bluuebunny/crossref_metadata_2025 huggingface dataset:
Continental margins and the extent and number of the continents
Authors: Cogley J. Graham
Published: February 2008

A new map of the extent of continental crust shows that the continents are more extensive and less dispersed than they are conventionally thought to be. Their total area is 210.4 × 106 km², or 41% of the earth's surface, and there are 14 of them. Continent‐ocean contacts are mapped on the basis of a literature survey, and continent‐continent contacts are drawn along plate boundaries; thus although Eurasia is still the largest continent, it does not include the separate continents India and Arabia and surrenders northeastern Siberia to North America. Much of the Arctic Ocean is underlain by North American continental crust, and North America is the second‐largest continent. Central America and New Zealand are recognized as continents, the latter being nine‐tenths submerged, and there are four microcontinents: Rockall, Seychelles, Agulhas, and Jan Mayen. Several oceanic windows, enclaves within the continents, must be recognized if the extent of the continents is to be accurately delineated. The average thickness of continental crust is 36 km; small continents seem to be thinner than large ones, and height and thickness are strongly interdependent. The total volume of continental crust is 7.2 × 109 km³, somewhat less than the 8.0 × 109 km³ estimate of Ronov and Yaroshevskiy (1977). Mean heights are significantly lower than the 875 m commonly quoted from Kossinna (1933), because submerged parts of the continents (30.6% of their total area) are correctly accounted for; the new estimate of mean height is ∼120 m. The relationship between mean height and area for individual continents, however, is still observed. Modal heights are not correlated with area and are ∼ 250 m for most of the large continents. Certain continents have anomalous mean or modal heights, suggesting that the mean height‐area relationship arises from greater relative lengths of orogenic belts around and internal tectonism within larger continents; anomalous modal heights suggest anomalous epeirogenic activity. These results may have implications for the hypsometry of ancient continents: in particular, Pangea need not have had a modal height much different from that observed today.
The geology of the Persian Gulf‐Gulf of Oman Region: A synthesis
Authors: Ross David A. , Uchupi Elazar , White Robert S.
Published: February 2008

During the Mesozoic most of the Arabian Peninsula, Persian Gulf, south‐western Iran, and eastern Iraq constituted the Arabian platform. Deformation of the Musandam Peninsula in the Late Cretaceous and mid‐Tertiary by compression (subduction) from the east and southwest, collision of the Arabian platform and Eurasian plate along the Zagros Crush zone during the Oligocene or early Miocene, and emplacement of the Zagros Mountains by gravitational sliding during the Neogene and Pleistocene have reduced the platform area to the Persian Gulf. Other factors that contributed to the reduction of the Arabian platform include the uplift of the Arabian Peninsula during the opening of the Red Sea in the Tertiary, tectonism of the Infracambrian Hormuz salt, upwarp of the platform sediment cover by basement uplift and/or salt tectonics, and a 600‐ to 400‐m drop in sea level since the Cretaceous. At present, tectonism in the region is restricted to the northern edge of the Gulf of Oman where the Arabian plate is subducting the Eurasian plate from the south and to the Zagros Crush zone where the Arabian and Eurasian plates are colliding with one another.
Research on flood frequency analysis: 1983–1986
Authors: Potter Kenneth W.
Published: February 2008

What follows is a selective review of research on flood frequency analysis published in U.S. journals or by U.S. investigators in other journals during the period 1983 through 1986. It is selective in two ways. First, it excludes research on the use of historical and paleoflood data, which is reviewed in an accompanying paper. Second, it highlights what I believe to be the most critical areas.
Upper ocean dynamics
Authors: Price James F. , Terray Eugene A. , Weller Robert A.
Published: February 2008

Research on upper ocean dynamics has contributed steady progress towards a more complete and useful understanding of the ocean's role in air/sea interaction and climate. High resolution models of the upper ocean thermal structure are now quite realistic, and in some applications are limited as much by uncertainties in the surface heat flux as by uncertainties in model physics. Low resolution upper ocean thermal models are routinely included in most climate models, and have been shown to give far better simulations than do simple slab or heat conduction models. Some of these climate models have provided important new insights into the mechanisms by which the ocean may respond to a future greenhouse‐imposed surface heating anomaly.

In contrast, the structure of the upper ocean wind‐driven velocity is not so well understood. Some models and high quality field data show significant sustained vertical shears within the temperature mixed‐layer, while other data sets show organized, energetic, three‐dimensional flow structures. Still other field data appear to reveal a velocity mixed‐layer very much like the thermal mixed‐layer. Thus the structure of the upper ocean velocity is an important and still open issue for dynamics and models, and also for the interpretation of the large and rapidly growing drifting buoy data base.

