Engineering topics

Cravens, T. E.

Publications and source records attributed to Cravens, T. E..

At least 55 records · Page 3

NASA NTRS1990

Electrons in the ionopause boundary layer of Venus

Measurements made by the retarding potential analyzer on board the Pioneer Venus Orbiter (PVO) have indicated the presence of both magnetosheath electrons and atmospheric photoelectrons in the plasma mantle region of Venus, which lies just outside the ionopause. A theoretical model of the spatial and energy distribution of electrons in the plasma mantle has been constructed using the two-stream electron transport method and the time-dependent electron energy equation. Magnetic field lines parallel to the ionopause are assumed. Transport and collisional processes, including electron scattering by neutrals and ions, are taken into consideration. The calculated electron spectra in the mantle show signatures of both solar wind electrons and photoelectrons. Cold ionospheric electrons are present only for altitudes below the ionopause. The calculated electron temperatures agree very well with the PVO data both in the mantle region and in the magnetized ionosphere.

Gan, LU↗

NASA NTRS1990

A two-dimensional kinematic dynamo model of the ionospheric magnetic field at Venus

The results of a high-resolution, two-dimensional, time dependent, kinematic dynamo model of the ionospheric magnetic field of Venus are presented. Various one-dimensional models are considered and the two-dimensional model is then detailed. In this model, the two-dimensional magnetic induction equation, the magnetic diffusion-convection equation, is numerically solved using specified plasma velocities. Origins of the vertical velocity profile and of the horizontal velocities are discussed. It is argued that the basic features of the vertical magnetic field profile remain unaltered by horizontal flow effects and also that horizontal plasma flow can strongly affect the magnetic field for altitudes above 300 km.

Cravens, T. E.↗

NASA NTRS1990

Electron energetics in the inner coma of Comet Halley

A quasi-two-dimensional model of the spatial and energy distribution of electrons in the inner coma of Comet Halley has been constructed from a spherically symmetric ion density profile based on Giotto measurements, using the two-stream electron transport method and the time-dependent electron energy equation. A sharp jump in the electron temperature was found to be present at a cometocentric distance of about 15,000 km. This thermal boundary separates an inner region where cooling processes are dominant from an outer region where heat transport is more important. Both thermal and suprathermal electron populations exist inside the thermal boundary with comparable kinetic pressures. Outside the thermal boundary, a cloud electron population does not exist, and the electrons are almost isothermal along the magnetic field lines.

Gan, LU↗

NASA NTRS1990

Angular distribution of electrons elastically scattered from CH4

Differential elastic (vibrationally) scattering cross sections of CH4 by electron impact have been measured using a modulated crossed-beam method. The energy and angular range covered were from 5 to 50 eV and from 12 to 156 deg, respectively. The integrated and momentum transfer cross sections were obtained from the differential cross sections. The present results are compared with earlier data. Some discrepancies were found in the measurements and theoretical results.

Shyn, T. W.↗

NASA NTRS1989

A magnetohydrodynamical model of the inner coma of Comet Halley

The magnetometer onboard the Giotto spacecraft observed a diamagnetic cavity surrounding the nucleus of Comet Halley. The location of the boundary of this diamagnetic cavity is determined by a balance between an inward magnetic pressure gradient force and an outward ion-neutral drag force, associated with collisions between the outwardly flowing neutrals and the stagnated ions. A one-dimensional time-dependent MHD model has been developed for the inner coma of Comet Halley, and includes ion-neutral collisions, photochemical production and loss of plasma, and finite conductivity. This model is used to investigate the plasma dynamics in the vicinity of the diamagnetic cavity boundary surface. A narrow transition layer with enhanced plasma density is shown to exist just inside the boundary, although a full understanding of this layer will require a two- or three-dimensional MHD model. The flux of cometary ions into this shocklike layer is removed by electron-ion recombination. The thickness of this layer is determined by the Mach number of the incident flow.

Cravens, T. E.↗

NASA NTRS1989

Test particle calculations of pick-up ions in the vicinity of Comet Giacobini-Zinner

Ionization of cometary neutral molecules produces ions which are picked-up by the solar wind. The cometary ion pick-up process for Comet Giacobini-Zinner is studied using a test particle method in which trajectories are numerically calculated for several thousand ions whose initial locations were chosen randomly with probability proportional to the neutral density. The cometary ion distribution function was calculated at several locations upstream of the bow shock. Several types of magnetic fluctuations were included. Circularly and linearly polarized monochromatic waves result in less pitch-angle scattering of pick-up ions than turbulent fluctuations which have about the same amplitude. Some anisotropy remains in the distribution function just upstream of the shock, even for the turbulent fluctuations.

