bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Background Winds on Atmospheric Gravity Waves
Authors: Shere K. D. , Bowhill S. A.
Published: February 2008

A dispersion relation is obtained for gravity waves propagating in an atmosphere with a horizontal background wind that is variable with altitude. Also, a full‐wave solution valid in a region containing a critical level is obtained for the case of a background wind varying linearly with altitude.
Commission 3: Progress in Ionospheric Radio
Authors: Villard O. G. , Aarons J. , Bibl K. , da Rosa A. V. , Kelso J. M. , Leadabrand R. L. , Nagy A. F. , Reid G. C. , Sechrist C. F. , Smith E. K. , VanZandt T. E. , Wait J. R.
Published: February 2008

The major advances in our understanding of the morphology of the D region in the last three years have come about through the application of rocket techniques. A series of rocket measurements carried out at Wallops Island [Mechtly and Smith, 1968a] at different seasons but at roughly the same solar zenith angle have shown the principal features of the seasonal variation in the electron density profile between 50 and 100 km. It appears that electron densities are lower by a factor of about 3 below 82 km on ‘normal’ winter days as compared with summer days for a solar zenith angle of 60°. The anomalous winter days of high absorption have been the subject of a separate rocket investigation [Sechrist et al., 1969], which has shown that the electron density profile is remarkably similar to that of a normal summer day and that in at least one case was associated with a pronounced temperature inversion near 70 km.
Commission 6: Progress in Radio Waves and Transmission of Information: 1. Radio Waves
Authors: Felsen L. B. , Marcuvitz N. , Mittra R. , Rhodes D. R. , Senior T. B. A.
Published: February 2008

In this area, by far the greatest effort has been focused on arrays, particularly phased arrays; the results have been summarized in three books [Hansen, 1966; King et al., 1968] and two special journal issues [Radio Science, 1968; IEEE, 1968]. Complete design formulas for scannable Baklanov‐Chebyshev arrays [Tseng and Cheng, 1968] and new approximations for long Dolph‐Chebyshev arrays [Drane, 1968] were developed. Infinite array models dominated in numerous studies of large phased arrays. Grating lobe effects were investigated by the induced emf method [Stark, 1966] and by use of impedance craters [Wheeler, 1966] Important and unexpected element pattern nulls not attributable to grating lobes have been observed and explained as coupling accumulation [Lechtreck, 1968] and as external resonances in dielectric sheets [Knittel et al., 1968; Wu and Galindo, 1968]. They have also been observed as mismatches in array simulators [Hannan, 1967a]. Proof of the possibility of impedance matching for all scan angles has been offered [Hannan, 1967b] and contested [Varon and Zysman, 1968] Limited control of coupling for wide angle impedance matching has been achieved by means of dielectric sheets [Magill and Wheeler, 1966], hole coupling of waveguides [Amitay, 1966], multiple modes in waveguides [Tang and Wong, 1968], and parasitic chokes [Dufort, 1968], and the form of asymptotic decay of coupling with distance has been established [Galindo and Wu, 1968]. Systematic procedures for the design of phased array elements have appeared [Wheeler, 1968; Amitay et al., 1968], as have methods of analysis of waveguide phased arrays without [Farrell and Kuhn, 1968], and/or with [Borgiotti, 1968a] dielectric sheets. Finally, there was developed an important geometrical description of phased array behavior, based on the fundamental principle of planar apertures, that unifies much of the existing knowledge of infinite arrays [Diamond, 1968]. A thorough application of this development to arrays of rectangular waveguides shows clearly the individual roles of the lattice and the elements [Ehlenberger et al., 1968]. The same principle was used to compute mutual admittances between slots [Borgiotti, 1968b], and formulas for observable stored energies of planar apertures have been derived from it [Rhodes, 1966] and contested [Borgiotti, 1967; Collin, 1967].
Commission 7: Progress in Radio Electronics
Authors: Heffner H. , Siegman A. E. , Whinnery J. R.
Published: February 2008

The advances of the past 3 years in the science and technology of quantum electronics are well summarized by the technical programs of the May 1968 International Conference on Quantum Electronics, held in Miami [IEEE, 1968a], and of the May 1969 Conference on Laser Engineering and Applications, held in Washington, D. C. [IEEE, 1969a]. The pace of development of new laser devices, new techniques, and new basic quantum electronic phenomena does not seem to be slowing, even though the maser concept has been with us for two decades, and working laser devices for one decade. On the technological side, the development of practical laser applications has obviously reached and passed the take‐off point, with new and different practical applications of lasers emerging at a rapid rate in an almost uncountable number of applications areas.
Review of Techniques in Transhorizon Propagation Research and their Relationship to Parameters of the Troposphere
Authors: Cox Donald C.
Published: February 2008

Transhorizon propagation techniques and their usefulness in determining atmospheric parameters are discussed. It is concluded that the state or structure of the atmosphere cannot be inferred unambiguously from individual measurement techniques used in experiments reported to date. However, because of the sensitivity of the transhorizon radio signals to changes in the parameters of the troposphere, it is suggested that combinations of available techniques be pursued to determine the parameters uniquely.
Structure of Vertical Wind Profiles
Authors: Reiter Elmar R.
Published: February 2008

