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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 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

The new magnetic white dwarf PG 1031 + 234 - Polarization and field structure at more than 500 milion Gauss

The discovery and study of a PG white dwarf which shows strong linear and circular polarization modulated with a rotation period of 3 hr 24 min is reported. The new star, PG 1031 + 234, is more highly magnetic than even Grw +70 deg 8247, with absorption components of hydrogen in its optical and UV spectra corresponding to fields from 200 to more than 500 MG. An asymmetric dependence of polarization on rotational phase shows that the surface field structure on the star is more complex than a simple centered dipole. The basic observational features can be well reproduced by a rotating star containing an oblique centered dipole punctuated near its magnetic equator by a high-field spot containing primarily radial field lines.

Schmidt, Gary D.↗
NASA NTRS1980

Equivalent source modeling of the main field using Magsat data

A graphic software package was developed to plot the dipole positions for a particular model on a world map. An arrow is drawn at each dipole in the direction of the horizontal magnetization vector, with length proportional to the horizontal magnitude. A contouring package represents the radical component of the magnetization vector. Based on quiet Magsat data, several dipole models were derived. These are being verified against each other and against MGST (6/80) data set as to goodness of fit and stability of solution. The spatial power spectra computed from spherical harmonic expansions of the dipole models is being analyzed relative to crustal and core content. The power spectra for one of the dipole models and a spherical harmonic model based on Magsat data through degree and order 23 is presented.

Source record↗
NASA NTRS1984

Distribution of aurora and ionospheric currents observed simultaneously on a global scale

The instantaneous spatial distribution of auroral emissions is observed with auroral imaging photometers on board the spacecraft Dynamics Explorer 1 (DE 1) as ground-based meridian chains of magnetometers simultaneously detect the magnetic signatures of ionospheric and field-aligned currents flowing at northern polar latitudes. Ionospheric conductivities at nighttime auroral latitudes are estimated from the measured auroral luminosities and used with the measured polar magnetic variations to compute model distributions of ionospheric and field-aligned currents. Temporal resolution for the coordinated observations and model calculations is 12 minutes. Model ionospheric and field-aligned current distributions are overlayed on global auroral images to illustrate spatial relations on a global scale at the maximum epoch of an auroral substorm. Eccentric-dipole-latitude and magnetic-local-time coordinates are used. A model field-aligned current distribution is compared quantitatively with the distribution of field-aligned currents inferred from simultaneous observations by the DE-2 magnetometer.

Craven, J. D.↗
NASA NTRS1982

Jovimagnetic secular variation

The history of Jupiter's planetary magnetic field is of great interest, in particular, because it represents the first opportunity to observe secular variations of a planetary dynamo other than the earth's. Indirect observations of the Jovian magnetic field began with the discovery of nonthermal radio emissions by Burke and Franklin (1955). In situ observations of the Jovian planetary magnetic field are limited to the Pioneer 10 and 11 flybys at epochs 1973.9 and 1974.9 and the Voyager 1 and 2 flyby encounters at epochs 1979.2 and 1979.5. Jovian magnetic field models are presented in a table, and a graph is provided in which the secular decrease of the earth's main dipole term is compared with estimates of Jupiter's main dipole term. The in situ magnetic field observations are consistent with no secular variation of the jovimagnetic dipole field from 1974.9 to 1979.2.

Connerney, J. E. P.↗
NASA NTRS2008

Optimized Geometry for Superconducting Sensing Coils

An optimized geometry has been proposed for superconducting sensing coils that are used in conjunction with superconducting quantum interference devices (SQUIDs) in magnetic resonance imaging (MRI), magnetoencephalography (MEG), and related applications in which magnetic fields of small dipoles are detected. In designing a coil of this type, as in designing other sensing coils, one seeks to maximize the sensitivity of the detector of which the coil is a part, subject to geometric constraints arising from the proximity of other required equipment. In MRI or MEG, the main benefit of maximizing the sensitivity would be to enable minimization of measurement time. In general, to maximize the sensitivity of a detector based on a sensing coil coupled with a SQUID sensor, it is necessary to maximize the magnetic flux enclosed by the sensing coil while minimizing the self-inductance of this coil. Simply making the coil larger may increase its self-inductance and does not necessarily increase sensitivity because it also effectively increases the distance from the sample that contains the source of the signal that one seeks to detect. Additional constraints on the size and shape of the coil and on the distance from the sample arise from the fact that the sample is at room temperature but the coil and the SQUID sensor must be enclosed within a cryogenic shield to maintain superconductivity.

