摘要:

Laser range, doppler and altimeter data collected from 14 near‐Earth satellites have been combined to determine the value of the geocentric gravitational coefficient (GM) of the Earth. A long‐arc solution using 3 years of laser range data to Lageos was used in a separate determination in which the effects of general relativity were investigated. The value of GM (including the mass of the atmosphere) was determined to be 398600.4405 km³/sec² when all corrections for general relativity are taken into account. The uncertainty (1‐σ) in the value of GM is estimated to be 0.001

ISSN:0094-8276    

DOI:10.1029/GL016i004p00271

年代:1989

数据来源: WILEY

摘要:

Inversions of seismic body waves indicate that most large earthquakes involve spatial and temporal heterogeneity of the moment release, but seismic body waves usually lack sufficient bandwidth to constrain the long‐period component of the radiation. Analysis of long‐period surface waves and free oscillations can constrain the overall source duration and moment; however, most procedures assume simple trapezoidal or boxcar source‐time histories, inconsistent with the body wave complexity. We find that source‐time function complexity can affect long‐period surface waves sufficiently to impact estimates of the source moment tensor, rupture duration and centroid depth. We present a procedure to objectively determine the long‐period component of complex source‐time functions in which we directly utilize results from body wave analysis. The method is applied to two great normal fault events of June 22, 1977 (Tonga,Mw=8.2) and August 19, 1977 (Indonesia,Mw=8.5). Standard long period analysis procedures underestimate the total slip duration in both cases. The overall source process times of both earthquakes are long

ISSN:0094-8276    

DOI:10.1029/GL016i004p00275

年代:1989

数据来源: WILEY

摘要:

The hypothesis that particular physical properties of the earth at the source can affect the radiation of high‐frequency waves was numerically investigated. Two factors have been considered: a non‐elastic behavior of the material at the crack tip (i.e presence of a cohesive zone) and the kinematics of the rupture front. Using the discrete wave number method, we calculated complete near‐source synthetic spectra radiated by a circular shear crack model. The two processes can explain an exponential decrease of the acceleration spectrum at high frequencies. Moreover, they can be differentiated. In the case of a final smooth deceleration of the rupture front, the rate of decay of the acceleration spectra is equivalent in all the azimuths. A clear azimuthal dependence exists in the presence of a cohesive zone, with the maximum rate of decay being observed in the direction of the fault strike. The parameter fmaxvaries as the ratio of the rupture velocity over the length of the cohesive zone for the non‐elastic model and as the inverse of the duration of the deceleration of the rupture front when the rupture front stops smoothly. These two models result in a decay of spectral amplitude at high frequency that mimics the effect of the anelastic atte

ISSN:0094-8276    

DOI:10.1029/GL016i004p00279

年代:1989

数据来源: WILEY

摘要:

Magnetotelluric (MT) measurements were carried out at 20 sites, extending 450 km across southern Mauritania in order to study lithospheric structures related to the West African craton (WAC) margin. The MT profile starts to the west on the Senegal‐Mauritania basin (S‐M basin), traverses across the Mauritanides orogenic belt, and terminates on the western border of the WAC (Taoudeni basin). Distortion effects due to local shallow inhomogeneities are present in nearly all of the basin data. In such a situation, the preliminary interpretation of the data was done by using 1D inversions based upon rotationally invariant parameters. Such distortion is not apparent for the belt and craton sites, and 1D inversions were followed by 2D modeling. The models produced reveal a clear crustal subdivision into a resistive upper crust underlain by a two‐layer lower crust with two conductors, one at mid‐crustal depths (supposed fluid‐produced) beneath the S‐M basin and the second at the base of the crust beneath the WAC. The 14‐km‐thick conductive material below the Mauritanides belt is interpreted as large imbricated thrusts representing the deep roots of the Mauritanides nappes. The models also show that significant contrasts in resistivity extend deep in the lithosphere between the cratonic area and the Sen

ISSN:0094-8276    

DOI:10.1029/GL016i004p00283

年代:1989

数据来源: WILEY

摘要:

