bluuebunny/crossref_metadata_2025 huggingface dataset:
Plasma wave observations with the Dynamics Explorer 1 spacecraft
Authors: Gurnett D. A. , Inan U. S.
Published: February 2008

This paper reviews the results from the plasma wave instrument on the Dynamics Explorer 1 (DE 1) spacecraft. The DE 1 spacecraft was launched on August 3, 1981, into an elliptical polar orbit with initial perigee and apogee radial distances of 1.09 and 4.65 RE. In the roughly 6 years since the launch of the spacecraft, DE 1 has provided basic new information on a wide variety of magnetospheric plasma wave phenomena. These include auroral kilometric radiation, auroral hiss, Z mode radiation, narrow‐band electromagnetic emissions associated with equatorial upper hybrid waves, whistler mode emissions, wave‐particle interactions stimulated by ground VLF transmitters, equatorial ion cyclotron emissions, ion Bernstein mode emissions, and electric field turbulence along the auroral field lines. We first give a brief review of the basic plasma wave modes that can exist in the equatorial and polar regions of the magnetosphere. After the basic terminology is established, each of the above areas of plasma wave research is discussed in detail, first by reviewing the state of knowledge at the time of the DE 1 launch and then by describing the contribution made by DE 1 in the 6 years since the spacecraft was launched.
Atmospheric trace gas studies in Antarctica
Authors: Cronn D. R. , Schilling K. J.
Published: February 2008

Atmospheric trace gas studies in Antarctica have led to an increased understanding of the global effects from anthropogenic emissions of various compounds. These studies have also contributed to the identification of various atmospheric circulation patterns and mixing mechanisms. Time trend analyses have revealed that although the atmospheric concentrations of F‐11, F‐12, CH3CCl3, and other halocarbons are still increasing, their rate of increase has slowed from peak increasing rates observed in the 1970s. For CO2, CO, and CH4, time trend analyses have also been used to analyze seasonal concentration cycles and to establish relationships between sources and sinks at various times throughout the year. Latitudinal gradient studies have generally revealed that for most compounds with significant anthropogenic sources, atmospheric concentrations are higher in the northern hemisphere than in the southern hemisphere. This is mainly a result of the overwhelming majority of anthropogenic sources existing in the northern hemisphere. Vertical concentration profiles in Antarctica are dependent upon the seasonal variations in circulation patterns associated with final warming and breakup of the polar vortex. Since final warming and breakup of the polar vortex may not always occur at the same time each year, profiles for a given time period from separate years may be different. Stratospheric‐tropospheric air exchange over Antarctica occurs primarily as a result of (1) general circulation patterns with subsidence over the polar regions, (2) stratospheric air injection in the vicinity of jet streams and injection from tropopause folding upstream of troughs, and (3) mountain waves of sufficient magnitude to displace air parcels across the tropopause.
Rocket investigations of electron precipitation and VLF waves in the Antarctic upper atmosphere
Authors: Sheldon W. R. , Benbrook J. R. , Bering E. A.
Published: February 2008

The two Antarctic rocket campaigns conducted by American scientists during the past 10 years are described, and the results of these efforts are reviewed. Although the primary objective of both campaigns, the observation of precipitating electrons triggered by the Siple VLF transmitter, was not achieved, significant advancements in understanding upper atmosphere phenomena were made. Standing wave patterns in the ionosphere were observed for the first time by wave experiments flown aboard Nike‐Tomahawk rockets. The same detectors were able to monitor a continuous signal from the transmitter through the neutral atmosphere and into the ionosphere. This in situ observation of a radio wave traversing a neutral and a plasma environment provided unique data for comparison to theoretical studies of wave propagation. SuperArcas measurements provided two distinct types of energetic electron precipitation data: one type was continuous on the time scale of > 1 min, and the other was in X ray microbursts created by bursts of energetic electrons stopping in the upper atmosphere with durations < 1 s. The burst data were compared to theoretical models of wave‐particle interactions. The observations of continuous electron precipitation with sufficient energy to penetrate the atmosphere to an altitude of 60–80 km were found to be consistent with a model of electron transport from the trapped population to the atmosphere in which gradient and curvature drift in the drift cone plays a principal role. The model requires slow electron pitch angle diffusion of the geomagnetically trapped population and predicts that almost all energetic electron precipitation occurs in the South Atlantic Anomaly. The electron precipitation model is shown to organize all of the SuperArcas data, which include not only measurements at Siple Station, Antarctica, but also those at Kerguelen, another L = 4 location in the southern hemisphere, and at northern locations at higher latitudes. A prediction of the model which involves the effect of the dawn‐to‐dusk convection electric field on energetic electron precipitation is described: diurnal modulation is predicted with the maximum electron precipitation intensity when the South Atlantic Anomaly is at local dawn. A test of this prediction made by SuperArcas flights in 1981 provided support for the slow diffusion model. The SuperArcas results on energetic electron precipitation are shown to disagree with a satellite result at L = 4 which requires fast diffusion. Reasons for the discrepancy between the two data sets are discussed, and steps which could be taken to resolve the apparent controversy are suggested. Finally, the suggestion is made that the two rocket campaigns conducted by American scientists in Antarctica during the past decade be regarded as a feasibility study leading to an active rocket research program in Antarctica for the remainder of the century.
The Gaia hypothesis: Can it be tested?
Authors: Kirchner James W.
Published: February 2008

