Engineering topics

Cravens, T. E.

Publications and source records attributed to Cravens, T. E..

At least 109 records · Page 6

NASA NTRS1981

The ionospheric peak on the Venus dayside

The behavior of the ionospheric peak on the dayside of Venus is described and interpreted by combining radio occultation measurements with theoretical calculations. The theoretical models are shown to be able to reproduce the measured electron densities very accurately when careful consideration is given to such parameters as the level of solar activity, the electron temperature, and the neutral density. What is more, the models are able to provide a check on the accuracy of neutral atmospheric models in the vicinity of 140 km. Chemical equilibrium is assumed for the calculation of ion and electron densities. A table giving Pioneer Venus radio occultation measurements of the Venus dayside ionosphere is included.

Cravens, T. E.↗

NASA NTRS1981

The role of charge exchange in the solar wind absorption by Venus

The amount of solar wind absorbed because of charge exchange processes in the dayside ionosheath of Venus is calculated. The calculations suggest the existence of a lower limiting ionopause altitude, below which all solar wind particles are removed from the flow by charge exchange. The cold, slow ions resulting from this interaction are thought to play an important role in building the magnetic barrier observed just outside the ionopause and in creating the dayside 'mantle' and downstream 'penumbra' regions. The total absorption caused by charge exchange is typically 2-5%, although when the solar wind dynamic pressure is very high, it can reach 16%.

Gombosi, T. I.↗

NASA NTRS1981

Vibrational and rotational cooling of electrons by molecular hydrogen

It is noted that the cooling of electrons by vibrational and rotational excitation of molecular hydrogen plays an important role in the thermal balance of electrons in atmospheres containing significant amounts of H2. Calculations are described of vibrational and rotational cooling rates of electrons by H2. Results for a wide range of electron and neutral temperatures are presented, and analytical formulas for some to the cooling rates are given.

Waite, J. H., Jr.↗

NASA NTRS1981

The global distribution of nitric oxide at 200 km

Measurements of nitric oxide at 200 km by the ultraviolet nitric oxide experiment on Atmosphere Explorer D are used to demonstrate the dependence of NO on latitude, longitude, and magnetic activity. NO is more abundant in the summer hemisphere than in the winter hemisphere and is more abundant during magnetically active times than during quiet times. A simple photochemical theory is used to show that the knowledge of local composition and temperature is sufficient to explain the variations of nitric oxide at this altitude.

Cravens, T. E.↗

NASA NTRS1981

Hot oxygen atoms in the upper atmosphere of Venus

Using data obtained by an ultraviolet spectrometer on the Pioneer-Venus Orbiter, energy and altitude distribution of nonthermal oxygen atoms in the Venus atmosphere for typical daytime and nighttime conditions are calculated using two different numerical methods. Agreement is found between results obtained by the two-stream transport method and the diffusion equation technique. Dissociative recombination of molecular oxygen ions and charge exchange of atomic oxygen ions with the neutral hydrogen and oxygen gas are the source terms, and calculations show that the dissociative recombination term is the dominant one. It is shown that measured hot atom concentrations exceed estimates of the thermal hydrogen atom concentrations derived from Brinton et al. (1980) over most of the daytime exosphere, and the calculated concentrations do so to an even greater extent. It is concluded that hot oxygen atoms are an important component of the dayside exosphere of Venus, their most important effect being the absorption of solar wind.

Nagy, A. F.↗

NASA NTRS1980

Solar wind absorption by Venus

The portion of solar wind interacting with the dayside ionosphere and atmosphere of Venus was determined based on magnetic field fluctuations in the ionosheath and the interaction with the upper neutral atmosphere above the ionopause. Fluctuations with the ratio of the number of particles intersecting the daytide ionopause to the total number of particles of 0.3 suggest that about 0.3% of solar wind may be absorbed. Most of fast H atoms resulting from the charge exchange interactions with the atmosphere escape; some of the energy deposition processes produce observable signatures (such as a narrow Lyman alpha emission region), but penetrating solar wind particles do not control the physical and/or chemical structure of the daytime Venus ionosphere.

Gombosi, T. I.↗

NASA NTRS1980

Model calculations of the dayside ionosphere of Venus - Energetics

A model of the energy balance of the dayside ionosphere of Venus is presented. Calculations of the dayside electron and ion temperature profiles are carried out and compared with data from experiments on the Pioneer Venus orbiter. The coupled heat conduction equations for electrons and ions are solved for several values of the solar zenith angle. It is shown that thermal conductivities are inhibited by the presence of a horizontal magnetic field. A realistic model of the magnetic field that includes fluctuations is employed in deriving an appropriate expression for the thermal conductivity. The contributions of photoelectrons, ion chemistry, Joule heating, and solar wind heating to the energy balance of the ionosphere are considered.

