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At least 775 records · Page 43
All sourcesNASA NTRSDOE OSTI
NASA NTRS1974

GSFC magnetic field experiment Explorer 43

The magnetic field experiment flown on Explorer 43 is described. The detecting instrument is a triaxial fluxgate magnetometer which is mounted on a boom with a flipping mechanism for reorienting the sensor in flight. An on-board data processor takes successive magnetometer samples and transmits differences to the telemetry system. By examining these differences in conjunction with an untruncated sample transmitted periodically, the original data may be uniquely reconstructed on the ground.

Seek, J. B.↗
NASA NTRS1973

Variations of the occurrence rate of discontinuities in the interplanetary magnetic field

A statistical study of magnetic field discontinuities for the interval from February to October 1968 using data from the NASA Goddard Space Flight Center/University of Rome experiment on Pioneer 8 has been conducted. High-resolution data (10-sec averages) have been employed, stringent criteria being utilized to select more than 16,000 events, about 50 per cent being identified as disturbances that are probably tangential discontinuities (TD). Near 1 AU the average occurrence rate of discontinuities is about 3.6/hr, whereas that of tangential-like discontinuities is about 1.6/hr. The rate of occurrence of discontinuities is found to be correlated with the directional change of the field across the discontinuity surface. The normal to the TD discontinuity plane is most often perpendicular to the Archimedean spiral field configuration. A decrease in the rate of occurrence of TD is found with increasing heliocentric distance and/or with decreasing heliocentric latitude. Possible interpretations of the effect are discussed.

Mariani, F.↗
NASA NTRS1975

Magnetic field of the magnetosheath

The magnetic field of the magnetosheath is most naturally discussed in terms of its steady state and its fluctuating components. Theory of the steady state field is quite well developed and its essential features have been confirmed by observations. The interplanetary field is convected through the bow shock where its magnitude is increased and its direction changed by the minimal amount necessary to preserve the normal component across the shock. Convection within the magnetosheath usually increases the magnitude still further near the subsolar point and further distortes the direction until the field is aligned approximately tangent to the magnetopause. Fluctuations of the magnetosheath field are very complex, variable and not well understood. Spectral peaks are common features which occur at different frequencies at various times. Perturbation vectors of hydromagnetic waves tend to be aligned with the shock and magnetopause surfaces. Magnetosheath waves may be generated upstream, within the magnetosheath, at the bow shock, or at the magnetopause, but the relative importance of these sources is not known.

Fairfield, D. H.↗
NASA NTRS1975

Magnetic fields experiment

The Explorer 45 magnetic fields instruments were designed, constructed, and tested; research in the program is summarized.

Cahill, L. J., Jr.↗
NASA NTRS1976

Magnetic fields of the magnetosheath

The magnetic field of the magnetosheath is most naturally discussed in terms of its steady state and its fluctuating components. The theory of the steady-state field is quite well developed, and its essential features have been confirmed by observations. The interplanetary field is convected through the bow shock, where its magnitude is increased and its direction changed by the minimal amount necessary to preserve the normal component across the shock. Convection within the magnetosheath usually increases the magnitude still further near the subsolar point and further distorts the direction until the field is aligned approximately tangent to the magnetopause. Fluctuations of the magnetosheath field are very complex, variable, and not well understood. Transverse waves are often dominant at frequencies below 0.002 Hz, and compressional waves are often dominant at somewhat higher frequencies. Perturbation vectors of hydromagnetic waves tend to be aligned with the shock and magnetopause surfaces. Magnetosheath waves may be generated upstream, within the magnetosheath, at the bow shock, or at the magnetopause.

Fairfield, D. H.↗
NASA NTRS1976

Intense magnetic fields and umbral dots

Intense magnetic fields (of one or two kilogauss) situated in a background of weak field and umbral dots (regions of weak field) situated in a background of intense field (the sunspot) have both received considerable, though largely unrelated, discussion in the recent literature. It is suggested that the two phenomena may in fact be less disparate than hitherto presumed. Applying a simple (hydrostatic) model calculation to both phenomena and assuming temperature differences to be constant with height, the depth of each of these structures can be estimated.

Roberts, B.↗
NASA NTRS1980

A modeling of magnetic field variations during magnetospheric substorms

Magnetic field variations in the noon-midnight plane during the magnetospheric substorm are studied in terms of changes of three current systems: the dynamo-driven current on the magnetopause, the cross-tail current and the field-aligned current-auroral electrojet system. The field-aligned current is assumed to be generated as a result of interruption and subsequent diversion of the cross-tail current to the ionosphere. It is concluded that the available observations are consistent with a large increase of the three currents.