The role of surface waves in generating and maintaining this current structure also remains poorly understood. Waves certainly play a central role in the mechanics of momentum transfer across the air‐sea interface, both through surface roughness and more directly as a consequence of breaking. The combined system of waves and wind‐driven currents is also known to be unstable, and their interaction may be an important mechanism for producing certain types of organized convection in the mixed‐layer.
Magmatic processes
Authors: Marsh Bruce
Published: February 2008

The dream of having physics and chemistry contribute equally in unraveling magmatic processes has nearly come true. A rigorous, logical infrastructure of forceful, quantitative investigation has not yet fully emerged, but most all the bits and pieces are hovering about, fidgeting for their rightful places.

This quadreinnium is marked by the general quantitative investigation of well defined magmatic process as opposed to construction of detailed physicochemical sceneries to explain specific field problems, although some of this has also produced good results. The surest sign of significant progress is the steady implementation of newly investigated physical processes into petrologic thinking. Regardless of inclination, petrologists are actively seeking evidence for or against various processes, are learning the essentials of the mechanics, and, perhaps best of all, are (most often) accurately using the new vocabulary. We are in the midst of an extremely fertile burst of activity, and the next four years promise, in terms of quantitatively solving problems, to be even better.
Metallogenesis
Authors: Brimhall George H.
Published: February 2008

Topics of importance in metallogensis (the genesis of non‐fuel mineral deposits) include an expansive array of active disciplines in Earth sciences. Mineral deposits are recognized in rocks of sedimentary, igneous and metamorphic origin which date from the oldest recognized supracrustal environments of the early Archean up to the present. The breadth of relevant topics is further enlarged through continuing developments in paleoclimatology, paleo‐oceanography, and organic geochemistry. These, in turn, are largely hybrid fields combining a physical or biological science with the historical perspectives and complexities of geology. These integrative disciplines offer important new insights into the formation and long‐term preservation of mineral deposits in the crust. Conversely, mineral deposits, which have long been recognized as being reactive, multi‐element geochemical, and in certain cases bio‐chemical systems, present challenging problems in their own right as subjects for these multidisciplinary evolutionary studies. The unparalleled opportunities that exist in mines for access and comprehensive data acquisition are only recently being realized and used in solving problems of general scientific importance.
Mechanisms and observations for isotope fractionation of molecular species in planetary atmospheres
Authors: Kaye Jack A.
Published: February 2008

Chemical and physical processes which may give rise to isotope fractionation of molecular species in the atmospheres of both Earth and other planets are reviewed, along with observations of isotopically substituted molecules in planetary atmospheres. Mechanisms for production of isotope fractionation considered include escape and effect of isotope substitution on equilibrium constants (including those of phase changes), photolysis rates, and chemical reaction rates. The isotopes considered for compounds in the terrestrial atmosphere include D, T, 13C, 14C, 15N, 18O, and 34S. Compounds for which data about isotopic composition in the terrestrial atmosphere are summarized include CO, CO2, O3, N2O, NH3, SO2, H2S, H2O, H, H2, and CH4. Planetary atmospheres discussed include those of Venus, Mars, Jupiter, Saturn, Uranus, and Titan; isotopes reviewed are D, 13C, 15N, and 18O. Suggestions for additional research in the area of isotopically substituted molecules in atmospheres are offered.
Comments on “Determination of the intensity of the Earth's magnetic field during archaeological times: Reliability of the Thellier Technique”
Authors: Walton D.
Published: February 2008

After indicating the basis of the technique, its use with a cryogenic SQUID magnetometer is outlined, particular emphasis being given to the need for stringent reliability criteria. Comparison of results for some samples is made with results obtained using the Shaw alternating‐field technique, and the agreement is satisfactory. Determinations made on 14 samples fired during the last 150 years give results which on average are in close agreement (within 2%) with observatory‐based values, the standard deviation of a sample from the predicted value being ±6%.
Southern hemisphere thermospheric dynamics: A review
Authors: Smith R. W. , Rees D. , Stewart R. D.
Published: February 2008

The dynamics of the upper thermosphere in the southern polar cap, auroral, and mid‐latitude zones is reviewed. Information has been drawn from published measurements from the DE 2 and AE‐C spacecraft and from ground‐based stations and interpreted using the latest thermospheric general circulation models. Necessarily, frequent reference is made to the F region of the ionosphere, which permeates the upper thermosphere. Great emphasis is placed on the coupling and feedback mechanisms which exist between these interpenetrating fluids. It is shown that to obtain close approximation to observation, a complex model is required which incorporates these mechanisms in a self‐consistent way. Published results from general circulation models show that there is a broad measure of agreement at both high and middle latitudes between simulation and observation over a wide range of geomagnetic activity for the sunlit southern middle and high latitudes near solar maximum. However, there are comparatively few data for a full investigation to be made, and present conclusions must be regarded as strictly interim. Strong driving forces in the auroral zones and polar caps cause a wind regime which is distinctly different from that observed at middle latitudes. Geomagnetic activity has been shown to have a strong effect on the extent of the high‐latitude wind field, its strength, and its penetration to middle latitudes. The orientation of the interplanetary magnetic field has also been shown to be important. The coupling mechanisms between ions and neutrals heat the thermosphere and drive its circulation using energy from magnetospheric sources, but as yet the details of the thermosphere's variation with geomagnetic and solar activity are not clear from observations. The strong vertical winds reported in the literature are certainly indications of powerful heating, but the connection between observations of vertical winds and the production of gravity wave activity has not been established. Halley, Antarctica, is a critical station in the evaluation of model simulations of the winds in the upper thermosphere since it is a high‐latitude station geographically but middle latitude geomagnetically and is subject to an unusual blend of driving forces. The observations are modeled surprisingly well considering the incomplete accounting of asymmetry caused by the large offset of the geomagnetic and geographic poles, and also of the South Atlantic anomaly. It is probable that, globally, there are detailed disagreements between the actual circulation and the simulations which have not yet come to light for the lack of observations. In the absence of new spacecraft experiments, new ground‐based stations are required in the Antarctic region to enable simultaneous measurements to be made over a wide range of the southern polar cap.
The occurrence and origin of Late Paleozoic remagnetization in the sedimentary rocks of North America
Authors: McCabe Chad , Elmore R. Douglas
Published: February 2008

