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NASA NTRS1986

Magnetized thermal conduction fronts

The evolution of planar thermal conduction fronts in the presence of a dynamically weak, but otherwise self-consistent, magnetic field is considered. The field is assumed to be connected and untangled. In the diffusion limit for the thermal conductivity, these fronts exhibit self-similar behavior, even in the presence of a field. The role of the field is restricted to channeling the heat flux along its lines of force, and it enters into the problem as a dimensionless angle variable. 'Combing' (or opening) of insulating field lines by the evaporative flow is explicitly demonstrated. Unless the field is nearly perpendicular to the front normal in the hot gas, insulating effects are not profound. Self-similarity breaks down if the front becomes saturated, and under certain conditions magnetized saturated conduction fronts cannot propagate: the solution characteristics of the wave equation form caustics. The physical resolution is the advent of two-fluid (nonlocal) heating. Such Coulomb-heated fronts are expected to be relatively rare in typical astrophysical systems. The large-scale effects of a magnetic field on cloud evaporation in the interstellar medium are briefly discussed, and it is suggested that these fields preclude the presence of time-independent evaporative solutions. Thermal interfaces may then continue to evolve until radiative cooling halts their development; large tracts of warm 10,000 K gas may result.

Balbus, S. A.↗

NASA NTRS1986

Modeling of ion acceleration through drift and diffusion at interplanetary shocks

A test particle simulation designed to model ion acceleration through drift and diffusion at interplanetary shocks is described. The technique consists of integrating along exact particle orbits in a system where the angle between the shock normal and mean upstream magnetic field, the level of magnetic fluctuations, and the energy of injected particles can assume a range of values. The technique makes it possible to study time-dependent shock acceleration under conditions not amenable to analytical techniques. To illustrate the capability of the numerical model, proton acceleration was considered under conditions appropriate for interplanetary shocks at 1 AU, including large-amplitude transverse magnetic fluctuations derived from power spectra of both ambient and shock-associated MHD waves.

Decker, R. B.↗

NASA NTRS1989

Electron velocity distributions and plasma waves associated with the injection of an electron beam into the ionosphere

An electron beam was injected into earth's ionosphere on August 1, 1985, during the flight of the Space Shuttle Challenger as part of the objectives of the Spacelab 2 mission. In the wake of the Space Shuttle a magnetically aligned sheet of electrons returning from the direction of propagation of the beam was detected with the free-flying Plasma Diagnostics Package. The thickness of this sheet of returning electrons was about 20 m. Large intensifications of broadband electrostatic noise were also observed within this sheet of electrons. A numerical simulation of the interaction of the electron beam with the ambient ionospheric plasmas is employed to show that the electron beam excites electron plasma oscillations and that it is possible for the ion acoustic instability to provide a returning flux of hot electrons by means of quasi-linear diffusion.

Frank, L. A.↗

DOE OSTI2020

Decoherence in Molecular Electron Spin Qubits: Insights from Quantum Many-Body Simulations

Quantum states are described by wave functions whose phases cannot be directly measured, but which play a vital role in quantum effects such as interference and entanglement. The loss of the relative phase information, termed decoherence, arises from the interactions between a quantum system and its environment. Decoherence is perhaps the biggest obstacle on the path to reliable quantum computing. Here we show that decoherence occurs even in an isolated molecule although not all phase information is lost via a theoretical study of a central electron spin qubit interacting with nearby nuclear spins in prototypical magnetic molecules. The residual coherence, which is molecule-dependent, provides a microscopic rationalization for the nuclear spin diffusion barrier proposed to explain experiments. The contribution of nearby molecules to the decoherence has a non-trivial dependence on separation, peaking at intermediate distances. Molecules that are far away only affect the long-time behavior. Because the residual coherence is simple to calculate and correlates well with the coherence time, it can be used as a descriptor for coherence in magnetic molecules. This work will help establish design principles for enhancing coherence in molecular spin qubits and serve to motivate further theoretical work

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Learning effective stochastic differential equations from microscopic simulations: Linking stochastic numerics to deep learning

We identify effective stochastic differential equations (SDEs) for coarse observables of fine-grained particle- or agent-based simulations; these SDEs then provide useful coarse surrogate models of the fine scale dynamics. We approximate the drift and diffusivity functions in these effective SDEs through neural networks, which can be thought of as effective stochastic ResNets. The loss function is inspired by, and embodies, the structure of established stochastic numerical integrators (here, Euler–Maruyama and Milstein); our approximations can thus benefit from backward error analysis of these underlying numerical schemes. They also lend themselves naturally to “physics-informed” gray-box identification when approximate coarse models, such as mean field equations, are available. Existing numerical integration schemes for Langevin-type equations and for stochastic partial differential equations can also be used for training; we demonstrate this on a stochastically forced oscillator and the stochastic wave equation. Our approach does not require long trajectories, works on scattered snapshot data, and is designed to naturally handle different time steps per snapshot. We consider both the case where the coarse collective observables are known in advance, as well as the case where they must be found in a data-driven manner.

