Engineering topics

Prasad, S. S.

Publications and source records attributed to Prasad, S. S..

At least 19 records

NASA NTRS1993

Theoretical studies in interstellar cloud chemistry

This final report represents the completion of the three tasks under the purchase order no. SCPDE5620,1,2F. Chemical composition of gravitationally contracting, but otherwise quiescent, interstellar clouds and of interstellar clouds traversed by high velocity shocks, were modeled in a comprehensive manner that represents a significant progress in modeling these objects. The evolutionary chemical modeling, done under this NASA contract, represents a notable advance over the 'classical' fixed condition equilibrium models because the evolutionary models consider not only the chemical processes but also the dynamical processes by which the dark interstellar clouds may have assumed their present state. The shock calculations, being reported here, are important because they extend the limited chemical composition derivable from dynamical calculations for the total density and temperature structures behind the shock front. In order to be tractable, the dynamical calculations must severely simplify the chemistry. The present shock calculations take the shock profiles from the dynamical calculations and derive chemical composition in a comprehensive manner. The results of the present modeling study are still to be analyzed with reference to astronomical observational data and other contemporary model predictions. As far as humanly possible, this analysis will be continued with CRE's (Creative Research Enterprises's) IR&D resources, until a sponsor is found.

Chiu, Y. T.↗

NASA NTRS1990

Modeling the chemistry of the dense interstellar clouds. I - Observational constraints for the chemistry

A search for correlations arising from molecular line data is made in order to place constraints on the chemical models of interstellar clouds. At 10 to the 21st H2/sq cm, N(CO) for dark clouds is a factor of six greater than the value for diffuse clouds. This implies that the strength of the UV radiation field where CO shields itself from dissociation is about one-half the strength of the average Galactic field. The dark cloud data indicate that the abundance of CO continues to increase with A(V) for directions with A(V) of 4 mag or less, although less steeply with N(H2) than for diffuse clouds. For H2CO, a quadratic relationship is obtained in plots versus H2 column density. The data suggest a possible turnover at the highest values for A(V). NH3 shows no correlation with H2, C(O-18), HC3N, or HC5N; a strong correlation is found between HC5N and HC3N, indicating a chemical link between the cyanopolyynes.

Federman, S. R.↗

NASA NTRS1985

Precipitating electron interaction with the atmosphere. II - The dayside cusp region

Interaction of precipitating low-energy magnetosheath electrons with the atmosphere in the dayside cusp region has been studied. Both pitch angle and energy distributions of the fluxes as well as excitation functions for selected N2 and O UV emissions were obtained by numerically solving the multiangle equations of electron transport. There is some possibility that atmospheric emissions may be used for remote measurements of incident soft energy flux, because the ratios of molecular to atomic emission line intensities in the low-energy region are quite different from those in the high-energy region.

Prasad, S. S.↗

NASA NTRS1985

The thermal structure of the dayside upper atmosphere of Venus above 125 km

A one-dimensional model of the Venus thermosphere has been constructed which includes computation of the heating efficiency of solar ultraviolet radiation, heat loss by radiation to space of infrared-active species, thermal transport by molecular and eddy conduction, and viscous dissipation. By comparing model predictions with results obtained from the Pioneer Venus Orbiter spacecraft, the results indicate that energy transport parameterized by eddy heat conduction plays a dominant role in determining thermospheric temperature. It is suggested that there exists a feedback mechanism linking heating and thermospheric circulation such that eddy cooling maintains an asymptotic temperature of about 300 K for both solar-maximum and solar-minimum conditions. The variation in thermospheric temperature with solar zenith angle, atomic oxygen-mixing ratio, rate of vibrational excitation of CO2 by ground-state O atoms, and the assumed transfer of O(1D) electronic energy to CO2 vibrational energy are also studied.

Hollenbach, D. J.↗

NASA NTRS1985

Chemistry in dynamically evolving clouds

A unified model of chemical and dynamical evolution of isolated, initially diffuse and quiescent interstellar clouds is presented. The model uses a semiempirically derived dependence of the observed cloud temperatures on the visual extinction and density. Even low-mass, low-density, diffuse clouds can collapse in this model, because the inward pressure gradient force assists gravitational contraction. In contrast, previous isothermal collapse models required the low-mass diffuse clouds to be unrealistically cold before gravitational contraction could start. Theoretically predicted dependences of the column densities of various atoms and molecules, such as C and CO, on visual extinction in diffuse clouds are in accord with observations. Similarly, the predicted dependences of the fractional abundances of various chemical species (e.g., CO, H2CO, HCN, HCO(+)) on the total hydrogen density in the core of the dense clouds also agree with observations reported to date in the literature. Compared with previous models of interstellar chemistry, the present model has the potential to explain the wide spectrum of chemical and physical properties of both diffuse and dense clouds with a common formalism employing only a few simple initial conditions.

