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At least 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

The Nonlinear Coupling of Alfven and Lower Hybrid Waves in Space Plasma

Space plasmas support a wide variety of waves, and wave-particle interactions as well as wave-wave interactions which are of crucial importance to magnetospheric and ionospheric plasma behavior. The excitation of lower hybrid waves (LHWs) in particular is a widely discussed mechanism of interaction between plasma species in space and is one of the unresolved questions of magnetospheric multi-ion plasmas. It is demonstrated that large-amplitude Alfven waves may generate LHWs in the auroral zone and ring current region and in some cases (particularly in the inner magnetosphere) this serves as the Alfven wave saturation mechanism. We present several examples of observational data which illustrate that the proposed mechanism is a plausible candidate to explain certain classes of LHW generation events in the ionosphere and magnetosphere and demonstrate electron and ion energization involving these processes. We discuss the morphology dynamics and level of LHW activity generated by electromagnetic ion cyclotron (EMIC) waves during the May 2-7 1998 storm period on the global scale. The LHWs were calculated based on a newly developed self-consistent model (Khazanov et. al. 2002) that couples the system of two kinetic equations: one equation describes the ring current (RC) ion dynamic and another equation describes the evolution of EMIC waves. It is found that the LHWs are excited by helium ions due to their mass dependent drift in the electric field of EMIC waves. The level of LHW activity is calculated assuming that the induced scattering process is the main saturation mechanism for these waves. The calculated LHWs electric fields are consistent with the observational data.

Khazanov, George V.↗
NASA NTRS1987

Pointing compensation system for spacecraft instruments

A closed loop system reduces pointing errors in one or more spacecraft instruments. Associated with each instrument is a electronics package (3) for commanding motion in that instrument and a pointing control system (5) for imparting motion in that instrument in response to a command (4) from the commanding package (3). Spacecraft motion compensation logic (25) compensates for instrument pointing errors caused by instrument-motion-induced spacecraft motion. Any finite number of instruments can be so compensated, by providing each pointing control system (5) and each commanding package (3), for the instruments desired to be compensated, with a link to the spacecraft motion compensation logic (25). The spacecraft motion compensation logic (25) is an electronic manifestation of the algebraic negative of a model of the dynamics of motion of the spacecraft. An example of a suitable model, and computer-simulated results, are presented.

Plescia, Carl T.↗
NASA NTRS2017

Marshall Space Flight Center's Solar Wind Facility

Historically, NASA's Marshall Space Flight Center (MSFC) has operated a Solar Wind Facility (SWF) to provide long term particle and photon exposure to material samples. The requirements on the particle beam details were not stringent as the cumulative fluence level is the test goal. Motivated by development of the faraday cup instrument on the NASA Solar Probe Plus (SPP) mission, the MSFC SWF has been upgraded to included high fidelity particle beams providing broadbeam ions, broadbeam electrons, and narrow beam protons or ions, which cover a wide dynamic range of solar wind velocity and flux conditions. The large vacuum chamber with integrated cryo-shroud, combined with a 3-axis positioning system, provides an excellent platform for sensor development and qualification. This short paper provides some details of the SWF charged particle beams characteristics in the context of the Solar Probe Plus program requirements. Data will be presented on the flux and energy ranges as well as beam stability.

Wright, K. H.↗
NASA NTRS2000

Molecular Dynamics Simulations of the Thermal Conductivity of Single-Wall Carbon Nanotubes

Carbon nanotubes (CNT) have very attractive electronic, mechanical. and thermal properties. Recently, measurements of thermal conductivity in single wall CNT mats showed estimated thermal conductivity magnitudes ranging from 17.5 to 58 W/cm-K at room temperature. which are better than bulk graphite. The cylinderical symmetry of CNT leads to large thermal conductivity along the tube axis, additionally, unlike graphite. CNTs can be made into ropes that can be used as heat conducting pipes for nanoscale applications. The thermal conductivity of several single wall carbon nanotubes has been calculated over temperature range from l00 K to 600 K using non-equilibrium molecular dynamics using Tersoff-Brenner potential for C-C interactions. Thermal conductivity of single wall CNTs shows a peaking behavior as a function of temperature. Dependence of the peak position on the chirality and radius of the tube will be discussed and explained in this presentation.

