Search NASA⌕ Search

SEARCH · Search NASA

Results for “MAGNETIC DIPOLES”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 811 records · Page 45
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Magnetic observations during the recent declining phase of solar activity

Changes in the heliospheric magnetic field during the recent declining phase in solar activity are reviewed and compared with observations during past sunspot cycles. The study is based principally on data obtained by IMP-8 and Ulysses. The field magnitude is found to have increased during the declining phase until it reached a maximum value of 11.5nT in approximately 1991.5, approximately two years after sunspot maximum. The field of the sun's south pole became negative after a reversal in early 1990. The sector structure disappeared at Ulysses in April 1993 when the latitude of the spacecraft was -30 deg revealing a low inclination of the heliospheric current sheet. A large outburst of solar activity in March 1991 caused four Coronal Mass Ejections (CMEs) and numerious shocks at the location of Ulysses. Following a delay of more than a year, a series of recurrent high speed streams and Corotating Interaction Regions commenced in July 1992 which were observed by IMP-8, Ulysses and Voyager 2. In all these respects, the behavior of the magnetic field mimics that seen in the two earlier sunspot cycles. The comprehensive data set suggests a correlation between the absolute value of B and sunspot number. The major solar cycle variations in the radial component (and magnitude) of the field have been successfully reproduced by a recent model consisting of a tilted solar dipole, whose strength and tilt undergo characteristic changes over the sunspot cycle, and the heliospheric current sheet. The large outbursts of activity in mid-1972, mid-1982 and the first quarter of 1991 may represent a characteristic last 'gasp' of solar activity before the sun evolves to a different state. The recurrent high speed streams in 1973, 1984 and 1992 accompany the developemnt of large asymetrical polar coronal holes and the growth in intensity of the polar cap fields. After they endure for about one year, the polar coronal holes recede and the high speed streams are replaced by weaker streams more characteristic of solar minimum.

Smith, E. J.↗
NASA NTRS1985

Empirical model for the Earth's cosmic ray shadow at 400 KM: Prohibited cosmic ray access

The possibility to construct a unit sphere of access that describes the cosmic radiation allowed to an Earth-orbiting spacecraft is discussed. It is found that it is possible to model the occluded portion of the cosmic ray sphere of access as a circular projection with a diameter bounded by the satellite-Earth horizon. Maintaining tangency at the eastern edge of the spacecraft-Earth horizon, this optically occluded area is projected downward by an angle beta which is a function of the magnetic field inclination and cosmic ray arrival direction. This projected plane, corresponding to the forbidden area of cosmic ray access, is bounded by the spacecraft-Earth horizon in easterly directions, and is rotated around the vertical axis by an angle alpha from the eastern direction, where the angle alpha is a function of the offset dipole latitude of the spacecraft.

Humble, J. E.↗
NASA NTRS1985

Theoretical model of the solar corona during sunspot minimum. II - Dynamic approximation

The theoretical quasi-static model of the solar corona during sunspot minimum developed in an earlier paper (Osherovich et al., 1984) is extended to include a quasi-radial outflow. The requirement imposed in the above paper on a plasma flow is relaxed. The dynamic equation derived for the combination of a dipole-like and a radial field is used to calculate the electric current density around the sun in the region 1.5-5 solar radii. Comparison with the current density given by the quasi-static model shows that the outflow decreases the current density only slightly in the dynamic case. The dynamic coronal model with a magnetic quadrupole field is also considered, in relation to the north-south asymmetry in the solar corona.

Osherovich, V. A.↗
NASA NTRS1986

A palaeomagnetic perspective of Precambrian tectonic styles

The considerable success derived from palaeomagnetic studies of Phanerozoic rocks with respect to the tectonic styles of continental drift and plate tectonics, etc., have not been repeated by the many palaeomagnetic studies of Precambrian rocks. There are 30 years of research with results covering the major continents for Precambrian times that overlap considerably yet there is no concensus. There is good evidence that the usual assumptions employed by palaeomagnetism are valid for the Precambrian. The exisence of magnetic reversals during the Precambrian, for instance, is difficult to explain except in terms of a geomagnetic field that was predominantly dipolar in nature. It is a small concession to extend this notion of the Precambrian geomagnetic field to include its alignment with the Earth's spin axis and the other virtues of an axial geocentric dipole that characterize the recent geomagnetic field. In terms of greenstone terranes it is obvious that tectonic models postulated to explain these observations are paramount in understanding Precambrian geology. What relevance the current geographical relationships of continents have with their Precambrian relationships remains a paradox, but it would seem that the ensialic model for the development of greenstone terranes is favored by the Precambrian palaeomagnetic data.

