摘要:

Images of the earth's dayglow emissions at ultraviolet wavelengths, primarily those of atomic oxygen at 130.4 nm, reveal regions of transient intensity decreases to ∼ 5% to 20% of surrounding values over areas estimated to be ∼ 2,000 km². The duration of these transient decreases in intensities is ∼ several minutes. Approximately 10 of these events occur each minute in the dayside upper atmosphere. The diurnal variation of the rate of occurrence is qualitatively similar to that for radar m

ISSN:0094-8276    

DOI:10.1029/GL013i004p00303

年代:1986

数据来源: WILEY

摘要:

Large, transient decreases of atmospheric dayglow intensities at ultraviolet wavelengths, primarily the atomic oxygen emissions at 130.4 nm, are interpreted in terms of an influx of heretofore undetected comet‐like objects. The primary composition of these comet‐like objects is water snow or clathrate in the form of a fluffy aggregate. These small comets are covered with a dust mantle and the tensile stress at fracture is estimated to be ∼ 0.1 dyne/cm². The water molecules that form the absorbing blanket for ultraviolet emissions arrive at the top of the earth's atmosphere as a piston of gas with bulk speed ≲ 20 km/sec. The mass of each of these comet‐like objects is ∼ 108gm, or ∼ 100 tons. The global influx rate is ∼ 20 comets per minute. The global mass accretion rate by the earth's atmosphere is ∼ 1012kg/year, and sufficient to replace the atmospheric mass in ∼ 5 × 106years. The earth and the other bodies in the solar system would be thus more strongly coupled to cometary matter t

ISSN:0094-8276    

DOI:10.1029/GL013i004p00307

年代:1986

数据来源: WILEY

摘要:

Zonal‐mean temperatures from the LIMS experiment are compared with a temperature "climatology" derived from 4 years of Rayleigh backscatter lidar measurements at 44 N and for the months of March, April, and May, when wave activity and, thus, atmospheric variability is very weak. The mean difference between the two data sets is less than 3.5 K between 37 and 64 km and is consistent with the theoretical estimates of accuracy reported for the LIMS data. The effects of longitudinal and year‐to‐year variations in this comparison were considered, but they do not change the conclusions significantly. These validation results extend the altitude range of accurate LIMS temperatures in the mesosphere some 10 km higher than previously reported. It also means that the LIMS ozone and NO2retrievals in the lower mesosphere should be free of any major temperature bias error ef

ISSN:0094-8276    

DOI:10.1029/GL013i004p00311

年代:1986

数据来源: WILEY

摘要:

To improve the lateral resolution of the first global 3 ‐ dimensional models of seismic wave velocities, regional studies have to be undertaken. The dispersion of Rayleigh waves along 86 paths across the Indian Ocean and surrounding regions is investigated in the period range 40 ‐ 300 s. The regionalization of group velocity according to the age of the sea floor shows an increase of velocity with age up to 150 s only, similar to the results in the Pacific Ocean. But here, this relationship vanishes more quickly at long period. Therefore the correlation of the deep structure with surface tectonics seems to be shallower in the Indian Ocean than in the Pacific Ocean. A tomographic method is applied to compute the geographical distributions of group velocity and azimuthal anisotropy and then the 3‐D structure of S‐wave velocity. Horizontal wavelengths of 2000 km for velocity and 3000 km for azimuthal anisotropy distribution can be resolved. Except for the central part of the South East Indian ridge which displays high velocities at all depths, the inversion corroborates a good correlation between lithospheric structure down to 120 km and surface tectonics: low velocities along the central and southeast Indian ridges, velocity increasing with the age of the sea floor, high velocities under African, Indian and Australian shields. At greater depths, the low velocity zones under the Gulf of Aden and the western part of the Southeast Indian ridges hold but the low velocity anomaly of the Central Indian ridge is offset eastward. The low velocity anomalies suggest uprising material and complex plate b

ISSN:0094-8276    

DOI:10.1029/GL013i004p00315

年代:1986

数据来源: WILEY

摘要:

The role of the Arctic Ocean in Earth's climatic and oceanographic development is significant but has become controversial because of disagreements concerning a reliable Arctic chronology. The first Arctic Ocean chronology based on U‐Th and14C data was in agreement with magnetostratigraphy developed later but these data have been challenged by recent amino acid diagenesis dates. New accelerator14C data support the earlier U‐Th‐magnetostratigraphic dates and confirm the reliability of the established Arctic Ocean chron

