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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.

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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Satellite sweeping in offset, tilted dipole fields

The paper presents a theory for the longitudinally averaged satellite sweeping rate in an offset, tilted dipole magnetic field. It includes the reductions in the sweeping rate when the moon radius is not large compared to the gyroradius or the azimuthal drift distance during a bounce period. With a large tilt angle between the magnetic and rotation axes, moons make large excursions in dipole L value, and more than one moon can sweep at a given L. The sweeping rate peaks at the minimum L for each moon. If the gyroradius is large, additional peaks can occur when the particle mirrors near the moon latitude. To illustrate the theory, sweeping rates are evaluated for parameters relevant to the observations at Uranus by the Voyager 2 Low Energy Charged Particles Experiment. Calculated sweeping rates for ions and electrons are typically two or three orders of magnitude less than the strong-diffusion loss rate. The observation of sweeping signatures at Uranus would imply that pitch-angle scattering there occurs at a rate far below the strong-diffusion limit, contrasting with the situation for energetic ions in the inner Jovian magnetosphere.

Paonessa, Mark T.↗
NASA NTRS1999

Discovery of a New Soft Gamma Repeater, SGR 1627-41

We report the discovery of a new soft gamma repeater (SGR), SGR 1627-41, and present BATSE observations of the burst emission and BeppoSAX Narrow-Field Instrument observations of the probable persistent X-ray counterpart to this SGR. All but one burst spectrum are well fit by an optically thin thermal bremsstrahlung model with kT values between 25 and 35 keV. The spectrum of the X-ray counterpart, SAX J1635.8-4736, is similar to that of other persistent SGR X-ray counterparts. We find weak evidence for a periodic signal at 6.41 s in the light curve for this source. Like other SGRs, this source appears to be associated with a young supernova remnant, G337.0-0.1. Based upon the peak luminosities of bursts observed from this SGR, we find a lower limit on the dipole magnetic field of the neutron star of B(sub dipole) approximately > 5 x 10(exp 14) G.

Woods, Peter M.↗
NASA NTRS2012

Pair Cascades and Deathlines in Magnetic Fields with Offset Polar Caps

We present results of electron-positron pair cascade simulations in a dipole magnetic field whose polar cap is offset from the dipole axis. In such a field geometry, the polar cap is displaced a small fraction of the neutron star radius from the star symmetry axis and the field line radius of curvature is modified. Using the modified parallel electric field near the offset polar cap, we simulate pair cascades to determine the pair deathlines and pair multiplicities as a function of the offset. We find that the pair multiplicity can change dr;unatically with a modest offset, with a significant increase on one side of the polar cap. Lower pair deathlines allow a larger fraction of the pulsar population, that include old and millisecond pulsars, to produce cascades with high multiplicity. The results have some important implications for pulsar particle production, high-energy emission and cosmic-ray contribution.

Harding, Alice K.↗
NASA NTRS1972

Vanishing of dipole matrix elements at level crossings.

Demonstration that the vanishing of certain coupling matrix elements at level crossings follow from angular momentum commutation relations. A magnetic dipole transition having delta M = plus or minus 1, induced near a crossing of the levels in a nonzero magnetic field, is found to have a dipole matrix element comparable to or smaller than the quotient of the level separation and the field. This result also applies in the analogous electric field electric dipole case.

Kocher, C. A.↗
NASA NTRS1990

Global magnetic anomaly and aurora of Neptune

The large offset and tilt of Neptune's dipole magnetic field combine to create a global magnetic anomaly, analogous to but much more important than earth's South Atlantic Anomaly. Energetic particle precipitation loss within the Neptune anomaly creates 'atmospheric drift shadows' within which particle fluxes are greatly reduced. The energetic particle dropout observed by Voyager near closest approach occurred near the predicted times when Voyager passed within the atmospheric drift shadow. Extremely soft, structured bursts of ions and electrons within the drift shadow may result from plasma wave-induced pitch angle scattering of trapped particles confined near the magnetic equator. The dropout does not necessarily imply that Voyager passed through an earth-like discrete auroral zone, as earlier reported. The ion and electron fluxes observed within the dropout period correspond to particles that must precipitate to Neptune's atmosphere within the anomaly region. This anomaly precipitation can account for a major portion of the ultraviolet emissions previously identified as Neptune aurora.

