bluuebunny/crossref_metadata_2025 huggingface dataset:
VLF Phase Changes Produced by Particle Precipitation into the Geomagnetic Anomaly During Solar Proton Events
Authors: Mendes A. , Ananthakrishnan S.
Published: February 2008

Solar protons in the events of August 28 and September 2, 1966, produced phase disturbances on the 21.4–kHz VLF transmissions from station NSS, Annapolis, Maryland, to São Paulo, Brazil, and also on the 26.1‐kHz VLF transmissions from station NPM, Hawaii, to São Paulo, Brazil. Both these paths lie well below the polar zone, but partially traverse the geomagnetic anomaly. A quantitative connection between the observed phase advances and satellite observations of the proton flux is used to calculate the length of path affected and the height changes in the disturbed portion of the waveguide. The VLF variations are accounted for by exponential ionospheric models with a reference height equal to the effective reflecting height of the sharply bound model.
Green's Functions for Surfaces of Revolution
Authors: Harrington Roger F. , Mautz Joseph R.
Published: February 2008

A solution is developed to calculate the electric field intensity at one point in space due to an electric dipole at another point in space, subject to boundary conditions on a surface of revolution. The solution is valid both external and internal to the surface. Hence, it may be used to compute not only radiated and scattered fields, but also fields internal to resonant cavities. The current on the surface is obtained as an intermediary step. The solution is effected by the method of moments applied to the potential integral equation, and involves inversion of a generalized impedance matrix. The principal limitations to the solution are due to the matrix computation and inversion, which requires that the generating contour be at most several wavelengths long. Some examples of computations for spheres, disks, and conespheres are given.
Foreword
Authors: Rawer K.
Published: February 2008

During the 1969 General Assembly of URSI at Ottawa, Canada, Commission III (Ionosphere) of URSI accepted a recommendation to cosponsor a symposium entitled, ‘Waves and Resonances in Plasmas.’ The symposium should have carefully controlled attendance and be organized in cooperation with URSI Commissions IV, VI, and VII and with the International Union on Pure and Applied Physics (IUPAP). On invitation of the Canadian URSI Committee this symposium was held from July 5 through 9 at St. John's, Newfoundland.
Attention and Two‐Stream Instability: Summary Report
Authors: Larenz R. W.
Published: February 2008

The last session with invited papers and contributions dealt with two topics: (a) attenuation and (b) two‐stream instability. It also gave an occasion to include a few other contributions, which could not be presented during the preceding sessions.

K. Suchy presented an invited paper on topic a dealing with collisional damping for linear wave coupling. Questions concerned with the physical meaning of this coupling mechanism and with some difficulties concerning the notations for describing the polarization of waves in a magnetoplasma have been answered in the printed version of this paper which is found in this issue.
A Practical Evaluation of Earth‐Backscatter Simulation and an Estimate of the HF Ground‐Scatter Coefficient
Authors: Althouse Edwin L. , Davis John R.
Published: February 2008

An attempt has been made in this work to perform an appraisal of the degree to which fixed‐frequency amplitude versus time‐delay simulations can be made to agree in detail with actual HF‐radar ground‐back‐scatter observations taken with azimuthally broad beam antennas. The physical constraints and effects of propagation phenomena such as the spatial distribution of the ionospheric electron density, D‐layer absorption, the antenna pattern, rough‐surface scattering of the surface of the earth, and cross‐polarization losses are discussed.

A composite backscatter‐simulation method, based upon a weighted sum of individual simulations (separate raytraces and backscatter simulations) from different electron density distributions, is used to account for downrange geographical variation in electron density profiles, and the multiplicity of propagation modes which may occur during conditions of nonblanketing sporadic‐E or spread‐F ionization. Tabular descriptions of electron density versus height, from actual ionospheric soundings, are used instead of ionospheric models. Both isotropic and anisotropic scattering models are used. Order of magnitude estimates are given for the radar cross section per unit area (σ0) of land illuminated by one‐hop 16‐MHz radiation westward from Washington, D.C.
Axial‐Radar Cross Section of Finite Cones by the Equivalent‐Current Concept with Higher‐Order Diffraction
Authors: Burnside W. D. , Peters L.
Published: February 2008

In a recent paper, Senior and Uslenghi [1971] further developed the radar cross section of a finite cone. For the axial radar cross section, they evaluated the fields on the axial caustic using the first‐ and second‐order solution of Keller [1960]. It is the intent of this paper to show that the equivalent‐current method of Ryan and Peters [1969, 1970] yields the same results as those of Senior and Uslenghi. The equivalent‐current technique is, however, a more straightforward procedure to implement for the treatment of the scattered fields in the vicinity of the caustic and should be included in the collection of diffraction solutions.

The equivalent‐current technique is required only in the vicinity of the axial caustic. The results merge with the more conventional procedures as the observation point moves away from the axial caustic region. Thus, the numerical results discussed in this paper are concerned with the axial backscattered fields.

