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At least 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

Changes in the X-Ray Emission from the Magnetar Candidate 1E 2259+586 During its 2002 Outburst

An outburst of more than 80 individual bursts, similar to those seen from Soft Gamma Repeaters (SGRs), was detected from the anomalous X-ray pulsar (AXP) 1E 2259+586 in 2002 June. Coincident with this burst activity were gross changes in the pulsed flux, persistent flux, energy spectrum, pulse profile, and spin-down of the underlying X-ray source. We present Rossi X-Ray Timing Explorer and X-Ray Multi-Mirror Mission observations of 1E 2259+586 that show the evolution of the aforementioned source parameters during and following this episode and identify recovery timescales for each. Specifically, we observe an X-ray flux increase (pulsed and phase-averaged) by more than an order of magnitude having two distinct components. The first component is linked to the burst activity and decays within approx. 2 days, during which the energy spectrum is considerably harder than during the quiescent state of the source. The second component decays over the year following the glitch according to a power law in time with an exponent -0.22 +/- 0.01. The pulsed fraction decreased initially to approx. 15% rms but recovered rapidly to the preoutburst level of approx. 23% within the first 3 days. The pulse profile changed significantly during the outburst and recovered almost fully within 2 months of the outburst. A glitch of size Delta(sib (nu)max) = (4.24 +/- 0.11) x 10(exp -6) was observed in 1E 2259+586, which preceded the observed burst activity. The glitch could not be well fitted with a simple partial exponential recovery. An exponential rise of approx. 20% of the frequency jump with a timescale of approx. 14 days results in a significantly better fit to the data; however, contamination from a systematic drift in the phase of the pulse profile cannot be excluded. A fraction of the glitch (approx. 19%) was recovered in a quasi-exponential manner having a recovery timescale of approx. 16 days. The long-term postglitch spin-down rate decreased in magnitude relative to the preglitch value. The changes in the source properties of 1E 2259+586 during its 2002 outburst are shown to be qualitatively similar to changes seen during or following burst activity in two SGRs, thus further solidifying the common nature of SGRs and AXP's as magnetars. The changes in persistent emission properties of 1E 2259+586 suggest that the star underwent a plastic deformation of the crust that simultaneously impacted the superfluid interior (crustal and possibly core superfluid) and the magnetosphere. Finally, the changes in persistent emission properties coincident with burst activity in 1E 2259+586 enabled us to infer previous burst-active episodes from this and other AXP's. The nondetection of these outbursts by all-sky gamma-ray instruments suggests that the number of active magnetar candidates in our Galaxy is larger than previously thought.

Woods, P. M.↗
NASA NTRS2012

The Origin of the EUV Late Phase: A Case Study of the C8.8 Flare on 2010 May 5

Since the launch of NASA's Solar Dynamics Observatory on 2010 February 11, the Extreme ultraviolet Variability Experiment (EVE) has observed numerous flares. One interesting feature observed by EVE is that a subset of flares exhibit an additional enhancement of the 2-3 million K emission several hours after the flares soft X-ray emission. From the Atmospheric Imaging Assembly (AIA) images, we observe that this secondary emission, dubbed the EUV late phase, occurs in the same active region as the flare but not in the same coronal loops. Here, we examine the C8.8 flare that occurred on 2010 May 5 as a case study of EUV late phase flares. In addition to presenting detailed observations from both AIA and EVE, we develop a physical model of this flare and test it using the Enthalpy Based Thermal Evolution of Loops (EBTEL) model.

Hock, R. A.↗
NASA NTRS2011

Simulation of the Indian Summer Monsoon Using Comprehensive Atmosphere-land Interactions, in the Absence of Two-way Air-sea Interactions

Community Land Model version 2 (CLM2) as a comprehensive land surface model and a simple land surface model (SLM) were coupled to an atmospheric climate model to investigate the role of land surface processes in the development and the persistence of the South Asian summer monsoon. Two-way air-sea interactions were not considered in order to identify the reproducibility of the monsoon evolution by the comprehensive land model, which includes more realistic vertical soil moisture structures, vegetation and 2-way atmosphere-land interactions at hourly intervals. In the monsoon development phase (May and June). comprehensive land-surface treatment improves the representation of atmospheric circulations and the resulting convergence/divergence through the improvements in differential heating patterns and surface energy fluxes. Coupling with CLM2 also improves the timing and spatial distribution of rainfall maxima, reducing the seasonal rainfall overestimation by approx.60 % (1.8 mm/d for SLM, 0.7 mm/dI for CLM2). As for the interannual variation of the simulated rainfall, correlation coefficients of the Indian seasonal rainfall with observation increased from 0.21 (SLM) to 0.45 (CLM2). However, in the mature monsoon phase (July to September), coupling with the CLM2 does not exhibit a clear improvement. In contrast to the development phase, latent heat flux is underestimated and sensible heat flux and surface temperature over India are markedly overestimated. In addition, the moisture fluxes do not correlate well with lower-level atmospheric convergence, yielding correlation coefficients and root mean square errors worse than those produced by coupling with the SLM. A more realistic representation of the surface temperature and energy fluxes is needed to achieve an improved simulation for the mature monsoon period.

