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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 415 records · Page 23
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NASA NTRS2018

Magnetic Fields of Extrasolar Planets: Planetary Interiors and Habitability

Jupiter’s radio emission has been linked to its planetary-scale magnetic field, and spacecraft investigations have revealed that most planets, and some moons, have or had a global magnetic field. Generated by internal dynamos, magnetic fields are one of the few remote sensing means of constraining the properties of planetary interiors. For the Earth, its magnetic field has been speculated to be partially responsible for its habitability, and knowledge of an extrasolar planet’s magnetic field may be necessary to assess its habitability. The radio emission from Jupiter and other solar system planets is produced by an electron cyclotron maser, and detections of extrasolar planetary electron cyclotron masers will enable measurements of extrasolar planetary magnetic fields.

Brain, D.A.↗
NASA NTRS1994

Gravitational instabilities in a proto-planetary disk including the effects of magnetic fields

We investigate the gravitational instability of a thin, Keplerian protoplanetary disk including the effects of a largely azimuthal magnetic field. The model follows that of our previous work (Noh, Vishniac, & Cochran 1991) except for the inclusion of a magnetic field. The disk is assumed to consist of neutral and ionized gas and neutral dust which are coupled by gravity and friction. The growth rates and eigenfunctions are calculated numerically using nonaxisymmetric linear perturbation methods. The results show that the growth rate has a maximum at some intermediate azimuthal number m, but for each value of m it is reduced relative to the unmagnetized case. The effects of the magnetic field appear more strongly on small scales. As the strength of the equilibrium magnetic field increases the growth rates decrease, and the maximum instability occurs at a lower value on m due to the increasing magnetic pressure. The response of each component to the magnetic field is discussed using the behavior of the eigenfunctions in the radial direction. With the inclusion of the magnetic field, the effects of the ionization fraction and friction on the growth rates also appear to be important for high m modes. Increasing the ionization fraction or the friction suppresses instability, but only slightly changes the maximally unstable azimuthal scales. The enhanced growth rates due to a dust component for which thermal pressure is negligible are somewhat reduced by the inclusion of a magnetic field. The effects of different boundary conditions (reflecting and transmitting) on the growth rates are also shown.

Noh, Hyerim↗
NASA NTRS2003

Magnetic Field Would Reduce Electron Backstreaming in Ion Thrusters

The imposition of a magnetic field has been proposed as a means of reducing the electron backstreaming problem in ion thrusters. Electron backstreaming refers to the backflow of electrons into the ion thruster. Backstreaming electrons are accelerated by the large potential difference that exists between the ion-thruster acceleration electrodes, which otherwise accelerates positive ions out of the engine to develop thrust. The energetic beam formed by the backstreaming electrons can damage the discharge cathode, as well as other discharge surfaces upstream of the acceleration electrodes. The electron-backstreaming condition occurs when the center potential of the ion accelerator grid is no longer sufficiently negative to prevent electron diffusion back into the ion thruster. This typically occurs over extended periods of operation as accelerator-grid apertures enlarge due to erosion. As a result, ion thrusters are required to operate at increasingly negative accelerator-grid voltages in order to prevent electron backstreaming. These larger negative voltages give rise to higher accelerator grid erosion rates, which in turn accelerates aperture enlargement. Electron backstreaming due to accelerator-gridhole enlargement has been identified as a failure mechanism that will limit ionthruster service lifetime. The proposed method would make it possible to not only reduce the electron backstreaming current at and below the backstreaming voltage limit, but also reduce the backstreaming voltage limit itself. This reduction in the voltage at which electron backstreaming occurs provides operating margin and thereby reduces the magnitude of negative voltage that must be placed on the accelerator grid. Such a reduction reduces accelerator- grid erosion rates. The basic idea behind the proposed method is to impose a spatially uniform magnetic field downstream of the accelerator electrode that is oriented transverse to the thruster axis. The magnetic field must be sufficiently strong to impede backstreaming electrons, but not so strong as to significantly perturb ion trajectories. An electromagnet or permanent magnetic circuit can be used to impose the transverse magnetic field downstream of the accelerator-grid electrode. For example, in the case of an accelerator grid containing straight, parallel rows of apertures, one can apply nearly uniform magnetic fields across all the apertures by the use of permanent magnets of alternating polarity connected to pole pieces laid out parallel to the rows, as shown in the left part of the figure. For low-temperature operation, the pole pieces can be replaced with bar magnets of alternating polarity. Alternatively, for the same accelerator grid, one could use an electromagnet in the form of current-carrying rods laid out parallel to the rows.

