bluuebunny/crossref_metadata_2025 huggingface dataset:
Assessment of the contribution of differential polarization to improved rainfall measurements
Authors: Ulbrich Carlton W. , Atlas David
Published: April 2008

A description is given of the effects of variations in the shape or breadth of the drop size distribution (DSD) on rainfall parameters deduced from a measurement technique which employs the differential reflectivity factor ZDR and the reflectivity factor at horizontal polarization ZH. The mathematical form of the DSD used is a gamma distribution. Justification for such a form is given through consideration of varying DSD shape in nature as implied by the results of empirical analyses of other workers. Theoretical expressions are derived for rainfall rate R, liquid water content W, and median volume diameter D0 in terms of ZDR, ZH, and size distribution dependent factors. The latter calculations assume backscattering cross sections for oblate, nonoscillating raindrops falling in still air with equilibrium shapes. These expressions are used to assess quantitatively the effects of changes in DSD breadth on values of R, W, and D0 deduced from ZDR and ZH. They are also used to show the effects of measurement errors in ZDR and ZH on R, W, and D0. The potential improvement in accuracy which is possible when account is taken of DSD shape variations is shown by simulating a (ZDR, ZH) dual‐measurement method using experimental raindrop size spectra. Methods by which DSD shape variations could be detected through the use of a third remote measurable are discussed.
Differential reflectivity and circular depolarization ratio radar signals and related drop oscillation and propagation effects in rainfall
Authors: Seliga T. A. , Aydin K. , Bringi V. N.
Published: April 2008

Differential reflectivity (ZDR) and circular depolarization ratio (CDR) radar signals are directly related to each other and to the median drop size diameter of rainfall when the drop sizes are exponentially distributed. These polarization parameters are examined for both static and oscillating raindrop shapes along with their relationships with rainfall rate and liquid water content estimates derived from ZDR or CDR and reflectivity factor (Z) measurements. Propagation effects at S band wavelengths are also considered and shown to be particularly important in CDR measurements.
Interpretation of simulated polarization diversity radar spectral functions
Authors: Metcalf James I.
Published: April 2008

A numerical model was developed to simulate the power spectra, cross‐spectrum, and derived functions obtainable from a coherent polarization‐diversity radar. The model was used to investigate the effects of air velocity variance, radar system noise, and differential propagation on the spectral functions. Input parameters include radar elevation angle, air velocity variance, rainfall rate, propagation distance, relative receiver noise level, and the fraction of scatterers having a preferred orientation. Results of calculations of 8.6‐mm wavelength backscatter are illustrated. These are compared with the functional forms obtained from theory and discussed in terms of the meteorological information derivable from them. The forms of the spectral power ratio and the cross‐spectral amplitude ratio are strongly affected by air velocity variance and by receiver noise. However, it appears possible to derive a good estimate of the Doppler air velocity from the power spectra. The cross‐spectral amplitude ratio may be of more general value in analysis than previously thought, because its Doppler velocity domain is greater than that of the spectral power ratio in the presence of receiver noise. The absolute error of the propagation term estimated from the cross‐spectral amplitude ratio is not strongly dependent on the rainfall rate or propagation distance. However, it may be difficult to extract detailed quantitative backscatter information when significant propagation effects are present.
S band radar differential reflectivity measurements in multiple polarization planes along satellite slant paths
Authors: Marshall R. E. , Pratt T. , Manus E. A. , Stapor D. P. , Andrews J. H.
Published: April 2008

A novel approach to making differential reflectivity measurements in multiple polarization planes has been proposed. This paper describes a versatile technique using a mechanical polarization switch and the systematic errors in differential reflectivity measurements which result. Initial results of simultaneous X band slant path measurements of attenuation and depolarization at 11.6 GHz and differential reflectivity measurements at 2.8 GHz using a slow polarization switch are presented.
Precipitation particle identification with centimeter wavelength dual‐polarization radars
Authors: Hendry A. , Antar Y. M. M.
Published: April 2008

Results from recent measurements made with the National Research Council 3.1‐cm dual‐channel polarization diversity radar are presented and related to the precipitation form. The paper also includes some results previously obtained at 1.8 cm wavelength. The use of intrinsic polarization characteristics for precipitation particle identification is demonstrated. Propagation effects, and the way in which they may be used to assist in the identification process, are also discussed. A qualitative classification chart relating the polarization characteristics to precipitation type is presented.
Identification of hydrometeors and other targets by dual‐polarization radar
Authors: Hall Martin P. M. , Goddard John W. F. , Cherry Stephen M.
Published: April 2008

