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At least 1,405 records · Page 78

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NASA NTRS1978

The distortion of the moon due to convection

Numerical calculations of the dynamical ellipticity of the moon due to finite-amplitude solid-state convection indicate that convection could be the cause of the nonhydrostatic gravitational figure, but only if the lunar lithosphere is capable of resisting global scale deformation. Thus lithospheric inhomogeneities and surface loads could also contribute substantially to the disequilibrium of the gravity figure. The calculations also show that it is unlikely that the geometrical distortion is due to convection.

Cassen, P.↗

NASA NTRS1979

Subsolidus convective cooling histories of terrestrial planets

The subsolidus convective cooling histories of terrestrial planets evolving from hot initial states are investigated quantitatively. A simple analytic model simulating average heat flux from a vigorously convecting mantle and incorporating a mantle viscosity proportional to mantle temperature and a lithosphere which thickens as the planet cools is employed. Heat flux from the convecting mantle is calculated on the basis of a power law relation between Nusselt number and Rayleigh number. The temperature distribution in the lithosphere is assumed to be linear throughout the cooling history of the planet. Cooling histories have been determined for the earth, Mars, Mercury and the moon and the mantle temperature decreases, mantle viscosity increases and decreases of heat flux to the surface and to the base of the lithosphere and of Nusselt and Rayleigh numbers are illustrated for each planet. It is found that primordial heat can contribute substantially to the present surface heat flux of a planet.

Schubert, G.↗

NASA NTRS1979

Convection in the ice crusts of satellites and its effect on surface relief

A theoretical analysis applied to the outer Galilean satellites shows that the mean temperature within the convecting ice crust is about 180 K. The time scale of convection, thought to be a measure of a period of tectonic activity, is of the order of 5 million years and the velocities are of the order of 1-20 cm per year depending on the thickness of the convecting layers (50-1000 km). Due to low temperatures T at the surface and strong dependence of the viscosity on T, the time scale of smaller topographic features is estimated (after Johnson and McGetching, 1973) to be of the order of several hundred million years to several billion years. It is suggested that comparatively short tectonic processes, repeating every few million years, may serve as an erosion factor in shaping the form of the surfaces.

Golitsyn, G. S.↗

NASA NTRS1979

Sunspots and the physics of magnetic flux tubes. IV - Aerodynamic lift on a thin cylinder in convective flows

The aerodynamic lift exerted on a long circular cylinder immersed in a convective flow pattern in an ideal fluid is calculated to establish the equilibrium position of the cylinder. The calculations establish the surprising result that the cylinder is pushed out the upwellings and the downdrafts of the convective cell, into a location midway between them. The implications for the intense magnetic flux tubes in the convection beneath the surface of the sun are considered.

Tsinganos, K. C.↗

NASA NTRS1979

Ionospheric convection at high latitudes

The characteristics of the high latitude ionospheric convection pattern that results from the transmission of electric fields from the magnetosphere was examined by gathering data from ground based and in situ measurements. The Atmosphere Explorer C satellite measurements of the motion of the high latitude ionosphere show that the dayside and nightside convection patterns are influenced by the interplanetary magnetic field and that the relationships between the energetic particle precipitation and the convection reversal boundaries are very complex. The existence of a magnetospheric boundary layer is consistent with the satellite observations, but its relationship to open and closed field lines cannot be determined.

Heelis, R. A.↗

NASA NTRS1980

Convective flows of viscous fluid in spherical layers. Certain astrophysical applications

The convective stability of a viscous liquid in spherical layers is investigated taking into consideration rotation, the latitudinal temperature gradient, and shear flow. The results of calculating nonlinear convective motion in spherical layers are examined. A discussion is given of the applicability of the results obtained to studying convection in astrophysical objects.

Yavorskaya, I. M.↗

NASA NTRS1979

The magnetospheric electric field and convective processes as diagnostics of the IMF and solar wind

Indirect measurements of the convection field as well as direct of the ionospheric electric field provide a means to at least monitor quanitatively solar wind processes. For instance, asymmetries in the ionospheric electric field and ionospheric Hall currents over the polar cap reflect the solar wind sector polarity. A stronger electric field, and thus convective flow, is found on the side of the polar cap where the y component of the IMF is parallel to the y component of the geomagnetic field. Additionally, the magnitude of the electric field and convective southward B sub Z and/or solar wind velocity, and thus may indicate the arrival at Earth of an interaction region in the solar wind. It is apparent that processes associated with the convention electric field may be used to predict large scale features in the solar wind; however, with present empirical knowledge it is not possible to make quantitative predictions of individual solar wind or IMF parameters.

