and high-z relations, possibly occurring universally at z is approximately greater than 1 or in the early stages of the cluster formation, prior to their observation. The properties of the intracluster medium at high redshift are found to be similar to those in the local Universe. The mean surface-brightness profile slope for the sample is Beta = 0.66 +/- 0.05, the mean gas mass fractions within R(sub 2500(z)) and R(200(z)) are 0.069 +/- 0.012 and 0.11 +/- 0.02, respectively, and the mean metallicity of the sample is 0.28 +/- 0.11 Z(sub solar).

Maughan, B. J.↗
NASA NTRS2009

The Lunar Regolith

A thick layer of regolith, fragmental and unconsolidated rock material, covers the entire lunar surface. This layer is the result of the continuous impact of meteoroids large and small and the steady bombardment of charged particles from the sun and stars. The regolith is generally about 4-5 m thick in mare regions and 10-15 m in highland areas (McKay et al., 1991) and contains all sizes of material from large boulders to sub-micron dust particles. Below the regolith is a region of large blocks of material, large-scale ejecta and brecciated bedrock, often referred to as the "megaregolith". Lunar soil is a term often used interchangeably with regolith, however, soil is defined as the subcentimeter fraction of the regolith (in practice though, soil generally refers to the submillimeter fraction of the regolith). Lunar dust has been defined in many ways by different researchers, but generally refers to only the very finest fractions of the soil, less than approx.10 or 20 microns. Lunar soil can be a misleading term, as lunar "soil" bears little in common with terrestrial soils. Lunar soil contains no organic matter and is not formed through biologic or chemical means as terrestrial soils are, but strictly through mechanical comminution from meteoroids and interaction with the solar wind and other energetic particles. Lunar soils are also not exposed to the wind and water that shapes the Earth. As a consequence, in contrast to terrestrial soils, lunar soils are not sorted in any way, by size, shape, or chemistry. Finally, without wind and water to wear down the edges, lunar soil grains tend to be sharp with fresh fractured surfaces.

Noble, Sarah↗
NASA NTRS2013

Application of Statistically Derived CPAS Parachute Parameters

The Capsule Parachute Assembly System (CPAS) Analysis Team is responsible for determining parachute inflation parameters and dispersions that are ultimately used in verifying system requirements. A model memo is internally released semi-annually documenting parachute inflation and other key parameters reconstructed from flight test data. Dispersion probability distributions published in previous versions of the model memo were uniform because insufficient data were available for determination of statistical based distributions. Uniform distributions do not accurately represent the expected distributions since extreme parameter values are just as likely to occur as the nominal value. CPAS has taken incremental steps to move away from uniform distributions. Model Memo version 9 (MMv9) made the first use of non-uniform dispersions, but only for the reefing cutter timing, for which a large number of sample was available. In order to maximize the utility of the available flight test data, clusters of parachutes were reconstructed individually starting with Model Memo version 10. This allowed for statistical assessment for steady-state drag area (CDS) and parachute inflation parameters such as the canopy fill distance (n), profile shape exponent (expopen), over-inflation factor (C(sub k)), and ramp-down time (t(sub k)) distributions. Built-in MATLAB distributions were applied to the histograms, and parameters such as scale (sigma) and location (mu) were output. Engineering judgment was used to determine the "best fit" distribution based on the test data. Results include normal, log normal, and uniform (where available data remains insufficient) fits of nominal and failure (loss of parachute and skipped stage) cases for all CPAS parachutes. This paper discusses the uniform methodology that was previously used, the process and result of the statistical assessment, how the dispersions were incorporated into Monte Carlo analyses, and the application of the distributions in trajectory benchmark testing assessments with parachute inflation parameters, drag area, and reefing cutter timing used by CPAS.