The Gaia hypothesis' central theme is that biological processes homeostatically maintain, on a planetary scale, geochemical and climatic conditions favorable for life. A number of distinct hypotheses have been proposed, spanning a range from the self‐evident to the highly speculative. The self‐evident forms of Gaia reiterate the well‐documented fact that biological processes are critical to biogeochemical cycles, adding the straightforward (though important) point that the coupling between biotic and physical processes should create feedback loops. The speculative forms of Gaia assert that biological processes regulate the physical environment, keeping Earth's climate and surface geochemistry stable and favorable for life. As metaphors, these versions of Gaia are intriguing, untestable, and, if taken literally as a basis for research, potentially misleading. As hypotheses, they are ill‐defined, unparsimonious, and unfalsifiable.
Identifying flow paths in models of surface water acidification
Authors: Beck M. B. , Kleissen F. M. , Wheater H. S.
Published: February 2008

A clear identification of the movement of water through soils is of central importance to the characterization of surface water acidification. One currently popular view is that the chemical composition of stream water is a function of the mixing of varying volumes of water deriving from different parts of the catchment, all with their own individual chemical “signatures.” The development of models describing this process, and their use for predictive purposes, is a controversial issue. Some would shun the use of models altogether; others would insist on their development based rigorously on the laws of physics; and yet others would accept the aggregation and approximation of simpler conceptual models. As such, this controversy is common to many of the disciplines of geophysics. The paper explores in detail the problem of how a uniquely best set of values for the coefficients of simple conceptual models can be estimated from field observations. In particular, we investigate the role of tracer observations in reducing the uncertainty (and hence ambiguity) associated with the model's constituent hypotheses about hydrological mechanisms. Results from one of the field sites of the (U.K.) Royal Society’s Surface Water Acidification Programme show that even simple conceptual models can lead to quite different interpretations of the dominant flow paths in a catchment. The skill of predicting future behavior will lie therefore in locating this almost inevitable ambiguity and in seeking to make predictive statements that are minimally sensitive to it.
Cosmogenic nuclides in extraterrestrial materials
Authors: Vogt S. , Herzog G. F. , Reedy R. C.
Published: February 2008

The nuclides made in extraterrestrial materials by cosmic rays help reveal the histories both of the irradiated objects and of the cosmic rays. Improvements in measurement techniques, especially in accelerator mass spectrometry, have greatly reduced detection limits. Thanks to several extensive series of measurements in meteorites, a comprehensive picture has taken shape of how cosmogenic nuclide production depends on size, shape, and composition. Complementary to this work, (1) the irradiation in space of meteoroids has been simulated by means of accelerator experiments both with very thick and with spherical targets, and (2) various models for calculating production rates of cosmogenic nuclides have been developed or refined. New classes of material—meteorites recovered in Antarctica and tiny meteorites from the stratosphere and deep‐sea sediments, for example—have become more widely available for analysis. From their cosmogenic nuclide contents we have learned about the exposure histories of lunar meteorites, of SNC meteorites, which may come from Mars, and of interplanetary dust particles. Cosmogenic nuclides have been put to use in unfolding increasingly complex exposure histories. Some grains in gas‐rich meteorites were irradiated in at least two episodes, and others perhaps even before the solar system formed. Continuing measurements of lunar samples reveal the past behavior of energetic solar particles. We now know from cosmogenic nuclide measurements that many meteorites found in Antarctica have been there for ∼0.1–1 million years and that the age distributions vary from site to site.
Radiation Resistance of a Short Dipole Immersed in a Cold Magnetoionic Medium
Authors: Wang T. N. C. , Bell T. F.
Published: February 2008

Electromagnetic radiation from sources immersed in a cold magnetoplasma is analyzed by the use of ‘principal‐polarized’ wave coordinates; and the wave fields, the mean complex radiated power, and the Poynting vector are systematically expressed in terms of ‘polarized’ wave modes. For each polarized wave mode, the medium is effectively isotropic, since the Fourier‐transformed wave equation for each principal mode is decoupled and is similar to the one obtained from the usual scalar Helmholtz operator. In application of the theory, consideration is made of the radiation resistance of a linear electric antenna of moderate length oriented both parallel and perpendicular to the static magnetic field. Approximate closed form expressions for the radiation resistance are obtained for the VLF frequency range by using a plasma model appropriate to the magnetosphere. These approximate closedform expressions are compared with the results obtained previously by other workers using a quasi‐static approximation, and it is shown that excellent agreement exists between the two methods of analysis. It is concluded that the quasi‐static approximation can accurately predict the radiation resistance of a linear antenna in the magnetosphere, given certain moderate restrictions on the antenna length.
A Subtraction Procedure in Antenna Theory
Authors: Latham R. W. , Lee K. S. H.
Published: February 2008

