bluuebunny/crossref_metadata_2025 huggingface dataset:
Relation between transverse mode control and longitudinal mode behavior in the injection laser
Authors: Yamada Minoru , Suematsu Yasuharu
Published: February 2008

Stabilization and control of the operating field mode in injection lasers are very important problems for wider application of the devices. This paper gives a theoretical analysis of the relation between transverse mode control and longitudinal mode behavior in the injection lasers which have undoped active regions. The guided modes are classified into three types, normal guided mode, active guided mode, and leaky mode, owing to guiding mechanism. Cutoff conditions for all types of modes are mentioned. Gain saturation was analyzed on the basis of the density matrix equation. The so‐called weakly (partially) inhomogeneous property of the gain saturation was taken into account. The single longitudinal mode operation is expected when the transverse higher modes are in cutoff condition, because the suppression effect among the resonant modes is promoted. The normal guided mode is the most suitable for the single longitudinal mode operation, because the field profile and diffraction loss are invariable with increasing injection current. The transverse mode control is the most important characteristic in obtaining the single longitudinal mode operation in the injection lasers.
Wave theoretical analysis of spot excitation near‐field scanning method for optical fiber index profiling: Effect of finite spot size
Authors: Kitsuregawa M. , Nakada H. , Kamiya T. , Yanai H.
Published: February 2008

A family of near‐field spot excitation or detection techniques have recently been developed to measure the refractive index profile of optical fibers. The present paper discusses the effect of finite spot size of the illuminating beam on the accuracy of the reconstructed profile. A wave theoretical analysis is performed, and a series of numerical examples are given. Experiments of the spot excitation near‐field scanning method are performed for a nearly parabolic index fiber, resulting in a satisfactory agreement with the numerical analysis. Furthermore, a quantity is introduced which is a function of the refractive index, wavelength, and spot size of the illuminating beam. This quantity serves to determine the suitable spot size for measuring the optical fibers having different parameters.
Nonlinear optical phenomenon in a three‐dimensional LiNbO3 optical waveguide
Authors: Uesugi Naoshi
Published: February 2008

The nonlinear optical phenomena in a three‐dimensional LiNbO3 optical waveguide are discussed. The phase‐matching condition and conversion efficiency of second harmonic generation and difference frequency generation are derived theoretically. The temperature dependence of birefringence of the LiNbO3 crystal is used to satisfy the phase‐matching condition of nonlinear interactions. It is revealed that the temperature‐tuning method is very useful in satisfying the phase‐matching condition over a wide wave‐length range. The several percent conversion efficiency is expected with 100‐mW fundamental or pumping power and a 1‐cm‐long optical waveguide. The second harmonic generation and difference frequency generation experiments using Ti in‐diffused LiNbO3 optical waveguides were also shown. The maximum conversion efficiency for second harmonic generation (SHG) was measured to be 0.77% for 65‐mW fundamental input power. Infrared waves ranging from 1.035 to 1.19 μm were generated by the difference frequency generation.
Depolarization of VHF geostationary satellite signals near the equatorial anomaly crests
Authors: Lee M. C. , Das Gupta A. , Klobuchar J. A. , Basu S. , Basu S.
Published: February 2008

The nighttime polarization fluctuations of linearly polarized 136 MHz satellite signals received at Ascension Island, located near the southern crest of the equatorial anomaly, have been shown to be the manifestation of depolarization effect due to the diffractive scattering by small‐scale (<200 m) density irregularities with power law spectra. The theory can explain its coexistence with L band scintillation. The absence of this phenomenon at equatorial locations off the anomaly crests is attributed to two factors: (1) the ambient plasma densities are relatively low, and (2) the propagation angles of satellite signals are more nearly perpendicular to the geomagnetic field.
Characteristics of 13.9 GHz radar scattering from oil films on the sea surface
Authors: Johnson J. W. , Croswell W. F.
Published: February 2008

An aircraft remote sensing project to study the response of a number of active and passive microwave and optical remote sensors to an oil‐covered sea surface was conducted by NASA Langley Research Center in 1979. Included was a 13.9‐GHz Doppler scatterometer, with a fan beam antenna and coherent detection, to measure radar backscatter as a function of incidence angle. The purpose of this paper is to present a comparison between the radar scattering signature of the clear surface and signatures of the surface while covered with a variety of crude oil films. The films were deposited on the surface from ships. Both in situ data and infrared aerial photography provided surface truth. Scattering changes were observed over a range of incidence angles from nadir to approximately 60°. Results show up to 14‐dB reduction in backscattered power between the clear surface and the films for horizontal polarization at an incidence angle of approximately 30°. In all cases, the greatest reduction in backscatter due to the oil was observed at approximately 30°, which suggests an optimum incidence angle for oil detection and mapping at 13.9 GHz (λ0 = 2.16 cm). Maximum reduction in backscatter also appears to occur where the oil is the thickest.
The mutual coherence function for transionospheric radiowaves
Authors: Rino C. L. , Owen J.
Published: February 2008

