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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Evidence for magnetic field reconnection at the Earth's magnetopause

Eleven passes of the ISEE satellites through the frontside terrestrial magnetopause were identified, where the plasma velocity in the magnetopause and boundary layer was substantially larger than in the magnetosheath. The nature of the plasma flow, magnetic field, and energetic particle fluxes in these regions were examined, with a view to determining whether the velocity enhancements can be explained by magnetic field reconnection.

Sonnerup, B. U. O.↗
NASA NTRS1980

Ionospheres of the terrestrial planets

The theory and observations relating to the ionospheres of the terrestrial planets Venus, the earth, and Mars are reviewed. Emphasis is placed on comparing the basic differences and similarities between the planetary ionospheres. The review covers the plasma and electric-magnetic field environments that surround the planets, the theory leading to the creation and transport of ionization in the ionospheres, the relevant observations, and the most recent model calculations. The theory section includes a discussion of ambipolar diffusion in a partially ionized plasma, diffusion in a fully ionized plasma, supersonic plasma flow, photochemistry, and heating and cooling processes. The sections on observations and model calculations cover the neutral atmosphere composition, the ion composition, the electron density, and the electron, ion, and neutral temperatures.

Schunk, R. W.↗
NASA NTRS1981

Momentum and energy exchange collision terms for interpenetrating bi-Maxwellian gases

For application to aeronomy and space physics problems involving strongly magnetized plasma flows, we derived momentum and energy exchange collision terms for interpenetrating bi-Maxwellian gases. Collision terms were derived for Coulomb, Maxwell molecule, and constant collision cross-section interaction potentials. The collision terms are valid for arbitrary flow velocity differences and temperature differences between the interacting gases as well as for arbitrary temperature anisotropies. The collision terms had to be evaluated numerically and the appropriate coefficients are presented in tables. However, the collision terms were also fitted with simplified expressions, the accuracy of which depends on both the interaction potential and the temperature anisotropy. In addition, we derived the closed set of transport equations that are associated with the momentum and energy collision terms.

Barakat, A. R.↗
NASA NTRS1981

Tearing modes at the magnetopause

This paper examines the possible occurrence of tearing modes in the dayside magnetopause. First, the expected magnetic signature of tearing, as obtained from existing theory, is reviewed. Magnetometer data from one terrestrial magnetopause crossing of Jupiter's magnetopause are then examined in detail. Magnetic field oscillations are found in three subsegments of the terrestrial crossing at a frequency of 0.1-0.2 Hz and with peak amplitudes of 5-10 nanotesla (nT), and in one segment of the Jovian crossing, at 0.05-0.1 Hz and with 2-nT amplitude. The frequency range, as well as the orientation of the magnetic field perturbation vectors, agrees with a model in which tearing-produced magnetic islands are convected past the satellite with the plasma flow in the current layer. In both cases the magnetopause structure was of the rotational discontinuity type with a nonvanishing normal magnetic field component. Hence, if the tearing structures were active, i.e., growing, at the observation site, ion tearing (or perhaps resistive tearing, with the resistivity provided by microturbulence) must be invoked. But it is also possible that the structures were passive, consisting of 'debris' from active tearing elswhere on the magnetopause surface, this debris being convected along the magnetopause past the observation site.

Greenly, J. B.↗
NASA NTRS1981

A three-ring circuit model of the magnetosphere

The magnetosphere is modeled by superimposing a dipole field, a uniform field and a perturbation field due to a simple current system. This current system comprises a ring current in the neutral line of the dipole plus uniform fields, together with vertical currents representing field-aligned currents to the neutral line. The current circuit is closed through two additional ring currents above and below the equatorial plane representing distributed adiabatic perpendicular currents. This system produces many magnetospheric features, among them a magnetopause, bending of magnetic field lines in the anti-solar direction, a magnetotail, and cusps in the day-side of the earth. The objective is to demonstrate that it is not necessary to think of the magnetic field topology as being caused by the flowing plasma carrying field lines. The primary physical problem is to derive the current system from the self-consistent interaction of the solar-wind and magnetospheric plasmas and fields.

Whipple, E. C., Jr.↗
NASA NTRS1981

Solar wind pickup of ionized Venus exosphere atoms

Previous calculations of electrostatic and electromagnetic growth rates for plasma instabilities have neglected the thermal spread of the distribution function of the planetary ions. We consider the effects of finite temperatures for exospheric ions borne in the solar wind. Specifically, growth rates are calculated for electromagnetic instabilities in the low-frequency case for Alfven waves and the intermediate frequency case for whistlers. Also, electrostatic growth rates are calculated for the intermediate frequency regime. From these growth rates, estimates are derived for the pickup times of the planetary ions. The electromagnetic instabilities are shown to produce the most rapid pickup. In the situation where the angle between the local Venus magnetic field and the plasma flow direction is small, the pickup times for both electromagnetic and electrostatic instabilities become very long. A possible consequence of this effect is to produce regions of enhanced planetary ion density in favorable Venus magnetic field-solar wind flow geometries.

