bluuebunny/crossref_metadata_2025 huggingface dataset:
Performance of VLF, LF, and MF telecommunication systems in a simulated satellite power system environment ascertained by experimental means
Authors: Rush C. M. , Violette E. J. , Espeland R. H. , Carroll J. C. , Allen K. C.
Published: February 2008

The purpose of this paper is to describe the results of experiments undertaken to assess the potential impact of the operation of the Satellite Power System on the D and E regions of the ionosphere, and on telecommunication systems that are dependent upon the structure of the lower ionosphere. Using the high‐power, high‐frequency transmitter facility located at Platteville, Colorado, power densities equivalent to the Satellite Power System can be delivered to heights of 70 to 100 km above the surface of the earth. Observations of the performance of telecommunication systems that operate in the VLF, LF, and MF portions of the spectrum have been investigated during times when the ionosphere was modified with power densities comparable to the Satellite Power System and when it was not. The results obtained indicate that the Satellite Power System as currently designed with a peak power density of 23 mW/cm2 is not likely to impact in an adverse manner upon the performance of VLF, LF, and MF telecommunication systems.
Reflection of oblique electron thermal modes in an inhomogeneous magnetized plasma
Authors: Ohnuma T. , Watanabe T. , Sanuki H.
Published: February 2008

In an inhomogeneous magnetoplasma, reflection of an oblique electron thermal mode radiated from a local source is investigated experimentally and theoretically near the electron plasma frequency layer. The experimental observation of reflection in the plasma density region lower than the ƒp layer is found to be in qualitative accord with the theoretical reflection, which is obtained from a kinetic theory in an inhomogeneous magnetoplasma. The reflection of the thermal mode is also compared with that of an electromagnetic mode at the ƒp layer.
Modifying effects of a strong electromagnetic wave upon a weakly ionized plasma: A kinetic description
Authors: Stubbe P.
Published: February 2008

The Boltzmann equation for a weakly ionized plasma under the action of a strong electromagnetic wave is solved numerically, assuming that the wave frequency is much larger than the electron gyrofrequency. Elastic collisions between electrons and neutral particles as well as rotational, vibrational, and electronic excitation are taken into account. The solutions are used to investigate to which extent the electron temperature and the propagation characteristics of strong electromagnetic waves in the lower ionosphere are affected by deviations of the electron distribution function from a Maxwellian distribution.
Scattering and diffraction of electromagnetic waves by inhomogeneous dielectric cylinder
Authors: Miyazaki Y.
Published: February 2008

Dielectric cylinders with inhomogeneous distribution of dielectric constants are very useful for high‐capacity optical communications as optical fibers with small dispersions. The measurements of refractive index distribution are indispensable for the development of manufacture of long‐distance optical glass fibers with high qualities. In order to develop these scattering pattern methods, this paper discusses exact analysis of asymptotic scattering and diffraction fields of vertically incident optical waves by inhomogeneous dielectric cylinder with arbitrary graded index profile. The scattering and diffraction fields are represented by a summation of fast convergent residue value series and the steepest descent integrals, by Watson transformation, that correspond to creeping waves propagating along boundary surfaces of the cladding and direct refracted and reflected waves in core and cladding, respectively. The physical meaning of each term is studied, comparing scattering pattern experiments of graded index fibers with arbitrary inhomogeneous index distributions.
Dielectric feeds with low cross polarization
Authors: Hombach Volker
Published: February 2008

Dielectric feeds, consisting of a specially shaped dielectric inserted into a waveguide aperture, can radiate patterns which are circular or elliptical in cross section and provide suitable cross‐polar performance. The patterns have been calculated approximately by applying twice the Kirchhoff‐Huygens principle. Results of theoretical and experimental investigations are presented for several dielectric feeds with good cross‐polarization behavior.
Roots of the modal equation for em wave propagation in a tropospheric duct
Authors: Ott R. H.
Published: February 2008

The solution of the modal equation for electromagnetic wave propagation in a tropospheric duct, having a trilinear refractivity profile, shows the existence of two sets of roots in the complex propagation plane. One of these is of the whispering gallery type with very low attenuation and the other is of the waveguide variety with relatively high attenuation. When the contrast in refractive index is small, the lower‐order waveguide modes have an attenuation comparable with the attenuation of the whispering gallery modes. A large contrast in refractive index causes the whispering gallery modes to dominate (surface or elevated ducts). Several examples of the altitude charts for the modal equation for propagation at 200 MHz through a medium having various contrasts in the refractive index give some physical insight into these two classes of modes.
Mean dyadic Green's function for a two layer random medium
Authors: Zuniga M. A. , Kong J. A.
Published: February 2008

