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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.

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All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Quasi-static magnetospheric MHD processes and the 'ground state' of the magnetosphere

The possible existence of a theoretically well-defined ground state of the earth's magnetosphere is discussed in the context of magnetic equilibrium and convection theories. The quasi-static MHD theory is reviewed and a two-dimensional model is used to show that the convecting magnetosphere can reach a steady state. It is suggested that, under the influence of convection, magnetic substorms occur periodically in the magnetosphere and are an integral part of the entire convection cycle. The concept of the ground state of the magnetosphere is examined. The term 'average magnetosphere', is proposed for defining a baseline configuration that corresponds to average solar wind conditions.

Voigt, G.-H.↗
NASA NTRS2022

Ultraviolet Spectropolarimetry with Polstar: Interstellar Medium Science

Continuum polarization over the UV-to-microwave range is due to dichroic extinction (or emission) by asymmetric, aligned dust grains. Scattering can also be an important source of polarization, especially at short wavelengths. Because of both grain alignment and scattering physics, the wavelength dependence of the polarization, generally, traces the size of the aligned grains. Similarly because of the differing wavelength dependencies of dichroic extinction and scattering polarization, the two can generally be reliably separated. Ultraviolet (UV) polarimetry therefore provides a unique probe of the smallest dust grains (diameter<0.09 μm ), their mineralogy and interaction with the environment. However, the current observational status of interstellar UV polarization is very poor with less than 30 lines of sight probed. With the modern, quantitative and well-tested, theory of interstellar grain alignment now available, we have the opportunity to advance the understanding of the interstellar medium (ISM) by executing a systematic study of the UV polarization in the ISM of the Milky Way and near-by galaxies. The Polstar mission will provide the sensitivity and observing time needed to carry out such a program (probing hundreds of stars in the Milky Way and dozens of stars in the LMC/SMC), addressing questions of dust composition as a function of size and location, radiation- and magnetic-field characteristics as well as unveiling the carrier of the 2175 Å extinction feature. In addition, using high-resolution UV line spectroscopy Polstar will search for and probe the alignment of, and polarization from, aligned atoms and ions - so called “Ground State Alignment”, a potentially powerful new probe of magnetic fields in the diffuse ISM.

B-G Andersson↗
NASA NTRS1980

Pair annihilation in superstrong magnetic fields

The kinematical and dynamical aspects of the annihilation processes in superstrong magnetic fields are studied. The feasibility and potential significance of detecting from magnetic neutron stars are discussed. The discussion proceeds from the derivation of the fully relativistic differential cross sections and annihilation rates for both one- and two-photon emission from a ground-state gas of electrons and positrons in a static, uniform magnetic field.

Daugherty, J. K.↗
NASA NTRS1983

Pair Production near Threshold in Pulsar Magnetic Fields

In pulsar polar cap models, curvature radiation gamma rays produce e(+)e(-) pairs in the strong magnetic fields near the surface of the neutron star. While these gamma rays have energies E sub gamma mc(2), they also propagate at very small angles to the fields, such that the threshold condition, E gamma 2mc(2)/sin theta is just barely satisfied when they pair produce. Threshold effects on the pair production attenuation coefficient, which are due to the discreteness of the e(+)e(-) Landau states, must therefore be considered when computing the mean free paths of curvature radiation photons in pulsar magnetic fields. These effects, which are not incorporated in the asymptotic expression for the attenuation coefficient, have some interesting consequences for pulsar models. Since pair production is suppressed near threshold, the photon mean free paths are longer than previously thought. In magnetic fields approximately 6x10 approximately G, the pairs tend to be produced in the ground state Landau level and will not synchrotron radiate. Since synchrotron radiation is an essential ingredient in the electromagnetic cascades which produce low energy pairs above the acceleration region, pulsars with very high magnetic fields may not produce many pairs.

Harding, A. K.↗
NASA NTRS1977

Crystal field and magnetic properties

Magnetization and magnetic susceptibility measurements have been made in the temperature range 1.3 to 4.2 K on powdered samples of ErH3. The susceptibility exhibits Curie-Weiss behavior from 4.2 to 2 K, and intercepts the negative temperature axis at theta = 1.05 + or - 0.05 K, indicating that the material is antiferromagnetic. The low field effective moment is 6.77 + or - 0.27 Bohr magnetons per ion. The magnetization exhibits a temperature independent contribution, the slope of which is (5 + or - 1.2) x 10 to the -6th Weber m/kg Tesla. The saturation moment is 3.84 + or - 1 - 0.15 Bohr magnetons per ion. The results can be qualitatively explained by the effects of crystal fields on the magnetic ions. No definitive assignment of a crystal field ground state can be given, nor can a clear choice between cubically or hexagonally symmetric crystal fields be made. For hexagonal symmetry, the first excited state is estimated to be 86 to 100 K above the ground state. For cubic symmetry, the splitting is on the order of 160 to 180 K.

