bluuebunny/crossref_metadata_2025 huggingface dataset:
A Transportable 16‐GHz Solar Telescope for Atmospheric Transmissivity Measurements
Authors: O'Brien Kevin C. , Heidt Richard C. , Owens Gerald J.
Published: February 2008

A transportable, 16‐GHz solar telescope is now being used to investigate the atmospheric transmission of solar radiation, with special emphasis on the effects of precipitation. An equatorial mount and digital‐clock drive direct the pointing angle of the antenna to follow the sun's path to within 0.05° of arc. A 40‐inch offset paraboloid antenna with twin prime focus feeds is used. The dual feeds alternately sample (at a 1‐KHz rate) the transmitted solar energy and the atmospheric emission, so that the detected difference signal is a direct measure of the atmospheric transmissivity. The superheterodyne, double‐sideband receiver has an approximate overall noise temperature of 740°K and IF bandwidth of 225 MHz. Attenuations up to 35 db can be measured. The fast switching rate along with a high‐speed data‐acquisition system allow measurements with a time constant less than 0.1 sec. The transportability of the instrument permits studies of attenuation in various climatic regions.
Radiation From Conical Surfaces Used for High‐Speed Spacecraft
Authors: Balanis Constantine A.
Published: February 2008

The analysis for the radiation pattern of an infinitesimal dipole mounted on the tip of a perfectly conducting, finite length cone is presented. The pattern is constructed from the superposition of a wave radiated directly from the linear element (in the presence of an infinite cone) and a wave diffracted from the edge of the cone. The method is verified by comparing computed values with experimental data.
Coupling: Summary Report
Authors: Felsen L. B.
Published: February 2008

W. R. Larenz' review paper (printed in this issue) presented a systematic categorization of various linear and nonlinear coupling mechanisms encountered in the hydrodynamically approximated warm plasma. As on several occasions during this symposium, the range of validity of the fluid model was questioned; the discussion provided the usual answers of the adequacy of the model as long as the wave numbers of electron or ion waves involved in plasma processes are well below the Debye wave number kD. The utility of the fluid model, when adequate, was emphasized in connection with radiation and boundary value problems since the complexities introduced by kinetic theory tend to render much of the required analysis intractable. For better understanding of the influence of anisotropy, it was pointed out that substantial anisotropy in the particle velocity distribution may be accommodated by an anisotropic pressure tensor within the hydrodynamic description.
Final Discussion: Summary Report
Authors: Rawer K.
Published: February 2008

Some remaining contributions were read in a first part of this session. R. F. Benson (Goddard Space Flight Center, GSFC, Greenbelt, Md., USA) read two papers concerned with resonances observed in topside sounders. The first paper considered the small frequency shifts which are found for successive harmonics of the electron gyro frequency, ƒB. When the magnetic field is computed from observed frequencies, there is agreement with the result of the well‐known computer programs (using Legendre development) for the sixth‐order resonance.
Bistatic‐Radar Detection of High‐Altitude Clear‐Air Atmospheric Targets
Authors: Doviak R. J. , Goldhirsh J. , Miller A. R.
Published: February 2008

The Valley Forge‐Wallops Island bistatic radar has routinely identified high‐altitude clear‐air atmospheric targets. Bistatically detected target signatures are interpreted in terms of layer and diffuse distributions of scatterers. Simplified numerical models appear to explain well the layer signature characteristics. Locations of bistatically detected targets are compared with locations of meteorological features and monostatically detected atmospheric targets. Comparison shows the advantage gained when bistatic radars complement monostatic observations of clear‐air atmospheric targets.
Net‐Field Polarization in a Magnetically Biased Plasma
Authors: Rusch W. V. T. , Stelzried C. T.
Published: February 2008

The net‐field polarization of a plane wave propagating in a magnetically biased cold plasma has been treated in a systematic fashion. Under quasi‐longitudinal conditions the polarization transformations are particularly simple and as such have been treated in the literature. However, under more general conditions the polarization transformations are relatively complex. The axial ratio, tilt angle, rotation rate, and polarization sense all become functions of the initial conditions and the constants of the medium. Under quasi‐longitudinal conditions the polarization ellipse rocks between two extremes, while the axial ratio also changes periodically between a minimum and a maximum value (Cotton‐Mouton effect). The wide variety of polarization transformations available suggests the usefulness of slabs of magnetically biased plasma to synthesize microwave polarizers and depolarizers. The theory of net‐field polarization when applied to data obtained during the 1968 solar occultation of Pioneer 6 verified that the usual quasi‐longitudinal approximation (Faraday rotation) is adequate to describe S‐band polarization in the solar corona.
Propagation of Electromagnetic Waves in Media Which Vary Slowly With Position and Time
Authors: Fante Ronald L.
Published: February 2008

Using WKB methods we have considered the propagation of electromagnetic waves in isotropic lossless media which vary slowly with both position and time. It is found that in such media the meaning of various quantities, such as the group velocity, must be reinterpreted. The theory is applied to study propagation in space‐time, varying dielectrics, and plasmas.
Imaging properties of monostatic and bistatic troposcatter radars
Authors: Lammers Uve H. W.
Published: February 2008