Romero, Leah M.↗
NASA NTRSDate not supplied

Lunar Communications and Navigation: Lunar Communications, Position, Navigation, and Time (CPNT) Architecture

The NASA Moon-to-Mars (M2M) Architecture will enable the return of humans to the Moon, establish a long-term presence there, and open more of the lunar surface to exploration than ever before. This growth of lunar activity requires robust and resilient communications, position, navigation, and time capabilities for crew safety, the command and control of spacecraft, the return of science data, and the precise telerobotic maneuvering of assets both in space and on the lunar surface. Within the M2M Architecture, the sub-architecture for Communications, Position, Navigation, and Time (CPNT) details the specific CPNT systems and infrastructure required to meet the M2M objectives. Two objectives most fundamental to the CPNT sub-architecture are: (1) develop a lunar surface, orbital, and Moon-to-Earth communications architecture that scales to support long term science, exploration, and industrial needs and (2) develop a lunar position, navigation, and timing (PNT) architecture that also scales to support long term needs. The subject presentation provides an overview of the M2M Architecture, planned mission activities, and infrastructure development underway by NASA. The presentation provides overview of the surface wireless network and the challenges associated with operating on the lunar surface.

architecture↗
DOE OSTI2020

A Novel Low-Cost Method of Manufacturing Nb3Sn Superconductors with Multiple-Tin-Tube Sources (CRADA FRA-2008-0001 Final Report)

In order to successfully sustain a fusion reaction, peak magnetic fields on the order of 12-13 Tesla will be required. Magnetic fields of this magnitude can only be accomplished by advanced superconductors such as Nb3Sn. However, the economic success of a fusion machine will depend on further improvements in the cost-performance of the Nb3Sn conductor. This project will develop a novel, low-cost, multiple-tin-tube process as a new manufacturing approach for large-scale Nb3Sn-conductor production. The process will be suitable for the efficient production of larger strands (0.83 mm), consistent with ITER (International Thermonuclear Experimental Reactor) specifications. In Phase I, Cu/Nb composites were manufactured and shaped into sub-elements. These sub-elements were tin-coated by electroplating, assembled into a precise-fit restack billet tube, and drawn to 0.83 mm diameter. In Phase II, the process will be scaled-up to full production levels. The improved Nb3Sn conductor should have an immediate benefit for high-field magnet applications. A prototype fusion machine, based on a cost effective Nb3Sn conductor, would have enormous economic and social benefits. In addition, the conductor should be applicable to nuclear magnetic resonance (NMR), which has requirements for use in chemical research, biochemistry, pharmaceutical chemistry, polymer science, petroleum research, agricultural chemistry, and medicine. Leszek Motowidlo, Principal Investigator, will be responsible for the overall coordination of the effort. He and others of SupraMagnetics staff will design and fabricate protoype PIT Nb3Sn conductors for evaluation at Fermilab. Emanuela Barzi will be responsible for the Fermilab subcontract and will coordinate and supervise critical current testing Nb3Sn strands and cable fabrication and evaluation.

43 PARTICLE ACCELERATORS↗
DOE OSTI2022

Impacts of Sub‐Grid Topographic Representations on Surface Energy Balance and Boundary Conditions in the E3SM Land Model: A Case Study in Sierra Nevada

Abstract Sub‐grid topographic heterogeneity has large impacts on surface energy balance and land‐atmosphere interactions. However, the impacts of representing sub‐grid topographic effects in land surface models (LSMs) on surface energy balance and boundary conditions remain unclear. This study analyzed and evaluated the impacts of sub‐grid topographic representations on surface energy balance, turbulent heat flux, and scalar (co‐)variances in the Energy Exascale Earth System Model (E3SM) land model (ELM). Three sub‐grid topographic representations in ELM were compared: (a) the default sub‐grid structure (D), (b) the recently developed sub‐grid topographic structure (T), and (c) high spatial resolution (1KM). Additionally, two different solar radiation schemes in ELM were compared: (a) the default plane‐parallel radiative transfer scheme (PP) and (b) the parameterization scheme (TOP) that accounts for sub‐grid topographic effects on solar radiation. A series of offline simulations with the three grid discretization structures (D, T, and 1KM) and two schemes of solar radiation (TOP and PP) were carried out using the Sierra Nevada, California. 1KM simulations with TOP well capture the spatial heterogeneity of surface fluxes compared to Moderate Resolution Imaging Spectroradiometer remote sensing data. There are significant differences between TOP and PP in the 1‐km simulated surface energy balance, but the differences in mean values and standard deviations become small when aggregated to the grid scale (i.e., 0.5°). The T configuration better mimics the 1KM simulations with TOP than the D configuration and better captures the sub‐grid topographic effects on surface energy balance and boundary conditions. These results underline the importance of representing sub‐grid topographic heterogeneities in LSMs and motivate future research to understand the sub‐grid topographic effects on land‐atmosphere interactions over mountainous areas.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2020