The mean dyadic Green's function for a two‐layer random medium with arbitrary three‐dimensional correlation functions has been obtained with the zeroth‐order solution to the Dyson equation by applying the nonlinear approximation. The propagation of the coherent wave in the random medium is similar to that in an anisotropic medium with different propagation constants for the characteristic transverse electric and transverse magnetic polarizations. In the limit of a laminar structure, two propagation constants for each polarization are found to exist.
New image representation for dipoles near a dissipative earth 1. Discrete images
Authors: Mahmoud Samir F. , Metwally Ahmed D.
Published: February 2008

The image theory for dipoles above an imperfectly conducting earth which has been developed by Wait, among others, is used to compute the change in the input impedance of a vertical magnetic dipole (VMD) due to the presence of the earth. Comparison with a previously developed rigorous solution is made and a good agreement is obtained, particularly when the conduction currents in the earth dominate over the displacement currents. Extension of the‐single image theory is made in two ways. First, multi‐images which are spread in space are used to represent the effect of the earth. Second, multipole sources having the same location are used. Both representations have shown a noticeable improvement over the single image in relation to computational results on the input impedance of a VMD.
Field of separable electric source current distribution in free space
Authors: Richmond Jack H.
Published: February 2008

Simple rigorous expressions are developed for the fields of an electric source current distribution that is separable in cartesian coordinates. The medium is unbounded free space, and the time‐harmonic source current exists in the slab region z1 < z < z2.
Using radar to estimate dissipation rates in thin layers of turbulence
Authors: Weinstock J.
Published: February 2008

A method is described for estimating the energy dissipation rate, ε, from radar measurements of the structure parameter, C2n, in stably stratified layers of turbulence. This estimate applies to horizontal layers of turbulence whose thickness, Lt, is less than the vertical resolution, 2Δ, of the radar (Lt, < 2Δ). Both ε within the turbulence layer and
the average of ε over the radar volume, are determined. The derived expression for ε is based on a theoretical relation of turbulence layer thicknesses to the buoyancy length.
Multiple scattering analysis of optical wave propagation through inhomogeneous random media
Authors: Tateiba Mitsuo
Published: February 2008

A moment equation is obtained in the small angle approximation for optical waves propagated through such a random medium that the material parameter fluctuates inhomogeneously in the direction of propagation and homogeneously in the direction transverse to the propagation path. The equation is an extension of the conventional moment equation and is derived by the same method as used in a homogeneous random medium. The limit of applicability of the equation is clear, and the equation can be applied to most of the practical cases of optical propagation through inhomogeneous random media. The inhomogeneous randomness is contained merely within the coefficient of the equation. Some effects of the inhomogeneous randomness are precisely given by solving the moment equation. A multifrequency moment equation can also be given.
A theory for near‐normal incidence microwave scattering from first‐year sea ice
Authors: Brown Gary S.
Published: February 2008

A theory is presented to explain near‐normal incidence microwave scattering from first‐year sea ice. The theory is based upon the hypothesis of an abnormal number of relatively large (in terms of wavelength) flat areas on the surface which are all at the same elevation relative to mean sea level. These flat areas are assumed to be randomly located circular spots with random radii. The incoherent power scattered by these equiheight flat areas is a result of their random transverse location while the coherent scattered power is a consequence of their equal elevation. The incoherent power is found to vary as the average number of flat spots per unit area while the coherent power varies as the second moment of the number per unit area. Application of this theory to typical radar altimeter data produces parameter estimates which are in reasonable agreement with the available surface truth. Finally, it is conjectured that this model may also explain certain anomalous scattering data obtained from other types of terrain and planetary surfaces.
Frequency response of a synthesis array: Performance limitations and design tolerances
Authors: Thompson A. R. , D'Addario L. R.
Published: February 2008

The frequency responses of the receiving channels of a radio astronomy synthesis array and, in particular, the variations between them for different antennas are factors that limit the accuracy and sensitivity of the instrument. A response that approximates a rectangular passband as closely as possible is advantageous from considerations of maximizing sensitivity within a given frequency band and minimizing the smearing of detail in the outer regions of a map. Tolerances on slopes, ripples, and other bandpass distortions can be determined by examining their effects with regard to loss in sensitivity and the introduction of errors in the assignment of complex gain factors for individual antennas by using calibration observations. These calibration errors are generally larger in amplitude than in phase and their effect is usually more serious than the loss in sensitivity. Velocity dispersion in waveguide transmission systems and the accuracy in adjustment of compensating time delays also affect the phase responses of the signal channels, and examination of these effects also leads to limits on system design parameters. Implementation of the tolerances derived, including some details of the filter specifications, is discussed for the very large array radio telescope of the National Radio Astronomy Observatory. Appendices contain an algorithm for performing a least squares solution for antenna gains and an analysis of the variation of the signal‐to‐noise ratio over the channels of a spectral line correlator system.
Characteristics of a power line used as a VLF antenna
Authors: Dazey M. H. , Koons H. C.
Published: February 2008