Schmidt, P. W.↗
NASA NTRS1993

Radiative properties of a plasma moving across a magnetic field. I - Theoretical analysis. II - Numerical results

The early-time evolution of plasmas moving across a background magnetic field is addressed with a 2D model in which a plasma cloud is assumed to have formed instantaneously with a velocity across a uniform background magnetic field and with a Gaussian density profile in the two dimensions perpendicular to the direction of motion. This model treats both the dynamics associated with the formation of a polarization field and the generation and propagation of electromagnetic waves. In general, the results indicate that, to zeroth order, the plasma cloud behaves like a large dipole antenna oriented in the direction of the polarization field which oscillates at frequencies defined by the normal mode of the system. Radiation damping is shown to play an important role in defining the plasma cloud evolution, causing a rapid decay of the polarizaiton field and a loss of plasma kinetic energy and momentum on time scales comprable to several ion gyroperiods. Scaling laws are derived for the plasma momentum and energy loss rates, and predictions for the braking time, the amplitude and spectrum of the radiation field, and the total radiated power are presented for conditions relevant to the recent Combined Release and Radiation Effects Satellite experiments.

Roussel-Dupre, Robert↗
NASA NTRS1993

Solar magnetic cycle

Using NSO/KP magnetograms, the pattern and rate of the emergence of magnetic flux and the development of the large-scale patterns of unipolar fields are considered in terms of the solar magnetic cycle. Magnetic flux emerges in active regions at an average rate of 2 x 10(exp 21) Mx/day, approximately 10 times the estimated rate in ephemeral regions. Observations are presented that demonstrate that the large-scale unipolar fields originate in active regions and activity nests. For cycle 21, the net contribution of ephemeral regions to the axial dipole moment of the Sun is positive, and is of opposite sign to that of active regions. Its amplitude is smaller by a factor of 6, assuming an average lifetime of ephemeral regions of 8 hours. Active regions larger than 4500 Mm(sup 2) are the primary contributor to the cycle variation of Sun's axial dipole moment.

Harvey, Karen L.↗
NASA NTRS1999

What is the Relationship between the Solar Wind and Storms/Substorms?

The interplanetary magnetic field (IMF) carried past the Earth by the solar wind has long been known to be the principal quantity that controls geomagnetic storms and substorms. Intervals of strong southward IMF with durations of at least a significant fraction of a day produce storms, while more typical, shorter intervals of less-intense southward fields produce substorms. The strong, long-duration southward fields are generally associated with coronal mass ejections and magnetic clouds or else they are produced by interplanetary dynamics initiated by fast solar wind flows that compress preexisting southward fields. Smaller, short-duration southward fields that occur on most days are related to long period waves, turbulence, or random variations in the IMF. Southward IMF enhances dayside reconnection between the IMF and the Earth's dipole with the reconnected field lines supplementing open field lines of the geomagnetic tail and producing an expanded polar cap and increased tail energy. Although the frequent storage of solar wind energy and its release during substorms is the most common mode of solar wind/magnetosphere interaction, under certain circumstances, steady southward IMF seems to produce intervals of relatively steady magnetosphere convection without substorms. During these latter times, the inner magnetosphere remains in a stressed tail-like state while the more distant magnetotail has larger northward field and more dipolar-like field lines. Recent evidence suggests that enhanced magnetosphere particle densities associated with enhanced solar wind densities allow more particles to be accelerated for the ring current, thus creating larger storms.

Fairfield, D. H.↗
NASA NTRS2001

ASCA Observations of PSR 1929+10 and PSR 0950+08

The ASCA X-ray spectra of the old pulsars PSR 1929+10 and PSR 0950+08 are adequately fitted by simple blackbodies. Between 0.5 and 5.0 keV, PSR 1929+10 is best fitted with T = (5.14 +/- 0.53) x 10(exp 6) K and L = 1.54 x 10(exp 30) ergs/s; PSR 0950+08 has T = (5-70 +/- 0.63) x 10(exp 6) K and L = 4.67 x 10(exp 29) ergs/s. The inferred areas are 2 orders of magnitude smaller than canonical polar cap sizes. There is some evidence for a dearth of photons above 5 keV, which would disfavor alternative power-law models for the spectra. Pulsed emission is detected from PSR 1929+10 with 99.8% significance and pulsed fraction 0.35 +/- 0.18. Marginal evidence of pulsed X-ray emission from PSR 0950+08 is present with 99.0% significance and pulsed fraction 0.34+/- 0.18. Relativistic ray tracing shows that if the geometry deduced from the polarization of the radio pulse is adopted for the surface thermal emission, a central dipole cannot reproduce the large pulsed modulations observed unless the radius of the neutron star is as large as 18 km. A mechanism of polar cap heating by the impact of relativistic particles is suggested that can produce small areas with T approx. 5 x 10(exp 6) K on old, spun-down pulsars with weak magnetic fields (B approx. 3 x 10(exp 11) G).

