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NASA NTRS1987

Finite Larmor radius effect on ion pickup at Venus

The interaction of the solar wind with Venus is influenced by the pickup of newly born exospheric oxygen ions by the convecting magnetosheath plasma. The flow and field configuration of the magnetosheath plasma, together with the large gyroradius of the pickup ions, cause mass loading to occur preferentially on one side of the magnetosheath. The observed hemispherical asymmetry in the magnetic field in the near-planet magnetosheath, attributed to this pickup process, is confirmed by direct observation of the picked-up planetary particles. Test particle calculations show that a current system created by ion pickup has the appropriate location and magnitude to account for the magnetic field asymmetry. The results indicate that a fluid treatment of the Venus mass-loading problem is not entirely appropriate; a hybrid or kinetic model is necessary to incorporate the finite Larmor radius of the pickup particles which produces the observed asymmetry.

Phillips, J. L.↗

NASA NTRS1987

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 NTRS1987

Eclipse measurements of Io's sodium atmosphere

The satellites of Jupiter eclipsed each other in 1985, and these events allowed an unusual measurement of the sodium in Io's extended atmosphere. Europa was used as a mirror to look back through the Io atmosphere at the sun. The measured column abundances suggest that the atmosphere is collisionally thin above 700 km and may be collisionally thin to the surface. The sodium radial profile above 700 km resembles a 1500 K exosphere with a surface density near 20,000 sodium atoms per cubic centimeter, but a complete explanation of the dynamics requires a more complex nonthermal model: the calculated loss rates suggest that the atmosphere is being replaced on a time scale of hours.

Schneider, Nicholas M.↗

NASA NTRS1987

Ion escape fluxes from the terrestrial high-latitude ionosphere

In this paper, the hydrodynamic transport equations for H(+) and O(+) are solved, including the important dynamic, collisional, and chemical effects that operate in the F region ionosphere below regions of ion acceleration. It is found that the most important parameter controlling the amount of O(+) in plasma outflows is the total ion flux demand imposed on the ionosphere by the higher-altitude acceleration region. The O(+) content is further modulated by the temperature of the exosphere and the resultant composition in the topside ionosphere, and by the location of the lower boundary of the ion acceleration region relative to the crossover altitude, where O and H have equal densities. As solar activity increases, the limiting O(+) escape flux increases, while the limiting H(+) escape flux decreases.

Barakat, A. R.↗

NASA NTRS1988

Preshock region acceleration of implanted cometary H(+) and O(+)

A self-consistent, three-fluid model of plasma transport and implanted ion acceleration in the unshocked solar wind is presented. The solar wind plasma is depleted by charge exchange with the expanding cometary exosphere, while implanted protons and heavy ions are produced by photoionization and charge transfer and lost by charge exchange. A generalized transport equation describing convection, adiabatic and diffusive velocity change, and the appropriate production terms is used to describe the evolution of the two cometary ion components, while the moments of the Boltzmann equation are used to calculate the solar wind density and pressure. The flow velocity is obtained self-consistently by combining the conservation equations of the three ion species. The results imply that second-order Fermi acceleration can explain the implanted spectra observed in the unshocked solar wind. Comparison of measured and calculated distribution indicates that spatial diffusion of implanted ions probably plays an important role in forming the energetic particle environment in the shock vicinity.

Gombosi, Tamas I.↗

NASA NTRS1987

Planetary studies; Proceedings of the Topical Meeting C3, Workshop III, and Symposium 8 of the Twenty-sixth COSPAR Plenary Meeting, Toulouse, France, June 30-July 11, 1986

Topics discussed in this issue include the atmospheres and ionospheres of planets and comets, future planetary missions, and latest results on Venus and Uranus missions. Papers are presented on the structure of the atmospheres of Mars and Venus below 100 kilometers, the structure of the atmospheres of Mars and Venus from the analysis of Fourier spectrometer measurements aboard Venera 15, physical parameters for the Saturn atmosphere computed by using Voyager data, hot hydrogen and deuterium in the exosphere of Venus, the theory and observations of cometary ionospheres, and the scientific goals of the Phobos mission. Additional papers are on the geological interpretation of a segment of Venus from radar images of the USSR spacecrafts Venera-15 and -16, the low-energy plasma in the Uranian magnetosphere, the initial estimates of Venus winds from ground-based radio tracking of the Vega balloons, the results of the Vega 1 balloon nephelometer experiment, and meteorological variations in the middle clouds of Venus from Vega balloon in situ measurements.

