摘要:

Evidence indicating lateral heterogeneity in the deep structure of island arcs has been collected from seismological literature. Abundant evidence from seismological studies in Japan during the last fifty years generally supports the large‐scale anomalous structure beneath this typical island‐arc region, i.e., the existence of a high‐Q, high‐velocity zone about 100 km thick that dips from the vicinity of the trench beneath the arc to a depth of several hundred kilometers, and two low‐Q, low‐velocity zones in the upper mantle on the inner (continental) and outer (oceanic) sides of this dipping zone. Most of the seismic activity is confined to this dipping zone and to the crust near and on the inner side of the volcanic belt. In other island‐arc regions, such as Kurile‐Kamchatka, Indonesia, Tonga‐Kermadec, and New Zealand, a number of seismological studies have indicated the existence of similar anomalous structure. At present, more detailed, more accurate determination of the configuration and physical parameters of these anomalous zones by seismological techniques is in progress, providing important information for understanding the global tectonic process and the origins of regional geophysical and geological features found in i

ISSN:8755-1209    

DOI:10.1029/RG009i004p00839

年代:1971

数据来源: WILEY

摘要:

A number of magnetic variation anomalies observed in geomagnetic deep‐sounding studies can be attributed to inhomogeneities of electrical resistivity in the upper crust. The electromagnetic induction in isolated conductors is usually too small to explain anomalies associated with crustal inhomogeneities, and anomalous variation fields mainly arise from concentration and channeling of currents induced elsewhere. These systems of currents are induced over a large region of the earth and have dimensions comparable to those of the source field. Most of the crustal anomalies are associated with deep conducting sedimentary basins or with regions of rapid change of sedimentary conductivity.Examples of anomalies of this origin are the North German basin, the Anadarko basin in Oklahoma and a region in north‐central Texas where more resistive paleozoic sediments are adjacent to conductive sediments in the Gulf coast plains. Other anomalies are related to currents concentrated in channels of conducting sea water located between two resistive continental blocks. Anomalous fields arising in this way have been observed on both sides of the Bonifacio Straits between Corsica and Sardinia. It is also thought that the Alert anomaly in the Canadian Arctic is closely related to the channeling of current through the Robeson Channel. An additional inland anomaly seems to be due to leakage of currents induced in the oceans into conductive structures in the upper crust. A magnetic variation anomaly with an amplitude larger than any other reported in the literature has been found in the northern Great Plains province of the United States. It extends along the eastern edge of the Black Hills north to the Williston basin. Higher conductivities than those characteristic of sediments are needed to explain this anomaly, which has been tentatively attributed to a zone of conducting graphite schists in the basement. Examples of magnetotelluric observations near current concentrations compared with two‐dimensional model calculations show that plane‐layered solutions are rarely adequate as representations of magnetotelluric resistivity curves in the vicinity of these features. In spite of these difficulties, the geomagnetic deep‐sounding technique can yield information about the lateral variations of resistivities of the upper mantle, providing that the conductivities at the surface are not too high or are relatively uniform

ISSN:8755-1209    

DOI:10.1029/RG009i004p00891

年代:1971

数据来源: WILEY

摘要:

Within the WKB (Wentzel‐Kremers‐Brillouin) or geometrical optics approximation, a wave packet can be taken to be made up of phonons transporting energy, momentum, and angular momentum. The energy‐momentum tensor formalism provides single generalized expressions for the conservation of these quantities and for the exchange of energy and momentum between the wave packet and the background medium. The formalism also leads to a clearer interpretation of wave energy and wave momentum; in many problems of geophysical interest, it is necessary to introduce modified concepts of the momentum density of a p

ISSN:8755-1209    

DOI:10.1029/RG009i004p00917

年代:1971

数据来源: WILEY

摘要:

This paper reviews present understanding of the internal structure of the thin boundary layer, termed the magnetopause, that separates the distorted geomagnetic field in the magnetosphere from the flow of solar plasma in the magnetosheath. The fundamental theoretical concepts of the subject are introduced by considering the structure of the boundary layer that exists when a cold, unmagnetized stream of ions and electrons impinges normally on a vacuum magnetic field. This idealized model indicates how the earth's magnetic field is confined by the impact pressure of the solar wind, the geomagnetic field being terminated by induced shielding currents flowing in the magnetopause. The various idealizations and approximations in this model, such as the assumption of a cold solar plasma and the neglect of the interplanetary magnetic field, are examined critically. The presence of thermal plasma in the magnetosphere modifies the structure of the boundary layer so that the ions penetrate substantially deeper (∼100 km) than the electrons into the geomagnetic field. Complications then arise if the solar plasma has a component of velocity parallel to the confined field, as occurs in the downstream magnetopause that bounds the geomagnetic tail. This additional component of velocity generates electric currents parallel to the magnetic field that may destroy the small‐scale equilibrium of the magnetopause and result in a tangential drag on the geomagnetic cavity. The available experimental information on the magnetopause structure is summarized and related to the theoretical investigations. Although the observations confirm some of the theoretical predictions, much of the detailed theory remains speculative at the present t

ISSN:8755-1209    

DOI:10.1029/RG009i004p00953

年代:1971

数据来源: WILEY

摘要:

Since Birkeland's observations of high‐latitude magnetic perturbations led him to postulate the existence of geomagnetically aligned electric currents, theoretical studies have led to a general understanding of the mechanisms that are capable of driving such currents, and of the processes associated with formation and control of current configurations. Recent advances in experimental techniques have yielded information on the spatial and temporal behavior of such field‐aligned Birkeland currents and have established their association to an individual auroral arc and energetic precipitating particles. This paper gives a brief summary of ionospheric effects related to production, maintenance, and control of Birkeland current systems. Available experimental data are discussed in relation to some of these, effe

ISSN:8755-1209    

DOI:10.1029/RG009i004p00987

年代:1971

数据来源: WILEY

摘要:

The status ofD‐region positive‐ and negative‐ion chemistry is reviewed. In the case of the positive‐ion chemistry, the presently accepted ion production rates and ion‐neutral reaction scheme are incompatible with the observed qualitative ion composition measurements. The observations indicate that water‐ion clusters, H3O+(H2O)n, particularly H5O2+, are dominant below 80 km, whereas calculations predict that NO+and its hydrates would be dominant. It seems rather likely that an essential element of the physics involved has so far escaped attention. In the case of the negative ions, the few observations only recently obtained are insufficent to critically test the detailed reaction schemes available from laboratory studies. Uncertainties in the concentrations of minor neutral constituents severely limit the ability to predict theoretical negative‐ion profiles at present. The first rocket negative‐ion observations do support the laboratory‐derived prediction of a dominant role for NO3−and its hydr

ISSN:8755-1209    

DOI:10.1029/RG009i004p00997

年代:1971

数据来源: WILEY