ISSN:0094-8276    

DOI:10.1029/GL013i001p00001

年代:1986

数据来源: WILEY

摘要:

Two apparent contradictory sets of facts exist concerning the presence of a magnetosphere at Uranus. The Voyager planetary radio astronomy experiment has not detected a Uranus signal at a range<0.7 AU, whereas IUE satellite observations show relatively strong emission indicating the presence of substantial particle excitation of the atmosphere. The character of the EUV emission implies the presence of an ionosphere, and mass‐loading of the extended system comparable to that of Saturn. If the ingredient for production of an active magnetosphere is present, the non detection of radio emission then suggests that Uranus has a very weak or non existent magnetosphere. The apparent paradox of an excited atmosphere in the absence of an active magnetosphere may possibly be explained in terms of the peculiar characteristics of the excited sunlit equatorial exospheres of Jupiter and Saturn. We suggest that 1) the observed Uranus EUV emission may be a similar phenomenon to those observed in the sub‐solar equatorial regions of Jupiter and Saturn, which appear to be disconnected from auroral or magnetospheric activity, and 2) the Uranus intrinsic magnetic field is probably weak or nonexistent because of the availability of substantial mass for producing an active magnetosphere as derived from the nature of the EUV emission. We predict a substantial escape rate of atomic hydrogen (∼2 × 10

ISSN:0094-8276    

DOI:10.1029/GL013i001p00002

年代:1986

数据来源: WILEY

摘要:

A one‐dimensional model has been used to study the effects of exospheric temperature, methane and water influx, ionospheric outflow, and electron precipitation on the composition and structure of the ionosphere of Uranus. Peak ion concentrations range from 10³ to 106cm−3with a wide variation in peak altitude, which depends strongly on the exospheric temperature. In all the cases we considered, H+is the major ion in the topside ionosphere. At altitudes near or below the peak, H3+and CH5+can dominate, depending on the magnitude of CH4and H2O influx. Atomic hydrogen column depths above the methane absorbing layer exceed 1017cm−2and can produce large (400 R) emissions of resonantly scattered Lyman‐alpha. In the sunlit polar cap, electron precipitation with energy fluxes of 0.6 to 1.0 erg cm−2s−1results in direct production of Lyman‐alpha emissions th

ISSN:0094-8276    

DOI:10.1029/GL013i001p00006

年代:1986

数据来源: WILEY

摘要:

A simple, two‐dimensional, spectral model of heat transport in the Venus ionosphere has been constructed. Numerical experiments with the model suggest that a nocturnal heat source about an order of magnitude smaller than the daytime source is required to simulate the observed ion and electron temperatures. The solar zenith angle dependence of the high‐altitude ion temperature appears to be caused by expansion followed by compression of the plasma as it flows from the dayside to the nightside. Thermal‐flux saturation appears to be significant on the nightside, but the formation of a standing shock wave does not seem to be required to explain the broad features of the observed, nocturnal ion temper

ISSN:0094-8276    

DOI:10.1029/GL013i001p00010

年代:1986

数据来源: WILEY

摘要:

Unlike on Earth, long‐wavelength gravity anomalies and topography correlate well on Venus. Venus's admittance curve from spherical harmonic degree 2 to 18 is inconsistent with either Airy or Pratt isostasy but is consistent with dynamic support from mantle convection. A model using whole mantle flow and a high viscosity near‐surface layer overlying a constant viscosity mantle reproduces this admittance curve. On Earth, the effective viscosity deduced from geoid modeling increases by a factor of 300 from the asthenosphere to the lower mantle. These viscosity estimates may be biased by neglect of lateral variations in mantle viscosity. The different effective viscosity profiles for Earth and Venus may reflect their convective styles, with tectonism and mantle heat transport dominated by hot plumes on Venus and by subducted slabs on Ea

ISSN:0094-8276    

DOI:10.1029/GL013i001p00014

年代:1986

数据来源: WILEY

摘要:

Properties of the potential are considered for applications in gravity, geomagnetic, and thermal field studies. It is found that classical theory is somewhat limited because of the common difficulty in evaluating potential within a material body containing the sources. The theory of a new type of representation of disturbing potential, a biharmonic form, is given which eliminates this difficulty. It is shown that multiquadric equations provide us with a physically valid numerical approximation of the formal integral representation. Error bounds are derived. The results of tests with real gravity anomalies are given, which compare classical methods with the new biharmonic form. In summary, the new approach eliminates the classical singularities associated with collocation of points of measurement (or prediction) and the sources of disturbing potential. It also improves computational efficiency.

ISSN:0094-8276    

DOI:10.1029/GL013i001p00018

年代:1986

数据来源: WILEY

摘要:

The algorithm of the original Reflectivity Method has been vectorized and implemented on a CDC CYBER 205 computer. Calculation times are shortened by a factor of 20 to 30 compared with a general purpose computer with a capacity of several million floating point operations per second (MFLOP). The rapid calculation of synthetic seismograms for complex models, high frequency sources and all offset ranges is a provision for modeling not only particular phases but the whole observed wavefield. As an example we model refraction data of the Black Forest, Southwest Germany and are able to derive rather tight constraints on the physical properties of the lower crust.

ISSN:0094-8276    

DOI:10.1029/GL013i001p00022

年代:1986

数据来源: WILEY

摘要:

We describe a formalism for waveform fitting using the conjugate gradient method for nonlinear optimization. The method is tested on synthetic Love waves, and on Rayleigh wave observations from the portable, broad‐band NARS arra

ISSN:0094-8276    

DOI:10.1029/GL013i001p00026

年代:1986

数据来源: WILEY

摘要:

Lateral variations of shear velocity structure in the western United States are studied using observations of Rayleigh‐ and Love‐wave phase velocity dispersion. We measured Rayleigh‐wave dispersion in the 6‐60 sec period range on over 80 paths using both single‐and two‐station methods. Love‐wave dispersion was also measured on about 50 paths. A pure‐path regionalization method employing a block‐province grid is used to determine lateral variations of dispersion in the Basin and Range and adjacent geologic regions. Structural inversions of the regionalized Rayleigh‐wave dispersion curves indicate significant lateral variations in the crust and upper mantle structure of the Basin and Range and transition areas. A pronounced low‐velocity zone in the upper mantle is found under the Eureka heat flow province in the central Basin and Range. The lithospheric thickness in this province is 45 km in contrast to the Great Basin model of Priestley and Brune (1978; ∼65 km). Regionalization of Love‐wave data, together with the Rayleigh‐wave results, suggest the existence of polarization anisotropy in many areas of the Basin and Range. If verified, anisotropy must be accounted for in phase velocity inversions for the struc

ISSN:0094-8276    

DOI:10.1029/GL013i001p00030

年代:1986

数据来源: WILEY

摘要:

Two problematic aspects of deep seismic zones are investigated: the maximum depth (about 700 km) of seismic activity and the relative maximum in seismic energy release in the 500‐700 km depth range. It is shown that both features can be accounted for by a model based on thermal assimilation. This implies that no barrier is required at a depth of 650‐700 km to explain the observati

ISSN:0094-8276    

DOI:10.1029/GL013i001p00034

年代:1986

数据来源: WILEY