The mathematical properties of the mutual coherence function for radiowaves propagating in a randomly irregular medium have been known since the early development of the theory. Recent satellite experiments that transmit multifrequency, phase‐coherent signals have facilitated direct measurement of the mutual coherence function for transionospheric radiowaves. In this paper we report the results of an extensive series of such measurements using the Wideband Satellite multifrequency beacon as a signal source. The measured mutual coherence functions show systematic departures from their simplest theoretical form, particularly under weak‐scatter conditions. The results are nonetheless consistent with a power law spectrum for the irregularities but show evidence of an abrupt change in the power law index at high frequencies. This spectral feature has been detected in rocket data, but it is evidently masked by diffraction effects in scintillation phase data. The measurements also verify the relationship between the intensity autocorrelation function and the mutual coherence function that applies under conditions of saturated scintillation.
Simulation of D and E region high‐power microwave heating with HF ionospheric modification experiments
Authors: Meltz G. , Rush C. M. , Violette E. J.
Published: February 2008

Fluid and kinetic theory calculations predict that the microwave beam from a solar power satellite could be intense enough to cause large, but localized changes in the properties of the lower ionosphere by ohmic heating. A 5‐GW system, radiating a peak flux of 23 mW/cm2 at a frequency of 2.45 GHz, could raise the electron temperature by a factor of 2–3, increase the E region density by 10–20%, and cause up to a 50% reduction in the D layer electron concentration. Doubling the flux could lead to a fivefold temperature rise and a 40% increase in the E region density. Since power is absorbed from the beam and heats the ionosphere at a rate that is proportional to the ratio of the flux to the square of an effective frequency, the effects of microwave heating can be studied at much lower power fluxes with high‐frequency radio waves. To investigate these effects, a series of ionospheric heating experiments were conducted at Platteville, Colorado, using the high‐frequency, high‐power facility. A pulsed ionosonde probe, located below the most intense portion of the high‐power beam, was used to observe the changes in the D and lower E regions. Phase and amplitude measurements were recorded during both CW and pulsed heating. Prompt (less than 50 ms) and sustained changes were observed in both the phase and attenuation. Perhaps most interesting are the large phase variations of several cycles at 3.4 MHz that were seen following turn‐on and turn‐off of a 5.2‐MHz heating wave. These changes occur relatively slowly, but steadily, in about 60–90 s. The observed changes in the probe amplitude and phase have been compared with theoretical estimates derived by integrating the coupled equations for power flux, electron temperature, and density as a function of altitude and time for both time‐varying and steady heating fluxes. The agreement is generally close, considering the differences between the ambient electron density model and the actual distributions.
A review of depolarization modeling for earth‐space radio paths at frequencies above 10 GHz
Authors: Bostian C. W. , Stutzman W. L. , Gaines J. M.
Published: February 2008

Depolarization limits the performance of dual‐polarized satellite communication systems at frequencies above 10 GHz. Caused by scattering from raindrops and ice crystals, it can be measured in any of several related ways. Theoretical models for depolarization may be classified as transmission, attenuation‐derived, and scaling models. This paper describes all three and discusses their relative merits.
Space diversity performance prediction for earth‐satellite paths using radar modeling techniques
Authors: Goldhirsh Julius
Published: February 2008

Both single‐terminal and two‐terminal joint probability rain fade distributions for earth‐satellite paths at 28.56 GHz are derived for Wallops Island, Virginia, by using radar modeling techniques. An array of parallel paths is modeled in the vertical plane which contained the COMSTAR satellite and a terminal receiving the 28.56 GHz COMSTAR beacon. The paths are displaced 1 km apart (8–100 km from the radar) at a fixed elevation angle (θ0 = 45°). They pass through the volume whose rain environment has been monitored as radar rain reflectivity levels and stored on magnetic tape. An attenuation at 28.56 GHz and a ground rain rate level is calculated for each path and for each radar scan. Both single and joint terminal conditional fade statistics are derived for an ensemble of such arrays, where the conditioning constant is the long‐term rain rate distribution. The single and joint terminal distributions are calculated by multiplying the conditional fade statistics by the measured long‐term rain rate distribution. As a partial check, the radar‐derived single‐terminal fade distribution at Wallops Island is compared with the directly measured levels obtained over a 3‐year period showing very good agreement. A family of diversity gain curves is calculated from the joint probability distributions corresponding to different probability levels. These are demonstrated to be represented by a single diversity gain function defined as the ‘relative diversity gain.’ The radar‐derived relative diversity gain is compared with those obtained from other investigations as well as with derived autocorrelation function results showing good agreement.
Airborne biological particles and electric fields
Authors: Benninghoff William S. , Benninghoff Anne S.
Published: February 2008