The feedpoint singularity in the current on an axially symmetric antenna with a slice generator is removed by a well‐defined subtraction procedure involving a new integral equation for the antenna current. In contrast to the usual linear antenna theory in which an open‐ended tube is chosen as the basic model for the antenna, this paper considers a model in which the antenna is either a sliced closed hollow shell or a sliced solid conductor. The nonsingular portion of the current is shown to have several desirable features. One of the major desirable features is that it yields an input admittance of an infinite cylindrical antenna comparable to that obtained by Duncan's extrapolation procedure. Another is that the reciprocity relation between the transmitting and receiving properties of an antenna can be formulated rigorously in terms of the finite parts of the currents.
Ionospheric Absorption of Radio Waves During Sunrise and Sunset Hours
Authors: Rai R. K.
Published: February 2008

The variation of nondeviative absorption of radio waves in the ionosphere with solar zenith angle has been studied at different spot frequencies. The experimental results suggest that the theoretical expression for the nondeviative absorption in the morning and evening hours should contain a constant absorption term along with the term dependent on the solar zenith angle. In the present communication it has been shown that the constant absorption term is present throughout the day, but its effect is not apparent toward noon hours, and at these hours the experimentally observed values of absorption at a fixed frequency appear to be a function of solar zenith angle only. The probable origin of the constant absorption term has been discussed.
Radio Wave Propagation for Irregular Boundaries
Authors: Dougherty H. T.
Published: February 2008

A new, vectorial, approximate solution is presented for a particular class of problems: that of a vector wave propagated through a linear, isotropic, piecewise‐homogeneous medium with irregular interfaces and boundaries. This class of problems lacks not only an exact method of solution but also a reliable method of approximation for tropospheric radio propagation at VHF and higher frequencies. A method is proposed that incorporates a recent expansion of the Helmholtz integral to evaluate the Stratton‐Chu integral by the methods of stationary phase. To demonstrate its potential, the method is applied to two of the simplest thin‐screen problems.
Wave‐Hop Series for an Anisotropic Ionosphere
Authors: Berry Leslie A. , Gonzalez Guillermo , Lloyd John L.
Published: February 2008

Johler has pointed out an error in a previous derivation of the wave‐hop series for an anisotropic ionosphere. The error, which results in inaccurate field strengths for some ionospheric models (especially at night), is corrected in this paper.
Call for Papers Symposium on Very Long Baseline Interferometry Sponsored by URSI and the NRAO Charlottesville, Virginia, April 13‐15, 1970
Authors: Findlay Dr. J. W.
Published: February 2008

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Call for Papers 1970 USNC/URSI‐IEEE Spring Meeting
Authors: Johnson Francis S.
Published: February 2008

Send original abstract and two copies, prepared in accordance with the instructions below, to: Dr. Francis S. Johnson, USNC/URSI, University of Texas at Dallas, P.O. Box 30365, Dallas, Texas 75230
Diffraction by Irregular Apertures
Authors: Dougherty H. T.
Published: February 2008

This is a companion paper to two other papers, one of which developed an approximate, closed‐form solution of the Stratton‐Chu vectorial integral equation for irregular boundaries. That development involved the evaluation of several integrals by the stationary phase method and assumed that the points of stationary phase were well separated. A second paper presented the correction factor for definite integrals that are evaluated by the stationary phase method. Here, this correction factor is incorporated as a weighting function to permit a superposition of the closed‐form solutions for irregular screens or apertures without limitation on the spacing between adjacent points of stationary phase. This extended solution is then applied to evaluate the diffraction effects for a bilinear screen and compared with experimental data.
Integral Equation Approach to the Radiation From a Vertical Antenna Over an Inhomogeneous Ground Plane
Authors: Wait James R. , Spies Kenneth P.
Published: February 2008

The fields of a dipole source located over a variable impedance surface are considered. A method of obtaining solutions of the integral equation for the tangential field is outlined. Numerical results are given for the resulting radiation patterns for an exponentially tapered variation of the surface impedance. It is shown that the tapering has the advantage of reducing the lobe structure of the low‐angle radiation.
Studies of the Surface of Mars (Very Early in the Era of Spacecraft Reconnaissance)
Authors: Pollack J. B. , Sagan Carl
Published: February 2008

There is probably no moment in the history of studies of Mars, including the year 1877, in which our knowledge of that planet is changing more rapidly. Prior to 1965 it was generally believed that the surface pressure on Mars was about 85 mb. As a result of ground‐based spectroscopy, rocket ultra‐violet spectroscopy, and the Mariner 4 flyby mission we now know that the actual pressure is about a factor of ten smaller. Prior to 1966 most students of the subject believed that the topographical relief on Mars was quite minimal.
The Microwave Spectrum of Jupiter
Authors: Dickel John R. , Degioanni Joseph J. , Goodman Gary C.
Published: February 2008