Osman, M.↗
NASA NTRS2010

Free-Free Transitions in the Presence of Laser Fields at Very Low Incident Electron Energy

We study the free-free transition in electron-hydrogenic systems in ground state in presence of an external laser field at very loud incident energies. The laser field is treated classically while the collision dynamics is treated quantum mechanically. The laser field is chosen to be monochromatic, linearly polarized and homogeneous. The incident electron is considered to be dressed by the laser in a nonperturbative manner by choosing a Volkov wave function for it. The scattering weave function for the electron is solved numerically by taking into account the effect of the electron exchange, short-range as well as of the long-range interactions to get the S and P wave phase shifts while for the higher angular momentum phase shifts the exchange approximation has only been considered. We calculate the laser assisted differential cross sections (LADCS) for the aforesaid free-free transition process for single photon absorption/emission. The laser intensity is chosen to be much less than the atomic field intensity. A strong suppression is noted in the LADCS as compared to the field free (FF) cross sections. Unlike the FF ones, the LADCS exhibit some oscillations having a distinct maximum at a low value of the scattering angle depending on the laser parameters as well as on the incident energies.

Bhatia, A. K.↗
NASA NTRS2009

Free-Free Transitions in the Presence of Laser Fields at Very Low Incident Electron Energy

We study the free-free transition in electron-hydrogenic systems in ground state in presence of an external laser field at very low incident energies. The laser field is treated classically while the collision dynamics is treated quantum mechanically. The laser field is chosen to be monochromatic, linearly polarized and homogeneous. The incident electron is considered to be dressed by the laser in a nonperturbative manner by choosing a Volkov wave function for it The scattering wave function for the electron is solved numerically by taking into account the effect of the electron exchange, short-range as well as of the long-range interactions to get the S and P wave phase shifts while for the higher angular momentum phase shifts, the exchange approximation has only been considered. We calculate the laser-assisted differential cross sections (LADCS) for the aforesaid free-free transition process for single photon absorption/emission. The laser intensity is chosen to be much less than the atomic field intensity. A strong suppression is noted in the LADCS as compared to the field free (FF) cross sections. Unlike the FF ones, the LADCS exhibit some oscillations having a distinct maximum at a low value of the scattering angle depending on the laser parameters as well as on the incident energies.

Bhatia, Anand K.↗
NASA NTRS1985

Polar plasmas as observed by Dynamics Explorers 1 and 2

Plasma measurements from the Dynamics Explorer 1 and 2 satellites were used to characterize the polar cap environment. Analysis of numerous polar-cap passes indicate that, in general, three major regimes of plasma exist: (1) polar rain--electrons with magnetosheath-like energy spectra but much lower densities, most intense near the cusp and weakening toward the central polar cap; (2) polar wind--low energy upward flowing ions with both field-aligned and conical distributions; and (3) acceleration events--sporadic events consistent with the acceleration of electrons and positive ions by parallel electric fields. (1) to (3) were observed at high altitudes by Dynamics Explorer 1, while (1) and (3) were also observed at low altitudes by Dynamics Explorer 2. The plasma parameters associated with these plasma regimes are presented and discussed in terms of source and acceleration mechanisms.