Foster, John E.↗
NASA NTRS2000

The Effect of a Rotating Magnetic Field on Flow Stability During Crystal Growth

The effect of a rotating magnetic field on the stability of flow in crystal growth configurations has been experimentally modeled using liquid gallium contained in a finite cylinder and heated from below. Several distinct flow regions were determined as a function of the Rayleigh and Hartmann numbers. At low values of the Rayleigh and Hartmann numbers, a region of stationary flow exists. As the rotating magnetic field is increased, the critical Rayleigh number bounding the stationary flow region can increase by a factor of 10. However, the rotating magnetic field itself induces an instability at a critical value of the Hartmann number independent of the Rayleigh number. In the stationary flow region, the rotating magnetic field can induce fluid motion with velocities several orders of magnitude larger than typical semiconductor crystal growth velocities. Thus, a rotating magnetic field can be used to achieve the benefits of forced convection without triggering deleterious instabilities.

Volz, Martin P.↗
NASA NTRS1992

Internal dynamics of a plasma propelled across a magnetic field

When a plasma is pushed across a magnetic field by some nonelectromagnetic force, ions and electrons get turned in opposite directions by the magnetic field. An exact analysis of that process is presented here for the internal region of the plasma. The energy provided by the initial push is used, in part, to create the electric field and in part to create some gyrations inside the plasma. When the rest energy density of the plasma exceeds twice the magnetic energy density (or when the Alfven speed is less than c), there will be enough energy to spare for the plasma to continue across the magnetic field at half its initial momentum. Two cases are considered: an impulsive start and a gentle push such as provided by gravity. The amplitude of the resulting internal gyrations becomes small in the second case. The frequencies of the gyrations are those of extraordinary modes of very long spatial wavelength.

Buneman, Oscar↗
NASA NTRS2006

3-D RPIC Simulations of Relativistic Jets: Particle Acceleration, Magnetic Field Generation, and Emission

Recent PIC simulations using injected relativistic electron-ion (electro-positron) jets into ambient plasmas show that acceleration occurs in relativistic shocks. The Weibel instability created in shocks is responsible for particle acceleration, and generation and amplification of highly inhomogeneous, small-scale magnetic fields. These magnetic fields contribute to the electron's transverse deflection in relativistic jets. The "jitter" radiation from deflected electrons has different properties than the synchrotron radiation which is calculated in a uniform magnetic field. This jitter radiation may be important to understand the complex time evolution and spectral structure in relativistic jets and gamma-ray bursts. We will present recent PIC simulations which show particle acceleration and magnetic field generation. We will also calculate associated self-consistent emission from relativistic shocks.

Nishikawa, K.-I.↗
NASA NTRS2021

Maps of Magnetic Field Strength in the OMC-1 Using HAWC+FIR Polarimetric Data

Far-infrared dust polarimetry enables the study of interstellar magnetic fields via tracing of the polarized emission from dust grains that are partially aligned with the direction of the field. The advent of high-quality polarimetric data has permitted the use of statistical methods to extract both the direction and magnitude of the magnetic field. In this work, the Davis–Chandrasekhar–Fermi technique is used to make maps of the plane-of-sky (POS) component of the magnetic field in the Orion Molecular Cloud (OMC-1) by combining polarization maps at 53, 89, 154 and 214 μm from HAWC+/SOFIA with maps of density and velocity dispersion. In addition, maps of the local dispersion of polarization angles are used in conjunction with Zeeman measurements to estimate a map of the strength of the line-of-sight (LOS) component of the field. Combining these maps, information about the threedimensional magnetic field configuration (integrated along the LOS) is inferred over the OMC-1 region. POS magnetic field strengths of up to 2 mG are observed near the BN/KL object, while the OMC-1 bar shows strengths of up to a few hundred μG. These estimates of the magnetic field components are used to produce maps of the mass-to-magnetic-flux ratio (M/Φ)—a metric for probing the conditions for star formation in molecular clouds— and determine regions of sub- and supercriticality in OMC-1. Such maps can provide invaluable input and comparison to MHD simulations of star formation processes in filamentary structures of molecular clouds. Unified Astronomy Thesaurus concepts: Molecular clouds (1072); Giant molecular clouds (653); Interstellar magnetic fields (845); Far infrared astronomy (529)

Jordan A Guerra↗
NASA NTRS1992

The magnetic field of Neptune

A model is given of the planetary magnetic field of Neptune based on a spherical harmonic analysis of the observations obtained by the Voyager 2. Generalized inverse techniques are used to partially solve a severely underdetermined inverse problem, and the resulting model is nonunique since the observations are limited in spatial distribution. Dipole, quadrupole, and octupole coefficients are estimated independently of other terms, and the parameters are shown to be well constrained by the measurement data. The large-scale features of the magnetic field including dipole tilt, offset, and harmonic content are found to characterize a magnetic field that is similar to that of Uranus. The traits of Neptune's magnetic field are theorized to relate to the 'ice' interior of the planet, and the dynamo-field generation reflects this poorly conducting planet.

