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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Large-scale solar and heliospheric magnetic fields

The magnetic structure of the extended solar corona varies with the changing photospheric field during the solar cycle. A simple potential model of the corona using solar surface observations from 1976 to the present shows how the large-scale coronal field evolves over more than a solar activity cycle. These predictions match well with large stable structures inferred from measurements of coronal electron density and the IMF, though dynamic changes are poorly modeled. The sun's polar field was about 25 stronger at solar minimum in 1986 than in 1976; the heliospheric current sheet was also flatter. In Cycle 21 the coronal and photospheric large-scale long-lived field patterns rotated every 26.9 days in the northern hemisphere; the southern field rotated every 28 days. Similar periods have been present in the IMF and in the occurrence of solar flares during the last several solar cycles.

Hoeksema, J. Todd↗
NASA NTRS1971

The extended coronal magnetic field.

The coronal magnetic field should contain many field lines connecting the photosphere to interplanetary space. A sharp boundary separates two adjacent sectors of opposite polarity. The large-scale structure of the corona is related to the photospheric sector pattern. The corona may frequently contain transient magnetic loops reaching out to five to ten solar radii.

Wilcox, J. M.↗
NASA NTRS1979

The magnetic effects of brecciation and shock in meteorites. II - The ureilites and evidence for strong nebular magnetic fields. III - The achondrites

Magnetic studies were conducted on samples of the ureilitic meteorites Goalpara, Havero, Novo Urei, and Kenna. Measurements included bulk magnetic susceptibility, natural remanent magnetization (NRM), saturation remanence, and the demagnetization behavior of NRM and saturation remanence. An approximate 'fossil' magnetic field intensity was determined by analysis of coercivity spectra of NRM and saturation remanence after reheating. It was found that the magnetic susceptibility of ureilites increased with severity of shock, and the NRM intensity decreases while its coercivity spectrum generally hardens with increasing shock. Goalpara and Kenna are distinguished by unexpectedly homogeneous, strong, stable, and directionally clustered NRM, indicative of a unique original magnetizing event. The results support the assertion that both ureilites and carbonaceous chondrites have formed from related nebular material and provide further evidence for a strong external magnetic field during the accretional stages and throughout an early bombardment stage.

Brecher, A.↗
NASA NTRS1981

Anisotropic magnetohydrodynamic turbulence in a strong external magnetic field

A strong external dc magnetic field introduces a basic anisotropy in incompressible MHD turbulence. The modifications that this is likely to produce in the properties of the turbulence are investigated for high Reynolds numbers. It is found that the turbulent spectrum splits into two parts: (1) an essentially two-dimensional spectrum with both the velocity field and the magnetic fluctuations perpendicular to the dc magnetic field, and (2) a generally weaker and more nearly isotropic spectrum of Alfven waves. These results are discussed in relation to measurements from the Culham-Harwell Zeta pinch device and the UCLA Macrorotor tokamak, as well as in relation to measurements of MHD turbulence in the solar wind.

Montgomery, D.↗
NASA NTRS2017

Parsec-Scale Obscuring Accretion Disk with Large-Scale Magnetic Field in AGNs

A magnetic field dragged from the galactic disk, along with inflowing gas, can provide vertical support to the geometrically and optically thick pc (parsec) -scale torus in AGNs (Active Galactic Nuclei). Using the Soloviev solution initially developed for Tokamaks, we derive an analytical model for a rotating torus that is supported and confined by a magnetic field. We further perform three-dimensional magneto-hydrodynamic simulations of X-ray irradiated, pc-scale, magnetized tori. We follow the time evolution and compare models that adopt initial conditions derived from our analytic model with simulations in which the initial magnetic flux is entirely contained within the gas torus. Numerical simulations demonstrate that the initial conditions based on the analytic solution produce a longer-lived torus that produces obscuration that is generally consistent with observed constraints.

galaxies↗
NASA NTRS1987

Studies of the configuration of the Venus ionospheric magnetic field

The dayside ionospheric magnetic field of Venus has been modeled from two different points of view. The Cloutier et al. electrodynamic model makes specific predictions about the behavior of the global magnetic field configuration that can be compared with those expected from the alternate diffusion/convection model. Although the diffusion/convection model is currently only one-dimensional, it is found that it is consistent with the observations in several areas where the three-dimensional electrodynamic model is not.

Luhmann, J. G.↗
NASA NTRS1985

The global structure of magnetic fields which support quiescent prominences

Magnetic fields in quiescent prominences were observed, but only recently has it become possible to measure the full magnetic field vector. The component of the field along the line of sight, B (parallel) can be uniquely determined, whereas for the component perpendicular to the line of sight B (perpendicular) and -B (perpendicular) are indistinguishable. An ambiguity remains in the actual magnetic field vector, in particular with respect to its orientation relative to the prominence axis. A sample of more than 100 prominences were studied. A more detailed analysis of 10 prominences are presented, and then set these prominence fields into relation to the underlying photospheric fields. It is found from statistical analysis of several hundred prominences that in 25% of the cases the field penetrates the prominence directly, whereas in 75% the field orientation in the prominence is reversed.

Anzer, U.↗
NASA NTRS1988

Mercury's magnetic field and interior

The magnetic-field data collected on Mercury by the Mariner-10 spacecraft present substantial evidence for an intrinsic global magnetic field. However, studies of Mercury's thermal evolution show that it is most likely that the inner core region of Mercury solidified or froze early in the planet's history. Thus, the explanation of Mercury's magnetic field in the framework of the traditional planetary dynamo is less than certain.

Connerney, J. E. P.↗
NASA NTRS1975

A survey of long term interplanetary magnetic field variations

Interplanetary magnetic field data from 10 IMP, AIMP, and HEOS spacecraft were merged into a composite data set spanning 1963 to 1974. A consideration of the mutual consistency of the individual data sets reveals agreement typically to within 0.2 gamma. Composite data set analysis reveals: (1) whereas the yearly averaged magnitudes of all field vectors show virtually no solar cycle variation, the yearly averaged magnitudes of positive- and negative-polarity field vectors show separate solar cycle variations, consistent with variations in the average azimuthal angles of positive- and negative-polarity field vectors, (2) there is no heliolatitude dependence of long time average field magnitudes, (3) field vectors parallel to the earth-sun line are on the average 1 gamma less in magnitude than field vectors perpendicular to this line, and (4) the heliolatitude-dependent dominant polarity effect exhibits a complex sign reversal in the 1968 to 1971 period and a measure of symmetry in 1972 to 1974 not found in earlier data.

King, J. H.↗