Barfield, J. N.↗
NASA NTRS2009

Virtex-4VQ dynamic and mitigated single event upset characterization summary

This report is the result of funding by the NASA Electronic Parts and Packaging Program (NEPP) and the combined efforts of members within the Xilinx Radiation Test Consortium (XRTC), sometimes known as the Xilinx Single Event Effects (SEE) Test Consortium. The XRTC is a voluntary association of aerospace entities, including leading aerospace companies, universitie and national laboratories, combining resources to characterize reconfigurable, field programmable gate arrays (FPGAs) for aerospace applications. Previous publications of Virtex-4 radiation results are for commercial (non-epitaxial) devices; see, for example, Refs. 1–5. A notable exception is Ref. 6, which presents XRTC upset measurements of storage elements in the PowerPC405s in the XQR4VFX60. This work represents a continuation of the efforts reported in the “Virtex-4QV Static SEU Characterization Summary” [7]. The contents of this report describe various Single Event Functional Interrupt (SEFI) and Single Event Upset (SEU) modes seen while dynamically exercising the clocked resources within Virtex-4 devices and the corresponding mitigation techniques related to the aforementioned observed SEFI modes.

Allen, Gregory↗
NASA NTRS1990

Solar cycle variations in F-region Te in the vicinity of the midlatitude trough based on AE-C measurements at solar minimum and DE-2 measurements at solar maximum

Magnetospheric energy deposited in the plasmasphere produces large enhancements in the electron temperature in the nightside ionosphere at the foot of the geomagnetic L shell that traverses the plasmapause. This temperature peak, which is associated with the midlatitude trough in electron density, often has a great enough amplitude to produce 630 nm emission known as a Sar-arc. The Atmosphere Explorer-C measurements made at solar minimum and the Dynamics Explorer-2 measurements made at solar maximum are used to illustrate how this signature of F-region electron heating changes with solar activity. Global empirical models of the electron temperature and density have not been able to resolve these features thus far because of their large movements with geomagnetic activity and because of the large bin sizes used in the models. It is not yet clear how this major feature of the F-region temperature structure could be included easily in the IRI model.

Brace, Larry H.↗
NASA NTRS1990

Spatial relationships between region 2 field-aligned currents and electron and ion precipitation in the evening sector

The equatorward cutoff of ion and electron precipitation in relation to the evening region 2 field-aligned current during isolated substorms has been investigated using the magnetic field and plasma data obtained from the Dynamics Explorer 2 satellite. The equatorward boundaries of the region 2 currents relative to those of central plasma sheet (CPS) electron precipitation are determined predominantly by magnetic local time and subsequently change with substorm phases. With approaching midnight, the equatorward boundary of CPS electron precipitation extends toward and eventually equatorward of that of the region 2 current. On the other hand, the equatorward boundary of the region 2 current coincides well with that of 10-20 keV ion precipitation during the whole course of substorms. It is proposed that these ions originate in the so-called Alfven layer and that the location of this inner boundary determines the lower latitude boundary of the region 2 current.

Fujii, R.↗
NASA NTRS1984

Electrostatic hiss and the beam driven electron acoustic instability in the dayside polar cusp

In this study it is shown that the upward moving electron beams observed by DE-1 in the dayside polar cusp at 2 to 5 R sub E drive an electrostatic electron acoustic mode, rather than the whistler mode near resonance, as previously concluded. The characteristics of this instability are compared with the properties of hiss in the polar cusp observed by the Dynamics Explorer (DE-1) satellite. The hiss frequencies and funnel shaped frequency-time spectra are consistent with the electron acoustic mode. However, because the hiss often has a fluctuating magnetic field component near the axis of the funnel shape, the hiss is probably composed of both electron acoustic and whistler mode waves.