Three tropospheric scatter systems, featuring antenna‐beam angular discrimination, or a combination of both antenna‐beam angular discrimination and signal‐delay temporal discrimination, are compared for sensitivity and spatial resolution as determined by theoretical calculations. Two of the radars used are bistatic; the third is monostatic or quasi‐monostatic. Whenever practical, parameters are chosen equal for all three radars. The linear dimensions of the scatter volume and its size as a function of position in space are determined. The imaging properties of each radar are analyzed after the scatter integral (received power level) is numerically evaluated using a tropospheric multiple‐layer model of turbulent refractive‐index structure. Results are presented for a 500‐km baselength in the bistatic case. Double layers of 100‐m vertical width and 500‐m vertical separation are displayed as seen with the angular beam resolution of 0.15° (between 3‐db points) and 40‐nsec temporal resolution corresponding to a 25‐MHz system bandwidth.
Excitation of the earth‐ionosphere waveguide by point dipoles at satellite heights
Authors: Pappert Richard A.
Published: February 2008

Formulas for the excitation of the earth‐ionosphere waveguide by electric and magnetic point dipoles at satellite heights are developed along the lines of Budden's [1961, 1962] general theory. The formulation, based on horizontal stratification, allows in an approximate manner for a vertically inhomogeneous, anisotropic ionosphere as well as for earth curvature along the direction of propagation. Although emphasis is on theory, some results pertinent to ELF are given. In particular, numerical comparisons with reciprocity predictions are included.
Cross‐focusing of two coaxial laser beams in a dielectric
Authors: Sodha M. S. , Tewari D. P. , Kamal Jyoti , Tripathi V. K.
Published: February 2008

The possibility of focusing a laser beam by the optical inhomogeneities induced by a second strong coaxial laser beam has been investigated. In a particular case in which the power of the strong beam equals the critical power, the medium provides an oscillatory waveguide for the weak beam. When both beams are strong the nonlinearities induced by them are additive, and both beams become focused even when their powers are below the critical powers.
Commission 5. Radio and radar astronomy
Authors: Cohen M. H.
Published: February 2008

This session was limited to observations at wave‐lengths above 1 cm. The review by B. M. Zuckerman (USA) began with spectral‐line equipment developments. Front ends can now be built anywhere in the range 1 to 30 GHz with system temperatures around 20 to 60 K, but these low‐noise receivers are all rather inflexible in frequency. There is now widespread use of correlators as back ends, and further improvements in performance can be expected.
Magnetospheric field morphology at magnetically quiet times
Authors: Sugiura Masahisa
Published: February 2008

The magnetospheric morphology is reviewed using the method of the ΔB topology. Typical examples of ΔB profiles along individual passes of OGO‐5 in the morning hours and near noon are given, supplementing similar data presented earlier. These results demonstrate the significance of the equatorial current system in the distortions of the magnetospheric field in its quiet state. A preliminary study of the differences between the observed inclination, I, and I calculated from a reference field, and the corresponding differences in declination, D, shows that ΔI and ΔD are larger than expected from the existing models. The magnitudes of ΔI at low latitudes are larger in the dusk sector than in the dawn sector, suggesting that the equatorial inner magnetosphere is more inflated near dusk than near dawn. The ΔD data show that the bending of the field lines toward the tail is greater in the dusk than in the dawn sector. To account for the large ΔD values at midlatitudes it is suggested that they are caused by field‐aligned currents flowing from the dusk end of the cross‐tail current to the ionosphere and returning to the tail from the ionosphere on the morning side.
Rate of erosion of dayside magnetic flux based on a quantitative study of the dependence of polar cusp latitude on the interplanetary magnetic field
Authors: Burch J. L.
Published: February 2008

By considering only those periods when the delay time from the interplanetary observing position to the magnetosphere is less than about 5 min, it is found that irrespective of substorm activity: (a) the 45‐min average value of interplanetary Bz predicts the latitudes of the poleward and equatorward boundaries of polar cusp electron precipitation with rms errors of 1.34° and 1.16° respectively; (b) both boundaries move equatorward by about 5° as Bz varies from 0 to −6γ, the cusp remaining about 4° wide; (c) the amount of flux added to the polar cap is about 9.2% of the total southward flux impingent on the magnetosphere in the previous 45 min; and (d) as Bz becomes more positive, the equatorward boundary moves only slightly more poleward (½° between Bz = 2γ and Bz = 6γ), while the poleward boundary moves significantly toward higher latitudes, resulting in a cusp approximately 7° wide for Bz = 6γ. This dependence of the width of the cusp on positive Bz suggests that reconnection between the IMF (interplanetary magnetic field) and open geomagnetic field lines (which are southward near the dayside magnetopause) is facilitated when the IMF is northward, allowing deeper penetration of magnetosheath plasma into the high latitude magnetosphere.
VLF ray trajectories in a latitude‐dependent model of the magnetosphere
Authors: Walter F. , Scarabucci R. R.
Published: February 2008

