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Combi, M. R.

Publications and source records attributed to Combi, M. R..

34 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Extended atmospheres of outer planet satellites and comets

The lifetimes of hydrogen atoms in the environment of Saturn's magnetosphere can be expected to be quite long (approx. 10 to the 8th power s). Preliminary testing of a model of the Titan hydrogen torus for these long-lived orbits was begun. A draft of the paper describing the analysis of Pioneer Venus observations of Comet P/Encke was completed. Plans to analyze Comet P/Halley data, as well as the development of models for cometary carbon and oxygen, are also discussed.

Smyth, W. H.↗
NASA NTRS1985

Cometary atmospheres: Modeling the spatial distribution of observed neutral radicals

An algorithm for the random walk problem of multiple elastic collisions between newly formed non-thermal neutral cometary radicals and the outflowing cometary molecules was incorporated into the Monte Carlo particle-trajectory model. Preliminary model analysis has shown that the effects of collision on the observed spatial distribution of cometary radicals becomes important for the larger bright comets, especially at smaller values of the helicocentric distance. The model and early results are discussed herein.

Combi, M. R.↗
NASA NTRS1985

Cometary atmospheres: Modeling the spatial distribution of observed neutral radicals

The dependencies on heliocentric velocity for the solar photodissociation lifetime and the solar fluorescence rate of the OH radical were incorporated into the Monte Carlo particle trajectory model for cometary radicals. For the case of CN a study was initiated on the effects of using several different adopted models on the calculation of production rates from filter photometry. Progress in these areas is discussed. A comparison of two different sets of scale length data for cometary C2 and CN is also presented.

Combi, M. R.↗
NASA NTRS1984

Extended atmospheres of outer planet satellites and comets

The cometary hydrogen particle-trajectory model was used successfully to analyze observations of Comet P/Encke. The Pioneer Venus Orbiter Ultraviolet Spectrometer observed the comet at 1216A (hydrogen Lyman-alpha) on 15 April 1984, when the comet was .58 AU from the Sun and 1.02 AU from Venus. The analysis implies a production rate at .58 AU of 3.3 x 10 to the 28th power/sec of the water molecules which photodissociate to produce the observed hydrogen.

Smyth, W. H.↗
NASA NTRS1984

Io's sodium directional features - Evidence for a magnetospheric-wind-driven gas escape mechanism

Elongated features in Io's sodium cloud, directed away from Jupiter and inclined both to the north and to the south of the satellite's orbital plane, have been observed. The north/south directions of the features are correlated with Io's magnetic longitude, suggesting a formation mechanism involving the oscillating plasma torus. It is shown by means of a model analysis that the features can result from a source of high-velocity (about 20 km/s) sodium combined with the oscillating neutral sodium sink provided by the plasma. The phase relationship between the features' directions and Io's magnetic longitude can be understood if escaping sodium is initially directed at near right angles to Io's orbital motion. The directionality of the features requires that the sodium flux from equatorial regions be higher than that from the poles. The initial directions and speeds of sodium atoms escaping Io to form the directional features can be understood in terms of a magnetospheric-wind-driven escape mechanism. The one sequence of directional feature observations that has been analyzed in detail implies a high-speed sodium source rate of about 10 to the 26th atoms/s.

Pilcher, C. B.↗
NASA NTRS1984

Outer satellite atmospheres: Their nature and planetary interactions

Significant insights regarding the nature and interactions of Io and the planetary magnetosphere were gained through modeling studies of the spatial morphology and brightness of the Io sodium cloud. East-west intensity asymmetries in Region A are consistent with an east-west electric field and the offset of the magnetic and planetary-spin axes. East-west orbital asymmetries and the absolute brightness of Region B suggest a low-velocity (3 km/sec) satellite source of 1 to 2 x 10(26) sodium atoms/sec. The time-varying spatial structure of the sodium directional features in near Region C provides direct evidence for a magnetospheric-wind-driven escape mechanism with a high-velocity (20 km/sec) source of 1 x 10(26) atoms/sec and a flux distribution enhanced at the equator relative to the poles. A model for the Io potassium cloud is presented and analysis of data suggests a low velocity source rate of 5 x 10(24) atoms/sec. To understand the role of Titan and non-Titan sources for H atoms in the Saturn system, the lifetime of hydrogen in the planetary magnetosphere was incorporated into the earlier Titan torus model of Smyth (1981) and its expected impact discussed. A particle trajectory model for cometary hydrogen is presented and applied to the Lyman-alpha distribution of Comet Kohoutek (1973XII).

Smyth, W. H.↗
NASA NTRS1984

Cometary atmospheres: Modeling the spatial distribution of observed neutral radicals

The Monte Carlo particle trajectory model for the saptial distribution of cometary radicals was modified to include the heliocentric distance dependence of the parent molecule velocity, and the heliocentric velocity dependence for CN fluorescence and radiation pressure. Available data on the observed spatial distributions of cometary radicals were studied and a preliminary comparison of newly published data from previous studies is discussed.

Combi, M. R.↗
NASA NTRS1984

Outer satellite atmospheres: Their extended nature and planetary interactions

Model calculations for the brightness of the sodium cloud in Region A were performed to clarify the role played by the plasma torus sink in producing the east-west intensity asymmetry observed in the sodium D-lines. It was determined that the east-west electric field, proposed by Barbosa and Kievelson (1983) and Ip and Goertz (1983) to explain the dawn-dusk asymmetry in the torus ion emissions measured by the Voyager UVS instrument, could also produce the east-west sodium intensity asymmetry discovered earlier by Bergstralh et al. (1975, 1977). Model results for the directional features of the sodium cloud are also reported. The completion of the development of the Io potassium cloud model, progress in improving the Titan hydrogen torus model, and efforts in developing our model for hydrogen cometary atmospheres are also discussed.

