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At least 811 records · Page 45
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Plasma flow and magnetic field characteristics near the midtail neutral sheet

Using IMP 6, 7, and 8 magnetic field and plasma data, we have determined statistical occurrance properties of bulk flow and magnetic field orientation near the midtail neutral sheet. Characteristics of bulk plasma flow and magnetic field significantly change according to the radial distance down the tail. High-speed flow events (v greater than 300 km/s) are essentially restricted to the region tailward of X = -2.5 R(sub E) and are predominatly sunward or tailward. The low-speed flows were nearly equally likely to be in any direction, with the occurace rate of dustward and sunward flow being larger than that of tailward and dawnward flow. Dustward flow occurrence is highest in the region Earthward of X = -2.5 R(sub E), while sunward flow occurrence is highest in the region tailward of X = -2.5 R(sub E). The significance of the dawn-to-dust flow in the near-Earth region obtained in our study supports the idea that there exists a very effective mechanism to accelerate ions in the dawn-to-dust direction and hence the relief of pressure buildup in the near-Earth region. During high-speed flow events the relationship between B(sub Z) polarity and plasma flow direction is largely consistent with that expected from the magnetic reconnenection processes associted with substorms. There are also significant numbers of negative B9sub Z) events that are not associated with tailward flow. Mechanism other than substorm neutral line should therefore also taken into account to explain general B(sub Z) polarity in the midtail region.

Nakamura, R.↗
NASA NTRS1991

The resonant absorption of p-modes by sunspots with twisted magnetic fields

A simplified inhomogeneous sunspot model with an axial current (twisted magnetic field) is considered. The absorption of incoming acoustic modes in a narrow resonance layer inside the sunspot flux tube is investigated, and the energy loss is estimated. For nonaxisymmetric modes the results are consistent with previous calculations. However, contrary to previous work, it is demonstrated that the existence of an azimuthal component of the magnetic field can lead to significant absorption of even the axisymmetric modes. If the absorption rate calculated in this paper is used in conjunction with the observed wavelength dependence of the absorption coefficient, it is found that the sunspot flux tube must have significant twist in the subsurface layers. Furthermore, the presence of twist in the magnetic field leads to a natural explanation for the observed dependence on m, the azimuthal wave mode number, and the magnitude of the absorption coefficient can be accounted for in a self-consistent way.

Chitre, S. M.↗
NASA NTRS1977

On the origin of the magnetic field in type-1 comet tails - A reply to Ershkovich

Ershkovich (1976) employed the observed geometry of the type 1 tail of comet Arend-Roland 1957 III to show that any magnetic field present there cannot be of 'internal' origin but must have been 'captured' from the magnetized solar wind. Such an 'external' origin for the magnetic field in type 1 comet tails is not disputed, but several other points raised by Ershkovich are discussed. These concern the generation of substantial magnetic fields within the coma during periods when the comet's heliocentric distance is typically not greater than 1 AU, Ershkovich's use of the pressure balance across the tail 'tangential discontinuity' to estimate that the 'captured' magnetic field is of the order of the solar wind-field, his interpretation of certain observations of motions in the tail of comet Arend-Roland, and his assertion that the distant comet tail will be transformed into a turbulent wake due to the Kelvin-Helmholtz instability. It is argued that the 'captured' magnetic field should be substantially amplified relative to the solar-wind field, that the pressure-balance equation is of limited use, and that the real configuration of a type 1 tail must be treated in detail before any definite conclusions can be reached about the Kelvin-Helmholtz instability.

Mendis, D. A.↗
NASA NTRS1994

Quasi-periodic oscillations of the magnetopause during northward sheath magnetic field

The Geotail satellite quasi-periodically crossed the dawn flank of magnetopause more than ten times during an interval of 1.5 hours on November 4, 1992. Magnetopause crossings were characterized by quasi-periodic pulses of a sawtooth wave form in the magnetic field and the plasma flow components tangential to the magnetopause. The magnetic field strength in the magnetosheath was larger than that in the magnetosphere. The direction of magnetic field outside the magnetopause current layer was northward with antisunward tilt, indicating the draping of magnetic field on the magnetopause. Boundary normals of wavy magnetopause systematically incline sunward on the upstream side, while they tend to incline antisunward with considerable deviation on the downstream side. Comparison with other multiple crossing events suggests that the November 4 event exhibits wavy structure of the dawn flank magnetopause associated with the northward interplanetary magnetic field (IMF).