The 100‐kW transportable very low frequency transmitter system was used at Kafjord, Norway, for transmissions to the SCATHA and GEOS spacecraft. A 22‐kV, 10.6‐km long transmission line was used as an antenna. Modifications were made in the line to reduce telephone interference. Components were designed and installed to reduce the resonant frequency and increase the antenna current. The final practical operating current was 45 A at 1280 Hz. This resulted in power dissipation of 72 kW and an estimated radiated power of 0.17 to 0.79 W.
Theoretical and calculational aspects of the radio refractive index of water vapor
Authors: Hill R. J. , Lawrence R. S. , Priestley J. T.
Published: February 2008

The radio refractive index is calculated by summing the refraction due to the infrared resonances of the water vapor monomer. Although this calculated refraction agrees well with measured values, its temperature dependence departs significantly from the measured variation with temperature. The rotational resonances of water vapor give a contribution to the radio refractive index that does not vary precisely as e/T2; theoretical reasons are given for this departure from the classical refraction by permanent dipoles.
Multiple scattering calculations of rain effects
Authors: Ishimaru Akira , Woo Richard , Armstrong J. W. , Blackman Doris C.
Published: February 2008

At millimeter wave frequencies the scattering cross section of raindrops becomes comparable to the absorption cross section, and incoherent intensity due to multiple scattering may become significant. This paper presents calculations of the incoherent intensity at 30, 60, 90, and 120 GHz. The scattering and absorption characteristics of the rain are calculated using the Mie solution and the Laws‐Parsons distribution. Incoherent intensities for the horizontal and vertical polarizations are obtained using the equation of transfer and the Stokes parameters. The obliquely incident wave is linearly polarized. The equation of transfer is solved by using the Gauss quadrature formula and the matrix eigenvalue technique. The matrix elements are calculated using the formulations developed by Sekera, and the vertical and horizontal components and their correlations are used in the Stokes parameter representations. The received incoherent intensity depends on the field of view of the receiving antenna. The ratio of the copolarized incoherent intensity to the copolarized coherent intensity is defined as the incoherent copolarized discrimination I‐CPD, and the ratio of the cross‐polarized incoherent intensity to the copolarized coherent intensity is defined as the incoherent cross‐polarized discrimination I‐XPD. These are computed in terms of rain rate, field of view, and copolarized attenuation. It is shown that multiple scattering effects may become significant during heavy rains. Backscattering results are also presented.
Scattering by single ice needles and plates at 30 GHz
Authors: Yeh C. , Woo R. , Ishimaru A. , Armstrong J.
Published: February 2008

Because of the small loss tangent of ice at 30–50 GHz, significant depolarization of satellite‐to‐ground communication signals could occur due to the presence of cirrus clouds in their paths. It is known that cirrus clouds basically consist of ice needles and plates. Extensive calculations have been performed for the problem of scattering by a family of ice needles and plates for various length‐to‐width ratios ranging from 1 to 10. Scattering results with special attention to induced cross‐polarized fields will be presented.
Studies of microwave absorption in liquids by optical heterodyne detection of thermally induced refractive index fluctuations
Authors: Davis Christopher C. , Swicord Mays L.
Published: February 2008

A new method for obtaining the microwave absorption spectrum of optically transparent condensed media is described. Phase fluctuations impressed on a single‐frequency laser beam passing through a sample are detected by optical heterodyning. These phase fluctuations result from density fluctuations induced by the absorption of microwave energy. Preliminary experiments on water and ethanediol yield absorption spectra which are consistent with those obtained in previous investigations using conventional techniques.
Extension of Lin empirical formula for the prediction of rain attenuation
Authors: Kanellopoulos J. D.
Published: February 2008

Lin's empirical expression for calculating rain attenuation is applicable to 5‐min point rain rate distributions. A modification of this empirical formula is presented, for application to rain rate with arbitrary integration time. The applicability of this technique is tested against experimental data taken from several locations in the United States and western Europe.
Direct‐ and cross‐polarized scatter from a turbulent laboratory plasma
Authors: Tremain Dennis E. , Graf Kurt A. , Guthart Harold
Published: February 2008

This paper presents backscatter data from a turbulent‐plasma scattering experiment that is principally concerned with depolarization of the incident signal. Results of calculations using models for the direct‐ and cross‐polarized cross sections are presented. A model based on reflection from an ‘overdense core’ accounts well for the observed results when the plasma is refractively overdense. Measurements show that a circularly polarized incident signal depolarizes more than a linearly polarized incident signal. The depolarization depends on the angle of incidence and electron density.
Multiple scattering theory for waves in discrete random media and comparison with experiments
Authors: Varadan V. K. , Bringi V. N. , Varadan V. V. , Ishimaru A.
Published: February 2008

Attenuation of electromagnetic waves by a random distribution of pair‐correlated dielectric spheres is studied as a function of frequency and volume concentration of spheres. The main aim of this paper is to compare theoretical results obtained using a self‐consistent multiple‐scattering formulation and measured values of attenuation for latex spheres in water. The agreement between theory and experiment is very good.
Gigahertz scintillations associated with equatorial patches
Authors: Aarons J. , Klobuchar J. A. , Whitney H. E. , Austen J. , Johnson A. L. , Rino C. L.
Published: February 2008