Curtis, S. A.↗
NASA NTRS1981

Catalytic surface effects experiment on the Space Shuttle

A Space Shuttle experiment planned to measure the surface catalytic efficiency of the baseline high-temperature reusable surface insulation (HRSI) during earth entry is described. A spray-on overcoat, with high catalytic efficiency, will be used as a comparative basis for determining the HRSI surface catalytic efficiency through surface temperature measurement. Catalytic efficiency, as well as aerothermal response of the overcoat, was evaluated, using various models made of HRSI material in arc-plasma flow environments. Agreement is obtained between the measured and computed heating rise of the coated surfaces. Computed predictions for the flight case are presented.

Stewart, D. A.↗
NASA NTRS1981

Planetary magnetospheres

A concise overview is presented of our understanding of planetary magnetospheres (and in particular, of that of the Earth), as of the end of 1981. Emphasis is placed on processes of astrophysical interest, e.g., on particle acceleration, collision-free shocks, particle motion, parallel electric fields, magnetic merging, substorms, and large scale plasma flows. The general morphology and topology of the Earth's magnetosphere are discussed, and important results are given about the magnetospheres of Jupiter, Saturn and Mercury, including those derived from the Voyager 1 and 2 missions and those related to Jupiter's satellite Io. About 160 references are cited, including many reviews from which additional details can be obtained.

Stern, D. P.↗
NASA NTRS1981

On the origin of the MeV energy nucleon flux associated with CIRs

Measurements of recurrent enhancements of interplanetary nucleon flux in the MeV energy range are presented and interpreted. The features recur at the solar rotation period in association with stream-stream plasma interaction regions corotating with the sun. At distances from the sun less than 1 AU, the maximum intensities of the hydrogen and helium components increase with increasing distance between 300%/AU and 600%/AU. A model is proposed which predicts the acceleration of nuclei from keV plasma energies to MeV energies by means of transit time damping of magnetosonic waves as solar wind plasma flows from the sun. Numerical solutions of the transport equation are derived to demonstrate that the model does not reproduce radial variations of hydrogen and helium fluxes, and estimates are made of the radial diffusive mean free path. Finally, the observations are found to be consistent with nucleon acceleration at the CIR shocks beyond 1 AU with subsequent diffusion toward the sun.

Christon, S. P.↗
NASA NTRS1981

The orientation and motion of the predawn current sheet and Jupiter's magnetotail

From the observed times when Pioneer 10 and Voyager 1 observe particle flux maxima and minima and when the magnetic field strength is at minimum a geometric description of the current sheet in Jupiter's magnetosphere is derived. It lags the planetary rotation by an angle delta which varies as a function of radial distance as delta equals 22 deg + 0.80 (r - 16.3). The maximum latitude of the current sheet is roughly 10 deg inside of 60 Jovian radii and decreases beyond. Thus there is a hinge point near 60 Jovian radii. The position of the hinge point moves in response to the external solar wind pressure variation. The variation of delta(r) is discussed theoretically and how it is related to the magnetic field topology and plasma flow is shown. It is concluded that the distant field lines do not corotate with Jupiter and that the Alfven speed at 50 Jovian radii is of the order of 1500 km/s. The topology of the current sheet is affected by centrifugal forces, outward mass transport and solar wind-magnetosphere interaction. It is shown how these effects can be related to different current systems.

Goertz, C. K.↗
NASA NTRS1981

Hollow cathode startup using a microplasma discharge

Attention is given to a microplasma discharge to initiate a hollow cathode discharge for such applications as plasma flow experiments, the electric propulsion of space vehicles, and as a replacement for filament cathodes in neutral beam injector ion sources. The technique results in a cathode that is easy to start, simple in design, and which does not require external RF exciters, inserts or heating elements. Future applications may include ion beam milling and ion implantation.

Aston, G.↗
NASA NTRS1982

On the structure of the Io Torus

Since the plasma flow near Io is reduced, neutrals originating in charge exchange are not energetic enough to leave the Jovian system and are therefore distributed over an extensive region, as indicated by the sodium cloud. New ions subsequently created in the distributed neutral atomic cloud as a result of charge exchange or electron impact ionization are picked up by the corotating magnetic field, and the radial current driven by the pickup process cannot close in the Io torus, so that it must instead be connected to the planetary ionosphere by field-aligned currents. These field-aligned currents will flow away from the equator at the outer edge of the neutral cloud, and towards it at the inner edge. It is found that the Jovian ionospheric photoelectrons also cannot supply the current flowing away from the equator, so that torus ions accelerated by a parallel electric field could be involved. The parallel potential drop is large enough to push the torus into the Jovian atmosphere, explaining both the sharp, discontinuous change of flux tube content and ion temperature at L equals 5.6, as well as the generation of Auroral-type hiss at that point.