Kokubun, S.↗
NASA NTRS1985

An interplanetary magnetic field ensemble at 1 AU

A method for calculation ensemble averages from magnetic field data is described. A data set comprising approximately 16 months of nearly continuous ISEE-3 magnetic field data is used in this study. Individual subintervals of this data, ranging from 15 hours to 15.6 days comprise the ensemble. The sole condition for including each subinterval in the averages is the degree to which it represents a weakly time-stationary process. Averages obtained by this method are appropriate for a turbulence description of the interplanetary medium. The ensemble average correlation length obtained from all subintervals is found to be 4.9 x 10 to the 11th cm. The average value of the variances of the magnetic field components are in the approximate ratio 8:9:10, where the third component is the local mean field direction. The correlation lengths and variances are found to have a systematic variation with subinterval duration, reflecting the important role of low-frequency fluctuations in the interplanetary medium.

Matthaeus, W. H.↗
NASA NTRS1986

An interplanetary magnetic field ensemble at 1 AU

A method for calculation ensemble averages from magnetic field data is described. A data set comprising approximately 16 months of nearly continuous ISEE-3 magnetic field data is used in this study. Individual subintervals of this data, ranging from 15 hours to 15.6 days comprise the ensemble. The sole condition for including each subinterval in the averages is the degree to shich it represents a weakly time-stationary process. Averages obtained by this method are appropriate for a turbulence description of the interplanetary medium. The ensemble average correlation length obtained from all subintervals is found to be 4.9 x 10 to the 11th cm. The average value of the variances of the magnetic field components are in the approximate ratio 8:9:10, where the third component is the local mean field direction. The correlation lengths and variances are found to have a systematic variation with subinterval duration, reflecting the important role of low-frequency fluctuations in the interplanetary medium.

Matthaeus, W. H.↗
NASA NTRS1991

Facility Measures Magnetic Fields

Partly automated facility measures and computes steady near magnetic field produced by object. Designed to determine magnetic fields of equipment to be installed on spacecraft including sensitive magnetometers, with view toward application of compensating fields to reduce interfernece with spacecraft-magnetometer readings. Because of its convenient operating features and sensitivity of its measurements, facility serves as prototype for similar facilities devoted to magnetic characterization of medical equipment, magnets for high-energy particle accelerators, and magnetic materials.

Honess, Shawn B.↗
NASA NTRS1993

Solar magnetic field studies using the 12 micron emission lines. II - Stokes profiles and vector field samples in sunspots

Polarimetric observations at 12 microns of two sunpots are reported. The horizontal distribution of parameters such as magnetic field strength, inclination, azimuth, and magnetic field filling factors are presented along with information about the height dependence of the magnetic field strength. Comparisons with contemporary magnetostatic sunspot models are made. The magnetic data are used to estimate the height of 12 micron line formation. From the data, it is concluded that within a stable sunspot there are no regions that are magnetically filamentary, in the sense of containing both strong-field and field-free regions.

Hewagama, Tilak↗
NASA NTRS1977

Magnetic field experiment for Voyagers 1 and 2

The magnetic field experiments of the Voyager program involve studies of the planetary fields of Jupiter, Saturn, possibly Uranus, and several satellites; the solar wind and satellite interactions with the planetary fields, as well as large- and micro-scale features of the interplanetary magnetic field will also be investigated. Dual low field and high field magnetometer systems with dynamic ranges of + or - 0.5 G and + or - 20 G respectively provide high reliability for the missions and permit the separation of the spacecraft and ambient fields. Quantization uncertainty, rms noise levels and data compaction schemes of the magnetometer systems are also mentioned.

Behannon, K. W.↗
NASA NTRS1982

ISEE-1 and -2 observations of magnetic field strength overshoots in quasi-perpendicular bow shocks

According to the Rankine-Hugoniot jump conditions, the magnetic field strength increases in passing from upstream to downstream of a magnetohydrodynamic shock. At the earth's bow shock the magnetic field often increases significantly more than is required by the Rankine-Hugoniot conditions before decaying in an oscillatory fashion to its downstream average value. Recent OGO-5 and ISEE studies, together with the realization that overshoots also occur in the bow shocks of Venus, Jupiter, and Saturn have focussed attention on these phenomena. It is found that none of the nonlinear dispersive wave theories in the shock literature can account for the overshoot. The present investigation documents the dependence of the overshoot amplitude and thickness on solar wind parameters. Magnetic field overshoots are found to be characteristic of supercritical quasi-perpendicular shocks. The overshoot thickness scales as the ion Larmor radius based upon the solar wind speed and magnetic field.

