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61.
Three major geometric factors which are likely to influence theoretical interpretation of planetary polarization measurements, viz., observer—planet distance, horizontal inhomogeneity of planetary disk, and deviation from a spherical body, are investigated.The distance effect is examined for regional as well as global polarizations. For convenience of analysis, the expressions for zenith and azimuth angles of incident and emergent light appropriate for a snap-shot observation are derived as explicit functions of distance between observer and planet. Sample computations for Venus indicate that regional polarization near the planetary limb is significantly affected by the observer's distance. This effect should be particularly noticeable when an observation is made at a phase angle around which the single scattering polarization of atmospheric scattering agents exhibits a steep variation. The global polarization at large phase angles (measured at disk-center) is gradually moved toward smaller phase angles, as the observer approaches the planet. Any narrow polarization features such as rainbow and glory at small phase angles are heavily smoothed out.The effects of horizontal inhomogeneity are investigated with a planetary disk having highly polarizing regions at high latitudes. Comparison of theoretical global polarization computed for such a disk with the Pioneer Venus OCPP measurements shows a possible change in cloud-haze stratification approximately at 50° latitude, consistent with other imaging observations. An approximate analytical representation of residual polarization at zero phase angle is then derived to compare to the numerical results for Venus. An attempt is also made to explain the relatively large magnitude of residual polarization observed on Jupiter.Finally, to study the effects of nonsphericity of planetary body, the global polarizations are computed for a spheroidal planet. The global polarization tends to increase as the planet's oblateness increases. However, for Jupiter and Saturn, such effect may be of secondary importance.  相似文献   
62.
The slowly varying components (S-components) of the solar radio emission were observed by a 17 Gc/s grating interferometer. In combination with 4 Gc/s data, it is deduced that the source of the 17 Gc/s S-component is optically thin.  相似文献   
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64.
Examining the evolution of fracture permeability under stressed and temperature-elevated conditions, a series of flow-through experiments on a single rock fracture in granite has been conducted under confining pressures of 5 and 10 MPa, under differential water pressures ranging from 0.04 to 0.5 MPa, and at temperatures of 20–90 °C, for several hundred hours in each experiment. Measurements of fluid and dissolved mass fluxes, and post-experimental microscopy, were conducted to constrain the progress of mineral dissolution and/or precipitation and to examine its effect on transport properties. Generally, the fracture aperture monotonically decreased with time at room temperature, and reached a steady state in relatively short periods (i.e., <400 h). However, once the temperature was elevated to 90 °C, the aperture resumed decreasing and kept decreasing throughout the rest of the experimental periods. This reduction may result from the removal of the mineral mass from the bridging asperities within the fracture. Post-experimental observations by scanning electron microscopy, coupled with energy dispersive X-ray spectroscopy (SEM-EDX), revealed the formation of several kinds of secondary minerals such as silica and calcite. However, the precipitated minerals seemed to have had little influence on the flow characteristics within the fracture, because the precipitation was limited to quite local and small areas. The evolving rates and ultimate magnitudes of the fracture aperture are likely to be controlled by the stress exerted over the contacting asperities and temperatures, and by the prescribed flow conditions. Thus, this complex behavior should be attributed to the coupled chemically- and mechanically-induced effect. A coupled chemo–mechano conceptual model, accounting for pressure and free-face dissolutions, is presented in this paper to follow the evolution of the fracture permeability observed in the flow-through experiments. This model addresses the two dissolution processes at the contacting asperities and the free walls within the fractures, and is also capable of describing multi-mineral dissolution behavior. The model shows that the evolution of a fracture aperture (or related permeability) and of element concentrations may be followed with time under arbitrary temperature and pressure conditions. The model predictions for the evolving fracture aperture and elements concentrations show a relatively good agreement with the experimental measurements, although it is not possible to replicate the abrupt reduction observed in the early periods of the experiments, which is likely to be due to an unaccounted mechanism of more stress-mediated fracture compaction driven by the fracturing of the propping asperities.  相似文献   
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66.