New polarimetric observations of Jupiter at 15.4 and 6.65 GHz have been combined with previously published values to determine the microwave spectrum of the planet. All of the data have been normalized to a consistent scale that should be good to 7% on a relative and, hopefully, on an absolute scale. The compound emission has then been separated into its thermal and polarized nonthermal components. The results show that a considerable range of models for the atmosphere of Jupiter will match the spectrum of the thermal component.The most reasonable model probably has an effective temperature near 130°K. The non‐thermal synchrotron emission from the Van Allen belts of Jupiter has a peak in its spectrum near 800 MHz and a discontinuity at higher frequencies that suggests two radiation belts surrounding the planet. The polarized fraction of this synchrotron emission reaches a peak of about 30% near a frequency of 3000 MHz but drops on either side of this frequency.
VLF Ground‐Based Measurements in Antarctica: Their Relationship to Stratifications in the Subsurface Terrain
Authors: Webber G. E. , Peden I. C.
Published: February 2008

The phase and amplitude characteristics of the near‐field broadside to a long horizontal dipole operated close to ground level at very low frequencies can be used to obtain useful information about the nature of the subsurface media. This study is about antarctic glacial terrain and makes use of data collected recently in the Byrd glacier area to draw tentative conclusions about the nature of the glacial ice and underlying earth. Data obtained by use of this method suggest that the depth of the ice layer is in the neighborhood of 2250 meters, that the bulk relative complex dielectric constant of the ice may be approximated by the value 6.0‐j14 at 12.8 kHz, and that the conductivity of the underlying earth approaches 0.10 mho meter−1. A reflection coefficient of approximately 0.60 is indicated when plane waves arrive at vertical incidence at the surface of the ice. These figures are shown to be reasonable when compared with available data from other sources.
Focusing Nonparabolic Mirrors by Using Phase‐Corrected Multiple‐Beam Sources
Authors: Ancona C.
Published: February 2008

The author describes and analyzes a method for correcting the aberration of nonparabolic mirrors by using a phase‐corrected multiple‐beam source consisting of an array of radiating elements fed through a Butler matrix. This source, which is identical to a phase‐modulated point source, is shown to be able to correct any mirror profile, the only limitation being the dimension of the source. The case of a plane mirror is examined first. An experimental mockup that uses an offset, illuminated plane mirror is measured. This mockup shows clear focusing of the mirror and magnification of the directivity of the source. The case of a mirror of circular profile is then analyzed theoretically, and the possibilities of correction as a function of the geometrical parameters are computed, taking into account the accepted phase error and the obstruction ratio obtained.
Currents on a Semi‐Infinite Tube Illuminated by Electromagnetic Waves
Authors: Kao Cheng C.
Published: February 2008

Exact Wiener‐Hopf solutions of the integral equations are presented. A simple relation is then proposed to relate the solutions of a semi‐infinite tube and long finite tubes of the same radius. This relation is useful in a practical sense as a means of obtaining numerical solutions for one case from the solutions of the other.
Low‐Latitude Ionospheric High‐Frequency Doppler Dispersion Study
Authors: Sears R. D.
Published: February 2008

Studies of the Doppler frequency dispersion on a multiple high‐frequency phase stable propagation link were made at a low‐latitude site under a variety of geomagnetic disturbance conditions. Frequency stability of the system enabled us to measure ionospheric‐induced Doppler frequency changes as low as 0.01 Hz. The average Doppler frequency changes during development of the reflecting region at dawn can be characterized consistently by an exponential decrease of positive frequency versus time as the critical frequency of the F‐region increases. The power spectral density of the fluctuations in Doppler frequency was usually nearly white over the fluctuation period range 1 < τ < 20 minutes. The power spectral density magnitude correlates well with the planetary magnetic disturbance index Kp summed over the period of the observations. This result suggests that the observed effects are related to irregular movements of the F region. Occasional coherent‐appearing spectral peaks were observed that may be a manifestation of wave energy in the acoustic window propagated to ionospheric altitudes. No obvious relationship between the appearance of these coherent disturbances and the tropospheric weather pattern could be found.
Electric Current Systems for Polar Substorms and Their Magnetic Effect Below and Above the Ionosphere
Authors: Fukushima Naoshi
Published: February 2008

For polar magnetic substorms, the two‐dimensional current system in the ionosphere and the three‐dimensional systems including the field‐aligned currents in the magnetosphere, are conjectured; both systems have the same geomagnetic effect on the ground but a different effect in and above the ionosphere. The intense westward electrojet along the auroral zone, or oval, is concluded to be mainly the Hall current in the ionosphere, and this is always accompanied by the field‐aligned current system in the meridional plane with a north‐south current segment in the ionosphere. The direction of current flow in the meridional plane depends on the mechanism of its production, whether it is of ionospheric origin or of magnetospheric origin. Recent rocket and satellite measurements of the magnetic field in and above the ionosphere indicate the presence of both these conditions.
High‐Frequency Backscattering From a Finite Cone
Authors: Senior T. B. A. , Uslenghi P. L. E.
Published: February 2008

