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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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At least 433 records · Page 24
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NASA NTRS1990

Observations of the earth's polar cleft at large radial distances with the Hawkeye 1 magnetometer

Based on 364 spacecraft passes through the dayside region, the position of the polar cleft at large radial distances was determined with the magnetometer flown on Hawkeye 1. This data set is ideal for the study of the cusp and cleft region. Identification of the cleft depended on noting strong negative deviations of the magnetic field strength in the region from that of the dipole field. In solar magnetic coordinates, cleft observations were found between 40 deg and 70 deg latitude and + or - 75 deg latitude and + or - 75 deg longitude, while in geocentricmagnetospheric coordinates, these observations were found between 20 deg and 75 deg latitude and + or - 75 deg longitude. The extreme longitudinal extent of 150 deg is larger than those reported in some previous studies. Large magnetic depressions associated with the cleft extend out to 12 earth radii.

Farrell, W. M.↗
NASA NTRS1984

Remanent magnetization and 3-dimensional density model of the Kentucky anomaly region

A three-dimensional model of the Kentucky body was developed to fit surface gravity and long wavelength aeromagnetic data. Magnetization and density parameters for the model are much like those of Mayhew et al (1982). The magnetic anomaly due to the model at satellite altitude is shown to be much too small by itself to account for the anomaly measured by Magsat. It is demonstrated that the source region for the satellite anomaly is considerably more extensive than the Kentucky body sensu stricto. The extended source region is modeled first using prismatic model sources and then using dipole array sources. Magnetization directions for the source region found by inversion of various combinations of scalar and vector data are found to be close to the main field direction, implying the lack of a strong remanent component. It is shown by simulation that in a case (such as this) where the geometry of the source is known, if a strong remanent component is present its direction is readily detectable, but by scalar data as readily as vector data.

Mayhew, M. A.↗
NASA NTRS1985

Magnetization models for the source of the 'Kentucky anomaly' observed by Magsat

Both the aeromagnetic data and magnetic anomaly data obtained by Magsat indicate the presence of a very magnetic source region within the crust beneath Kentucky and Tennessee. A source model was previously developed to fit surface gravity and long-wavelength aeromagnetic data, using limited seismic constraint. For the present study the model was further developed, and it is demonstrated that the source region for the satellite anomaly is considerably more extensive than the Kentucky body sensu stricto. The extended source region is modeled using both prismatic model sources and dipole array sources. Magnetization directions for the source region found by inversion of various combinations of scalar and vector data are found to be close to the main field direction, implying the lack of a strong remanent component. It is shown by simulation that in a case (such as this) where the geometry of the source is known, if a strong remanent component is present its direction is determinable, but by scalar data as readily as vector data. Magnetization magnitude for the extended source region is about 3 A/m if the vertical extent of the source includes the whole of the crust.

Mayhew, M. A.↗
NASA NTRS1991

The source surface and photospheric magnetic field models

It is possible to reproduce the configuration of the neutral line on the solar source surface by the axial dipole at the center of the sun and a few fictitious dipoles on the photosphere. An attempt is made to identify the nature of such fictitious dipoles in the photospheric magnetic fields. It is shown that large-scale photospheric dipole fields can be identified clearly at the locations indicated by the fictitious dipoles when the photospheric field is very simple. They are found to be active regions.

Saito, T.↗
NASA NTRS2009

Fundamental Limits on the Imaging and Polarisation Properties of Far-Infrared Detectors

Far-infrared bolometric detectors are used extensively in ground-based and space-borne astronomy, and thus it is important to understand their optical behaviour precisely. We have studied the intensity and polarisation response of free-space bolometers, and shown that when the size of the absorber is reduced below a wavelength, the response changes from being that of a classical optical detector to that of a few-mode antenna. We have calculated the modal content of the reception patterns, and found that for any volumetric detector having a side length of less than a wavelength, three magnetic and three electric dipoles characterize the behaviour. The size of the absorber merely determines the relative strengths of the contributions. The same formalism can be applied to thin-film absorbers, where the induced current is forced to flow in a plane. In this case, one magnetic and two electric dipoles characterize the behaviour. The ability to model easily the intensity, polarisation, and straylight characteristics of electrically-small detectors will be of great value when designing high-performance polarimetric imaging arrays.