An exponential relationship between the vertical wind shear and the layer thickness over which this shear is measured has been derived. From an analysis of the Richardson number it appears that shallow layers reaching maximum shearing conditions under the established power law should be turbulent, irrespective of the prevailing thermal stability conditions.
Spectral Interdependence of the Radio Refractivity and Water Vapor in the Atmosphere
Authors: Bean B. R. , Emmanuel C. B.
Published: February 2008

Continuous measurements of temperature, water vapor, and radio refractivity were taken over several days at a height of 11 meters above ground over the high plains of Colorado. A spectral analysis indicates that temperature fluctuations contribute little to radio refractivity fluctuations, which are primarily caused by variations in water vapor density over a wide range of spectral frequencies.
Troposcatter Signal Characteristics Simulated by Random Layered Scatterer Arrays
Authors: Birkemeier W. P. , Fontaine A. B. , Gerks I. H. , Pittle R. D.
Published: February 2008

Phase and amplitude of simulated troposcatter signals have been computed as functions of time, using thin layers of randomly spaced point scatterers moving horizontally and uniformly, normal to the path. The layer heights and cross‐wind speeds were chosen to agree with the atmospheric conditions known to exist in two radio experiments. Comparisons of the simulated and actual radio signals show good agreements in amplitude fading rate and average Doppler shift versus antenna pointing angle in one case when the scattering angular dependence used in the simulation matched the classical −11/3 power‐law isotropic refractivity spectrum. In the other case, where slower fading was observed, an anisotropic model of the refractivity spectrum produced excellent agreement between the simulated and radio data. Knowledge of the cross‐wind speed allowed the degree of anisotropy for the model to be evaluated.
Nonlinear ‘Explosive’ Instabilities: Do They Exist in the Atmosphere?
Authors: Wilhelmsson H.
Published: February 2008

The question whether or not nonlinear explosive instabilities exist in the atmosphere is raised.
Spectra of Anisotropic Turbulence in the Atmosphere
Authors: Borkowski Janusz
Published: February 2008

In this paper the spectral distribution of turbulent energy is considered. The turbulence is assumed to be homogeneous and axisymmetric with a vertically directed axis of symmetry.The spectral representation of the velocity correlation equation was the starting point for determining the spectra of turbulent energy. Transfer of energy in axisymmetric turbulence was approximated by adopting a Heisenberg‐type hypothesis. The general solution obtained presents as special cases the −5/3 law and the −11/5 law for the spectrum of total energy.It is shown that the spectrum of the vertical energy behaves like k−5/3 for large wave numbers and like k−5/3 for buoyancy subrange. The spectrum of temperature fluctuations is also investigated.
Irregularities in Ionization of the Nighttime D Region Deduced from Vertically Incident VLF Radio Waves
Authors: Gossard Earl E.
Published: February 2008

Results of a spaced‐receiver experiment using several very low radio frequencies to probe the irregular structure of the lower ionosphere at night are presented. A multiple frequency sounding facility was used whose transmissions were received via a nearly vertically incident first‐hop sky wave. The present paper describes some of the statistical characteristics of the phase and amplitude fluctuations in the diffraction pattern at the ground. The data are compared with a theoretical analysis of the diffracting properties of an irregular screen [Bowhill, 1961] to determine the feasibility of using a single receiver near a standard VLF transmitter to continuously monitor the scale size of the irregularities in the nighttime D region by measuring the phase and amplitude of the first‐hop sky wave. Some implications are pointed out regarding the functional forms of the spatial spectrum and the correlogram of the irregularities at the height of reflection.
Inferences From the Optical Properties of the Moon Concerning the Nature and Evolution of the Lunar Surface
Authors: Hapke Bruce
Published: February 2008

The unusual optical properties of the moon are summarized. They imply that the fine‐grained, compressible material observed by the Surveyor television cameras is representative of the entire lunar surface and very probably of the surfaces of Mercury, Mars, and the asteroids also. The soil is the result of repeated meteorite impacts and is not an ash flow. Evidence for darkening is examined, and it is concluded that a process is acting on the lunar surface to alter the albedo and color of the rock powder. Materials of chondritic or granitic composition are not prevalent on the moon. Differences in the optical properties of the maria and the highlands imply that the soil cover is thicker in the highlands than in the maria and that the high‐land soils are laterally more homogeneous than are those of the maria.
Preliminary Results of Measurements by Space Probes Venera 5 and Venera 6 in the Atmosphere of Venus—Summary*
Authors: Avduevsky V. S. , Marov M. Ya , Rozhdestvensky M. K.
Published: February 2008

The automatic interplanetary stations Venera 5 and Venera 6 reached Venus on May 16 and 17, respectively, and made measurements of the nightside atmosphere in the equatorial plane at a distance of 2700 km from the dawn terminator. The most probable distance between the points where the stations entered the atmosphere was about 300 km. The detailed description of the probe design may be found in Pravda [1969].

