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At least 505 records · Page 28
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NASA NTRS1993

Evidence for Day-to-Night Ion Transport at Low Solar Activity in the Venus Pre-Dawn Ionosphere

Periapsis of the Pioneer Venus spacecraft 2 dropped below 180 km on August 28, 1992 near midnight, and 42 orbits of low altitude data at moderately low solar activity in the pre-dawn sector were obtained before contact was lost to the spacecraft in October, 1992. Through a combination of analysis of data from the PV orbiter ion mass spectrometer (OIMS) and modeling, we consider here what can be learned about the relative importance of plasma transport from the dayside and electron precipitation in maintaining the nightside ionosphere during the re-entry period. In particular, we examine here the atomic ion density profiles. We compute the average peak density of O(+) as a function of solar zenith angle and determine what fluxes of atomic ions or precipitating electrons would he necessary to produce those values. We then compare model calculations of the ion densities to those observed during the re-entry period. We find that the low solar activity nightside ionosphere shows evidence of significant day-to-night plasma transport.

Brannon, J. F.↗
NASA NTRS1993

Evidence for Day-to-Night Ion Transport at Low Solar Activity in the Venus Pre-Dawn Ionosphere

Periapsis of the Pioneer Venus spacecraft dropped below 180 km on August 28, 1992 near midnight, and 42 orbits of low altitude data at moderately low solar activity in the pre-dawn sector were obtained before contact was lost to the spacecraft in October, 1992. Through a combination of analysis of data from the PV orbiter ion mass spectrometer (OIMS) and modeling, we consider here what can be learned about the relative importance of plasma transport from the dayside and electron precipitation in maintaining the nightside ionosphere during the re-entry period. In particular, we examine here the atomic ion density profiles. We compute the average peak density of O(+) as a function of solar zenith angle and determine what fluxes of atomic ions or precipitating electrons would be necessary to produce those values. We then compare model calculations of the ion densities to those observed during the re-entry period. We find that the low solar activity nightside ionosphere shows evidence of significant day-to-night plasma transport.

Brannon, J. F.↗
NASA NTRS1984

Deflagration plasma thruster

This paper introduces the application of the magnetized plasma deflagration process to space propulsion. The deflagration process has the unique capability of efficiently converting input energy into kinetic energy in the accelerating direction. To illustrate the totally divergent characters of 'snowplow' detonation and deflagration discharges, examples of the differences between deflagration and detonation 'snowplow' discharges are expressed in terms of current densities, temperature, and particle velocities. Magnetic field profiles of the deflagration mode of discharges are measured. Typical attainable plasma characteristics are described in terms of velocity, electron temperature, and density, as well as measurement techniques. Specific impulses measured by piezo-electric probe and pendulum methods are presented. The influence of the transmission line in the discharge circuits on plasma velocity is measured by means of a microwave time-of-flight method. The results for the deflagration thruster are compared with other space thrusters. Further research areas are identified.

Cheng, D. Y.↗
NASA NTRS1986

Solutions to bi-Maxwellian transport equations for SAR-arc conditions

The first subsonic solutions of the bi-Maxwellian-based 16-moment set of transport equations for stable auroral red (SAR) arc conditions are presented. These are compared with the solutions obtained from the Maxwellian-based 13-moment transport equations for the same boundary conditions. Close agreement between the 16-moment and 13-moment solutions was obtained for the drift velocity, total electron temperature, total proton heat flow, and total electron heat flow profiles. On the other hand, significant discrepancies were found. Thus, the 16-moment density profile falls off more rapidly with increasing altitude than that computed with the 13-moment equations; the total proton temperature is less in the 16-moment case than in the 13-moment case by several thousand degrees at most altitudes; and differences exist in the ratios of the proton and electron temperature anisotropies with the altitude. A simplified set of transport equations was obtained by dropping terms which remain relatively small at all altitudes.

Demars, H. G.↗
NASA NTRS1987

Electron dynamics and potential jump across slow mode shocks

In the de Hoffmann-Teller reference frame, the cross-shock electric field is simply the thermoelectric field responsible for preserving charge neutrality. As such, it gives information regarding the heating and dissipation occurring within the shock. The total cross-shock potential can be determined by integrating a weighted electron pressure gradient through the shock, but this requires knowledge of the density and temperature profiles. Here, a recently proposed alternative approach relying on particle dynamics is exploited to provide an independent estimate of this potential. Both determinations are applied to slow mode shocks which form the plasma sheet boundary in the deep geomagnetic tail as observed by ISEE 3. The two methods correlate well. There is no indication of the expected transition from resistive to viscous shocks, although the highest Mach number shocks show the highest potentials. The implications of these results for the electron dissipation mechanisms and turbulence at the shock are discussed.