Monsoon Precipitation↗
NASA NTRS2021

Advances in Land Surface Models and Indicators for Drought Monitoring and Prediction

Millions of people across the globe are affected by droughts every year, and recent droughts have highlighted the considerable agricultural impacts and economic costs of these events. Monitoring the state of droughts depends on integrating multiple indicators that each capture particular aspects of hydrologic impact and various types and phases of drought. As the capabilities of land-surface models and remote sensing have improved, important physical processes such as dynamic, interactive vegetation phenology, groundwater, and snow pack evolution now support a range of drought indicators that better reflect coupled water, energy and carbon cycle processes. In this work, we discuss these advances, including newer classes of indicators that can be applied to improve the characterization of drought onset, severity and duration. We utilize a new model-based drought reconstruction to illustrate the role of dynamic phenology and groundwater in drought assessment. Further, through case studies on flash droughts, snow droughts, and drought recovery, we illustrate the potential advantages of advanced model physics and observational capabilities, especially from remote sensing, in characterizing droughts.

Drought↗
NASA NTRSDate not supplied

Timing and Geochemical Evolution of Triassic and Jurassic Magmatism during the Break-up of Pangea along the Eastern North American Margin

While our understanding of continental break-up has seen dramatic advances over the past decades, the impact of syn-rift melt generation on the transition from mechanical stretching to seafloor spreading remains enigmatic. Early Mesozoic continental break-up along the U.S. Eastern North American Margin (ENAM) is characterized by protracted Triassic lithospheric stretching and syn-rift deposition interrupted by voluminous Central Atlantic Magmatic Province (CAMP) magmatism. This magmatic phase is followed by ~50 Myr of protracted rifting prior to steady-state seafloor spreading. Mesozoic magmatism recognized in outcrop and aeromagnetic datasets along the ENAM has been summarily attributed to CAMP, although extensive regional dikes preserved in the ENAM proximal rift domain remain largely undated. This study provides new constraints on the timing, duration, and geochemical evolution of syn-rift ENAM magmatism using in-situ apatite U-Pb geochronology and whole-rock geochemistry and integrates these data into a tectonic model. We present in-situ apatite U-Pb ages and whole-rock geochemical data for 16 diabase dikes and sills from NJ, PA, VA, NC, and SC. A Palisades Sill sample yielded an age of 201 ± 8 Ma and agrees with previous 40 Ar/39 Ar and U-Pb ages. Our new in-situ apatite U-Pb ages fall into three clusters and suggest three main pulses of magmatism along the ENAM, with an initial pulse of magmatism at ~205-195 Ma attributed to CAMP. Whole-rock geochemistry of this pulse shows silica (46-53 wt.%), total alkalis (2-4 wt.%), and enrichment of LREEs relative to HREEs, comparable to previously reported geochemical data for CAMP-related rocks along the ENAM. A second magmatic pulse occurred between ~181-175 Ma and shows slightly higher silica and total alkalis, compared to the older CAMP-age phase. The third pulse between ~166-145 Ma generally shows slightly higher silica and total alkalis. LOI and Sr/Yb values of the two younger pulses are similar to CAMP-age samples suggesting no post-crystallization alteration. These new data demonstrate long-lived syn-rift off-axis magmatism persisted ~50 Myr after the onset of CAMP. This post-CAMP off-axis magmatism is likely related to slow magma-limited spreading during the protracted transition to symmetric seafloor spreading along the ENAM.

Z. Foster-Baril↗
NASA NTRS1997

Boundary Conditions for the Paleoenvironment: Chemical and Physical Processes in the Pre-Solar Nebula

The basic theme of this program is the study of molecular complexity and evolution in interstellar clouds and in primitive solar system objects. Research has included the detection and study of a number of new interstellar molecules and investigation of reaction pathways for astrochemistry from a comparison of theory and observed molecular abundances. The latter includes studies of cold, dark clouds in which ion-molecule chemistry should predominate, searches for the effects of interchange of material between the gas and solid phases in interstellar clouds, unbiased spectral surveys of particular sources, and systematic investigation of the interlinked chemistry and physics of dense interstellar clouds. In addition, the study of comets has allowed a comparison between the chemistry of such minimally thermally processed objects and that of interstellar clouds, shedding light on the evolution of the biogenic elements during the process of solar system formation.