In a program designed to compare measurements of scintillation activity in the equatorial anomaly region, observations were made of satellite beacon signals transmitting at frequencies ranging from 137 MHz to 7 GHz. Recordings were made at Ascension Island in January–February 1981, during a month of very high solar flux and a high occurrence of scintillations. Saturation was noted in the VHF‐UHF range with levels of 8 dB peak to peak at 4 GHz and 3 dB peak to peak at 7 GHz. Statistics of occurrence of various levels for 1.5 and 4 GHz are given in the paper. The hypotheses of vertical or horizontal irregularity sheets or rods within the patches of irregularities were examined with data from the Global Positioning System satellites. Vertical sheets were eliminated as a possibility. A comparison of scintillations with 6300‐Å airglow images, which map regions of depleted electron density over the entire sky, showed that in the anomaly region, maximum scintillation activity occurs within the patch and not at the walls of the patch.
Spaced antenna drift at Jicamarca, mesospheric measurements
Authors: Røyrvik Ola
Published: February 2008

A spaced antenna drifts experiment was implemented for the Jicamarca mesosphere‐stratosphere‐troposphere radar. The whole radar antenna was used to transmit a radio signal vertically, and three separate quarters of the antenna were used to receive the scattered radio signal. Drifts data from the mesosphere were analyzed by the method of similar fades. This technique appears not to be as well adapted to measure horizontal velocities as the Doppler beam swinging technique usually used at Jicamarca. It was shown that for large antennas the error in the estimated horizontal velocity increases with increasing antenna dimensions. It follows directly from this finding that an increase in aspect sensitivity of the scattered power will also cause an increase in the error in the measured horizontal velocity. It was shown that short‐period (<15 min) oscillations in the calculated drift of the irregularity pattern over the ground are most likely to be caused by horizontal variations in the vertical velocity within the scattering volume in the mesosphere.
Observations of enhanced plasma lines by EISCAT during heating experiments
Authors: Hagfors T. , Kofman W. , Kopka H. , Stubbe P. , Äijanen T.
Published: February 2008

Some initial observations of heater‐induced plasma line enhancements as observed by EISCAT are described. The most striking differences of these results from those previously observed at Arecibo are in their much more pronounced overshoot at the time of the heater turn‐on and the way in which the higher‐order spectral features show a clear line structure.
SABRE observations of a sequence of Pc 5 micropulsations
Authors: Waldock J. A. , Thomas E. C. , Stewart C. P. , Jones T. B. , Nielsen E.
Published: February 2008

Observations of a Pc 5 micropulsation event, using the Wick half of the new SABRE auroral radar, are presented. During a 3‐hour period in the early morning of September 19, 1981, a train of pulsations were recorded. Analysis revealed that there were three main events, rather than one continuous disturbance. The first event appeared as a perturbation of only one or two cycles, extending throughout the latitude of the viewing area. It was also visible in the STARE region, but the perturbation drift velocity was in the north‐south direction as opposed to the east‐west perturbation drift of a more conventional toroidal mode field line resonance. The second disturbance was an isolated, one‐cycle perturbation, also visible throughout both SABRE and STARE viewing areas and also having a dominant north‐south velocity component. Finally, a very localized monochromatic event, lasting five cycles or more, with a period of about 5 min, was recorded. The characteristics of the third event were found to be consistent with those predicted by field line resonance theory.
A comparative analysis of some prediction methods for rain attenuation statistics in earth‐to‐space links
Authors: Macchiarella Giuseppe
Published: February 2008

The results of a comparison among six prediction methods of rain attenuation statistics developed in the United States, Europe, and Japan are presented. The models have been tested through attenuation at 11.6 GHz and rainfall data collected in Italy at Fucino, Gera Lario, and Spino d' Adda during the SIRIO experiment during 1978–1982.
Modeling of equatorial multifrequency scintillation
Authors: Franke S. J. , Liu C. H.
Published: February 2008

Analytical and numerical techniques are used to model multifrequency amplitude scintillation data recorded in the equatorial region at Ascension Island. In particular, the behavior of the temporal coherence interval of multifrequency amplitude scintillation recorded at VHF, L band and C band is studied. The data cover a wide range of perturbation strengths corresponding to scintillation indices (S4) in the range 0.05–0.25 at C band (4 GHz). The dependence of the 50% decorrelation interval of the amplitude fluctuations on wave frequency and scintillation intensity is compared to analytical and numerical simulation results that are computed using a phase screen model. The observations are found to agree closely with simulated scintillation generated using a two‐component power law spectrum model for the ionospheric irregularities. A simple inverse relationship is found to exist between the correlation interval of saturated scintillations at VHF and the perturbation strength as measured by the C band scintillation index. The result is shown to be consistent with analytical results based on an asymptotic approximation to the covariance function for intensity scintillations.
Statistical model for low‐frequency propagation
Authors: Ferguson Jerry A. , Morfitt David G. , Hansen Peder M.
Published: February 2008