The present paper analyses parameters of the planetary atmosphere based on the results of the direct measurements of pressure and temperature. A tentative model of the atmosphere can be constructed on the basis of the Venera 5 and Venera 6 in‐situ measurements together with the earlier results of Venera 4 [Avduevsky et al., 1968, 1969] and Mariner 5 [Kliore et al., 1969].
Mercury: Recent Observations at 3.75‐cm Wavelength—Summary
Authors: Klein M. J.
Published: February 2008

This paper summarizes the results of 84 measurements of Mercury at 3.75‐cm wavelength. These data show that the microwave disk temperature varies with phase angle and with hermographic longitude and that the ratio of the electrical to thermal skin depths in Mercury's epilith is approximately unity at 1‐cm wavelength.
Radar Mapping of Venus With Interf Erometric Resolution of the Range‐Doppler Ambiguity
Authors: Rogers Alan E. E. , Ingalls Richard P.
Published: February 2008

A map of the surface reflectivity of Venus at a wavelength of 3.8 cm is obtained by using a fixed base line radar interferometer. The two‐fold hemispheric ambiguity in the range‐Doppler map has been resolved by interferometry. The map covers a region extending approximately from −80° to 0° longitude (Carpenter's definition) and from −50° to 40° in latitude. The map shows many new features in addition to delineating clearly features already observed. Large circular regions with a radar appearance similar to lunar maria are among the newly observed features. Analysis of the method employed in resolving the hemispheric ambiguity shows that the relatively small range of projected base line change available produces only small sidelobes in the noise level.
Heights of Ionospheric Irregularities at a Sub‐Auroral Latitude
Authors: Kidd William C.
Published: February 2008

The heights of ionospheric irregularities were measured at the Sagamore Hill Radio Observatory, 42.6°N, 70.8°W. Beacon signals at 40.01 MHz from Explorer 22 were recorded at spaced receivers during the summer and fall of 1968 and during a short period in the winter of 1969. By using the mean time differences between 1‐second samples at each receiver and by using the method of similar fades, the heights of dominant irregularities were found to vary between 130 and 900 km. Scale size (determined from the height measurement and the time between major peaks of the diffraction patterns) increased from 1 km at a height of 200 km to 4 km at a height of 800 km. Two large geomagnetic storms occurred during the test periods. Results were compared with simultaneous measurements of total electron content. The mean height of the irregularity region decreased by 100 km during periods of major depression of total electron content; this decrease suggests that the irregularities are distributed throughout the F region for both quiet and disturbed conditions.
ELF and VLF Propagation for Models of a Perturbed Ionosphere
Authors: Galejs Janis
Published: February 2008

Calculated parameters of wave propagation illustrate the effects of various choices of ion collision frequencies and molecular weights. Comparisons with atmospheric conductivity measurements tend to justify the use of ion collision frequencies that exceed the collision frequency between neutral particles and of molecular weights that are larger than the molecular weights for the usually assumed atmospheric constituents.
A Heat‐Equation Approach to Mixed Ray and Modal Representations of Green's Functions for ∇2 + κ2
Authors: Kurss Herbert
Published: February 2008

A Green's function G for ∇2 + κ2 is interpreted essentially as a Laplace transform of a Green's function H for ∇2 ‐ ∂/∂t. The Laplace integral is evaluated by selecting a mixing parameter T and representing H by rays in (0, T) and modes in (T, ∞). This procedure enables one to systematize, simplify, and extend the scope of the technique originated by Ewald (1916). As an illustration, G for a parallel‐plate wave guide is detailed.
Foreword
Authors: Findlay John W.
Published: February 2008

The articles in this issue were selected from 40 papers presented at a symposium in Charlottesville, Virginia, on April 13 through 15, 1970. The symposium was held at the National Radio Astronomy Observatory and was sponsored by URSI and the NRAO. The members of the organizing committee were: Prof. M. H. Cohen, California Institute of Technology; Dr. J. W. Findlay, National Radio Astronomy Observatory; Dr. D. L. Jauncey, Center for Radiophysics and Space Research, Cornell University; Dr. K. I. Kellermann, National Radio Astronomy Observatory; Dr. J. L. Locke, National Research Council, Ottawa, Canada; Prof. O. E. H. Rydbeck, Chalmers University of Technology, Gothenburg, Sweden; and Dr. V. V. Vitkevich, Lebedev Physical Institute, Moscow, USSR.