Although it has been known for 25 years that some Paleozoic sedimentary rock units in Europe and North America were remagnetized during Pennsylvanian or Permian time, it is only very recently that the complex and widespread nature of the late Paleozoic remagnetization phenomenon has been generally acknowledged. It is now recognized that many Paleozoic paleomagnetic poles for North America that were once considered reliable are in fact the result of remagnetization, and as a consequence the paleomagnetic data base for the Paleozoic is undergoing rapid and drastic revision. The causes of late Paleozoic remagnetization in North America are currently the focus of much interest and active research. The remagnetization can reside in either hematite or magnetite, and different remagnetization mechanisms have been important in different settings. Chemical remagnetization processes, some related to specific diagenetic events, are dominant in hematite‐bearing sandstones and carbonates. In magnetite‐bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important, but it is difficult to determine which process is the dominant one in some settings. Some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain‐building events that affected much of the continent during the late Paleozoic. Paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids.
Note on New Numerical Maps of ƒ0E8 for Solar Cycle Minimum and Maximum
Authors: Leftin M. , Ostrow S. M. , Preston C.
Published: February 2008

Limitations of numerical maps of lower decile, median, and upper decile values of ƒ0E8 derived for each month of both a solar cycle minimum and a solar cycle maximum year, and some applications to propagation problems are discussed briefly. The numerical map coefficients and contour charts for representative months and hours of the day in 1954 and 1958 are published.
Numerical Solutions for the Coupling Between Waveguides in Finite Arrays
Authors: Wu C. P.
Published: February 2008

Mutual coupling between the waveguides in finite arrays has been analyzed by using the integral equation method. Numerical solutions are obtained from which we find that the coupling in general decreases with the wavelength, as well as the distance s between the waveguides, and that for large separations the rate of decrease is inversely proportional to s½. However, when the waveguides are loaded with dielectric plugs, the characteristics of coupling may be substantially changed, owing to the excitation of modes that are propagating inside the dielectric but evanescent in the empty waveguide region. For example, the coupling may be greatly enhanced or suppressed depending on the choice of parameters such as the dielectric constant and plug thickness. In particular, it is found that under suitable conditions the coupling can exhibit resonance characteristics.
Wave Propagation in an Open, Periodic, Two‐Dimensional Iris Waveguide
Authors: Brown Gary S. , Mittra R.
Published: February 2008

The existence and character of modal fields on a two‐dimensional structure comprising an infinite periodic series of slotted metal planes is considered. The problem is two‐dimensional, since no variation in one transverse coordinate is assumed. Integral equations involving the unknown field in one iris and the propagation constant are formulated for both types of field polarization with the aid of appropriate Green's functions. The integral equations are reduced to infinite matrix equations by a novel procedure that involves the application of Galerkin's method in the Fourier transform domain. It is shown that radial Mathieu functions provide a rapidly convergent series representation for the iris field. For a given polarization, the first two modal propagation constants and iris field distributions are computed for even field symmetry. The real or attenuating part of the propagation constant is found to oscillate between a maximum and minimum value as a function of the distance beween adjacent planes when this distance is large compared with the iris width. The field distribution in the iris is also found to be very insensitive to plane spacing but becomes more peaked as the iris width is increased.
Effect of a Moving Dielectric Half‐Space on the Radiation From a Line Source
Authors: Hazama K. , Shiozawa T. , Kawano I.
Published: February 2008

This paper discusses the effect of a moving dielectric half‐space on the radiation from a line source. Specifically, let us assume that an electric line source is embedded in one of two dielectric half‐spaces separated by a plane of infinite extent, with either of the dielectric half‐spaces moving uniformly. Then, the problem of how the radiation pattern of the line source, as viewed from the side of a dielectric half‐space at rest, is affected by the motion of the other dielectric half‐space is investigated. After considering an electric line source located both in a stationary and in a moving dielectric half‐space, it is found that the radiation pattern is greatly modified by the motion of a dielectric medium, especially in the presence of the other dielectric medium at rest instead of in the presence of a vacuum.
Concurrent Equatorial and High‐Latitude Geomagnetic Pulsations
Authors: Campbell W. H.
Published: February 2008