Ray paths of VLF waves that propagate inside the magnetosphere are studied. The model of the magnetosphere is composed of electrons and ions, and includes latitudinal gradients of composition; the propagation is treated on the magnetic meridian. It is found that the presence of ions together with the latitudinal gradients generate ray paths with very peculiar features. The paths of the rays are combinations of trajectories influenced by the gradients of composition, magnetic reflections (ion effect), and reflections at the steep gradients of the plasmapause. The related whistler study provides an alternative explanation of several properties of whistler trains, without the use of field‐aligned ducts of enhanced ionization.
Far‐field transient backscattering by ice spheres
Authors: Ray Peter S. , Stephens J. J.
Published: February 2008

Approximate far‐field, backscattering transient responses are shown for three ice‐sphere sizes. Signal ringing continues for a longer time, but the effective transient decay time is less than a space‐equivalent time of six diameters. Glory, axial, and stationary rays in combination with an internal surface wave of one cut are the principal contributors to the scattering by ice, but the dipole must be included for small electrical sizes.
Evidence for the existence of nighttime F‐region polarization fields at Arecibo
Authors: Behnke R. A. , Hagfors T.
Published: February 2008

Recent measurements are presented of F‐region ion drifts made at Arecibo using a new continuous beamswinging technique. The measurements are applied to the question of the origin of the F‐region electric field over Arecibo. It is shown that the daytime electric fields are probably caused by E‐region tidal winds, while the nighttime electric fields appear to be generated in the F region itself by thermospheric winds.
Diurnal and long‐term behavior of the exospheric temperature as observed by incoherent scatter sounding in the F2 region
Authors: Alcayde D.
Published: February 2008

A spectral analysis has been performed on the exospheric temperature derived from incoherent scatter data gathered at St. Santin for two unusually long observation campaigns. It is found that the main components of the oscillations are diurnal and semidiurnal, with amplitudes of respectively 10 and 2.5% of the steady component. Based on this evidence the bulk of the data, corresponding to 84 shorter experiments of only about 40 hr, has been Fourier‐analyzed in terms of the abovementioned components. The long‐term variations of the characteristics of each steady state and of the oscillation components are then analyzed and their implications for the temperature structure are discussed.
Observation of neutral temperature tidal‐type oscillations in the F1 region
Authors: Fontanari J. , Alcayde D.
Published: February 2008

Incoherent scatter data gathered at St. Santin yield a determination of the ionic (neutral) temperature in the F1 region. Although large uncertainties are present in these data, a study of their short‐term variations is possible provided they have been statistically combined in typical periods. It is shown that waves of tidal type are present in the lower thermosphere, with dominant periods of 24 and 12 hr. Seasonal variations are found in good agreement with exospheric temperature observations. Comparisons of experimental results with relevant theoretical works show a rather good general agreement. As concerns the semidiurnal component, Lindzen and Hong's work shows the possible presence of two modes in the thermosphere: a trapped mode generated in situ, and a propagating one generated below 100 km; however, experimental results suggest a rapid damping of the propagating mode above 180 km, while the in‐situ excited mode dominates.
Observations of sounder‐pulse‐induced modulations on the reception of broadcast signals in the ionosphere
Authors: McAfee John R.
Published: February 2008

Observations of pulse‐stimulated transient effects on the behavior of received signals were made during a portion of a rocket‐borne RF sounder flight. These consisted of the amplitude modulation (often severe) of various ground‐based broadcasts which were received. Their occurrence was synchronous with the sounder pulses and dependent (at least qualitatively) upon the relationship between the broadcast frequency and the local plasma resonant frequencies. The exact nature of the physical processes which cause the modulation is unknown, but could lie in the modification of the sheath‐controlled antenna impedance by the pulse. The possibility that such modulations are present in experiments may be important in terms of explaining other observations and in rectifying misinterpretations.
Field‐aligned scattering from a heated region of the ionosphere—Observations at HF and VHF
Authors: Fialer P. A.
Published: February 2008

Radio scattering measurements have been made of a region of the ionosphere above the Institute for Telecommunication Sciences' ionospheric heating facility at Platteville, Colorado. Strong scattering was observed at frequencies in the HF and VHF bands. The scattering has been shown to originate from electron density fluctuations that are highly elongated in the direction of the geomagnetic field. Radar scattering cross sections of 70 to 80 dbsm are measured in the HF and low VHF bands. The scattering cross section decreases at a rate of 20 to 30 db per octave at the high end of the VHF band. The scattering irregularities are produced by operation of the heating facility with o‐mode polarization at frequencies that reflect within the ionosphere. The strength of the scattered energy decreases at a rate of at least 10 db per degree for observation geometries that do not match the geometry for specular reflections from long cylinders aligned with the geomagnetic field. The spatial distribution of irregularities has been estimated for both E‐ and F‐region heating. The temporal properties of the scattering have also been determined including the frequency spectrum of the scattered signals. The processes by which ionospheric heating produces these irregularities are not yet fully explained. Several potentially useful applications of radio scattering from such irregularities are discussed.
Suggestions for future research in HF ionospheric modification
Authors: Sweeney L. E.
Published: February 2008