The high‐frequency backscattered field produced by a plane electromagnetic wave at oblique incidence on a perfectly conducting, right circular cone with a flat base is considered. The first two terms of the asymptotic expansion are obtained by applying the geometrical theory of diffraction; these terms reduce to results derived earlier in the particular case of a circular disk. Owing to the axial caustic, functions must be introduced to match the wide‐angle formulas to the known results for nose‐on incidence. This matching is effected by employing Bessel functions and Fresnel integrals for the first‐ and second‐order terms, respectively. The resulting expressions are valid for all cone angles and for a wide range of aspect angles about nose‐on; they are also found to be in good agreement with experimental data.
Whispering‐Gallery Propagation in the E Region of the Ionosphere at HF and VHF
Authors: Chang Hsi‐Tien
Published: February 2008

By using a Chapman model and neglecting collisions, the maximum usable frequency (MUF) for whispering‐gallery propagation under the E region is found to be about fifteen time the critical frequency. This value is some three times higher than the MUF for ground‐to‐ground communication. However, collisions cause serious attenuation of trapped whispering‐gallery modes associated with the E region even at frequencies close to the MUF. Radio communication along the E‐region whispering gallery, unlike communication along the F‐region whispering gallery, is practical only for a distance of a few thousands of kilometers and is ineffective for propagation around the world.
Theory of HF and VHF Propagation Across the Rough Sea, 1, The Effective Surface Impedance for a Slightly Rough Highly Conducting Medium at Grazing Incidence
Authors: Barrick Donald E.
Published: February 2008

One method of analyzing radiation and propagation above a surface is to employ an effective surface impedance to describe the effect of the boundary. In this paper an expression is derived for the effective impedance at grazing incidence of a slightly rough interface between air and a finitely conducting medium for which the Leontovich boundary condition is applicable. The perturbation technique of Rayleigh and Rice is employed, and attention is restricted to vertical polarization. The resulting effective surface impedance consists of two terms, the impedance of the lower medium when the surface is perfectly smooth and a term accounting for roughness. The latter term can be complex in general and depends on the strengths of the roughness spectral components present. The result is applicable to either deterministic periodic surfaces or random rough surfaces. Various alternate definitions of the effective surface impedance are examined and are seen to be equivalent. The analysis of power flow at the surface permits the interpretation of the interaction process in terms of scattered fields.
Combined Differential Faraday and Differential Absorption Measurements at 40 Megahertz
Authors: Livingstone C. E. , Lyon G. F.
Published: February 2008

Differential Faraday rotation measurements of the 40‐ and 41‐MHz beacons of satellite 1964–64A/BE‐B were made at London, Ontario (43°N, 81°W) through the use of a continuous output polarimeter of the phase comparison type, and simultaneous differential absorption measurements were made by use of the ordinary and extraordinary mode amplitudes at 40 MHz. Sufficient measurement accuracy was obtained to employ the second‐order expansion of the magnetoionic refractive and absorption index equations to extract total electron contents, the ratio of the density squared integral to the total electron content, a mean ionospheric electron collision frequency, and a parameter sensitive to the height of an electron density irregularity, from the data. Two passes, orbit 17239, March 17, 1968, and orbit 17280, March 20, 1968, were selected for analysis. Peak electron densities, mean plasma scale heights, and mean electron temperatures were computed from the reduced data by the assumption of an electron density height profile; they were found to correspond to within experimental error with comparable published measurements. During both passes examined, F region electron density profile broadenings were detected that were, for March 20, attributed to adiabatic expansions of the F region electron gas, which was driven by localized neutral gas motions, and for March 17 the broadenings were attributed to thermal expansions of the F region plasma. On both occasions, the fractional errors in the data were sufficiently small over some portions of the pass to permit a determination of the dominant ionospheric absorption mechanism.
Diffraction of Electromagnetic Waves by a Moving Impedance Wedge
Authors: Lang K. C.
Published: February 2008

A formal solution to the problem of the diffraction of electromagnetic waves by a wedge with given surface impedances, moving parallel or perpendicular to its bisection plane with constant velocity, has been obtained. Characteristics of the diffracted field are studied, and approximate expressions for the Doppler shifts of the excited surface‐wave fields and the scattered field in the shadow region are evaluated for both relativistic and on‐relativistic considerations. Plots of these results for a 60° wedge and a half plane are shown to illustrate their dependence on the surface impedance, the wedge angle, and the angle of incidence. Also, the variation of the amplitude of surface waves with respect to the velocity of the moving wedge is discussed.
The Generalized Convergence Coefficient in the Earth‐Ionosphere Waveguide
Authors: Cory H. , Thiel J.
Published: February 2008

The geometrical optics convergence coefficient has been determined up to now only when the transmitter and the receiver are both situated on the ground or when the transmitter is at the base of the ionosphere and the receiver is on the ground. This paper presents formulas for the convergence coefficient regardless of the positions of the transmitter and the receiver in the earth‐ionosphere waveguide. There are four different possible paths of the ray. The convergence coefficients are calculated for each of them, and numerical results are given.
Scattering From a Turbulent Laboratory Plasma at 31 Gigahertz
Authors: Graf K. A. , Guthart H. , Douglas D. G.
Published: February 2008