Cheng, Andrew F.↗
NASA NTRS1985

Pulsars as cosmic ray accelerators: Energy development of protons

Results of numerical integrations of the Lorentz-Dirac-equation with Landau approximation for protons in the electromagnetic vacuum field of a magnetic dipole rotating with its angular velocity omega perpendicular to its magnetic moment mu will be discussed with special attention to the energy development of protons.

Thielheim, K. O.↗
NASA NTRS1985

Pulsars as cosmic ray particle accelerators: Dynamics of electrons

The Lorentz-Dirac-equation with Landau approximation has been solved numerically for electrons in the electromagnetic field of a magnetic dipole rotating with the angular velocity omega perpendicular to its magnetic moment mu. Results are discussed with respect to electron orbits and energy development.

Thielheim, K. O.↗
NASA NTRS1998

An X-ray Pulsar with a Superstrong Magnetic Field in the Soft Gamma-Ray Repeater SGR1806-20

Soft gamma-ray repeaters (SGRs) emit multiple, brief (approximately O.1 s) intense outbursts of low-energy gamma-rays. They are extremely rare; three are known in our galaxy and one in the Large Magellanic Cloud. Two SGRs are associated with young supernova remnants (SNRs), and therefore most probably with neutron stars, but it remains a puzzle why SGRs are so different from 'normal' radio pulsars. Here we report the discovery of pulsations in the persistent X-ray flux of SGR1806-20, with a period of 7.47 s and a spindown rate of 2.6 x 10(exp -3) s/yr. We argue that the spindown is due to magnetic dipole emission and find that the pulsar age and (dipolar) magnetic field strength are approximately 1500 years and 8 x 10(exp 14) gauss, respectively. Our observations demonstrate the existence of 'magnetars', neutron stars with magnetic fields about 100 times stronger than those of radio pulsars, and support earlier suggestions that SGR bursts are caused by neutron-star 'crust-quakes' produced by magnetic stresses. The 'magnetar' birth rate is about one per millenium, a substantial fraction of that of radio pulsars. Thus our results may explain why some SNRs have no radio pulsars.

Kouveliotou, C.↗
NASA NTRS1985

The microwave structure of hot coronal loops

The thermal cyclotron emission from model dipole magnetic loops is computed. It is shown that a simple, isothermal dipole loop can show a great deal of spatial and polarization structure at microwave frequencies. This structure is sensitive to the observation frequency and angle of observation. Two qualitatively distinct microwave loop structures can be distinguished: (1) 'thin loop,' observed as a string of independent microwave peaks, corresponding to different harmonics of the local electron gyrofrequency; and (2) 'thick loop,' the harmonics are merged, so that a more continuous microwave structure is observed. It is shown that the presence of an external plasma can result in a change in the observed mode of polarization along one leg of a loop, without a change in the sign of the line-of-sight magnetic field. Models such as these, along with high-resolution microwave and related observations, can provide an excellent diagnostic of the magnetic and plasma properties of coronal loops.

Holman, G. D.↗
NASA NTRS1992

Venus internal magnetic field and its interaction with the interplanetary magnetic field