ISSN:0094-8276    

DOI:10.1029/GL013i004p00319

年代:1986

数据来源: WILEY

摘要:

Two related factors affect the vertical distribution and inventory of7Be (53‐day half life) in salt marsh sediments: (i) the movement of pore water, and (ii) the presence of vegetation. Infiltration of rainwater into unsaturated sediment distributes7Be over several centimeters depth without disturbing the sediment fabric. The sediment inventory of7Be expected from its atmospheric flux could only be accounted for in vegetated areas where foliage traps7Be from rainfall and surface flow. These factors lead to heterogeneous vertical and lateral distributions of7Be throughout the marsh, and may also affect the sediment distribution of other trace substances, such as210Pb,137Cs, and heavy metal

ISSN:0094-8276    

DOI:10.1029/GL013i004p00322

年代:1986

数据来源: WILEY

摘要:

We have measured the velocity of sound in a sandstone saturated with oil and brine at different concentrations. The velocity variations with the concentration depend drastically on the way of entering the fluids in the sample : injection of non‐wetting fluid in a sandstone fully saturated with a totally wetting fluid (drainage) or vice versa (imbibition). We interpret our measurements with an extension of the Biot‐Gassmann theory to a wetted frame of the sandstone saturated with an effective oil‐brine

ISSN:0094-8276    

DOI:10.1029/GL013i004p00326

年代:1986

数据来源: WILEY

摘要:

Detailed geophysical surveys in the vicinity of the Galapagos 95.5°W propagating rift tip establish the opening history of the rift and its velocity of propagation. These data together with a theory for mantle upwelling into slowly widening lithospheric cracks constrain the viscosity of the asthenosphere beneath the propagating rift to be less than about 1017to 1018Pa s

ISSN:0094-8276    

DOI:10.1029/GL013i004p00329

年代:1986

数据来源: WILEY

摘要:

Epicenters of intermediate‐depth events located by the ISC, taken together with focal depths determined from the reflected phase pP, define a double‐planed seismic zone beneath northern Chile and northwestern Argentina. The lower zone of seismicity, however, is the result of a systematic misidentification of sP as pP at many North American seismograph stations. The lower seismic plane consists of 14 earthquakes located between 22° and 24.5° S that occurred between 1964 and 1984. Depths calculated from pP arrival times reported to the ISC are heavily biased by the abundance of reports from high‐gain stations in western North America. Reports from these stations place the lower zone events at 125‐150 km depth, 35‐45 km below the main Wadati‐Benioff zone. Short‐period seismograms produced by the lower zone earthquakes at North American stations show only one clear reflected phase following P, which is often incorrectly identified as pP. Our identification of this phase as sP results in a shallower depth determination that is in agreement with pP and sP arrival time reports from stations in Africa and Southern Europe. The absence of a strong pP phase from North American records for the 14 events can be explained by the proximity of its take‐off angle to a nodal plane on the focal sphere. The majority of intermediate depth events in the region lie in the upper plane; these earthquakes do produce recognizable pP phases. Hence, no clear evidence exists for a double‐planed seismic

ISSN:0094-8276    

DOI:10.1029/GL013i004p00333

年代:1986

数据来源: WILEY

摘要:

We have investigated the potentially important effects arising from lateral heterogeneities on postglacial uplifts. Time‐dependent solutions for Maxwell rheology with lateral variations in the upper mantle are obtained by finite‐element techniques in cylindrical geometry. The displacements in the central portion of the ice sheet are not at all influenced by these lateral variations, whereas the uplifts near the edge of the ice load are extremely sensitive to lateral heterogeneities in the upper mantle. For ice sheets characteristic of the Laurentide ice sheet, deformations around the peripheral bulge can still be affected by an asthenosphere as far as twenty‐five degrees away. The continental lithosphere under the former ice load would appear to be thicker with the presence of a neighboring asthenosphere. Recent estimates of the North American plate thickness would exceed 250 km under these circumst

ISSN:0094-8276    

DOI:10.1029/GL013i004p00337

年代:1986

数据来源: WILEY