Livesey, W. A.↗
NASA NTRS1992

Three dimensional magnetic fields in extra high speed modified Lundell alternators computed by a combined vector-scalar magnetic potential finite element method

A 3D finite element (FE) approach was developed and implemented for computation of global magnetic fields in a 14.3 kVA modified Lundell alternator. The essence of the new method is the combined use of magnetic vector and scalar potential formulations in 3D FEs. This approach makes it practical, using state of the art supercomputer resources, to globally analyze magnetic fields and operating performances of rotating machines which have truly 3D magnetic flux patterns. The 3D FE-computed fields and machine inductances as well as various machine performance simulations of the 14.3 kVA machine are presented in this paper and its two companion papers.

Demerdash, N. A.↗
NASA NTRS1975

The magnetic field of Mercury. I

An updated analysis and interpretation are presented of the magnetic field observations obtained during the Mariner 10 encounter with the planet Mercury on March 29, 1974. The combination of data relating to position of the detached bow shock wave and magnetopause and the geometry and magnitude of the magnetic field within the magnetosphere-like region surrounding Mercury lead to the conclusion that an internal planetary field exists with dipole moment approximately 5.1 times 10 to the 22nd G per cu cm. The dipole axis has a polarity sense similar to that of earth and is tilted 7 deg from the normal to Mercury's orbital plane. The magnetic field observations reveal a significant distortion of the modest Hermean field by the solar wind flow and the formation of a magnetic tail and neutral sheet which begins close to the planet on the night side. Presently, an active dynamo mechanism in the planetary interior appears to be favored in the interpretation of the field origin.

Ness, N. F.↗
NASA NTRS2004

Pumping Liquid Oxygen by Use of Pulsed Magnetic Fields

An effort is underway to develop a method of pumping small amounts of liquid oxygen by use of pulsed magnetic fields. This development is motivated by a desire to reduce corrosion and hazards of explosion and combustion by eliminating all moving pump parts in contact with the pumped oxygen. The method exploits the known paramagnetism of liquid oxygen. Since they both behave similarly, the existing theory of ferrofluids (liquids with colloidally suspended magnetic particles) is directly applicable to paramagnetic liquid oxygen. In general, the force density of the paramagnetic interaction is proportional to the magnetic susceptibility multiplied by the gradient of the square of the magnitude of the magnetic field. The local force is in the direction of intensifying magnetic field. In the case of liquid oxygen, the magnetic susceptibility is large enough that a strong magnetic-field gradient can lift the liquid in normal Earth gravitation.

Youngquist, Robert↗
NASA NTRS1972

Measurements of lunar magnetic field interaction with the solar wind.

Study of the compression of the remanent lunar magnetic field by the solar wind, based on measurements of remanent magnetic fields at four Apollo landing sites and of the solar wind at two of these sites. Available data show that the remanent magnetic field at the lunar surface is compressed as much as 40% above its initial value by the solar wind, but the total remanent magnetic pressure is less than the stagnation pressure by a factor of six, implying that a local shock is not formed.

Dyal, P.↗
NASA NTRS2010

Interplanetary Magnetic Field Power Spectrum Variations: A VHO Enabled Study

The newly reprocessed high time resolution (11/22 vectors/sec) Wind mission interplanetary magnetic field data and the solar wind key parameter search capability of the Virtual Heliospheric Observatory (VHO) affords an opportunity to study magnetic field power spectral density variations as a function of solar wind conditions. In the reprocessed Wind Magnetic Field Investigation (MFI) data, the spin tone and its harmonics are greatly reduced that allows the meaningful fitting of power spectra to the approx.2 Hz limit above which digitization noise becomes apparent. The power spectral density is computed and the spectral index is fitted for the MHD and ion inertial regime separately along with the break point between the two for various solar wind conditions . The time periods of fixed solar wind conditions are obtained from VHO searches that greatly simplify the process. The functional dependence of the ion inertial spectral index and break point on solar wind plasma and magnetic field conditions will be discussed

Szabo, A.↗
NASA NTRS2011

Interplanetary Magnetic Field Power Spectrum Variations: A VHO Enabled Study

The newly reprocessed high time resolution (11/22 vectors/sec) Wind mission interplanetary magnetic field data and the solar wind key parameter search capability of the Virtual Heliospheric Observatory (VHO) affords an opportunity to study magnetic field power spectral density variations as a function of solar wind conditions. In the reprocessed Wind Magnetic Field Investigation (MFI) data, the spin tone and its harmonics are greatly reduced that allows the meaningful fitting of power spectra to the ~2 Hz limit above which digitization noise becomes apparent. The power spectral density is computed and the spectral index is fitted for the MHD and ion inertial regime separately along with the break point between the two for various solar wind conditions. The time periods of fixed solar wind conditions are obtained from VHO searches that greatly simplify the process. The functional dependence of the ion inertial spectral index and break point on solar wind plasma and magnetic field conditions will be discussed

Szabo, A.↗