The purpose of the symposium was the exchange of information between radio astronomers and workers in disciplines such as geodesy and relativity, in which the very long baseline interferometry technique can be valuable.
Very Long Baseline Interferometry of Galactic OH Sources
Authors: Ronnang B. O. , Rydbeck O. E. H. , Moran J. M.
Published: February 2008

Several hydroxyl radical (OH) microwave emission sources were studied in July 1969 with very long baseline interferometry (VLBI). The elements of the interferometer were the 84‐foot telescope of the Onsala Space Observatory at Chalmers University of Technology, Sweden, and the 120‐foot telescope of the Haystack Microwave Research Facility at Lincoln Laboratory, MIT; a baseline of 31.1×106 wavelengths, or a fringe spacing of 0.0066 sec of arc at a frequency of 1665 MHz, resulted. Earlier spectral line interferometric observations have given detailed information on the angular sizes and spatial separations of the 1665‐MHz OH sources in the W3 source. The measurements reported here confirm these results and give additional information on the complex structure of the various features. Four other galactic OH emission sources with unknown angular sizes were also investigated. Only one (the 5‐km/sec component in the 1667‐MHz spectrum of W49) gave reliable fringes.
Oscillator Synchronization via Satellite
Authors: Wood L. E. , Thompson M. C.
Published: February 2008

A technique is proposed for the synchronization of widely separated microwave oscillators via satellite transponder. As a result of calculations of the phase shifts over the paths, computation of expected signal‐to‐noise ratios, and experiments on a 15‐km line of sight path, synchronization to within 0.05 cycle rms for 10‐GHz oscillators should be possible.
Plasma Sheet Convection Velocities Inferred From Electron Flux Measurements at Synchronous Altitude
Authors: Shelley E. G. , Johnson R. G. , Sharp R. D.
Published: February 2008

Large increases in the low energy electron fluxes (0.5 to 50 kev) are observed to occur at synchronous altitude on most nights in the local evening or midnight sectors. These increases are usually found to be associated with magnetic substorms. The time of increase in the fluxes is found to be energy dependent, with the lower energy electrons nearly always being observed first for the evening and midnight events. If one interprets these flux increases as resulting from an inward convection of the plasma sheet in connection with a substorm, one can estimate the convection velocity at synchronous altitude from the measured energy dependent time of flux increase and earlier measurements [Vasyliunas, 1968; Schield and Frank, 1970; Frank, 1970] of the energy dependent radial structure of the inner edge of the plasma sheet. The median velocity estimated from 38 cases by this technique is approximately 3 km/sec. This implies a westward electric field of approximately 0.36 mv/m, a value consistent with other measurements of this field.
The Mean Green's Function: A Nonlinear Approximation
Authors: Rosenbaum Shalom
Published: February 2008

In this paper, we examine the behavior of the coherent Green's function in a randomly fluctuating, unbounded, and statistically homogeneous medium. The starting point of the analysis is a ‘nonlinear’ approximation of the Dyson equation which has been pointed out by several investigators. Although exact solutions seem out of reach, owing to the inherent complexity of the problem, approximate solutions that are applicable in the limits of small and large‐scale fluctuations can be obtained. The large‐scale fluctuations are emphasized in this paper. Both one‐ and three‐dimensional fluctuations are considered, and explicit calculations are made for the case in which an exponential correlation governs the refractive‐index fluctuations. The solutions obtained are investigated in various limiting cases for which closed‐form results are derived.
Characteristic Signatures of the Midlatitude Ionosphere Observed With a Narrow‐Beam HF Backscatter Sounder
Authors: Hunsucker Robert D.
Published: February 2008

Over 18,000 frames of data acquired with a narrow‐beam azimuth and elevation scan high‐frequency backscatter sounder located at Boulder, Colorado, during the period October 1964 through June 1968 have been examined and classified. Analysis of these data has revealed that the ‘irregular structure’ of the midlatitude ionosphere is the rule rather than the exception. Irregularities of varying scale size and apparent motion were present in about 90% of the observations made during almost one‐half a sunspot cycle. The ‘signatures’ observed by this HF radar system have been categorized into eight generic types, which have been labeled with names roughly describing their appearance on the range‐azimuth scan record. The relative diurnal and seasonal occurrences, as well as the qualitative sunspot cycle and geomagnetic correlation of these signatures, are presented.
A Survey of HF Interferometry for Ionospheric Propagation Research
Authors: Sherrill W. M.
Published: February 2008

The transform relation between the field distribution and the angular spectrum forms the basis for the application of interferometry to propagation research. It is convenient to divide interferometry into instrumental combination and transform techniques which measure these Fourier relations to provide propagation mode resolution in terms of direction and angular width. Conventional direction‐finder instrumentation represents the limiting case of single plane wave solution to the interference field. Computer instrumented interferometers and sampling arrays provide the capability for more general wave field analysis based on non plane wave models. This paper is a summary of interferometry techniques for direction finding, angular spectrum, angular width, and coherence ratio measurements and reviews the evolution of interferometers for propagation research. The reference list includes works from the radio astronomy and propagation literature relevant to fundamental interference analysis and interferometer instrumentation.
Modified Born Backscattering from Turbulent Plasmas: Attenuation Leading to Saturation and Cross‐Polarization
Authors: Shkarofsky I. P.
Published: February 2008