Dynamics and drivers of land use and land cover changes in Bangladesh

Abstract Bangladesh has undergone dramatic land use and land cover changes (LULCC) in recent years, but no quantitative analysis of LULCC drivers at the national scale exists so far. Here, we quantified the drivers of major LULCC in combination with biophysical and socioeconomic observations at the sub-district level. We used Landsat satellite data to interpret LULCC from 2000 to 2010 and employed a Global Surface Water Dataset to account for the influences of water seasonality. The results suggest that major LULCC in Bangladesh occur between agricultural land and waterbodies and between forest and shrubland. Exclusion of seasonal waterbodies can improve the accuracy of our LULCC results and driver analysis. Although the gross gain and loss of agricultural land are large on the local scale, the net change (gross gain minus gross loss) at a country scale is almost negligible. Climate dynamics and extreme events and changes in urban and rural households were driving the changes from forest to shrubland in the southeast region. The conversion from agricultural land to standing waterbodies in the southwest region was mainly driven by urban household dynamics, population growth, distance to cities and major roads, and precipitation dynamics. This study, which is the first effort accounting for water seasonality and quantifying biophysical and socioeconomic drivers of LULCC at the national scale, provides a perspective on overall LULCC and underlying drivers over a decadal time scale and national spatial scale and can serve as a scientific basis for developing land policies in Bangladesh.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS2015

An Upper Bound on Orbital Debris Collision Probability When Only One Object has Position Uncertainty Information

Upper bounds on high speed satellite collision probability, P (sub c), have been investigated. Previous methods assume an individual position error covariance matrix is available for each object. The two matrices being combined into a single, relative position error covariance matrix. Components of the combined error covariance are then varied to obtain a maximum P (sub c). If error covariance information for only one of the two objects was available, either some default shape has been used or nothing could be done. An alternative is presented that uses the known covariance information along with a critical value of the missing covariance to obtain an approximate but useful P (sub c) upper bound. There are various avenues along which an upper bound on the high speed satellite collision probability has been pursued. Typically, for the collision plane representation of the high speed collision probability problem, the predicted miss position in the collision plane is assumed fixed. Then the shape (aspect ratio of ellipse), the size (scaling of standard deviations) or the orientation (rotation of ellipse principal axes) of the combined position error ellipse is varied to obtain a maximum P (sub c). Regardless as to the exact details of the approach, previously presented methods all assume that an individual position error covariance matrix is available for each object and the two are combined into a single, relative position error covariance matrix. This combined position error covariance matrix is then modified according to the chosen scheme to arrive at a maximum P (sub c). But what if error covariance information for one of the two objects is not available? When error covariance information for one of the objects is not available the analyst has commonly defaulted to the situation in which only the relative miss position and velocity are known without any corresponding state error covariance information. The various usual methods of finding a maximum P (sub c) do no good because the analyst defaults to no knowledge of the combined, relative position error covariance matrix. It is reasonable to think, given an assumption of no covariance information, an analyst might still attempt to determine the error covariance matrix that results in an upper bound on the P (sub c). Without some guidance on limits to the shape, size and orientation of the unknown covariance matrix, the limiting case is a degenerate ellipse lying along the relative miss vector in the collision plane. Unless the miss position is exceptionally large or the at-risk object is exceptionally small, this method results in a maximum P (sub c) too large to be of practical use. For example, assuming that the miss distance is equal to the current ISS alert volume along-track (+ or -) distance of 25 kilometers and that the at-risk area has a 70 meter radius. The maximum (degenerate ellipse) P (sub c) is about 0.00136. At 40 kilometers, the maximum P (sub c) would be 0.00085 which is still almost an order of magnitude larger than the ISS maneuver threshold of 0.0001. In fact, a miss distance of almost 340 kilometers is necessary to reduce the maximum P (sub c) associated with this degenerate ellipse to the ISS maneuver threshold value. Such a result is frequently of no practical value to the analyst. Some improvement may be made with respect to this problem by realizing that while the position error covariance matrix of one of the objects (usually the debris object) may not be known the position error covariance matrix of the other object (usually the asset) is almost always available. Making use of the position error covariance information for the one object provides an improvement in finding a maximum P (sub c) which, in some cases, may offer real utility. The equations to be used are presented and their use discussed.