Prediction of LF signal strengths is usually done by using a single “average” model of the ionosphere. However, this approach suffers from the fact that signal strength is a nonlinear function of ionospheric parameters. Hence the “average” ionosphere does not always give the correct average signal strength. In addition, the standard deviation of signal strength varies considerably with position, in general being larger in the vicinity of modal interference nulls. A model where the daytime ionosphere is treated statistically is presented. The propagation model used is an earth ionosphere wave‐guide. The ionospheric electron density profile is specified by a reference height h′ and slope β. These parameters are assumed to be distributed in a jointly normal distribution. Best fit values for this model are obtained using measurements made at two points on either side of a deep modal interference null located about 1000 km from a 59‐kHz transmitter. The measured signal strength mean, and standard deviation were used to obtain a best fit mean, standard deviation and correlation coefficient for the β and h′ distribution. The resulting ionospheric parameters give signal statistics that fit the measured mean and standard deviation at both monitoring points within 1 dB.
Unstable density gradients in the high‐latitude ionosphere
Authors: Cerisier J. C. , Berthelier J. J. , Beghin C.
Published: February 2008

Electron density irregularities at medium scale (from a few kilometers to tens of kilometers) have been observed in the high‐latitude topside F region by the satellite Aureol 3. These irregularities are field‐aligned structures, as evidenced by their ducting effect on downgoing ELF hiss. It is often observed that the density enhancements have an asymmetrical structure in the direction perpendicular to the magnetic field. On one side of the density enhancement, an intense electrostatic turbulence is observed together with large density fluctuations. The other side is usually steeper and appears stable. These observations suggest that the density irregularities are convected by the dc electric field and that the instabilities are of the gradient drift type.
Scattering from a periodic array of conducting bars of finite surface resistance
Authors: Rubin Barry J. , Bertoni Henry L.
Published: February 2008

In this paper we treat the scattering of electromagnetic waves from a two‐dimensional structure consisting of a periodic array of conducting bars of rectangular cross section. The E mode case is considered, in which current essentially flows around each bar, perpendicular to the bar axis. Numerical solutions are obtained based on approximating the currents on each bar by a set of P “triangle” functions. The electric field radiated by the periodic array of triangle current functions is found by standard Fourier series techniques. The field is required to satisfy the boundary conditions in an integral sense over P intervals associated with the triangle functions. The resulting P equations in P unknowns are solved numerically. Finally, a number of examples are analyzed, including transmission gratings and parallel plate polarizers.
Amplitude scintillations during the early and late phases of evolution of irregularities in the nighttime equatorial ionosphere
Authors: Bhattacharyya A. , Rastogi R. G.
Published: February 2008

Temporal variation of amplitude scintillations on VHF and UHF signals arising due to the presence of irregularities in the night time equatorial F region is studied during the early and late phases of evolution of these irregularities. Power spectra are calculated for the amplitude of 40, 140 and 360 MHz signals from a geostationary satellite ATS‐6 measured at an equatorial station Ootacamund (11.4°N, 76.7°E, magnetic dip 6°) for the growth and decay phases of three scintillation events. The roll‐off portions of the power spectra in the decay phase are found to be significantly steeper than those in the growth phase in all the cases. From a rough estimate of the irregularity drift speed obtained from the observed Fresnel frequency in each case, the steepening of the roll‐off portion in the decay phase can be attributed to the dissipation of irregularities with wavelengths of several hundred meters in this phase. The slope is found to be steepest for the decay phase of that event which is associated with the highest irregularity drift speed.
Covariant electromagnetic theory for inertial frames with substratum flow
Authors: Wilhelm H. E.
Published: February 2008

Based on the Galilean relativity principle and Maxwell's equations, electromagnetic field equations are derived for inertial frames, in which the substratum of the electromagnetic waves flows with arbitrary velocity | w | < c (velocity of light). It is demonstrated that the electromagnetic field equations with electromagnetic substratum flow are strictly covariant against Galilei transformations. Wave equations, conservation and invariance theorems, and boundary conditions are derived for the electrodynamic fields in presence of electromagnetic substratum flow. Initial‐boundary‐value problems are solved for electromagnetic signal propagation and induction in the substratum by an integral equation method. Physical effects for the measurement of the velocity field of the electromagnetic substratum are discussed. Maxwell's conception that his equations refer to a frame of reference with resting electromagnetic substratum is confirmed, and it is shown that Maxwell's equations are also applicable to inertial frames with small substratum velocities, | w | ≪ c.
Application of an offset mesh for the transient, explicit finite‐difference analysis of the perfectly conducting rectangular plate
Authors: Riley Douglas J. , Davis William A.
Published: February 2008