Connerney, J. E. P.↗
NASA NTRS1992

Evolution of vector magnetic fields and the August 27 1990 X-3 flare

Vector magnetic fields in an active region of the sun are studied by means of continuous observations of magnetic-field evolution emphasizing magnetic shear build-up. The vector magnetograms are shown to measure magnetic fields correctly based on concurrent observations and a comparison of the transverse field with the H alpha fibril structure. The morphology and velocity pattern are examined, and these data and the shear build-up suggest that the active region's two major footprints are separated by a region with flows, new flux emergence, and several neutral lines. The magnetic shear appears to be caused by the collision and shear motion of two poles of opposite polarities. The transverse field is shown to turn from potential to sheared during the process of flux cancellation, and this effect can be incorporated into existing models of magnetic flux cancellation.

Wang, Haimin↗
NASA NTRS2016

Penetration of Magnetosheath Plasma into Dayside Magnetosphere: Magnetic Field in Plasma Filaments - 2.

In this paper, we examined plasma structures (filaments), observed in the dayside magnetosphere but containing magnetosheath plasma. These filaments show the stable antisunward motion (while the ambient magnetospheric plasma moved in the opposite direction) and the existence of a strip of magnetospheric plasma, separating these filaments from the magnetosheath. These results, however, contradict both theoretical studies and simulations by Schindler (1979), Ma et al. (1991), Dai and Woodward (1994, 1998), and other researchers, who reported that the motion of such filaments through the magnetosphere is possible only when their magnetic field is directed very close to the ambient magnetic field, which is not the situation that is observed. In this study, we show that this seeming contradiction may be related to different events as the theoretical studies and simulations are related to the case when the filament magnetic field is about aligned with filament orientation, whereas the observations show that the magnetic field in these filaments may be rotating. In this case, the rotating magnetic field, changing incessantly its direction, drastically affects the penetration of plasma filaments into the magnetosphere. In this case, the filaments with rotating magnetic field, even if in each moment it is significantly inclined to the ambient magnetic field, may propagate through the magnetosphere, if their average (for the rotation period) magnetic field is aligned with the ambient magnetic field. This shows that neglecting the rotation of magnetic field in these filaments may lead to wrong results.

Lyatsky, Wladislaw↗
NASA NTRS1993

The generation of magnetic fields by the polarization electric field in the ionosphere of Venus

Measurements by the magnetometer on the Pioneer Venus orbiter have established that during conditions of low solar wind dynamic pressure, large-scale magnetic fields are not present in the ionosphere of Venus but that during conditions of high solar wind dynamic pressure the ionosphere of Venus is magnetized. The source of the magnetic field is thought to be currents induced in the ionosphere by the solar wind. We will show that ionospheric polarization electric field can act as a source, or 'battery', producing a small magnetic field, even without any initial magnetic field. We have calculated this polarization source as a function of altitude and solar zenith angle. The magnetic field was then determined using a 2D kinematic dynamo model of the ionosphere of Venus. The magnetic field attains a maximum strength of about 5 nT at a solar zenith angle of about 120 deg. This magnetic field might act as a 'seed' field for magnetic flux ropes and terminator waves.

Shinagawa, H.↗
NASA NTRS1994

Charged-particle motion in multidimensional magnetic-field turbulence

We present a new analysis of the fundamental physics of charged-particle motion in a turbulent magnetic field using a numerical simulation. The magnetic field fluctuations are taken to be static and to have a power spectrum which is Kolmogorov. The charged particles are treated as test particles. It is shown that when the field turbulence is independent of one coordinate (i.e., k lies in a plane), the motion of these particles across the magnetic field is essentially zero, as required by theory. Consequently, the only motion across the average magnetic field direction that is allowed is that due to field-line random walk. On the other hand, when a fully three-dimensional realization of the turbulence is considered, the particles readily cross the field. Transport coefficients both along and across the ambient magnetic field are computed. This scheme provides a direct computation of the Fokker-Planck coefficients based on the motions of individual particles, and allows for comparison with analytic theory.

Giacalone, J.↗