This paper summarizes suggestions for future research in the modification of the ionosphere by high powered HF radio waves. The research areas discussed include both theoretical and experimental work to: (1) better define and explain known phenomena, (2) verify the existence and establish the properties of effects that have been tentatively observed, (3) discover new effects, and (4) develop applications of known phenomena to science and engineering.
A multipulse correlator design for incoherent scatter radar
Authors: Rino C. L. , Baron M. J. , Burch G. H. , de la Beaujardiere O.
Published: February 2008

A multipulse correlator system for the Chatanika, Alaska, incoherent scatter radar is described. The system utilizes a hardware correlator unit that can deliver as many as 40 8‐bit by 8‐bit lagged products every 20 μsec. The particular lags at which products are computed are patch‐wire programmable, and the range gates at which data are taken are set arbitrarily under computer control. The system operates with a fixed bandwidth of 50 kHz, so that the maximum range resolution is 6 km. To obtain sufficient lag resolution for operation at 1290 MHz, we compute lagged products at other than the nominal subpulse spacing. An example of the correlator operation is given.
A numerical study of electromagnetic scattering from ocean‐like surfaces
Authors: Lentz R. R.
Published: February 2008

The integral equations describing electromagnetic scattering from cylindrical, perfectly conducting surfaces are formulated and numerical results are presented. The results are compared with those obtained using approximate methods such as physical optics, geometrical optics, and perturbation theory. The integral equation solutions show that the surface radius of curvature must be greater than 2.5 wavelengths for either the physical optics or geometrical optics to give satisfactory results. It has also been shown that perturbation theory agrees with the exact fields as long as the root‐mean‐square surface roughness is less than one tenth of a wavelength.
Geometrical optical approach for electromagnetic wave propagation in rectangular mine tunnels
Authors: Mahmoud Samir F. , Wait James R.
Published: February 2008

Electromagnetic wave propagation inside an empty rectangular mine tunnel with imperfect walls is considered. The modal expansion of the fields is complicated by the coupling of the basic modes by the imperfect walls. To avoid this difficulty, and in view of the large guide dimensions relative to the free space wavelength, a geometrical ray approach is proposed. To provide a theoretical foundation for the method, we first consider an idealized waveguide model with two perfectly reflecting side walls. The modal field expansion for this prototype model is fully analyzed to provide a satisfactory comparison between the modal and the geometrical ray sums. The proposed general ray method is then applied to the rectangular waveguide when all four walls are imperfectly conducting. Finally, the influence of wall roughness is considered by a relatively simple method.
Horizontal structure of midlatitude sporadic‐E layers observed by incoherent scatter radar
Authors: Miller K. L. , Smith L. G.
Published: February 2008

An investigation of the horizontal structure of sporadic‐E layers has been made using the incoherent scatter radar at the Arecibo Observatory. Data are presented for two observations, one with the radar beam held in a vertical position, and another with the beam scanning in azimuth across the sporadic‐E layer. The first observation was made at sunset and shows the passage of a large region of ionization, about 150 km in extent, having little small‐scale horizontal structure. The second, at midday, shows considerable variation in the value of the maximum electron density in the vertical cross section of an intense sporadic‐E layer. The horizontal dimensions of the features range in size down to the resolution of the radar (300 m). By considering the data statistically it is shown that small patches, 300 m or less in horizontal extent, exist in the sporadic‐E layer with densities great enough to account for the maximum frequency of the echo recorded on the ionosonde located at the observatory.
Chatanika radar investigation of high latitude E‐region ionization structure and dynamics
Authors: Hunsucker Robert D.
Published: February 2008

Simultaneous Chatanika incoherent scatter radar, auroral all‐sky camera, and photometric data are utilized to delineate the structure and dynamics of the auroral E layer for various types of disturbances. The vertical distribution of ionization (Ne(h) profiles) is presented and discussed for various disturbance phenomena, such as the great geomagnetic storm of August 2–9, 1972, intense auroral particle precipitation events (March 16, 1972, March 2, 1973), and auroral infrasonic wave disturbances, April 2, 1973. Ne(h) profiles for quiet conditions and for a slowly varying disturbance (the July 10, 1972, solar eclipse) are shown for comparison purposes.
Electron heating rates in the E and lower F regions
Authors: Schutz S. R. , Smith L. G. , Voss H. D.
Published: February 2008

Electron temperatures and electron densities measured on four rocket flights have been used to deduce electron heating rates in the E and lower F regions. Above 150 km heating rates at midday are found to be in good agreement with calculated values. Anomalies are found in heating rates at sunrise which are attributed to a source, such as joule heating, which does not produce ionization.
A canonical solution to the three‐dimensional electromagnetic prospecting problem
Authors: Howard Allen Q.
Published: February 2008