An expansion scheme, which converges rapidly, is developed for computing the backscattered cross section of electromagnetic waves from turbulent media. The scheme is based on the average rather than actual field with the use of the distorted‐wave Born approximation, in which the signal is modified to include attenuation due to scattering along its path length. If the receiver beamwidth is wide enough and the scattering is very directional, no net attenuation occurs. Net loss occurs when the receiver aperture is small and the scattering is diffusive. First‐order modified Born is derived in detail, and relations are evaluated. It has the potential for providing saturation in the backscattered power as the level in refractive‐index fluctuation is increased. Higher‐order corrections are then calculated. Second‐order modified Born provides a contribution to the transverse cross‐polarization that is consistent with a modified version of the result in work by R. S. Ruffine and D. A. deWolf. Both of these in turn are consistent, within the limits of directional scatter, with J. W. Strohbehn and S. F. Clifford's cross‐polarization, which results from fluctuations in refractive index gradients. It is also shown that the longitudinal cross‐polarized component is related to diffusion effects and angle‐of‐arrival deviations. However, in several experiments the postulated attenuation implicit in first‐order modified Born was found to be compensated for by other effects, probably by higher‐order terms.
The Radio Astronomy Explorer Satellite, a Low‐Frequency Observatory
Authors: Weber R. R. , Alexander J. K. , Stone R. G.
Published: February 2008

The Radio Astronomy Explorer satellite (RAE‐1) is the first spacecraft designed exclusively for radio astronomical studies. It is a small, but relatively complex, observatory including two 229‐meter antennas, several radiometer systems covering a frequency range of 0.2 to 9.2 MHz, and a variety of supporting experiments such as antenna impedance probes and TV cameras to monitor antenna shape. Since its launch in July, 1968, RAE‐1 has sent back some 1010 data bits per year on measurements of long‐wavelength radio phenomena in the magnetosphere, the solar corona, and the galaxy. In this paper we describe the design, calibration, and performance of the RAE‐1 experiments in detail.
Measuring the Vertical Angles of Arrival of HF Skywave Signals With Multiple Modes
Authors: Kelso John M.
Published: February 2008

The measurement of the vertical angle of arrival of HF skywave, multimode signals generally requires either highly complicated mathematical manipulations or observing techniques that necessitate a waiting period until the relative phases among the modes either reach a desired condition or pass through a desired range of variations. This paper utilizes an extenson of a very old method (the Prony method, c. 1795) for approximating sums of exponential functions. The technique uses relatively simple calculations that are based on an instantaneous (or nearly so) set of samples from a uniformly spaced, vertically disposed array of antenna elements. Unlike many other methods, this technique requires neither time variations of phase nor a perfectly reflecting ground plane, and is valid for arbitrary polarization of the (similar) antenna elements and for any general wave polarization. Experimental results from some field tests of the process for vertical angle estimation are presented.
Synthesis of Data Obtained at Jicamarca, Peru, During the September 11, 1969, Eclipse
Authors: Sterling D. L. , Hanson W. B. , Woodman Ronald F.
Published: February 2008

Continuous measurements of electron temperatures, electron concentrations, and vertical plasma drift were made at Jicamarca during the September 11, 1969, solar eclipse. Control measurements were made on September 13, 1969. Synthesis of the observed electron concentrations was made, using a model that solved the time‐dependent electron‐ion continuity equations. Ions O+, NO+, O2+, and N2+ were treated. Physical and chemical processes pertinent to the F region were considered in a realistic way. At noon, 300 km above Jicamarca, we obtained the values q = 290−3 sec−1 for the O+ production rate, β = 1.94 × 10−4 sec−1 for the loss coefficient, and H = 59 km for the atomic oxygen scale height. All large‐scale features observed in the F2 region were synthesized by the model. This synthesis was not sensitive to a simultaneous 30% reduction of q and β
Modeling of Atmospheric Noise
Authors: Giordano Arthur A. , Haber Fred
Published: February 2008

Atmospheric noise resulting from lightning discharges is modeled as a Poisson‐related process that depends on two major physical factors: (a) the world distribution of thunderstorms and (b) the prevailing propagation conditions. In this work theoretical models of atmospheric noise have been developed by postulating physically motivated combinations of source distributions and propagation conditions. Then the noise statistics at the output of a narrow‐band receiver for each combination are calculated. Particular noise statistics that are computed for several cases include the amplitude‐probability distribution, the average envelope‐level crossing rate, and the pulse‐spacing distribution. The models postulated are found to be consistent with both experimental and prior theoretical results.
The Atmosphere Explorer spacecraft system
Authors: Spencer N. W. , Brace L. H. , Grimes D. W.
Published: February 2008

A short description is presented of the design goals, the spacecraft, and the data system and analysis concept for the Atmosphere Explorer mission.
A wave‐optical field solution for a line source over nonparallel stratified earth
Authors: Wait James R.
Published: February 2008

We consider a wedge geometry for a model of a nonuniform layered earth. The plane wave spectrum of a line source excitation is allowed to reflect successively at the nonparallel interfaces. The semiconvergent series is truncated when the lowermost image term has been accounted for. The partial sum, so obtained, is considered to give a useful approximation to the resultant fields. It is found that there is a significant dependence on the tilt angle of the subsurface interface.
An improved equation for the radio refractive index of air
Authors: Thayer Gordon D.
Published: February 2008