The scattering of electromagnetic waves from a turbulent plasma has been examined experimentally at 31 GHz. The 31‐GHz cross section data agree well with a scattering model proposed earlier and with 9.4‐GHz data measured earlier. Measurements were made to determine the dependence of the cross section on bistatic angle and electron density. Comprehensive diagnostic measurements were made on the plasma so that the radar measurements could be compared to the scattering model. A synthetic‐aperture, or coherent processing, experiment was also performed to relate the instantaneous scattered field in a small volume (specified by the resolution obtainable by coherent processing techniques) to the instantaneous distribution of electrons in the same volume. The measurements confirm the validity of first‐order Born theory when applied to instantaneous rather than time‐averaged quantities.
The Transient Fields of a Hertzian Dipole in the Presence of a Dielectric Half‐Space
Authors: Hill David A.
Published: February 2008

The radiation of Hertzian dipoles (vertical electric and magnetic and horizontal electric and magnetic) with impulsive currents in the presence of a nondispersive dielectric half‐space is considered. By taking the inverse Fourier transform of the Sommerfeld integrals, exact closed‐form solutions are obtained for particular locations of source and observation points. These solutions contain doublet and delta singularities whose magnitudes and arrival times are determined by geometrical optics followed by smoothly decaying waveforms whose final values are determined by static theory.
Surface Waves on Two‐Dimensional Corrugated Structures
Authors: Lee S. W. , Jones W. R.
Published: February 2008

The characteristic equation is derived for the propagation of surface waves guided by a planar two‐dimensional corrugated structure. This equation is solved numerically to obtain the propagation wave number as a function of the direction of propagation on the surface. The data obtained illustrate the anisotropic nature of the wave propagation. In addition, an isolated surface wave is found to exist with its propagation wave number in a ‘forbidden’ region of wave number space. The relevance of this wave to a transmission null in a phased array antenna is discussed.
Announcement and Call for Papers
Authors: Evans John V.
Published: February 2008

A number of sessions will be organized around special topics as follows: (Comm. 1) Measurement on and Related to Antennas Operating between 10 and 100 GHz, RF Techniques for Measurement of Pollution of Air and Water; (Comm. 2) Propagation Measurements with ATS5, Remote Sensing of Surfaces, Remote Sensing of Atmospheres; (Comm. 3) Ionospheric Perturbation; (Comm. 3 and 4) Conjugate Effects in the Ionosphere and Magnetosphere; (Comm. 4) Magnetospheric Perturbation; (Comm. 5) Spectral Line Interferometry, Long‐baseline Interferometry; (Comm. 6) Time Harmonic and Transient Radiation and Diffraction, Stochastic and Nonlinear Effects, Adaptive Arrays, Beams and Open Resonators.
Reflection of Pulses From a Linearly Varying Ionosphere Model with a Vertical Magnetic Field
Authors: Hill D. A. , Wait J. R.
Published: February 2008

The reflection of a linearly polarized plane electromagnetic wave by an idealized cold‐plasma ionosphere model, with a constant vertical magnetic field, is examined. A linear electron density profile and a constant collision frequency are assumed. For vertical incidence, the exact time‐harmonic solution for the parallel and cross‐polarized reflected fields are expressible in terms of Airy functions. Then, using a numerical inverse Fourier transform, the impulse response of the model is obtained. It is shown that causality is satisfied for both the in‐polarized and the cross‐polarized component of the reflected transient wave. The cross‐polarized component owes its existence to the presence of the dc magnetic field.
Redistribution of Ionization in the Auroral and Equatorial Ionosphere
Authors: Closs R. L.
Published: February 2008

It is shown that the electric fields that can exist in the auroral and equatorial ionospheres should produce a convergent flow of metallic ions. It is suggested that this convergent flow of ions is responsible for some of the sporadic E with high ƒbEs observed in these regions. It is further suggested that the existence of meridional currents in the equatorial ionosphere might be detected by their production of sporadic E, whereas in the auroral ionosphere the direction of the electric field and the molecular weight of the ions responsible for the Es layer can be found from height measurements and the separation between the striations in the sporadic layer.
Observations of Radio Refractivity Gradients in the Mekong Delta
Authors: Samson C. A. , Maloney L. J.
Published: February 2008

Beyond‐the‐horizon 2‐GHz radio paths in the Mekong Delta have shown large diurnal and seasonal propagation variations, which sometimes resulted in serious degradation of system performance. Measurements of refractivity gradients, in January and February 1970, using a high‐resolution radiosonde technique showed a higher incidence of subrefraction than was indicated by the available climatological statistics. Low‐based elevated layers with extreme superrefractive gradients were occasionally observed over wide areas.
RF Sheath and Admittance Characteristics of a Spherical Plasma Probe
Authors: Kan J. R.
Published: February 2008

A radio‐frequency sheath model for a spherical probe in a collisionless plasma is presented. The method of solution is based on the quasi‐static approximation and the electrostatic probe theory of Bernstein and Rabinowitz. The resistive part of the admittance is ascribed to the sheath transit‐time collisionless dissipation mechanism suggested by Mayer and developed by Gould. Expressions are obtained for the effective sheath thickness and the equivalent resistance of the transit‐time dissipation. The sheath model and, hence, the admittance are completely determined in terms of the bias potential, the probe radius, the plasma frequency, and the Debye length; i.e., there are no adjustable parameters in our theory which are to be determined by experiment. Our results agree favorably with Cohen and Bekefi's experimental data on the conductance resonant frequency and the width of the conductance peak.
Rayleigh Scattering Cross Sections
Authors: Kleinman R. E. , Senior T. B. A.
Published: February 2008