Eom, Byeong Ho↗
NASA NTRS1976

Possibility of detecting magnetospheric radio bursts from Uranus and Neptune

The intensity of magnetospheric radio bursts (MRBs) is scaled to solar-wind input into planetary magnetospheres and the frequency of emission is scaled to polar surface magnetic-field strength in order to estimate the possibility of detecting MRBs from Uranus and Neptune. A scaling law is derived which relates the ratio of power radiated in MRBs to the solar-wind input for earth, Jupiter, and Saturn. Power-flux spectra of MRBs from these three planets are plotted, and it is shown that Jupiter and Saturn may radiate 1% to 5% of the solar-wind energy input into their magnetospheres. The properties of MRBs from Uranus and Neptune are estimated by assuming a conversion efficiency of 1% to 5%, a bandwidth of half the peak frequency, and conformity of Uranus' and Neptune's dipole moments with the magnetic Bode's law. Based on the results, it is suggested that detection of MRBs from these two planets may be a reasonable cruise-mode radio-astronomy objective on future missions to the outer solar system.

Kennel, C. F.↗
NASA NTRS1980

IMP 8. Volume 1: EM field experiment

The electromagnetic fields experiment on IMP-J used two electric dipole antennas and a triaxial search coil magnetic antenna to sense the electric and magnetic field of plasma waves in space. The electric dipole antennas consisted of a fine wire, 0.021 inches in diameter, with a nominal extended tip-to-tip length of 400 ft. The outermost 50 ft. of each element was conducting and the rest of the antenna was covered with an insulating coating. The search coil antennas each consisted of a high mu core with two separate windings of 40,000 turns each to sense ac magnetic fields. The search coils had a length of 18 inches tip-to-tip and are mounted on the end of a boom. The axes of the x prime and y prime search coil antennas were parallel to the x prime and y prime electric antenna axes.

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NASA NTRS1987

The magnetosphere of Saturn

Pioneer 11 and Voyager 1 and 2 magnetic field measurements over the entire flyby of Saturn's magnetic field have been analyzed by fitting a magnetospheric dipole field (i.e., a dipole field plus the field due to currents in the magnetopause), higher moments of the internal field aligned with the dipole along the rotation axis, and the field due to an equatorial sheet current to the magnetic measurements. A dipole moment of 21,431 nT R(s) exp 3, a quadrupole moment of 2403 nT R(s) exp 4, an octopole moment of 2173 nT R(s) exp 5, and an equatorial sheet current of half thickness 2.0 R(s) from about 5 R(s) to the solar edge of the magnetopause, fit the measurements over the entire magnetosphere with an rms deviation of 3.2 nT where R(s) is the planet radius, 66,330 km. The primary feature of the present analysis is the explicit inclusion of the calculated magnetopause current field, which reduces the overall rms deviation over the entire flyby from sigma values of 4.7 and 5.9 nT, using previous models, to 3.2 nT using the present.

Beard, David B.↗
NASA NTRS1988

The nature of the interior of Uranus based on studies of planetary ices at high dynamic pressure

Data from the Voyager II spacecraft showed that Uranus has a large magnetic field with geometry similar to an offset tilted dipole. To interpret the origin of the magnetic field, measurements were made of electrical conductivity and equation-of-state data of the planetary 'ices' ammonia, methane, and 'synthetic Uranus' at shock pressures and temperatures up to 75 gigapascals and 5000 K. These pressures and temperatures correspond to conditions at the depths at which the surface magnetic field is generated. Above 40 gigapascals the conductivities of synthetic Uranus, water, and ammonia plateau at about 20/(ohm-cm), providing an upper limit for the electrical conductivity used in kinematic or dynamo calculations. The nature of materials at the extreme conditions in the interior is discussed.

Nellis, W. J.↗
NASA NTRS1980

Equivalent source modeling of the main field using Magsat data

Progress is reported on software development for equivalent dipole source modeling of the main magnetic field. This includes a spatial statistical output capability, a subroutine to compute the equivalent spherical harmonic representation from the dipole distribution, and capability to plot the global locations of the dipoles and the values of the source magnetization.

Source record↗
NASA NTRS1986

A modeling analysis program for the JPL Table Mountain Io sodium cloud data

Progress and achievements in the second year are discussed in three main areas: (1) data quality review of the 1981 Region B/C images; (2) data processing activities; and (3) modeling activities. The data quality review revealed that almost all 1981 Region B/C images are of sufficient quality to be valuable in the analyses of the JPL data set. In the second area, the major milestone reached was the successful development and application of complex image-processing software required to render the original image data suitable for modeling analysis studies. In the third area, the lifetime description of sodium atoms in the planet magnetosphere was improved in the model to include the offset dipole nature of the magnetic field as well as an east-west electric field. These improvements are important in properly representing the basic morphology as well as the east-west asymmetries of the sodium cloud.

Smyth, W. H.↗
NASA NTRS1994

The source of Jovian auroral hiss observed by Voyager 1

Observations of auroral hiss obtained from the Voyager 1 encounter with Jupiter have been reanalyzed. The Jovian auroral hiss was observed near the inner boundary of the warm Io torus and has a low-frequency cutoff caused by propagation near the resonance cone. A simple ray tracing procedure using an offset tilted dipole of the Jovian magnetic field is used to determine possible source locations. The results obtained are consistent with two sources located symmetrically with respect to the centrifugal equator along an L shell (L approximately = 5.59) that is coincident with the boundary between the hot and cold regions of the Io torus and is located just inward of the ribbon feature observed from Earth. The distance of the sources from the centrifugal equator is approximately 0.58 +/- 0.01 R(sub J). Based on the similarity to terrestrial auroral hiss, the Jovian is auroral hiss is believed to be generated by beams of low energy (approximately tens to thousands of eV) electrons. The low-frequency cutoff of the auroral hiss suggests that the electrons are accelerated near the inferred source region, possibly by parallel electric fields similar to those existing in the terrestrial auroral regions. A field-aligned current is inferred to exist at L shells just inward of the plasma ribbon. A possible mechanism for driving this current is discussed.

