It is shown that the influence of the homogeneous conducting half-space can be represented approximately in terms of an image source located at a complex depth.
An account is given of some recent work on the mode theory of VLF ionospheric propagation. Attention is confined to the behavior of the attenuation coefficient of the dominant mode. The ionosphere is assumed to be a sharply bounded and homogeneous ionized medium. It is indicated that earth curvature increases the attenuation rate by as much as a factor of 2 as compared with the corresponding attenuation for a flat earth. The influence of the earth's magnetic field is also shown to be important. In fact, east‐to‐west propagation paths suffer much greater attenuation than west‐to‐east paths. The theoretical results in the present paper appear to agree well with the experimental data of W.L. Taylor.
When a conducting body is immersed in a time‐varying magnetic field, eddy currents are induced. These, in turn, produce a secondary magnetic field which may be detected by an observer external to the sphere. It has been demonstrated that a measurement of the external field can be used to estimate the conductivity of the body if certain assumptions are valid. For example, Ward (1953) has shown that the conductivity and permeability of geological core specimens from diamond drill holes may be determined by examining the frequency dependence of the time‐harmonic response of the specimen. In principle, the same information should also be available from the time response of the specimen for a suddenly applied magnetic field (Wait, 1951). In this paper, we wish to discuss the transient solution when a homogeneous conducting sphere is under the influence of a transient magnetic field. Also, as suggested earlier by Wait (1951), the actual time‐domain electromagnetic response of a massive sulfide body should exhibit features akin to this transient solution. Thus, the results should have application in mining geophysics. We consider both a nonpermeable and a permeable sphere, using a different approach for each. Because the former is a special case of the latter, a consistency check is obtained.
Numerical results are presented for the conductance and susceptance of a slotted-sphere antenna which is immersed in a compressible, isotropic, and lossless electron plasma. Previous series expansions for the parameters of interest are cast in a form suitable for numerical work. The results indicate that an appreciable fraction of the power radiated is in the form of electroacoustic waves. However, this fraction may be significantly reduced if the sphere is surrounded by an electron depletion layer (i.e., a sheath) whose thickness is somewhat greater than a Debye length.:! Strictly speaking. u./w.o= (y) 1~A tt where ~ is the classical De bye length and y is the ratio of specific heats.
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