Examples of various concurrent ULF field fluctuations observed at: Jicamarca (Peru), College (Alaska), and Byrd (Antarctica) are presented to demonstrate the similarity of geomagnetic micropulsations of both Pi and Pc types at equatorial latitudes. The beginning of one Pi 1 event showed as much as 10‐sec difference in onset times at a world distribution of stations. The apparent arrival of the signal varied according to the geomagnetic latitudes of the observatories. Magnetospheric sources of geomagnetic pulsations may be observed with equatorial station data.
Aperture Field Bounds for an Azimuthally Symmetric Slotted Cylinder
Authors: Mayhan Joseph T.
Published: February 2008

The problem of specifying the minimum magnitude of the maximum electric field over an azimuthally symmetric slotted cylinder subject to the constraint of a specified radiated power into a given wedge angle is treated. Aperture bounds are presented for both the TE, TM and the general TE/TM excitation of the slot. The degree to which these bounds are attainable is discussed, and their relationship to uniform illumination of the aperture is examined. The problem of synthesizing a specified field pattern over the given angle is also treated.
Spectra of Atmospheric Variables in the Boundary Layer
Authors: Panofsky Hans A.
Published: February 2008

This paper summarizes current knowledge about various types of spectra in the lowest few hundred meters of the atmosphere, with emphasis on measurements and interpretations since 1965.
Measurements on the Growth of Small Disturbances in a Stratified Shear Layer
Authors: Scotti R. S. , Corcos G. M.
Published: February 2008

A statically stable, stratified free shear layer was formed within the test section of a wind tunnel by merging two uniform streams of air of different temperature at the entrance to the contraction section. The resulting diffusion‐thickened interface was characterized by nearly self‐similar temperature (odd), velocity (odd), and Richardson number J (even) profiles that were adjustable over the ranges 0.07 ≤ J ≤ 0.76 and 50 ≤ Re ≤ 150, where Re is the Reynolds number. Small periodic disturbances were imposed on the interface by oscillating a taut, 0.004‐inch‐diameter steel wire within the interface in the contraction section, and subsequent development was deduced from measurements of the fluctuating temperature downstream. Results indicate complete infinitesimal stability for J > 1/4 and instability over a range of non‐dimensional wave numbers of the order of unity for smaller values of J. Growth rates are presented that agree reasonably well with theoretical predictions based on the assumption of inviscid parallel flow.
Experiments on the Stability of Stratified Shear Flows
Authors: Thorpe S. A.
Published: February 2008

Some experiments are described in which a shear flow is developed in a stratified brine solution contained in a long, closed tube of rectangular cross section. The fluid motion is produced by tilting the tube from an initial horizontal position, and the flow thus generated is found to be uniformly accelerating and easily expressed in terms of the initial density profile in the tube until disrupted by instability. (The details of the method are given in an earlier paper.) The particular case of a diffuse interface between two homogeneous layers of different densities that move with equal and opposite speeds is discussed in some detail, and the transition to turbulence is described. The transition is characterized by the formation of waves that develop into rolls of a very beautiful form. Measurements at the onset of the wave motion are compared with theoretical predictions.
Photoelectron Escape Fluxes Obtained by a Monte Carlo Technique
Authors: Cicerone R. J. , Bowhill S. A.
Published: February 2008

Photoelectrons in the ionosphere result from the action of solar ultraviolet radiation on atmospheric gases. It is extremely difficult to study the energy, pitch‐angle and spatial distributions of photoelectrons with a continuity equation approach because of the large variety and nature of the physical processes that must be considered. A Monte Carlo technique is well suited to simulate photoelectron diffusion through the atmosphere; one such method to deduce photoelectron distributions is described. Elastic and inelastic collisions with the neutral gases and Coulomb losses to thermal electrons are included. Illustrative results are presented for the photoelectron escape fluxes that result from the release of 20‐ev electrons from various ionospheric altitudes.
Frequency Dependence of Radar Ducting
Authors: Pidgeon V. W.
Published: February 2008

A series of tests were performed to investigate the effect of radar ducting on mircowave propagation at four wavelengths: 1, 3, 5, and 12 cm. A boat equipped with the four CW transmitters operated between a few miles and fifty nm off shore. The receiving station was at the shore thirty feet above sea level. Data recorded at the shore site gave signal strength as a function of boat range. Ducting conditions existed part of the time, giving ranges of four to five times the normal, 4/3 earth radius horizon. During these conditions, signal strengths as great as 10 db or more above free space were observed out to maximum range. All frequencies gave extended horizons during these conditions. During conditions of near‐normal horizon ranges, in the diffraction region the longer wavelength gave larger signals at greater ranges. Signal strength fall‐off rate in this region approximated 2 db/nm for each frequency.
Effects of Elevation and Ground Conductivity on Horizontal Dipole Excitation of the Earth‐Ionosphere Waveguide
Authors: Pappert Richard A.
Published: February 2008