In November and December 1977 at McMurdo Station in Antarctica we investigated the kinds, numbers, and deposition of airborne particles larger than 2 μm while measuring electric field gradient at 2.5 m above the ground. Elementary collecting devices were used: Staplex Hi‐Volume and Roto‐rod samplers, Tauber (static sedimentation) traps, petrolatum‐coated microscope slides, and snow (melted and filtered). The electric fields were measured by a rotating dipole (Stanford Radioscience Laboratory field mill number 2). During periods of blowing snow and dust the electric field gradient was + 500 to + 2500 V/m, and Tauber traps with grounded covers collected 2 or more times as much snow and dust as the ones with ungrounded covers. During falling snow the electric field gradient was −1000 to −1500 V/m, and the ungrounded traps collected almost twice as much snow and dust as those grounded. These observations suggest that under the prevailing weather conditions in polar regions the probable net effect is deposition of greater quantities of dust, including diaspores and minute organisms, on wet, grounded surfaces. This hypothesis needs examination for its use in explanation of biological distribution patterns.
Scattering from a random slab
Authors: Hubey H. M. , Sivaprasad K. , Vasudevan R.
Published: February 2008

The order‐of‐scattering solutions (Bellman, Vasudevan, and Ueno (BVU) series) of the Riccati equation for the reflection function of an inhomogeneous slab are used to calculate the mean power intensity for a medium having random fluctuations of its dielectric constant. A solution for a Fokker‐Planck‐like differential equation for the joint probability density of the real and imaginary parts of the reflection function is obtained using a technique due to Van Kampen for nonlinear equations with multiplicative noise. The mean reflected power calculated from the probability density function is in agreement with that derived from the BVU solutions.
Microscopic Lagrangian description of warm plasmas: 4. Macroscopic approximation
Authors: Kim H. , Crawford F. W.
Published: February 2008

The averaged‐Lagrangian method is applied to linear wave propagation and nonlinear three‐wave interaction in a warm magnetoplasma, in the macroscopic approximation. The microscopic Lagrangian treated in papers 1–3 is first expanded to third order in perturbation. Velocity integration is then carried out, before applying Hamilton's principle to obtain a general description of wave propagation and coupling. The results are specialized to the case of interaction between two electron plasma waves and an Alfvén wave. The method is shown to be more powerful than the alternative possibility of working from the beginning with a macroscopic Lagrangian density.
Solar power satellites and telecommunications
Authors: Gordon William E. , Duncan Lewis M.
Published: February 2008

The oil crisis in the early seventies stimulated an intensive search for energy alternatives. Among the alternatives proposed for the long range (year 2000 and beyond) as a source of base load electric power is P. Glaser's solar power satellite. In a bold concept, Glaser proposed collecting solar energy on a geosynchronous satellite and relaying it by a microwave beam to a large collector on the ground. The concept in principle is feasible but requires a number of advances in technology to make it cost competitive and raises some questions about the effects of microwave radiation and the impacts on telecommunications. We propose to describe the concept, outline the problems it raises, treat the telecommunications problems with care, and comment generally on the problems of dealing with a bold proposal to solve, in part, a major problem facing society.
Numerical separation of overlapping ordinary and extraordinary echoes in digital ionograms
Authors: Paul A. K.
Published: February 2008

Modern digital ionosondes have the capability to measure amplitude and phase of each echo with high precision. Recording these quantities at four antennas located at the corners of a square permits the numerical separation of overlapping ordinary and extraordinary echoes. The formulation of the nonlinear equation system to do this is given together with an iteration algorithm for its solution. The practical application of this process is demonstrated.
Ionospheric scintillations
Authors: Wu D. M. , Marcuvitz N.
Published: February 2008

A renormalization technique, employed in the spirit of the formal theory of scattering, is applied to the problem of ionospheric scintillations. In the forward scattering limit, using a Markov approximation for ionospheric fluctuations and renormalized moment equations, one derives the wave statistics for a plane wave or beam propagating through the ionosphere. Fluctuations in the latter are described by an arbitrary power spectrum. Peaks superposed on a power law spectrum are found to increase the scintillation index S4, and a focusing effect is observed in the strong scintillation limit. An experimental power spectrum, obtained from in situ measurements by the Explorer E satellite and corresponding to electron density fluctuations of 20%, is used to calculate S4 for nighttime equatorial scintillations. For an ionospheric slab model 200 km thick at an altitude of 300 km, numerical calculations yield an S4 of about 1.1, in approximate agreement with measurements of Basu.
Physical optics approach for scattering by thin helical wires
Authors: Sezginer A. , Kong J. A.
Published: February 2008

A physical optics formulation of scattering by thin wire helices of finite and infinite extent is presented. The high frequency limit of the integral representation of the physical optics field is obtained by the saddle point technique and is shown to be identical to the field computed by the geometrical optics approximation. For thin wire helices of finite extent, the physical optics approximation to the current should be supplemented by current waves traveling along the wire that are excited by the discontinuities at the ends of the wire. This modified physical optics approach is found to be a useful approximation to compute scattering by nonresonant helices that have a larger pitch than approximately 5 wavelengths. A hybrid method composed of the modified physical optics technique and the method of moments is applied, for the degenerate case of a linear wire, in order to extend the region of validity to cover the resonant cases. The modified physical optics approach is typically 5–10 times faster than Galerkin's method, and most important, it provides physical insight into the scattering phenomenon.
Cross modulation of VLF and LF waves by gravity waves
Authors: Rumi G. C.
Published: February 2008