Cravens, T. E.↗

NASA NTRS1989

Solar cycle variations of the electron densities near the ionospheric peak of Venus

Photochemical equilibrium calculations of electron and ion densities, appropriate for altitudes below about 180 km, were carried out for the Venus dayside ionosphere corresponding to solar cycle maximum and minimum conditions. The results were compared with data from radio occultation measurements. The agreement between the calculations and measurements was, in general, quite good. These comparisons indicate that the most commonly used neutral atmosphere model of Venus (Hedin et al., 1983) predicts densities which are somewhat low near the electron density peak for solar cycle maximum, but provides surprisingly good predictions for solar cycle minimum conditions.

Kim, J.↗

NASA NTRS1989

A one-dimensional multispecies magnetohydrodynamic model of the dayside ionosphere of Mars

A one-dimensional multispecies magnetohydrodynamic model of the Martian ionosphere is developed using methods similar to those used by Shinagawa and Cravens (1988) for Venus, and is used to examine the nature of the solar wind interaction with the Martian ionosphere. The four ion species included in the model are CO2(+), O2(+), O(+), and H(+). Scenarios with and without a small intrinsic field are modeled for the Viking conditions (solar minimum). It was found that the inclusion of an intrinsic magnetic field does not improve the agreement between the calculated ion density profiles and the measured ones. The results also indicate that large horizontal plasma motions must be present at high altitudes, indicating that the dynamics of the upper ionosphere of Mars is controlled by the solar wind.

Shinagawa, H.↗

NASA NTRS1989

Pickup ions in the unshocked solar wind at Comet Halley

Data obtained by the Tunde-M experiment aboard the Vega 1 spacecraft indicate that the energetic ion flux begins to increase at a distance of about 10 to the 7th km outside Halley's bow shock, with a number of large enhancements superimposed on the general flux level. The energy spectra of ions were determined in the solar wind reference frame by fitting Maxwellian distributions to the ion fluxes in the relevant energy range, between about 90 and 120 keV; they indicate that the temperatures of the Maxwellian at cometocentric distances of 1-2 million km are similar to the temperatures measured just outside the bow shock of Comet G-Z.

Kecskemety, K.↗

NASA NTRS1989

Cometary plasma boundaries

The solar wind starts to interact with comets at distances from the nucleus of several million kilometers. The nature of the interaction changes as a function of cometocentric distance. Several dynamically important boundaries have been observed in the cometary plasma environment by instruments on several spacecraft: (1) the bow shock marks the transition from supersonic to subsonic solar wind flow, (2) the cometopause was observed at a distance of about 100,000 km from Comet Halley where the flow begins to stagnate and where charge exchange with neutrals becomes important, (3) the diamagnetic cavity boundary (i.e., contact surface, ionopause) separates magnetized and unmagnetized cometary plasma, (4) the magnetotail boundary defines the tail lobes, (5) the plasma sheet boundary defines the extent of the plasma sheet, and (6) the density enhancement layer was observed at a distance of 10,000 km from Comet Halley and might be located where the neutrals and plasma thermally decouple.

Cravens, T. E.↗

NASA NTRS1988

Superthermal electron processes in the upper atmosphere of Uranus - Aurora and electroglow

Strong ultraviolet emissions from the upper atmosphere of Uranus suggest that both auroral and electroglow phenomena are of significant aeronomical consequences in the structure of the upper atmosphere. Combined modeling and data analysis were performed to determine the effect of electroglow and auroral phenomena on the global heat and atomic hydrogen budgets in the Uranus upper atmosphere. The results indicate that the auroral and electroglow heat sources are not adequate to explain the high exospheric temperature observed at Uranus, but that the atomic hydrogen supplied by these processes is more than sufficient to explain the observations. The various superthermal electron distributions modeled have significantly different efficiencies for the various processes such as UV emission, heating, ionization, and atomic hydrogen production, and produce quite different H2 band spectra. However, additional information on the UV spectra and global parameters is needed before modeling can be used to distinguish between the possible mechanisms for electroglow.

Waite, J. H., Jr.↗

NASA NTRS1988

A one-dimensional multispecies magnetohydrodynamic model of the dayside ionosphere of Venus

Using a modification of the one-dimensional multispecies 'one-major-ion' MHD model of Shinagawa et al. (1987), the behaviors of plasma and magnetic field in the dayside ionosphere of Venus was studied for both time-dependent and steady-state conditions. The present model is more complete than the one-major-ion model of Shinagawa et al., although a comparison of the results indicated that the one-major-ion treatment was a fairly good approximation. Two new cases are presented, including steady-state conditions for the magnetized ionosphere, and the inclusion of ion loss due to horizontal transport in the magnetized region. The resulting calculated profiles of the magnetic field and the electron density agree much better with the observations at high altitudes than those without the ion loss terms, indicating the importance of the horizontal transport processes in the ionosphere of Venus at high altitudes.