Horizontal electric dipole excitation of vertical E at the ground for 19.8‐kHz radiation beneath a hypothetical daytime ionosphere and beneath a highly anisotropic ionosphere has been numerically investigated as a function of ground conductivity and source elevation. East to west propagation is considered with a magnetic dip angle of 50°, and full allowance is made for earth curvature and ionospheric anisotropy. In horizontal dipole excitation between a highly anisotropic ionosphere and a good conducting ground, broadside launching, except for sources very close to the ground, is to be preferred to end‐on launching. Since TE waves are launched broadside, this is a direct consequence of mode coupling in the ionosphere. For source elevations comparable to or greater than a wavelength, the horizontal and vertical electric dipole excitations can become comparable. In excitation by a horizontal electric dipole beneath a daytime ionosphere, end‐on launching is to be preferred to broadside launching for a ground‐based horizontal dipole exciter. However, for an elevated horizontal dipole source, the preferred direction of launch depends upon the ground conductivity as well as transmitter‐receiver distance. Thus, even with weak ionospheric anisotropy, mode coupling can be significant. Up to elevations of 40 km (the highest considered in this paper), the elevated vertical dipole is a more efficient source of vertical E at the ground than is the elevated horizontal dipole.
An Alternative Integral Equation for Propagation Over Irregular Terrain
Authors: Ott R. H. , Berry L. A.
Published: February 2008

An elementary function that is closely related to the Sommerfeld flat‐earth attenuation function is used to derive an alternative integral equation for propagation over irregular terrain. This choice of elementary function is shown to satisfy a scalar ‘parabolic’ wave equation. For the special case of a spherical earth, the integral equation yields more accurate results than previous general formulations; in addition, the integral equation is numerically feasible for both vertical and horizontal polarization.
High Power VLF Transmitting Antennas Using Fast Wave Horizontal Dipole Arrays
Authors: Seeley E. W.
Published: February 2008

Horizontal VLF transmitting dipole arrays are much easier and less expensive to construct than are the vertical VLF transmitting antennas in use at the present time. The horizontal dipoles have much greater power‐radiating capability and bandwidth. The theoretical and experimental effects of mutual resistance on array efficiency gain over one dipole are presented. The radiation efficiency is greatly increased by increasing the wave velocity along the resonant dipole. Radiation characteristics of a theoretical 18‐dipole array on Hawaiian lava are shown. The antenna radiation pattern needed to get omnidirectionl coverage at VLF is presented. It is shown that directivity is needed because of the nonreciprocal east‐west propagation attenuation.
The Equatorial F‐layer Irregularity Extent as Observed From Huancayo, Peru
Authors: Bandyopadhyay P. , Aarons Jules
Published: February 2008

Simultaneous observations of propagation paths to two synchronous satellites transmitting at 137 MHz have revealed distinctive characteristics of the equatorial irregularity structure. Data were taken between July 1967 and February 1969. The abrupt onset of scintillations on a particular path within minutes led to the concept that a sharp boundary of about 10 km exists between quiet non‐scintillating regions and regions of irregularities. Once the scintillation has started, a region over 5000 km wide from east to west may contain irregularities. Within such a region there is strong localized control of irregularities. Comparison of scintillation onset on two paths spaced 1 to 2 hours apart at their subionospheric points reveals that, in the mean, scintillation onset is a function of local time. However, many days show nearly simultaneous onset on two paths that are 1 hour apart in local time. Other days show onset of scintillations on the westerly satellite path before onset at the eastern intersection; this variability is clearly a negation of the diurnal pattern. Maximum occurrence of scintillation has been observed in September, October, and November. A somewhat lower occurrence of high‐amplitude scintillation is noted in the December–January solstice period. A secondary maximum exists in March, according to other South American data. A very low occurrence of scintillation is observed in May, June, and July. The scintillation phenomenon is correlated in the mean with the downward motion of the F layer after 2100 local time at Huancayo. A hypothesis is advanced that after 2100 the westerly F‐layer maximum is higher in altitude and has a greater electron density than the easterly maximum. This west‐east gradient of electron density produces (either through electrodynamic forces or diffusion) the irregularities that show an apparent eastward drift. The diurnal pattern, with its nighttime maximum of scintillation occurrence, is a westward motion, but the irregularity drifts noted are clearly eastward in the direction of lower electron density profiles.
Studies on F‐region Irregularities at Low Latitude From Scintillations of Satellite Signals
Authors: Bhar J. N. , Gupta A. Das , Basu S.
Published: February 2008

The characteristics of F‐region irregularities at low latitudes have been studied from scintillations of signals from beacon satellites having different orbital inclinations. It has been found that the irregularities are embedded in distinct N‐S‐oriented clouds. A comparison of the experimental and theoretical results of scintillation has been made to determine the parameters of the irregularities.
Measurement of Irregularity Heights by the Spaced Receiver Technique
Authors: Paul L. M. , Yeh K. C. , Flaherty B. J.
Published: February 2008