Flood, D. J.↗
NASA NTRS1977

Crystal field and magnetic properties of ErH3

Magnetization and magnetic susceptibility measurements have been made in the temperature range 1.3 to 4.2 K on powdered samples of ErH3. The susceptibility exhibits Curie-Weiss behavior from 4.2 to 2 K, and intercepts the negative temperature axis at 1.05 + or - 0.05 K, indicating that the material is antiferromagnetic. The low field effective moment is 6.77 + or - 0.27 Bohr magnetons per ion. The magnetization exhibits a temperature independent contribution, the slope of which is (5 + or - 1.2) times 10 to the minus 6 Weber m/kg Tesla. The saturation moment is 3.84 + or - 0.15 Bohr magnetons per ion. The results can be qualitatively explained by the effects of crystal fields on the magnetic ions. No definitive assignment of a crystal field ground state can be given, nor can a clear choice between cubically or hexagonally symmetric crystal fields be made. For hexagonal symmetry, the first excited state is estimated to be 86 to 100 K above the ground state. For cubic symmetry, the splitting is of the order of 160 to 180 K.

Flood, D. J.↗
NASA NTRS2009

Non-ambipolar radio-frequency plasma electron source and systems and methods for generating electron beams

An electron generating device extracts electrons, through an electron sheath, from plasma produced using RF fields. The electron sheath is located near a grounded ring at one end of a negatively biased conducting surface, which is normally a cylinder. Extracted electrons pass through the grounded ring in the presence of a steady state axial magnetic field. Sufficiently large magnetic fields and/or RF power into the plasma allow for helicon plasma generation. The ion loss area is sufficiently large compared to the electron loss area to allow for total non-ambipolar extraction of all electrons leaving the plasma. Voids in the negatively-biased conducting surface allow the time-varying magnetic fields provided by the antenna to inductively couple to the plasma within the conducting surface. The conducting surface acts as a Faraday shield, which reduces any time-varying electric fields from entering the conductive surface, i.e. blocks capacitive coupling between the antenna and the plasma.

Hershkowitz, Noah↗
NASA NTRS2011

Non-ambipolar radio-frequency plasma electron source and systems and methods for generating electron beams

An electron generating device extracts electrons, through an electron sheath, from plasma produced using RF fields. The electron sheath is located near a grounded ring at one end of a negatively biased conducting surface, which is normally a cylinder. Extracted electrons pass through the grounded ring in the presence of a steady state axial magnetic field. Sufficiently large magnetic fields and/or RF power into the plasma allow for helicon plasma generation. The ion loss area is sufficiently large compared to the electron loss area to allow for total non-ambipolar extraction of all electrons leaving the plasma. Voids in the negatively-biased conducting surface allow the time-varying magnetic fields provided by the antenna to inductively couple to the plasma within the conducting surface. The conducting surface acts as a Faraday shield, which reduces any time-varying electric fields from entering the conductive surface, i.e. blocks capacitive coupling between the antenna and the plasma.

Hershkowitz, Noah↗
NASA NTRS1974

Technique for direct measurement of magnetic entropy of solids: Results for dysprosium titanium oxide

A measurement technique was devised which permits direct observation of the magnetic entropy of solids as a function of applied magnetic field. Measurements were made of the magnetic entropy, in the temperature range 2 to 20 K, of polycrystalline samples of dysprosium titanium oxide (Dy2Ti2O7) to determine its suitability for use as the working substance of a magnetic refrigerator. Magnetization measurements were also made at 4.2 K and below to provide additional information on the nature of the compound. The measurements indicated that crystalline electric fields perturbed the ground state of the dysprosium ions, removed the 16-fold degeneracy predicted by Hund's rules, and left only a twofold degeneracy in its place. A positive, temperature independent contribution to the magnetization was observed in the saturation region, which indicated that the doublet ground-state wave function was perturbed by a nearby unpopulated upper energy level.

Flood, D. J.↗
NASA NTRS1982

The far-infrared spectrum of the OH radical

It is thought likely that the study of spectral lines in the far-infrared might provide at least as much information about the physics and chemistry of the interstellar environment as radioastronomy. However, by comparison with the microwave region, the far-infrared is largely unexplored. There is a pressing need for good laboratory data to aid searches and assignments of spectra from the interstellar clouds and nebulae. Brown et al. (1981) have conducted a study of the laser magnetic resonance (LMR) spectrum of the OH radical in its ground state at far-infrared wavelengths. The present investigation is concerned with the computation of the frequencies of individual hyperfine transitions involving all rotational levels up to J = 4 1/2. The results of the calculation are presented in a table. The results are summarized in a diagram which shows the low-lying energy levels of OH. The frequencies of transitions between levels studied directly in the LMR spectrum are quite reliable.