Keating, G. M.↗

NASA NTRS1987

Neutral upper atmospheres of Venus and Mars

Results obtained over the last 21 years from satellite measurements of the neutral upper atmospheres of Venus and Mars are discussed. Particular attention is given to the chemistry of the neutral upper atmospheres of the two planets, the exospheric temperatures, and the sensitivity of their atmospheric temperatures to solar activity. Various theoretical models used to interpret the planetary atmospheric data are discussed.

Keating, G. M.↗

NASA NTRS1988

An analysis of satellite observations of the O I EUV dayglow

Observations of the latitudinal variation of the 1356-A, 1304-A, and 989-A O I earth dayglow, obtained with the USAF STP78-1 satellite at solar maximum on March 21, 1979, are reported and compared with the predictions of thermospheric models and published laboratory measurements of electron-impact excitation cross sections. The results are presented in extensive tables and graphs and characterized in detail, with particular attention to the 1304-A solar flux and the 989-A branching ratio. The STP78-1 data (as well as simultaneous AE-E satellite measurements ) are shown to indicate exospheric temperatures that were 220-240-K higher, but varied significantly less with latitude, than those predicted by the MSIS-83 model (Hedin, 1983) for the day of the observations.

Link, R.↗

NASA NTRS1988

Asymmetries in the location of the Venus ionopause

Four large data sets of Venus ionopause observations, based on different definitions and observations by Pioneer Venus Orbiter field and particle instruments, are compared. Regardless of the definition chosen, the ionopause is observed to be higher on average at dawn than at dusk. The results of an empirical ionosphere model and a data analysis technique which minimizes orbital bias are combined to show that most or all of this asymmetry results from the effects of orbital bias and uneven sampling of solar wind conditions. However, an additional asymmetry is found in which the ionopause is displaced in the direction opposite to the motional electric field of the solar wind. This effect is attributed to asymmetric pickup of exospheric ions by the electric and magnetic fields in the magnetosheath. This ion pickup interpretation can be understood in terms of a previous observed magnetosheath magnetic field asymmetry. Quantitative analysis indicates that the transverse momentum imparted by the solar plasma to pickup particles in their initial gyration is transmitted by the magnetosheath magnetic field to the ionosphere boundary.

Phillips, J. L.↗

NASA NTRS1988

A general model for Io's neutral gas clouds. II - Application to the sodium cloud

A previously developed neutral cloud model is applied to the neutral sodium cloud produced by Io. Past models for the cloud are reviewed and the motivation for developing better models is discussed. The interactions of sodium in the plasma torus are discussed. It is shown how the new model can be used to provide a consistent physical explanation for the basic physical characteristics of the spatially extended Io sodium cloud and for the east-west intensity asymmetry observed very near Io. This leads to definite conclusions regarding the flux distribution and density of sodium in Io's exosphere, the level of the collisional exobase in the atmosphere, the nature of the interaction of the corotating plasma torus and the satellite atmosphere/corona, and the presence of an east-west electric field in the planetary system.

Smyth, W. H.↗

NASA NTRS1988

Hot oxygen atoms in the upper atmospheres of Venus and Mars

This paper presents the results of calculations of the hot oxygen atom density values for the upper atmospheres of Venus (computed for solar cycle maximum) and for that of Mars (for solar cycle minimum conditions), using a two stream model to calculate the equilibrium fluxes and energy distribution of the nonthermal component of atomic oxygen. After the energy-dependent hot oxygen fluxes are calculated below the exosphere, the densities above the exobase are obtained using Liouville's equation. The values of hot oxygen atoms calculated for Venus were found to be in good agreement with the values observed by Pioneer Venus Orbiter; the Venus values are about a factor of twenty larger than the calculated values for Mars at the exobase.

Nagy, Andrew↗

NASA NTRS1988

Constraints on Titan's ionosphere

The near flyby of Saturn's moon Titan by Voyager 1 revealed a Venus-like interaction between the moon and Saturn's magnetospheric plasma. Although neither the radio science experiment occultation observation nor the in-situ measurements directly detected the ionosphere, plasma of ionospheric origin was observed as Voyager 1 passed through Titan's wake. Balancing the magnetic pressure in this low-beta region of Saturn's magnetosphere with ionospheric particle pressure yields an upper limit on the ionospheric density. Using an ionospheric temperature equal to the exospheric temperature of 200 K yields a charge density of about 3000/cu cm, which is consistent with the peak ionospheric electron density inferred from a balance of electron impact ionization of molecular nitrogen and recombination loss. Both of these quantities are consistent with limits derived from Voyager 1 observations. Good constraints on these quantities are important in planning the Cassini mission to orbit Saturn and probe Titan's ionosphere and atmosphere at the beginning of the next century.