New constants are derived for the equation relating the radio refractive index, n, at frequencies below 20 GHz to the pressure, temperature, and humidity of the air. The new equation is

urn:x-wiley:00486604:media:rds876:rds876-math-0001
where Pα is the partial pressure of dry air (mb), T is the absolute temperature (K), e is the partial pressure of water vapor (mb), and Zα−1 and Zw−1 are the inverse compressibility factors for dry air and water vapor, respectively. Improved values of the constants are obtained by considering the relationship between the radio and optical portions of the electromagnetic spectrum. With the use of the full three‐term equation including non‐ideal gas law effects, the accuracy of the new radio refractive index equation ranges from 0.018%, or about 0.05N, for dry air to 0.048%, or 0.21N, for extremely moist air.
Numerical results for a cross‐modulation problem in a rarefied nonlinear plasma
Authors: Bassanini Piero
Published: February 2008

The problem of cross‐modulation of the electromagnetic waves generated by two oscillating electric dipoles placed in the center of a spherical cavity surrounded by a nonlinear plasma is considered. One of the dipoles is amplitude‐modulated with low (acoustic) frequency, and the plasma is assumed slightly ionized, rarefied, and cold. Second‐harmonic generation is investigated by means of a perturbative technique in the frame of the “quasi‐hydrodynamic” approach, assuming a nonzero electron collision frequency. Numerical results for the field modulated upon the sum of the frequencies of the two dipoles are given, in correspondence to values of the parameters of relevance for radio wave propagation in the ionosphere, and a nonlinear effect of resonance is emphasized.
Enhanced airglow, electron acceleration, and parametric instabilities
Authors: Weinstock J.
Published: February 2008

A calculation is made of the acceleration of electrons during ionospheric modification experiments. The electrons are accelerated by Langmuir waves excited unstable by the parametric instability. This calculation results in an electron flux of about 109 cm−2 sec−1 for conditions over Boulder, and is sufficient to explain observations or airglow enhancements.
Seasonal and solar cycle variation of E‐region nitric oxide
Authors: Golshan N. , Sechrist C. F.
Published: February 2008

Measured electron density and [NO+]/[O2+] data are used to deduce the seasonal and solar activity variation of E‐region nitric oxide concentration. The results imply a strong dependence of [NO] on soft X rays leading to the possibility that dissociation of N2 by energetic photoelectrons is a major source of N(2D). Seasonal variations of the thermosphere apparently cause enhanced [NO] in summer in agreement with the summer enhancement of experimental [NO+]/[O2+].
Determination of nitric oxide concentrations from sunrise E‐region electron density measurements
Authors: Monro P. E. , Smith L. G.
Published: February 2008

Midlatitude sunrise electron density profiles have been analyzed in order to determine nitric oxide concentrations in the range 100–160 km. In general, the determinations are restricted to heights for which the ionization of nitric oxide by direct solar Lyman‐α radiation is the main contribution to the growth of the E region. The concentrations obtained are larger, by factors of approximately 3 to 4, than those obtained from the midlatitude airglow measurements of other workers. Concentrations have been obtained from one sunset measurement and these are significantly lower than the sunrise values. The results also give a clear indication of increasing nitric oxide concentrations above 120 km with increasing solar activity.
Concluding remarks, session 1: Instabilities and irregularities
Authors: Reid George C.
Published: February 2008

The topic of instabilities and irregularities is a comparatively recent field, in which a great deal of insight has been developed through interdisciplinary contacts between ionospheric and plasma physicists. Historically, the landmark year for the opening of the field was 1963, which saw the independent development by Farley [1963] and Bunemann [1963] of the theory of the two‐stream instability (now generally accepted as the cause of the Type I electro jet irregularities), and the development by Simon [1963] and Hoh [1963], in a laboratory‐plasma context, of what is now variously known as the gradient‐drift, crossfield, or E×B instability (which is becoming accepted as the source of Type II irregularities). In all fairness, however, it is worth remembering that Martyn [1959] had earlier published a qualitative description of the gradient‐drift instability and its possible application to ionospheric spread‐F.

Since 1963, a great deal of effort has gone into studying the application of these instabilities to the E region, particularly in the equatorial electro jet, but also more recently in the auroral electro jet, where conditions are much more complicated. As shown by several of the papers presented at this conference, we have come a long way since the early days, when only linear theory was available and all one could do was to predict thresholds for the instability. Unfortunately, as is usual when knowledge increases, the picture has also become somewhat fuzzier and less distinct than it was: even the distinction between the instabilities is not as clear as it was, since essentially the same equations are used to describe both, and the irregularities they create feed back on each other. It is also now clear that much of the time we are in fact seeing secondary irregularities, at least at the frequencies commonly used in radar observations. These irregularities are generated as nonlinear consequences of the growth of the primary irregularities, which may for the most part be visible only at much lower radio frequencies, such as those used by ionosondes.
Global features of lower ionospheric radio wave absorption in relation to middle atmospheric temperature variation and wind circulation
Authors: Samanta S.
Published: February 2008

