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NASA NTRS1981

Observations of Uranus, Neptune, and Titan by the International Ultraviolet Explorer

The ultraviolet spectra of Uranus, Neptune, and Titan do not reveal absorption features at 7-A resolution from 2100 to 3200 A. Upper limits of from 1 to 3 A are set for the equivalent widths of narrow absorptions, which corresponds to a CO/H2 mixing ratio less than 2 x 10 to the -4th in the case of Uranus. The slopes of the continuum reflectivities of Uranus and Neptune are consistent with the semi-infinite Rayleigh-Raman scattering model of Cochran, while the absolute levels are matched only if solar photometry is modified within acceptable limits. An alternative, but less satisfactory explanation of the new data is that the UV reflectivities of both Uranus and Neptune are depressed uniformly by a continuum absorber. The suggestion by Savage et al (1980) that the albedo of Uranus decreases by approximately 20% from 2200 to 1800 A is not confirmed. For Titan, the albedo decreases monotonically toward shorter wavelengths down to 2300 A. New limits are set for the pressure level in the atmosphere of Titan up to which the real, but presently unidentified, UV absorber there must extend, and for the fraction of Titan that must be covered by this absorber

Caldwell, J.↗

NASA NTRS1980

Galactic-scale civilization

Evolutionary arguments are presented in favor of the existence of civilization on a galactic scale. Patterns of physical, chemical, biological, social and cultural evolution leading to increasing levels of complexity are pointed out and explained thermodynamically in terms of the maximization of free energy dissipation in the environment of the organized system. The possibility of the evolution of a global and then a galactic human civilization is considered, and probabilities that the galaxy is presently in its colonization state and that life could have evolved to its present state on earth are discussed. Fermi's paradox of the absence of extraterrestrials in light of the probability of their existence is noted, and a variety of possible explanations is indicated. Finally, it is argued that although mankind may be the first occurrence of intelligence in the galaxy, it is unjustified to presume that this is so.

Kuiper, T. B. H.↗

NASA NTRS1980

Recent observations of energetic electrons in solar flares

The impulsive solar X-ray burst observed by the ISEE 3 X-ray spectrometer experiment on October 5, 1978 is discussed with consideration of energetic electrons in solar flares. The X-ray spectrum is consistent with a power law electron spectrum with no apparent low energy cut-off up to energies of around 5 keV. Although an explanation of the observed X-ray spectrum in terms of the emission from a multi-thermal electron spectrum cannot be ruled out, the observations lend support to the existence of nonthermal electron spectra during the impulsive phase of solar flares.

Kane, S. R.↗

NASA NTRS1980

An ICR study of an association reaction at low pressure

The reactions of the gaseous ion CH3+ with HCN are investigated, using the ion cyclotron resonance technique. The pressure range of the experiment was 1 x 10 to the -6 to 3 x 10 to the -4 Torr with a rate coefficient of 2 x 10 to the -10 cu cm/sec. Good agreement was found between rate coefficients determined in the trapping mode at the low end of the pressure range and the drift mode at the high end of the pressure range. It is shown that the association reaction of CH3 with HCN follows second order kinetics throughout the entire pressure range of the experiment. The reaction is interpreted in terms of both a saturated three-body and a radiative association mechanism. Some explanations for the disagreement of the rate coefficient with that reported previously are also given.

Mcewan, M. J.↗

NASA NTRS1981

The origin of the eccentricities of the rings of Uranus

The effect of gravitational perturbations from a nearby satellite on the eccentricity e of a narrow particulate ring is considered. The perturbations near a resonance in an eccentric ring may be divided into corotation and Lindblad terms. For small e, the corotation terms damp e, whereas the Lindblad terms excite e. In the absence of saturation the corotation terms win by a small margin, and e damps. However, if the perturbations open gaps at the strongest resonances, then the Lindblad terms win, and e grows. This result offers an explanation for the existence of both circular and eccentric rings around Uranus. It is also shown that eccentricity changes induced by circular rings on eccentric satellite orbits are similar to those induced by satellites with circular orbits on eccentric rings.

Goldreich, P.↗

NASA NTRS1981

Leakage of magnetospheric ions into the magnetosheath along reconnected field lines at the dayside magnetopause

Strong evidence is presented for escape of magnetospheric particles along reconnected field lines into the magnetosheath, using observations of approximately 30 to approximately 120-keV/charge protons and alpha particles made by the Max-Planck-Institut/University of Maryland instrument on Isee 1. During three magnetopause crossings, which have been identified from tangential stress balance analysis as reconnection events, the magnetospheric particle distribution extends well into the magnetosheath, and the particles in the magnetosheath show a strong anisotropy along the magnetic field. The proton to alpha particle ratio in this layer as well as in distinct bursts within the magnetosheath is the same as this ratio within the magnetosphere (at equal energy per charge). It is concluded that the most likely explanation for these observations is that magnetospheric particles are escaping along reconnected field lines into the magnetosheath. It is argued that magnetospheric particles are seen in the magnetosheath up to the reconnection separatrix, and the magnetosheath bursts are interpreted as multiple encounters of this magnetosheath layer by the satellite due to boundary motions.