The problem of three‐dimensional scattering from anomalies buried in the earth is considered. In the general situation, this requires the fields to be evaluated using two‐dimensional numerical integration. In this or the related inverse problem, subsurface characteristics are extracted from a two‐dimensional grid of surface field measurements. This grid of points is computed with time‐saving two‐dimensional digital Fourier transforms, i.e., with plane‐wave analysis. A simple method for selection of sampling windows and increments in x and κ (wavenumber) space which bounds and minimizes aliasing errors is presented and used. The theory is applied to the canonical problem of magnetic dipole excitation of a buried cylinder.
Radiation from nonuniform charged particle beams in ionospheric plasmas
Authors: Baker Dennis J. , Weil Herschel
Published: February 2008

The radiation from a class of nonuniform charged particle beams of finite dimensions moving through a magnetoplasma is studied. The beam structures include bunched beams which could result from natural interaction of plasma‐supported waves with the beam particles or those which could be generated by spacecraft‐borne particle accelerators. The generation of narrow band noise by beam structures generated by whistler wave‐beam particle interaction is illustrated. It is also shown how radiation by bunched particles remains finite in regions of refractive index resonances regardless of how slowly the particles move in contrast to the theoretical result for radiation from individual particles which unrealistically gives infinite radiated power. The theory developed includes thermal effects, including temperature anisotropy for the electrons and ions. Illustrative numerical results for the special case of a single electron with zero perpendicular velocity and a large axial velocity are given using two warm plasma theories. It is shown that near the upper hybrid resonance frequency, radiation into thermal and cold plasma modes is comparable.
Half plane edge diffraction
Authors: Senior T. B. A.
Published: February 2008

The concept of nonmetallic, resistive, and “conductive” sheets is discussed, and for a plane electromagnetic wave incident on a half plane composed of any one of these materials, the exact solution of the boundary value problem is obtained for incidence in a plane perpendicular to the edge. In each case the edge diffraction coefficient is proportional to a product of split functions arising from the Wiener‐Hopf method of solution. Since each function depends on only one angular variable, the coefficient for arbitrary directions of incidence and observation is expressible in terms of the backscattering coefficient for edge‐on incidence on a half plane with an equivalent impedance. This last is rather easily computed, and data are presented to illustrate the scattering behavior.
Diffraction tensors for imperfectly conducting edges
Authors: Senior T. B. A.
Published: February 2008

The imperfectly conducting edges considered are those of nonmetallic, resistive and “conductive“ half planes illuminated by a plane electromagnetic wave at arbitrary (oblique) incidence. By expressing the boundary conditions at the surface in terms of the normal components of the field and their normal derivatives, each problem is reduced to a combination of two scalar ones analogous to those for a plane wave incident in a plane perpendicular to the edge. From the explicit and exact solutions thus obtained, the edge diffracted fields are derived and expressed in terms of edge diffraction tensors. The tensors are relatively simple and compact. Their computation is discussed and data are presented to illustrate the behavior of the field.
The crossed antenna as an electric field probe with attached cable
Authors: King Ronold W. P.
Published: February 2008

The properties of an electric field probe that consists of two mutually perpendicular conductors attached to the end of a long coaxial line are investigated with a view to minimizing the pickup by the cable. The structure is treated as a junction‐loaded crossed‐wire antenna. Coupled integral equations for the currents are derived and solved approximately. It is shown that the contribution to the voltage across the load can be kept relatively small compared with the contributions by the active elements of the probe if a correct design is maintained. The analysis is carried out specifically for a normally incident plane wave with components of the electric field parallel to both the cable and the elements of the probe.
Determination of representative height distributions of the electron concentration in the ionosphere
Authors: Dudeney J. R. , Bradley P. A.
Published: February 2008

Median height distributions of electron concentration may be determined from ionograms by means of the composite ionogram technique of Wright, or by inserting tabulated parameters in the idealized model of Bradley and Dudeney. This paper discusses the complementary nature of these two approaches.
Experimental investigation of the scattering of electromagnetic waves from a model random medium of discrete scatterers
Authors: Olsen R. L. , Kharadly M. M. Z.
Published: February 2008

The paper describes a new three‐dimensional physical model of a random discrete‐scatterer medium and presents the results of an experimental investigation to determine the suitability of the model in studies aimed at the evaluation of approximate scattering theory. In contrast to previous models in which the scatterer statistics were indirectly controlled by a physical process, the present model allows the statistics to be directly controlled as in a Monte Carlo computer simulation. It also permits considerable flexibility in the type of scatterers used. The experimental model investigated consisted of a layered slab region of polystyrene foam (142 × 16 × 21 cm) in which polyethylene spheres of 1.27 cm diameter were embedded at random positions. These positions were generated by computer from a uniform distribution. The medium was scanned by a narrow 35‐GHz beam and the randomly varying transmitted field was recorded, sampled, and statistically analyzed. The accuracy of the first and second field moments, determined in experiments on the model with two different average densities of the spherical scatterers, is evaluated.
Wavelength dependence in radio‐wave scattering and specular‐point theory
Authors: Leonard Tyler G.
Published: February 2008

