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At least 451 records · Page 25
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NASA NTRS1978

Time delay occultation data of the Helios spacecraft for probing the electron density distribution in the solar corona

S-band time delay measurements were collected from the spacecraft Helios A and B during three solar occultations in 1975/76 within heliocentric distances of about 3 and 215 earth radius in terms of range, Doppler frequency shift, and electron content. Characteristic features of measurement and data processing are described. Typical data sets are discussed to probe the electron density distribution near the sun (west and east limb as well) including the outer and extended corona. Steady-state and dynamical aspects of the solar corona are presented and compared with earth-bound-K-coronagraph measurements. Using a weighted least squares estimation, parameters of an average coronal electron density profile are derived in a preliminary analysis to yield electron densities at r = 3, 65, 215 earth radius. Transient phenomena are discussed and a velocity of propagation v is nearly equal to 900 km/s is determined for plasma ejecta from a solar flare observed during an extraordinary set of Helios B electron content measurements.

Edenhofer, P.↗
NASA NTRS1977

Time delay occultation data of the Helios spacecrafts and preliminary analysis for probing the solar corona

S-band time delay measurements were collected from the spacecrafts Helios A and B during three solar occultations in 1975/76 within heliocentric distances of about 3 and 215 solar radii in terms of range, Doppler frequency shift, and electron content. A description is given concerning some characteristic features of the methods of measurement and data processing. Typical data sets are discussed to probe the electron density distribution near the sun (west and east limb as well) including the outer and extended corona. Steady-state and dynamical aspects of the solar corona are presented and compared with earth-bound K-coronagraph measurements. Using a weighted least squares estimation 3 parameters of an average coronal electron density profile are derived in a preliminary analysis to yield electron densities of about 130 billion; 100 million; 7 million/cu m at r?3; 65; 215 solar radii. Transient phenomena are discussed and a velocity of propagation v approximately 900 km/s for plasma ejecta from a solar flare is determined from an extraordinary set of Helios B electron content measurements on April 30/May 1, 1976.

Edenhofer, P.↗
NASA NTRS1977

Occultation of Epsilon Geminorum by Mars. II - The structure and extinction of the Martian upper atmosphere

The occultation of Epsilon Geminorum by Mars on April 8, 1976, was observed at three wavelengths and 4-ms time resolution with the 91-cm telescope aboard NASA's G. P. Kuiper Airborne Observatory. Temperature, pressure, and number-density profiles of the Martian atmosphere were obtained for both the immersion and emersion events. Within the altitude range 50-80 km above the mean surface, the mean temperature is about 145 K, and the profiles exhibit wavelike structures with a peak-to-peak amplitude of 35 K and a vertical scale of about 20 km. The ratio of the refractivity of the atmosphere at 4500 A and 7500 A is consistent with the atmospheric composition measured by Viking 1. From the 'central flash' - a bright feature in the light curve midway between immersion and emersion - an optical depth at 4500 A of 3.3 + or - 1.7 per km atm (about 0.23 per equivalent Martian air mass) is found for the atmosphere about 25 km above the mean surface near the south polar region. This large value and its weak wavelength dependence rule out Rayleigh scattering as the principal cause of the observed extinction.

Elliot, J. L.↗
NASA NTRS1977

Twilight ozone measurement by solar occultation from AE 5

The BUV on AE 5 was used for a solar occultation measurement of atmospheric ozone. An observation was carried out during the morning twilight near 5 deg N, December 17, 1976, at the fixed wavelength of 255.5 nm, and a profile between 49 and 82 km was obtained. The number densities determined by this measurement were 3.7 x 10 to the 10th cu cm at 50 km, 5.1 x 10 to the 9th at 60 km, 3.9 x 10 to the 8th at 70 km, and 3.0 x 10 to the 7th at 80 km. No evidence of a high altitude secondary maximum was found. These concentrations are between a factor of 4 and 20 smaller than those midnight results reported from a Copernicus measurement, and similar to the values from the Krueger-Minzer Mid-latitude Model above 55 km. These values may be as much as a factor of 2 less than the Krueger-Minzer model below 50.

Guenther, B.↗
NASA NTRS1978

The occultation of Epsilon Geminorum by Mars as observed at Princeton

Results are reported for two-color (red and blue) high-speed photometric observations of the April 8, 1976, occultation of Epsilon Gem by Mars. The observing system is briefly described, fits to an isothermal atmospheric model of the event are presented, and the blue data are used to derive temperature profiles of the Martian atmosphere in the manner of Wasserman and Veverka (1973). The results are compared with those of Elliot et al. (1977), which were obtained from the NASA Kuiper Airborne Observatory.

