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At least 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
NASA NTRS2019

The MERiT Onboard the CeREs: A Novel Instrument to Study Energetic Particles in the Earth's Radiation Belts

The Miniaturized Electron pRoton Telescope, MERiT, is a low‐mass, low‐power, compact instrument using an innovative combination of particle detectors, sensor electronics, and onboard processing. MERiT is flying on the Compact Radiation belt Explorer, CeREs, a 3U CubeSat launched into a low earth orbit of 500‐km altitude and inclination of 85° on 16 December 2018. The primary and secondary science goals of CeREs are to investigate electron microbursts and to study solar particles. MERiT comprises a stack of solid state detectors (SSD) behind space facing avalanche photo diodes (APDs) surrounded by W‐Al shielding to reduce side‐penetrating particle background. The APD‐SSD combination enables measurement of electrons from 5 to 200 keV and 1 to 8 MeV; protons from 200–400 keV and 7–100 MeV in differential channels with energy resolution ΔE/E≈30% for both electrons and protons. MERiT measures microbursts with a high time resolution ranging from 4 to 16 ms and solar particles with a cadence of 1 s. MERiT energy channels and cadences are software configurable via algorithms and lookup tables residing on a field‐programmable gate array. The lookup tables can be changed via ground commands. MERiT geometry factor is 31 sq.cm‐sr and optimized to measure microbursts with the instrument viewing the local zenith in orbit. MERiT enables investigation of dynamical processes of radiation belt electron energization and loss, solar electron and proton transport, and their access to the Earth's polar caps. We describe the MERiT sensor design, calibration, operational modes, data products, and science goals.

Kanekal, S. G.↗
NASA NTRS2014

The Origin of the Local 1/4-KeV X-Ray Flux in Both Charge Exhange and a Hot Bubble

The solar neighbourhood is the closest and most easily studied sample of the Galactic interstellar medium, an understanding of which is essential for models of star formation and galaxy evolution. Observations of an unexpectedly intense diffuse flux of easily absorbed 1/4-kiloelectronvolt X-rays coupled with the discovery that interstellar space within about a hundred parsecs of the Sun is almost completely devoid of cool absorbing gas, led to a picture of a 'local cavity' filled with X-ray-emitting hot gas, dubbed the local hot bubble. This model was recently challenged by suggestions that the emission could instead be readily produced within the Solar System by heavy solar-wind ions exchanging electrons with neutral H and He in interplanetary space, potentially removing the major piece of evidence for the local existence of million-degree gas within the Galactic disk. Here we report observations showing that the total solar wind charge-exchange contribution is approximately 40 percent of the 1/4-keV flux in the Galactic plane. The fact that the measured flux is not dominated by charge exchange supports the notion of a million-degree hot bubble extending about a hundred parsecs from the Sun.

Charge exhange and a hot bubble↗
NASA NTRS1973

An auroral breakup mechanism

A purely growing electrostatic drift instability driven by the electron temperature gradient at the inner edge of the plasma sheet can grow for large enough values of the temperature gradient. The parallel electric field associated with the instability is localized near the magnetic equator. The growth of the drift instability leads to enhanced whistler noise and increased electron pitch angle diffusion. If the current limit is exceeded in the ionosphere while the parallel electric field of the drift instability exists along the field line, rapid electron precipitation (the auroral breakup) can result.

Maggs, J. E.↗
NASA NTRS2003

Two-Dimensional Transport Studies for the Composition and Structure of the Io Plasma Torus

The overall objective of this project is to investigate the roles of local and spatially extended plasma sources created by Io, plasma torus chemistry, and plasma convective and diffusive transport in producing the long-lived S(+), S(++) and O(+) radial ribbon structures of the plasma torus, their System III longitude and local-time asymmetries, their energy sources and their possible time variability. To accomplish this objective, two-dimensional [radial (L) and System III longitude] plasma transport equations for the flux-tube plasma content and energy content will be solved that include the convective motions for both the east-west electric field and co-rotational velocity-lag profile near Io s orbit, radial diffusion, and the spacetime dependent flux-tube production and loss created by both neutral-plasma and plasma-ion reaction chemistry in the plasma torus. For neutral-plasma chemistry, the project will for the first time undertake the calculation of realistic three-dimensional, spatially-extended, and time-varying contributions to the flux-tube ion-production and loss that are produced by Io's corona and extended neutral clouds. The unknown two-dimensional spatial nature of diffusion in the plasma transport will be isolated and better defined in the investigation by the collective consideration of the foregoing different physical processes. For energy transport, the energy flow from hot pickup ions (and a new electron source) to thermal ions and electrons will be included in investigating the System III longitude and local-time temperature asymmetries in the plasma torus. The research is central to the scope of the NASA Sun-Earth Connection Roadmap in Quest II Campaign 4 "Comparative Planetary Space Environments" by addressing key questions for understanding the magnetosphere of planets with high rotation rates and large internal plasma sources and, in addition, is of considerable importance to the NASA Solar System Exploration Science Theme. In this regard, Jupiter is the most extreme example with its rapid rotation and with its inner Galilean satellite Io providing the dominant plasma source for the magnetosphere.