Mcnutt, Ralph L., Jr.↗

NASA NTRS1988

Io: IUE observations of its atmosphere and the plasma torus

Two of the main components of the atmosphere of Io, neutral oxygen and sulfur, were detected with the IUE. Four observations yield brightnesses that are similar, regardless of whether the upstream or the downstream sides of the torus plasma flow around Io is observed. A simple model requires the emissions to be produced by the interaction of O and S columns in the exospheric range with 2 eV electrons. Cooling of the 5 eV torus electrons is required prior to their interaction with the atmosphere of Io. Inconsistencies in the characteristics of the spectra that cannot be accounted for in this model require further analysis with improved atomic data. The Io plasma torus was monitored with the IUE. The long-term stability of the warm torus is established. The observed brightnesses were analyzed using a model of the torus, and variations of less than 30 percent in the composition are observed, the quantitative results being model dependent.

Ballester, G. E.↗

NASA NTRS1988

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

Waite, J. H., Jr.↗

NASA NTRS1989

Crustal diffusion of gases out of Mercury and the moon

An upper limit to the rate of diffusion of sodium and potassium out of Mercury and the moon was calculated, and the resulting flux was compared to that required to maintain the known exospheres. It was found that diffusion rates are inadequate by 12 orders of magnitude on the moon and 9 orders of magnitude on Mercury for diffusion out of orthoclase minerals. Diffusion will be more rapid out of pure glass by five to six orders of magnitude and out of shocked basalt by an amount depending on the microstructure of the mineral. The observed abundance and distribution of volatiles in small glass spherules on the moon indicates that diffusion is very inefficient after solidification and cooling. At Mercury, the limitation on sodium flux to the atmosphere is shown to be the rate at which new regolith is created. The discrepancy between the observed column abundance of sodium in the Mercurean atmosphere and the known sources may indicate that either Mercury's crust has a larger volatile content than the moon or that a recycling mechanism exists in the Mercurean atmosphere which is not present for the moon.

Killen, R. M.↗

NASA NTRS1989

The ionosphere of Neptune

Limited knowledge of ionospheric processes of the outer planets and practically no information on the upper atmosphere of Neptune result in a range of possible ionospheres for Neptune. Various cases are investigated in order to establish a theoretical framework of the ionospheric structure against which the radio occultation measurements can be evaluated. The peak electron densities could range from 1000/cu cm to 10 to the 6th/cu cm depending on ion loss processes and particle ionization processes. The scale height could also range from 300 km to 2000 km depending on the exospheric temperature. Although various assumptions must be made, it is predicted that an auroral ionosphere with large electron densities and a large scale height will be observed by the ingress radio occultation measurement, and that a compressed ionosphere with small electron densities will be seen during the egress measurement.

Shinagawa, H.↗

NASA NTRS1989

Stratospheric ozone measurements from the 1985 MAP/GLOBUS NO(x) campaign

Nine experiments obtained stratospheric ozone measurements during the fall 1985 Middle Atmosphere Program (MAP) Global Budget of Stratospheric Trace Constituents (GLOBUS) NO(x) campaign. Measurements are grouped into four sets from four air masses and compared. Agreement between individual experiments and weighted mean profiles is generally within 10 percent and within absolute accuracies. Dobson Umkehr values in layer 5 differ by more than absolute accuracies from weighted mean profiles. Brewer sonde values for one data set are outside absolute accuracies near the ozone peak. Solar UV occultation measurements are 30-35 percent less than the weighted mean profile and outside absolute accuracies. Measurements from the Exospheric Satellite backscattered ultraviolet experiment, not previously compared with other techniques, agree within 10-15 percent of weighted mean profiles. Results of the campaign are generally consistent with those of four previous intercomparison campaigns.

Robbins, D.↗

NASA NTRS1990

Monte Carlo simulation of nonadiabatic expansion in cometary atmospheres - Halley

Monte Carlo methods developed for the characterization of velocity-dependent collision processes and ballistic transports in planetary exospheres form the basis of the present computer simulation of icy comet atmospheres, which iteratively undertakes the simultaneous determination of velocity distribution for five neutral species (water, together with suprathermal OH, H2, O, and H) in a flow regime varying from the hydrodynamic to the ballistic. Experimental data from the neutral mass spectrometer carried by Giotto for its March, 1986 encounter with Halley are compared with a model atmosphere.

Hodges, R. Richard, Jr.↗