Groth, E. J.↗
NASA NTRS1978

Martian occultation of epsilon Gem as observed from the C. E. Kenneth Mees Observatory

Ground-based observations of the occultation of epsilon Gem by Mars on April 8, 1976 have been reduced to yield the scale height and temperature profiles of the Martian atmosphere for number densities between 10 to the 13th and 10 to the 15th per cu cm. The deduced variations in temperature are remarkably similar to the in situ measurements from the Viking landers.

French, R. G.↗
NASA NTRS1978

Effects of turbulence on average refraction angles in occultations by planetary atmospheres

Four separable effects of atmospheric turbulence on average refraction angles in occultation experiments are derived from a simplified analysis, and related to more general formulations by B. S. Haugstad. The major contributors are shown to be due to gradients in height of the strength of the turbulence, and the sense of the resulting changes in refraction angles is explained in terms of Fermat's principle. Because the results of analyses of such gradient effects by W. B. Hubbard and J. R. Jokipii are expressed in other ways, a special effort is made to compare all of the predictions on a common basis. We conclude that there are fundamental differences, and use arguments based on energy conservation and Fermat's principle to help characterize the discrepancies.

Eshleman, V. R.↗
NASA NTRS1978

The radii of Uranian rings alpha, beta, gamma, delta, epsilon, eta, 4, 5, and 6 from their occultations of SAO 158687

All available timing data for the occultations of SAO 158687 on March 10, 1977, by the cited rings of Uranus are analyzed. Least-squares fits to the data are performed using a model which postulates that rings alpha, beta, gamma, and delta are circular and coplanar. A solution obtained under the assumption that the ring plane coincides with the plane of the satellite orbits is adopted which yields radii of 44,844 km for ring alpha, 45,799 km for ring beta, 47,746 km for ring gamma, and 48,423 km for ring delta. The uncertainties in these values are discussed along with the apparent shapes and inclinations of these main rings. The mean radii estimated for the other rings are: 47,323 km for ring eta, 42,663 km for ring 4, 42,360 km for ring 5, and 41,980 km for ring 6.

Elliot, J. L.↗
NASA NTRS1978

Structure of the Martian atmosphere from Epsilon Gem occultation observations

Information has been collected on Martian atmospheric scale heights derived from observations of the occultation of Epsilon Gem by Mars on Apr. 8, 1976; the observations give data in the altitude range of approximately 50-80 km. A rough, unweighted average of results so far available yields a temperature of approximately 165 K. Excursions of plus or minus 40 K about this mean may be present as a function of both altitude and areographic coordinates.

Hubbard, W. B.↗
NASA NTRS1978

The occultation of Epsilon Gem by Mars as observed from Agassiz Station

Observations of the April 8, 1976, occultation of Epsilon Gem by Mars made at the Agassiz Station of the Harvard College Observatory have been analyzed to yield temperature profiles of the Martian atmosphere for number densities between 10 to the 13th and 10 to the 15th power per cu cm. Pronounced wavelike structure is evident in both immersion and emersion profiles, with a peak-to-peak variation of up to 50 K and a vertical scale of 20 km.

Liller, W.↗
NASA NTRS1978

Pressure sensing of the atmosphere by solar occultation - An application to remote sensing of atmospheric minor gases

The simultaneous inference of pressure and gas concentration in a remote-sensing experiment will increase the accuracy of inverted gas concentrations and minimize the dependence of the experiment on pressure or mass-path error resulting from use of climatological pressure data, satellite ephemeris, and instrument pointing accuracy. A technique for obtaining tangent-point pressure information needed for inversion of gas concentration in an IR solar occultation experiment is described. By measuring IR absorption in bands of atmospheric CO2 (e.g., 2.0, 2.7, or 4.3 microns) pressure values for each layer (2 km or less in the vertical) of the atmosphere can be obtained with rms errors of less than 3%.

Park, J. H.↗
NASA NTRS1979

The polar ionosphere of Venus near the terminator from early Pioneer Venus orbiter radio occultations

Fourteen profiles of electron density in the ionosphere of Venus were obtained by the dual-frequency radio occultation method with the Pioneer Venus orbiter between 5 and 30 December 1978. A region of almost constant electron density above approximately 250 km was detected. The ionopause height is found to vary from about 300 to 700 km. The structures of the profiles are consistent with models in which O2(+) dominates near the ionization peak and is replaced by O(+) at higher altitudes.