The significance of the electric and magnetic polarizability tensors in low frequency scattering is emphasized. In the particular case of perfectly conducting, rotationally symmetric bodies with plane‐wave illumination, it is shown how the entire Rayleigh‐scattered field can be expressed in terms of just three tensor elements, functions only of the geometry of the body. Inequalities satisfied by these elements are used to establish optimum lower bounds on the scattering cross sections, and, in addition, the elements themselves are examined analytically and computationally for a variety of shapes. Some of the implications of these results are discussed.
Height‐gain experimental data for groundwave propagation: 1, Homogeneous paths
Authors: King R. J. , Cho S. H. , Jaggard D. L. , Sokolov V.
Published: February 2008

Experimental height‐gain data for the magnitude of electromagnetic groundwaves propagating over flat homogeneous paths were obtained by microwave (4.765 GHz) modeling. The experimental results were compared with height‐gain theory, and the limitations of the theory were established in terms of surface impedance, height of the receiving point, and antenna separation distance. Earlier experimental studies of this problem have not been entirely conclusive due to an inadequate knowledge of the surface impedance and the influence of the surface upon the antenna impedance. Here, extensive corrective measures were taken to eliminate or account for these effects.

It is concluded that the theory for the total field is remarkably accurate as the height above or distance along the surface is varied, even at distances as close as 2λ0 from the source. Furthermore, the question as to the choice between the various forms of numerical distance seems academic. The restriction on height above the surface is eliminated, and the restriction upon the magnitude of the surface impedance is relaxed depending upon the presence of a significant trapped surface wave.

Part 2 is a companion paper which relates the homogeneous path height‐gain to mixed‐path experimental measurements versus height and distance.
Field‐aligned ionospheric E‐region irregularities and sporadic E
Authors: Basu Sunanda , Vesprini Robert L. , Aarons Jules
Published: February 2008

A continuous series of oblique backscatter observations at 19 MHz made at Plum Island, Massachusetts (56°N invariant), over nearly half a solar cycle is utilized to derive the occurrence characteristics of aspect‐sensitive field‐aligned irregularities in the E‐layer and their association with ground‐backscattered echoes which are propagated via sporadic E (E8) reflection. Under quiet magnetic conditions, it is found that the field‐aligned echo from E‐layer heights, FAE(E), displays a summer evening maximum in conjunction with the E8 echo. A weak secondary maximum is observed in the winter with no detectable field‐aligned structures being evident in the daytime E8. During disturbed magnetic conditions, the E8 ground‐backscattered echo is greatly suppressed together with a simultaneous increase of the FAE(E) which also appear in the daytime; no seasonal control is evident. One year's continuous observations at 49 MHz with a narrowbeam antenna showed the quiet time, E8‐associated FAE(E) to be of the discrete type, whereas the FAE(E) obtained during storms were found to be diffuse in nature. These observations are discussed from the point of view of current plasma instability theories.
The neutral‐atmosphere temperature instrument
Authors: Spencer N. W. , Niemann H. B. , Carignan G. R.
Published: February 2008

The determination of the temperature of the neutral gas at the location of the satellite is based on measurement of the velocity distribution of the molecular nitrogen. Measurement of the thermal‐velocity component in the presence of the free‐stream velocity will be obtained through application of the velocity‐scan technique and, independently, through use of a baffle technique. A 3‐cm diameter spherical sampling chamber with a 0.5‐cm diameter precisely‐knife‐edged orifice is located at the satellite equator to permit free diffusion of atmosphere gases between the chamber interior and the atmosphere. The spherical chamber is connected through a high‐conductance tube to a quadrupole mass‐spectrometer sensor to permit accurate quantitative evaluation of the density of the gas. The spectrometer, tuned to N2 for most of the measuring time, will also be tuned to other gases for exploratory purposes. The accuracy of the temperature data is expected to be better than 5%. The altitude range, determined primarily by system snr at the upper altitudes and mean‐free‐path limitations at the lower altitudes, is expected to be from 130 to 400 km.
The cylindrical electrostatic probes for Atmosphere Explorer ‐C,‐D, and ‐E
Authors: Brace L. H. , Theis R. F. , Dalgarno A.
Published: February 2008

This paper briefly describes the cylindrical electrostatic probes to be used on the new Atmosphere Explorer‐C, ‐D, and ‐E, and outlines the methods to be employed to analyze the experimental data in terms of electron temperature and ion and electron concentration. Two independent cylindrical sensors and partially redundant electronic systems permit greater reliability in measurements. Measurements are made at 1‐sec and 2‐sec intervals along the orbit. The sensors themselves have been modified from previous applications to cope with the lower electron temperature expected in these low‐altitude orbits and to counter possible surface contamination caused by the hydrazine engines used for orbit adjustments.
The photoelectron‐spectrometer experiment on Atmosphere Explorer
Authors: Doering J. P. , Bostrom C. O. , Armstrong J. C.
Published: February 2008

The Atmosphere Explorer (AE) photoelectron spectrometer experiment consists of electrostatic deflection‐type electron spectrometers designed to measure the low‐energy electron flux expected to be encountered along the AE orbit. The system has been designed to provide general information on the intensity, angular distribution, energy spectrum, and net flows along field lines as well as sufficient energy resolution to allow the fine details of the electron‐energy spectrum to be studied. The instrumentation consists of two separate electron‐spectrometer detector heads connected to a common electronics system for data processing and telemetry interfacing.