Whereas different hydrometeor types cannot be readily distinguished using radar reflectivity alone, this paper shows how the differential reflectivity available from dual‐polarization radars can be used as an indicator of hydrometeor type (when supported by reflectivity factor also), and that the spatial variability of differential reflectivity is a useful addition. Various hydrometeor types, ground targets, and burnt chaff from straw fires may be identified, whereas conventional radar reflectivity measurement alone would require considerable pattern recognition to make the same distinctions.
Circular depolarization ratio and Doppler velocity measurements with a 35‐GHz radar during the Cooperative Convective Precipitation Experiment
Authors: Kropfli R. A. , Moninger W. R. , Pasqualucci F.
Published: April 2008

All aspects of atmospheric convection on the North American High Plains were studied during the Cooperative Convective Precipitation Experiment. This 3‐month experiment conducted near Miles City, Montana, during the summer of 1981 made use of 14 instrumented aircraft and a network of seven Doppler radars. One of these radars, the National Oceanic and Atmospheric Administration/Wave Propagation Laboratory K band (35 GHz) radar had dual‐polarization capability. The circular depolarization ratio (CDR) from the log receiver of this radar was measured in many different meteorological situations, including nonprecipitating clouds, convective storms with light precipitation, and severe hail storms. The analysis discussed here focuses on measurements near the melting layer where a sharp peak in the CDR profile was observed. A method for classifying particles on the basis of their fall speeds is related to measurements of CDR near this level for more than a 1‐hour period. Preliminary aircraft data are used as an aid in the interpretation.
Scattering properties of hydrometeors as measured by dual‐polarization Doppler radar during CCOPE
Authors: Moninger W. R. , Kropfli R. A. , Pasqualucci F.
Published: April 2008

An 8.6‐mm‐wavelength, dual‐polarization Doppler radar was operated by the Wave Propagation Laboratory during the Cooperative Convective Precipitation Experiment. The complex cross correlation between the two received polarization channels (left‐ and right‐handed circular) provided information about the degree of common alignment and the scattering properties of the observed hydrometeors. The data show four distinct layers in a lightly precipitating stratiform cloud. These are (1) a subcloud layer consisting of raindrops and some non‐Rayleigh‐scattering graupel particles, (2) a narrow “depolarization band” containing highly non‐Rayleigh particles that are ice and ice‐water mixtures, (3) a main cloud region consisting of small ice particles, and (4) a cloud top region consisting of substantially larger particles than were found in the main cloud.
Analysis of aircraft hydrometeor spectra and differential reflectivity (ZDR) radar measurements during the Cooperative Convective Precipitation Experiment
Authors: Bringi V. N. , Seliga T. A. , Cooper W. A.
Published: April 2008

Radar measurements of ZDR are compared with ZDR deduced from aircraft graupel spectra during the Cooperative Convective Precipitation Experiment. On August 4, 1981, the University of Wyoming's Super King Air made two penetrations (at altitudes of 3 km and 1.8 km above mean sea level) through a convective cell located 90 km NE of the CHILL radar. The aircraft was equipped with two two‐dimensional PMS probes located orthogonally so as to view the falling hydrometeors along their symmetry axis (V probe) and along an axis orthogonal to it (H probe). Elliptical raindrop images were observed below cloud base (1.8 km altitude), and conical graupel images were observed at 3 km altitude. Because of beam blockage at the lowest elevation scans, radar data were not available in the rain region. Aircraft raindrop spectra were analyzed in detail, and it is shown that ZDR deduced from such spectra should be an excellent estimator of D0 provided the correct form for the drop size distribution is assumed. ZDR and ZH deduced from the graupel spectra are shown to be in the range measured by the radar.
Circular polarization for precipitation measurement
Authors: Torlaschi E. , Humphries R. G. , Barge B. L.
Published: April 2008

A polarization diversity S band radar has operated for the past 15 years in Alberta, Canada, to study precipitation, particularly severe storms. Theoretical and observational studies demonstrate that the radar reflectivity factor together with the degree of orientation of the precipitation can be used to determine the contributions rain and hail make to the total precipitation rate. As a result, measurements of total precipitation when mixtures of rain and hail are present are improved over measurements using solely the reflectivity factor.
Scattering by imperfectly conducting and impedance spheroids: A numerical approach
Authors: Sebak A. A. , Shafai L.
Published: April 2008