Smyth, William H.↗
NASA NTRS1994

Regulation of the solar wind electron heat fluxfrom 1 to 5 AU: Ulysses observations

In this study we use observations from the three-dimensional electron spectrometer and magnetometer aboard the Ulysses spacecraft to examine the solar wind electron heat flux from 1.2 to 5.4 AU in the ecliptic plane. Throughout Ulusses' transit to 5.4 AU, the electron heat flux decreases more rapidly (approximately R(exp -30)) than simple collisionless expansion along the local magnetic field and is smaller than expected for a thermal gradient heat flux, q(sub parallel e) (r) = - Kappa(sub parallel) del(sub parallel) T(sub e)(r). The radial gradients and magnitudes expected for a number of electron heat flux regulatory mechanisms are examined and compared to the observations. The best agreement is found for heat flux regulation by the whistler heat flux instability. The upper bound and radial scaling for the electron heat flux predicted for the whistler heat flux instability are consistent with the observations.

Scime, Earl E.↗
NASA NTRS2012

In-Flight Boundary-Layer Transition on a Large Flat Plate at Supersonic Speeds

A flight experiment was conducted to investigate the pressure distribution, local flow conditions, and boundary-layer transition characteristics on a large flat plate in flight at supersonic speeds up to Mach 2.0. The primary objective of the test was to characterize the local flow field in preparation for future tests of a high Reynolds number natural laminar flow test article. The tests used a F-15B testbed aircraft with a bottom centerline mounted test fixture. A second objective was to determine the boundary-layer transition characteristics on the flat plate and the effectiveness of using a simplified surface coating for future laminar flow flight tests employing infrared thermography. Boundary-layer transition was captured using an onboard infrared imaging system. The infrared imagery was captured in both analog and digital formats. Surface pressures were measured with electronically scanned pressure modules connected to 60 surface-mounted pressure orifices. The local flow field was measured with five 5-hole conical probes mounted near the leading edge of the test fixture. Flow field measurements revealed the local flow characteristics including downwash, sidewash, and local Mach number. Results also indicated that the simplified surface coating did not provide sufficient insulation from the metallic structure, which likely had a substantial effect on boundary-layer transition compared with that of an adiabatic surface. Cold wall conditions were predominant during the acceleration to maximum Mach number, and warm wall conditions were evident during the subsequent deceleration. The infrared imaging system was able to capture shock wave impingement on the surface of the flat plate in addition to indicating laminar-to-turbulent boundary-layer transition.

Banks, Daniel W.↗
NASA NTRS1999

Transmission Through Carbon Nanotubes with Polyhedral Caps

We study electron transport between capped carbon nanotubes and a substrate, and relate this transport to the local density of states in the cap. Our results show that that the transmission probability mimics the behavior of the density of states at all energies except those that correspond to localized states. For a capped carbon nanotube that is not connected to a substrate, the localized states do not couple to the coexisting continuum states. However, close proximity of a substrate causes hybridization between these states. As a result, new transmission paths open from substrate states to nanotube continuum states via the localized states in the cap. We show that the interference between various paths gives rise to transmission antiresonances with the minimum equal to zero at the energy of the localized state. The presence of defects in the tube places close to the cap transforms antiresonances into resonances. Depending on the spatial position of defects, these resonant states are capable of carrying a large current. The results of this paper are of relevance to carbon nanotube based studies on molecular electronics and probe tip applications.

Anantram, M. P.↗
NASA NTRS1982

Observation and modeling of energetic particles at synchronous orbit on July 29, 1977

In the 12 hours immediately after a worldwide storm sudden commencement at 0027 UT on July 29, there was a series of at least four magnetospheric substorms, the last and largest of which exhibited an expansion phase onset at approximately 1200 UT. Data from six spacecraft in three general local time groupings (0300, 0700, and 1300 LT) are examined, and vector magnetic field data and energetic electron and ion data from approximately 15 keV to more than 2MeV are employed. Four primary types of studies are carried out: (1) timing and morphology of energetic particle injections; (2) variation of particle phase space densities, using local magnetic field and particle flux data; (3) measurement of boundary motions, using high-energy ion gradient anisotropies; and (4) adiabatic modeling, which included injection, large-scale convection, corotation, and gradient drifts. For the 1200 UT substorms, it is concluded that there was a substantial flux dropout in a broad sector near local midnight because of a large-scale boundary motion, followed by a recovery to a predropout configuration.

Baker, D. N.↗
NASA NTRS1991

Hydrogen peroxide and the evolution of oxygenic photosynthesis

Possible pathways for the evolution of oxygenic photosynthesis in the early reducing atmosphere of the earth are discussed. It is suggested that the abiotic production of atmospheric oxidants could have provided a mechanism by which locally oxidizing conditions were sustained within spatially confined habitats thus removing the available reductants and forcing photosynthetic organisms to utilize water (rather than ferrous or sulfide ions) as the electron donor. It is argued that atmospheric H2O2 played the key role in inducing oxygenic photosynthesis, because, as peroxide concentrations local environments increased, primitive organisms would not only be faced with a loss of a reductant, but would be also forced to develop a biochemical apparatus (such as catalase) that would protect them against the products of oxygenic photosynthesis. This scenario allows for the early evolution of oxygenic photosynthesis at the time when global conditions were still anaerobic.