97 MATHEMATICS AND COMPUTING↗

NASA NTRS2001

Quasi-Decadal Oscillations Generated by the QBO

Quasi-decadal oscillations (QDO) have been observed in the stratosphere and have been linked to the equatorial Quasi-Biennial Oscillation (QBO) and to the 11-year solar activity cycle. With the use of a 2D version of our Numerical Spectral Model (NSM) that incorporates Hines' Doppler Spread Parameterization (DSP) for gravity waves (GW), we demonstrate that beat periods between 9 and 11 years can be generated by the QBO as it interacts through GW filtering with the Annual Oscillation (AO) and Semi-annual Oscillation (SAO). Results are discussed from computations covering up to 50 years, and our analyses leads to the following conclusions. The QDO as a stand-alone signature is largely confined to the upper mesosphere. Its largest signature appears in the form of amplitude modulations of the QBO, AO and SAO, and these extend into the lower stratosphere. The downward control that characterizes the QBO apparently comes into play, and the longer time constants for diffusion and radiative loss at lower altitudes facilitate the QDO response. Although excited by the QBO, which is confined to low latitudes, the QDO is shown to extend to high latitudes. The effect is particularly large for the QBO with period around 33.5 month (near the upper limit of observations), which interacts with the SAO to produce a hemispherically symmetric QDO. Our analysis indicates that the QDO is transferred to high latitudes by the meridional circulation, which prominently exhibits this periodicity particularly in the amplitude modulation of the AO.

Mayr, H. G.↗

NASA NTRS1983

Shock processing of interstellar grains

Theoretical and observational evidence is presented that shock processing of interstellar dust grains by supernova blast waves affects both heavy element depletions and ultraviolet extinction curves. By coupling a realistic model of grain sizes and populations with a radiative shock code, significant grain destruction at velocities as low as 40 km/s is demonstrated. Nonthermal sputtering and grain-grain collisions destroy relatively more large grains than small, and more silicates than graphite. Consequently, both the 2175 A extinction 'bump' and the far-ultraviolet normalized extinction are increased in strength. Ultraviolet extinction studies with the International Ultraviolet Explorer of nine stars near three supernova remnants (the Monoceros Loop, Shajn 147, and Vela) exhibit strong 2175 A bumps and normal or high far-ultraviolet extinction. Diffuse bands, if they are created by small grains, should show little correlation with such activity.

Seab, C. G.↗

NASA NTRS1993

Interstellar Dust: Physical Processes

Dust is formed in stellar environments, and destroyed by sputtering, shattering and vaporization in shock waves due to cloud-cloud collisions and supernova blast waves. Dust is also destroyed during star formation. We review the dust formation and destruction balance. The calculated destruction time-scale is less than or equal to one billion years and the star dust injection time-scale is approx. 2.5 billion years. Hence, the fractions of elemental carbon and silicon locked up in stardust are less than 0.3 and less than 0.15, respectively. An efficient ISM dust formation route is therefore implied. In particular, in dense clouds dust grows; through the processes of coagulation and the accretion of gas phase molecules e.g. H20, CO, CH4. These icy materials may then be photoprocessed to refractory materials in more diffuse regions. The resulting carbonaceous grain mantle may actually be the glue that holds the coagulated grains together.

Jones, A. P.↗

NASA NTRS1997

Premixed Gas Combustion: An Excitable System

Rotating spiral and target patterns have been observed experimentally on freely-propagating premixed gas flames in large diameter tubes at normal gravity (1-g). These modes of propagation occur in near-limit mixtures which have a Lewis number (Le, defined as the ratio of the thermal diffusivity of the cold mixture to the mass diffusivity of the scarce component into the mixture) sufficiently greater than one. However, at 1-g, buoyant flows strongly distort the flame curvature, hydrodynamics (thus stretch) and convective transport of species and heat. In turn, these alter the critical Le required for onset of instability. To isolate and better understand the mechanisms which drive the observed patterns and their dynamics, 1-g and microgravity (micro-g) experiments are being conducted to determine: (1) the structure and dynamics of the patterns, (2) a map of the critical Le and heat loss for their occurrence, (3) the relative significance of the chemical kinetics, and (4) the effect of curvature (local wave and global flame front) on wave propagation. With this in hand, we will be better prepared to discuss an additional mode, a state of 'chemical turbulence,' which seems to be the ultimate fate of many of these near-limit flames prior to extinction.