Radio‐wave scattering from natural surfaces contains a strong quasispecular component that at fixed wavelengths is consistent with specular‐point theory, but often has a strong wavelength dependence that is not predicted by physical optics calculations under the usual limitations of specular‐point models. Wavelength dependence can be introduced by a physical approximation that preserves the specular‐point assumptions with respect to the radii of curvature of a fictitious, effective scattering surface obtained by smoothing the actual surface. Wavelength dependence of the scattering, as expressed in terms of the mean‐square tilt of the smoothed surface, then depends on an integral of the two‐dimensional surface power spectral density. A uniform low‐pass filter model of the scattering process yields explicit results for the effective surface roughness versus wavelength. Interpretation of experimental results from planetary surfaces indicates that the asymptotic surface height spectral densities fall at least as fast as an inverse cube of spatial frequency. Asymptotic spectral densities for Mars and portions of the lunar surface evidently decrease more rapidly, at least as fast as an inverse fourth or fifth law of the spatial frequency.
Note on the theory of transmission of electromagnetic waves in a coal seam
Authors: Wait James R.
Published: February 2008

A coal seam is idealized as a resistive slab bounded by more conductive rock. The source is taken to be a vertical electric dipole. It is shown that the dominant mode has relatively low attenuation.
The history of the ITU, with particular attention to the CCITT and the CCIR, and the latter's relations with URSI
Authors: Smith Ernest K.
Published: February 2008

This paper is designed to provide members of URSI with background information about the International Telecommunication Union and its two international consultative committees (CCIs): the International Radio Consultative Committee (CCIR) and the International Telephone and Telegraph Consultative Committee (CCITT). Toward this end the history of the present ITU and its predecessor organizations, the International Telegraph Union and the Radio Telegraph Union, is reviewed. The history, structure, and schedule of the CCIs are then treated. Finally, it is suggested that, in the future, contacts between URSI and the CCIR will be both richer and of increasing importance.
The earth‐ionosphere cavity
Authors: Tran A. , Polk C.
Published: February 2008

To analyze ELF wave propagation in the earth‐ionosphere cavity, a flat earth approximation may be derived from the exact equations, which are applicable to the spherical cavity, by introducing a second‐order or Debye approximation for the spherical Hankel functions. In the frequency range 3 to 30 Hz, however, the assumed conditions for the Debye approximation are not satisfied. For this reason an exact evaluation of the spherical Hankel functions is used to study the effects of the flat earth approximation on various propagation and resonance parameters. By comparing the resonance equation for a spherical cavity with its flat earth counterpart and by assuming that the surface impedance Zi at the upper cavity boundary is known, the relation between the eigenvalue ν and Sν, the sine of the complex angle of incidence at the lower ionosphere boundary, is established as ν( ν + 1) = (kaSν)2. It is also shown that the approximation ν(ν + 1) ≅ (ν + ½)2 which was used by some authors is not adequate below 30 Hz. Numerical results for both spherical and planar stratification show that (1) planar stratification is adequate for the computation of the lowest three ELF resonance frequencies to within 0.1 Hz; (2) planar stratification will lead to errors in cavity Q and wave attenuation which increase with frequency; (3) computation of resonance frequencies to within 0.1 Hz requires the extension of the lower boundary of the ionosphere to a height where the ratio of conduction current to displacement current, (σ/ωε0), is less than 0.3; (4) atmospheric conductivity should be considered down to ground level in computing cavity Q and wave attenuation.
Model of equatorial scintillations from in‐situ measurements
Authors: Basu Sunanda , Basu S. , Khan B. K.
Published: February 2008

In‐situ measurements of F‐region irregularity amplitude and ambient electron density made by the retarding potential analyzer on OGO‐6 near the perigee altitude of 400 km have been utilized to derive the variation of electron density deviation over the equatorial region. Based on these measured electron density deviations and other assumed model parameters including a three‐dimensional power law form of irregularity spectrum with index 4, a model of equatorial scintillations is developed in the framework of diffraction theory. The percentage occurrence contours of estimated equatorial scintillations ≥ 4.5 dB at 140 MHz during 1900–2300 LMT for the period November–December 1969 and 1970 have been derived. The model is found to depict a pronounced longitude variation with the scintillation belt width and percentage occurrence being maximum over the African sector. The latitude extent of the spatial scintillation belt narrows over the American sector without much decrease in the scintillation occurrence whereas over the Indian and Far Eastern sectors both the extent and the occurrence are found to decrease. The percentage occurrence of scintillations estimated from this model is found to be consistent with VHF scintillation measurements at Ghana, Huancayo, and Calcutta. In addition, the model was found to be in qualitative agreement with GHz observations at various longitudes made by the COMSAT group. The effects of varying model parameters on scintillation estimates at VHF, UHF, and GHz are discussed. Implications of the observed longitudinal variation of scintillations on current theories of equatorial irregularity formation are indicated.
Some observations of anomalous depolarization on 19 and 12 GHz earth‐space propagation paths
Authors: Cox D. C. , Arnold H. W. , Rustako A. J.
Published: February 2008