Some experimental data concerning amplitude and phase of two waves, one on 16 kHz and the other on 60 kHz, emitted in Rugby and received in Turin, are first presented. The amplitudes are affected by almost sinusoidal undulations; the oscillations of the 60‐kHz amplitudes are synchronous with those of the 16‐kHz amplitudes; furthermore, when the 60‐kHz amplitude increases, the 16‐kHz amplitude decreases in a specular fashion. This trend is explained in terms of a cross modulation imposed by traveling gravity waves that progressively modulate the ionization profile in the nocturnal D region of the ionosphere and hence affect the absorption of the VLF and LF waves. From this phenomenon, information about the lower tail of the nocturnal ionospheric D region can be obtained.
A hysteresis effect in the generation of field‐aligned irregularities by a high‐power radio wave
Authors: Jones T. B. , Robinson T. , Stubbe P. , Kopka H.
Published: February 2008

The anomalous self‐absorption of a high‐power radio wave measured during recent ionospheric modification experiments exhibits a hysteresis effect. These results are consistent with the assumption that the growth of field‐aligned irregularities due to the interaction of a high‐power pump wave and the ionospheric plasma occurs in two stages. During the first stage, striation growth is due to an instability which has a threshold independent of striation amplitude. When the striation level is sufficiently high, the threshold of the second stage is reached. The second‐stage threshold is dependent on the striation amplitude. The state of background ionosphere is also found to be an important factor influencing the observed effects.
Radio studies of the main F region trough in Antarctica
Authors: Dudeney J. R. , Rodger A. S. , Jarvis M. J.
Published: February 2008

Case studies of the equatorward movement of the main F region trough are presented, using data recorded by a digital ionospheric sounder located at Halley Bay (76°S, 27°W; Λ = 61.8, L = 4.2). Three events were chosen to give a range of magnetic conditions and local times. Comparison of the results, projected into the equatorial plane, with theoretical predictions of the simple ‘teardrop’ model of the plasmapause, shows remarkably good agreement for the two occasions when the level of magnetic activity remained approximately constant over the observing period. Consequently, estimates of the strength of the dawn‐dusk convection electric field in the magnetosphere have been made. However, magnetic activity changed during the course of the other event and the comparison gave poor results. The main F region trough results are also compared with the predictions of contemporary empirical equations which express the invariant latitude of the trough minimum as a function of local time and magnetic index, Kp, based on global satellite surveys. It is shown that none of these equations adequately accounts for the observed equatorward motion of the trough for these three cases. In particular, they contain insufficient allowance for the observed strong dependence of equatorward velocity upon magnetic activity. Some suggestions upon possible improvements to the empirical models are made which, if implemented, would be of great value for improving the reliability of predictions for high frequency, transauroral radio wave communications.
Analysis of a vertical, tubular cylinder which penetrates an air‐dielectric interface and which is excited by an azimuthally symmetric source
Authors: Johnson William A.
Published: February 2008

A method of moments analysis of a hollow, tubular cylinder which perpendicularly penetrates an air‐dielectric interface is carried out using a potentiallike formulation. Currents are excited on this cylinder by means of an azimuthally symmetric source. Since the thin‐wire kernel is not used and real‐axis integration of Sommerfeld integrands is employed, this analysis provides an independent check on the numerical electromagnetics code for the case of thin wires which perpendicularly penetrate an air‐dielectric interface. Sample results are given for thin wires which penetrate an air‐water interface.
An analytical and numerical comparison between two methods for the prediction of distributions of attenuations due to rain on terrestrial paths
Authors: Costa Emanoel , Dias Manoel G. S.
Published: February 2008

It is shown that the integral proposed by Misme and Fimbel for the prediction of the distribution of attenuations due to rain on a terrestrial path from the knowledge of the rain rate distribution in its neighborhood can, under some simplifying assumptions, be analytically transformed. Although the final expression is relatively involved, it is shown that its most important term agrees with the expression proposed by Assis and Einloft to perform the same prediction. This indicates that the method proposed by the latter authors is an approximate solution for the Misme and Fimbel integral. Rain attenuation values are also calculated by each prediction method for the same percentage of time, assuming a large number of combinations of input parameters, in order to display the relative difference between the results yielded by them. The obtained results show that the simplification introduced by Assis and Einloft on the Misme‐Fimbel method has no bearing on the agreement of its prediction with those obtained by other techniques.
Delta function excitation of waves in the earth's ionosphere
Authors: Vidmar R. J. , Crawford F. W. , Harker K. J.
Published: February 2008

Excitation of the earth's ionosphere by delta function current sheets is considered, and the temporal and spatial evolution of wave packets is analyzed for a two‐component collisional F2 layer. Approximations of an inverse Fourier‐Laplace transform via saddle point methods provide plots of typical wave packets. These illustrate cold plasma wave theory and may be used as a diagnostic tool since it is possible to relate specific features, e.g., the frequency of a modulation envelope, to plasma parameters such as the electron cyclotron frequency. It is also possible to deduce the propagation path length and orientation of a remote radio beacon.
Macroscopic Lagrangian description of warm plasmas: 1. Formulation of the Lagrangian
Authors: Peng Y.‐K. M. , Crawford F. W.
Published: February 2008