Shinagawa, H.↗

NASA NTRS1988

Vibrational-excitation cross sections of water molecules by electron impact

A crossed-beam technique was used to measure absolute differential cross sections for the vibrational excitation of water-vapor molecules. The energy and angular range were from 2.2 to 20 eV and from 30 to 150 deg. Vibrational-excitation cross sections were determined for the bending (010) and stretching (100 and 001) modes of the electronic ground state. It is shown that the integral cross sections are generally larger than those of Seng and Linder (1976) by 10-20 percent for both the bending and stretching modes. It is noted that the results obtained are of interest in connection with the theoretical modeling of cometary ionospheres.

Shyn, T. W.↗

NASA NTRS1988

The Jovian Aurora - Electron or ion precipitation?

High signal-to-noise spectra of the Jovian aurora obtained at 1200 to 1500 A by the IUE Observatory were examined for the existence of sulfur and oxygen emissions that would be expected if the UV emissions were produced by precipitating heavy ions. The results of these measurements and subsequent spectral modeling, using a model of heavy aurora constructed by Horanyi et al. (1988), showed mixed evidence of the oxygen and sulfur emissions. It was noted that only the UV emissions which are produced above the UV absorbing hydrocarbon layer were observed by the IUE and Voyager UV spectrometers. This fact, combined with the recent observations of the longitudinal distribution of the Jovian UV aurora, indicates that electrons as well as ions play a role in Jovian auroral processes. Based on the observations, it is suggested that heavy-ion auroral energy deposition is concentrated at altitudes below the homopause, while electrons with energies of 10 to 30 keV are responsible for the bulk of the observable UV and EUV emissions.

Waite, J. H., Jr.↗

NASA NTRS1988

The precipitation of energetic heavy ions into the upper atmosphere of Jupiter

Evidence for auroral particle precipitation at Jupiter was provided by the ultraviolet spectrometers on board the Voyager 1 and 2 spacecraft and by the International Ultraviolet Explorer. Magnetospheric measurements made by instruments on board the Voyager spacecraft indicate that energetic sulfur and oxygen ions are precipitating into the upper atmosphere of Jupiter. A theoretical model describing the interaction of precipitating oxygen with the Jovian atmosphere was constructed. The auroral energy is deposited in the atmosphere by means of ionization, excitation, and dissociation and heating of the atmospheric gas. Energetic ion precipitation and electron precipitation are shown to have similar effects on the atmosphere and ionosphere of Jupiter.

Horanyi, M.↗

NASA NTRS1987

Vibrationally excited molecular hydrogen in the upper atmosphere of Jupiter

A comprehensive theoretical model for vibrationally excited H2 in the upper atmosphere of Jupiter is presented. Theoretical calculations demonstrate the probable existence of significantly enhanced populations of vibrationally excited H2 in the Jovian upper atmosphere, especially in the auroral regions. It is also shown that this H2 is an important chemical sink of H(+) ions in the Jovian ionosphere and thus has a significant effect on the calculated ionospheric electron densities.

Cravens, T. E.↗

NASA NTRS1987

Ion energetics in the inner coma of Comet Halley

The cometary plasma in the magnetic barrier just outside the diamagnetic cavity which surrounds the nucleus of Comet Halley is virtually stagnant. The outflowing neutral gas exerts an outward ion-neutral drag force on this plasma, which balances the inward magnetic pressure gradient force in the vicinity of the contact surface. The cometary ions are frictionally heated due to the relative motion of the ion and neutral gases. The ion flow velocity must have a few km/s nonradial component in order to explain the ion temperatures measured by the ion mass spectrometer on Giotto.

Cravens, T. E.↗

NASA NTRS1987

A one-dimensional time-dependent model of the magnetized ionosphere of Venus

The behavior and time evolution of the large-scale magnetic fields and plasma of the dayside Venus ionosphere are studied using a one-dimensional model. The coupled continuity, momentum, and Maxwell's equations are solved simultaneously for O(+), O2(+), and H(+), and the magnetic field. The calculated magnetic field profiles are in good agreement with Pioneer Venus orbiter magnetometer observations. The magnetic field structure is quasi-steady for slow changes of the solar wind dynamic pressure. The peak at 165 km is maintained by downward convection from higher altitudes. The time scale for the decay of the field by the pure one-dimensional vertical diffusion/convection process is several hours unless the flux is resupplied from the top of the ionosphere.

Shinagawa, H.↗