Tarafdar, S. P.↗

NASA NTRS1985

Ozone in the free atmosphere

The present book provides a summary of the state of scientific knowledge of stratospheric and free tropospheric ozone as it exists at the beginning of 1983. Ozone photochemistry in the stratosphere is discussed, taking into account fundamental molecular properties, the absorption spectrum of ozone, photodissociation, ozone formation and destruction in the upper atmosphere, the photochemistry of odd-hydrogen, the photochemistry of odd-nitrogen, the photochemistry of odd-chlorine, and photochemistry-temperature coupling. The observed distribution of atmospheric ozone and its variations are considered along with ozone transport, ozone in the troposphere, stratospheric ozone perturbations, and climatic and biological effects. Attention is given to the techniques of observing atmospheric ozone, horizontal-vertical ozone transport and conservative quantities, measurements of tropospheric ozone, the tropospheric ozone budget, ozone models, natural ozone variations, and anthropogenic ozone perturbations.

Whitten, R. C.↗

NASA NTRS1985

Interstellar clouds - From a dynamical perspective on their chemistry

The possibility is examined that in the course of its dynamical evolution, a single mass of interstellar gas would exhibit properties of diffuse clouds, dense clouds and finally also of clouds perturbed by shocks or intense UV or X-ray radiation generated by a star of its own creation. This concept provides a common thread through the bewildering diversity of physical and chemical compositional properties shown by interstellar clouds. From this perspective, instead of being static objects, interstellar clouds are possibly incessantly evolving from initially diffuse to later dense state and then to star formation which ultimately restructures or disperses the remaining cloud material to begin the whole evolutionary process once again. Based on a simplified study of interstellar chemistry from a dynamical perspective, the ideas are presented as an heuristic: to encourage thought on the future direction of molecular astrophysics and the need to consider the chemical behavior of interstellar clouds in conjunction with, rather than in isolation from, their dynamical behavior. A physical basis must be sought for the semiempirical temperature formula which has been given a critical role in the collapse of diffuse clouds. Self-shielding effects in the chemistry of CO were neglected and this drawback should be removed; the ability of the model to explain the fractional abundances of more complex molecules, such as cyanopolyynes, should be examined.

Prasad, S. S.↗

NASA NTRS1984

The chemistry of phosphorus in dense interstellar clouds

Laboratory experiments show that the ion-molecule chemistry of phosphorus is significantly different from that of nitrogen in dense interstellar clouds. The PH3 molecule is not readily formed by gas-phase, ion-molecule reactions in these regions. Laboratory results used in a simple kinetic model indicate that the most abundant molecule containing phosphorus in dense clouds is PO.

Thorne, L. R.↗

NASA NTRS1983

Galactic cosmic rays and N2 dissociation on Titan

The electromagnetic and particle cascade resulting from the absorption of galactic cosmic rays in the atmosphere of Titan is shown to be an important mechanism for driving the photochemistry at pressures of 1 to 50 mbar in the atmosphere. In particular, the cosmic ray cascade dissociates N2, a process necessary for the synthesis of nitrogen organics such as HCN. The important interactions of the cosmic ray cascade with the atmosphere are discussed. The N2 excitation and dissociation rates and the ionization rates of the principal atmospheric consituents are computed for a Titan model atmosphere that is consistent with Voyager 1 observations. It is suggested that HCN may be formed efficiently in the lower atmosphere through the photodissociation of methylamine. It is also argued that models of nitrogen and hydrocarbon photochemistry in the lower atmosphere of Titan should include the absorption of galactic cosmic rays as an important energy source.

Capone, L. A.↗

NASA NTRS1983

Auroral electron interaction with the atmosphere in the presence of conjugate field-aligned electrostatic potentials

In the present investigation, both pitch angle and energy distributions of auroral electrons are obtained, for the first time, by rigorous numerical solutions of a multiangle transport equation at all energies in a realistic three component atmosphere in the presence of a conjugate field-aligned electrostatic potential. The results confirm all previous demonstrations of the validity of Evan's (1974, 1975, 1976) hypothesis concerning the origin of low energy auroral electrons from collisional interactions of the auroral primaries with the atmosphere. The existence of a collisionally populated field-perpendicular flux at energies near the beam energy is also demonstrated.

Prasad, S. S.↗

NASA NTRS1983

UV radiation field inside dense clouds - Its possible existence and chemical implications

Interstellar UV radiation cannot penetrate into the interior of dense clouds, and cosmic ray ionization is thought to be the sole driver of the gas phase chemistry. However, cosmic ray energy deposition also involves electronic excitation of the absorbing gas. It appears, therefore, possible that emissions resulting from these excitations might maintain a significant flux of chemically effective UV photons in the interior of dense clouds. The present investigation is concerned with this possibility, taking into account a simplified approach. This approach involves a consideration of Lyman and Werner band photons of molecular hydrogen and their relevance to C and CO chemistry. The results of this investigation suggest that a chemically significant flux of UV photons may exist inside dense clouds due to cosmic ray excitation of the various band systems of hydrogen. These photons would recover C I from the CO reservoir in dense clouds via photodissociation at an order of magnitude faster rate than that possible in connection with the reaction of CO with He(+).