Schwartz, Steven J.↗
NASA NTRS1982

Time-dependent calculations of Jupiter's ionosphere

Time-dependent calculations of the vertical distribution of protons in Jupiter's ionosphere show that the accumulation of protons in the topside ionosphere produced from solar ionizing radiation overwhelms the loss to vibrationally excited molecular hydrogen at vibrational temperatures as high at 1600 K. At 2500 K the ionization is decreased over the entire planet with little diurnal variation. For Voyager 1 then, unless the H2 vibrational temperature is as high as thousands of degrees and the topside density of H2 is asymmetric and larger by orders of magnitude, dynamical processes are more likely causes of the low electron densities seen in the nightside upper ionosphere. A calculation of the H3(+) density profile showed that the distribution above the turbopause is controlled by diffusion.

Chen, R. H.↗
NASA NTRS1999

GHRS Observations of Cool, Low-Gravity Stars: The Outer Atmosphere and Wind of the Nearby K Supergiant Lambda Velorum - 5

UV spectra of lambda Velorum taken with the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope are used to probe the structure of the outer atmospheric layers and wind and to estimate the mass-loss rate from this K5 lb-II supergiant. VLA radio observations at lambda = 3.6 cm are used to obtain an independent check on the wind velocity and mass-loss rate inferred from the UV observations, Parameters of the chromospheric structure are estimated from measurements of UV line widths, positions, and fluxes and from the UV continuum flux distribution. The ratios of optically thin C II] emission lines indicate a mean chromospheric electron density of log N(sub e) approximately equal 8.9 +/- 0.2 /cc. The profiles of these lines indicate a chromospheric turbulence (v(sub 0) approximately equal 25-36 km/s), which greatly exceeds that seen in either the photosphere or wind. The centroids of optically thin emission lines of Fe II and of the emission wings of self-reversed Fe II lines indicate that they are formed in plasma approximately at rest with respect to the photosphere of the star. This suggests that the acceleration of the wind occurs above the chromospheric regions in which these emission line photons are created. The UV continuum detected by the GHRS clearly traces the mean flux-formation temperature as it increases with height in the chromosphere from a well-defined temperature minimum of 3200 K up to about 4600 K. Emission seen in lines of C III] and Si III] provides evidence of material at higher than chromospheric temperatures in the outer atmosphere of this noncoronal star. The photon-scattering wind produces self-reversals in the strong chromospheric emission lines, which allow us to probe the velocity field of the wind. The velocities to which these self-absorptions extend increase with intrinsic line strength, and thus height in the wind, and therefore directly map the wind acceleration. The width and shape of these self-absorptions reflect a wind turbulence of approximately equal 9-21 km/s. We further characterize the wind by comparing the observations with synthetic profiles generated with the Lamers et al. Sobolev with Exact Integration (SEI) radiative transfer code, assuming simple models of the outer atmospheric structure. These comparisons indicate that the wind in 1994 can be described by a model with a wind acceleration parameter beta approximately 0.9, a terminal velocity of 29-33 km/s, and a mass-loss rate approximately 3 x 10(exp -9) solar M/yr. Modeling of the 3.6 cm radio flux observed in 1997 suggests a more slowly accelerating wind (higher beta) and/or a higher mass-loss rate than inferred from the UV line profiles. These differences may be due to temporal variations in the wind or from limitations in one or both of the models. The discrepancy is currently under investigation.

Carpenter, Kenneth G.↗
NASA NTRS2000

Near-Simultaneous Polar and DMSP Measurements of Topside Ionosphere Field - Aligned Flows at High Latitudes

Near-simultaneous observations of topside 0' parallel flows are presented for four periods of measurement by POLAR and DMSP satellites during April 1996. The POLAR measurements are from Southern perigee measurements near 5000 km altitude, while the DMSP measurements were from 840 km altitude. These observations provide a view of wide-spread upward/downward ionospheric O(+) flows over the broad polar region along extended and multiple distinct satellite tracks. In general, the velocities are upward toward expected cleft and auroral latitudes, typically about a 2-10 km/s at 5000 km altitude, and 0-2 km/s at 840 km altitudes. At the highest, polar cap latitudes, downward velocities are more frequent at both altitudes, but especially at the lower 840 km altitude. The downward velocities were typically a few hundred d s at 840 altitude, and 0-1 km/s at 5000 km altitude. In some instances, apparently downward velocities were observed at 840 km altitudes while upward 0' flows were observed at 5000 km altitude on the same flux tube. The O+ density were characteristically 1-10 O(+)/cu cm at 5000 km altitude and 10(exp 3) - 10(exp 40 O(+)/cu cm at 840 km altitude, while the O(+) flux were characteristically 1 10(exp 5) - 10(exp 7) O(+)+/sq cm -sec at 5000 km altitudes and characteristically 10(exp 7) - 10(exp 9) O(+)/sq cm -sec at 840 km altitude. We have also examined the dual altitude parameter measurements for a polar cap field line, the POLAR and DMSP measurements approximately 30 minutes apart, and compared them with results from a transport simulation in which a flux tube was subjected to a brief pulse of soft electron precipitation and topside transverse ion heating. The simulated density and velocity altitude profiles for 5 minutes later generally matched the observations, except that the observed downward velocities (500 - 600 m/s) at 840 km altitude were much larger than those simulated.