Thomas, Christopher N.↗
NASA NTRS1974

Energy and pitch angle distributions for auroral ions using the current sheet acceleration model

Using a dipole-plus-tail magnetic field model, H(+), He(+2), and O(+6) ions are followed numerically, backward in time, from an output plane perpendicular to the axis of the geomagnetic tail, to their point of entrance to the magnetosphere as solar wind particles in the magnetosheath. An adiabatic or guiding center approximation is used in regions where the particles do not interact directly with the current sheet. A Maxwellian distribution with bulk flow is assumed for solar wind particles in the magnetosheath. Bulk velocity, density, and temperature along the magnetopause are taken from the fluid calculations of Spreiter. Using Liouville's theorem, and varying initial conditions at the output plane, the distribution function is found as a function of energy and pitch angle at the output plane. These results are then mapped to the auroral ionosphere using guiding center theory.

Jaeger, E. F.↗
NASA NTRS1985

Maser cavity servo-tuning system

Two collocated, weakly coupled probes, one loop and one dipole, detect the magnetic and electric fields inside a maser cavity. Signals from the probes are compared in phase, and the signal output from the phase detector is applied to a varactor, the reactance of which is coupled into the cavity by a microwave coupler. Alternatively, the varactor may be placed inside the cavity. Any deviation of phase from 90 deg as detected by the phase detector will then produce an error signal that will change the reactance coupled into the resonant cavity to change its reactance, and thus correct its resonance frequency. An alternative to using two probes is to use a single disk probe oriented to detect both the magnetic and electric fields, and thus provide the error signal directly.

Sydnor, R. L.↗
NASA NTRS1992

Electromagnetic scattering in clouds

Techniques used to explain the nature of the optical effects of clouds on the light produced by lightning include a Monte Carlo simulation, an equivalent medium approach, and methods based on Boltzmann transport theory. A cuboidal cloud has been considered using transform methods and a diffusion approximation. Many simplifying assumptions have been used by authors to make this problem tractable. In this report, the cloud will have a spherical shape and its interior will consist of a uniform distribution of identical spherical water droplets. The source will be modeled as a Hertz dipole, electric or magnetic, inside or outside the cloud. An impulsive source is used. Superposition may be employed to obtain a sinusoid within an envelope which describes a lightning event. The problem is investigated by transforming to the frequency domain, obtaining Green's functions, and then using the Cagniard-DeHoop method to symbolically recover the time domain solution.

Solakiewicz, Richard↗
NASA NTRS1992

The Lightcraft project: Flight technology for a hypersonic mass transit system

Rensselaer Polytechnic Institute has been developing transatmospheric 'Lightcraft' technology aimed at creating an efficient, economically affordable, hypersonic mass transportation system. The system utilizes laser-energized airbreathing engines to accelerate minimum-volume passenger capsules. The system gains a high level of reliability by using remote 'centralized' space power sources, e.g., satellite solar power stations. The most critical portion of the Lightcraft's acceleration trajectory involves flight propulsion at hypersonic velocities within the Earth's atmosphere, using a 'Magneto-Hydro-Dynamic (MHD) Fanjet' mode. Of all the propulsion modes proposed for the Lightcraft's combined-cycle engine, the MHD-Fanjet mode has received the least critical inquiry, largely because of its complexity. During the 1991-1992 academic year, Rensselaer's ADP teams produced a detailed conceptual design for the MHD-Fanjet engine, including the specific details of its integration with the other three propulsive modes. To facilitate this process, students built a full-scale mockup of a 1/12th section of this annular engine, complete with a working model of the shroud translation system. The class also made preliminary design calculations for the double-dipole, 'cuspfield' superconducting magnets that provide the external magnetic field needed by the MHD air accelerator, as well as for an onboard microwave power system to enhance the electrical conductivity of the air plasma working fluid. In addition, a large hypersonic model of the MHD accelerator was designed for future tests in RPI's Hypersonic Shock Tunnel in order to validate present analytical performance models. Another group continued design work on a full-sized prototype of a one-person 'Mercury Lightcraft' (a transatmospheric flight simulator), with major emphasis on the detailed design of the major structure, robotic landing gear, and exterior aeroshell.

Myrabo, Leik↗
NASA NTRS1972

Theoretical and observational analysis of spacecraft fields

In order to investigate the nondipolar contributions of spacecraft magnetic fields a simple magnetic field model is proposed. This model consists of randomly oriented dipoles in a given volume. Two sets of formulas are presented which give the rms-multipole field components, for isotropic orientations of the dipoles at given positions and for isotropic orientations of the dipoles distributed uniformly throughout a cube or sphere. The statistical results for an 8 cu m cube together with individual examples computed numerically show the following features: Beyond about 2 to 3 m distance from the center of the cube, the field is dominated by an equivalent dipole. The magnitude of the magnetic moment of the dipolar part is approximated by an expression for equal magnetic moments or generally by the Pythagorean sum of the dipole moments. The radial component is generally greater than either of the transverse components for the dipole portion as well as for the nondipolar field contributions.