The scintillation of radio signals from an artificial earth satellite has been analyzed by spaced receiver techniques to obtain the heights of the electron density irregularities responsible for scintillations observed at the ground. We report such data taken near the maximum of the present solar cycle. These results are also compared with results of data taken near the minimum phase of the solar cycle. Eighty‐four passes of BE‐B extending from January 1968 to July 1968 were reduced for height values. The heights were computed by measuring the time delays in the ground pattern by graphical cross‐correlation techniques. These delays were then substituted into a height equation for a spherical earth and ionosphere. The spherical formula was developed so that height values for sub‐ionospheric latitude from 36° to 50°N could be measured. The diurnal behavior and the latitude dependence of irregularity height are studied and compared with results obtained during the solar minimum.
Propagation of Radio Waves Over a Nonuniform Layered Medium
Authors: Bahar E.
Published: February 2008

Propagation of radio waves over the surface of a stratified medium with an overburden of arbitrarily varying depth is analyzed in terms of a complete local set of coupled wave amplitudes. The complete set consists of the surface wave and the radiation terms. To reduce the coupling between the component waves, the local modal expansion that is chosen reflects the varying characteristics of the surface wave structure. Comparisons with earlier solutions to somewhat idealized, related problems are given, together with a discussion of recent experimental results.
A Novel Directivity Expression for Planar Antenna Arrays
Authors: Forman B. J.
Published: February 2008

An equation that is useful for rapid machine computation of the directivity characteristics of a planar array with discrete identical elements and uniform phase progression is derived. Computer time and cost are reduced by an order of magnitude or more when this equation is used rather than the computational techniques that involve integration of the array power pattern. Parameters of the new equation are the number of elements and the interelement spacings along two principal axes, the beam‐pointing angles, the array excitation in amplitude, and an element voltage pattern of the form (cos θ)ν+1/2, where ν > −1. An example of the usefulness of the equation is given by means of its application to a uniformly excited rectangular array. Curves of directivity versus interelement spacing and scan angle for square arrays with a (cos θ) element power pattern are presented.
VLF Fields of Elevated Sources
Authors: Galejs Janis
Published: February 2008

An anisotropic ionosphere causes coupling between TE and TM modes; for any given mode the field components of a horizontal electric dipole remain finite for both the axial and endfire directions. During the daytime the ionospheric anisotropy is relatively weak, and the vertical electric fields Er of an elevated horizontal electric dipole exhibit a minimum near the broadside direction for west‐to‐east (WE) and (at short distances) for east‐to‐west (EW) propagation; during nighttime the fields exhibit maximum values near the broadside direction. For elevated sources and observation points, the transverse electric horizontal field component Eф of a horizontal electric dipole becomes comparable to Er of a vertical electric dipole. Inclined electric dipoles exhibit amplitude variations of the vertical electric fields Er with changes of direction relative to the dipole axis. The amplitude variations are particularly pronounced for the EW direction of nighttime propagation.
Backscattering From Two Identical Cylinders Illuminated by an Obliquely Incident Plane‐Wave
Authors: Lin Juang‐Lu
Published: February 2008

A study is presented of the backscattering from two identical, perfectly conducting, thin cylinders illuminated by a plane wave at an arbitrary angle of incidence. The theory is based on an integral equation method. By decomposing the induced currents into symmetric and antisymmetric modes, the simultaneous integral equations for the induced currents are converted into independent integral equations similar to the one for a single cylinder for which the solution has already been carried out. The induced current on the cylinders and the backscattering cross sections of the cylinders are determined. Comparisons are made between the calculated and the measured values of the echo area. The experimental results are in good agreement with theory.
Coherence Properties of a 9‐Gigahertz Propagation Path Near the Ground
Authors: d' Auria Giovanni , Solimini Domenico
Published: February 2008

Spatial statistical properties of a 39‐km X‐band propagation path developing near the ground have been investigated by means of the degree of coherence, which has been measured systematically over 3 months in the fall of 1968. A direct method of measurement has been worked out and has proved to be convenient in many respects. The method is based on the repeated plotting of the interference patterns obtained by a phase scanned interferometer. The measurements of the degree of coherence refer to three pairs of antennas that were permanently on site and were cyclically switched on the circuit of the interferometer. Results on the behavior of the degree of coherence with the time of day and with the progression of the autumnal season are reported. Distribution curves for the degree of coherence of the three pairs of antennas are also given. A practical use of the study is the appreciation of the gain deterioration of large antenna systems operating near the ground. This method may also be a novel approach to the electromagnetic probing of the atmosphere.
Excitation of the Terrestrial Wave Guide by Sources in the Lower Ionosphere
Authors: Galejs Janis
Published: February 2008