Cravens, T. E.↗

NASA NTRS1980

Model calculations of the dayside ionosphere of Venus - Ionic composition

Comprehensive model calculations of the dayside ion density distributions were carried out and compared with results from the Pioneer Venus ion mass spectrometer. The coupled continuity and momentum equations were solved for O2(+), O(+), CO2(+), C(+), N(+), He(+), and H(+) densities for altitudes well away from the ionopause, where the horizontal transport terms are negligible. Chemical equilibrium solutions, describing conditions below about 200 km, were also obtained for N2(+), NO(+), and CO(+). The agreement between the model calculations of ion density and the measurements is good for some species, such as O(+), and rather poor for others, such as CO(+), indicating that while a basic understanding of the major chemical and physical processes controlling the composition and vertical distribution of the dayside Venus ionosphere, well below the ionopause, has been achieved, there are many important details requiring further investigations.

Nagy, A. F.↗

NASA NTRS1980

Venus nighttime hydrogen bulge

The concentration of atomic hydrogen in the Venus thermosphere near 165 km altitude and approximately 18 deg north latitude has been derived from Pioneer Venus in situ measurements of H(+), O(+), O and CO2 concentrations, under the assumption of chemical equilibrium. Altitude profiles of derived H concentration suggest that chemical equilibrium prevails to an altitude of at least 200 km on the dayside and to 165 km on the nightside. Measurements below these limits were made by the ion and neutral mass spectrometers on the orbiter spacecraft between December 1978 and July 1979, while periapsis traversed a complete diurnal cycle. The hydrogen concentration is found to rise sharply at both terminators from a dayside value of approximately 50,000/cu cm, and to exhibit an asymmetric nightside distribution with a peak density in the predawn sector approximately 400 times greater than the dayside value. Analysis suggests that wind-induced diffusion, combined with exospheric return flow, can account for the observed hydrogen behavior. The large day-night temperature contrast enhances advective transport, which produces the large H concentration diurnal variation; the shift of the H concentration nighttime maximum toward dawn is caused by atmospheric superrotation.

Brinton, H. C.↗

NASA NTRS1980

Hot hydrogen in the exosphere of Venus

Lyman-alpha measurements of the hydrogen corona of Venus by Mariners 5 and 10 have been shown to be consistent with a two-temperature component model. Bertaux et al. (1978) have successfully fitted the Venera 9 exospheric Lyman-alpha data to an elevated (500 K) single temperature. Various source mechanisms have been proposed to explain the 'hot' (1000 K) energetic component of the hydrogen corona. In the present paper recent results from the Pioneer Venus Orbiter are used to establish the major sources of this hot hydrogen population.

Cravens, T. E.↗

NASA NTRS1979

A model of the neutral and ion nitrogen chemistry in the daytime thermosphere of Venus

Density profiles of N(4S), NO, N(2D), NO(+), and N(+) are calculated for the thermosphere of Venus. The results show that N(4S) is the dominant odd nitrogen species throughout the thermosphere and has a maximum density of 18 million atoms/cu cm at 132 km. The calculated NO(+) density agrees well with recent Pioneer Venus measurements, but the calculated N(+) densities are a factor of two to five less than the measurements. The production of N(4S) atoms generated in the model is adequate to explain recent measurements of the nitric oxide chemiluminescent emission on the night side of Venus.

Rusch, D. W.↗

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

Comparison of calculated and measured ion densities on the dayside of Venus

Data from the Pioneer Venus ion mass spectrometers are compared with model calculations of the ion density distributions appropriate for daytime conditions. The model assumes diffusive equilibrium upper boundary conditions for the major ions (O2(+), O(+), CO2(+), He(+), and H(+)); the agreement between the calculated and measured gross behavior of these ions is reasonably good except for H(+), which may be influenced strongly by convective transport processes. The distributions of five minor ions (C(+), N(+), NO(+), CO(+), and N2(+)) are also calculated for the chemically controlled region (less than approximately 200 km); the agreements are, in general, poor, an indication that the present understanding of the Venus minor ion chemistry is still incomplete.