Characteristic time scales relevant to the accumulation of planetesimals in a gaseous nebula are examined and the accumulation toward the planets is simulated by numerically solving a growth equation for a mass distribution function. The eccentricity and inclination of planetesimals are assumed to be determined by a balance between excitation due to mutual gravitational scattering and dissipation due to gas drag. Two kinds of mass motion in the radial direction, i.e., diffusion due to mutual scattering and inward flow due to gas drag, are both taken into account. The diffusion is shown to be effective in later stages with a result of accelerating the accumulation. As to the coalescent collision cross section, the usual formula for a binary encounter in a free space is used but the effect of tidal disruption which increases substantially the cross section is taken into account. Numerical results show that the gravitational enhancement factor (i.e., the so-called “Safronov number”), contained in the cross section formula, always takes a value of the order of unity but the accumulation proceeds relatively rapidly owing to the effects of radial diffusion and tidal disruption. That is, a proto-Earth, a proto-Jupiter, and a proto-Saturn with masses of 1×1027 g are formed in 5×106, 1×107, and 1.6×108 years, respectively. Also, a tentative numerical computation for the Neptune formation shows that a proto-Neptune with the same mass requires a long accumulation time, 4.6×109 years. Finally, the other effects which are expected to reduce the above growth times further are discussed.  相似文献   
67.
Astrophysics and Space Science - Electrostatic (ES) waves generated in space plasmas, e.g., Langmuir and ion-acoustic waves, are subject to multiple applications, such as plasma diagnosis,...  相似文献   
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69.
Iapetus, one of the saturnian moons, has an extreme albedo contrast between the leading and trailing hemispheres. The origin of this albedo dichotomy has led to several hypotheses, however it remains controversial. To clarify the origin of the dichotomy, the key approach is to investigate the detailed distribution of the dark material. Recent studies of impact craters and surface temperature from Cassini spacecraft data implied that sublimation of H2O ice can occur on Iapetus’ surface. This ice sublimation can change the albedo distribution on the moon with time.In this study, we evaluate the effect of ice sublimation and simulate the temporal change of surface albedo. We assume the dark material and the bright ice on the surface to be uniformly mixed with a certain volume fraction, and the initial albedo distribution to incorporate the dark material deposits on the surface. That is, the albedo at the apex is lowest and concentrically increases in a sinusoidal pattern. This situation simulates that dark materials existed around the Iapetus’ orbit billions of years ago, and the synchronously rotating Iapetus swept the material and then deposited it on its surface. The evolution of the surface albedo during 4.0 Gyr is simulated by estimating the surface temperature from the insolation energy on Iapetus including the effect of Saturn’s eccentricity and Iapetus’ obliquity precession, and evaluating the sublimation rate of H2O ice from the Iapetus’ surface.As a result, we found that the distribution of the surface albedo changed dramatically after 4.0 Gyr of evolution. The sublimation has three important effects on the resultant surface albedo. First, the albedo in the leading hemisphere has significantly decreased to approach the minimum value. Second, the albedo distribution has been elongated along the equator. Third, the edge of the low albedo region has become clear. Considering the effect of ice sublimation, the current albedo distribution can be reconstructed from the sinusoidal albedo distribution, suggesting the apex-antapex cratering asymmetry as a candidate for the origin of the albedo dichotomy. From the model analysis, we obtained an important aspect that the depth of the turn-over layer where the darkening process proceeded for 4 Gyr should be an order of 10 cm, which is consistent with evaluation from the Cassini radar observations.  相似文献   
70.
In this work, we first establish a simple procedure to obtain with 11-figure accuracy the values of Chandrasekhar’s H-function for isotropic scattering using a closed-form integral representation and the Gauss-Legendre quadrature. Based on the numerical values of the function produced by this method for various combinations of ? 0, the single scattering albedo, and μ, the cosine of the zenith angle θ of the direction of radiation emergent from or incident upon a semi-infinite scattering-absorbing medium, we propose a rational approximation formula with μ 1/4 and \(\sqrt{1-\varpi_{0}}\) as the independent variables. This allows us to reproduce the correct values of H(? 0,μ) within a relative error of 2.1×10?5 without recourse to any iterative procedure or root-finding process.  相似文献   
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