In this paper an offset mesh is used for the transient finite‐difference analysis of the rectangular plate. An explicit, or time‐marching, finite‐difference approximation for the electric field integral equation is used. It is demonstrated that the offset mesh represents a more natural discretization for obtaining the current response of the plate than the standard (nonoffset) mesh typically used, but a stability criterion which differs from the one associated with the standard scheme is required. Results obtained from both the standard mesh and the offset mesh are compared, and the difference in the stability criterion required for each mesh is discussed.
Analysis of the near and far fields of an offset paraboloid illuminating a rotatable flat passive reflector
Authors: Narasimhan M. S. , Philips Bobby
Published: February 2008

In this paper an analysis of the near‐ or far‐field patterns of an offset paraboloid (fed at its focus) illuminating a rotatable flat reflector for scanning the main beam of the offset reflector is presented. This analysis is based on the near‐field GTD analysis developed earlier and the known techniques for transforming near fields to far fields. Computed results on the far‐field patterns of a typical offset paraboloid illuminating a flat reflector are presented. Further, the computed results on the far‐field patterns of a typical paraboloid illuminating a square flat reflector satisfactorily correlate with the available measured results.
Comparison of evaporation duct and path loss models
Authors: Patterson Wayne L.
Published: February 2008

This paper will evaluate the current meteorological measurement techniques used to infer the evaporation‐ducting process. Included within this study is a comparison of relative performance, sensitivities to meteorological inputs, and ease of computation for three standard evaporation duct models and two electromagnetic wave propagation models. The evaporation duct models performed reasonably and equally well when considering statistically averaged meteorological inputs, but all were extremely sensitive to point‐observed meteorological input parameters. The measurement techniques employed by naval and transiting commercial vessels are not of sufficient quality to infer adequately the evaporation‐ducting process. The theory behind calculation of path loss values based upon current evaporation duct inputs is statistically sound. For studies of propagation phenomena or development of electromagnetic systems, both of the compared models perform reasonably well and are accurate predictors. However, the use of either model for estimation of a single propagation range based upon a point meteorological observation is severely limited by the accuracy of the data.
Measurement of turbulent energy dissipation rates in the middle atmosphere by radar techniques: A review
Authors: Hocking W. K.
Published: February 2008

Radars operating in the frequency band between 2 MHz and several hundred megahertz are capable of supplying a large data base of measurements of turbulent energy dissipation rates in the middle atmosphere. So far this has not been achieved; only occasionally have such radars been used to produce estimates of turbulence intensities. In order to encourage a greater emphasis on this aspect of radar studies of the middle atmosphere, this review summarizes the various techniques which can be used to measure turbulent energy dissipation rates. It is shown how absolute measurements of backscatter cross section can be used to measure turbulence intensities. A new theory is presented which shows that the power backscattered from the mesosphere depends on the turbulent energy dissipation rate, the electron density gradient, the neutral density scale height, the total electron density and the temperature gradient. The effects of turbulence on the width of signal spectra received by these radars are discussed, and it is shown how turbulence intensities may be extracted from spectral width measurements. The importance of removing nonturbulent processes which also broaden the width of the power spectra, such as wind shear broadening and beam width broadening, are stressed.
Lateral wave propagation in the presence of a conducting plate
Authors: Dunn John M.
Published: February 2008

Lateral waves can propagate when an antenna is placed close to an interface between two different media. In this paper, a study is made of how the lateral wave is affected by a conducting plate placed parallel to the interface. This changes the type of decay of the wave along the interface from that of algebraic to exponential. A new method is used in developing the formulas. Impedance boundary conditions are used to decouple the two media at each step of an iterative procedure. Single medium techniques can then be used in each region. The formulas are seen to agree well with numerical results from the exact integral representations of the fields.
International reference ionosphere: Recent developments
Authors: Bilitza Dieter
Published: February 2008

An introduction to the history and intentions of the International Reference Ionosphere (IRI) is given and future plans are outlined. The description of the topside electron density profile in the IRI is based on Bent's summary of topside sounder measurements, mainly Alouette. Instead of Bent's tabular form, the IRI has the advantage of an analytical description. Our comparison with AEROS satellite data shows that the IRI model is inaccurate in its description of the latitudinal behavior of the electron density in the topside ionosphere close to the magnetic equator. An improved IRI formula is proposed that closely follows the experimental data. The present IRI is restricted to solar activities reached during the last solar cycle 20. Measurements during the present solar cycle indicate a saturation effect for very high solar activities.
Effect of nonfrozen turbulence on scattering of electromagnetic waves
Authors: Dong Bao
Published: February 2008