Neubauer, F. M.↗
NASA NTRS1985

Infrared and far-infrared laser magnetic resonance spectroscopy of the GeH radical - Determination of ground state parameters

The GeH radical has been detected in its ground 2 Pi state in the gas phase reaction of fluorine atoms with GeH4 by laser magnetic resonance techniques. Rotational transitions within both 2 Pi 1/2 and 2 Pi 3/2 manifolds have been observed at far-infrared wavelengths and rotational transitions between the two fine structure components have been detected at infrared wavelengths (10 microns). Signals have been observed for all five naturally occurring isotopes of germanium. Nuclear hyperfine structure for H-1 and Ge-73 has also been observed. The data for the dominant isotope (/Ge-74/H) have been fitted to within experimental error by an effective Hamiltonian to give a set of molecular parameters for the X 2 Pi state which is very nearly complete. In addition, the dipole moment of GeH in its ground state has been estimated from the relative intensities of electric and magnetic dipole transitions in the 10 micron spectrum to be 1.24(+ or - 0.10) D.

Brown, J. M.↗
NASA NTRS2012

Low-degree Structure in Mercury's Planetary Magnetic Field

The structure of Mercury's internal magnetic field has been determined from analysis of orbital Magnetometer measurements by the MESSENGER spacecraft. We identified the magnetic equator on 531 low-altitude and 120 high-altitude equator crossings from the zero in the radial cylindrical magnetic field component, Beta (sub rho). The low-altitude crossings are offset 479 +/- 6 km northward, indicating an offset of the planetary dipole. The tilt of the magnetic pole relative to the planetary spin axis is less than 0.8 deg.. The high-altitude crossings yield a northward offset of the magnetic equator of 486 +/- 74 km. A field with only nonzero dipole and octupole coefficients also matches the low-altitude observations but cannot yield off-equatorial Beta (sub rho) = 0 at radial distances greater than 3520 km. We compared offset dipole and other descriptions of the field with vector field observations below 600 km for 13 longitudinally distributed, magnetically quiet orbits. An offset dipole with southward directed moment of 190 nT-R-cube (sub M) yields root-mean-square (RMS) residuals below 14 nT, whereas a field with only dipole and octupole terms tuned to match the polar field and the low-altitude magnetic equator crossings yields RMS residuals up to 68 nT. Attributing the residuals from the offset-dipole field to axial degree 3 and 4 contributions we estimate that the Gauss coefficient magnitudes for the additional terms are less than 4% and 7%, respectively, relative to the dipole. The axial alignment and prominent quadrupole are consistent with a non-convecting layer above a deep dynamo in Mercury's fluid outer core.

magnetometer↗
NASA NTRS1991

Model of Jovian F region ionosphere (Saturnian ionosphere in offset dipole approximation)

Researchers investigated the offset effect of Saturn's dipole on its ionosphere. The magnetic field of Saturn is primarily that of a dipole closely aligned to the rotational axis, but displaced northward from the center by a distance approximately equal to 0.05 R sub S, R sub S being the reference radius of Saturn. This offset effect would manifest itself most prominently between the ionospheric profiles in the Northern and Southern Hemispheres of Saturn.

Tan, A.↗
NASA NTRS1974

Theoretical and observational analysis of spacecraft fields

In order to investigate the dipolar and nondipolar contributions of spacecraft magnetic fields, a simple magnetic field model is proposed. This model consists of a number N of randomly oriented dipoles of strength M sub k in a given volume. Two sets of formulas are presented that give the rms multipole field components, first, for isotropic orientations of the dipoles at given positions and second, for isotropic orientations of the dipoles distributed uniformly throughout a cube or sphere. The statistical results for a cube (8 cu m in size) together with individual examples computed numerically show the following features. Beyond about 2- to 3-m distance from the center of the cube the field is dominated by an equivalent dipole. The magnitude of the magnetic moment of the dipolar part is approximated by N to the 1/2 power times M for equal magnetic moments or generally by the Pythagorean sum of the dipole moments. The radial component tend to be greater than either of the transverse components for the dipole portion as well as for the nondipolar field contributions.