Kaye, S. M.↗

NASA NTRS1980

An experimental investigation of convection in a fluid that exhibits phase change

Convection flows were systematically observed in a layer of fluid between two isothermal horizontal boundaries. The working fluid was a nematic liquid crystal, which exhibits a liquid-liquid phase change at which latent heat is released and the density changed. In addition to ordinary Rayleigh-Benard convection when either phase is present alone, there exist two types of convective motions initiated by the unstable density difference. When a thin layer of heavy fluid is present near the top boundary, hexagons with downgoing centers exist with no imposed thermal gradient. When a thin layer of light fluid is brought on near the lower boundary, the hexagons have upshooting centers. In both cases, the motions are kept going once they are initiated by the instability due to release of latent heat. Relation of the results to applicable theories is discussed.

Fitzjarrald, D. E.↗

NASA NTRS1979

Evolution of ion cyclotron instability in the plasma convection system of the magnetosphere

In the present paper, Liouville's theorem is used in combination with approximate, but extremely accurate, expressions which reflect the invariance of the magnetic moment and the longitudinal invariant to determine analytically the evolution of an adiabatically convecting energetic particle distribution. Features of the convecting distribution, reproduced by this model, include positive pitch angle anisotropy, regions in velocity space where the nonmonotonic perpendicular energy distribution is greater than zero, and the energy dependence of the degree of particle injection. The energy dependence of the injection yields upper and lower cutoffs to the distribution within the plasmasphere, and only an upper cutoff outside. This approach is used to study the evolution of ion cyclotron waves in a convecting particle distribution.

Kaye, S. M.↗

NASA NTRS1980

Effect of ice-albedo feedback on global sensitivity in a one-dimensional radiative-convective climate model

The feedback between the ice albedo and temperature is included in a one-dimensional radiative-convective climate model. The effect of this feedback on global sensitivity to changes in solar constant is studied for the current climate conditions. This ice-albedo feedback amplifies global sensitivity by 26 and 39%, respectively, for assumptions of fixed cloud altitude and fixed cloud temperature. The global sensitivity is not affected significantly if the latitudinal variations of mean solar zenith angle and cloud cover are included in the global model. The differences in global sensitivity between one-dimensional radiative-convective models and energy balance models are examined. It is shown that the models are in close agreement when the same feedback mechanisms are included. The one-dimensional radiative-convective model with ice-albedo feedback included is used to compute the equilibrium ice line as a function of solar constant.

Wang, W.-C.↗

NASA NTRS1980

High-latitude ion convection in the nighttime F region

A large data base of Atmosphere Explorer C measurements of the bulk ion velocity vector in the high-latitude F region has been examined. The concept of shear and rotational flow reversals is expanded upon to determine the extent to which the boundary between antisunward and sunward convection (i.e., a polar cap boundary) is an electrostatic equipotential. A most important factor in characterizing the nature of the large-scale geometrical configuration of the nighttime F region convection is the location and extent of quasi-equipotential regions of the convection reversal boundary.

Heelis, R. A.↗

NASA NTRS1980

The effect of convection on the propagation of relativistic galactic electrons

The paper presents exact analytical solutions which describe steady-state transport of relativistic electrons subject to diffusion, convection, and radiation losses. The diffusion coefficient is spatially and energy dependent; the bulk convection velocity is considered to be constant and directed outward from the galactic plane. The electron spectral behavior with respect to galactic height and energy indicates that convection provides a 'break' in the steady-state energy spectrum without recourse to distribution of sources. Radio observations of NGC 891 and NGC 4631 galaxies can be used to estimate their diffusion coefficient and its energy dependence. Application of analytical solutions to our Galaxy indicates that the diffusion coefficient and outflow velocity deduced from relativistic electron measurements and radio observations of their synchrotron radiation agree with values based on the transport of the cosmic ray nucleonic component.

Lerche, I.↗

NASA NTRS1980

Thermal convection of an internally heated infinite Prandtl number fluid in a spherical shell

A Galerkin technique is used to study the finite-amplitude axisymmetric steady convective motions of an infinite Prandtl number Boussinesq fluid in a spherical shell. Two types of heating are considered: in one case, convection is driven both by internal heat sources in the fluid and by an externally imposed temperature drop across the shell boundaries; in the other case, only internal heat sources drive convection and the lower boundary of the shell is adiabatic. Two distinct classes of axisymmetric steady states are found to be possible: states characterized by temperature and radial velocity fields that are symmetric about an equatorial plane; and a class of solutions that does not possess any symmetry properties about the equatorial plane.

