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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

Magnetopause characteristics at 0840-1040 hours local time

An analysis of three-dimensional energetic particle distributions for 14 consecutive ISEE satellite orbits during magnetopause crossings and close approaches is presented. The data were collected from the Nov. 10 through Dec. 11, 1977, time period and cover local times of 0840 to 1040 hours. It was found that the magnetopause in this period defined by energetic particles can be represented as sharp particles and a well defined boundary for magnetospherically trapped particles for a wide range of magnetospheric activity and magnetosheath field conditions. The magnetopause position, orientation, and velocity were determined for all identified magnetopause crossings and close approaches using the technique of Williams (1979); it was found that the magnetopause is nearly always in motion with velocities ranging from near zero to at least plus or minus 25 km/s. Correlations with published plasma, magnetic field, and plasma wave magnetopause identifications show the energetic particle results to be accurate and an important factor in determining magnetopause characteristics and behavior.

Williams, D. J.↗
NASA NTRS1983

ISEE-3 - A late entry in the great comet chase

The ISEE-3 spacecraft, currently in a figure-8 traverse of the geomagnetic tail between 80-220 earth radii, is being redirected to a flyby of the Giacobini-Zinner comet in 1985. The satellite carries experiments for the solar wind plasma, magnetic field data, plasma wave and composition, radio waves, and energetic protons, and can gather data on the solar wind-comet interaction. A double lunar swingby will be employed to establish the intercept trajectory, yielding a total velocity change of 100 m/sec, with a reserve of 100 m/sec for midcourse corrections. The comet will be encountered at a distance of 3000 km in September 1985, followed by passage through the Halley's comet tail 5000 km from the nucleus in March 1986.

Farquhar, R.↗
NASA NTRS1986

The Jovian magnetotail

The current understanding of the plasma, magnetic field, and plasma wave characteristics of the near and distant Jovian magnetotail is discussed. The properties of these two regions are compared, with particular emphasis given to the data on the distant tail obtained by Voyager 2. The Jovian tail has many unusual properties, such as the large scale sausage-string shape of its outer boundary, but some of its properties resemble those of the earth's magnetotail, such as its central current sheet and a surrounding region resembling a plasma sheet consisting of hot ions. Data are presented that support the view that plasma in the Jovian tail plasma sheet comes from Jupiter.

Lepping, R. P.↗
NASA NTRS1990

Discovery of a cyclotron absorption line in the spectrum of the binary X-ray pulsar 4U 1538 - 52 observed by Ginga

A cyclotron absorption line near 20 keV has been found in the spectrum of the massive eclipsing binary X-ray pulsar 4U 1538 - 52 in observations with the Ginga observatory. The line is detected throughout the 529 s pulse cycle with a variable equivalent width that has its maximum value during the smaller peak of the two-peak pulse profile. It is found that the profile of the pulse and the phase-dependence of the cyclotron line can be explained qualitatively by a pulsar model based on recent theoretical results on the properties of pencil beams emitted by accretion-heated slabs of magnetized plasma at the magnetic poles of a neutron star. The indicated field at the surface of the neutron star is 1.7 (1 + z) x 10 to the 12th G, where z is the gravitational redshift.

Clark, George W.↗
NASA NTRS1992

Reconnection of stressed magnetic fields

It is shown that magnetized plasma configurations under magnetic stress relax irreversibly to the state of minimum stress at a rate that is essentially Alfvenic provided a magnetic null is present. The relaxation is effected by the reconnection at the field null and proceeds at a rate proportional to the absolute value of ln(eta) exp-1, where eta is the resistivity. An analytic calculation in the linear regime is presented.

Hassam, A. B.↗
NASA NTRS1987

Emerging flux, magnetic reconnection, plasma turbulence and waves in the transition zone of the solar atmosphere

Ultraviolet spectroscopy of the solar transition zone, especially Spacelab 2 results, is reviewed. Emerging magnetic flux is considered as the energy source of the solar corona and the solar wind. The conversion of magnetic into kinetic energy is facilitated in the transition zone because of unique conditions. Radiation losses are sufficient to create fast instabilities. Observed nonthermal velocities in spectra of the transition zone indicate the existence of strong plasma oscillations and turbulence. The small filling factor of the transition zone indicates a highly filamentary structure of density, magnetic field strength, and currents, which results in rapid reconnection time scales. Particle acceleration and the heating of the corona are placed in the areas of strong plasma oscillations of the transition zone.