The A1 absorption data for a large number of stations have been utilized to study the global features of the lower ionospheric radio wave absorption. The seasonal index ns (defined in the relation L = Bs (cos χ)ns, where L is the monthly median value of absorption, and χ, the midmonth solar zenith angle), is found to decrease rapidly and smoothly with increasing latitude and also with increasing solar activity for all stations. The constant cos χ plot of seasonal values of absorption versus month shows summer as well as winter enhancements when considered on a global scale. Such plots versus latitude show maximum absorption values at around the tropical region, an increase in the absorption values at high latitudes above 50°N, with a minimum around 40°–50°N, and a fall toward the equator below 23°N during sunspot minimum. A secondary maximum with a strong peak is observed around 50°N with a minimum around 45°N during sunspot maximum. The plots of absorption values as well as temperatures for 30 and 50 km heights versus cos χ show loop‐like structuring, indicating a considerable time dependence between them. Absorption and temperature at 30 km show a similar seasonal trend, whereas temperatures at 30 and 50 km show an opposite trend in their seasonal variations. The direction of looping of both temperature and absorption changes near about 35°N between low and high latitudes. Both temperature and absorption show large variabilities during winter periods. These findings have been explained in terms of the seasonal variation of the middle atmospheric wind circulation, and the importance of the dynamics of the lower ionosphere in controlling the aeronomy has been demonstrated.
Comparison of theories for intensity fluctuations in strong turbulence
Authors: Fante Ronald L.
Published: February 2008

Approximate solutions to Tatarskii's equation for the intensity scintillations of a wave propagating through a thin, highly turbulent layer have recently been developed by both the optical and radio astronomy communities. It is demonstrated here that these results, which are at first glance dissimilar, are in fact identical.
Theory of electromagnetic scattering from a layered medium with a laterally varying substrate
Authors: Wait James R. , Chang David C.
Published: February 2008

An analysis is given for scattering of a plane wave from a two‐layer conducting medium where the upper layer is homogeneous but where the lower layer or substrate is characterized by a periodically varying surface impedance. The exact solution obtained is expressed in terms of continued fraction expansions that converge rapidly for most cases of physical interest. The motivation was to provide understanding of how electromagnetic waves can be used to determine the thickness of a layer of coal that is backed by a relatively highly conducting region of slate. It is shown that, if the slate varies sufficiently slowly in the horizontal direction, the observed surface impedance and wave tilt at the air/coal interface is related directly to the local properties at the point of measurement.
Vertically polarized waves in inhomogeneous media with critical coupling regions, energy conservation, and reciprocity relationships
Authors: Bahar E. , Agrawal B. S.
Published: February 2008

Using a generalized WKB approach rigorous full wave solutions are derived for the electromagnetic fields for vertically polarized waves propagating through inhomogeneous dielectric media with critical coupling regions. Both the real and imaginary parts of the permittivity profile are assumed to vary in the horizontally stratified media. The generalized WKB solutions are consistent with energy conservation and reciprocity relationships in electromagnetic theory. The reflection and transmission coefficients and the related characteristic surface impedance for inhomogeneous layers of finite thickness are computed as functions of the transverse wave number for several permittivity profiles. Excitation of propagating and evanescent waves are considered. For the trapped waveguide modes of the inhomogeneous layer the reciprocal of the reflection coefficient vanishes and the layer is characterized by a surface impedance. Some special permittivity profiles for which closed form analytic solutions of the electromagnetic fields are known are also considered. Computations derived from these closed form solutions are shown to be in good agreement with computations based on the generalized WKB solutions.
Backscatter from scattering cells located in a random medium
Authors: Kupiec I. , Rosenbaum S. , Felsen L. B.
Published: February 2008

Certain diagnostic methods, based on electromagnetic backscatter phenomena, seek to provide information on selected scattering volumes that are part of a larger randomly fluctuating medium. Under these circumstances, the effects of the fluctuating medium on the signal passing to and from the scattering cell cannot be ignored. Inhomogeneities in the background refractive index may also significantly affect the characteristics of the backscattered signal. Assuming large scale, small amplitude, refractive index fluctuations, the present study explores the influence of the propagation path on the scattering cross section, as well as on the first and second moments of the temporal spectrum of the backscattered signal. In a homogeneous background, fluctuations are found to provide the same mean frequency shift as the Born approximation, but to enhance the second Doppler moment. In an inhomogeneous background, the latter effect may be overshadowed by deterministic refraction, especially for scattering volumes located far from the source. These aspects are illustrated by application to a radially stratified plasma column.
A least‐squares polynomial analysis and its application to topside ionograms
Authors: Titheridge J. E. , Lobb R. J.
Published: February 2008