Mckay, C. P.↗
NASA NTRS1975

Photoelectron escape from the ionosphere of Jupiter

Photoelectron escape fluxes and ambient electron heating from the Jovian ionosphere are computed as a function of local time and latitude. Several differences for the fluxes expected from a hydrogen atmosphere, rather than a terrestrial type of atmosphere, are described, including an increase in structure in the energy spectra due to the paucity of ionic states entering the photo-ionization processes and lower escape fluxes above 10 eV than were expected from a simple scaling of earth fluxes.

Swartz, W. E.↗
NASA NTRS1977

Surface aspects of pitting and stress corrosion cracking

The pitting and stress corrosion cracking of a stable austenitic stainless steel in aqueous chloride environments were investigated using a secondary ion mass spectrometer as the primary experimental technique. The surface concentration of hydrogen, oxygen, the hydroxide, and chloride ion, magnesium or sodium, chromium and nickel were measured as a function of potential in both aqueous sodium chloride and magnesium chloride environments at room temperature and boiling temperatures. It was found that, under anodic conditions, a sharp increase in the chloride concentration was observed to occur for all environmental conditions. The increase may be associated with the formation of an iron chloride complex. Higher localized chloride concentrations at pits and cracks were also detected with an electron microprobe.

Truhan, J. S., Jr.↗
NASA NTRS1977

Modeling of spacecraft charging

Three types of modeling of spacecraft charging are discussed: statistical models, parametric models, and physical models. Local time dependence of circuit upset for DoD and communication satellites, and electron current to a sphere with an assumed Debye potential distribution are presented. Four regions were involved in spacecraft charging: (1) undisturbed plasma, (2) plasma sheath region, (3) spacecraft surface, and (4) spacecraft equivalent circuit.

Whipple, E. C., Jr.↗
NASA NTRS1978

Geomagnetic field fluctuations at synchronous orbit. II - Radial diffusion

Power spectra of geomagnetic-field variations measured at synchronous equatorial altitude (geomagnetic shell parameter about 6.6) in the magnetosphere are used to calculate the time dependence of the radial diffusion coefficient for particles in the radiation belts. The diffusion coefficients calculated are mainly applicable for relativistic electrons. The magnitudes of the derived diffusion coefficients using data only from local day observations are consistent with those reported from analyses of most particle observations and thus are slightly larger than those derived from magnetic sudden commencements. They are consistent with the diffusion coefficients calculated from power spectra of ground-based geomagnetic data measured near L = 4.

Lanzerotti, L. J.↗
NASA NTRS1987

Hemispheric differences in the morphology of the high latitude ionosphere measured at about 500 km

Ambient electron density measurements made by the radio frequency capacitance probe on Ariel 4 have been analyzed in order to study the structures present at about 500 km altitude at latitudes poleward of + or - 50 deg Lambda during the 1971 Northern Hemisphere winter (December solstice) and the 1972 Southern Hemisphere winter (June solstice); data obtained on 94 days centered on each solstice have been used. The general morphology of, and the extreme densities within, the midlatitude electron density trough, the polar cap depletions, and the electron density enhancements associated with the cusp-auroral zone were determined statistically. Analyses of data acquired during quiet magnetic conditions show that the Northern and Southern Hemisphere winter ionospheres were significantly different; in particular, the Southern Hemisphere densities were lower than those in the Northern Hemisphere. The maximum electron densities observed in the Northern Hemisphere occurred in a magnetic-local-time range symmetrical about the 02-14 MLT meridian, whereas in the Southern Hemisphere the maxima were symmetrical with respect to the midnight-noon magnetic meridian.

Tulunay, Y. K.↗
NASA NTRS1988

The dependence of crack closure on fatigue loading variables

Optical and electron microscopy and stereoimaging techniques are used to determine crack opening loads local to the crack tip in aluminum alloy 7091 and 304 stainless steel. It is found that the crack peels open nonlinearly such that significant load increases are necessary to open the crack the last few microns from the tip. The stress intensity range ratio is shown to depend on fatigue loading variables according to a given relationship.

Hudak, Stephen J., Jr.↗
NASA NTRS1990

An approximate method for the calculation of nonequilibrium radiative heat transfer

A simplified method for the calculation of radiative heat transfer in hypersonic reentry flows where local thermal nonequilibrium (LTNE) conditions exist is presented. This method has been incorporated into the computer code RADMC (RADiation/Modified Corrections), which utilizes an approximate 'two-state' model of the electronic levels of atoms and molecules in the determination of the populations of the excited states for both LTNE and local thermal equilibrium (LTE) conditions for each of the relevant chemical species. The population data are used to calculate a correction factor to the radiative cross sections in an eight-step nongray radiative heat-transfer model. Results have been obtained for various points along the proposed trajectory of the Aeroassist Flight Experiment and the Fire II trajectory, and compared with results obtained for the same flowfields from the NEQAIR radiative heat-transfer code.

Greendyke, Robert B.↗