Irvine, William M.↗
NASA NTRS2015

CFD Modeling of the Multipurpose Hydrogen Test Bed (MHTB) Self-Pressurization and Spray Bar Mixing Experiments in Normal Gravity: Effect of the Accommodation Coefficient on the Tank Pressure

A CFD model for simulating the self-pressurization of a large scale liquid hydrogen storage tank is utilized in this paper to model the MHTB self-pressurization experiment. The kinetics-based Schrage equation is used to account for the evaporative and condensi ng interfacial mass flows in this model. The effect of the accommodation coefficient for calculating the interfacial mass transfer rate on the tank pressure during tank selfpressurization is studied. The values of the accommodation coefficient which were considered in this study vary from 1.0e-3 to 1.0e-1 for the explicit VOF model and from 1.0e-4 to 1.0e-3 for the implicit VOF model. The ullage pressure evolutions are compared against experimental data. A CFD model for controlling pressure in cryogenic storage tanks by spraying cold liquid into the ullage is also presented. The Euler-Lagrange approach is utilized for tracking the spray droplets and for modeling the interaction between the droplets and the continuous phase (ullage). The spray model is coupled with the VOF model by performing particle tracking in the ullage, removing particles from the ullage when they reach the interface, and then adding their contributions to the liquid. Droplet-ullage heat and mass transfer are modeled. The flow, temperature, and interfacial mass flux, as well as droplets trajectories, size distribution and temperatures predicted by the model are presented. The ul lage pressure and vapor temperature evolutions are compared with experimental data obtained from the MHTB spray bar mixing experiment. The effect of the accommodation coefficient for calculating the interfacial and droplet mass transfer rates on the tank pressure during mixing of the vapor using spray is studied. The values used for the accommodation coefficient at the interface vary from 1.0e-5 to 1.0e-2. The droplet accommodation coefficient values vary from 2.0e-6 to 1.0e-4.

cryogenic fluid storage↗
NASA NTRS1976

Ring current loss mechanisms and composition as inferred from equatorial pitch angle distributions observed during a storm recovery phase

Explorer 45 measurements of pitch angle distributions (PAD's) of ring current ions were examined for evidence of charge exchange or pitch-angle diffusion as loss mechanisms for the ring current during a storm recovery phase. The observed rounding of the PAD's above a minimum parallel energy is seen as resulting from pitch angle diffusion driven by resonant interactions with ion cyclotron waves. Calculations of growth rates using observed ion fluxes and PAD's have shown that ion cyclotron waves can be significantly amplified by the ring current plasma. However, wave growth off the equator must be taken into account in order to understand the time evolution of the observed PAD's. Isotropic distributions at lower energies remain isotropic at L-values between 3 and 4 despite the presence of charge exchange losses. An explanation for this apparent inconsistency may be that ring current is dominated by some ion species other than protons during recovery phase.

Lyons, L. R.↗
NASA NTRS2012

Nature of Reduced Carbon in Martian Meteorites

Martian meteorites provide important information on the nature of reduced carbon components present on Mars throughout its history. The first in situ analyses for carbon on the surface of Mars by the Viking landers yielded disappointing results. With the recognition of Martian meteorites on Earth, investigations have shown carbon-bearing phases exist on Mars. Studies have yielded presence of reduced carbon, carbonates and inferred graphitic carbon phases. Samples ranging in age from the first approximately 4 Ga of Mars history [e.g. ALH84001] to nakhlites with a crystallization age of 1.3 Ga [e.g. Nakhla] with aqueous alteration processes occurring 0.5-0.7 Ga after crystallizaton. Shergottites demonstrate formation ages around 165-500 Ma with younger aqueous alterations events. Only a limited number of the Martian meteorites do not show evidence of significance terrestrial alterations. Selected areas within ALH84001, Nakhla, Yamato 000593 and possibly Tissint are suitable for study of their indigenous reduced carbon bearing phases. Nakhla possesses discrete, well-defined carbonaceous phases present within iddingsite alteration zones. Based upon both isotopic measurements and analysis of Nakhla's organic phases the presence of pre-terrestrial organics is now recognized. The reduced carbon-bearing phases appear to have been deposited during preterrestrial aqueous alteration events that produced clays. In addition, the microcrystalline layers of Nakhla's iddingsite have discrete units of salt crystals suggestive of evaporation processes. While we can only speculate on the origin of these unique carbonaceous structures, we note that the significance of such observations is that it may allow us to understand the role of Martian carbon as seen in the Martian meteorites with obvious implications for astrobiology and the pre-biotic evolution of Mars. In any case, our observations strongly suggest that reduced organic carbon exists as micrometer- size, discrete structures on Mars associated with clay and salt minerals. The Mars Science Laboratory s investigators should be aware of reduced organic carbon components within clay-bearing phases.

