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NASA NTRS1974

Twilight airglow. II - N2/+/ emission at 3914 A

One of the experiments aboard a rocket flight carrying instruments to measure the dawn airglow, the ion and electron densities, and the photoelectron spectrum is reported. For a solar zenith angle of 90 deg the emission at 3914 A from N2(+) peaks at about 260 km. The integrated intensity from model calculations suggests that resonance scattering of 3914-A solar photons off N2(+) produces 90% of the emission, whereas simultaneous photoionization excitation of N2(+) produces less than 10% of the emission. Photoelectron impact excitation is found to contribute about 1%.

Sharp, W. E.↗

NASA NTRS1974

Observations of He I 584 A nighttime radiation - Evidence for an interstellar source of neutral helium

The intensity and spatial variations of the He I 584 A radiation in the night sky were measured to an altitude of 264 km from a sounding rocket launched from Thumba, India. The flux varied in intensity with look angle from a high of 16 plus or minus 5 rayleighs to a low of less than 1 rayleigh. The data obtained are presented in the form of an all-sky map. Since the peak flux levels observed are approximately two orders of magnitude larger than that expected from geocoronal resonant scattering, we have compared our results with those expected from two interplanetary sources of neutral helium: the penetration of the local interstellar medium into the solar neighborhood and the dust deionization of solar-wind alpha particles. The dust deionization-model predictions using currently accepted input parameters appear to be inconsistent with the observations. The interstellar helium source is compatible with the data to within the uncertainties of the models.

Paresce, F.↗

NASA NTRS1974

Further evidence for an interstellar source of nighttime He I 584 A radiation

The intensity of He I, 584 A radiation in the night sky was measured from a sounding rocket at altitudes between 160 and 217 km. The results obtained confirm previous measurements of this radiation. The observed intensity and spatial variations can be entirely accounted for by resonance scattering of solar 584 A radiation from interstellar helium. The best fit to the observations in this case is obtained with an interstellar helium model in which the bulk velocity of the gas is 5 km/sec, the temperature is 10,000 K, and the helium number density is 0.032 per cu cm. Even if a geocoronal scattering component is present, the data can be used to set a lower limit to the temperature of the local interstellar medium of 1,500 (plus or minus 500) K, assuming a normal interstellar helium abundance.

Paresce, F.↗

NASA NTRS1974

The Mariner Jupiter/Saturn ultraviolet experiment

Atmospheric properties of Saturn's rings as measured by an ultraviolet spectrometer onboard the Mariner Jupiter/Saturn probe are studied. Measurements were made of planetary atmospheric radiation, resonance scattered by atomic or molecular constituents, solar spectra, and the absorption characteristics of the atmosphere. Measurements were made in the 400 to 1600 A region.

Broadfoot, A. L.↗

NASA NTRS1975

Laser applications to atmospheric sciences: A bibliography

A bibliography is given of 1460 references of the applications of lasers to atmospheric sciences. The subjects covered include: aerosols; clouds; the distribution and motion of atmospheric natural and man-made constituents; winds; temperature; turbulence; scintillation; elastic, Raman and resonance scattering; fluorescence; absorption and transmission; the application of the Doppler effect and visibility. Instrumentation, in particular lidar, is included, also data handling, and interpretation of the data for meteorological processes. Communications, geodesy and rangefinding are not included as distinct areas. The application to the atmosphere is covered, but not the ocean or its surface.

Harris, F. S., Jr.↗

NASA NTRS1974

An upper limit on the evolution of carbon monoxide from Comet Kohoutek

An upper limit on the rate of evolution of CO from Comet Kohoutek has been obtained from a search for resonant scattering of sunlight near 4.7 microns. The observations were made approximately two months after perihelion. The rate of evolution of CO at that time was apparently less than that of CH3CN observed before perihelion.

Wollman, E. R.↗

NASA NTRS1975

Sodium D-line emission from Io - Synoptic observations from Table Mountain Observatory

We report the first results of a program initiated at Table Mountain Observatory to study the time variation of the sodium D-line emission around Io. During 1974 July and August, 51 spectra were obtained. These data demonstrate that (1) the sodium emission is highly correlated with Io's orbital position, (2) the gross temporal and amplitude characteristics of the emission are explained if resonance scattering of sunlight is the dominant excitation mechanism, (3) the emission is not directly related to dekametric noise storms from Jupiter, (4) an upper limit of 130 kR is placed on any constant near-surface (auroral) source of emission, (5) a steady-state source of sodium operated during our 7-week observing interval, (6) the sodium cloud is probably distributed in a partial toroid, possibly with more sodium leading than trailing Io, and (7) the D-line emission does not depend strongly on the solar phase angle.