During the summer of 1976 anomalous depolarization events associated with precipitation have been observed on 19 and 12 GHz earth‐space propagation paths. These events are characterized by strong depolarization (> −20 dB) occurring with low signal attenuation (often <1 dB). They are not explained by contemporary atmospheric propagation theory. The anomalous events could significantly affect performance of satellite communications systems using the same frequencies on two orthogonal polarizations.
On the investigation of diffracted fields at the shadow boundaries of staggered parallel plates—A spectral domain approach
Authors: Rahmat‐Samii Y. , Mittra R.
Published: February 2008

The diffraction of plane waves by two staggered parallel plates is investigated using the spectral diffraction coefficient in the Fourier transform domain. It is shown that when the interaction between the plates is negligible, the total field can be represented in terms of a double Fourier integral, a result that is identical to the one derived by Jones via the Wiener‐Hopf technique. The double integral is asymptotically evaluated via the saddle‐point integration technique and compact expressions for the field are derived, up to the order k−1, for observation points located at the shadow boundaries and also away from the transition regions. These expressions are compared with the results of other asymptotic theories.
Antenna concepts for interstellar search systems
Authors: Basler Roy P. , Johnson George L. , Vondrak Richard R.
Published: February 2008

An evaluation is made of microwave receiving systems designed to search for signals from extraterrestrial intelligence. Specific design concepts are analyzed parametrically to determine whether the optimum antenna system location is on earth, in space, or on the moon. Parameters considered include the hypothesized number of transmitting civilizations, the number of stars that must be searched to give any desired probability of receiving a signal, the antenna collecting area, the search time, the search range, and the cost. This analysis suggests that (1) search systems based on the moon are not cost‐competitive, (2) if the search is extended only a few hundred light years from the earth, a Cyclops‐type array on earth may be the most cost‐effective system, (3) for a search extending to 500 light years or more, a substantial cost and search‐time advantage can be achieved with a large spherical reflector in space with multiple feeds, (4) radio frequency interference shields can be provided for space systems, and (5) cost can range from a few hundred million to tens of billions of dollars, depending on the parameter values assumed.
Stimulated plasma waves in the ionosphere
Authors: Benson Robert F.
Published: February 2008

The results of 15 years of plasma resonance observations from rocket‐ and satellite‐borne topside sounders are summarized. The major resonances are observed at the electron plasma frequency ƒN, the electron cyclotron frequency ƒH, the upper hybrid frequency ƒT = (ƒ2N+ƒ2H)1/2, the harmonics 2ƒT and nƒH with n > 1, and two series of resonances that appear between nƒH∶ the ƒQn resonances observed above ƒT (at the maximum frequency of the Bernstein modes) and the ƒDn resonances observed below ƒT (called the diffuse resonances). Most of the resonances can be interpreted in terms of the reception of longitudinal plasma waves stimulated by the sounder pulse. In some cases these waves, which travel with low group velocity (of the order of the electron thermal velocity or less) are received after being reflected in the nearby medium (within a distance of several kilometers). The reflections are due to the extreme sensitivity of the dispersion curves near the resonant frequencies to small changes in electron density (for the ƒN and ƒT resonances) or to small changes in magnetic field strength (for the lower‐order nƒH resonances). In other cases (ƒQn resonances and the higher‐order nƒH resonances) the signal reception is due to the matching of a component of the wave group velocity to the satellite velocity rather than to a wave reflection process. The ƒDn resonances involve a sounder‐stimulated electron temperature anisotropy leading to the Harris instability of the Bernstein modes and a nonlinear coupling between these wave‐modes. Since the interpretation of all of the resonances depends on warm plasma theory, the observations can be used to obtain the electron temperature T⊥ and T∥ (corresponding to electron motions perpendicular and parallel to the ambient magnetic field direction, respectively) as well as the electron density and the magnetic field intensity. These measurements pertain to a large volume (hundreds of meters to kilometers) around the antenna and thus are not seriously affected by spacecraft/plasma interactions. The results of the investigations of artificially stimulated plasma waves by sounder pulses have been applied to the interpretation of naturally occurring wave phenomena such as the (n+1/2)ƒH magnetospheric VLF emissions, the terrestrial kilometric radiation, and the Jovian decametric radiation. Among the major unexplained plasma resonance phenomena are the resonances observed at ƒH and 2ƒT, and the numerous ion effects associated with the electron plasma resonances. The advent of the Space Shuttle with subsatellites will enable active‐controlled experiments to be conducted with highly flexible radio‐frequency instrumentation in order to investigate problems of interest to many geophysical and astrophysical phenomena such as wave mode coupling and the evolution of nonlinear and plasma wave instability processes.
Microscopic Lagrangian description of warm plasmas: 3. Nonlinear wave‐particle interaction
Authors: Galloway J. J. , Crawford F. W.
Published: February 2008