Morgan, D. D.↗
NASA NTRS1999

Mercury's Magnetosphere

Among the major discoveries made by the Mariner 10 mission to the inner planets was the existence of an intrinsic magnetic field at Mercury with a dipole moment of approx. 300 nT R(sup 3, sub M). This magnetic field is sufficient to stand off the solar wind at an altitude of about 1 R(sub M) (i.e. approx. 2439 km). Hence, Mercury possesses a 'magnetosphere' from which the so]ar wind plasma is largely excluded and within which the motion of charged particles is controlled by the planetary magnetic field. Despite its small size relative to the magnetospheres of the other planets, a Mercury orbiter mission is a high priority for the space physics community. The primary reason for this great interest is that Mercury unlike all the other planets visited thus far, lacks a significant atmosphere; only a vestigial exosphere is present. This results in a unique situation where the magnetosphere interacts directly with the outer layer of the planetary crust (i.e. the regolith). At all of the other planets the topmost regions of their atmospheres become ionized by solar radiation to form ionospheres. These planetary ionospheres then couple to electrodynamically to their magnetospheres or, in the case of the weakly magnetized Venus and Mars, directly to the solar wind. This magnetosphere-ionosphere coupling is mediated largely through field-aligned currents (FACs) flowing along the magnetic field lines linking the magnetosphere and the high-latitude ionosphere. Mercury is unique in that it is expected that FACS will be very short lived due to the low electrical conductivity of the regolith. Furthermore, at the earth it has been shown that the outflow of neutral atmospheric species to great altitudes is an important source of magnetospheric plasma (following ionization) whose composition may influence subsequent magnetotail dynamics. However, the dominant source of plasma for most of the terrestrial magnetosphere is the 'leakage'of solar wind across the magnetopause and more direct entry through the northern and southern cusps. Although Mariner 10 did not return plasma composition measurements, the Hermean magnetosphere should be ideal for measuring the manner and rate of solar wind plasma entry due to the lack of strong internal atmospheric sources. Finally, the solar wind conditions experienced by Mercury as it orbits the Sun at 0.31 to 0.47 AU are quite different from those typically encountered by the Earth. This may allow for new understanding of the external factors affecting the transfer of mass, momentum and energy from the solar wind to planetary magnetospheres. This article provides a brief overview of what is now known about Mercury's magnetosphere and why it is a priority target for future planetary missions.

Slavin, J. A.↗
NASA NTRS1976

Pulsar extinction

The radiation properties of pulsars are reinvestigated in the context of the 'PCFB' model, according to which the radiation originates at the polar caps and the magnetic-field lines change from a closed to an open configuration at the 'force-balance' or 'corotation' radius. Major attention is given to the condition for electron-positron pair creation, which leads, in turn, to an extinction condition whereby any pulsar will cease to be a radio emitter after its period has increased beyond a certain value. This extinction condition is derived on the basis of a model where the magnetic field is the same as that of a point dipole located at the center of the star; effects of dipole distortion are also considered. A comparison of the results with observational data shows that most pulsars satisfy or nearly satisfy the pair-creation condition for undistorted dipoles and seem to satisfy the extinction condition. It is noted that pulsars which should be extinguished according to the undistorted-dipole model need not be if the magnetic field is sufficiently distorted at the polar caps.

Sturrock, P. A.↗
NASA NTRS2018

The Varying Core Magnetic Field from a Space Weather Perspective

This paper summarizes recent advances in our understanding of geomagnetism, and its relevance to terrestrial space weather. It also discusses specific core magnetic field features such as the dipole moment decay, the evolution of the South Atlantic anomaly, and the location of the magnetic poles that are of importance for the practice of space weather.

Core Field↗
NASA NTRS1981

Magnetic field configuration of the heliosphere in interstellar space

An improved model of the heliospheric magnetic field configuration is presented by including a uniform interstellar magnetic field and by adopting different values for a few magnetic field parameters. All the basic expressions for the magnetic field components remain the same as in Akasofu et al. (1980). In the magnetic arcade field, the magnetic field of the spherical dipoles is changed from 500 to 50 G. In the field of an extensive current disk around the sun lying in the ecliptic plane, the solar current disk extends from a distance of 1.5 solar radii instead of 10 solar radii. A uniform and southward directed interstellar magnetic field of 0.22 gamma is included. The total magnetic field configuration is constructed for the northward and southward orientation of the solar dipole field.

Akasofu, S.-L.↗