Wang, F. Y.-H.↗
NASA NTRS2003

Polar, Cluster and SuperDARN Evidence for High-Latitude Merging during Southward IMF: Temporal/Spatial Evolution

Magnetic merging on the dayside magnetopause often occurs at high latitudes. Polar measured fluxes of accelerated ions and wave Poynting vectors while skimming the subsolar magnetopause. The measurements indicate that their source was located to the north of the spacecraft, well removed from expected component merging sites. This represents the first use of wave Poynting flux as a merging discriminator at the magnetopause. We argue that wave Poynting vectors, like accelerated particle fluxes and the Walen tests, are necessary, but not sufficient, conditions, for identifying merging events. The Polar data are complemented with nearly simultaneous measurements from Cluster in the northern cusp, with correlated observations from the SuperDARN radar, to show that the locations and rates of merging vary. Magnetohydrodynamic (MHD) simulations are used to place the measurements into a global context. The MHD simulations confirm the existence of a high-latitude merging site and suggest that Polar and SuperDARN observed effects are attributable to both exhaust regions of a temporally varying X-line. A survey of 13 merging events places the location at high latitudes whenever the interplanetary magnetic field (IMF) clock angle is less than approximately 150 degrees. While inferred high-latitude merging sites favor the antiparallel merging hypothesis, our data alone cannot exclude the possible existence of a guide field. Merging can even move away from equatorial latitudes when the IMF has a strong southward component. MHD simulations suggest that this happens when the dipole tilt angle increases or when IMF B(sub X) increases the effective dipole tilt.

Maynard, N. C.↗
NASA NTRS1975

Scientific results from the Pioneer 11 mission to Jupiter

Results concerning the magnetosphere, the magnetic field, and the radiation belts of the planet Jupiter obtained by Pioneer 10 were supplemented by the observations made during the passage of Pioneer 11. Pioneer 11 provided further evidence that the disk model is inadequate as a description of the Jovian magnetosphere. The new data have made it possible to develop a field model based on an internal dipole, quadrupole, and possibly octopole and an external dipole and quadrupole.

Opp, A. G.↗
NASA NTRS1980

Saturnian trapped radiation and its absorption by satellites and rings - The first results from Pioneer 11

Preliminary results from Pioneer 11 concerning the acceleration and trapping of charged particles in the magnetic field of Saturn are reported. The identification and measurement of the intensities and spectra of charged particle species was performed by an experiment including four charged particle sensor systems, within 20 Saturn radii of the planet. Increases in the intensity of 0.5- to 1.8-MeV protons within 15 Saturn radii indicate the trapping and acceleration of particles in the dipole field region, while a decrease in proton intensity between seven and four Saturn radii is attributed to absorption by Dione and Enceladus and possibly ring material as well. Proton and electron intensity distributions are found to be axially symmetric within four Saturn radii, indicating a centered dipole aligned with the planetary rotation axis. Trapped radiation absorption at the orbit of Mimas is analyzed to obtain an upper limit of 4 x 10 to the -8th Saturn radii-squared/sec to the inward diffusion coefficient; an absorption-like feature observed at L = 2.5 is attributed to a previously unidentified satellite of diameter less than 200 km and semimajor axis 2.51 Saturn radii. Radiation absorption by the newly discovered F ring was also observed, however beneath the A, B and C rings a low flux of high-energy electrons was detected.

Simpson, J. A.↗
NASA NTRS1985

DE-1 measurements of AKR wave directions

In addition to its wave rotation sense, the direction of auroral kilometric radiation (AKR) can also be measured with the plasma wave instrument on Dynamics Explorer 1, from the relative phase of the signals received by its orthogonal electric dipole antennas. By this method, which differs in principle from the previous spin-null method for measuring wave directions, it has been found possible to pinpoint the AKR source by triangulation, using measurements from different points along the DE-1 orbit. The resulting apparent source, in one instance, seemed to occupy a well-defined auroral-zone invariant magnetic latitude and showed the expected increase of altitude with decreasing frequency. An analysis of the method also confirmed the validity of the previous rotation sense measurements.