In our development, the second‐order diffracted ray across the base of the cone includes a factor of one‐half not included in the Senior‐Uslenghi solution. The reason for this factor is discussed in detail. The solution to a simpler wedge problem with and without this factor is compared to an integral equation solution in the Appendix.

The experimental and theoretical results for the cone differ consistently by approximately 2 db for a wide range of cone‐base radii and indicate that an additional mechanism contributes significantly to the scattered fields. One is suggested and indeed is required in the similar wedge solution of the Appendix.

The equivalent‐current technique is seen to be applicable to cones which have base radii as small as 0.3λ.
Automatic N (h, t) Profiles of the Isonosphere With a Digital Ionosonde
Authors: Wright J. W. , Laird A. R. , Obitts D. , Violette E. J. , McKinnis D.
Published: February 2008

A method is described to accomplish automatic data selection and profile inversion to obtain ionospheric electron‐density profiles from digitized radio soundings. The profile inversion is based upon the well‐established formulation of Paul [1960] by which the optimum radio‐frequency sounding intervals can be specified from an approximate knowledge of the profile. The expected virtual height coordinates (h') at these frequencies (ƒ) are likewise estimated, and procedures are then used to select h'(ƒ) observations nearest the predicted coordinates from a subsequent digital ionogram. From these the next profile is obtained. The process adaptively follows the changing shape and detail of the profile. The procedure requires an average of 20 sec per profile on a standard data‐processing computer and can be adapted, with benefit to on‐line real‐time use in our ‘dynasonde,’ digital ionosonde.
On the Reflection of FM Electromagnetic Pulses From the Ionosphere
Authors: Sodha M. S. , Rattan I. , Chakravarti A. K.
Published: February 2008

We have derived tractable analytical results for a more complete understanding of the distortion of the amplitude, the pulse length, and the rate of frequency shift of an FM Gaussian pulse after it has undergone reflection from an ionospheric model with a linear electron‐density profile and constant collision frequency.
Input Resistance of a Short Dipole Antenna in a Warm Uniaxial Plasma
Authors: Baenziger G. , Kuehl H. H.
Published: February 2008

The input resistance of a short dipole antenna immersed in a warm magnetoplasma is calculated on the basis of the kinetic theory (Boltzmann equation). The plasma is uniaxial; i.e., the cyclotron frequency is much larger than the operating frequency of the antenna and the plasma frequency. The plasma is homogeneous, and collisions are included although the sheath is neglected. The dipole is assumed to be infinitesimally thin (producing an infinite input reactance( except for the case in which the dipole axis is parallel to the static magnetic field when we allow the dipole to have a nonzero radius and calculate the input impedance. It is found that at frequencies much less than the plasma frequency the input resistance differs markedly from the hydrodynamic and cold‐plasma results because of collisionless absorption processes. The input resistance is generally smaller when the dipole axis is inside the resonance cone than when it is outside. The resonance cone anomaly in the input resistance is reduced in magnitude from that of the cold‐plasma theory and is shifted to a higher frequency.
On the Electric Field of a Moving Test Charge
Authors: Yu M. Y. , Stenflo L. , Shukla P. K.
Published: February 2008

The electric field at large distances from a moving test charge in a collisionless plasma is calculated. Significant differences from the well‐known inverse fourth‐power dependence of the electric field on the distance between the observer and the charged particle are obtained.
A System‐Synthesis Approach to the Inverse Problem of Scattering by Smooth, Convex‐Shaped Scatterers for the High‐Frequency Case
Authors: Vandenberghe Francis H. , Boerner Wolfgang M.
Published: February 2008

The solution of the inverse problem of electromagnetic scattering by smooth, convex‐shaped and perfectly conducting scatterers is viewed and analysed following a system‐synthesis approach. The formulation of the associated iterative averaging method is based on the fact that a knowledge of the variation of the scattered field's magnitude about the monostatic angle necessarily reflects some information on the curvature of the obstacle within the illuminated region. For the two‐dimensional case this cognizance permits one to assume that for any smooth, slowly and uniformly varying, convex‐shaped scatterer, an equivalent circular cylinder of identical curvature about the monostatic direction gives rise to an identical backscattered field magnitude. The method leads to the recovery of the equivalent electrical radii of curvature about the specular point which are in fair agreement with the exact values of elliptically cylindrical scatterers employed here for computational verification. This approach could be, therefore, useful in the portrayal of unknown scatterers, by curve fitting the various values of the electrical radii of curvature, obtained via the iterative averaging method as applied to data measured about the monostatic angle, for different directions of illumination with respect to the center of the scatterer. Although only the cases of circularly and elliptically cylindrical objects are examined here, it is anticipated that this technique is also applicable, in principle, for the portrayal of three‐dimensional scatterers of convex, smooth shape, provided that high‐frequency measurement data are available.
Diffraction by a dielectric‐loaded wedge
Authors: Mohsen A. , Hamid M. A. K.
Published: February 2008