Gibson, Everett K., Jr.↗
NASA NTRS1982

Spatial and temporal evolution of soft and hard X-ray emission in a solar flare

Hard X-ray burst spectrometer and imaging spectrometer data are used to study the spatial and temporal characteristics of the 3.5-30.0 keV emission in an Apr. 10, 1980 solar flare. It is found that: (1) continuous energy release is needed to sustain the increase of the emission through the flare's rising phase, before and after the impulsive phase in hard X-rays, and the release is characterized by the production of 50 million-150 million K thermal regions within the flare loop structures; (2) the observational parameters which characterize the impulsive burst indicate that it is probably associated with nonthermal processes, such as particle acceleration; and (3) the continuous energy release is associated with strong chromospheric evaporation, in view of spectral line behavior. Both particle acceleration and chromospheric evaporation stop just before flare maximum, and the subsequent evolution is probably governed by the radiative cooling of the flare plasma.

Machado, M. E.↗
NASA NTRS2022

The Hyper-inflation Stage in the Coronal Mass Ejection Formation: A Missing Link That Connects Flares, Coronal Mass Ejections, and Shocks in the Low Corona

We analyze the formation and three-dimensional (3D) evolution of two coronal mass ejections (CMEs) and their associated waves in the low corona via a detailed multi-viewpoint analysis of extreme-ultraviolet observations. We analyze the kinematics in the radial and lateral directions and identify three stages in the early evolution of the CME: (1) a hyper-inflation stage, when the CME laterally expands at speeds of ∼1000 km/s, followed by (2) a shorter and slower expansion stage of a few minutes and ending with (3) a self-similar phase that carries the CME into the middle corona. The first two stages coincide with the impulsive phase of the accompanying flare, the formation and separation of an EUV wave from the CME, and the start of the metric type II radio burst. Our 3D analysis suggests that the hyper-inflation phase may be a crucial stage in the CME formation with wide-ranging implications for solar eruption research. It likely represents the formation stage of the magnetic structure that is eventually ejected into the corona, as the white-light CME. It appears to be driven by the injection of poloidal flux into the ejecting magnetic structure, which leads to the lateral (primarily) growth of the magnetic flux rope. The rapid growth results in the creation of EUV waves and eventually shocks at the CME flanks that are detected as metric type II radio bursts. In other words, the hyper-inflation stage in the early CME evolution may be the "missing" link between CMEs, flares, and coronal shocks.

Solar coronal waves↗
NASA NTRS2014

Constraints On the Emission Geometries and Spin Evolution Of Gamma-Ray Millisecond Pulsars

Millisecond pulsars (MSPs) are a growing class of gamma-ray emitters. Pulsed gamma-ray signals have been detected from more than 40 MSPs with the Fermi Large Area Telescope (LAT). The wider radio beams and more compact magnetospheres of MSPs enable studies of emission geometries over a broader range of phase space than non-recycled radio-loud gamma-ray pulsars. We have modeled the gamma-ray light curves of 40 LAT-detected MSPs using geometric emission models assuming a vacuum retarded-dipole magnetic field. We modeled the radio profiles using a single-altitude hollow-cone beam, with a core component when indicated by polarimetry; however, for MSPs with gamma-ray and radio light curve peaks occurring at nearly the same rotational phase, we assume that the radio emission is co-located with the gamma rays and caustic in nature. The best-fit parameters and confidence intervals are determined using amaximum likelihood technique.We divide the light curves into three model classes, with gamma-ray peaks trailing (Class I), aligned (Class II), or leading (Class III) the radio peaks. Outer gap and slot gap (two-pole caustic) models best fit roughly equal numbers of Class I and II, while Class III are exclusively fit with pair-starved polar cap models. Distinguishing between the model classes based on typical derived parameters is difficult. We explore the evolution of the magnetic inclination angle with period and spin-down power, finding possible correlations. While the presence of significant off-peak emission can often be used as a discriminator between outer gap and slot gap models, a hybrid model may be needed.