Neubauer, F. M.↗
NASA NTRS1994

Multiwavelength study of the magnetically active T Tauri star HD 283447

We observed the luminous T Tauri star HD 283447 = V773 Tauri simultaneously at X-ray, ultraviolet, optical photometric and spectroscopic, and radio wavelengths for several hours on UT 1992 September 11. ROSAT, IUE, Very Large Array (VLA) and an intercontinental Very Long Baseline Interferometry (VLBI) network, and three optical observatories participated in the campaign. The star is known for its unusually high and variable nonthermal radio continuum emission. High levels of soft X-ray and Mg II line emission are discovered, with luminosity L(sub x) = 5.5 x 10(exp 30) ergs/s (0.2 - 2 keV) and L(sub Mg II) = 1 x 10(exp 29) ergs/s, respectively. Optically, the spectrum exhibits rather weak characteristics of `classical' T Tauri stars. A faint, broad emission line component, probably due to a collimated wind or infall, is present. During the campaign, the radio luminosity decreased by a factor of 4, while optical/UV lines and X-ray emission remained strong but constant. The large gyrosynchrotron-emitting regions are therefore decoupled from the chromospheric and coronal emission. Five models for the magnetic geometry around the star are discussed; solar-type activity, dipole magnetosphere, star-disk magnetic coupling, disk magnetic fields, and close binary interaction. The data suggest that two magnetic geometries are simultaneously present: complex multipolar fields like those on the Sun, and a large-scale field possibly associated with the circumstellar disk.

Feigelson, Eric D.↗
NASA NTRS1976

On the global TRM of the lunar lithosphere

A global thermoremanent magnetization (TRM) theory is developed which takes into account the direction and magnitude of a hypothetical source dipole field, the distribution of magnetic permeability and the coefficient of TRM and the cooling rate in the lithosphere, and models the low-field TRM characteristics of the material. When applied to the moon in conjunction with lunar magnetic parameter ranges now available, the theory gives a permanent lunar dipole moment which is uncertain to a factor of about 10 to the 5th power.

Srnka, L. J.↗
NASA NTRS1988

Substorm variations in the magnitude of the magnetic field - AMPTE/CCE observations

Using energetic-particle data taken in the near-earth tail by the AMPTE/Charge Composition Explorer (CCE) satellite, 167 ion injection events, that were essentially dispersionless over a 25-285 keV energy range, were identified, and the variations in the total magnetic field strength over the course of these events were examined in order to determine the dependence of the magnetic field strength on dipole latitude. Results indicate that, during periods of substorm activity, the latitudinal position of the current sheet varied significantly within the 32-deg wedge centered on the dipole equator traversed by CCE. Results also suggest that, even in the near-earth magnetotail out to 8.8 R(E) (CCE apogee), the local field measurements are a better guide to the determination of satellite's position relative to the current shield during a substorm, than is the magnetic latitude.

Lopez, R. E.↗
NASA NTRS1994

Kinetic and thermodynamic properties of a convecting plasma in a two-dimensional dipole field

Charged particle guiding center motion is considered in the magnetic field of a two-dimensional ('line') dipole on which is superimposed a small, static, perpendicular electric field. The parallel equation of motion is that of a simple harmonic oscillator for cos theta, the cosine of magnetic colatitude theta. Equations for the perpendicular electric and magnetic drifts are derived as well as their bounce-averaged forms. The latter are solved to yield a bounce-averaged guiding center trajectory, which is the same as that obtained from conversation of magnetic moment mu, longitudinal invariant J, and total (kinetic plus electrostatic) energy K. The algebraic simplicity of the trajectory equations is also manifest in the forms of the invariants. An interesting result is that guiding centers drift in such a way that they preserve the values of their equatorial pitch angles and (equivalently) mirror latitudes. The most general Maxwellian form of the equilibrium one-particle distribution function f is constructed from the invariants, and spatially varying density and pressure moments, parallel and perpendicular to the magnetic field, are identified. Much of the paper deals with the more restricted problem in which f is specified as a bi-Maxwellian over a straight line of finite length in the equatorial plane of the dipole and perpendicular to field lines. This might be thought of as specifying a cross-tail ion injection source; our formalism then describes the subsequent spatial development. The distribution away from the source is a scaled bi-Maxwellian but one that is cut off at large and small kinetic energies, which depend on position. Density and pressure components are reduced from the values they would have if the total content of individual flux tubes convected intact. The equatorial and meridional variations of density and pressure components are examined and compared systematically for the isotropic and highly anisotropic situations. There appears to be little qualitative difference due to anisotropy. An anisotropy measure is defined, and its spatial variation determined as a signature of possible MHD instability. Extreme values are found, larger than at the source, but the plasma beta in such regions is probably so low as to render the effect inconsequential energetically. Finally, the possible consequence of 'nonadia- batic' pressure profiles on electrostatic interchanges is considered, and a boundary delineating stabilizing and destabilizing regions determined.

Huang, T. S.↗