Zeng, W.↗
NASA NTRS1972

Mars lower atmosphere - Some new implications.

Based on occultation data from the Mariner spacecraft, there are some preliminary indications of an apparent systematically higher surface pressure on the night side of Mars than on the day side. If confirmed by additional data currently becoming available, this apparent surface pressure difference could be attributed to several causes, including topography effects as has been assumed previously, an actual increase of pressure on the night side caused by diurnal effects, an apparent increase on the night side due to some mechanisms such as a dust layer, or an apparent lower pressure on the day side due to the existence of a low-lying electron layer. By calculating the effect of a low-lying electron layer on the combined neutral atmosphere and electron layer refractivity profile, it is found that the day side and night side surface pressures would be in agreement if a layer with maximum density of about 80,000 electrons per cu cm below 15 km altitude were present on the day side.

Michael, W. H., Jr.↗
NASA NTRS2003

On the Determination of Ejecta Structure and Explosion Asymmetry from the X-ray Knots of Cassiopeia A

We present a detailed analysis of Chandra X-ray spectra from individual ejecta knots in the supernova remnant Cassiopeia A. The spectra are fitted to give the electron temperature T(sub e), and (single) ionization age n(sub e)t. These quantities are compared with the predictions of self similar hydrodynamic models incorporating time dependent ionization and radiation losses, and Coulomb electron-ion equilibration behind the reverse shock, for a variety of different ejecta density profiles described by a uniform density core and a power law envelope. We find that the ejecta close to the 'jet' region in the NE, but not actually in the jet itself, have a systematically shallower outer envelope than ejecta elsewhere in the remnant, and we interpret this as being due to more energy of the initial explosion being directed in this polar direction as opposed to equatorially. The degree of asymmetry we infer is at the low end of that generally modelled in asymmetric core-collapse simulations, and may be used to rule out highly asymmetric explosion models.

Laming, J. Martin↗
NASA NTRS1996

Results of Detailed Modeling of the Narrow-Line Region of Seyfert Galaxies

We present model line profiles of [O II] lambda3727, [Ne III] lambda3869, [O I] lambda5007, [Fe VII] lambda6087, [Fe X] lambda6374, [O I] lambda6300, H(alpha) lambda6563, and [S 2] lambda6731. The profiles presented here illustrate explicitly the pronounced effects that collisional de-excitation, and that spatial variations in both the ionization parameter and cloud column density, have on Narrow-Line Region (NLR) model profiles. The above effects were included only qualitatively in a previous analytical treatment by Moore and Cohen. By making a direct correspondence between these model profiles and the analytical model profiles of Moore and Cohen, and by comparing with the observed profiles presented in a companion paper and also with those presented elsewhere in the literature, we strengthen some of the conclusions of Moore and Cohen. Most notably, we argue for constant ionization parameter, uniformly accelerated outflow of clouds that are individually stratified in ionization, and the interpretation of emission-line width correlations with ionization potential as a column density effect. For comparison with previous observational studies, such as our own in a companion paper, we also calculate profile parameters for some of the models, and we present and discuss the resulting line width correlations with critical density (n(sub cr)) and Ionization Potential (IP). Because the models we favor are those that produce extended profile wings as observed in high spectral resolution studies, the line width correlations of our favoured models are of particular interest. Line width correlations with n(sub cr) and/or IP result only if the width parameter is more sensitive to extended profile wings than is the Full Width at Half-Maximum (FWHM). Correlations between FWHM and n(sub cr) and/or IP result only after convolving the model profiles with a broad instrumental profile that simulates the lower spectral resolution used in early observational studies. The model in agreement with the greatest number of observational considerations has electron density decreasing outward from n(sub e) approx. equals 10(exp 6)/cu cm to n(sub e) approx. equals 10(exp 2)/cu cm and, due to collisional de-excitation effects in the lowest velocity clouds, it generates broad flat-topped profile peaks in the lines of lowest critical density (e.g., [O II] lambda3727 and [S II] lambda(lambda)6716, 6731). Because the observed profile peaks of both low and high critical density lines are often very similar, our favored model requires a contribution to NLR emission-line spectra from low-velocity, low-density, and low-ionization gas not included in the model NLR.