The problem of diffraction of an E‐polarized plane wave by a perfectly conducting wedge, having the illuminated side loaded with a thin dielectric slab of which the relative permittivity is not much larger than unity, is treated. The solution is given in an integral form, consisting of geometrical optics and associated diffraction terms, by applying an approximate boundary condition at the dielectric surface. The diffraction integrals involved are evaluated asymptotically in terms of Fresnel's integrals. Using the reciprocity principle, the results are extended to the cylindrical‐wave excitation. The validity of the results is verified by comparison with experiment for the case of a dipole‐fed dielectric‐loaded strip.
Bistatic‐radar estimation of surface‐slope probability distributions with applications to the moon
Authors: Parker M. N. , Tyler G. L.
Published: February 2008

A method for extracting surface‐slope frequency distributions from bistatic‐radar data has been developed and applied to the lunar surface. Telemetry transmissions from orbiting Apollo spacecraft were received on the earth after reflection from the lunar surface. The echo‐frequency spectrum was related analytically to the probability distribution of lunar slopes. Standard regression techniques were used to solve the inverse problem of finding slope distributions from observed echo‐frequency spectra. Data taken simultaneously at two wavelengths, 13 and 116 cm, have yielded diverse slope statistics. In the lunar highlands, results obtained at the two wavelengths either can be quite similar in form and scale or can indicate markedly differing functions. Within the mare, the distributions obtained at the two wavelengths are quite similar in form, but the rms slopes from the 116‐cm data are systematically less than those obtained from the 13‐cm data. Within the highlands the rms slopes obtained at the two wavelengths are nearly the same.
The open‐source neutral‐mass spectrometer on Atmosphere Explorer‐C, ‐D, and ‐E
Authors: Nier A. O. , Potter W. E. , Hickman D. R. , Mauersberger K.
Published: February 2008

The open‐source mass spectrometer will be used to obtain the number densities of the neutral atmospheric gases in the mass range 1 to 48 amu at the satellite location. The ion source has been designed to allow gas particles to enter the ionizing region with the minimum practicable number of prior collisions with surfaces. This design minimizes the loss of atomic oxygen and other reactive species due to reactions with the walls of the ion source. The principal features of the open‐source spectrometer and the laboratory calibration system are discussed.
Commission 8. Radio noise of terrestrial origin
Authors: Hagn G. H.
Published: February 2008

The review paper, given by E. L. Maxwell (USA), concentrated on recent developments in three areas: (a) prediction of atmospheric noise levels and characteristics, (b) calculation of system performance in the presence of atmospheric noise, and (c) measurement of atmospheric noise.

The development of a digital‐computer model for predicting the characteristics of VLF atmospheric‐noise characteristics was described. The model provides for the calculation of (a) mean values of vertical electric‐field intensity which can be converted to mean noise power available from an ideal short vertical antenna, Fa, (b) the standard deviation from the mean values, (c) mean values of voltage deviation Vd) and (d) the direction of arrival of the noise energy. It uses both thunderstorm and lightning distributions derived from thunderstorm‐day observations at more than 7000 locations and lightning characteristics to calculate the noise energy radiated per square kilometer over the entire earth surface. An earth‐ionosphere waveguide model is used to calculate the noise energy propagated to any location. Statistical characteristics of noise are calculated from those of thunderstorms and lightning and from propagation parameters. It was emphasized that measured VLF noise data were not used to generate the model. The accuracy of the noise model was good when compared with atmospheric radio‐noise (ARN‐2) data but not better than that of the empirical model of CCIR Report 322 [1964] when compared with data from the 16 locations used for that model. For very high latitudes and other remote locations the results from the current model differ from those of CCIR Report 322 by up to 20 db. It seemed desirable to update the report for VLF by using the results from the noise‐prediction model and all the noise data collected by the NBS/NOAA network. It was suggested that a separate map for VLF for three‐month periods for the world might be preferable to the presentation in CCIR Report 322. Some continuing measurements with ARN‐2 equipment were remommended.
The importance of tilted layers in the tropospheric ducting of radio waves over the Timor sea
Authors: Barton Ian J.
Published: February 2008

A solid state microwave refractometer was mounted in a Royal Australian Air Force Dakota aircraft in order to measure vertical profiles of the atmospheric radio‐refractive index over the Timor Sea at times of anomalous radio wave propagation. Computer raytracing techniques, applied to model atmospheres based on these measurements, have indicated the importance of tilted layers in the tropospheric ducting of short‐wave radio energy.
Subsurface electromagnetic fields of a line source on a two‐layer earth
Authors: Wait James R. , Spies Kenneth P.
Published: February 2008

A previous analysis for a line source on a homogeneous half‐space model of the earth is extended to allow for the presence of stratification. The formal integral solution is reduced to a form suitable for numerical integration. Also, a number of special cases and limiting forms are considered. It is shown that the subsurface field has a structure that can be significantly modified when the half‐space is no longer homogeneous. The results have application to the design of electromagnetic techniques in mine rescue operations.
A method for predicting the F1 layer critical frequency based on the Zurich smoothed sunspot number
Authors: DuCharme E. D. , Petrie L. E. , Eyfrig R.
Published: February 2008