An approximate numerical method for treating the problem of scattering from spheroids with impedance boundary conditions is presented. Initially, the electric surface currents are determined from the solution of the H field integral equation and neglecting the effect of the magnetic surface currents. The impedance boundary conditions are then used to obtain the magnetic surface currents from the computed electric currents. The method is applied to both spherical and spheroidal objects of varying surface impedances and the computed results are compared with the analytic or the direct integral equation solutions. It is found that the accuracy of the results is reasonable, but depends on both the object's geometry and its surface impedance. The results also show that the method predicts the backscattered field accurately, for all surface impedance types. Representative computed data for the bistatic and the backscattered fields of spheroids with different axial ratios and surface impedances are also presented.
Simulation of wide bandwidth signals that have propagated through random media
Authors: Knepp Dennis L. , Wittwer Leon A.
Published: April 2008

A numerical technique is described to generate realizations of the received signal after propagation of a wide bandwidth waveform through a layer of strongly turbulent media. These signal realizations are generated to have Rayleigh amplitude statistics and to have spatial and frequency correlation properties which obey the parabolic wave equation in the strong scatter limit. This technique is based upon the solution for the two‐frequency mutual coherence function Γ for spherical wave propagation with transmitter and receiver located on opposite sides of a finite layer of ionized electron density irregularities. An analytic solution may be obtained for Γ in the strong scatter regime by use of the quadratic approximation for the phase structure function. The thin phase screen approximation to the thick layer is then utilized, and great simplification to the analytic expression for Γ is obtained. The relationship between the impulse response function of the propagation channel and the two‐frequency mutual coherence function and its Fourier transform is then used to directly obtain statistical realizations of wide bandwidth signals. These statistical realizations may be used as direct input to digital receiver modems to represent the effect of propagation through an ionospheric environment disturbed by barium releases or other sources of enhanced ionization. Although these statistical signal realizations apply only to the case of strong scattering, their generation requires only a fraction of the computer resources required for signal generation by wide bandwidth multiple phase screen calculations.
Attenuation rates of coastal radar signals at 25 MHz
Authors: Lyons R. S. , Barrick D. E.
Published: April 2008

The attenuation rate of the ground wave signal with range is a factor limiting the performance of coastal radars. We show that observed attenuation rates are less than theoretically predicted rates at 25 MHz. This result, contrary to earlier findings at lower frequencies, suggests the onset of tropospheric ducting above 20 MHz. The attenuation rates for various sea states and distances are tabulated to allow estimates of system performance near 25 MHz.
Reflection and scatter formula for anisotropically turbulent air
Authors: Doviak R. J. , Zrnic' D. S.
Published: April 2008

Existing formulations assume that the correlation lengths of refractive index irregularities, generated by turbulence, are small in comparison to the Fresnel length. However, there is experimental evidence that the contrary may be true. This paper extends the existing formulations for the case where the Fresnel zone radius is comparable to or smaller than the correlation length and develops a statistical solution that embraces several echoing mechanisms. Conditions are specified under which Fraunhofer and Fresnel reflection and scatter from turbulent air can be distinguished. An integral expression for echo power is developed which shows that echo intensity depends not only on a resolution volume weighting function, but also on a more important Fresnel term. The spectral sampling function demonstrates that for resolution volumes in the antenna's far field this function is independent of the location of the resolution volume. The conditions under which the echo power is proportional to the square of the pulse width are based upon the statistical approach herein adopted for Fresnel scattering.
Quasi‐complementary codes: A new technique for MST radar sounding
Authors: Sulzer M. P. , Woodman R. F.
Published: April 2008

The binary phase complementary code set has been used in stratospheric radar sounding. The quasi‐complementary code technique presented here has nearly complementary properties and offers significant reduction of side lobes caused by imperfections in the radar transmitter. The new technique and its application are described; a simulation of the performance of the two techniques when certain imperfections are present in the transmitter is presented. The results are compared with the actual performance of the Arecibo 430‐MHz radar transmitter.
Impedance measurements on a VLF multiturn loop antenna in a space plasma simulation chamber
Authors: Koons H. C. , Dazey M. H. , Edgar B. C.
Published: April 2008

The Space Sciences Laboratory of The Aerospace Corporation is presently defining an experiment to test a loop antenna configuration for a VLF transmitter in the ionosphere. The primary objectives of the experiment are to validate existing models for radiation by a loop antenna and to study the performance of the antenna in the ionospheric plasma. A one‐third scale model of the antenna has been constructed. Impedance measurements have been made on the model in a 5‐m diameter space plasma simulation chamber at NASA Lewis Research Center. The measurements confirm that the reactance of the antenna in an ionospheric plasma is essentially identical to its free space self‐inductance. The effective series resistance of the circuit increases with frequency. The losses are attributed to power transferred to plasma turbulence.
The calibration of an HF Radar used for ionospheric research
Authors: From W. R. , Whitehead J. D.
Published: April 2008