Smyth, W. H.↗
NASA NTRS1983

Outer satellite atmospheres: Their extended nature and planetary interactions

Significant progress in model analysis of data for the directional features of the Io sodium cloud is reported and appears to provide some support for a satellite emission mechanism that is driven by a magnetospheric wind. A number of model calculations for the two dimensional intensity morphology of the Io sodium (region B) cloud are compared with six observations. Results of this comparison support tentative conclusions regarding the satellite emission conditions, the role of the plasma torus and the sodium atom escape flux. Progress in updating the Titan hydrogen torus model is also discussed.

Smyth, W. H.↗
NASA NTRS1982

Outer satellite atmospheres: Their nature and planetary interactions

Significant progress is reported in early modeling analysis of observed sodium cloud images with our new model which includes the oscillating Io plasma torus ionization sink. Both the general w-D morphology of the region B cloud as well as the large spatial gradient seen between the region A and B clouds are found to be consistent with an isotropic flux of sodium atoms from Io. Model analysis of the spatially extended high velocity directional features provided substantial evidence for a magnetospheric wind driven gas escape mechanism from Io. In our efforts to define the source(s) of hydrogen atoms in the Saturn system, major steps were taken in order to understand the role of Titan. We have completed the comparison of the Voyager UVS data with previous Titan model results, as well as the update of the old model computer code to handle the spatially varying ionization sink for H atoms.

Smyth, W. H.↗
NASA NTRS1980

Neutral cometary atmospheres. III - Acceleration of cometary CN by solar radiation pressure

The acceleration of cometary CN radicals due to solar radiation pressure has been determined by fitting Monte Carlo models to nine observed sunward-tailward pairs of brightness profiles of the (0-0) band of CN at 3883 A. The profiles were determined from spectrograms of comets Bennett 1970 II and West 1976 VI. The values of the observed acceleration agree with those computed from resonance fluorescence calculations to within the expected uncertainties. This provides an independent confirmation of the identification of the observed scale lengths with the photochemical lifetimes and velocities associated with the production of observed cometary CN by the photodissociation of HCN. The ratio of the intensity of the (0-1) band of CN at 4216 A to the (0-0) band at 3883 A has been determined from spectrograms of comet West, and is compared with theoretical values.

Combi, M. R.↗
NASA NTRS1980

Brightness profiles of CO/+/ in the ionosphere of Comet West /1976 VI/

The observed sunward and tailward brightness profiles of the (2-0) band of the (A2Pi-X2Sigma) system of CO(+) (4260 A) in Comet West (1976 VI) are established over a range of heliocentric distance from 0.440 to 0.842 AU. From these data the variation with heliocentric distance of the CO(+) production rate is investigated along with the size of the contact surface (the diameter of the onset of the tail). A photoionization model is proposed for the production of CO(+) in Comet West in which CO photoionization is considered to be the principle source of CO(+); the model is consistent with the observational data and the MHD theories.

Combi, M. R.↗
NASA NTRS1980

Neutral cometary atmospheres. I - An average random walk model for photodissociation in comets

A method for constructing physically realistic photochemical models, taking into account the isotropic ejection of dissociated molecular fragments as well as radiation pressure acceleration, has been developed using Monte Carlo techniques. The effect of the isotropic ejection, as opposed to the radial motion arbitrarily assumed in Haser's model, is adequately described by a simple average random walk model. It is shown that measured radial (Haser) scale lengths are in fact only lower limits to a range of possible true scale lengths for a given brightness profile, which explains the current discrepancies between observed scale lengths and those predicted by photochemistry.

Combi, M. R.↗
NASA NTRS1980

Neutral cometary atmospheres. II - The production of CN in comets

Brightness profiles of the CN (0-0) band at 3883 A have been constructed from spectrograms of comets Bennett (1979 II) and West (1976 VI). The subsequent analysis of these profiles shows that the parent molecule of CN had a radial scale length of (2.19 plus or minus 0.07) x 10 to the 4th r (H-squared), in kilometers. This, combined with current theories as well as photochemistry, is consistent with production by simple photodissociation of HCN. The rather random variation of the radial scale length for the apparent decay of CN is indicative not of a true decay scale length, but of the fact that no steady state in the CN parent production rate is achieved for the time scale necessary to build up the observed profile. A revision of the average CN production law with heliocentric distance from published photometry indicates an r(H to the -2nd) law for comet West out to at least 2.555 AU. This implies that the vaporization of this comet was controlled by some species more volatile than water. Based on this and other evidence, CO2 is suggested, but even more volatile molecules like CO cannot be ruled out.

Combi, M. R.↗
NASA NTRS1979

O/1D/ and H2O/+/ in comet Bennett 1970. II

The spatial distribution of O(1D) in comet Bennett on April 18 implies the existence of an average scale length of about 10,000 km for the source of one or several parents, and an average scale length also of about 10,000 km for their destruction. The spatial distribution of H2O(+) has also been studied sunward and tailward for the same comet and the same date. Two spectra taken 20 minutes apart show recognizable wave patterns moving tailward with a velocity of 17 km/sec. The interaction of the H2O(+) ions with the solar wind gives clues to the onset of the plasma tail. A characteristic time of 800 sec seems to be associated with the instabilities seen at the onset of the tail.

Delsemme, A. H.↗