A series of expressions to calculate the monthly median F1 layer penetration frequency (ƒoF1) as a function of the ionospheric index IF2 was reported by DuCharme et al., [1971]. Using similar techniques and the same data base, development of a series of expressions to calculate ƒoF1 as a function of the Zurich smoothed sunspot number R12 is described. The derived expressions are evaluated for the period of maximum solar activity 1967 through 1969 and compared with observed ƒoF1 data for this period, which are outside the 1954 through 1966 data base used to develop the expressions.
Self‐induced motions of thermal plasma in the magnetosphere and the stability of the plasmapause
Authors: Richmond A. D.
Published: February 2008

Significant small‐scale convective motions can be generated by the presence of thermal plasma in the magnetosphere. An isolated plasma‐filled flux tube at L = 4, of density 3 ×108 m−3 and temperature 3000 K, can convect away from the earth at a velocity on the order of 0.7 RE hr−1 at night, due to the electric field it creates. An interchange instability occurs at the plasmapause, with a growth rate on the order of 1.9 × 10−3 sec−1 at night, creating turbulence which tends to destroy the steep plasma density gradient. This instability can explain the gradual decrease of the density gradient observed during periods of decreasing magnetic activity. In order to maintain a steep density gradient, three possibilities are discussed: (a) reduction of the instability growth rate due to enhanced ionospheric conductance; (b) reduction of the growth rate due to the presence of radiation belt plasma and/or a sharp temperature increase at the plasmapause; and (c) rapid radial convergence of large‐scale flow due to the presence of an Alfvén layer at the plasmapause. The self‐induced convective motions may also be important in the production and/or redistribution of small‐scale irregularities of thermal plasma observed both inside and outside the plasmapause.
Incoherent scatter observations of the ionospheric response to a large solar flare
Authors: Mendillo Michael , Evans John V.
Published: February 2008

Incoherent scatter observations at Millstone Hill were made during the importance 3B solar flare which occurred at 15:00 UT on 7 August 1972. Measurements were obtained of the electron density profile over the height range 125 to 1200 km, together with observations of the electron temperature, ion temperature, and vertical drift velociy above 225 km. The Ne(h) results showed electron density enhancements of nearly 100% at 125 km and 60% at 200 km; Ne enhancements were also observed throughout the topside, with the percentage enhancement increasing with height above 400 km. The total electron content up to 1200 km showed an increase of 3.8 × 1016 el m−2 (≃30%).

The temperature and drift data represent the first Thomson scatter measurements of these parameters during flare conditions. While the electron temperature results showed substantial increases at all altitudes examined, the ion temperatures were only slightly perturbed. The drift data provided the most interesting results. In response to the flare, a dramatic upward surge was observed in which the drifts increased nearly linearly with height over the height range 300 to 750 km.

In discussing the overall results obtained during this event, estimates are made of the flare‐induced electrodynamic and thermal processes which contribute to the observed effects.
Neutral composition in the lower thermosphere
Authors: Scialom G.
Published: February 2008

A photochemical model of the F1 region was set up from rocket‐borne mass spectrometric measurements of the ionic and neutral composition, and from reaction rates determined in laboratory experiments. This model was applied to determine the seasonal variations of the neutral composition in the lower thermosphere from incoherent scatter ionic composition data. We find an atomic oxygen winter maximum, in agreement with satellite drag and energy balance studies. Moreover, we put in evidence a molecular oxygen winter minimum, which confirms results obtained from Lyman‐α absorption measurements on O2, and from twilight 6300 Å airglow studies.
Midlatitude thermospheric winds from incoherent scatter radar and theory
Authors: Salah J. E. , Holt J. M.
Published: February 2008

Ionospheric plasma vertical drifts measured by the Millstone Hill incoherent scatter radar are decomposed into estimates of meridional neutral particle motion by accounting for ion diffusion and semidiurnal electric fields. For two equinoctial days with relatively high F‐region electron densities the resulting neutral circulation at 300 km is composed of equatorward winds of 100 to 150 m sec−1 at night and poleward winds of 25 to 50 m sec−1 during the day. Harmonic analysis of the wind pattern yields a diurnal northward component of 80 to 100 m sec−1 with a maximum near noon, a semidiurnal component of 35 m sec−1 at 0600 LT and a prevailing southward wind of about 20 m sec−1. The observations are compared with tidal theory predictions and are interpreted by means of a simplified integration of the horizontal equations of motion through which effects of zonal advection, ion drag, viscosity, and pressure gradients are studied. Although the observed wind pattern is consistent in general with that derived from global models based on satellite drag observations, various dicrepancies concerning the amplitude and phase of the diurnal bulge are noted and discussed.
A new wideband, fully steerable, decametric array at Clark Lake
Authors: Erickson William C. , Fisher J. Richard
Published: February 2008

A new, fully steerable, decametric array for radio astronomy is under construction at the Clark Lake Radio Observatory near Borrego Springs, California. This array will be a “T” of 720 conical spiral antennas (teepee‐shaped antennas, hence the array is called the TPT), 3.0 by 1.8 km capable of operating between 15 and 125 MHz. Both its operating frequency and beam position will be adjustable in less than one msec, and the TPT will provide a 49‐element picture around the central beam position for extended‐source observations.