Pearlman, Howard↗

NASA NTRS2001

Quasi-Decadal Oscillations Generated by the QBO

Quasi-decadal oscillations (QDO) have been observed in the stratosphere and have been linked to the equatorial Quasi-Biennial Oscillation (QBO) and to the 11-year solar activity cycle. With the use of a 2D version of our Numerical Spectral Model (NSM) that incorporates Hines'Doppler Spread Parameterization (DSP) for gravity waves (GW), we demonstrate that beat periods between 9 and I I years can be generated by the QBO as it interacts through GW filtering with the Annual Oscillation (AO) and Semi-annual Oscillation (SAO). Results are discussed from computations covering up to 50 years, and our analyses leads to the following conclusions. The QDO as a stand-alone signature is largely confined to the upper mesosphere. Its largest signature appears in the form of amplitude modulations of the QBO, AO and SAO, and these extend into the lower stratosphere. The downward control that characterizes the QBO apparently comes into play, and the longer time constants for diffusion and radiative loss at lower altitudes facilitate the QDO response. Although excited by the QBO, which is confined to low latitudes, the QDO is shown to extend to high latitudes. The effect is particularly large for the QBO with period around 33.5 month (near the upper limit of observations), which interacts with the SAO to produce a hemispherically symmetric QDO. Our analysis indicates that the QDO is transferred to high latitudes by the meridional circulation, which prominently exhibits this periodicity particularly in the amplitude modulation of the AO. The propensity for beat periods around 10 years may help to increase significantly the efficiency for generating solar cycle signatures in the atmosphere, and we discuss numerical experiments related to that.

Mayr, Hans G.↗

NASA NTRS1985

The 50-MHz meteor radar observation at Syowa Station, Antarctica

The 50-MHz Doppler radar installed at Syowa Station (69 deg 00'S, 39 deg 35'E), Antarctica, in 1982 can detect continuously a meteor echo if an operator assigns the meteor mode operation to the radar. The radar has two narrow antenna beams (4 deg in the horizontal plane), one toward geomagnetic south and the other toward approximately geographic south, with a crossing angle of about 33 deg. The minicomputer annexed to the radar controls the transmission and reception of a 50-MHz wave. If the receiver detects a meteor echo, the flag signal is sent to the computer. Then the computer begins to determine the echo range with a time resolution of 1 micro s and to sample every 200 microns/s for 1 s the Doppler signal and echo intensity at the particular range (R). The line-of-sight velocity (V sub D) of the echo trail is calculated from the output from the Doppler signal detection circuit having an offset frequency by using the so-called zero-crossing method. The echo amplitude decay time calculated by a least-mean square method is used to obtain the ambipolar diffusion coefficient (D) and then to calculate the echo height (H). About 120 day observations were made during 1982-1983. Some early results are presented. magnetic tapes together with V sub D, D, H and R for later analysis in Japan. About 120 day observation were made during 1982-1983. Some early results are presented.

Tanaka, T.↗

DOE OSTI2021

Evaluating the Feasibility of Measuring and Analyzing Strain Caused by Changes in Barometric Pressure in the Vadose Zone