Schubert, G.↗

NASA NTRS1980

Mantle convection pattern and subcrustal stress field under South America

The tectonic, igneous and metallogenic features of South America are discussed in terms of the crustal deformation associated with stresses due to mantle convection as inferred from the high degree harmonics in the geopotential field. The application of Runcorn's model for the laminar viscous flows in the upper mantle to satellite and gravity data results in a convection pattern which reveals the ascending flows between the descending Nazca plate and the overlying South American plate as well as segments of the descending Nazca plate beneath South America. The arc volcanism in South America is shown apparently to be related to the upwelling of high-temperature material induced by the subduction of the Nazca plate, with the South American basin systems associated with downwelling mantle flows. The resulting tensional stress fields are shown to be regions of structural kinship characterized by major concentrations of ore deposits and related to the cordillera, shield and igneous systems and the upward Andean movements. It is suggested that the upwelling convection flows in the upper mantle, coupled with crustal tension, have provided an uplift mechanism which has forced the hydrothermal systems in the basement rocks to the surface.

Liu, H.-S.↗

NASA NTRS1981

Mesoscale convection in the clouds of Venus

A theory explaining the high aspect ratios for cloud level convection in the Venusian atmosphere is presented. The apparent flatness of large-scale convection cells in the Venusian clouds is accounted for by anisotropic eddy diffusion and radiative transfer effects. Horizontal eddy diffusivities must be at least ten-fold greater than vertical diffusivities. Anisotropy ratios greater than or approximately equal to 1,000 are sufficient to explain the flattening in cases where the vertical eddy diffusivity is greater than or approximately equal to 100,000 sq cm/s and the effects of radiative transfer are negligible. For vertical diffusivity greater than or approximately equal to 100,000 sq cm/s, radiative transfer contributes to the flattening. Radiative transfer alone can not account for the apparent aspect ratios, as this would require vertical heat diffusivities large enough to give a nonphysically high value of convective heat transport.

Covey, C. C.↗

NASA NTRS1981

A variable mixing-length ratio for convection theory

It is argued that a natural choice for the local mixing length in the mixing-length theory of convection has a value proportional to the local density scale height of the convective bubbles. The resultant variable mixing-length ratio (the ratio between the mixing length and the pressure scale height) of this theory is enhanced in the superadiabatic region and approaches a constant in deeper layers. Numerical tests comparing the new mixing length successfully eliminate most of the density inversion that typically plagues conventional results. The new approach also seems to indicate the existence of granular motion at the top of the convection zone.

Chan, K. L.↗

NASA NTRS1981

Effect of gravity on coupled convective and interfacial instabilities during directional solidification

Instabilities occurring during the directional constant-rate solidification of a dilute solution of tin in lead are investigated as a function of gravity with particular emphasis on small constant gravitational accelerations. Two types of instabilities have been identified: (1) convective type that corresponds to long wavelengths, and (2) morphological type that corresponds to short wavelengths. These instabilities are coupled but while the convective instabilities depend strongly on gravity, the morphological instabilities are practically independent of gravity. For low velocities, the convective instabilities occur at much lower solute concentrations than the morphological instabilities, whereas at high velocities the reverse is true.

Coriell, S. R.↗

NASA NTRS1981

Mesospheric heating due to convectively excited gravity waves - A case study

A series of at least daily rocket soundings of the mesosphere at Wallops Island, Virginia (37 deg 50 min N, 75 deg 29 min W), in August and September 1976 reveal near simultaneity between rapid temperature rises and tropospheric convection in the form of squall lines. A multilevel numerical model is developed to test the hypothesis that the convection and warmings are related via internal gravity waves. Some features of the model are (1) the wave energy source is expressed in terms of cloud-base mass flux, plume diameter and buoyant updraft velocity; (2) the turbulent-viscous gravity wave dissipation is limited to above 55 km and is parameterized on the basis of findings by Hines (1965). Significant findings are: (1) mesospheric heating rates of the same order as those observed result for reasonable values of the convective parameters and in situ dissipation time scales; (2) only gravity waves confined to a well-defined wavelength and frequency interval are able to propagate upward to mesospheric altitudes; (3) heating rates are strongly dependent on plume diameter and updraft velocity; and (4) for a given cloud-base mass flux, heating rates are optimized for a plume updraft velocity of 10 m/s.

Clark, J. H. E.↗