Frisbee, Joseph H., Jr.↗
NASA NTRS1994

Highly ionized gas absorption in the disk and halo toward HD 167756 at 3.5 kilometers per second resolution

High-resolution spectra of interstellar Si IV, C IV, and N V absorption lines along the 4 kpc path to the inner Galaxy star HD 167756 at z = -0.85 kpc are presented. The spectra were obtained with the echelle mode of Goddard High Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope (HST) and have signal-to-noise ratios ranging from 23 to 38. The high resolution of the measurements full width at half maximum (FWHM = 3.5 km/s) results in fully resolved line profiles for the highly ionized gas absorption. The measurements provide information on the column density per unit velocity, N(v), as a function of velocity for Si IV, C IV, and N V. The C IV and N V profiles extend from -70 to +70 km/s, while the Si IV profiles extend from -40 to +70 km/s. The integrated logarithmic column densities are long N(Si IV) = 13.09 +/- 0.02, log N(C IV) = 13.83 +/- 0.02, and log N(N V) = 13.56 +/- 0.03. The N V profile is broad, asymmetric, and featureless, while the Si IV profile contains narrow absorption components near V(sub LSR) = -19, 0, +20, and +52 km/s with Doppler spread parameters, b about = 10-12 km/s. The C IV profile contains both broad and narrow structure. The high ion feature near +52 km/s is also detected in the low-ionization lines of Ca II, O I, Si II, and Fe II. The other narrow Si IV and C IV components occur within several km/s of components seen in low-ionization species. The sight line contains at least two types of highly ionized gas. One type gives rise to a broad N V profile, and the other results in the more structured Si IV profile. The C IV profile contains contributions from both types of highly ionized gas. The broad but asymmetric N V profile is well represented by a large Galactic scale height gas which is participating in Galactic rotation and has a combination of thermal and turbulent broadening with b(sub tot) about = 42 km/s. The C IV to N V abundance ratio of 1.0 +/- 0.3 for the gas implies T about 1.6 x 10(exp 5) K or about 8 x 10(exp 5) K if the gas is in collisional ionization equilibrium and has a solar carbon to nitrogen abundance ratio. This absorption may be associated with cooling hot gas situated in Galactic shells and supershells along the sight line. The gas producing the narrow Si IV and C IV absorption components has line widths that are compatible with origins in conductive interfaces between the warm and hot interstellar medium. Kinematic flows associated with the photoionized edges of clouds might also produce Si IV and C IV lines with Doppler spread parameters similar to those observed, but the C IV to Si IV ratio in this gas is 3.5, which leads us to favor the conductive interface interpretation.