Prasad, S. S.↗

NASA NTRS1983

Dependence of interstellar depletion on hydrogen column density - Possibilities and implications

A reexamination of the observed column densities of various elements in diffuse clouds suggests that almost all elements including oxygen, nitrogen, sulfur, and argon may be depleted with respect to hydrogen in interstellar clouds with large hydrogen column density. The amount of depletion varies from element to element and increases with increasing column density of hydrogen nuclei. This result is in qualitative agreement with the depletion of oxygen and sulfur independently inferred from the gas phase chemistry of sulfur in dense clouds. The rate of increase of depletion with hydrogen column density implied by the present study is large. It is possible that observational selection effects may have amplified the real dependence on N(H). A broad spectrum of C/O ratios ranging from values greater than unity to values less than unity appears possible for interstellar clouds, which would have the effect of a large variation in chemical composition from cloud to cloud.

Tarafdar, S. P.↗

NASA NTRS1982

Loss of CO/+/ ions by reaction with H2 in OMC-1

The temperature dependence of the rate constant for the reaction CO(+)+H2 yields HCO(+) has been measured in the temperature range 100-390 K. The rate constant has no dependence on temperature in this range, indicating that at the temperatures prevalent in OMC-1, the rate of loss of CO(+) by reaction with H2 is much too large to allow for the observed column abundance under steady-state conditions.

Huntress, W. T., Jr.↗

NASA NTRS1982

Sulfur chemistry in dense interstellar clouds

A model is presented for the gas phase chemistry of molecules containing sulfur in dense interstellar clouds. The sulfur chemistry is different from that used in previous models as a result of an extensive search of the recent literature and the availability of new laboratory data. The changes have a significant effect on the calculated abundance of sulfur compounds. The linked chemistry of sulfur and oxygen in the present model requires a severe depletion of sulfur and low fractional abundances of both O and O2 in the dense clouds. In contrast, the high abundance of SO and the low abundance of CS relative to SO in the HVS in the KL may indicate an oxygen-rich, high temperature environment compared to OMC-1. The formation of S-H bonds is slow because of the absence of radiative association between S(+) and H2. The present model underestimates the abundance of H2S unless a radiative association reaction between HS(+) and H2 is postulated.

Prasad, S. S.↗

NASA NTRS1982

A mesospheric source of nitrous oxide

In the terrestrial atmosphere, nitrous oxide (N2O) has a major role in the chemistry of ozone. Current atmospheric models assume that N2O is produced only by fixation at the earth's surface and that there are no local sources in the stratosphere or mesosphere. It is pointed out here that a significant in situ N2O source does exist above 20 km due to the excitation of the metastable N2(A 3Sigma u +) state by resonance absorption of solar UV photons that penetrate deeply into the atmosphere through the 1,800-2,200 A O2-O3 window. This source significantly affects the NO altitude distribution in the mesosphere and, in the earth's prebiological atmosphere, made N2O an important stratospheric constituent.

Zipf, E. C.↗

NASA NTRS1982

Modelling of auroral electrodynamical processes: Magnetosphere to mesosphere

Research conducted on auroral electrodynamic coupling between the magnetosphere and ionosphere-atmosphere in support of the development of a global scale kinetic plasma theory is reviewed. Topics covered include electric potential structure in the evening sector; morning and dayside auroras; auroral plasma formation; electrodynamic coupling with the thermosphere; and auroral electron interaction with the atmosphere.

Chiu, Y. T.↗

NASA NTRS1981

Formation of organic molecules on Titan

A mechanism is proposed for the formation of complex organic nitrogen compounds in the dense lower atmosphere of Titan. The mechanism is based on three-body association reactions with HCNH(+) ions formed by the reaction of N(+) with CH4, which lead to the production of ethyl cyanide, vinyl cyanide and cyanoacetylene. Calculations for a model atmosphere consistent with the preliminary interpretation of Voyager 1 data for the region of maximum cosmic ray activated chemistry, corresponding to a temperature between 150 and 160 K and a pressure of 20 mbar, are presented which show substantial organic nitrile and hydrogen cyanide production rates. Based on these production rates, it is expected that significant equilibrium concentrations of these compounds will be found on Titan.

Capone, L. A.↗

NASA NTRS1981

Atmospheric nitrous oxide produced by solar protons and relativistic electrons

A mechanism by which solar proton (SP) events and relativistic electron precipitation (REP) events may lead to the production of stratospheric NO is described. The process comprises the production of N2O in the mesosphere, its downward migration, and conversion in the stratosphere to NO by the reaction N2O + O(D) yields 2NO. This process would amplify direct NO production by more than 10%.

Prasad, S. S.↗