Considerable experience has been gained in the operation of completed portions of the array, and successful operation of the final array is assured. This paper describes the results of the tests which have been conducted with the conical spirals and outlines the planned electronics.
Radiation from a microwave source in the intermediate zone
Authors: Lombardini P. P. , Fiscella B.
Published: February 2008

The vertical distribution of the field radiated by a microwave transmitter in the proximity of a smooth earth is obtained by direct application of Huygens' principle. For points at either side of the horizon and on the horizon itself this new approach offers uniform results which are free from the interpolations which characterize the methods currently used.
A certain behavior of the ionospheric F2 region at low latitudes
Authors: Huang Chun‐ming
Published: February 2008

The continuity equation of electron density is solved for the conditions of the F2 region at low latitude along 69°W meridian in the equinox of high solar activity. The transport term includes the electromagnetic drift observed at Jicamarca, the plasma diffusion, the vertical motion due to thermal expansion and contraction, and the neutral wind for which the direction reversal in the morning is rather late. The large‐scale irregularity which forms a bifurcated F2 layer is generated at the geomagnetic equator and travels as far as 15° latitude with a speed of about 10 km min−1. Other features of this bifurcation and the probable mechanism of its formation are described. The height of maximum electron density is strongly controlled by the electromagnetic drift at very low latitude from 0700 to midnight. The computation also shows that the large postsunset peak of NmF2 at a subtropical region is caused by the rapid sunset increase of upward drift at the equator. The late reversal of neutral wind favors the observed diurnal pattern of NmF2. Other effects of neutral wind and that of loss rate on the low latitude ionosphere are discussed.
Corrections and error estimates of electron density profiles obtained from ionograms
Authors: Paul A. K.
Published: February 2008

Electron density profiles derived from ionograms have limited accuracy due to technical and natural reasons. In the following we show how corrections and error estimates can be obtained on the basis of the propagation theory of radio waves and the general knowledge of the ionospheric structure.
Amplitude variation of Pc 5 pulsations at high latitudes
Authors: Gupta Jagdish Chandra
Published: February 2008

Normal‐run magnetograms for the year 1967 from seven Canadian observatories were scaled to study the amplitude characteristics of Pc 5 pulsations. The results of the statistical analysis are: (1) Large amplitude Pc 5's occur in a limited region located on the poleward side of the auroral zone. (2) The diurnal variation of Pc 5 amplitude maximizes in the postnoon period at Resolute Bay and this peak tends to appear earlier and earlier in the day at the lower latitude stations, e.g., it appears in the morning at Great Whale River. An additional midnight peak in the diurnal variation of amplitude is noted at auroral zone stations. (3) The amplitude attains a maximum value at Great Whale River near the central line of the auroral zone. It decreases at a faster rate toward the lower latitudes than toward the higher latitudes. In quiet intervals an additional peak in the amplitude‐latitude profile is noted near the cusp region at Baker Lake. (4) The diurnal variation pattern of amplitude shows no clear relationship with the seasons or with magnetic activity. (5) Under disturbed magnetic conditions a much larger amplification of the amplitude of Pc 5's has been at Great Whale River than at high or low latitude stations. Pc 5 amplitudes correlate very well with Kp up to a level of 6−. (6) With increasing solar elevation angle the region of maximum amplitude of Pc 5 is found to move toward the northern border of the auroral zone. (7) From the data analyzed, amplitudes of Pc 5 do not show a 27‐day recurrence tendency.
Two days in the life of the ionosphere over Arecibo
Authors: Swartz Wesley E. , Ioannidis George A. , Shen Jenny S. , Brice Neil M. , Rowe John F.
Published: February 2008

High resolution incoherent scatter electron density data using Barker‐coded pulses are presented for two separate 24‐hr periods of very different magnetic conditions. The first period in April 1972 was magnetically quiet while the second period spanned parts of two of the five most magnetically disturbed days in July 1972. Solar activity was low with 2800‐MHz indices of 129 and 128 for the April days and indices of 109 and 117 for the July days. Contour plots and selected profiles are presented which show larger sporadic‐E layers and stronger gravity wave effects during the disturbed night than during the quiet night.
Inversion of sweep‐frequency sky‐wave backscatter leading edge for quasiparabolic ionospheric layer parameters
Authors: Rao N. Narayana
Published: February 2008

A method is presented for the derivation of quasiparabolic ionospheric layer parameters from a set of three data points on the leading edge of a sweep‐frequency sky‐wave backscatter ionogram. The method is illustrated with a numerical example. Possible extensions of the method are discussed.
Propagation in conducting waveguides having slowly varying cross sections
Authors: Nayfeh Ali H. , Asfar Omar R.
Published: February 2008