Brueckner, G. E.↗
NASA NTRS1995

Radial energy transport by magnetospheric ULF waves: Effects of magnetic curvature and plasma pressure

The 'radial' transport of energy by internal ULF waves, stimulated by dayside magnetospheric boundary oscillations, is analyzed in the framework of one-fluid magnetohydrodynamics. (the term radial is used here to denote the direction orthogonal to geomagnetic flux surfaces.) The model for the inhomogeneous magnetospheric plasma and background magnetic field is axisymmetric and includes radial and parallel variations in the magnetic field, magnetic curvature, plasma density, and low but finite plasma pressure. The radial mode structure of the coupled fast and intermediate MHD waves is determined by numerical solution of the inhomogeneous wave equation; the parallel mode structure is characterized by a Wentzel-Kramer-Brillouin (WKB) approximation. Ionospheric dissipation is modeled by allowing the parallel wave number to be complex. For boudnary oscillations with frequencies in the range from 10 to 48 mHz, and using a dipole model for the background magnetic field, the combined effects of magnetic curvature and finite plasma pressure are shown to (1) enhance the amplitude of field line resonances by as much as a factor of 2 relative to values obtained in a cold plasma or box-model approximation for the dayside magnetosphere; (2) increase the energy flux delivered to a given resonance by a factor of 2-4; and (3) broaden the spectral width of the resonance by a factor of 2-3. The effects are attributed to the existence of an 'Alfven buoyancy oscillation,' which approaches the usual shear mode Alfven wave at resonance, but unlike the shear Alfven mode, it is dispersive at short perpendicular wavelengths. The form of dispersion is analogous to that of an internal atmospheric gravity wave, with the magnetic tension of the curved background field providing the restoring force and allowing radial propagation of the mode. For nominal dayside parameters, the propagation band of the Alfven buoyancy wave occurs between the location of its (field line) resonance and that of the fast mode cutoff that exists at larger radial distances.

Kouznetsov, Igor↗
NASA NTRS1985

Plasma-dominated magnetic field configurations for the magnetosphere of Uranus

There is significant indirect evidence that the planet Uranus possesses a magnetic field. This evidence is based on the observation of hydrogen Lyman alpha emission from Uranus with the aid of the International Ultraviolet Explorer (IUE) spacecraft. The detection of water ice on the Uranian moons led Cheng (1984) to suggest that charged particle sputtering of the icy satellites could provide a significant internal source of oxygen ions and protons to the Uranian magnetosphere. Cheng concluded that this mechanism would predict aurorae around both magnetic poles of Uranus. Cheng's idea of the presence of a continuous internal plasma supply to the Uranian magnetosphere is further pursued in the present investigation. Questions are considered regarding the evolution of Uranus' magnetosphere from a vacuum configuration toward a plasma pressure dominated equilibrium configuration, taking into account the amount of the thermal plasma pressure as a free parameter.

Ip, A. K.↗
NASA NTRS1989

Currents between tethered electrodes in a magnetized laboratory plasma

Laboratory experiments on important plasma physics issues of electrodynamic tethers were performed. These included current propagation, formation of wave wings, limits of current collection, nonlinear effects and instabilities, charging phenomena, and characteristics of transmission lines in plasmas. The experiments were conducted in a large afterglow plasma. The current system was established with a small electron-emitting hot cathode tethered to an electron-collecting anode, both movable across the magnetic field and energized by potential difference up to V approx.=100 T(sub e). The total current density in space and time was obtained from complete measurements of the perturbed magnetic field. The fast spacecraft motion was reproduced in the laboratory by moving the tethered electrodes in small increments, applying delayed current pulses, and reconstructing the net field by a linear superposition of locally emitted wavelets. With this technique, the small-amplitude dc current pattern is shown to form whistler wings at each electrode instead of the generally accepted Alfven wings. For the beam electrode, the whistler wing separates from the field-aligned beam which carries no net current. Large amplitude return currents to a stationary anode generate current-driven microinstabilities, parallel electric fields, ion depletions, current disruptions and time-varying electrode charging. At appropriately high potentials and neutral densities, excess neutrals are ionized near the anode. The anode sheath emits high-frequency electron transit-time oscillations at the sheath-plasma resonance. The beam generates Langmuir turbulence, ion sound turbulence, electron heating, space charge fields, and Hall currents. An insulated, perfectly conducting transmission line embedded in the plasma becomes lossy due to excitation of whistler waves and magnetic field diffusion effects. The implications of the laboratory observations on electrodynamic tethers in space are discussed.