pulsars: general↗
NASA NTRS2014

Phase Equilibrium Experiments on Potential Lunar Core Compositions: Extension of Current Knowledge to Multi-Component (Fe-Ni-Si-S-C) Systems

Numerous geophysical and geochemical studies have suggested the existence of a small metallic lunar core, but the composition of that core is not known. Knowledge of the composition can have a large impact on the thermal evolution of the core, its possible early dynamo creation, and its overall size and fraction of solid and liquid. Thermal models predict that the current temperature at the core-mantle boundary of the Moon is near 1650 K. Re-evaluation of Apollo seismic data has highlighted the need for new data in a broader range of bulk core compositions in the PT range of the lunar core. Geochemical measurements have suggested a more volatile-rich Moon than previously thought. And GRAIL mission data may allow much better constraints on the physical nature of the lunar core. All of these factors have led us to determine new phase equilibria experimental studies in the Fe-Ni-S-C-Si system in the relevant PT range of the lunar core that will help constrain the composition of Moon's core.

Righter, K.↗
NASA NTRS1979

A search for the interplanetary quantity controlling the development of geomagnetic storms

An historical account is presented concerning the evolution of our present concept of geomagnetic storms. The present concept was formulated by Chapman (1927) in his magnetic data statistical studies of 'the initial rise' (now termed the initial phase) 'and subsequent larger decrease' (now termed the main phase) in H, followed by 'slow recovery'. The concept introduced by Alfven in 1940 of guiding center motions of a charged particle in a nonuniform magnetic field (ring currents) is also discussed. By 1963 it became quite certain that the ring current, namely a storm-time Van Allen belt, is formed in the magnetosphere during the storm's main phase. The search then began for the solar wind quantity controlling the development of the main phase. The author then gives a personal account of how our concept of geomagnetic storms has advanced and how new findings based on satellite and ground-based observations have made it possible to arrive at a first-approximation expression for the interplanetary quantity controlling the development of geomagnetic storms. Since a geomagnetic storm is a magnetic manifestation of a magnetospheric storm, which is a nonlinear superposition of intense magnetospheric substorms, the main emphasis is shifted toward the understanding of magnetospheric substorms in order to arrive at the parameters controlling the development of geomagnetic storms.

Akasofu, S.-I.↗
NASA NTRS1982

Research on the noise produced by large scale structure in supersonic jets

Using a dual electrode glow discharge excitation device, a method of mode selection into either the n = 0 or n = + or - 1 azimuthal modes was developed and tested by Fourier analyzing experimental azimuthal phase and amplitude data. On a M = 2.1 moderate Reynolds number (Re = 68,000) jet with a laminar boundary layer, under n = 0 or n = +1 excitation, coherent axial wave evolution, mean flow, and sound pressure level directivity measurements were performed. The boundary layer of this same jet was excited usng a grit coating on the nozzle wall, and bandpassed axial wave evolution data obtained and compared to the masurements made by previous workers on a conventional high Reynolds number jet (Re = 5,000,000) using a hot-film type probe.

Mclaughlin, D. K.↗
NASA NTRS1989

Damage analysis of a crack layer

Damage analysis of a crack layer in polystyrene is carried out by employing optical microscopy and principles of quantitative stereology. The results show that, within the quasistatic phase of crack layer propagation, the average crazing density, along the trailing edge of the active zone, is constant. This is consistent with a self-similarity hypothesis of damage evolution employed by the crack layer theory. The average crazing densities within the active zone and along its trailing edge are found to be practically equal. A layer of constant crazing density, adjacent to the crack planes, accompanies the crack during its quasi-static growth. This suggests that: (1) a certain level of crazing density should be reached, around the crack tip, prior to crack advance; (2) the specific energy, associated with this 'core' of damage, could be considered as a Griffith's type energy. The results are in favor of certain hypothesis adopted by the crack layer theory.

Botsis, J.↗
NASA NTRS1990

Space Station Freedom commercial infrastructure

Several approaches to initiating the provision of the Space Station Freedom (SSF) commercial infrastructure are discussed, including proposals from the private sector, the commercial development of infrastructure, and the commercial operation of infrastructure. Specific options for SSF commercial infrastructure which are currently being studied by NASA are described. One candidate for commercial service is the supplemental power for SSF beyond the Assembly Complete phase. The methods which a company could use in providing supplemental power are discussed, with special attention given to the use of solar dynamic power elements attached ot the SSF evolution structure. Another option under evaluation is commercial provision of SSF logistics services using ELVs.

Barquinero, Kevin↗