Calvert, W.↗
NASA NTRS2022

How a Realistic Magnetosphere Alters the Polarizations of Surface, Fast Magnetosonic, and Alfvén Waves

System-scale magnetohydrodynamic (MHD) waves within Earth's magnetosphere are often understood theoretically using box models. While these have been highly instructive in understanding many fundamental features of the various wave modes present, they neglect the complexities of geospace such as the inhomogeneities and curvilinear geometries present. Here, we show global MHD simulations of resonant waves impulsively excited by a solar wind pressure pulse. Although many aspects of the surface, fast magnetosonic (cavity/waveguide), and Alfvén modes present agree with the box and axially symmetric dipole models, we find some predictions for large-scale waves are significantly altered in a realistic magnetosphere. The radial ordering of fast mode turning points and Alfvén resonant locations may be reversed even with monotonic wave speeds. Additional nodes along field lines that are not present in the displacement/velocity occur in both the perpendicular and compressional components of the magnetic field. Close to the magnetopause, the perpendicular oscillations of the magnetic field have the opposite handedness to the velocity. Finally, widely used detection techniques for standing waves, both across and along the field, can fail to identify their presence. We explain how all these features arise from the MHD equations when accounting for a non-uniform background field and propose modified methods that might be applied to spacecraft observations.

ULF Waves↗
NASA NTRS1978

The 3-dimensional radio mapping experiment /SBH/ on ISEE-C

The SBH experiment on ISEE-C will provide maps of the large scale structure of the interplanetary magnetic field from ten solar radii altitude to the earth orbit, in and out of the ecliptic. The SBH instrument will track type III solar radio bursts at 24 frequencies in the range 30 kHz-2 MHz thus providing the positions of 24 points along the line of force which guides the electrons producing the radio radiation. The antennas are two dipoles: one (90 m long) in the spin plane, the other (15 m long) along the spin axis. The receiver was designed for high sensitivity (0.3 microV in 3 kHz BW), high intermodulation rejection (80 dB/1 microV input for order 2 products), large dynamic range (70 dB), high selectivity (-30-dB response 6.5 kHz away from the center frequency of 10.7 MHz for the 3 kHz BW channels), and high reliability (expected orbital life: 3 years).

Knoll, R.↗
NASA NTRS2003

Distributed magnetic field positioning system using code division multiple access

An apparatus and methods for a magnetic field positioning system use a fundamentally different, and advantageous, signal structure and multiple access method, known as Code Division Multiple Access (CDMA). This signal architecture, when combined with processing methods, leads to advantages over the existing technologies, especially when applied to a system with a large number of magnetic field generators (beacons). Beacons at known positions generate coded magnetic fields, and a magnetic sensor measures a sum field and decomposes it into component fields to determine the sensor position and orientation. The apparatus and methods can have a large `building-sized` coverage area. The system allows for numerous beacons to be distributed throughout an area at a number of different locations. A method to estimate position and attitude, with no prior knowledge, uses dipole fields produced by these beacons in different locations.

Prigge, Eric A.↗
NASA NTRS1982

Interplanetary magnetic field control of high-latitude activity on July 29, 1977

Multisatellite particle and magnetic field data for the substorms of July 29, 1977, show auroral-like activity above 80 deg invariant latitude during the recovery period. The movement of auroral zone activity to high latitudes followed the substorm sequence, at which time the inferred interplanetary magnetic field (IMF) was strongly northward. Electron energy spectra indicative of a field-aligned potential drop, and the absence of supporting precipitating ions, are found at latitudes greater than 80 deg. The north-south symmetry of these observations suggests that the events are on closed field lines. It is noted the very strong northward IMF connected to the sunward tilted geomagnetic dipole field plays a role in the driving of strong Birkeland and ionospheric current systems in the northern polar regions, while eliminating them from the southern polar regions.

Zanetti, L. J.↗
NASA NTRS1984

Electromagnetic damping of neutron star oscillations

A simple model of magnetic field perturbations driven by neutron star oscillations is used to estimate the electromagnetic power radiated by g-modes and torsional oscillations. The calculation assumes that the neutron star has a frozen-in magnetic field which is perturbed by the oscillatory motions of the surface. The disturbances propagate into the vacuum as outgoing electromagnetic waves. The relative effectiveness of Joule heating of the neutron star crust by pulsation-induced electric currents is estimated. It is concluded that electromagnetic damping is the dominant energy dissipation mechanism for quadrupole g-mode oscillations of neutron stars. For dipole spheroidal modes, both electromagnetic radiation and Joule heating are important, and there is no gravitational radiation emitted by these modes.

Mcdermott, P. N.↗
NASA NTRS1980

Initial geomagnetic field model from MAGSAT

Magsat data from magnetically quiet days were used to derive a thirteenth degree and order spherical harmonic geomagnetic field model, MGST(3/80). The model utilized both scalar and vector data and fit that data with standard deviations of 8, 52, 55 and 97 nT for the scalar magnitude, B sub r, B sub theta and B sub phi respectively. When compared with earlier models, the Earth's dipole moment continues to decrease at a rate of about 26 nT/year. Evaluation of earlier models with Magsat data shows that the scalar field at the Magsat epoch is best predicted by the POGO(2/72) model but that the AWC/75 and IGS/75 are better for predicting vector fields.

Langel, R. A.↗