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At least 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

The magnetic fields of Mercury, Venus and Mars

Just as clearly as Mariner 10 established that Mercury has an intrinsic magnetic field, the Pioneer Venus orbiter has established that Venus has no significant intrinsic field. This is perhaps the opposite of what might be expected. Mercury, a small planet, might be expected to cool rapidly and its internal dynamo to cease, while Venus, which is almost as large as the earth, should not have lost much heat. On the contrary the source of energy of the Mercury dynamo appears to be extant whereas that of Venus appears to be extinct. The existence of a Martian magnetic field is controversial. No unambiguous signature of a Martian magnetic field has been reported. If the field on the nightside of Mars is of planetary rather than solar origin the Russian Mars spacecraft observations indicate the Martian dipole lies near the planetary equator rather than its pole.

Russell, C. T.↗
NASA NTRS1992

Levitation of YBa2Cu3O(7-x) superconductor in a variable magnetic field

The influence of both a linear alternating and rotational magnetic field component on the levitation behavior of a YBa2Cu3O(7-x) superconductor was examined. The transition from a plastic regime of levitation to an elastic one, induced by an alternating field component, was observed. An elastic regime in contrast to a plastic one is characterized by the unique position of stable levitation and field frequency dependence of relaxation time to this position. It was concluded that the vibrations of a magnet levitated above the superconductor can induce a transition from a plastic regime of levitation to an elastic one. It was found that a rotational magnetic field component induced rotations of a levitated superconductor. Rotational frictional motion of flux lines is likely to be an origin of torque developed. A prototype of a motor based on a levitated superconductor rotor is proposed.

Terentiev, Alexander N.↗
NASA NTRS1993

A global analytical representation of the magnetic field produced by the region 2 Birkeland currents and the partial ring current

A quantitative model is developed of the magnetic field produced by the electric current system of region 2 Birkeland currents, closed via the partial ring current. The distribution of j-perpendicular is computed from a given axially asymmetric spatial distribution of hot isotropic magnetospheric plasma over an infinitely thin L shell in an axisymmetric purely dipolar geomagnetic field, while the field-aligned current density is found from the continuity of the net electric current. The magnetic field distribution is derived by a Biot-Savart integral over the electric current system. An assumed cosine dependence of the plasma pressure on local time makes it possible to reduce the problem of analytical representation of the B field to two dimensions. The obtained numerical fits for the partial ring current/region 2 Birkeland current magnetic field are relatively simple, continuous, and valid throughout the whole extraterrestrial space from ionospheric heights up to tens of earth radii.

Tsyganenko, N. A.↗
NASA NTRS1982

The influence of stray magnetic fields on ion beam neutralization

An experimental investigation is described of a comparison between the ion beam neutralization characteristics of a local neutralizer (within approximately 5 cm of the beam edge) and those associated with a distant one (approximately 1 meter away from the thruster). The influence of magnetic fields in the vicinity of the neutralizer cathode orifice which are either parallel or normal to the neutralizer axis is assessed. The plasma property profiles which reflect the influence of the magnetic fields are measured. The results suggest that magnetic fields at the region of a neutralizer cathode orifice influence its ability to couple to the ion beam. They reveal that there is a potential jump from the neutralizer cathode orifice to the plasma which exists close to the orifice. This potential drop is found to increase as the axial component of magnetic flux density increases. A magnetic field perpendicular to the neutralizer axis induces a potential rise a few centimeters downstream from the neutralizer cathode.

Feng, Y.-C.↗
NASA NTRS1992

Current collection through the transport of electrons across magnetic field lines

Numerical simulations are used to investigate the collection of electrons by a long conducting cylinder in a flowing plasma. The plasma flow simulates the relative motion between a spacecraft and the plasma. The sheath structures and the levels of electron current collections for the cases with and without an ambient magnetic field are examined. It is found that for the flow perpendicular to the magnetic field, the current is considerably enhanced depending on the relative drift velocity. In the case of a nonzero magnetic field perpendicular to the cylinder axis, the potential structure is a 2D double layer with dimensions L-perpendicular much less than L-parallel. L-perpendicular is found to be the current limiting radius given by the Parker-Murphy model (1967). For the flow along the ambient magnetic field, the electron current is found to be smaller than that for the flow perpendicular to this field. This is explained in terms of the potential structures.

Singh, N.↗
NASA NTRS1991

Oxide superconductors under magnetic field

One of the current most serious problems for the oxide superconductors from the standpoint of practical application is the various novel features derived mainly from their extremely short coherence. In particular, the coherence length so far observed in the cuprate superconductors is in the range of 0.1 nm perpendicular to the CuO2 plane. This seems to be creating most of the difficulties in the device fabrication and in the performance under the magnetic field. Some of the superconducting properties under the magnetic field will be discussed in terms of the short coherence length. A model will be presented based on the gradual strengthening of the pinning force with decrease in temperature and the weak coupling at the grain boundaries. Secondly, the broadening of the superconducting transition under the magnetic field is discussed. This is observed significantly only when the field is applied perpendicular to the basal plane and the relative orientation of the current to the field is insignificant in determining the extent of broadening. Besides, the change in the strength of the pinning force does not affect the width of the broadening. From these observations discussions will be made on a model based on the giant fluctuation. Based on this model, it is predicted that the coherence length along the c-axis will be the single most important material parameter to determine the performance of the superconductor under a strong magnetic field. It seems that BYCO is superior in this regard to Bi- or Tl-systems as far as the performance at 77 K is considered, although another material with the coherence length slightly longer along the c-axis is still highly desired.