The single polynomial analysis of ionograms is adapted to give a least‐squares procedure in which the number of scaled virtual heights is greater than the number of terms in the real height expansion. More terms can then be used in the polynomial without incurring unphysical oscillations. The detail required in the real height profile is set by the number of terms independently of the number of scaled virtual heights. Any mixture of ordinary and extraordinary ray virtual heights can be used. The accuracy with which the calculated profile fits the observations is given by an rms fitting error; the accuracy can also be checked at each scaled frequency, to detect (and reject) incorrect points. The method is particularly appropriate for the analysis of topside ionograms since full use can be made of fragmentary ordinary and extraordinary ray traces. These are incorporated into a single analysis which produces a real height curve interpolating smoothly across any unobserved regions. An 8 or 10 term polynomial, fitted to 10 to 20 virtual heights, is suitable for routine work. The resulting analytic expression can give any required number of points on the real height profile.
Synthesis of rectangular planar arrays for sum patterns with ring side lobes of arbitrary topography
Authors: Elliott R. S.
Published: February 2008

A simplified derivation and generalization of a design procedure due to Tseng and Cheng is presented. It permits determination of the discrete current distribution in a rectangular planar array corresponding to a sum pattern with a central main beam and concentric (but noncircular) ring side lobes, the latter having individually arbitrary heights. A practical example illustrates the theory.
Spacecraft observations of man‐made whistler‐mode signals near the electron gyrofrequency
Authors: Dunckel N. , Helliwell R. A.
Published: February 2008

Whistler‐mode signals from low‐frequency ground‐based transmitters in the 60–100 kHz range are observed by the OGO‐1 satellite at frequencies unusually close to the local electron gyrofrequency, ƒH. The intensity of signals observed on outbound passes near 40° geomagnetic latitude remains high until the decrease in ƒH causes the normalized frequency ƒ/ƒH to approach 0.9. Here the signal disappears abruptly into the background noise, a change of at least 20 dB. Ray‐tracing calculations provide a simple explanation for the sudden loss of signal. Rays in this frequency range which are launched just inside the plasmapause reach an altitude where ƒ/ƒH ≃ 0.9 before abruptly bending inward toward the earth's equator. Rays launched outside the plasmapause are reflected in the form of a “Spitze” at the point where the plasma frequency decreases to the wave frequency. Thus the primary features of the observed LF propagation in the magnetosphere can be explained using classical cold‐plasma expressions for the refractive index.
A statistical simulation of ground‐wave atmospherics generated by lightning return strokes
Authors: Lefferts Robert E.
Published: February 2008

A statistical model of lightning return strokes was used in a simulation program to determine the statistical properties of the atmospherics generated. Although the highly simplified Bruce‐Golde model of the return stroke was used, the results of this study compare well with recent experimental investigations. Statistical data were obtained for the α, β, and γ parameters of the Bruce‐Golde model which show that the variance of these parameters is too large to allow characterization by a single set of parameter values. The simulation program developed provides a simple means of evaluating the statistical effects of such factors as receiver bandwidth, changes in the nature of the return strokes, or propagation conditions which would be very difficult to evaluate either analytically or experimentally. Two examples of the application of the simulation program are given, the first of which illustrates the importance of the ground‐wave correction factor in determining the peak value of the electric field waveform. The second example demonstrates the effect of receiver bandwidth on the observed electric field waveform, and this simulation is equivalent to multiple observations, under different conditions, of exactly the same atmospherics at different propagation distances.
Atmospheric attenuation statistics at 15 and 35 GHz for very low elevation angles
Authors: Altshuler E. E. , Gallop M. A. , Telford L. E.
Published: February 2008

Since it is becoming increasingly difficult to obtain frequency assignments below 10 GHz for ground‐to‐aircraft wideband data links, frequencies in the window regions of the millimeter wave spectrum are now being considered for this application. In order to estimate system performance at these frequencies, the atmospheric attenuation along the path must be known. In this paper, 440 sets of slant‐path attenuation data at frequencies of 15 and 35 GHz are presented. The attenuations are measured with an 8.8 m paraboloidal antenna located at Prospect Hill, Waltham, Massachusetts, using the sun as a source. Meteorological data such as surface temperature, pressure and absolute humidity were recorded at the time of the measurements along with somewhat subjective descriptions of cloud conditions along the path. The systematic measurement and recording of a substantial amount of attenuation data at many elevation angles, under variable atmospheric conditions and at two frequencies, has provided a data base which permits an in‐depth statistical analysis of the relationships between attenuation, elevation angle, and atmospheric conditions. The mean attenuations at both 15 and 35 GHz decrease smoothly with increasing elevation angle and at 0°range from 5.44 to 8.80 dB and 15.47 to 24.12 dB respectively for clear to cloudy conditions. At an angle of 5° the mean attenuations at 15 and 35 GHz have a range of .69 to 1.08 dB and 2.45 to 9.02 dB respectively. For regions having a climatology comparable to that of the Boston area, the mean attenuations can be estimated from regression lines derived from the data.
Polar D‐region electron density profiles during a solar proton event
Authors: Helms Ward J.
Published: February 2008

Polar D‐region electron density profiles during a moderate solar proton event are calculated using a full‐wave computer solution on VLF step frequency sounder data from Byrd Station, Antarctica. VLF reflection height variation with solar proton flux, particle precipitation from the magnetosphere, and solar zenith angle are shown. It is suggested that the mesospheric pressure during this event is less than that extrapolated from an atmospheric model.