A macroscopic Lagrangian is derived which includes a pressure tensor, heat conduction, and elastic collisions. Its Euler‐Lagrange equations are shown to be the Maxwell equations and the macroscopic force law. The corresponding Hamiltonian is derived, and Hamilton's canonical equations are also demonstrated to lead to the Maxwell equations and the macroscopic force law. The treatment is facilitated by working in a mixture of Eulerian coordinates (for the fields) and Lagrangian coordinates (for the particle motions), and the introduction of a macroscopic potential expressed in terms of the plasma thermal energy and the energy losses by heat conduction. In paper 2 the macroscopic Lagrangian is applied to the description of nonlinear three‐wave interactions.
Scattering from an infinite array of periodic broken wires buried in a dielectric sheet
Authors: Gallagher J. G. , Brammer D. J.
Published: February 2008

The scattering of a plane wave from a periodic broken‐wire grid buried in a dielectric sheet is studied for a general angle of incidence and for arbitrary linear polarization. The scattered field is expanded in a set of Floquet modes, and the modal coefficients are obtained through the current expansion for a reflecting antenna. The general formulation of the problem is applicable for grids in an infinite dielectric and when located at the boundary interface of two dielectrics. A moment method is used to determine the current on the grid elements and numerical results for the transmission characteristics of several different grid configurations are presented. In addition, the effect on the resonant frequency of parametric variations in the grid geometry is determined. Predicted results are compared with measured transmission characteristics and show excellent agreement.
Wave scattering by an irregularity slab embedded in a stratitfied medium: Applications to ionospheric propagation
Authors: Kiang Yean‐Woei , Liu C. H.
Published: February 2008

Plane wave scattering by an irregularity slab embedded in a linearly stratified isotropic medium is studied under the single scatter approximation. In addition to regular reflection from the stratified background, both the incident and the reflected waves are scattered by the random irregularities. The behavior of the unperturbed field in the neighborhood of the turning point is accurately taken into account by using the full wave solution. The scattered fields can be interpreted in terms of the Bragg condition similar to the case of the well‐known Booker‐Gordon form, but with four terms accounting for the reflection effect of the turning point on both the unperturbed and scattered waves. The case of elongated irregularities is studied in detail. Analytical expression for the angular spectrum of the average scattered field intensity is derived, and its physical meaning discussed. In comparison with the case where the background medium is homogeneous, the relation to the two‐dimensional Booker‐Gordon formula is clarified. Applications to high‐frequency propagation in the disturbed ionosphere will be discussed.
Field distributions of trapped electron Bernstein waves launched by a point source in an inhomogeneous magnetoplasma
Authors: Ohnuma Toshiro , Watanabe Tsuguhiro
Published: February 2008

In an inhomogeneous magnetized plasma, field distributions of trapped electrostatic cyclotron harmonic waves are investigated numerically when an external point source is located outside or inside the plasma. The trapped field amplitudes and wave patterns are obtained spatially in the inhomogeneous plasma.
Doppler spectrum anisotropy observed in 1‐m irregularity auroral backscatter
Authors: Haldoupis C. , Hoeg P. , Nielsen E.
Published: February 2008

We present some atypical Doppler spectral observations made with STARE (≈140 MHz) during very disturbed magnetic conditions. The most striking feature is an anisotropy in spectrum width which is especially pronounced at times when the drift velocity is directed nearly along the bisector of the angle formed by the two radar lines of sight. Instead of both spectra being identical during these times, they were found to be quite different with one being systematically wider than the other even though their corresponding echo power is the same. The observing geometry in conjunction with spectral and drift velocity evidence support the notion that both radars observe secondary irregularities associated with longer wavelength two‐stream‐generated primaries propagating perpendicular to the observing directions. Even though the variations in spectrum width are closely correlated to drift velocity changes, it is clear that a parameter other than the ambient electric field must play the key role in the observed anisotropy. Following the linearized fluid approach proposed by Sudan et al. (1973), we can explain the observations if we assume an ambient destabilizing electron density gradient to exist in the plane perpendicular to the earth's magnetic field. By using the observed variations in drift velocity direction and assuming a power law dependence of spectrum width on the characteristic gradient length, we obtain a remarkably good agreement between the computed and the observed variations in spectrum width ratio. A consequence of our interpretation is that at times the secondary waves propagating at large angles relative to the main current flow can be highly structured in space resulting in narrow Doppler spectra.
EISCAT studies of F region irregularities using beam scanning
Authors: Hargreaves J. K. , Burns C. J. , Kirkwood S. C.
Published: February 2008