Savage, Blair D.↗
NASA NTRS1993

The connection between Venus' free obliquity and its CMB oblateness

The most striking feature of Venus rotational state is its slow retrograde rotation which is apparently maintained by a balance between solid tidal friction and thermal tidal torques. Solid tides tend to drive the spin toward synchronous rotation while thermal tides drive it away. A balance is achieved at a specific rate because of the inverse frequency dependence of the thermal tide to the semi-diurnal heating. Atmospheric models have been constructed to estimate the thermal tidal torque based on ground heating. The solid friction dissipation factor Q approximately equal to 50 can be deduced assuming rotation has achieved steady state. The most perplexing feature of Venus orientation is its non-zero free obliquity epsilon approximately equal to 1.5 deg relative to its orbit. Although solid tides and perhaps atmospheric tides tend to increase the free obliquity on a time 1/K(sub t) approximately equal to 1 x 10(exp 8) yr, viscous friction (CMF) at a core-mantle boundary (CMB) resulting from the differential angular orientation Delta-epsilon of the core and mantle spin axes should have damped the free obliquity on a time scale as short as 10(exp 6) yr. One means of achieving a balance similar to that controlling rotation is to introduce a comparatively large CMB ellipticity e(sub c) to reduce Delta-epsilon such that there is a balance between solid-thermal tides and CMF. The balance depends not so much on the potential frequency dependence on the tides as on the quadratic dependence of CMF on Delta-epsilon if the layer is turbulent.

Yoder, C. F.↗
NASA NTRSDate not supplied

Space Weather Investigation Frontier (SWIFT) Mission Concept: Continuous Distributed Observations of Geo-Effective, Heliospheric Structures from the Vantage Points of Sun-Earth L1 and Sub- L1

Continuous, in-situ, multi-point observations along the Sun-Earth line at and inside the Lagrange point L1 (sub-L1) will enable a better understanding of the three-dimensional structure and temporal evolution of heliospheric structures that drive terrestrial space weather. The proposed SWIFT (Space Weather Investigation Frontier) mission will use a new solar sail propulsion system developed by NASA to enable a suite of science instruments onboard a smallsat to maintain observations along the Sun-Earth line, sub-L1, for extended periods. Three identically instrumented small spacecraft at L1 will fly in concert with the sailcraft at sub-L1 in an optimized tetrahedron constellation, covering distances between 10 to 100’s of Earth radii. This viewing geometry will enable scientists to distinguish between local and global processes driving space weather by revealing the spatial characteristics, temporal evolution, and geo-effectiveness of small- to meso-scale solar wind structures and substructures of macro-scale structures, such as interplanetary coronal mass ejections (ICMEs) and stream interaction regions (SIRs). In addition, real time measurements of earth-bound heliospheric structures from sub-L1 will improve our current forecasting lead-times by up to 35 percent. This paper will provide an overview of the proposed SWIFT science and mission concept.

Spacecraft Propulsion↗
NASA NTRSDate not supplied

Space Weather Investigation Frontier (SWIFT)

Continuous, in-situ, multi-point observations along the Sun-Earth line at and inside the Lagrange point L1 (sub-L1) will enable a better understanding of the three-dimensional structure and temporal evolution of heliospheric structures that drive terrestrial space weather. The proposed SWIFT (Space Weather Investigation Frontier) mission will use a new solar sail propulsion system developed by NASA to enable a suite of science instruments onboard a smallsat to maintain observations along the Sun-Earth line, sub-L1, for extended periods. Three identically instrumented small spacecraft at L1 will fly in concert with the sailcraft at sub-L1in an optimized tetrahedron constellation, covering distances between 10 to 100’s of Earth radii. This viewing geometry will enable scientists to distinguish between local and global processes driving space weather by revealing the spatial characteristics, temporal evolution, and geo-effectiveness of small-to meso-scale solar wind structures and substructures of macro-scale structures, such as interplanetary coronal mass ejections (ICMEs) and stream interaction regions (SIRs). In addition, real time measurements of earth-bound heliospheric structures from sub-L1 will improve our current forecasting lead-times by up to 35 percent. This paper will provide an overview of the proposed SWIFT science and mission concept.