Bergstralh, J. T.↗

NASA NTRS1975

The aeronomy of the upper atmosphere of Venus

It is shown that rather tight constraints can be placed on the mixing ratio of atomic oxygen in the upper atmosphere of Venus, as well as that of CO and molecular oxygen, unless extraordinary processes which lead to a very large efflux of hydrogen from the planet are postulated. The analysis conducted is similar to the investigation carried out for the earth by Liu and Donahue (1974). If Jeans escape provides the main escape mode for hydrogen from Venus, the concentration ratio of atomic oxygen to carbon dioxide at 130 km is much less than 1% and some mechanism other than resonance scattering plus photoelectron excitation must account for the oxygen airglow.

Liu, S. C.↗

NASA NTRS1975

He 584 A airglow emission from Venus - Mariner 10 observations

Observations were made across the bright limb of Venus by the ultraviolet spectrometer on Mariner 10 as the spacecraft flew by this planet on February 5, 1974. The altitude variation of the brightness of 584-A emission is interpreted with respect to the helium distribution in the atmosphere. Resonance scattering of solar radiation is considered as the source of this emission. The comparison of model calculations with the data shows that the observed altitude profile is consistent with a He density of about 2 million/cu cm at 145-km altitude and with an exospheric temperature of the order of 375 K. If the major loss of helium is by solar-wind scavenging only, the present helium content on Venus can be maintained by an outgassing rate of about 200,000 per sq cm per sec from the planet crust. By using these neutral helium measurements, it is found that the He(+) ion density in the outer ionosphere would be of the order of 200/cu cm.

Kumar, S.↗

NASA NTRS1975

A model for Io's atmosphere and sodium cloud

Observations of the sodium D-line emissions associated with the Galilean satellite Io suggest that the emitting region has two components. The first is an atmosphere which extends 1000 km above Io's surface. This is a relatively dense region which is optically thick in the sodium D lines. The second component is a surrounding optically thin cloud spread out along Io's orbit to form part of a torus. No emission was detected during any of three eclipses observed, although Io's sodium emission was consistently present when Io was near orbital elongation. This indicates that resonant scattering is the principal emission mechanism in both components. For the atmosphere, measurements of the D-line intensity, D-2/D-1 ratio, and line width indicate a temperature of 500 to 1000 K and a sodium column abundance of 3 by 10 to the 13th power per sq cm. Upper limits to the abundance of Mg, K, Ca, and Li indicate a high sodium abundance ratio in the atmosphere. This may also be true for Io's surface if sputtering by energetic particles is the source of the atmosphere. The sodium cloud has been detected as far as 60 arcsec from Io in the orbital plane and at least 6 arcsec above and below this plane. The cloud can be sustained by atoms escaping from the critical level at the top of the atmosphere.

Macy, W., Jr.↗

NASA NTRS1976

Far ultraviolet excitation processes in comets

Recent observations of atomic oxygen and carbon in the far ultraviolet spectrum of comet Kohoutek have demonstrated the existence of these atomic species in the cometary coma. However, in order to identify the source of their origin, it is necessary to relate the observed ultraviolet flux to the atomic production rate. Analyses of observed OI wavelength 1304 and CI wavelength 1657 A multiplets have been carried out using high resolution solar spectra. Also examined is the possibility of observing ultraviolet fluorescence from molecules such as CO and H2, as well as resonance scattering either from atomic ions for which there are strong corresponding solar lines (CII) or from atoms for which there is an accidental wavelength coincidence (SI).

Feldman, P. D.↗

NASA NTRS1976

Interstellar helium flow experiment MA-088

The Apollo Soyuz Test Project Interstellar Helium Glow Experiment (MA-088) studied the motion of helium in the local interstellar medium as that medium passed through the solar system to determine several poorly known properties of the local interstellar gas. The instrument used was a photometer sensitive to two solar extreme ultraviolet spectral lines that are resonantly scattered by helium gas. The instrument surveyed the entire celestial sphere during a series of slow, rolling maneuvers by the Apollo spacecraft. The equipment operated properly, and usable data were obtained.

Bowyer, S.↗

NASA NTRS1976

Satellite measurements of high-altitude twilight Mg/plus/ emission

Observations made by the ultraviolet spectrometer on board the orbiting geophysical observatory OGO 4 confirmed the presence of resonance scattering at 2800 A of Mg(plus) ions in the twilight subtropical ionosphere. The column density reached 4 billion ions/sq cm above 160 km. Photometric measurements by the ESRO TD 1 satellite revealed a maximum of the Mg(plus) abundance at equinoxes in the top side F region. The interhemisphere asymmetries observed in the intensity distribution are essentially attributed to the effect of eastward thermospheric winds. The 2800-A doublet was also detected by OGO 4 at middle and high latitudes from 110 to 250 km. The brightness of the emission and other evidence indicate that evaporation of meteoritic matter cannot explain the abundance of ions at 200 km. Therefore Mg(plus) ions are probably transported upward from the 100-km permanent source layer.