A beam scanning program to study the F region irregularities of smaller scale has been run on the EISCAT UHF radar. At the 300‐km level the scan covers 128 km in about 8½ min, there being north‐south and east‐west scans as well as periods with the beam stationary and directed along the local magnetic field direction. The paper gives examples of the variations of electron density observed at given ranges during a geomagnetically disturbed period. Power spectra suggest the presence of periodicities from one to several minutes during some sections of the runs. It is demonstrated that the velocities of field‐aligned irregularities can be determined from their observed range‐time behavior, and measurements thus made are compared with the plasma drift determined from tristatic reception. The irregularity motions appear to vary considerably during the time of one 8½‐min scan and in some cases may indicate a propagating wave.
Direct evidence of plasma density structuring in the auroral F region ionosphere
Authors: Tsunoda R. T. , Häggström I. , Pellinen‐Wannberg A. , Steen Å. , Wannberg G.
Published: February 2008

We investigate the hypothesis that large‐scale (≈ 100 km) plasma density enhancements (or “blobs”) found in the auroral F layer become structured via a magnetic‐flux‐tube interchange (MFTI) process. In such a process, plasma structure is produced when spatially irregular electric fields transport higher number density plasma (from within the blob) into a region containing lower number density plasma (the background ionosphere), and vice versa. Direct experimental evidence of this process can be obtained by measuring concurrently the spatial distributions of F region plasma density and electric field. Using the tristatic EISCAT radar facility, we measured these quantities in a two‐dimensional plane transverse to the geomagnetic field, at 300‐km altitude. We show, in a case study, that plasma density structure found along the poleward wall of a blob was indeed accompanied by similar‐scale variations in the ionospheric electric field, and that the sense of relative motion between high and low number density plasma is consistent with ongoing structuring of the plasma via an MFTI process. From the estimated growth rate of 3×10−3 s−1, the observed plasma structure could have been produced in several minutes by the irregular electric field pattern. The source of the MFTI process, however, is not clear. The MFTI process did not appear to be driven by F region polarization electric fields, a conclusion based on (1) the apparent lack of inverse correlation between plasma density and “slip” velocity (i.e., difference velocity between plasma and neutrals) patterns, and (2) the positive growth rate found along the poleward wall of the blob in the presence of a westward Pedersen current. This conclusion excludes (at least for this data set) the gradient drift and current‐convective instabilities as primary sources of the ongoing structuring process.
Comparative evaluation of several rain attenuation prediction models
Authors: Crane Robert K.
Published: February 2008

A comparative evaluation of several rain attenuation prediction models claiming global applicability was performed using data from 126 propagation paths in the CCIR data bank and 76 paths in the Thayer School data bank. The best predictive model relative to the available observations was determined, and the strengths of the different modeling techniques were evaluated. The path‐to‐path deviations between measurements and model predictions were found to obey a lognormal distribution. This result was obtained for the five prediction models subject to analysis: the Global and revised two‐component models by Crane, the revised CCIR model, a modified version of the lognormal model of Morita and Higuti, and a revised version of the “simple” model of Stutzman and Dishman. The root‐mean‐square differences between measurement and model prediction were model dependent. Overall, the revised CCIR model performed best. For applications in North America the Global model performed best, for Europe the revised CCIR model performed best, and for Asia the revised CCIR model was best. The models were tested in combination with modeled or measured rain rate distributions. The Global rain rate model produced better results than the CCIR rain rate model. When long‐term rain rate measurements were available, the best results were obtained by using the measurements.
A reexamination of the range resolution dependence of backscattered power observed by VHF radars at vertical incidence
Authors: Green J. L. , Gage K. S.
Published: February 2008

Experiments have been conducted using the 40‐MHz Sunset radar in Colorado and the 50 MHz Poker Flat, Alaska, MST radar to reexamine the range resolution (Δr) dependence of backscattered power for Fresnel scatter. Sunset radar observations now show a (Δr)1 dependence for backscattered power provided Δr > 1 km. For Δr < 1 km, results vary from day to day but are closer to (Δr)1 than (Δr)2. In an independent experiment the Δr dependence of backscattered power observed by the Poker Flat MST radar was also found to be close to (Δr)1.
Some useful expressions for spatial coherence functions propagated through random media
Authors: Tateiba Mitsuo
Published: February 2008

When a Gaussian wave beam propagates through an inhomogeneous random medium where the intensity and scale size of turbulence vary as functions of propagation distance, the coherence function is expressed in a compact form. It agrees entirely with the exact coherence functions for plane and spherical waves in the limiting cases where the minimum spot size of the wave beam tends to infinity and zero. Some expressions presented here are useful for many practical applications of the coherence function.
Singularity expansion method formulation for impulse response of a perfectly conducting thick cylinder
Authors: Chuang C.‐I. , Nyquist D. P. , Chen K.‐M. , Drachman B. C.
Published: February 2008