Scholer, M.↗

NASA NTRS1981

Neutral winds and electric fields in the dust auroral oval. II - Theory and model

A two-dimensional numerical model of the thermosphere is applied to the auroral zone neutral wind, electric field, and plasma density data set, presented in an earlier paper. The model shows that the action of the Lorentz force can be responsible to a great extent for the large zonal velocities near the 150-km altitude. Model equations are described, an explanation of the use of the geophysical conditions is given, and model integrations are compared to the wind measurements. However, for the two-dimensional model to be effective, the atmosphere must not cross too many meridians of local time during the integration period, so that the background state should remain fairly uniform. It is concluded that the two-dimensional model cannot accurately explain the details of the wind profiles because of the three-dimensional character of the physical situation. Thus it is noted that the observed winds were part of a large-scale three-dimensional flow which is only weakly coupled to short-term variations in magnetospheric conditions.

Mikkelsen, I. S.↗

NASA NTRS1981

Limit on galactic 6.13 MeV gamma-ray line

The University of New Hampshire large gamma-ray detector observed the galactic center region during a balloon flight from Alice Springs, Australia on 1977 November 21-22. The absence of any observable line at 6.13 MeV or its escape-peak energy makes it possible to place an upper limit of 8.1 x 10 to the -4th photons/(sq cm s) at the 99% confidence level on the 0-16 de-excitation line at this energy from the galactic disk in the direction of the center. This limit restricts the interpretation given by Willett et al. (1979) of the line at 6.13 MeV which they observed while viewing the galactic anticenter. The present results indicate that it is highly unlikely that the line which these authors report is due to dark nebulae or the quiet sun. Possible explanations for their observation are atmospheric background, local production in the detector, a localized cosmic source in the direction of the galactic anticenter, or a statistical fluctuation.

Dunphy, P. P.↗

NASA NTRS1981

A meteor-ablation model of the sodium and potassium layers

Recent results on meteor ablation are used as input to a model in which ablation (or evaporation) is the source and deposition on dust particles is the sink. The dust comes from recondensation of the rest of the meteor vapor. An excellent match is found to the observed characteristics of the sodium layer. It is argued that the high-latitude winter maximum of abundance (and layer height) is due to the much lower rate of ionization there and over the polar cap. The computations show the required factor of 4-5 and also match the height variation. To keep the ion density from being too large, downward electrodynamic transport at about 50 cm/sec must be invoked. The different behavior and low abundance of potassium require an additional sink. The one proposed is Penning ionization by metastable excited O2. Inherent in the model is that sodium and its compounds reside almost entirely on dust particles below 80 km. Large abundances in the free form at stratospheric heights are very unlikely and are not a good explanation of observed ambient ions.

Hunten, D. M.↗

NASA NTRS1981

The ultraviolet spectrum of Herbig-Haro Object 1

Continuum and emission line fluxes for HH 1 in the range of 1200-1300 A have been determined using IUE spectra, and it is found that the luminosity of HH 1 in this wavelength interval is slightly larger than 1 solar luminosity, or 20 times greater than for the wavelength range 3200-11,000 A. If averaged over intervals of 100 A, it is shown that the UV continuum rises almost monotonically toward shorter wavelengths over the entire interval 1300-3000 A. The UV continuum energy distribution cannot be explained by a T Tauri stellar continuum that is scattered by dust. Emission line identifications and flux measurements strongly confirm the statement that the high-ionization emission lines are too strong to be explained by shock-wave models derived from the emission line spectrum in the interval 3700-11,000 A. A possible explanation for this fact is discussed.

Boehm, K. H.↗

NASA NTRS1981

Titan's hydrogen torus

A model of Titan's hydrogen torus, capable of describing its time evolution under the influence of the gravitational fields of both the satellite and the planet, is presented. Estimated lifetimes for hydrogen atoms near Titan's orbit of the order of 10 to the 7th s, based on recent Pioneer 11 measurements, suggest that the torus completely encircles Saturn and is angularly unsymmetric, having an enhanced gas density near the satellite. New model calculations confirm this and provide an explanation for the torus detected by the Copernicus satellite and the UV instrument of Pioneer 11. Agreement between calculated and observed Lyman alpha intensities suggests a hydrogen escape flux between 1 x 10 to the 9th/sq cm-s and 3 x 10 to the 9th/sq cm-s should be operative at Titan. This produces a torus containing some 10 to the 34th hydrogen atoms.