Goertz, C. K.↗
NASA NTRS1983

Comparison of model high-lititude electron densities with Millstone Hill observations

The predictions of a high-latitude ionospheric model are compared with the diurnal variations of plasma convection velocities and electron densities observed at Millstone Hill on a geomagnetically moderately active day near equinox. The observed convection pattern was consistent with a two-cell, asymmetric pattern with enhanced plasma flow in the dusk sector, with flow speeds reaching 1.5 km/s. In the dusk strong convection cell, the falloff of the magnetospheric potential with latitude was proportional to the inverse of the sine of colatitude to the fourth power. On the dayside, a region of high density occurred at 500 km in the 1000-1900 LT sector. The nocturnal midlatitude trough was deepest and widest and reached its most equatorward position in the morning sector. The model, which is based on average auroral precipitation fluxes, can describe the gross features of the enhanced densities in the auroral zone.

Sojka, J. J.↗
NASA NTRS1983

Theory of hydromagnetic turbulence

The present state of MHD turbulence theory as a possible solar wind research tool is surveyed. The theory is statistical, and does not make statements about individual events. The ensembles considered typically have individual realizations which differ qualitatively, unlike equilibrium statistical mechanics. Most of the theory deals with highly symmetric situations; most of these symmetries have yet to be tested in the solar wind. The applicability of MHD itself to solar wind parameters is highly questionable; yet it has no competitors, as a potentially comprehensive dynamical description. The purpose of solar wind research require sharper articulation. If they are to understand radial turbulent plasma flows from spheres, laboratory experiments and numerical solution of equations of motion may be cheap alternative to spacecraft. If "real life" information is demanded, multiple spacecraft with variable separation may be necessary to go further. The principal emphasis in the theory so far has been on spectral behavior for spatial covariances in wave number space. There is no respectable theory of these for highly anisotropic situations. A rather slow development of theory acts as a brake on justifiable measurement, at this point.

Montgomery, D.↗
NASA NTRS1983

Interaction of the plasma tail of comet Bradfield 1979L on 1980 February 6 with a possibly flare-generated solar-wind disturbance

Solar wind plasma data from the ISEE-3 and Helios 2 spacecraft were examined to explain a uniquely rapid 10 deg turning of the plasma tail of comet Bradfield 1979L on 1980 February 6. It was suggested that the tail position angle change occurred in response to a solar wind velocity shear across which the polar component changed by approx. 50 km s-1. The present activity was caused by noncorotating, disturbed plasma flows probably associated with an Importance 1B solar flare.

Niedner, M. B., Jr.↗
NASA NTRS1984

Plasma and magnetospheric research

Methods employed in the analysis of plasmas and the magnetosphere are examined. Computer programs which generate distribution functions are used in the analysis of charging phenomena and non maxwell plasmas in terms of density and average energy. An analytical model for spin curve analysis is presented. A program for the analysis of the differential ion flux probe on the space shuttle mission is complete. Satellite data analysis for ion heating, plasma flows in the polar cap, polar wind flow, and density and temperature profiles for several plasmasphere transits are included.

Comfort, R. H.↗
NASA NTRS1983

Interaction of the plasma tail of comet Bradfield 1979L on 1980 February 6 with a possibly flare-generated solar-wind disturbance

Solar-wind plasma data from the ISEE-3 and Helios 2 spacecraft were examined in order to explain a uniquely rapid 10 deg turning of the plasma tail of comet Bradfield 1979l on 1980 February 6. An earlier study conducted before the availability of in situ solar-wind data (Brandt et al., 1980) suggested that the tail position angle change occurred in response to a solar-wind velocity shear across the polar component changed by approximately 50 km/s. The present contribution confirms this result and further suggests that the comet-tail activity was caused by non-corotating, disturbed plasma flows probably associated with an Importance 1B solar flare.

Niedner, M. B., Jr.↗
NASA NTRS1984

Plasma and magnetospheric research

Research activities on the following topics were summarized: (1) software for the Space Plasma computer Analysis Network (SPAN), (2) plasmaspheric field-aligned temperature gradients, (3) the shift in spacecraft potential as a function of plasma density, (4) plasma flow, (5) the Fabry-Perot interferometer, and (6) the Differential Ion Flux Probe (DIFP).

Comfort, R. H.↗