The effectiveness of a dipole source within the ionosphere is determined relative to a ground based source of the same dipole moment. The earth‐to‐ionospheric cavity is excited best if dip angles of the geomagnetic field are in excess of 15° to 20°. In the optimum orientation the axes of electric and magnetic dipoles are transverse to the geomagnetic field; axial electric dipoles are particularly ineffective. For frequencies in the VLF range the relative effectiveness of an ionospheric source is about 10% at nighttime and 1% at daytime. In the ELF range the relative effectiveness is from 20 to 100%.
Reflection of Electromagnetic Pulses From a Perturbed Linear Ionosphere Model
Authors: Hill D. A. , Wait J. R.
Published: February 2008

The reflection of a horizontally polarized, impulsive plane wave by a weakly perturbed ionosphere model is considered. A linear electron density profile and a constant collision frequency are assumed for the unperturbed model, and magnetic field effects are neglected. The transient response is formulated as an inverse Fourier transform of the first‐order frequency domain solution. For the special case of a thin‐layer irregularity, a numerical inverse transform is used to obtain responses that satisfy causality. It is shown that the presence of the thin‐layer irregularity is to modify the waveform of the reflected transient.
Aperture Effects on the Spectrum of Amplitude Scintillation
Authors: Lee R. W.
Published: February 2008

Spectra of amplitude and amplitude‐difference fluctuations have been obtained over a millimeter‐wave propagation path, demonstrating substantial aperture‐filtering effects.
Directive Gain of Circular Taylor Patterns
Authors: Rudduck R. C. , Wu D. C. F. , Hyneman R. F.
Published: February 2008

The directive gain of circular Taylor patterns is determined. It is shown that so‐called optimum patterns, i.e., uniform sidelobes in all planes, are severely limited for planar apertures because of excessive sidelobe power. The optimum directive gain of the circular Taylor pattern with a given sidelobe level can be obtained by appropriate design.
Some Recent Work on Sporadic‐E Theory
Authors: Whitehead J. D.
Published: February 2008

Recent developments in wind‐shear theory and other midlatitude ES theory, with a discussion of relevant experimental work, is given. The role of metallic ions and especially their production and loss is not understood. The gradient instability is specifically excluded from this discussion though it is likely to be important.
Lunar Influences on Sporadic E
Authors: Tarpley J. D. , Matsushita S.
Published: February 2008

Studies of sporadic‐E data have shown that both the altitude and density of ES layers vary in lunar time. Possible sources of the lunar effect are motions of the ES layer produced by the electrostatic field associated with the lunar ionospheric current system, and wind shears in the lunar atmospheric tide. From studies of lunar atmospheric tides and the lunar ionospheric dynamo, values of the electrostatic field and wind shear may be estimated. Simple models are used to determine the effects of these electric‐field and wind distributions on ES layers, and the results are compared with the observed lunar variations in ES.
Cross‐Field Instability for the Formation of Sporadic E
Authors: Kato Susumu
Published: February 2008

The cross‐field instability is an important process in the explanation of small‐scale sporadic E. Differences in the theories presented by different authors are apparent. It is true that there is no one‐to‐one correspondence of each Fourier component of perturbation between the electric field and the electron density about the direction of the ambient density gradient. The question arises as to how to choose properly the perturbation in the ambient charge gradient; but the difficulty does not seem serious. Computer experiments using realistic perturbations are desirable for the final answer on this question. The short‐circuiting effect along the geomagnetic field does not damp the electrical polarization with small transverse size, i.e., small east‐west wavelength. The upper limit of this size is a few hundred meters.
Nonlinear Sheath Admittance, Currents, and Charges Associated With High Peak Voltage Drive on a VLF/ELF Dipole Antenna Moving in the Ionosphere
Authors: Shkarofsky I. P.
Published: February 2008

We consider a bare antenna consisting of two line elements driven by a VLF source and immersed in the ionosphere. Nonlinear effects associated with high voltages up to 1000 volts and with an additional induced v×B·1 voltage are included. The applied voltage is visualized as a dc effect. For various instantaneous voltage values, we obtain the distribution along the antenna of voltage and resis'ive current, sheath size, and charge per unit length. At the feed points, waveforms for the charge and both resistive and displacement currents are illustrated, assuming an applied cosine voltage form. Averaging over a cycle yields, as a function of peak bias voltage, the average power dissipated in collection of particles from the medium, the average work done in moving charge between the two half‐antennas, and the average sheath conductance and capacitance. Numerical results are given for three altitudes and two antenna lengths.
Focusing of Intense Electromagnetic Waves in Ducts With Saturating Nonlinearity
Authors: Virmani C. J. K. , Dubey P. K.
Published: February 2008

A theory of self‐focusing of intense electromagnetic waves in ducts with parabolic and saturating nonlinearity has been developed. The relative effects of inhomogeneity in the refractive index of the medium, parabolic nonlinearity due to small electrical polarization of the medium, and saturating nonlinearity due to considerable electrical polarization on self‐focusing have been studied.
In‐situ measurements on the conductivity and surface impedance of sea ice at VLF
Authors: McNeill Duncan , Hoekstra Pieter
Published: February 2008

An experimental program to measure in‐situ values of the electrical resistivity and surface impedance of sea ice at VLF was carried out at Point Barrow, Alaska. Temperature, salinity, and resistivity were measured as a function of depth in the ice for both first‐year and multiyear sea ice by means of cored samples. All three quantities varied with the age of the ice, and, in addition, the resistivity varied with age from 102 ohm m to 104 ohm m at the surface and in general down to a few ohm m at the sea‐water interface.