The experiment will measure the electron‐energy spectrum from 2 to 500 ev (and possibly to less than 0.1 ev), The energy resolution (ΔE/E) of the electron spectrometers will be approximately 2.5% full width half maximum. The geometrical factor of the analyzers will be approximately 10−3 cm2 ster, independent of electron energy. Placement on the spacecraft will be arranged so as to give as little interference as possible from shadowing of electron trajectories by the spacecraft and contamination of the observed spectra by locally generated photoelectrons.
Scattering from a moving cylinder: Oblique incidence
Authors: LeVine D. M.
Published: February 2008

The scattering of plane waves from a moving perfectly conducting cylinder is treated in the general case of arbitrary velocity and oblique incidence. This is a generalization of the cases previously treated in the literature and includes discussion of an additional phenomenon, mode coupling, which does not occur in the cases treated previously. Attention is focused on the depolarized component of the scattered field as it is one of the more apparent manifestations of the mode coupling. In addition, Doppler shift is discussed, and scattered power is calculated in the far fields of a thin wire. It is shown that, to first order in β = v/c, Doppler shift depends only on the component of velocity which is perpendicular to the cylinder and in the plane of incidence and that an azimuthal component of scattered power exists which does not occur in the absence of motion. The treatment is relativistically correct.
Variations of the ray path and phase path: A Hamiltonian formulation
Authors: Bennett J. A.
Published: February 2008

Using a formulation based on Hamilton's ray equations, the variations of the path of a ray, and of the corresponding phase path, due to changes in the ray end points or the medium properties, are investigated. General expressions are obtained which represent the variations in terms of the solution of systems of ordinary differential equations. These may be integrated along the unvaried rays in order to evaluate the variations. The results, which may be applied to the analysis of a number of propagation problems, represent a simplification for anisotropic media and are also particularly well adapted to numerical applications.
Scattering of HF radio waves by elliptical electron‐density distribution
Authors: Corbin Victor L. , Klein Milton M.
Published: February 2008

The differential cross sections for an ellipsoid and elliptic cylinders having Gaussian electron‐density distributions have been obtained by a ray‐tracing procedure. Calculations for the case of an external magnetic field were restricted to the ellipsoidal distribution. The results show that the scattering is extremely sensitive to the orientation of the body. A peak in cross section occurs at the scattering angle corresponding to the ray normal to the critical surface, and increases as the surface becomes flatter. The cross section is sensitive to the ratio of peak density to critical density for moderate values but becomes relatively insensitive when the ratio exceeds 3. The total cross section (shown only for the elliptic cylinder) is a very sensitive function of both orientation and ratio of major to minor axes. The introduction of a magnetic field decreases the ordinary ray cross section; the extraordinary ray exhibits higher cross‐sectional values than the nonmagnetic field case. Comparison of the Gaussian ellipsoid with the corresponding conducting ellipsoid shows that the Gaussian has a larger cross section in the forward region but considerably lower values in the back‐scatter region.
A qualitative study of the reconnection between the Earth's magnetic field and an interplanetary field of arbitrary orientation
Authors: Cowley S. W. H.
Published: February 2008

To the present date only the reconnection process for exactly antiparallel fields has been discussed in detail. In magnetospheric terms this restricts us to the consideration only of southward interplanetary fields. The qualitative study presented here shows how reconnection takes place between arbitrarily oriented fields in infinite and finite geometries. The process may best be thought of as a continuous exchange of field‐line partners during the time a field line maps into the diffusion region. In a finite geometry the diffusion regions lie on field lines which connect neutral points of the field configuration, and along which a potential drop is imposed. Although the discussion here centers principally on the magnetosphere, the described reconnection process and field topology should also be applicable to other astrophysical problems.
Contribution of magnetospheric field‐aligned current to geomagnetic bays and Sq fields: A comment on partial ring‐current models
Authors: Fukushima Naoshi , Kamide Yohsuke
Published: February 2008

Curved field‐aligned currents in the magnetosphere produce a noticeable geomagnetic effect on the earth's surface not only in the D component but also in the H component. For geomagnetic bays or longitudinal asymmetry of ΔH (deviation in H from its quiet level) during magnetic storms, the contribution from the field‐aligned currents is greater than that of a partial ring current. The Sq field also may be appreciably influenced by the field‐aligned currents associated with the magnetospheric plasma convection.