A quantitative model of the migrational history of any free water in the martian satellite Phobos is presented. Results include predicted depths of retained ice and current water fluxes as a function of latitude, regolith porosity and effective pore size. It is found that, despite Phobos’ small size, low albedo and proximity to the sun, ice may be present within the sounding depth of the GRUNT experiment on the Soviet Phobos Mission—at least at mid to high latitudes. Implications for other experiments on the Phobos mission are also discus

ISSN:0094-8276    

DOI:10.1029/GL016i004p00287

年代:1989

数据来源: WILEY

摘要:

Recent investigations of CO photoabsorption demonstrate that photodissociation longward of the ionization threshold at 88.5 nm occurs primarily through line absorptions rather than continuous processes. We have constructed high resolution photoabsorption cross sections for CO at rotational temperatures near 250 K from the improved data on dissociating transitions. We examine the effects of the new cross sections on the rate of solar photodissociation of CO in the thermosphere of Venus and compare the results to values obtained with the lower resolution cross sections available previously. We find that the photodissociation profile peaks slightly higher in the atmosphere and the peak value and integrated total rate both decrease by about a factor of two.

ISSN:0094-8276    

DOI:10.1029/GL016i004p00291

年代:1989

数据来源: WILEY

摘要:

Using eight months of magnetotail plasma data, we have done a statistical survey on the relation between ion density and pressure in the Earth's plasmasheet. More than 270,000 spin averaged (4.5s) samples of ion density and thermal pressure obtained in the central plasma sheet and the plasma sheet boundary layer were cross‐correlated in order to obtain typical values of the polytropic index γ for different tail regions and disturbance conditions. The plasma sheet ion population behaves, on average, adiabaticly both in the central plasma sheet and the plasma sheet boundary layer. However, a polytropic index of about 1.4 for the quiet plasma sheet indicates that the latter behaves like a poorly insulated vessel. Hence, there seems to be no quiet time magneto‐tail equilibrium (“ground state”), but rather continuous cooling until new energy is entering from the

ISSN:0094-8276    

DOI:10.1029/GL016i004p00295

年代:1989

数据来源: WILEY

摘要:

Codes for the prediction of spacecraft environments are currently in widespread use; however, few if any comparisons of code results with experimental measurements have been performed. In this paper we report the results of calculations performed for the Atmosphere Explorer‐D satellite at an altitude of approximately 250 km. Measurements of the atmospheric composition at the ram surface of the spacecraft were obtained by a closed source mass spectrometer before and during the operation of a return flux experiment in which a neon plume was directed into the spacecraft velocity vector. Calculations using the Integrated Spacecraft Environment Model (ISEM) reproduced the measured fluxes of the ambient species immediately before and during the venting operation. The presence of the neon plume reduced the flux of ambient species reaching the detector. The model reproduced these trends and gave a range of calculated neon return flux with a mean approximately equal to the value derived from the measurements. The model predicted a surface density of ambient species approximately 33 times the ambient level with and without the plume and nonnegligible scattering of neon into the wake of the spacecraft during plume operatio

ISSN:0094-8276    

DOI:10.1029/GL016i004p00299

年代:1989

数据来源: WILEY

摘要:

The excitation functions for the seasonal components (annual and semiannual) of polar motion are calculated based on the atmospheric antisymmetric Rossby‐Haurwitz wave (1;−3). The meridional structures of geopotential and horizontal wind of this wave are reasonable approximations of the quasi‐standing seasonally varying Rossby waves propagating within the westerly background wind. Both wind and pressure loading contribute to the seasonal components of polar motion. The calculated displacement vector of the annual component agrees well with the observations. The semiannual component, estimated to be smaller by a factor of about ten, is not inconsistent with the

ISSN:0094-8276    

DOI:10.1029/GL016i004p00303

年代:1989

数据来源: WILEY

摘要:

Analysis of geomagnetic data from several stations in the equatorial region shows that the amplitude of the lunar semimonthly tide frequently does not have the expected symmetry about noon in its diurnal variation. There is a distinct tendency for larger amplitudes in the morning. At many stations this effect is largest in equinoctial months, though there are regional differences. Analysis of data from different years confirms its presence. The interaction between solar and lunar tides in the middle atmosphere may be responsible for the observed modulation.

ISSN:0094-8276    

DOI:10.1029/GL016i004p00307

年代:1989

数据来源: WILEY