Moore, David↗
NASA NTRS1981

Free electron lasers for transmission of energy in space

A one-dimensional resonant-particle model of a free electron laser (FEL) is used to calculate laser gain and conversion efficiency of electron energy to photon energy. The optical beam profile for a resonant optical cavity is included in the model as an axial variation of laser intensity. The electron beam profile is matched to the optical beam profile and modeled as an axial variation of current density. Effective energy spread due to beam emittance is included. Accelerators appropriate for a space-based FEL oscillator are reviewed. Constraints on the concentric optical resonator and on systems required for space operation are described. An example is given of a space-based FEL that would produce 1.7 MW of average output power at 0.5 micrometer wavelength with over 50% conversion efficiency of electrical energy to laser energy. It would utilize a 10 m-long amplifier centered in a 200 m-long optical cavity. A 3-amp, 65 meV electrostatic accelerator would provide the electron beam and recover the beam after it passes through the amplifier. Three to five shuttle flights would be needed to place the laser in orbit.

Segall, S. B.↗
NASA NTRS1973

High-speed spectrograph for shock tube studies

Instrument provides information on spatial distribution of electron density of gas flow in high-performance shock tube. System permits measurement of profiles of hydrogen-alpha and -beta lines with enough spectral range to record spectral features from the near ultraviolet to the long-wavelength cutoff of photographic film.

Borucki, W. J.↗
NASA NTRS1993

On the generation of plasma waves in Saturn's inner magnetosphere

Voyager 1 plasma wave measurements of Saturn's inner magnetosphere are reviewed with regard to interpretative aspects of the wave spectrum. A comparison of the wave emission profile with the electron plasma frequency obtained from in situ measurements of the thermal ion density shows good agreement with various features in the wave data identified as electrostatic modes and electromagnetic radio waves. Theoretical calculations of the critical flux of superthermal electrons able to generate whistler-mode waves and electrostatic electron cyclotron harmonic waves through a loss-cone instability are presented. The comparison of model results with electron measurements shows excellent agreement, thereby lending support to the conclusion that a moderate perpendicular anisotropy in the hot electron distribution is present in the equatorial region of L = 5-8.

Barbosa, D. D.↗
NASA NTRS1982

Theoretical studies of photoexcitation and ionization in H2O

Theoretical studies using Franck-Condon and static-exchange approximations are reported for the complete dipole excitation and ionization spectrum in H2O, where (1) large Cartesian Gaussian basis sets are used to represent the required discrete and continuum electronic eigenfunctions at the ground state equilibrium geometry, and (2) previously devised moment-theory techniques are employed in constructing the continuum oscillator-strength densities from the calculated spectra. Comparisons are made of the calculated excitation and ionization profiles with recent experimental photoabsorption studies and corresponding spectral assignments, electron impact-excitation cross sections, and dipole and synchrotron-radiation studies of partial-channel photoionization cross sections. The calculated partial-channel cross sections are found to be atomic-like, and dominated by 2p-kd components. It is suggested that the latter transition couples with the underlying 1b(1)-kb(1) channel, accounting for a prominent feature in recent synchrotron-radiation measurements.

Diercksen, G. H. F.↗
NASA NTRS1994

Electric currents in the subsolar region of the Venus lower ionosphere

The ion and electron momentum equations, along with Ampere's law, are solved for the ion and electron drift velocities and the electric field in the subsolar Venus ionosphere, assuming a partially ionized gas and a single ion species having the ion mean mass. All collision terms among the ions, electrons and neutral particles are retained in the equations. A general expression for the evolution of the magnetic field is derived and compared with earlier expressions. Subsolar region data in the altitude range 150-300 km from the Pioneer Venus Orbiter are used to calculate altitude profiles of the components of the current due to the electric field, gradients of pressure, and gravity. Altitude profiles of the ion and electron velocities as well as the electric field, electrodynamic heating, and the energy density are determined. Only orbits having a complete set of measured plasma temperatures and densities, neutral densities, and magnetic field were considered for analysis; the results are shown only for orbit 202. The vertical velocity at altitudes above 220 km is upgoing for orbit 202. This result is consistent with observations of molecular ions at high altitudes and of plasma flow to the nightside, both of which require upward velocity of ions from the dayside ionosphere. Above about 230 km the momentum equations are extremely sensitive to the altitude profiles of density, temperature, and magnetic field.

Cole, K. D.↗