Kliore, A. J.↗
NASA NTRS1979

Liquid content of the lower clouds of Venus as determined from Mariner 10 radio occultation

S- and X-band radio occulation data obtained during the Mariner 10 flyby of Venus are analyzed to determine the effects of signal attenuation due to the clouds of Venus. The signal-amplitude data are corrected for errors introduced by spacecraft motion and antenna steering during the occultation, absorption-coefficient profiles are computed, and the liquid content of the Venusian clouds is estimated. Excess attenuation effects are found and ascribed to absorption by small liquid particles, consisting of a water-sulfuric acid mixture containing 75% + or - 25% sulfuric acid, in the clouds. Three absorbing cloud layers at respective heights of 68, 60, and 48 km are suggested by the X-band absorptivity profile, but only the 68- and 48-km layers are clearly indicated by the S-band profile. The relatively slow S-band absorption at 60 km is tentatively attributed to an unknown constituent that is much more absorbing in the X-band than in the S-band. The results are compared with nephelometer measurements performed at 0.9 micron by the Venera 9 and 10 probes.

Kliore, A. J.↗
NASA NTRS1979

On the wavelength dependence of the effects of turbulence on average refraction angles in occultations by planetary atmospheres

The dependence of the effects of planetary atmospheric turbulence on radio or optical wavelength in occultation experiments is discussed, and the analysis of Hubbard and Jokipii (1977) is criticized. It is argued that in deriving a necessary condition for the applicability of their method, Hubbard and Jokipii neglect a factor proportional to the square of the ratio of atmospheric or local Fresnel zone radius and the inner scale of turbulence, and fail to establish sufficient conditions, thereby omitting the square of the ratio of atmospheric scale height and the local Fresnel zone radius. The total discrepancy is said to mean that the results correspond to geometrical optics instead of wave optics, as claimed, thus being inapplicable in a dicussion of wavelength dependence. Calculations based on geometrical optics show that the bias in the average bending angle depends on the wavelength in the same way as does the bias in phase path caused by turbulence in a homogeneous atmosphere. Hubbard and Jokipii comment that the criterion of Haugstad and Eshleman is incorrect and show that there is a large wave optical domain where the results are independent of wavelength.

Haugstad, B. S.↗
NASA NTRS1979

The epsilon Geminorum occultation - Evidence for waves or turbulence

Intercomparison of occultation data from various observatories shows poor correlation of the raw data but apparent correlations in the derived temperature profiles, as discussed in a paper by French and Elliott. We point out that the data are at least as consistent with Martian scintillation as they are with a semiglobal wave model.

Hubbard, W. B.↗
NASA NTRS1979

Initial observations of the nightside ionosphere of Venus from Pioneer Venus Orbiter radio occultations

Results of radio occultation measurements of electron density profiles of the nightside ionosphere of Venus at solar zenith angles from 90 to 164 deg, obtained from the Pioneer Venus Orbiter, are reported. Data were derived from closed-loop S- and X-band signals received by the Deep Space Network upon ionospheric entry and exit of the spacecraft. Nightside electron density profiles are found to be rather uniform in the solar zenith angle range of from 95 to 107 deg, with peak electron densities ranging from 23,000 to 40,000/cu cm, while between 110 and 164 deg, profiles exhibit a high degree of variability and peak electron densities vary from 7,600 to 31,800/cu cm. A possible mechanism for the maintenance of the nightside Venus ionosphere during the long Venus night, which is consistent with the observed spatial and temporal variability of deep ionospheric electron density profiles, is proposed to be impact ionization by precipitating particles, although transport processes from the dayside may also be important.

Kliore, A. J.↗
NASA NTRS1979

Mission analysis for earth atmospheric measurements using solar occultation experiments on Shuttle Spacelabs

The maximum geographical coverage of solar occultation experiments for various Shuttle-Spacelab mission concepts is defined and an analysis that includes trade-offs between parameters such as launch time, season, orbital inclination and altitude is presented as well as the mission design data for the Spacelab-3 flight. The effects of orbital ranges from 220 to 600 km on geographical coverage are examined with inclinations up to 97 deg for sun-synchronous orbit. Results show that the widest band of latitude coverage in the tropics and the temperate zones can be achieved with a mid-inclined (i.e., 57 deg) orbit and a mid-morning or late-night launch time.

Harrison, E. F.↗
NASA NTRS1980

Illinois occultation summary, 1. 1977 - 1978

Instrumentation and data acquisition techniques used to record lunar occultations at the University of Illinois Prairie Observatory are described. Tables and graphs summarize data from 64 events which include 30 observations of stars brighter than 7th magnitude, 40 reappearances, 4 angular diameter measurements, 8 observations of binary stars, and 6 observations which may indicate multiplicity.

Radick, R.↗