The present analysis treats the problem of wave propagation in circular waveguides taking into account the effects of both large wall conductivity and slowly varying cross section. The problem is formulated in terms of the Hertz vector for the case of TM waves. The method of multiple scales is used to find a first‐order uniformly valid expansion for the Hertz vector in both the transmission and attenuation regions of the waveguide. The solutions for the perfectly conducting and the circular waveguides are obtained as special cases from the general solution.
a model for RF scattering from field‐aligned heater‐induced irregularities
Authors: Rao P. B. , Thome G. D.
Published: February 2008

The paper presents a model for radio wave scattering from the ionospheric irregularities generated by the intense radio wave transmissions from the Platteville heater. The model is based upon the HF‐UHF radar backscatter observations for a physical description of the scattering medium and upon the theory of weak scattering (Born approximation) from anisotropic irregularities for mathematical development. According to the model, the electron density fluctuations responsible for radio wave scattering having an rms value of 1 to 1.5% are aligned along the earth's magnetic field and are distributed in a diffuse pancake‐shaped volume of 15 km gaussian thickness vertically and of the size of the heater beam which is 100 km in gaussian diameter horizontally. At radio frequencies the irregularities within the disturbed volume are highly aspect sensitive and consequently only those scatterers which lie on the surface of specularity where a radar views transverse to the magnetic field count in contributing to the received signal. Model computations have been made of the scattering cross sections for monostatic as well as bistatic radar configurations and they are found to be in good agreement with the observations. The radar cross section at a given frequency for bistatic geometry is substantially greater than that for monostatic geometry, all else being equal. Finally, it is shown that the width of the scatter‐illuminated bands on the ground is set primarily by the finite dimensions of the scattering volume rather than by the finite aspect sensitivity of the individual scatterers.
Ionospheric heating by powerful radio waves
Authors: Meltz G. , Holway L. H. , Tomljanovich N. M.
Published: February 2008

In addition to the communication applications that stem from the generation of field‐aligned irregularities, ionospheric heating by powerful HF waves also holds promise for establishing new techniques in D‐, E‐, and F‐region aeronomy. This paper concentrates on the application of the theory of plasma heating by ohmic and nonlinear dissipation to the special conditions of the lower ionosphere where the electron collision and disturbing wave frequencies are comparable and the upper ionosphere where electron thermal transport is large and restricted by the earth's magnetic field. We predict the magnitude and distribution of large‐scale changes in the electron temperature and density in both the D and F layers under a variety of conditions. These results are compared with direct measurements or inferred changes whenever possible.

We also examine the effects of specular‐height mismatch, multiple frequency operation involving heater‐power sharing, and antenna beam tilt on the total cross section for field‐aligned scattering. The analysis is based on the concept that the energy‐density distribution on the heater reflectrix computed by means of a ray‐tracing technique can be related directly to the density‐fluctuation intensity through an appropriate scattering model. Lastly, potential applications to aeronomy are discussed, particularly in regard to D‐region measurements of the effective recombination coefficient magnitude and temperature dependence and the relative water cluster ion concentration.
Nighttime and sunrise variations of absorption
Authors: Ganguly S.
Published: February 2008

The average behavior of the MF A1 absorption data at night and sunrise times at Calcutta has been described with a view to understanding the maintenance of the nighttime absorbing region. The nighttime variation of absorption which attains a minimum around one hour after local midnight could easily be accounted for in terms of the scattered UV fluxes.

A small decrease in absorption before sunrise is evidenced. The analysis using the partially reflecting Es echo suggests this is caused by a decrease in deviative loss at F‐region heights. The onset time for a further increase in absorption agrees very well with that observed from other techniques.
Evaluation of nighttime exponential ionospheric models using VLF atmospherics
Authors: Hughes H. G. , Gallenberger R. J. , Pappert R. A.
Published: February 2008

Measurements of very low frequency (VLF) atmospherics were made simultaneously at a station close to the lightning sources (Norman, Oklahoma) and at stations 1787 km (La Posta, California) and 5912 km (Kamuela, Hawaii) from Norman. Timing synchronization was maintained between stations to better than 10 μsec which, coupled with the source bearing from Norman, allowed accurate fixing of source locations. Using the numerical techniques of Pappert et al. for propagation parameter calculations, an appropriate exponential ionospheric conductivity model was chosen which best fit the modal interference structure near 10 kHz as a function of source distance as measured at La Posta. From source current moment spectra measured at Norman, the electric field strengths at La Posta and Kamuela were predicted for a wide frequency band (2 to 30 kHz) using the selected conductivity profile. For β = 0.6 km−1 and h′ = 84 km (in the notation of Wait and Spies) good agreement between the numerical calculations and the average measured field strengths was obtained.
Expansions applicable to a nonstationary scattering medium
Authors: v. d. Woude P. , Bremmer H.
Published: February 2008