Akasofu, S.-I.↗
NASA NTRS1982

Parity nonconservation and the origin of cosmic magnetic fields

Three mechanisms of cosmic magnetic field generation are discussed: (1) asymmetric decay of particles emitted by rotating black holes; (2) asymmetric proton emission by black holes due to weak radiative corrections, and (3) equilibrium parity-violating currents. It is shown that all three mechanisms can produce a seed field sufficiently strong to account for the present galactic fields.

Vilenkin, A.↗
NASA NTRS1986

Damping Melt Convection With A Magnetic Field

Application of 3-kG magnetic field reduces thermal and solutal convection in Bi/MnBi melt in Bridgman-Stockbarger (moving-vertical-thermalgradient) furnace operating in normal gravitational field. Resulting Bi/MnBi alloy samples had properties similar to samples grown under nearly zero gravity. New technique proves useful in growing more uniform, defect-free semiconductor materials from such other electrically conductive melts.

Pirich, R. G.↗
NASA NTRS1986

Dynamic evolution of coronal magnetic fields

The response of coronal magnetic fields to photospheric motion is investigated using a time-dependent, two-dimensional MHD simulation. Starting with an initially uniform field, a circular section of the loop base is slowly rotated to represent the photospheric motion. The field lines at the base move with this flow in a manner consistent with the generated electric fields. The subsequent evolution of the field and flow can be characterized as passing through several distinct configurations. In the earliest phase the kinetic energy is negligible, and the current and field are parallel throughout most of the cylinder. This is followed by a period in which the field rotation increases, the axial field at and near the axis increases, and the acial field decreases in two cylindrical regions away from the axis. When the field in an appreciable portion of the cylinder has undergone one complete rotation, a rapid change in field configuration occurs with a large portion of the field making several rotations at large radii and a corresponding large reduction in the axial field.

Steinolfson, Richard S.↗
NASA NTRS1987

Mercury's magnetic field - A thermoelectric dynamo?

Permanent magnetism and conventional dynamo theory are possible but problematic explanations for the magnitude of the Mercurian magnetic field. A new model is proposed in which thermoelectric currents driven by temperature differences at a bumpy core-mantle boundary are responsible for the (unobserved) toroidal field, and the helicity of convective motions in a thin outer core (thickness of about 100 km) induces the observed poloidal field from the toroidal field. The observed field of about 3 x 10 to the -7th T can be reproduced provided the electrical conductivity of Mercury's semiconducting mantle approaches 1000/ohm per m. This model may be testable by future missions to Mercury because it predicts a more complicated field geometry than conventional dynamo theories. However, it is argued that polar wander may cause the core-mantle topography to migrate so that some aspects of the rotational symmetry may be reflected in the observed field.

Stevenson, D. J.↗
NASA NTRS1995

Radial evolution of the high/low frequency breakpoint in magnetic field spectra

The spectra of magnetic field variations in the solar wind show different behavior in two frequency regions; a high frequency region in which the spectral exponent is about -5/3 and a low frequency region in which it is typically -1. The two types of variations must arise from different processes and a clue to the relationship between the spectral regions lies in understanding the behavior of the breakpoint between the spectral regions. Studies of the average behavior of spectra have shown that the break point occurs at about 3.5 hours at 1 AU. It is also known that, on average, the breakpoint occurs at lower frequencies with larger heliocentric distances. Ideally however, instead of the average properties of the spectra, we would like to know how the breakpoint evolves in particular samples of the solar wind as they propagate to larger heliocentric distances. In the study reported here we take advantage of the fact that, in 1974, Pioneer 10 (4.4 AU) and Pioneer 11 (5.6 AU) were close to being co-aligned and being aligned with the Earth. Solar wind observed at Earth can be closely matched with solar wind later observed at P10 and P11. We here compare the breakpoint observed at Earth with that observed at Pioneers 10 and 11 for matched samples of the wind.

Feynman, J.↗
NASA NTRS1998

Numerical Simulation of THM Growth of CdTe in Presence of Rotating Magnetic Fields

The influence of rotating magnetic fields (RMF) on the flow pattern and compositional uniformity in the solution zone of a traveling heater method (THM) system for growth of CdTe is numerically investigated. The analysis is conducted at the 10(exp -6) and 10(exp -1) g(sub 0) as representative of space and ground processing conditions. It is shown that under microgravity conditions, application of RMF can be used to overwhelm residual buoyancy-induced convection and to control the uniformity of solution-zone composition at the growth front without appreciable modification of the growth interface shape. At high gravity levels, RMF is found not to be able to completely dominate buoyancy-induced convection. In this regime, for the range of field strengths studied, RMF is found to result in a) complex flow structures in the solution zone, b) enhancement of compositional non-uniformities at the growth front, and c) increased convexity of the growth interface. A scaling analysis of convection in the solution zone is used to generate a non-dimensional map delineating the RMF- and gravity-dominated flow regimes.

Ghaddar, Chahid K.↗