Tokar, R. L.↗
NASA NTRS1989

An investigation of a mathematical model of an optically pumped Ti(3+):Al2O3 laser system

During the last several years, solid state lasers were developed that have the potential for meeting rigorous performance requirements for space-based remote sensing of the atmosphere. In order to design a stable and efficient laser and to understand the effect on laser output of changes in the physical and design parameters, an understanding of the development of the dynamical processes of the laser is necessary. Typically, the dynamical processes in a laser system are investigated via rate equations describing the evolution of the occupancy in the electronic levels and of the photon density in the laser cavity. There are two approaches to this type of study. Most often, for the sake of simplicity, the spatial variations of the dynamic variables in the laser system are disregarded and the mathematical model consists of a system of first order nonlinear ordinary differential equations (ODE). The second approach is to take into account both spatial and temporal variations in the dynamic variables in the laser cavity. The resulting model consists of a first order semilinear system of partial differential equations (PDE). The model which was studied was studied was generic in the sense that it was a four-level laser system, but the parameters used in the numerical study were specific to Titanium-doped sapphire. For simplicity, a constant, spatially uniform pumping scheme was considered. In addition, a simplification of the model was made so that it treats a single lasing wavelength with a narrow bandwidth. The purpose was to investigate both versions of the mathematical model and to determine whether the numerical solutions are similar both qualitatively and quantitatively. The systems of ordinary differential equations were solved numerically using a Runge-Kutta-Fehlberg algorithm which was very efficient for typical values of the physical parameters. A numerical scheme, based on the Modified Euler method, for computing solutions to the system of partial differential equations was developed and implemented. The PDE model was solved numerically at the expense of greatly increased computer time.

Roberts, Lila F.↗
NASA NTRS1984

F layer ionization patches in the polar cap

Ground-based optical and digital ionosonde measurements were conducted at Thule, Greenland to measure ionospheric structure and dynamics in the nighttime polar cap F layer. These observations showed the existence of large-scale (800-1000 km) plasma patches drifting in the antisunward direction during a moderately disturbed (Kp greater than or equal to 4) period. Simultaneous Dynamics Explorer (DE-B) low-altitude plasma instrument (LAPI) measurements show that these patches with peak densities of about 10 to the 6th el per cu cm are not locally produced by structured particle precipitation. The LAPI measurements show a uniform precipitation of polar rain electrons over the polar cap. The combined measurements provide a comprehensive description of patch structure and dynamics. They are produced near or equatorward of the dayside auroral zone and convect across the polar cap in the antisunward direction. Gradients within the large scale, drifting patches are subject to structuring by convective instabilities. UHF scintillation and spaced receiver measurements are used to map the resulting irregularity distribution within the patches.

Weber, E. J.↗
NASA NTRS1967

Dynamical friction.

Chandrasekhar dynamical friction theory for star in cluster adapted to Fokker-Plank electron in two-component plasma theory and including multiple encounters

STAR CLUSTER↗
NASA NTRS1968

Technique increases storage capacity in camera tube target

Technique increases the signal current, where direct beam readout is used, in Secondary Electron Conduction /SEC/ camera tubes. Increasing the storage capacity and therefore the dynamic range of the SEC target permits satisfactory operation at reduced frame rates.

Boll, K. F.↗
NASA NTRS1969

Charged particles in the magnetosphere.

Charged particle population dynamics in outer radiation zone of magnetosphere, discussing ring current particles, protons, electrons, alpha particles and electric field

Van Allen, J. A.↗
NASA NTRS1977

Strong convective and shock wave behavior in solar flares

A model has been developed to study the gasdynamics of a flare region heated by a stream of energetic electrons. It is shown that the energy deposition can introduce strong chromospheric dynamical effects. As a result of fluid motion into rarefied regions, there is considerable redistribution of mass causing a profound influence on the emitted line radiation.

Bloomberg, H. W.↗
NASA NTRS1988

Adiabatic criteria for outer-sphere bimolecular electron-transfer reactions

A model is presented for outer-sphere bimolecular electron-transfer reactions which is correct in the adiabatic, nonadiabatic, and intermediate dynamical regimes for an overdamped solvent coordinate. From this model, the conditions for the transfer to be adiabatic or nonadiabatic are deduced. The time-scale separations needed to adequately describe the process as an average over (distant dependent) unimolecular rates are described.

Onuchic, Jose Nelson↗