Gerard, J.-C.↗

NASA NTRS1976

Global atomic oxygen density derived from OGO-6 1304 A airglow measurements

Results are presented for analysis of data on the atomic oxygen 1304-A triplet in the earth's dayglow between 400 and 1100 km which were obtained with the OGO-6 UV photometer during a 40-day period that included both quiet and disturbed conditions. Variations in the atomic oxygen column density are analyzed by obtaining best-fit models in which the 1304-A emission is produced by solar resonance scattering and photoelectron excitation. It is shown that the column density can be determined uniquely from the measured 1304-A intensity, provided the excitation processes can be described quantitatively. The values of the excitation parameters are determined directly from the data, and the deduced variations in column density over the daytime atmosphere are found to agree well with the Jacchia (1971) models. The latitudinal dependence of the column-density variations during a geomagnetic storm are discussed, the results are compared with recent measurements of the solar 1304-A fluxes as well as with calculations of the photoelectron excitation, and a method is suggested for determining the absolute atomic oxygen densities.

Strickland, D. J.↗

NASA NTRS1976

Spectroscopic studies of the solar corona at X-ray wavelengths

The spatial distribution of emission in several X-ray lines is discussed with emphasis on temperature dependence and association with active regions. Results are presented for the trio of helium-like O VII lines which demonstrate (1) a spatial variation in the density-dependent forbidden-to-intersystem line ratio, and (2) a strong spatial variation in the intensity of the O VII resonance line relative to the optically forbidden transitions. The second effect appears to be caused by resonance scattering by material in the line of sight.

Acton, L. W.↗

NASA NTRS1976

Optical emissions from the mid-day aurora

Intensities of various optical emissions from mid-day auroras and spectral profile of N2(+) ING bands measured aboard a jet aircraft are presented. The data are compared with simple calculations based on present knowledge of the types and energy distributions of particles precipitating in the mid-day auroral region and the available cross-sections for the excitation of various optical emissions in air. The analysis concerns the interaction of magnetosheath protons with the atmosphere, electron interaction with the atmosphere, resonant scattering of sunlight by N2(+) ions, and auroral emissions around 3886 A. It is shown that most of the OI red and green line emissions in the mid-day aurora are excited by the low-energy electrons precipitating in the cusp region, but only part of the N2 and N2(+) emissions can be so accounted, while precipitating protons excite the rest of N2 and N2(+) emission. The emission feature around 3886 A observed in mid-day aurora is most likely He 3888 A blended with N2(+) ING(1, 1) band.

Sivjee, G. G.↗

NASA NTRS1976

Observations of interstellar helium with a gas absorption cell - Limits on the bulk velocity of the interstellar medium

Results are reported for observations of solar 584-A flux resonantly scattered by the 1s(2)-1s2p transition of neutral interstellar helium. A photometer equipped with a helium gas-absorption cell and flown aboard a sounding rocket to a peak altitude of 185 km was employed to observe the sky in Perseus. The data reduction procedure is described, including subtraction of the terrestrial atmospheric background, calculation of the solar flux, and reduction of the number density of scatters to a function of phase-space parameters of the local interstellar medium. The ratio of 584-A fluxes observed with the gas cell full and empty is computed and compared with numerical models of the interstellar-helium flow through the solar system. The results show that the bulk speed of the distant interstellar medium with respect to the sun is unlikely to be less than 10 to 15 km/s, at the 2-sigma level. Since this value is inconsistent with results obtained from Lyman-alpha observations, it is suggested that either the total ionization rate for helium is variable or present models of the behavior of the local interstellar medium need further refinement.

Freeman, J.↗

NASA NTRS1977

Some optical and kinetic properties of the nearby interstellar gas

A model for resonant scattering of sunlight by the nearby interstellar gas is developed which takes into account the modification of a Maxwell-Boltzmann velocity distribution by a central force. While the formalism applies to both hydrogen and helium, the latter is emphasized. Theoretical line profiles and integrated emission rates for helium are compared with those of approximate models and with existing observations. As a result of this analysis, an upward revision of the bulk flow velocity of interstellar helium relative to the solar system, previously deduced from He 584-A observations, is required. Velocities of 15-20 km/s, temperatures of the order of 5000-10,000 K, and densities of the order of 0.008-0.02 per cu cm are sufficient to explain the (helium) observations. However, higher-precision measurements are required in order to place more stringent limits on the gas parameters. Calculations of the flux of interstellar helium atoms impinging on earth's upper atmosphere show that the resulting impact ionization is small compared with terrestrial processes.

Meier, R. R.↗