Stenzel, R. L.↗
NASA NTRS2011

Solar Eruptions, CMEs and Space Weather

Coronal mass ejections (CMEs) are large-scale magnetized plasma structures ejected from the Sun and propagate far into the interplanetary medium. CMEs represent energy output from the Sun in the form of magnetized plasma and electromagnetic radiation. The electromagnetic radiation suddenly increases the ionization content of the ionosphere, thus impacting communication and navigation systems. The plasma clouds can drive shocks that accelerate charged particles to very high energies in the interplanetary space, which pose radiation hazard to astronauts and space systems. The plasma clouds also arrive at Earth in about two days and impact Earth's magnetosphere, producing geomagnetic storms. The magnetic storms result in a number of effects including induced currents that can disrupt power grids, railroads, and underground pipelines. This lecture presents an overview of the origin, propagation, and geospace consequences of solar storms.

Gopalswamy, Nat↗
NASA NTRS1989

Nonlinear twist-kink instability of a coronal loop

Three-dimensional magnetoinductive particle simulations are used to demonstrate that the mechanical twisting motion applied to a magnetized plasma column induces a current aligned to the external magnetic field direction, pinches the plasma and magnetic fields, and stores the energy in poloidal magnetic fields. As the twist motion continues, the field lines locally begin to wrap around the plasma more than one revolution. A strong MHD instability sets in that is a mixture of kink and ballooning modes, releasing the magnetic energy and causing destruction of coherent column structure and flows of turbulent plasma. A similar episode ensues, exhibiting relaxation oscillations. The buildup of poloidal fields and structure and its sudden release driven by the twist motion may be a model for the solar coronal loop dynamics which exhibits a slow energy buildup with some photospheric motion and a sudden energy release by flares.

Zaidman, E. G.↗
NASA NTRS2003

Magnetic Flux Compression Experiments Using Plasma Armatures

Magnetic flux compression reaction chambers offer considerable promise for controlling the plasma flow associated with various micronuclear/chemical pulse propulsion and power schemes, primarily because they avoid thermalization with wall structures and permit multicycle operation modes. The major physical effects of concern are the diffusion of magnetic flux into the rapidly expanding plasma cloud and the development of Rayleigh-Taylor instabilities at the plasma surface, both of which can severely degrade reactor efficiency and lead to plasma-wall impact. A physical parameter of critical importance to these underlying magnetohydrodynamic (MHD) processes is the magnetic Reynolds number (R(sub m), the value of which depends upon the product of plasma electrical conductivity and velocity. Efficient flux compression requires R(sub m) less than 1, and a thorough understanding of MHD phenomena at high magnetic Reynolds numbers is essential to the reliable design and operation of practical reactors. As a means of improving this understanding, a simplified laboratory experiment has been constructed in which the plasma jet ejected from an ablative pulse plasma gun is used to investigate plasma armature interaction with magnetic fields. As a prelude to intensive study, exploratory experiments were carried out to quantify the magnetic Reynolds number characteristics of the plasma jet source. Jet velocity was deduced from time-of-flight measurements using optical probes, and electrical conductivity was measured using an inductive probing technique. Using air at 27-inHg vacuum, measured velocities approached 4.5 km/s and measured conductivities were in the range of 30 to 40 kS/m.