In this work the limitation of using a frozen model to describe a turbulent medium is discussed. Among other assumptions, the frozen model requires the statistically independent velocity fluctuations to have infinitely large scales in comparison with scales of the refractive index fluctuations. When the scale of velocity fluctuations becomes comparable with or smaller than the scale of refractive index fluctuations, the frozen model is no longer valid and has to be modified. Upon three hypotheses, a non‐frozen model is proposed in this paper. These hypotheses are generalizations of the ones used in the past. They include: (1) Both the refractive index and the velocity field of the turbulence are isotropic, stationary, homogeneous, and jointly stationary and homogeneous with each other. (2) The medium is incompressible. (3) The refractive index is conservative. Based on this non‐frozen model several formulas for the space‐time autocorrelation function and the wave‐number frequency spectrum of the refractive index are derived. Electromagnetic wave scattering from a turbulent medium characterized by this non‐frozen model is studied. The width of frequency spectrum and the mean frequency shift have been obtained. The results show that the power spectrum of the scattered field can no longer be regarded as equal to the velocity spectrum of the turbulence in general. The modification is important especially when the scale of velocity fluctuations is small in comparison with the electromagnetic wavelength. In Appendix the effects of finite radar pulse volume and radar beam broadening on the width of frequency spectrum of radar echos from a non‐frozen turbulence drifting perpendicularly to the axis of the radar beam are investigated.
Waves obliquely incident upon a stratified anisotropic slab: A variational reaction approach
Authors: Jeng Shyh‐Kang , Wu Ruey‐Beei , Chen Chun Hsiung
Published: February 2008

The propagation problems of waves obliquely incident upon a stratified anisotropic slab are analyzed through a variational reaction approach. A “partial” variational equation is first derived from the variation of reactions, and is then solved via the finite element method. Advantages of such an approach are simplicity in formulation and efficiency in numerical solution. Also included are numerical results for the problem of waves incident upon a stratified magnetoplasma.
Time domain Born approximation to the far‐field scattering of plane electromagnetic waves by a penetrable object
Authors: Quak Dirk , de Hoop Adrianus T.
Published: February 2008

The low‐contrast or (first‐order) Born approximation is applied to the time domain scattering of a plane electromagnetic wave by a penetrable object of bounded extent, embedded in an isotropic and homogeneous medium. Closed‐form analytic expressions are obtained for the spherical wave far‐field scattering amplitude related to homogeneous objects of the following shapes: an ellipsoid, an elliptic cone of finite height, an elliptic cylinder of finite height, and a tetrahedron. Relaxation effects and anisotropy of the scattering object are included. The results are, among others, useful as test cases for time domain inverse scattering algorithms.
Reduction of dyadic Green's functions to scalar Hertz potentials for gyrotropic media
Authors: Weiglhofer Werner
Published: February 2008

Electromagnetic fields in anisotropic media can be represented in terms of dyadic Green's functions. The four dyadic Green's functions (Green's tensors) are solutions of systems of coupled tensor equations of first order which can be transformed to second‐order tensor equations for each of the Green's tensors. For the special case of gyrotropic media (anisotropic media with a distinguished axis and electromagnetic parameters having a rotational invariance with respect to this axis, like a cold ionospheric electron plasma under the influence of earth's magnetic field) it is shown how the Green's tensors can be represented in terms of scalar Hertz potentials. So the problem of solving tensor equations can be reduced to the solution of a system of two coupled partial differential equations of second order for only two scalar potentials.
Millimeter wave moisture sounding: The effect of clouds
Authors: Isaacs Ronald G. , Deblonde Godelieve
Published: February 2008

The effect of water clouds on the retrieval of atmospheric water vapor profiles using simulated millimeter wave sounder data is investigated. Synthetic nadir brightness temperatures are evaluated for a five‐channel hypothetical millimeter wave moisture sounder (91.655 ± 1.25, 150.0 ± 1.25, 183.31 ± 1, 183.31 ± 3, and 183.31 ± 7 GHz) using an appropriate radiative transfer model. Clouds filling the field of view are shown to significantly distort the weighting functions for each channel. The effect of cloud on simulated brightness temperatures over ocean and dry land backgrounds is investigated by para‐metrically varying cloud liquid water content for two cloud types. In general, typical clouds increase brightness temperatures over the ocean and decrease them over land. By applying a simple statistical inversion method to obtain moisture profiles, it is demonstrated that cloud presence degrades the accuracy of water vapor profile retrievals performed in simulation over both land and ocean backgrounds.
Estimation of tropospheric delay for microwaves from surface weather data
Authors: Askne J. , Nordius H.
Published: February 2008

We have developed two methods to determine the tropospheric path delay for microwaves. One uses a closed form model of the atmosphere in which two parameters are used to describe the decrease in temperature with height and the relation between total pressure and water vapor partial pressure, respectively. The other method uses numerical integration of refractivity profiles, which are computed from temperature and humidity profiles obtained by statistical regression. The two methods have been tested on two sets of observations and compared with simultaneous radio soundings. The best results show an improvement of 10% in one case and 30% in the other compared with a presently used model. Possibilities to improve the statistical model further are discussed.
Magnetic elliptical polarization of Schumann resonances
Authors: Sentman D. D.
Published: February 2008