The HF radar on Bribie Island, Australia, uses crossed‐fan beams produced by crossed linear transmitter and receiver arrays of 10 elements each to simulate a pencil beam. The beam points vertically when all the array elements are in phase, and is steerable by up to 20° off vertical at the central one of the three operating frequencies. Phase and gain changes within the transmitters and receivers are compensated for by an automatic system of adjustment. The 10 transmitting antennae are, as nearly as possible, physically identical as are the 10 receiving antennae. Antenna calibration using high flying aircraft or satellites is not possible. A method is described for using the ionospheric reflections to measure the polar diagram and also to correct for errors in the direction of pointing.
The modal spectrum of a grounded gyromagnetic slab with a perpendicular or parallel axis of magnetization
Authors: Uzunoglu N. K. , Tsalamengas J. L. , Fikioris J. G.
Published: April 2008

Electromagnetic wave propagation along a lossless, grounded, gyromagnetic slab, magnetized perpendicular or parallel to the ferrite‐air interface, is considered analytically for an arbitrary direction of propagation parallel to the slab surface. In addition to the usual surface waves, the present structure is found to support proper complex ones, which, corresponding to evanescent modes, carry no real power, but may store reactive energy. Propagation conditions are discussed and, also, dispersion relations and field intensity distributions for the supported modes are derived. A variety of related numerical results is presented in several plots.
Laboratory simulation of low‐energy electron beam injection by a Japanese sounding rocket in space
Authors: Tsutsui Minoru , Matsumoto Hiroshi , Kimura Iwane
Published: April 2008

Wave‐particle interactions produced by a low‐energy electron beam injected into the ionosphere are experimentally examined in a space simulation plasma chamber. The energy range of the beam is from thermal (∼0.1 eV) to 4 eV. An electrostatic VLF wave with a monochromatic frequency well below the LHR frequency is excited. The beam‐excited wave propagates almost perpendicular to the beam flow direction. The phase velocity of the excited wave is much slower than the ion sound speed in the background plasma. A standing wave is observed within a hollow region surrounded by a conic beam in the direction perpendicular to the conic axis, but the wave inside the hollow cone is found to propagate along the conic axis. The frequency changes of the emissions observed in the previous rocket experiment are successfully reproduced when the beam injection angle from the magnetic field is changed.
A prediction method for the received power from rain scattering
Authors: Awaka Jun
Published: April 2008

This paper presents a prediction method for the received power from rain scattering which is based on the assumption that the rainfall rate and the rain attenuation are described by lognormal distributions. Predicted values agree quite well with the long‐term experimental values at 14.3 GHz.
Interaction of an acoustic wave of artificial origin with the ionosphere as observed by vertical HF sounding at total reflection levels
Authors: Blanc Elisabeth
Published: April 2008

Interaction of a pressure wave with the ionosphere observed by vertical HF total reflection is described in terms of characteristic changes in the echo phase path, group path, and amplitude. The interaction also produces distortion of the frequency modulation and envelope of the echo pulses. Observations of this interaction are presented for acoustic waves generated by ground level explosions. The experiment used Gaussian pulses at 2.411 and 5.060 MHz reflected in the E and F regions. Since the interaction lasts only a few seconds, the sounding data have been analyzed for each pulse, individually recorded at the frequency of 25 pulses per second, and digitized with a group time resolution of 0.3 μs. An analysis of all the sounding parameters has been performed to determine the profile of the ionization ripple produced by the acoustic wave in the ionosphere. Group time measurements of the pulse maxima and a sound speed model are used to retrieve gradient changes during vertical propagation of the ripple. When ionospheric curvature is not significant, the time distribution of the instantaneous frequencies within the pulse is expressed in terms of the electron density profile gradient at the reflection level, and this is used to improve the reconstructed profile. The simulation results of the ripple effects on the echo group path, phase path, and amplitude are compared with the experimental data and found to be in good agreement. The ripple travels in the E region at 300 m/s and in the F region at 600 m/s. It is sinusoidal and characterized by relative amplitudes of 4 and 0.5% with wavelengths of 2 and 5 km, respectively.
MITHRAS: A brief description
Authors: de la Beaujardière O. , Wickwar V. B. , Baron M. J. , Holt J. , Wand R. M. , Oliver W. L. , Bauer P. , Blanc M. , Senior C. , Alcaydé D. , Caudal G. , Foster J. , Nielsen E. , Heelis R.
Published: April 2008