Turner, M. W.↗
NASA NTRS2012

Three-Dimensional Magnetohydrodynamic Modeling of the Solar Wind Including Pickup Protons and Turbulence Transport

To study the effects of interstellar pickup protons and turbulence on the structure and dynamics of the solar wind, we have developed a fully three-dimensional magnetohydrodynamic solar wind model that treats interstellar pickup protons as a separate fluid and incorporates the transport of turbulence and turbulent heating. The governing system of equations combines the mean-field equations for the solar wind plasma, magnetic field, and pickup protons and the turbulence transport equations for the turbulent energy, normalized cross-helicity, and correlation length. The model equations account for photoionization of interstellar hydrogen atoms and their charge exchange with solar wind protons, energy transfer from pickup protons to solar wind protons, and plasma heating by turbulent dissipation. Separate mass and energy equations are used for the solar wind and pickup protons, though a single momentum equation is employed under the assumption that the pickup protons are comoving with the solar wind protons.We compute the global structure of the solar wind plasma, magnetic field, and turbulence in the region from 0.3 to 100 AU for a source magnetic dipole on the Sun tilted by 0 deg - .90 deg and compare our results with Voyager 2 observations. The results computed with and without pickup protons are superposed to evaluate quantitatively the deceleration and heating effects of pickup protons, the overall compression of the magnetic field in the outer heliosphere caused by deceleration, and the weakening of corotating interaction regions by the thermal pressure of pickup protons.

Usmanov, Arcadi V.↗
NASA NTRS1986

Detailed observations of the plasma sheet during a substorm on April 24, 1979

Plasma, magnetic field, and energetic particle data obtained by ISEE 1 and 2 satellites for the April 24, 1979 substorm are studied in relation to the neutral line model and the boundary layer model. The ISEE 1 and 2 instruments and experiments utilized to collect the data are discussed. The major reconfiguration of the tail plasma and magnetic field plasma region, and the plasma ion flows observed support the neutral line model for interpreting substorms. The plasma ion distribution function and plasma flow are examined. The region of lobe-plasma sheet interface referred to as the separatrix layer is identified. The differences in times of plasma sheet dropout and recovery, and absence or presence of flux anisotropies are investigated. Energetic particle measurements are analyzed to study the relationship between energetic ions and plasma ions, and the velocity distributions. The data support the application of the neutral line model to the evaluation of substorms; however, the data are inconsistent with the boundary layer dynamics model.

Hones, E. W., Jr.↗
NASA NTRS1987

The electron distribution function upstream from the earth's bow shock

A general analytic theory for the distribution function of particles backstreaming from an arbitrary shock in a magnetized plasma is presented. Particle motions are shown to be restricted to two-dimensional planes. A general form for the source term describing upstreaming particles accelerated at the shock is presented along with explicit source terms for planar and parabolic shocks. The origin and form of the escape velocity cutoff for a shock in a magnetized plasma are discussed. The distribution function upstream of a finite planar shock and a parabolic shock in two dimensions is calculated, and an analytic approximation of the cutoff velocity at points in the upstream region is derived. The theory is then applied to the earth's bow shock, calculating distribution functions and the nature and spatial variation of the cutoff velocity. The theory for the particle distribution upstream of the bow shock is compared with the model of Filbert and Kellogg (1979).

Cairns, Iver H.↗
NASA NTRS1987

Plasma and magnetic field variations in the distant magnetotail associated with near-earth substorm effects

Examination of many individual event periods in the ISEE 3 deep-tail data set has suggested that magnetospheric substorms produce a characteristic pattern of effects in the distant magnetotail. During the growth, or tail-energy-storage phase of substorms, the magnetotail appears to grow diametrically in size, often by many earth radii. Subsequently, after the substorm expansive phase onset at earth, the distant tail undergoes a sequence of plasma, field, and energetic-particle variations as large-scale plasmoids move rapidly down the tail following their disconnection from the near-earth plasma sheet. ISEE 3 data are appropriate for the study of these effects since the spacecraft remained fixed within the nominal tail location for long periods. Using newly available auroral electrojet indices (AE and AL) and Geo particle data to time substorm onsets at earth, superposed epoch analyses of ISEE 3 and near-earth data prior to, and following, substorm expansive phase onsets have been performed. These analyses quantify and extend substantially the understanding of the deep-tail pattern of response to global substorm-induced dynamical effects.

Baker, D. N.↗