Nagy, A. F.↗

NASA NTRS1979

The latitudinal gradient of nitric oxide in the thermosphere

Theoretical calculations of nitric oxide altitude profiles are made at five different latitudes by using neutral temperatures and composition primarily from the MSIS (mass spectrometer and incoherent scatter) model. The nitric oxide calculated for an altitude of 105 km remains nearly constant with increasing latitude. Observations made by the ultraviolet nitric oxide instrument on the Atmosphere Explorer C satellite show that at low magnetic activity (Ap value of approximately 4), the NO density at 105 km agrees with the theory; however, at moderate levels of activity it increases with latitude. This discrepancy between the theoretical and observed latitudinal gradients of nitric oxide suggests the transport of NO from a high latitude source to lower latitudes. At 200 km the theoretical and observed latitudinal gradients are in reasonable agreement, an indication that the knowledge of the local composition and temperature is sufficient to model nitric oxide at this altitude.

Cravens, T. E.↗

NASA NTRS1979

The energetics of the ionosphere of Venus - A preliminary model based on Pioneer Venus observations

A model of the energy balance of the dayside ionosphere of Venus is presented. The coupled energy equations for electrons and ions are solved numerically and the calculated temperatures are compared with temperatures measured by instruments on the Pioneer Venus Orbiter. Neutral and ion density profiles consistent with those measured by various Pioneer Venus experiments are used in the model. It was found that using standard EUV sources and thermal conductivities the calculations produce temperatures that are much lower than the measured ones. Consequently, further calculations were performed in which (1) the thermal conductivities were inhibited by means of an almost horizontal magnetic field and/or (2) heat inflow into both the electron and ion gas at the top of the ionosphere was assumed due to the solar wind interaction. It was found that the calculated and measured temperatures were in reasonably good agreement if the thermal conductivities were inhibited by a near horizontal field of about 10 gammas and if the electron and ion heat fluxes were taken to be 4 x 10 to the 9th eV/sq cm/sec and 10 to the 7th eV/sq cm/sec, respectively.

Cravens, T. E.↗

NASA NTRS1979

Plasma diffusion into the wake of Venus

A model of the diffusion of ionosheath plasma into the wake region of Venus is presented. It is shown that particle diffusion, which is assumed to be a consequence of the fluctuating magnetic field observed in the wake of Venus by Pioneer Venus and Veneras 9 and 10, can explain the plasma observations made in the wake by Veneras 9 and 10. The pressure due to these diffusing particles when balanced against the ionospheric pressure yield ionopause heights less than 1000 km for zenith angles less than about 135 deg. The model also predicts significant fluxes of low energy electrons and ions for zenith angles less than 135 deg, which are capable of producing the observed nightside ionosphere.

Gombosi, T.↗

NASA NTRS1978

The Martian ionosphere in light of the Viking observations

A theoretical model has been constructed in which the ion density and the ion and electron temperature distributions are calculated by solving the coupled continuity-momentum equations and the coupled energy equations. The latest experimental results from the Viking 1 and 2 landers are used to vary some of the parameters in the model in order to obtain agreement between the theoretical and experimental results. It is found that solar EUV radiation alone is not able to maintain the observed high ion temperatures. It was also established that the energy coupling between the electron and ion gas is insufficient to account for the measured ion temperatures even in the presence of very large electron temperatures. Direct heat input to the ion gas, probably due to solar wind-ionosphere interactions, can result in ion temperature values in reasonable agreement with the observations. The ion densities calculated with the present model agree well with the Viking observations in the chemically controlled region, but at higher altitudes, dynamic transport processes need to be invoked to achieve consistency among the observed and calculated temperature and density values.

Chen, R. H.↗

NASA NTRS1978

The ionosphere and airglow of Venus - Prospects for Pioneer Venus

The paper presents model calculations for the Cytherean nighttime and daytime ionosphere. It is shown how some of the proposed mechanisms can be tested with the aid of the Pioneer Venus observations scheduled for December 1978. Theoretical calculations of the energetics of the Cytherean ionosphere are performed, and it is concluded that the Project Venus measurements will find elevated ion and electron temperatures, resulting primarily from energy fluxes associated in some manner with the solar wind. According to this model, the energy flux will act directly on the ion gas. Ultraviolet dayglow intensities were calculated, and it is anticipated that hundreds of kR's of CO2-related emission features such as the CO Cameron bands will be observed. Nightside ionosphere calculations were made assuming the precipitation of energetic electrons as an ionization source, and the intensities of some of the resulting emission features are calculated.

Cravens, T. E.↗