An analysis of the expressions for the magnetic signatures of the transverse magnetic (TM) normal modes of the earth‐ionosphere cavity shows that the side multiplets are elliptically polarized traveling waves. Measurements of the orthogonal, horizontal components of the magnetic field in the extremely low frequency (ELF) range 3–60 Hz performed during September 1985 reveal that the Schumann resonance eigenfrequencies determined separately from the north–south and east–west magnetic components often differ by as much as 0.5 Hz, indicating that the underlying magnetic signal is not linearly polarized at such times. Determination of the corresponding magnetic polarization reveals a high degree of magnetic ellipticity, with both the sense of polarization and the ellipticity varying diurnally. The dominant sense of polarization over the measurement passband is identified to be right‐handed during local daylight hours, with the maximum ellipticities greater than 0.5 occurring just after local sunrise and before local sunset. The frequencies of maximum ellipticity during the daylight hours coincide with the Schumann resonance intensity peaks at 7, 14, 20 and 26 Hz. During the local nighttime hours the dominant sense of polarization is identified to be left‐handed, with a maximum ellipticity greater than 0.5. The frequencies of maximum ellipticity during the nighttime coincide with the intensity nulls between the Schumann resonances. The high degree of magnetic ellipticity suggests that the side multiplets of the Schumann resonances corresponding to azimuthally inhomogeneous normal modes are strongly excited in the highly asymmetric earth‐ionospheric cavity.
Propagation of artificially excited Langmuir waves in the ionosphere
Authors: Kohl H. , Kopka H. , LaHoz C. , Stubbe P.
Published: February 2008

The MPAE heater near Tromsö, Norway was used to excite parametric decay instabilities in the ionosphere, which then were observed by means of the EISCAT radar. During unfavorable geophysical conditions the heater power may not be sufficient to exceed the threshold of excitation for wavelengths matched to the observing radar. Evidence is presented that during such conditions instabilities of longer wavelengths are produced that cannot be seen by the radar immediately, but that become observable after they have propagated for some distance and changed their wavelength according to the dispersion relation.
Three‐dimensional computational analysis of radomes
Authors: Uno Toru , Adachi Saburo , Sawaya Kunio
Published: February 2008

Most of earlier investigations concerning radomes have been restricted to the cases that the radome is two dimensional, or the radome and the primary antenna are not too large compared to a wave‐length, although there are increasing necessity in analysis and design of practical three dimensional large radomes. Recently, it has become possible to respond to this requirements by the appearance of high speed large computers. The purpose of this report is to analyze the radiation characteristics of an aperture antenna covered by an arbitrarily curved three dimensional large radome. In this analysis, the aperture antenna is also arbitrarily shaped, illuminated and located. Moreover, it is permitted that the radome is multilayered or even inhomogeneous. The analysis is essentially based on the method of local slab model approximation. The radiation patterns for various parameters of radomes are shown.
Geometrical optics methods for mode conversion in anisotropic media
Authors: Suchy K.
Published: February 2008

Mode conversion can occur if two (or more) solutions of the dispersion equation coalesce. If the corresponding polarization vectors do not coincide (e.g., in lossless media) and the group velocities are finite and parallel, a coupled system of transport equations is derived for the amplitudes of the coinciding modes, valid in a small “conversion region.”
Developments in Arctic long‐wave propagation theory and experiments
Authors: Kelly F. J. , Martin A. J. , Knowles S. H. , Byrd E. S. , Andrews M. , DeBlasio L. , Siegel J. , Clamons D. , Sheeley N. , Deebel M. , Priddy T. , Quinn L. , Rhoads F. J.
Published: February 2008

During 1985 a flight expedition for signal and noise measurements of long‐wavelength radio waves was undertaken to the Arctic. Ground‐based measurements have been made in Greenland and Norway. The data suggest the revision of long‐wave coverage prediction programs to satisfy the special requirements of the Arctic. Polar cap absorption events and the structure of the Greenland ice cap must be taken into account.
A study of C band equatorial scintillation in the Asian sector
Authors: Mollen T. A. , Liu C. H. , Fang D. J.
Published: February 2008

Ionosphere scintillation data at 4 GHz received in Hong Kong are used to study the statistics of scintillating signals. The data were taken during the last solar maximum (1979–1980) and covered two ionospheric locations, POR (123°E, 20°N) and IOR (108°E, 20°N). The study includes two parts. First, the cumulative distribution functions of the signal are generated from the data base and compared with theoretical distributions. Signal statistics such as the decibel levels of maximum peak, 5% peak, 95% fade and maximum fade are recorded and compared to the scintillation index S4. Empirical relations between S4 and these decibel levels are obtained through statistical analysis. It will be shown that simple linear relations exist between S4 and peak/fade decibel levels. These relations were compared with the results obtained from the “third peak” method for determining the SI index. The second part of the study involved a statistical investigation of the variations of the decorrelation interval of the signal and the roll‐off slope of the power spectrum. Diurnal and seasonal variations are investigated. Geophysical interpretation of the results are presented.