In a previous study, Knudsen et al. suggested that Venus has a weak internal magnetic dipole field of the order of 7 x 10 + 20 G cm(exp -3) that is manifested in the form of magnetic flux tubes threading the ionospheric holes in the Venus nightside ionosphere. They pointed out that any internal field of Venus, dipole or multipole, would be weakened in the subsolar region and concentrated in the antisolar region of the planet by the supersonic transterminator convection of the dayside ionosphere into the nightside hemisphere. The inferred magnitude of the dipole field does not violate the upper limit for an internal magnetic field established by the Pioneer Venus magnetometer experiment. The most compelling objection to the model suggested by Knudsen et al. has been the fact that it does not explain the observed interplanetary magnetic field (IMF) control of the polarity of the ionospheric hole flux tubes. In this presentation I suggest that a magnetic reconnection process analogous to that occurring at earth is occurring at Venus between the IMF and a weak internal dipole field. At Venus in the subsolar region, the reconnection occurs within the ionosphere. At Earth it occurs at the magnetopause. Reconnection will occur only when the IMF has an appropriate orientation relative to that of the weak internal field. Thus, reconnection provides a process for the IMF to control the flux tube polarity. The reconnection in the subsolar region takes place in the ionosphere as the barrier magnetic field is transported downward into the lower ionosphere by downward convection of ionospheric plasma and approaches the oppositely directed internal magnetic field that is diffusing upward. The reconnected flux tubes are then transported anti-Sunward by the anti-Sunward convecting ionospheric plasma as well as by the anti-Sunward-flowing solar wind. Reconnection will also occur in the Venus magnetic tail region, somewhat analogously to the reconnection that occurs in the magnetotail of the Earth. The possibility that reconnection is occurring between the IMF and an internal dipole field may be tested by measuring the orientation of the IMF projected into a plane perpendicular to the solar wind velocity during time intervals for which ionospheric holes are observed. The orientations of the IMV components should fall within a 180 deg angle.

Knudsen, W. C.↗
NASA NTRS1979

Quantum-mechanical calculation of three-dimensional atom-diatom collisions in the presence of intense laser radiation

A formalism is presented for describing the collision of fluorine with the hydrogen molecule in the presence of intense radiation. For a laser frequency on the order of the spin-orbit splitting of fluorine, the interaction of the molecular system with the radiation occurs at relatively long range where, for this system, the electric dipole is vanishingly small. Hence the interaction occurs due to the magnetic dipole coupling. Even so, at low collision energies a substantial enhancement of the quenching cross section is found for a radiation intensity of 10 to the 11th W/sq cm.

Devries, P. L.↗
NASA NTRS1988

Laboratory behavior of a plasma plume injected into the magnetized plasma flow

Plasma plumes are injected into the tail of the simulated earth's magnetosphere produced by an interaction between the simulated solar wind and a magnetic dipole. The behavior of laboratory artificial plasma plumes injected into the magnetized plasma flow is discussed in conjunction with the AMPTE artificial comet experiments (Minami et al., 1986) and other active chemical release experiments in space.

Minami, Shigeyuki↗
NASA NTRS1992

Large-gap magnetic positioning system having advantageous configuration

A magnetic configuration was devised in which the positioned object is maintained in a stable orientation and position on one side of an opaque plane surface entirely by means of magnetic components on the other side of the plane. The system is effective with or without gravity, and can operate in any orientation. In this system, the positioned object need only contain a simple dipole magnet. The positioning components consist of a group of permanent magnets creating a magnetic field configuration which stabilizes the levitated dipole in all but one degree of freedom, and a magnetic position sensing and force feedback system to actively stabilize the object in the one unstable direction. The system utilizes very low power at equilibrium and can maintain gaps of 50 mm.

Chong, Paul↗
NASA NTRS2004

Rotational Sweepback of Magnetic Field Lines in Geometrical Models of Pulsar Radio Emission