The averaged‐Lagrangian method is applied to nonlinear wave‐particle interactions in an infinite, homogeneous, magnetic‐field‐free plasma. The specific example of Langmuir waves is considered, and the combined effects of four‐wave interactions and wave‐particle interactions are treated. It is demonstrated how the latter lead to diffusion in velocity space, and the quasilinear diffusion equation is derived. The analysis is generalized to the random phase approximation. The paper concludes with a summary of the method as applied in Parts 1–3 of the paper.
Radio‐frequency radiation levels in urban areas
Authors: Janes D. E. , Tell R. A. , Athey T. W. , Hankin N. N.
Published: February 2008

Data on environmental levels of radio‐frequency energy have been obtained for Atlanta, Boston, Miami, and Philadelphia; about 5% of the population of the United States lives within these Standard Metropolitan Statistical Areas. Data are collected at 15 to 30 sample sites within each metropolitan area. Earlier investigations led to the working hypothesis that the principal sources of environmental radio‐frequency radiation are contained within the broadcast service. The frequency bands measured are the standard AM broadcast band (0.5 to 1.6 MHz), the VHF‐television bands (54 to 88 and 174 to 216 MHz), the FM‐radio band (88 to 108 MHz), two land‐mobile bands (150 and 450 MHz), and the UHF‐television band (470 to 890 MHz). Representative spectra are presented. Values of power density integrated over the frequency range from 54 to 900 MHz generally fall into the range between 0.001 and 1 μ/cm2 with most values between 0.01 and 0.1 μW/cm2.
Currents and charges induced by a normally incident plane wave on single and crossed tubular cylinders with ka = 2
Authors: King R. W. P. , Blejer D. J. , Wan S.‐K. , Burton R. W.
Published: February 2008

The theoretical and experimental determination of the surface densities of current and charge induced on tubular conducting cylinders and crossed cylinders by a normally incident plane wave is described when the cylinders are two wavelengths in circumference (ka = 2 ). Graphs of calculated and measured distributions of charge and current on single cylinders in E‐ and H‐polarized fields and on crossed cylinders are displayed and discussed with special reference to earlier work on cylinders and crossed cylinders with ka = 1. Difficulties arising from propagating waveguide modes and a nonplanar incident field are considered.
Polarization dependence of transient signals in evaporation ducts
Authors: Zimmer H. , Langenberg K. J. , Becker K.‐D.
Published: February 2008

The evaporation duct model used is that of Kahan and Eckart and consists of a discontinuous drop of the otherwise constant relative permittivity at the upper duct boundary. The earth is assumed to be a perfect conductor and ideally plane. We determine the electrical field strength exactly at some fixed point within the duct layer, having chosen a certain polarization of the primary source whose moment is allowed to vary arbitrarily in time. The method used for solution is essentially based on the application of two functional transforms and Cagniard's method for their inversion. Physically, the applicability of Cagniard's idea is based on the evaluation of the field in a series of image sources of the primary source. The field strengths of the image sources combine to the total field strength with different signs for the two polarizations apart from the fact that they are of different mathematical structure. Hence we can give a physically intuitive description of the polarization dependence of the time history of the electrical field strength. Furthermore, a number of numerical examples clearly show the different effects of the evaporation duct on differently polarized carrier‐modulated pulses. It turns out that for certain values of the carrier frequency the evaporation duct causes different arrival times for TE and TM polarized pulses, which is explained by their different modal cutoff frequencies.
Vegetation modeled as a water cloud
Authors: Attema E. P. W. , Ulaby Fawwaz T.
Published: February 2008

Because the microwave dielectric constant of dry vegetative matter is much smaller (by an order of magnitude or more) than the dielectric constant of water, and because a vegetation canopy is usually composed of more than 99% air by volume, it is proposed that the canopy can be modeled as a water cloud whose droplets are held in place by the vegetative matter. Such a model was developed assuming that the canopy “cloud” contains identical water droplets randomly distributed within the canopy. By integrating the scattering and attenuation cross‐section contributions of N droplets per unit volume over the signal pathlength through the canopy, an expression was derived for the backscattering coefficient as a function of three target parameters: volumetric moisture content of the soil, volumetric water content of the vegetation, and plant height. Regression analysis of the model predictions against scattering data acquired over a period of four months at several angles of incidence (0°–70°) and frequencies (8–18 GHz) for HH and VV polarizations yields correlation coefficients that range from .7 to .99 depending on frequency, polarization, and crop type. The corresponding standard errors of estimate range from 1.1 to 2.6 dB.
The height distribution of refractive index structure parameter in an atmosphere being modified by spatial transition at its lower boundary
Authors: Gossard Earl E.
Published: February 2008

An approximate method for solution of problems in turbulent diffusion is described and applied to radio wave propagation in the lower atmosphere for some boundary value problems not previously examined in the literature. The results of the approximate method are compared with results of a special case for which the diffusion equation can be solved exactly. The height distribution of the refractive index structure parameter and the thickness of the radio duct are calculated for air moving over a lower surface that (a) changes refractive index abruptly (e.g., a shoreline), (b) changes refractive index continuously, or (c) suddenly increases or decreases surface roughness.