The wavetilt of a VLF plane wave propagating over sea ice is theoretically linearly dependent on the thickness. Measurements of the quadrature phase wavetilt at 18.6 kHz give values that are of the right order of magnitude but erratic in local behavior. Short‐spacing Wenner array resistivity measurements and telluric current measurements at VLF demonstrated that the erratic behavior was due to significant horizontal variations of the sea‐ice resistivity over distances of a few m.
Radiation of a buried dipole in the presence of a semi‐infinite metallic tube
Authors: Chang David C.
Published: February 2008

The problem of a buried axial Hertzian dipole placed at the end of a semi‐infinitely long metallic tube is associated with the investigation of electromagnetic through‐the‐earth propagation. Under a thin‐tube approximation explicit analytic expression of the far‐field is obtained by employing the Wiener‐Hopf technique. Numerical results are presented in this paper for various dipole locations and ground conductivities. The improvement over the result based upon a Kirchhoff's approximation is noted.
Effects of anisotropy on quasi‐static fields of a vertical electric dipole buried in a layered anisotropic earth
Authors: Negi Janardan G. , Saraf Padmakar D.
Published: February 2008

The radiation from a vertical electric dipole located in the upper anisotropic layer of a stratified conductive earth is analysed in detail. The variations of the near fields in the anisotropic medium and outside zone have an appreciable dependence on the anisotropy factor in the neighbourhood of the dipole. However, the amplitude gradients (with respect to the numerical distance) remain almost uniform for varying coefficients of anisotropy and for the conductivity contrast between the isotropic lower half‐space and the longitudinal conductivity of the upper anisotropic conducting layer. The results have applications in underground communications and submarine detection.
An empirical model for average F‐layer scintillation at VHF/UHF
Authors: Fremouw E. J. , Rino C. L.
Published: February 2008

An empirical approach to modeling the electron‐density irregularities in the F layer that are primarily responsible for amplitude scintillation of VHF/UHF signals has been devised and tested. An irregularity model was postulated as a function of geomagnetic latitude, local time of day, season, and sunspot number. The primary parameters of the irregularities that were postulated were their strength and transverse scale‐size. The irregularities were assumed to be aligned along the geomagnetic field, and their axial ratio was taken as constant, as were the height and thickness of the irregular layer.

The model was tested by computing the fractional rms fluctuation in received power to be expected in a given situation, under the weak‐scatter assumption, and comparing the results against values of this or related quantities reported in the literature. The model then was improved by iteration. The development made use of 12 data sets, and final testing employed those 12 plus an independent one. Lack of appropriate data precluded testing poleward of about 70°geomagnetic latitude.

The model is offered as a tool for VHF/UHF communication‐systems planning, to the extent that the average value of scintillation in a specified circumstance is of engineering value. Geophysical application should be limited to such uses as experiment planning, guiding of intuition, and serving as a basis for more refined modeling.
Determination of atmospheric water‐vapor densities from measurements of the 6943.8‐Å absorption line strength
Authors: Johnson M. M. , LaGrone A. H.
Published: February 2008

The effects of pressure and temperature variations on the determination of water‐vapor densities from ruby‐laser radar returns are considered. At the center of the 6943.8‐Å water‐vapor line, the thermal dependence of the absorption cross section is almost negligible while the pressure dependence is not. Water‐vapor densities should be deduced from the line strength, which is independent of pressure, rather than the absorption coefficient as has been done previously. The 6943.8‐Å line strength and line width were measured over a fixed‐length atmospheric path using a ruby laser as 0.17 cm−1 g−1 cm2 and 0.098 cm−1 bar−1 (295 K), respectively, with standard error estimates of 16 and 9%.
Modeling the D‐region partial‐reflection experiment
Authors: Cohen D. J. , Ferraro A. J.
Published: February 2008

Recently published results on D‐region partial‐reflection amplitude distributions indicate that below 80 km, the distributions are Rayleigh‐like in character. Because of this, this paper describes a model of the partial‐reflection mechanism which employs multiple reflectors as opposed to the single‐reflector concept now in use by most experimenters using the partial‐reflection experiment. The particular new feature incorporated by this model is the variation of the reflection coefficient throughout the reflecting volume; previous multiple‐reflector models considered only the variation of the differential absorption in the reflecting volume. It is shown, for example, that for 50‐μsec pulse lengths, serious errors in electron density can result in the analysis of partial‐reflection data for which multiple reflectors are present.