Changes in barometric pressure propagate into the subsurface where they can affect water level measurements in wells and cause vertical strain. Previous strain sensors were developed to measure vertical strain, but they were limited to measurements at a single depth, making it difficult to evaluate strains from a migrating pressure wave. Technology known as Coherence-length-gated Microwave Photonics Interferometry (CMPI) uses optical fiber sensors to measure strain at multiple locations. It has the potential to detect strain caused by variations in barometric pressure at multiple depths in the subsurface. The goal of this study is to evaluate the feasibility of using this technology to record the changes in vertical strain in the vadose zone from small fluctuations in air pressure. Propagation of these air pressure fluctuations is affected by hydrogeological properties such as water content and permeability. A second goal of this study is to evaluate how these properties affect the air pressure distribution, strain, and gas-phase diffusivity with a long-term goal of using strain to monitor the vadose zone. The focus of this study is on a suite of laboratory experiments that used a sand-filled column with an open head space. The CMPI fiber was packaged and embedded along the axis of the column to measure the strain at multiple locations while air pressure transducers were installed through the wall of the column. Small periodic fluctuations in air pressure, similar to barometric pressure fluctuations, were created in the head space of the column using an audio driven speaker driven at 4 Hz using a sinusoidal signal created by a function generator. Then, the fluctuations propagated along the column where they were measured by the transducers and recorded as functions of time. Initial tests were conducted using dry sand, but then additional tests were conducted after injecting water into the column, which changed the hydrologic and mechanical properties. Another experiment was conducted after creating a thin barrier to air flow, which was designed to simulate a thin layer of saturated ground during rainfall. A final experiment was conducted by changing the grain size of the material to create a heterogeneity in the upper half of the column. The suite of six experiments was conducted to highlight effects of variations in water content and heterogeneities on air pressure and vertical strain. Numerical simulations were also created to evaluate the laboratory data and to provide a baseline analysis for the results. The numerical simulations used the governing equations to linear poroelasticity and two-phase flow with boundary conditions representing the experiments. The conceptual model for strain caused by barometric pressure recognizes that a fluctuating barometric pressure causes fluctuations in the air pore pressure that decrease in amplitude and lag in time with increase depth. The pressure distribution results in two different loads that cause strain: 1.) a mechanical load as the barometric pressure acts on the ground surface; and 2.) pressure loading in the pore space. The results show that the total vertical strain is a contribution of both of these loads, and this is the strain that was observed in the laboratory. The lab experiments show that both air pressure and strain vary as sinusoids that propagate with depth. The amplitude of the air pressure and strain decrease as a function of depth in all the conditions that were evaluate. For example, in the dry sand, the amplitude of the air pressure decreases from 25 Pa at the head space to 5 Pa at a depth of 70 cm, whereas the amplitude of the strain decreases from 0.15 to 0.06 µε over the same depth interval. In many cases the amplitudes decrease is approximately negative exponential functions of depth. When water was injected into the sand and the saturation increased, the amplitude of the air pressure increased in the partially saturated sand but decreased sharply to zero where the soil was saturated. The strain decreased with depth, but the transition from partially to fully saturated conditions had little effect on the strain profile, even though it had a major effect on the air pressure. This apparently is because strain is caused by variations in both water and air pressure. The fluctuating air pressure caused the water pressure to fluctuate, which caused strain throughout the column. Phase delay of the air pressure and the strain both increase as linear to bi-linear functions of depth. The phase delays indicate that pressure propagates at a velocity of 7 to 8 m/s in dry or partially saturated sand, but it drops by more than an order of magnitude in the vicinity of the capillary fringe. The phase delays indicate the strain propagates faster than the pressure. The data indicate that the strain propagates from 1.2 to more than 4x faster than the pressure. Switching from sand to silt in the column caused the velocity of the pressure to drop by roughly half for both pressures, but curiously, it had little effect on the velocity of the strain.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2022

Exploring the MeV sky with a combined coded mask and Compton telescope: the Galactic Explorer with a Coded aperture mask Compton telescope (GECCO)

The sky at MeV energies is currently poorly explored. Here we present an innovative mission concept that builds upon the heritage of past and current missions improving the sensitivity and, very importantly, the angular resolution. This consists in combining a Compton telescope and a coded-mask telescope. We delineate the motivation for such a concept and we define the scientific goals for such a mission. The Galactic Explorer with a Coded Aperture Mask Compton Telescope (GECCO) is a novel concept for a next-generation telescope covering hard X-ray and soft gamma-ray energies. The potential and importance of this approach that bridges the observational gap in the MeV energy range are presented. With the unprecedented angular resolution of the coded mask telescope combined with the sensitive Compton telescope, a mission such as GECCO can disentangle the discrete sources from the truly diffuse emission. Individual Galactic and extragalactic sources are detected. This also allows to understand the gamma-ray Galactic center excess and the Fermi Bubbles, and to trace the low-energy cosmic rays, and their propagation in the Galaxy. Nuclear and annihilation lines are spatially and spectrally resolved from the continuum emission and from sources, addressing the role of low-energy cosmic rays in star formation and galaxy evolution, the origin of the 511 keV positron line, fundamental physics, and the chemical enrichment in the Galaxy. Such an instrument also detects explosive transient gamma-ray sources, which, in turn, enables identifying and studying the astrophysical objects that produce gravitational waves and neutrinos in a multi-messenger context. By looking at a poorly explored energy band it also allows discoveries of new astrophysical phenomena.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2022

Exploring the MeV Sky With A Combined Coded Mask and Compton Telescope: the Galactic Explorer With A Coded Aperture Mask Compton Telescope (GECCO)