Heliophysics↗
DOE OSTI2025

Attosecond Transient Grating Spectroscopy with Near-Infrared Grating Pulses and an Extreme Ultraviolet Diffracted Probe

Transient grating spectroscopy has become a mainstay among metal and semiconductor characterization techniques. Here, we extend the technique toward the shortest achievable time scales by using tabletop high-harmonic generation of attosecond extreme ultraviolet (XUV) pulses that diffract from transient gratings generated with sub-5 fs near-infrared (NIR) pulses. We demonstrate the power of attosecond transient grating spectroscopy (ATGS) by investigating the ultrafast photoexcited dynamics in an Sb semimetal thin film. ATGS provides an element-specific, background-free signal unfettered by spectral congestion, in contrast to transient absorption spectroscopy. With ATGS measurements in Sb polycrystalline thin films, we observe the generation of coherent phonons and investigate the lattice and carrier dynamics. Among the latter processes, we extract carrier thermalization, hot carrier cooling, and electron-hole recombination, which are on the order of 20 fs, 50 fs, and 2 ps time scales, respectively. Furthermore, the simultaneous collection of transient absorption and transient grating data allows us to extract the total complex dielectric constant in the sample dynamics with a single measurement, including the real-valued refractive index, from which we are also able to investigate carrier-phonon interactions and longer-lived phonon dynamics. The outlined experimental technique expands the capabilities of transient grating spectroscopy and attosecond spectroscopies by providing a wealth of information concerning carrier and lattice dynamics with an element-selective technique at the shortest achievable time scales.

Quintero-Bermudez, Rafael↗
NASA NTRS1994

Optimizing the Zeldovich approximation

We have recently learned that the Zeldovich approximation can be successfully used for a far wider range of gravitational instability scenarios than formerly proposed; we study here how to extend this range. In previous work (Coles, Melott and Shandarin 1993, hereafter CMS) we studied the accuracy of several analytic approximations to gravitational clustering in the mildly nonlinear regime. We found that what we called the 'truncated Zeldovich approximation' (TZA) was better than any other (except in one case the ordinary Zeldovich approximation) over a wide range from linear to mildly nonlinear (sigma approximately 3) regimes. TZA was specified by setting Fourier amplitudes equal to zero for all wavenumbers greater than k(sub nl), where k(sub nl) marks the transition to the nonlinear regime. Here, we study the cross correlation of generalized TZA with a group of n-body simulations for three shapes of window function: sharp k-truncation (as in CMS), a tophat in coordinate space, or a Gaussian. We also study the variation in the crosscorrelation as a function of initial truncation scale within each type. We find that k-truncation, which was so much better than other things tried in CMS, is the worst of these three window shapes. We find that a Gaussian window e(exp(-k(exp 2)/2k(exp 2, sub G))) applied to the initial Fourier amplitudes is the best choice. It produces a greatly improved crosscorrelation in those cases which most needed improvement, e.g. those with more small-scale power in the initial conditions. The optimum choice of kG for the Gaussian window is (a somewhat spectrum-dependent) 1 to 1.5 times k(sub nl). Although all three windows produce similar power spectra and density distribution functions after application of the Zeldovich approximation, the agreement of the phases of the Fourier components with the n-body simulation is better for the Gaussian window. We therefore ascribe the success of the best-choice Gaussian window to its superior treatment of phases in the nonlinear regime. We also report on the accuracy of particle positions and velocities produced by TZA.

