摘要:

A brief discussion of, and a little speculation about, the relevance of the polar regions on climate is given. The main body of the paper gives a survey of the known deep convection areas of the world ocean. There are two distinct types of convection. The first is the classic sinking occurring on continental shelf slope systems, as typified by various locations around the Antarctic coast. The freezing of sea ice, and resulting brine ejection, creates dense salty water on the shelf which descends the slope under a balance of Coriolis, gravity, and frictional forces, entraining the surrounding warm deep water as it goes. The second process is the more recently observed open‐ocean convection, occurring in locations such as the Mediterranean, the Labrador Sea, and two locations in the Weddell gyre, and is hypothesized to occur in the Greenland Sea. Open‐ocean convection has many overall similarities in all these areas: it occurs in narrow (20–50 km) areas; it forms about 10 m³ s−lof deep water; it occurs only in regions of cyclonic mean circulation; more than one water mass in the mean circulation is involved; a preconditioning seems to be required; some surface forcing (cooling or sea ice formation) is necessary; a violent breakup of the water mass frequently occurs on time scales of

ISSN:8755-1209    

DOI:10.1029/RG021i001p00001

年代:1983

数据来源: WILEY

摘要:

Since 1967, when the U.S. Navy Navigation System was released to the general public, the accuracy of geodetic and geodynamic satellite Doppler positioning using satellite Doppler has improved steadily. This improvement is due mainly to refinements in hardware, refraction, and orbital modeling. Point positioning, using a minimum of 30 passes and the U.S. Defense Mapping Agency (DMA) precise ephemeris, has reached submeter accuracy. Relative positioning among several simultaneously observing stations separated by distances up to 250 km and using a minimum of 30 passes has a typical accuracy below 0.5 m (1σ). Current hardware design and errors in modeling of delays in the wet portion of the troposphere limit the achievable accuracy in relative positioning at the 0.1‐ to 0.5‐m (1σ) level, unless errors in receiver phase and tropospheric refraction delays are continuously measured (calibrated). Also, a crystal oscillator of lower short‐term stability (≥ 5 × 10−12/100 s) increases the Doppler data noise and causes a positive correlation persisting over several minutes. Geodetic and geodynamic Doppler data reductions usually involve meticulous modeling for instrumental, atmospheric, and orbital errors as well as careful Doppler data editing. A short‐arc orbit computation employed in such satellite Doppler reductions requires a gravitational potential expanded to at least degree and order 10 if the orbit shape accuracy is to be comparable to DMA precise ephemeris. Satellite Doppler positioning has had a significant impact on geodetic datum definition and the establishment of national and continental networks of unprecedented accuracy. Satellite Doppler methods, apart from providing one of the most accurate continuous methods of polar motion monitoring, have also been found useful in other geodynamical and ice motion studies. This review concentrates mainly on system performance, models, and computa

ISSN:8755-1209    

DOI:10.1029/RG021i001p00027

年代:1983

数据来源: WILEY

摘要:

The role of deformation analysis is discussed with respect to its existing or possible future applications in geodesy and geodynamics. Expressions for strain tensors are given in the more general case of Riemannian spaces and specialized for Euclidean spaces and the case of infinitesimal deformation. Among the various applications, special emphasis is given to the study of crustal deformations of the earth, deformations of the gravity field, and gravity field related deformations. Other applications are also considered.

ISSN:8755-1209    

DOI:10.1029/RG021i001p00041

年代:1983

数据来源: WILEY

摘要:

Density and elasticity data are consistent with, but do not require, a uniform upper‐ and lower‐mantle composition. Such data cannot at present resolve the small changes in physical properties that would be required to keep distinct mantle reservoirs dynamically separated. Alternatively, the issue of chemical stratification in the mantle can be addressed by analyzing the details of the phase equilibria of the appropriate minerals. In order to do this we construct an internally consistent pressure calibration scale which is based in part on the coesite‐stishovite transition pressurePtr(GPa) = 7.45(±0.3) + 2.1(±0.3) × 10−3T(°C) according to our reanalysis of the available data. We conclude that a discontinuous reaction occurs in the olivine component (α+γ = α+β) of the mantle at the conditions of the 400‐km seismological discontinuity; however, no discontinuous reactions corresponding to the 670‐km discontinuity have yet been identified. The only reaction observed in diamond cell experiments at approximately the pressures existing at 670‐km depth, the breakdown of γ‐spinel to form a silicate perovskite assemblage, appears not to satisfy the observed sharpness of this discontinuity. Thus it may be necessary to invoke either a univariant reaction that has not yet been observed experimentally or a chemical discontinuity at this depth. As the mantle is likely to be at temperatures higher than those of the experiments (estimated to be near 1000°C), additional univariant reactions involving the silicate ilmenite or garnet phases that are predicted to occur may be significant. One possible interpretation is that the mantle is of uniform composition and that such (hypothetical) reactions produce the 670‐km discontinuity. This would imply that the phase assemblages so far studied in high‐pressure‐temperature experiments are not those occurring in the mantle. Alternatively, the mantle