The impulse response of an infinite, perfectly conducting thick cylinder to normally incident, transversely polarized, impulsive plane wave illumination is determined. Spectral‐domain analysis based upon the singularity expansion method reveals that this response consists of a discrete series of natural resonance modes (natural frequencies are computed) augmented by a series of continuous‐spectrum terms. The resultant late‐time response demonstrates the correct “creeping wave” behavior as predicted by the Fourier synthesis technique, but with far fewer terms required for convergence.
Two‐frequency radiative transfer equation for scalar waves in a random distribution of discrete scatterers with pair correlations
Authors: Tsolakis A. I. , Besieris I. M. , Kohler W. E.
Published: February 2008

The Dyson equation and the two‐frequency Bethe‐Salpeter equation for vector‐valued electromagnetic waves in the presence of a random distribution of absorptive discrete scatterers with pair correlations are derived on the basis of the Twersky multiple scattering formalism. These equations are subsequently “scalarized” in the case of a tenuous scatterer distribution and within the framework of the Rayleigh‐Debye condition. A systematic transition is then made to a two‐frequency radiative transfer equation via a phase‐space approach. The main strength of the radiative transfer theory expounded here stems from the fact that it is applicable under conditions of large‐angle scattering, statistical inhomogeneities and statistical anisotropies. It accounts, also, for a variable scatterer density, absorption, and frequency offsets.
Aperture synthesis using orbiting telescopes
Authors: Kuiper T. B. H. , Synnott S. P. , Linfield R. P. , Resch G. M. , Tubbs E. F.
Published: February 2008

A study was carried out to determine the feasibility, with current technology, of performing aperture synthesis using two telescopes orbiting the earth in coordinated orbits separated by ∼ 10 m to 1 km. The objective was to determine whether there is a practical alternative to a very large, deployed, servo‐controlled submillimeter telescope (i.e., the Large Deployable Reflector) for obtaining high resolution submillimeter images of astronomical sources. We find that suitable classes of orbits exist which can provide good uv coverage over the entire sky and that real‐time correlation of wideband signals can be performed in orbit using current technology. The most difficult task appears to be the real‐time determination of the orientation of the baseline vector in a stable coordinate system. We have identified a plausible scheme for the determination of an arbitrary direction to within 0.003 arc sec in an astrometric coordinate system. This scheme not only makes submillimeter interferometric image reconstruction possible but should also have numerous other applications.
Air and precipitation particle motions within a cold front measured by the MU VHF radar
Authors: Wakasugi Koichiro , Fukao Shoichiro , Kato Susumu , Mizutani Akiyoshi , Matsuo Masaru
Published: February 2008

The VHF Doppler radar at Shigaraki, Japan, is shown to be capable of investigating mesoscale meteorological phenomena in the troposphere. Three dimensional velocities of air and precipitation particles in a cold frontal system are measured directly. The time and altitude resolution obtained with this system are unequalled by other conventional instruments. A strong and deep updraft associated with the frontal passage is observed extending above 10 km. Enhanced echoes accompanying the updraft are also detected with a meteorological radar.
High‐resolution MST observations of turbulence by using the MU radar
Authors: Sato Toru , Tsuda Toshitaka , Kato Susumu , Morimoto Shigeki , Fukao Shoichiro , Kimura Iwane
Published: February 2008

The full MU (middle and upper atmosphere) radar system has been in operation since late 1984. Among various possible observations, the standard tropospheric, stratospheric, and mesospheric observations have been carried out so far. Preliminary results of turbulence observations are presented. These observations demonstrate the capability of this system and also reveal some basic scattering properties.
The diurnal variation of backscattered power from VHF Doppler radar measurements in Colorado and Alaska
Authors: Nastrom G. D. , Ecklund W. L. , Gage K. S. , Strauch R. G.
Published: February 2008

Results from a statistical analysis of backscattered signal power measurements at 4–18 km by the clear‐air Doppler radars (MST radars, or wind profilers ) in the Colorado Profiler Network and at Poker Flat, Alaska, show a systematic diurnal variation during the summer months. These diurnal variations have a high level of statistical significance. In the midtroposphere the diurnal range is near 7 dB with largest values in the afternoon, probably associated with the enhanced turbulence and moisture levels due to afternoon convection. In the stratosphere the diurnal range is about 5 dB over Colorado, with smallest values in the afternoon. The stratospheric cycle is interpreted as a response to modulation of the amplitude of gravity waves propagating upward from the troposphere. The diurnal cycle at all heights becomes indistinct during the winter months.
Es structure using an HF radar
Authors: From W. R. , Whitehead J. D.
Published: February 2008

By using an HF radar which produces a steerable beam about 4° wide and measures angle of arrival and Doppler shift of radio echoes, the structure of various types of mid‐latitude sporadic E (Es) has been determined. Totally reflecting Es is a very smooth layer, tilted less than 1° from the horizontal. Partially reflecting Es consists of clouds of ionization. These clouds vary in size from a few kilometers to 25 km in the direction of movement and larger in the transverse direction. Echoes often disappear rapidly: the clouds either disappear quickly or have sharp edges. Spread Es has a curious structure of small clouds, each of which reflects only for a few seconds, but each cloud moves with the same velocity, typically 100 m/s, even though the heights of the clouds vary up to 10 km. It is difficult to reconcile this finding with the presence of wind shears.
Anomalous vertical magnetic field for electromagnetic induction in a laterally varying thin conductive sheet
Authors: Hill David A. , Wait James R.
Published: February 2008