This paper concerns the wave equation for an irregular time‐dependent nondispersive medium contained in a slab, while a plane monochromatic primary wave passes through the latter in a perpendicular direction. The equation is reduced to an equivalent integral relation in which the influence of the medium in directions perpendicular to that of the incident wave is accounted for by an operational expression. The application of a corresponding forward‐scattering approximation, the reliability of which is discussed, enables the derivation of two alternative versions for the integral equation that refers to a mainly forward‐scattering medium. The second version contains as a factor the geometric optical approximation that is associated with a rectilinear propagation of the incident wave toward the point of observation. It is shown that the Neumann‐Liouville series for the solution of the two mentioned versions correspond, respectively, to the well‐known Born series (but including the time dependence of the slab medium) and to a series containing corrections to the above‐mentioned geometric optical factor.
Electric field spectrum and intensity covariance of a wave in a random medium
Authors: Fante Ronald L.
Published: February 2008

Numerical results are presented for the effect of the strength of turbulence on the temporal frequency spectrum of a focussed beam propagating in a random medium having a von Karman spectrum for the index of refraction fluctuations.

We also present a new theory for the normalized covariance BI(z, r) in the intensity fluctuations of a plane wave and find that forσ12 ≡ 1.23 k07/6 Cn2z11/6 » 1,B1 (z, R) ≃ (−11.2σ12R5/3) + (σ12)−2/5ƒ[R/(σ12)3/5]

where k0 is the signal wave number, Cn2 is the refractive index structure constant, z is the path length, R = (k0/2πz)1/2r, and f3(···) is a function given in section 3. Note that the first term is sharply peaked about R = 0 and corresponds to the rapid drop‐off in BI(R) observed experimentally. The second term varies very slowly with R and corresponds to the long tail of BI(R) seen in experiments. Unlike other theories this theory is completely self‐contained, and does not involve the introduction of any arbitrary cut‐off parameters.
Antenna investigation of a statistically inhomogeneous atmosphere
Authors: Armand N. A. , Lomakin A. N. , Sarkisyanz V. A.
Published: February 2008

Results of experimental investigations of fluctuations of phase front wave propagations in the turbulent atmosphere are presented. The correctness of the Kolmogorov‐Obukhov spectrum has been confirmed for inhomogeneities with horizontal sizes up to dozens of kilometers, and the results have been detected dependent essentially upon duration. The latter has been explained in the limits of the hypothesis of “frozen‐in” turbulence. The problem of gain losses of circular antennas was solved by using a parabolic equation for a coherent function. Inhomogeneities of a refractive index are assumed to have the Kolmogorov one‐dimensional spectrum and to be anisotropic.
Comparison of Arecibo E‐region data and sporadic‐E theory: a measurement of the diffusion coefficient
Authors: Harper R. M. , Wand R. H. , Whitehead J. D.
Published: February 2008

Incoherent scatter measurements using multiple‐pulse techniques were performed at Arecibo to study the E region with a height resolution of 2.4 km. These simultaneous measurements of ion temperature, ion‐neutral collision frequency, and electron density, as well as horizontal and vertical components of ion drift velocity, have allowed comparison between theories of sporadic‐E and experiment. Sporadic‐E layers were consistently observed to descend along the curve where the vertical ion drift velocity (Vz) was zero and its gradient ∂ Vz/∂z was negative, as predicted by redistribution theory. Assuming no diffusion, the calculated effective recombination coefficient at the layer peak is more than an order of magnitude too small for NO+ or O2+, and varies with height in a manner which is unacceptable for a recombination coefficient. This indicates that the layer is composed of long‐lived ions (probably metallic), and that the peak layer density is controlled by diffusion rather than recombination. By ignoring recombination, the diffusion coefficient was estimated from the layer thickness and the vertical gradient in the vertical ion velocity. The results were compared with independent values calculated from the measurements of ion temperature and ion‐neutral collision frequency. The two estimates were in good agreement over the range 110 to 120 km, showing that electron densities in sporadic‐E layers are primarily determined by molecular diffusion.
The propagation of gaussian beams very near the source
Authors: Lewin L.
Published: February 2008

The field of a gaussian beam is examined in detail. It is shown that the usual formula for the field does not satisfy the wave equation within the Rayleigh distance except in the limit of large waist diameter. For small waist sizes the departure from the results of the usual formula can be substantial within a distance of about 1.3 wavelengths from the source. An exact formula is found for the on‐axis radiation. In order to make this study, certain features such as the obliquity factor, which is commonly ignored, have to be considered in detail. This leads to the need for a physical characterization of the source, which is not capable of a unique specification. The choice is probably not critical—the usual formulas for the transformation of gaussian beams do not need to refer to it at all—and the one used here pertains closely to the condition of radiation from the dielectric‐air interface of a solid state laser.
The relative accuracy of ionogram analysis techniques
Authors: Titheridge J. E.
Published: February 2008