Smyth, W. H.↗

NASA NTRS1981

Observations of Saturn in the 5- to 8-micron spectral region

A spectrum of Saturn obtained from the Kuiper Airborne Observatory exhibits an emission peak at 6.8 microns attributed to ethane, but is otherwise dominated by absorption from 5.3 to 7.2 microns. While the large absorption in this spectral region is consistent with the presence of ammonia gas or ammonia ice, or both, such an explanation is inconsistent with the lack of a major absorption near 3.0 microns.

Witteborn, F. C.↗

NASA NTRS1981

The evolution of a coronal streamer and the photospheric magnetic field

A large equatorial coronal streamer observed in the outer corona grew in brightness and size during successive limb passages between October 6, 1973 and January 10, 1974 (solar rotations 1606-1611). Unlike previous studies of streamers and their photospheric associations, no definite surface feature could be identified in the present case. This suggests that the streamer is associated with the large scale photospheric magnetic field. Comparison of the streamer growth with observed underlying photospheric magnetic flux changes indicated that as the streamer increased in brightness, areal extent, and density, the photospheric magnetic flux decreased. Three possible explanations for the streamer's growth are presented, the conceptually simplest being that the decrease in photospheric field results in an opening of the flux tubes under the streamer, which permits an increase mass flux through the streamer.

Poland, A. I.↗

NASA NTRS1981

The solar origins of solar wind interstream flows - Near-equatorial coronal streamers

A class of low-speed solar wind flows with velocities of 450 km/s and less, ion temperatures of 40,000 K and less, and heavy ion distributions indicating moderate coronal freezing in temperatures in the range from 1 million to 2.5 millions K is considered. For brevity this class is termed interstream. Interstream flows have as yet, not firm identification with a coronal origin. The considered investigation is concerned with the identification of the coronal origins of interstream flows. It is found that major sources of low speed solar wind are the quiescent, near-equatorial coronal streamers. Such an identification provides a natural explanation for the long term variations of solar wind electron temperature and density observed at 1 AU by Feldman et al. (1979) in terms of the concurrent long term morphological variation in the coronal equatorial streamer belt observed using the Mauna Loa K-coronameters.

Feldman, W. C.↗

NASA NTRS1981

Thermal response of Saturn's ring particles during and after eclipse

The investigation is based on data which were obtained on May 18, 1978, at the 5-m Hale telescope on Palomar Mountain. A Ge:Ga bolometer was used at the f/70 Gregorian focus; a cold interference filter limited the wavelength response to between 16 and 26 micrometers. A brightness asymmetry is observed between the ansae of all three rings; the largest asymmetry occurs in the C ring. The simplest explanation of such asymmetries is just the eclipse cooling and subsequent heating of similar particles at different radial distances from Saturn, coupled with the different travel times from eclipse exit to east ansa. The observed eclipse cooling and subsequent heating in Saturn's B ring support the idea that the uppermost surface of the particles is of low-conductivity water frost, similar to the uppermost surface of the Galilean satellites (omitting Io).

Froidevaux, L.↗

NASA NTRS1981

Visual fatigue - The need for an integrated model

Two experiments were conducted to examine possible decrements in performance over time on two visual tracking tasks. In Experiment 1, six subjects tracked a horizontal, sinusoidally moving target in the picture plane for 6.5 min. In Experiment 2, six subjects tracked a target sinusoidally changing focus in the depth plane between 0 and 4 D over 6.5 min. Results indicated a linearly decreasing amplitude of both pursuit eye movements (.29 deg of visual angle per minute) and accommodation (.11 D/min). These visual fatigue effects are discussed in the context of several competing explanations.

Malmstrom, F. V.↗

NASA NTRS1981

Moonlets in Saturn's rings

The brightness structure within Cassini's division in Saturn's rings is explained in terms of perturbations produced by moonlets embedded within an optically thin disk of smaller ring particles. The moonlets exert gravitational torques on neighboring ring particles and create gaps; diffusion acts to fill the gaps. A new explanation is offered for the inner edge of the Cassini division being located at the 2:1 resonance with Mimas.

Lissauer, J. J.↗

NASA NTRS1981

Dynamics and drift minimization of a momentum biased spacecraft in the lower atmosphere

This paper addresses the dynamic characteristics of a momentum biased spacecraft (Magsat) which was placed in an elliptic, sun synchronous, low earth orbit. It discusses various dynamic phenomena observed, develops models for their plausible explanation, and suggests techniques for preventing or minimizing adverse phenomena. An aerodynamic coefficient model which accounts for atmospheric superrotation is proposed. This model is used to conduct analysis and simulations, the results of which are shown to be compatible with data taken by Magsat. A control philosophy is developed, and analytic expressions for the long term motion of the momentum biased axis for special cases are derived.

Saxena, A. K.↗