Kitazawa, K.↗
NASA NTRS1990

Oxide superconductors under magnetic field

One of the current most serious problems for the oxide superconductors from the standpoint of practical application is the various novel features derived mainly from their extremely short coherence. In particular, the coherence length so far observed in the cuprate superconductors is in the range of 0.1 nm perpendicular to the CuO2 plane. This seems to be creating most of the difficulties in the device fabrication and in the performance under the magnetic field. Some of the superconducting properties under the magnetic field will be discussed in terms of the short coherence length. A model will be presented based on the gradual strengthening of the pinning force with decrease in temperature and the weak coupling at the grain boundaries. Secondly, the broadening of the superconducting transition under the magnetic field is discussed. This is observed significantly only when the field is applied perpendicular to the basal plane and the relative orientation of the current to the field is insignificant in determining the extent of the broadening. Besides, the change in the strength of the pinning force does not affect the width of the broadening. From these observations discussions will be made on a model based on the giant fluctuation. Based on this model, it is predicted that the coherence length along the c-axis will be the single most important material parameter to determine the performance of the superconductor under a strong magnetic field. It seems that BYCO is superior in this regard to Bi- or Tl-systems as far as the performance at 77 K is considered, although another material with the coherence length slightly longer along the c-axis is still highly desired.

Kitazawa, K.↗
NASA NTRS1974

Cross correlation between cosmic-ray fluctuations and interplanetary magnetic-field fluctuations

Study of the cross-correlation between cosmic-ray intensity fluctuations and the interplanetary magnetic field, using the low-frequency limit of the theory of the interplanetary scintillations of cosmic rays. Cross-spectral analysis of the flux of the Alert neutron monitor and simultaneous values of the interplanetary magnetic field components are presented which support the theory. A model of cosmic-ray fluctuations is described, with effects of gradients and interplanetary sector structure dominating for very low frequencies (less than or about equal to .000005 Hz) and magnetic-field induced interplanetary scintillations of cosmic rays dominating for higher frequencies.

Jokipii, J. R.↗
NASA NTRS2004

Subsurface Structure of the Ismenius Area and Implications for Evolution of the Martian Dichotomy and Magnetic Field

The Martian dichotomy divides the smooth, northern lowlands from the rougher southern highlands. The northern lowlands are largely free of magnetic anomalies, while the majority of the significant magnetic anomalies are located in the southern highlands. An elevation change of 2-4 km is typical across the dichotomy, and is up to 6 km locally. We examine a part of the dichotomy that is likely to preserve the early history of the dichotomy as it is relatively unaffected by major impacts and erosion. This study contains three parts: 1) the geologic history, which is summarized below and detailed in McGill et al., 2) the study of the gravity and magnetic field to better constrain the subsurface structure and history of the magnetic field (this abstract), and 3) modeling of the relaxation of this area. Our overall goal is to place constraints on formation models of the dichotomy by constraining lithospheric properties. Initial results for the analysis of the geology, gravity, and magnetic field studies are synthesized in Smrekar et al..

Smrekar, S. E.↗
NASA NTRS1978

Coronal magnetic fields

The observational evidence on the strength of the coronal magnetic field above active regions is reviewed. Recent advances in observations and plasma theory are used to determine which data are the more reliable and to revise some earlier estimates of field strength. The results from the different techniques are found to be in general agreement, and the relation where magnetic field strength (in gauss) equals 0.5 times the -1.5 power of distance (in units of solar radius) minus 1 for distances between 1.02 and 10 solar radii is consistent with all the data to within a factor of about 3.

Dulk, G. A.↗
NASA NTRS1971

Lunar magnetic field measurements, electrical conductivity calculations and thermal profile inferences

Steady magnetic field measurements of magnitude 30 to 100 gamma on the lunar surface impose problems of interpretation when coupled with the nondetectability of a lunar field at 0.4 lunar radius altitude and the limb induced perturbations of the solar wind at the Explorer orbit. The lunar time-varying magnetic field clearly indicates the presence of eddy currents in the lunar interior and permits calculation of an electrical conductivity profile. The problem is complicated by the day-night asymmetry of the moon's electromagnetic environment, the possible presence of the transverse magnetic mode, and the variable wave directions of the driving function. The electrical conductivity is calculated to be low near the surface, rising to a peak of .006/ohm meter at 250 km, dropping steeply inwards to a value of about .00005/ohm meter, and then rising toward the interior. A transition at 250 km depth from a high conductivity to a low conductivity material is inferred, suggesting an olivine-like core at approximately 800 C, although other models are possible.

Colburn, D. S.↗