Increasing accuracy in the conversion of h′(f) to N(h) profiles is obtained by increasing the order of the polynomial used to interpolate between measured points. Linear and parabolic lamination techniques correspond to first‐ and second‐order interpolation. Fourth‐order interpolation (as in the 5‐term overlapping polynomial method) is about optimum. In comparing different methods, it is essential that fixed boundary conditions be employed; when this is done an adjacent polynomial technique is much less accurate than overlapping polynomials. All methods (including least‐squares procedures) are equally sensitive to errors in the virtual height data. Possible procedures for reducing the errors caused by underlying and valley ionization are critically reviewed. It is concluded that, in general, only a single parameter is necessary or desirable to describe these regions. This parameter can be determined from ordinary and extraordinary ray measurements, at any latitude.
Numerical calculation of electromagnetic waves in an anisotropic multilayered medium
Authors: Nagano Isamu , Mambo Masayoshi , Hutatsuishi Godo
Published: February 2008

An improved matrix method of calculating the solutions of wave equations in a horizontally stratified anisotropic medium is described. An inhomogeneous medium is divided into a number of thin horizontal and homogeneous slabs. The solutions of a differential equation with constant coefficients in each slab are connected by applying the boundary conditions, and the wave fields are given as the power series expansion of wave fields. In calculating the wave fields, a matrix is divided into two matrices for two independent solutions and Gram‐Schmidt orthogonalizing process is applied to prevent numerical swamping. Comparisons are made between this and other full wave methods regarding step size and computer time, and this method is found to be more efficient than the full wave method especially at high frequency wave fields. The magnetoionic modes separated from the resultant wave are shown for a model ionosphere.
Marconi, radio waves, and the ionosphere
Authors: Beynon W. J. G.
Published: February 2008

The life story of Marconi is too well documented to need detailed repetition, but there are certain facts about his early years which bear directly upon his quite remarkable success and indeed which relate to the early development of the subject of wireless telegraphy.

Anyone looking at the early history of radio wave propagation and of Marconi's major role in it, cannot fail to be impressed with certain facts. Thus it is surely remarkable that in February 1896 when he first took his black wireless boxes to England, Marconi was only 21 years old and it is to be remembered that not only was he a very young man but he had had very little formal training in science. The second remarkable fact is that on 2 June 1896, i.e., within a few weeks of arriving in England (he was now just 22) he filed with the British Patent Office a provisional patent specification (the first of many) to safeguard his commercial interests in wireless communication. The third striking fact is that just one year later, in the summer of 1897, at the age of 23 he and his friends formed the Wireless Telegraph and Signal Company Limited (soon to be renamed the Marconi Wireless Telegraph Company) with an initial capital of £100,000. The shares in the new company were quickly subscribed; Marconi himself received £15,000 in cash and also held a large number of the shares.
Research on statistical aspects of tropospheric propagation
Authors: Barzilai Giorgio
Published: February 2008

In microwave links, atmospheric inhomogeneities and terrain irregularities produce random fluctuations of the electromagnetic field whose comprehensive description requires a statistical approach. To this purpose several functions can be defined and measured. Among them, the normalized mutual coherence function, or complex degree of coherence, has proved to be the most convenient both for direct applications and for simple measuring procedure. The Istituto di Elettronica of the University of Rome is carrying on measurements of the degree of coherence of microwave fields. It has been measured for different paths at 9.4 GHz by using waveguide interferometric systems of various configurations which have been applied to the determination of the coherence function for several distances in the horizontal plane, and to the evaluation of its anisotropy in a plane transverse to the direction of propagation for different meteorological conditions.
Theory of EM wave propagation through tunnels
Authors: Wait James R.
Published: February 2008

A rather ingenious communication technique is being developed for use in Belgian coal mines. The idea is to use a two‐wire transmission line or equivalent loosely braided coaxial cable that can be suspended from the upper wall. A transmitter placed in the vicinity of the line excites a strongly unbalanced mode that normally would propagate like a coaxial or TEM mode with relatively high attenuation. The key step in the Belgian system, and the closely related French system, is to convert this unbalanced mode to a balanced mode that is much less attenuated because the return current flows mostly in the second wire rather than through the surrounding rock. The physical basis of this system has many similarities with the leaky feeder device described by Martin and used in British coal mines. We summarize the analysis of the propagation characteristics for an idealized mine tunnel of circular cross section. The extension to a rectangular tunnel is also considered. Some comparison is made with relevant experimental data. Actually, we deal only with the twin open wire pair that is the basic ingredient of the so‐called INIEX/Deryck system. However, as suggested above, the leaky braided cable system and the slotted cylindrical shielded cables have common features. The important point is that propagation modes which depend on current flow in the tunnel walls will suffer excessive attenuation while modes which utilize the second conductor as a return current path will have less attenuation.
Excitation of an infinite antenna above a dissipative earth
Authors: Chang David C. , Olsen Robert G.
Published: February 2008

The antenna problem consisting of exciting an infinitely long horizontal thin wire with an arbitrary distribution of impressed electric field along the axis of the wire is discussed. The special case of excitation by a delta function voltage source is solved. The current on the antenna is found as a sum of two residue (discrete) contributions and two branch integration (continuous) contributions. The far field and orthogonality properties of both the discrete and continuous modes are discussed. Finally, expressions are given for the current on the antenna. From this current the input conductance of the antenna can be obtained.