Elena Orlando↗

DOE OSTI2021

Standoff Detection of Oil and Powder Mixtures at 12 Meters Using a Tunable Quantum Cascade Laser-Based System with a Close Focus Telescope and Uncooled Infrared Detector

We have designed and demonstrated a quantum cascade laser (QCL) based standoff system that utilizes an uncooled mercury cadmium telluride (MCT) detector with lock-in signal processing for chemical identification at a distance of 12.5 meters in indoor ambient light conditions. In the system, a tunable quad-QCL operating (1 MHz) in quasi-continuous wave mode between 8.45 and 10.03 μm (~1182 to 1000 cm –1 ) serves as the active mid-infrared source for remotely interrogating mineral, powder, and thin film oil samples including powder mixtures (6, 12.5, 25, and 50%) of crystalline quartz (SiO 2 ) in KBr. Light as reflected from a given sample is collected using a 10-inch (25.4 cm) Dall Kirkham telescope and coupled with ZnSe optics to an uncooled MCT detector. Furthermore, the mixture dependence of the highly transparent KBr and strongly absorbing quartz was found to fit a modified version of the Schatz reflectance model for compacted powder mixtures. All reflectance spectra reported are relative to an Au-coated diffuse reflector. A NIST traceable polystyrene standard reflector was also used to determine the QCL wavelength tuning range and calibration.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRS1979

Turbulence characteristics of compressor discharge flows

Turbulence measurements were conducted in a large gas turbine engine (JT9D) at the entrance to the diffuser duct, joining the compressor discharge to the combustor inlet. Hot film probe and hot wire probe measurements were obtained at temperatures from 450K (350F) (idle) to 608K (635F) (rich approach). At I.D. (25 percent span) and mid-span locations, the turbulence intensity increased slightly from 6 + or - percent at idle condition to 7 or - 1 percent at rich approach. At O.D. (75 percent span) the turbulent intensity increased more rapidly, from 7.5 + or - 0.5 percent at idle to 15 + or - 0.5 percent at rich approach. The spectra showed turbulent energy distributed uniformly over a 0.1 to 5 KHz bandwidth (down 3db) at all operating conditions, corresponding to random turbulence with velocity wave lengths of 2 cm to 1 meter travelling at the mean velocity of 100 m/sec. Tests results are given in tables and graphs.

Grant, H. P.↗

NASA NTRS1984

The Soft X-ray Diffuse Background: Implications for the Nature of the Local Interstellar Medium

Observations of the diffuse X-ray background in the B and C bands (130-188 eV and 160-284 eV, respectively) provide convincing evidence for the existence of high-temperature interstellar gas. Since the opacity of normal interstellar material is very high, it is assumed that the soft X-ray flux observed in the galactic plane originates within a few hundred parsecs of the Sun. The intensity and B/C ratio of this low-latitude flux can be provided by emission from an equilibrium plasma with normal abundances, T =10 to the 6th power K, and 0.0019 cm(-6) pc emission measure. More sophisticated nonequilibrium models of material heated by a supernova blast wave would reduce the required emission measure somewhat, but not by so much as a factor of two. Arbitrarily limiting the pressure to 10 to the 4th power cm(-3) K gives a maximum density of 0.005 cm(-3) and a minimum radius for the emitting region of 75 pc. This fits in well with ultraviolet interstellar absorption measurements which indicate that the interstellar medium is very deficient in neutral hydrogen out to approx. 100 pc from the Sun.

Mccammon, D.↗

NASA NTRS1990

Ion precipitation from the inner plasma sheet due to stochastic diffusion

Plasma sheet ions do not conserve their first adiabatic invariant when the magnetic field is appreciably tail-like. They do conserve a different adiabatic invariant but only to linear, rather than exponential, accuracy in the appropriate small parameter. Thus significant stochastic diffusion can occur for particles crossing the separatrix dividing the segments of orbits on which the particles cross and do not cross the tail midplane. Such ions can escape the plasma sheet and precipitate into the atmosphere. Stochastic scattering is strongest from those field lines where the ion's Larmor period in the normal component of the neutral sheet magnetic field approximately equals its bounce period. By comparing the rates of stochastic ion loss and convection in the tail, it is possible to estimate the location and thickness of the inner edge of the ion plasma sheet created by stochastic ion loss. Ions of different masses precipitate into the atmosphere at slightly different locations. Since wave particle interactions are not needed, this precipitation will always occur and should be particularly evident during quiet geomagnetic conditions, when it is less likely to be masked by other precipitation mechanisms.

Zelenyi, L.↗