We employ a simple model to show how the natural electromagnetic field on the surface of the earth, which has a strong horizontal magnetic field component, can be converted to a significant vertical magnetic field at the surface. Such a conversion mechanism will be caused by lateral variations of the subsurface conductivity structure. Our idealized model is a thin conducting sheet with a periodic variation of the conductivity‐thickness product in one horizontal direction only.
Twenty years of international satellite communication
Authors: Evans J. F.
Published: February 2008

This paper reviews the development of international satellite communications over the past two decades. To date over 100 satellites have been launched to provide both domestic and overseas commercial (i.e., non‐military) communications services and this paper largely confines itself to describing the technical developments involved in creating the INTELSAT system which are representative of the field as a whole. INTELSAT, is an international organization established in 1964 to provide overseas communications. It has developed a global system designed to provide (1) a high degree of connectivity and (2) a large communications capacity. Connectivity has been achieved using relatively broad (albeit “shaped”) satellite antenna patterns permitting earth stations in many countries that are illuminated by the same beam to be interconnected. This desirable attribute has, however, required the use of large earth stations and one future technical direction is likely to entail using satellites that generate narrow pencil beams permitting the use of smaller earth stations; on‐board switching will then be required to restore connectivity between beams. A large capacity has been achieved through the use of “frequency reuse,” i.e., techniques to utilize the C‐band frequencies assigned for satellite communications several‐fold. This has been accomplished through satellite designs that employ isolated antenna beams to illuminate different parts of the earth and/or beams that view overlapping parts but employ opposite senses of (circular) polarization. Associated with this approach have been efforts to use the assigned bands as efficiently as possible through “channelization,” i.e., subdividing them into frequency intervals, each of which is served by a transponder amplifier designed to yield minimum interference with its neighbors. The paper concludes with a brief discussion of the likely trends in the design of satellite systems as these evolve in competition with fiber‐optic cables.
Experiments with maximum entropy and maximum likelihood spectra of VHF radar signals
Authors: Klostermeyer J.
Published: February 2008

Maximum entropy and maximum likelihood methods (MEM, MLM) are applied to estimate wind‐induced Doppler shifts of VHF radar signals and to compare the accuracies of both methods with the accuracy of the periodogram method used with the SOUSY VHF Radar. At large signal‐to‐noise ratios (S/N ≳ 100), differences between the three Doppler shift estimates are one order of magnitude smaller than the frequency resolution of the periodogram. At moderate signal‐to‐noise ratios (0.3 ≤ S/N ≤ 10), MEM and MLM estimates have significantly higher accuracies than the periodogram estimates with the largest differences in accuracy occurring just above S/N = 0.3; MLM tends to be slightly more accurate than MEM. In contrast to the periodogram method, MEM and MLM clearly indicate a limiting signal‐to‐noise ratio (S/N ≈ 0.3) below which the Doppler shift estimates are dominated by noise rather than radar signals. For any routine application it is of basic importance that there are stochastic relationships between signal‐to‐noise ratio and the prediction error filter lengths of MEM and MLM that yield optimum Doppler shift estimates. Since the accuracy of Doppler shifts obtained by MEM and MLM increases with decreasing spectral width, both methods have their greatest potentials when they are applied to radar signals measured with small antenna beam widths, small pulse lengths and short coherent integration times.
Shallow sounding of crustal regions using electromagnetic surface waves
Authors: Brown M. F. , King R. W. P.
Published: February 2008

A specific physical interpretation of the effect of relative permittivities εr and conductivities σ upon the amplitude and phase of propagating electromagnetic surface waves is given, where ϵr and σ characterize the layered material regions of a planetary lithosphere. The interpretation is made possible by the availability of simple formulas for the fields of vertical and horizontal unit electric dipoles near interfaces between regions of matter; the earth's lithosphere at its interface with salt water is used as an example. The ranges of εr, σ, frequency ƒ, and radial distance ρ over which the formulas apply are shown to be quite broad. The E1ρ component (the radial electric field in region 1, characterized by complex wave number K1 and adjoined at a planar interface to region 2, characterized by k2, where |k1|2 » |k1|2) is discussed in detail. The attenuation of 20 log10 |E1ρ| as a function of ρ and ρ/δ2, Where δ is the skin depth, is illustrated for conductivity ratios σ2/σ1 = 10−1 to 10−3, over a range of ƒ from 1 Hz to 105 Hz. |E1ρ| is shown to be significant for determining permittivities ε2 at the higher frequencies in this range. The use of |E1ρ| to infer conductivities σ2 at lower frequencies in shallow sounding of the earth's sea floor is discussed in terms of specific lithological and sediment‐related parameters. The technology required to transmit and receive 20 log10 |E1ρ| at freshwater and saltwater interfaces with the earth's crust is addressed. A specific experiment is proposed for testing a specialized insulated antenna for seafloor use in shallow coastal waters at 1 kHz.