Brown, J. M.↗
NASA NTRS1983

Pair production near threshold in pulsar magnetic fields

In Pulsar polar cap models, curvature radiation gamma-rays produce e(+) e(-) pairs in the strong magnetic fields near the surface of the neutron star. While these gamma rays havea energies E sub gamma mc(2), they also propagate at very small angies to the fields, such that the threshold condition, E gamma 2mc(2)/sin theta is just barely satisfied when they pair produce. Threshold effects on the pair production attenuation coefficient, which are due to the discreteness of the e(+) e(-) Landau states, must therefore be considered when computing the mean free paths of curvature radiation photons in pulsar magnetic fields. These effects, which are not incorporated in the asymptotic expression for the attenuation coefficient, have some interesting consequences for pulsar models. Since pair production is suppressed near threshold, the photon mean free paths are longer than previously thought. In magnetic fields greater than approximately 6 x 10 to the 12th G, the pairs tend to be produced in the ground state Landau level and will not synchrotron radiate. Since synchrotron radiation is an essential ingredient in the eiectromagnetic cascades which produce low energy pairs above the acceleration region, pulsars with very high magnetic fields may not produce many pairs.

Harding, A. K.↗
NASA NTRS1986

Electron-impact ionization of laser-excited Ba-138 (... 5p6 6s 6p) and Ba-138 (... 5p6 6s 5d) atoms

Electron-impact ionization cross sections for laser-excited Ba-138 (... 5p6 6s 6p; 1P1, M = - 1) and cascade-populated Ba-138 (... 5p6 6s 5d; 1D + 3D) atoms were measured in the threshold to 10 eV energy range. The peak cross sections for the excited species are about a factor of 2 larger than that for ground-state Ba. In addition, it was demonstrated that ionization from individual magnetic substates of various hyperfine levels can be studied. The ionization cross sections in the case of Ba-138 (1P1) were found to be equal for the M = 0 and for the M = + or - 1 sublevels within the experimental error limit.

Trajmar, S.↗
NASA NTRS1974

Magnetization and magnetic entropy of Dy2Ti2O7

Isothermal measurements of the magnetic entropy, magnetization, and differential susceptibility of Dy2Ti2O7 have been made. Absolute values of magnetic entropy have been determined as a function of applied magnetic field for several temperatures in the range 2-20 K, and several isofield lines in the magnetic entropy-temperature plane have been determined. Magnetization measurements, which extended into the saturation region, yield a powder-average magnetic moment of about 4.7 Bohr magnetons per ion. A van Vleck contribution to M was also observed. The results are consistent with the assumption of a nearly pure Jz equals + or - 15/2 ground-state Kramers doublet for the Dy(+++) ion. Comments on the suitability of Dy2Ti2O7 for use in a magnetic refrigerator cycling between 4.2 and 20 K are included.

Flood, D. J.↗
NASA NTRS1992

Magnetic cloud passage at Earth and associated substorm activity

An approach to the study of the solar wind-magnetosphere interaction by signal type, that is, by examining the effect in the magnetosphere of well defined interplanetary structures, is presented. Focus is on the response of the magnetosphere to interplanetary magnetic clouds. Among their properties are: the slow and smooth variation of the magnetic field vector, with fluctuation level well below common interplanetary values; the similarly well behaved bulk flow; the wide range of field and flow parameters; and the longevity of passage (1 to 2 days). If the magnetic cloud is oriented such that a long period of uninterruptedly northward pointing field is followed by a long interval of continuously southward pointing field, then the transition of the magnetosphere from a quiescent state (the 'ground state') to a very active state can be studied, the latter being sustained by continued forcing from the magnetic cloud. A synopsis of the main findings of a recent study in such an interaction is given, concentrating on the substorm activity attending the second part of cloud passage.

Farrugia, C. J.↗
NASA NTRS1963

Command and telemetry systems

The command and telemetry portions of the Telstar system provide necessary support functions for the basic communications experiment and the radiation experiment. By means of the command system, the states of 9 magnetic latching relays in the satellite are controlled from the ground. Commands are sent to the satellite by coded signals modulated on a carrier in the VHF band. The telemetry system also uses a VHF carrier to transmit encoded information from the satellite. Data on 112 items are provided once each minute. This paper discusses the over-all command and telemetry systems and considers the general objectives, system aspects and detailed implementation.

TELEMETRY↗
NASA NTRS1996

State modeling and pass automation in spacecraft control

The integrated monitoring and control commercial off-the-shelf system (IMACCS), which demonstrates the feasibility of automating spacecraft monitoring and control activities through the use of state modeling, is described together with its use. The use of the system for the control and ground support of the solar, anomalous and magnetic particle explorer (SAMPEX) spacecraft is considered. A key component of IMACCS is the Altair mission control system which implements finite state modeling as an element of its expert system capability. Using the finite state modeling and state transition capabilities of the Altair mission control system, IMACCS features automated monitoring, routine pass support, anomaly resolution and emergency 'lights on again' response. Automatic orbit determination and the production of typical flight dynamics products exists. These functionalities are described.

Klein, J.↗
NASA NTRS1973

Satellite studies of magnetospheric substorms on August 15, 1968. I - State of the magnetosphere.

The state of the magnetosphere on August 15, 1968, as defined by magnetic indices and ground magnetograms, is described. Onset times of various phases of two magnetospheric substorms are determined. These substorms occurred while the OGO 5 satellite was inbound on the midnight meridian through the cusp region of the geomagnetic tail. It is concluded that at least two worldwide substorm expansions were preceded by growth phases.

Mcpherron, R. L.↗