Melott, Adrian L.↗
NASA NTRS1991

Non-Gaussian microwave background fluctuations from nonlinear gravitational effects

Whether the statistics of primordial fluctuations for structure formation are Gaussian or otherwise may be determined if the Cosmic Background Explorer (COBE) Satellite makes a detection of the cosmic microwave-background temperature anisotropy delta T(sub CMB)/T(sub CMB). Non-Gaussian fluctuations may be generated in the chaotic inflationary model if two scalar fields interact nonlinearly with gravity. Theoretical contour maps are calculated for the resulting Sachs-Wolfe temperature fluctuations at large angular scales (greater than 3 degrees). In the long-wavelength approximation, one can confidently determine the nonlinear evolution of quantum noise with gravity during the inflationary epoch because: (1) different spatial points are no longer in causal contact; and (2) quantum gravity corrections are typically small-- it is sufficient to model the system using classical random fields. If the potential for two scalar fields V(phi sub 1, phi sub 2) possesses a sharp feature, then non-Gaussian fluctuations may arise. An explicit model is given where cold spots in delta T(sub CMB)/T(sub CMB) maps are suppressed as compared to the Gaussian case. The fluctuations are essentially scale-invariant.

Salopek, D. S.↗
DOE OSTI2023

Radiation Transport Two-temperature GRMHD Simulations of Warped Accretion Disks

In many black hole (BH) systems, the accretion disk is expected to be misaligned with respect to the BH spin axis. If the scale height of the disk is much smaller than the misalignment angle, the spin of the BH can tear the disk into multiple, independently precessing "sub-disks." This is most likely to happen during outbursts in black hole X-Ray binaries (BHXRBs) and in active galactic nuclei (AGNs) accreting above a few percent of the Eddington limit, because the disk becomes razor-thin. Disk tearing has the potential to explain variability phenomena including quasi-periodic oscillations in BHXRBs and changing-look phenomena in AGNs. Here, we present the first radiative two-temperature general relativistic magnetohydrodynamic (GRMHD) simulation of a strongly tilted (65°) accretion disk around an M BH = 10 M ⊙ BH, which tears and precesses. This leads to luminosity swings between a few percent and 50% of the Eddington limit on sub-viscous timescales. Surprisingly, even where the disk is radiation-pressure-dominated, the accretion disk is thermally stable over t ≳ 14,000 r g /c. This suggests warps play an important role in stabilizing the disk against thermal collapse. The disk forms two nozzle shocks perpendicular to the line of nodes where the scale height of the disk decreases tenfold and the electron temperature reaches T e ~ 10 8 –10 9 K. In addition, optically thin gas crossing the tear between the inner and outer disk gets heated to T e ~ 108 K. This suggests that warped disks may emit a Comptonized spectrum that deviates substantially from idealized models.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2018

Decadal Changes in Summertime Reactive Oxidized Nitrogen and Surface Ozone over the Southeast United States

Widespread efforts to abate ozone (O3) smog have significantly reduced emissions of nitrogen oxides (NOx) over the past 2 decades in the Southeast US, a place heavily influenced by both anthropogenic and biogenic emissions. How reactive nitrogen speciation responds to the reduction in NOx emissions in this region remains to be elucidated. Here we exploit aircraft measurements from ICARTT (International Consortium for Atmospheric Research on Transport and Transformation - July-August 2004), SENEX (Southeast Nexus - June-July 2013), and SEAC4RS (Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys - August-September 2013) and long-term ground measurement networks alongside a global chemistry-climate model to examine decadal changes in summertime reactive oxidized nitrogen (RON) and ozone over the Southeast US. We show that our model can reproduce the mean vertical profiles of major RON species and the total (NO (sub y)) in both 2004 and 2013. Among the major RON species, nitric acid (HNO3) is dominant (approximately 42-45 percent), followed by NOx (31 percent), total peroxy nitrates (Sigma PNs; 14 percent), and total alkyl nitrates (Sigma ANs; 9-12 percent) on a regional scale. We find that most RON species, including NOx, Sigma PNs, and HNO3, decline proportionally with decreasing NOx emissions in this region, leading to a similar decline in NO (sub y). This linear response might be in part due to the nearly constant summertime supply of biogenic VOC (Volatile Organic Compounds) emissions in this region. Our model captures the observed relative change in RON and surface ozone from 2004 to 2013. Model sensitivity tests indicate that further reductions of NOx emissions will lead to a continued decline in surface ozone and less frequent high-ozone events.

Li, Jingyi↗