ISSN:8755-1209    

DOI:10.1029/RG021i001p00051

年代:1983

数据来源: WILEY

摘要:

A review and critique of present‐day kinetic theory models of planetary exospheres is presented. Models of ionized exospheres, specifically the solar and the terrestrial polar wind, are also discussed. The objective of the paper is to point out the need for a rigorous kinetic theory treatment of the atmosphere in the altitude region between the thermosphere and the exosphere. This is the region where the atmosphere undergoes a transition from a collision‐dominated to a collisionless situation. The various aspects of the exospheric problem are introduced and developed around this main theme. The exospheric problem and its considerable overlap with several related problems in physics and chemistry are noted. The calculation of the velocity distribution function of exospheric constituents with the spherically symmetric collisionless model is then presented. Various modifications of this standard model with regard to planetary rotation, nonuniform exobasic density and temperature distributions, and time‐dependent effects are subsequently discussed and compared. The extent to which collisionless models suffice to explain observational data of planetary exospheres is assessed. This assessment is given in terms of a comparison of density profiles or reduction factors determined with hydrogen and/or helium cells. A comparison of experimental measurements of the diurnal variation of terrestrial atomic hydrogen densities and theoretical models to explain the asymmetry is presented. The collisionless approach is criticized on theoretical grounds for the neglect of a consideration of the transition region and the resulting artificial boundary between the thermosphere and the exosphere. The aim of collisional models with regard to escape‐induced non‐Maxwellian effects, modifications of the temperature structure, nonthermal escape processes, and satellite particle populations from Monte Carlo simulations and/or kinetic theory models based on a Boltzmann equation is discussed, and the various results are compared. The various attempts to introduce collisional concepts are shown to be incomplete, each considering the problem with different objectives. Throughout, the importance of a more rigorous collisional treatment is stressed, in particular with respect to the description of nonthermal escape processes and satellite particle populations which cannot be considered in a collisionless context. The effects of charge exchange reactions, photoionization, and solar radiation pressure on satellite particles are examined. With regard to nonthermal escape processes, the absence of a detailed kinetic theory of product velocity distributions based on a collisional theory is a

ISSN:8755-1209    

DOI:10.1029/RG021i001p00075

年代:1983

数据来源: WILEY

摘要:

A qualitative explanation is proposed for the main features of quiet‐time Birkeland currents. The major conclusions are as follows: (1) The source of region 1 currents is the interplanetary electric field, linked to the polar ionosphere directly along open field lines near noon but indirectly, via the plasma sheet, at most other longitudes. (2) Region 2 arises from convective charge separation, owing to guiding center drifts. On the dayside, secondary charge separation may occur, extending the current pattern sunward. (3) The magnetopause boundary layer flow is not a major energy source ofj11∥, and neither are dynamo processes in convected plasmas or in the plasma sheet. (4) The convection reversal is expected to occur (as observed) in the interior of region 1, often near its poleward edge. (5) The cusp currents are associated with an inhomogeneity of the polar electric field, correlated with interplanetaryByand first observed by Heppner, though contributions by the boundary layer are not ruled out. (6) The four‐lobed polar electric field patterns occasionally observed during times of northward interplanetaryBzare generated by a boundary layer dynamo and signify the temporary existence of a closed magnetospheric configuration. (7) The branching ratio between two routes by which magnetospheric space charge may be neutralized, via the ionosphere and via polarization currents, is estimated, with and without considerations of particle mirr

ISSN:8755-1209    

DOI:10.1029/RG021i001p00125

年代:1983

数据来源: WILEY