摘要:

Seismic surface wave data have been used to infer details of earth structure over various propagation paths. Experimental methods for measuring surface wave dispersion include the Fourier phase method, the time correlation method, the band‐pass filtering method, the group delay time method, and various digital ‘moving window’ techniques. Surface wave data exhibit distinctive observational characteristics for different tectonic provinces such as shields, aseismic continental platforms, rifts, oceans, and mountains. Inversion procedures used to determine velocity models from dispersion measurements utilize trial and error procedures or application of partial derivatives of phase and group velocity with respect to model parameters combined with linear inverse theory. The observed surface wave data emphasize that there are pronounced lateral variations in crustal and upper mantle structure and the continents and oceans are themselves inhomogeneous. Shield areas possess the highest values of shear wave velocities with depth and a relatively weak mantle low‐velocity zone. On the other hand, rift areas have much lower shear velocities and a very pronounced low‐velocity zone. Under the oceans the lid overlying the low‐velocity zone is variable in thickness and thickens with the increasing age of the ocean floor. The average velocity above 170–200 km is less under oceans than under continents. Surface waves propagating over oceanic paths are attenuated more than continental paths, but detailed regional studie

ISSN:8755-1209    

DOI:10.1029/RG016i001p00001

年代:1978

数据来源: WILEY

摘要:

This paper concerns the linear response of the ocean to forcing at a specified frequency and wave number in the absence of mean currents. It discusses the details of the forcing function, the general properties of the equations of motion, and possible simplifications of these equations. Two representations for the oceanic response to forcing are described in detail. One solution is in terms of the normal modes of the ocean. The vertical structure of these modes corresponds to that of the barotropic and baroclinic modes; their latitudinal structure corresponds to that of inertia‐gravity and Rossby waves. These waves are eigenfunctions of Laplace's tidal equations (LTE) with the frequency as eigenvalue. The description in terms of vertically standing modes is particularly useful if the forcing is nonlocal, because only these modes can propagate into undisturbed regions. The principal result is that it is extremely difficult for baroclinic (but not barotropic) disturbances to propagate horizontally away from a forced region. Instabilities of the Gulf Stream excite disturbances that are confined to the immediate neighborhood of the current; disturbances due to instabilities of equatorial currents do not propagate far latitudinally. A second representation of the oceanic response to forcing is in terms of vertically propagating, or vertically trapped, latitudinal modes. These modes are eigenfunctions of LTE with the equivalent depthh(not the frequency) as eigenvalue. Both positive and negative eigenvalueshare necessary for completeness. The modes withh>0 consist of an infinite set of inertia‐gravity waves and a finite set of Rossby waves which either propagate vertically or form vertically standing modes. The latitudinally gravest modes are equatorially trapped and have been observed in the Atlantic and Pacific oceans. The modes withh<0 are necessary to describe the oceanic response to nonresonant forcing. In the vertical this response attenuates with increasing distance from the forcing region. Because of the shallowness of the ocean the large eastward traveling atmospheric cyclones in mid‐latitudes and high latitudes force a response down to the ocean floor. Interaction with the bottom topography will result in smaller‐scale disturbances and will affect the frequency spectrum of the response when bottom‐trapped waves ar

ISSN:8755-1209    

DOI:10.1029/RG016i001p00015

年代:1978

数据来源: WILEY

摘要:

We give a detailed review of developments in atmospheric acoustics of the last decade. These developments include new ways to use refractive effects, studies of phase and amplitude fluctuations during propagation of sound along a path, nonlinear effects near high‐powered acoustic antennas, problems related to noise, insights into large‐scale atmospheric processes gained from infrasound, applications dependent on the Doppler frequency shift, and hybrid devices using both acoustic and electromagnetic waves. The introduction of the echosonde approximately 10 years ago results in the considerable emphasis given to advances in active acoustic remote sensing of atmospheric struct

ISSN:8755-1209    

DOI:10.1029/RG016i001p00047

年代:1978

数据来源: WILEY

摘要:

We review here the recent works on low‐latitude whistler propagation based on ground measurements with a reference to space observations and point out the role of low‐latitude ground whistlers in the general study of whistler propagation. Then we identify the main unsolved problems of low‐latitude whistlers: (1) whether or not ground whistlers propagate along field lines, (2) the characteristics of transmission through the ionosphere, and (3) the properties of low‐latitude whistler ducts (formation and decay times) and their structure. These fundamental problems also concern high‐latitude whistlers, but they have less influence, since the rate of duct occurrence is higher and the duct trapping is much easier at high latitudes. On the other hand, considering that the smaller dip angle of the geomagnetic field at low latitudes makes the duct trapping and the ionospheric transmission very hard and that the rate of duct occurrence itself is not so high, we can say that low‐latitude ground whistlers have suffered severe conditions during the course of their propagation. In other words, the fuller study of low‐latitude whistlers will contribute to solving the most poorly understood problem: how the whistler propagates upward from its source, excites ducts, and leaks from ducts, penetrating to the ground. Furthermore, the characteristics of whistler ducts may be studied effectively by using low‐latitude whistlers. Hence these questions should be investigated during this International Magnetospher

ISSN:8755-1209    

DOI:10.1029/RG016i001p00111

年代:1978

数据来源: WILEY

摘要:

Recent and ongoing planetary missions have provided and are continuing to provide extensive observations of the variations of the interplanetary magnetic field (IMF) both in time and with heliocentric distance from the sun. Large time variations in both the IMF and its fluctuations are observed. These are produced predominantly by dynamical processes in the interplanetary medium associated with stream interactions. Magnetic field variations near the sun are propagated to greater heliocentric distances, a process also contributing to the observed variability of the IMF. Temporal variations on a time scale comparable to or less than the corotation period complicate attempts to deduce radial gradients of the field and its fluctuations from the various observations. However, recent measurements inward to 0.46 AU and outward to 5 AU suggest that the radial component of the field on average decreases approximately asr−2, as was predicted by Parker, while the azimuthal component decreases more rapidly than ther−1dependence predicted by simple theory. Three sets of observations are consistent with anr−1.3dependence for |Bϕ|. The temporal variability of solar wind speed is most likely the predominant contributor to this latter observational result. The long‐term average azimuthal component radial gradient is probably consistent with the Parkerr−1dependence when solar wind speed variations are taken into account. The observations of the normal component magnitude |Bθ| are roughly consistent with a heliocentric distance dependence ofr1.4. The observed radial distance dependence of the total magnitude of the IMF is well described by the Parker formulation. There is observational evidence that amplitudes of fluctuations of the vector field with periods less than 1 day vary with heliocentric distance as approximatelyr3/2, in agreement with theoretical models by Whang and Hollweg. In relation to total field intensity, the amplitude of directional fluctuations is on average nearly constant with radial distance, at most decreasing weakly with increasing distance, although temporal variations are large. There is evidence that fluctuations in field intensity grow in relation to those in field direction with increasing distance. More observations are needed to confirm these conclusions. The number of directional discontinuities per unit time is observed to decrease with increasing distance from the sun. The apparent decrease may possibly be caused by geometric or selection effects. The relationship between fluctuations of the field and the corotating stream structure is still not understood in detail, and therefore the origins of the various mesoscale and microscale features are at presen

ISSN:8755-1209    

DOI:10.1029/RG016i001p00125

年代:1978

数据来源: WILEY

摘要:

Low‐altitude satellite observations of the low‐energy electron fluxes that populate the polar regions are summarized and classified into two groups: class 1, the very low intensity distributions, and class 2, the more intense, often structured distributions observed during magnetically disturbed conditions. High‐altitude observations of electron fluxes, including the solar wind and the tail lobes, are presented to suggest that class 1 observations are the result of direct access of interplanetary electrons through the lobes into the polar regions. Class 2 observations may be due in part to magnetospheric proc

ISSN:8755-1209    

DOI:10.1029/RG016i001p00147

年代:1978

数据来源: WILEY