We study the rotational distortions of the vacuum dipole magnetic field in the context of geometrical models of the radio emission from pulsars. We find that at low altitudes the rotation deflects the local direction of the magnetic field by at most an angle of the order of r(sup 2 sub n), where r(sub n) = r/R(sub lc), r is the radial distance and R(sub lc) is the light cylinder radius. To the lowest (i.e. second) order in r(sub n) this distortion is symmetrical with respect to the plane containing the dipole axis and the rotation axis ((Omega, mu) plane). The lowest order distortion which is asymmetrical with respect to the (Omega, mu) plane is third order in r(sub n). These results confirm the common assumption that the rotational sweepback has negligible effect on the position angle (PA) curve. We show, however, that the influence of the sweep back on the outer boundary of the open field line region (open volume) is a much larger effect, of the order of r(sup 1/2 sub n). The open volume is shifted backwards with respect to the rotation direction by an angle delta(sub o nu) approx. 0.2 sin alpha r(sup 1/2 sub n) where alpha is the dipole inclination with respect to the rotation axis. The associated phase shift of the pulse profile Delta phi(sub o nu) approx. 0.2 r(sup 1/2 sub n) can easily exceed the shift due to combined effects of aberration and propagation time delays (approx. 2r(sub n)). This strongly affects the misalignment of the center of the PA curve and the center of the pulse profile, thereby modifying the delay radius relation. Contrary to intuition, the effect of sweepback dominates over other effects when emission occurs at low altitudes. For r(sub n) < or approx. 3 x 10(exp -3) the shift becomes negative, i.e. the center of the position angle curve precedes the profile center. With the sweepback effect included, the modified delay-radius relation predicts larger emission radii and is in much better agreement with the other methods of determining r(sub n).

Dyks, J.↗
NASA NTRS1988

Numerical simulation of mass injection for the formation of prominence magnetic field configurations. II - Symmetric injection

A two-dimensional MHD model simulating the formation of Kippenhahn-Schluter (1957) quiescent prominence (QP) magnetic field configurations is used to explore symmetric mass injection into a dipole magnetic field. An optimum magnetic field strength for QP formation by mass injection is obtained. It is found that a weaker magnetic field strength is more favorable for the condensation of the injected plasma but that a stronger field is more favorable for supporting the condensed plasma against gravity.

An, C.-H.↗
NASA NTRS2012

Pulsar Pair Cascades in Magnetic Fields with Offset Polar Caps

Neutron star magnetic fields may have polar caps (PC) that are offset from the dipole axis, through field-line sweepback near the light cylinder or non-symmetric currents within the star. The effects of such offsets on electron-positron pair cascades are investigated, using simple models of dipole magnetic fields with small distortions that shift the PCs by different amounts or directions. Using a Monte Carlo pair cascade simulation, we explore the changes in the pair spectrum, multiplicity and energy flux across the PC, as well as the trends in pair flux and pair energy flux with spin-down luminosity, L(sub sd). We also give an estimate of the distribution of heating flux from returning positrons on the PC for different offsets. We find that even modest offsets can produce significant increases in pair multiplicity, especially for pulsars that are near or beyond the pair death lines for centered PCs, primarily because of higher accelerating fields. Pair spectra cover several decades in energy, with the spectral range of millisecond pulsars (MSPs) two orders of magnitude higher than for normal pulsars, and PC offsets allow significant extension of all spectra to lower pair energies. We find that the total PC pair luminosity L(sub pair) is proportional to L(sub sd), with L(sub pair) approximates 10(exp -3) L(sub sd) for normal pulsars and L(sub pair) approximates 10(exp -2) L(sub sd) for MSPs. Remarkably, the total PC heating luminosity for even large offsets increases by less than a factor of two, even though the PC area increases by much larger factors, because most of the heating occurs near the magnetic axis.

Harding, Alice K.↗
NASA NTRS1992

Structure of the magnetopause current layer at the subsolar point

Attention is given to a 1D electromagnetic particle simulation model developed for the magnetopause current layer between the shocked solar wind and the dipole magnetic field at the subsolar point, which was extended to include the IMF in the solar wind. Interaction of the solar wind with the vacuum dipole field as well as the dipole field filled with a low-density magnetospheric plasma are investigated. The width and structure of the magnetopause current layer are found to differ markedly depending on the direction of the IMF. When the IMF is pointing southward, the current layer between the solar wind and the dipole field is narrow, and the magnetic field has a single ramp structure caused by the reflection of the solar wind at that point. The current layer becomes several times wider and the magnetic field develops a multiple ramp structure when the IMF is northward. Comparisons of these simulation results with the recent spacecraft data of the magnetopause crossing near the subsolar point are provided.

Okuda, H.↗