Between May 1981 and June 1982 an intensive campaign of 33 coordinated observations was carried out using the three incoherent‐scatter radars capable of probing the auroral zone. During this period the groups operating the Dynamic Explorer satellites and the STARE radar made special efforts to acquire data coincident with the radar observations. The objective of these MITHRAS experiments and subsequent analysis is to further our understanding of the interactions of the magnetosphere, the ionosphere, and the thermosphere, with special emphasis on local time/universal time variations. Three experimental modes with different time resolution and spatial coverage were used to examine different aspects of these interactions. The analysis of the extensive data set involves collaboration among groups of experimenters as well as between experimenters and theoreticians.
Observations of fast Faraday polarization fluctuations at VHF and amplitude scintillations at VHF and UHF near the equatorial anomaly crest in the Indian sector
Authors: Maitra A. , Das S. K. , DasGupta A.
Published: April 2008

Simultaneous measurements of fast Faraday polarization fluctuations (FPF's) and amplitude scintillations at 136 MHz during April 1977 to February 1980 and of amplitude scintillations at 136 MHz and 244 MHz during August 1981 to December 1982 made at Calcutta (27° dip subionospheric) have been utilized to investigate the nature of ionospheric irregularities during different seasons near the equatorial anomaly crest in the Indian sector. The irregularities occurring in the equinoxes under high solar activity conditions are found to be responsible for intense VHF/UHF scintillations and FPF's. The June solstice is characterized by irregularities producing slow and moderate to weak VHF scintillations but no significant FPF's and UHF scintillations. The occurrence of FPF's is mostly confined to premidnight hours and reduces significantly with respect to the amplitude scintillation occurrence in postmidnight hours. The results are assessed in terms of the strength of irregularities and the longitudinal behavior of global equatorial scintillation activities.
Equatorial scintillations during the major magnetic storm of April 1981
Authors: Aarons J. , DasGupta A.
Published: April 2008

The geomagnetic storm of April 11–14, 1981, was the most severe storm reported since the August 1972 events. Observations of equatorial scintillations in the VHF through microwave range of frequencies (250 MHz to 1.5 GHz) by two stations, Huancayo, Peru, situated near the magnetic equator, and Ascension island, close to the equatorial anomaly crest, are presented. Unusually high levels of activity were recorded at both the stations even around sunrise. At Huancayo, triggered by a reversal of electric field and therefore a rise in the height of the F layer, the 1.5‐GHz transmissions from the satellite MARISAT (15°W) exhibited a scintillation index of 9 dB, which is one of the highest levels recorded at Huancayo over a period of several years. At Ascension island, severe depletions in ionospheric electron content associated with scintillations were detected for the first time in our observations near sunrise. The dual‐station multisatellite observations are discussed in terms of plasma processes at the equator. The reversal of direction after a deep magnetic bay caused the reversal of direction of the electric field, produced an upward bubble velocity, and generated F layer irregularities. This bay, however, was unable to produce an upwelling after sunrise because of E layer conductivity. The postmidnight and sunrise behavior of the F layer irregularity region has been verified by observations of additional severe magnetic storms during years of high sunspot activity. One example, observations on March 5, 1981, at Manila, is shown. The localized generation of patches during periods of magnetic activity indicates the possibility of bubble formation during magnetically quiet periods without a triggering wave. A hypothesis is proposed for instabilities forming from longitudinal (east‐west) electron density gradients existing in this case during the magnetic bay; in the case of magnetically quiet postsunset activity, steep and irregular gradients in electron density may exist across the longitudinal plane at the magnetic equator, producing conditions for the generation of instabilities.
Protonospheric depletion and recovery during geomagnetic storms
Authors: Joshi H. P. , Iyer K. N. , Janve A. V.
Published: April 2008

The average response of protonospheric electron content, as measured by the ATS 6 radio beacon experiment, to geomagnetic storms is determined for a ray path/L shell geometry with L = 1.35 being the lowest intersected and the transmission path almost transverse to the L shells. The results show maximum absolute positive and negative departures of NP from mean levels, associated with storms and a recovery time of ∼6 days in agreement with previous results for other geometries. The NP variation seems to be modulated by modifed ionospheric fluxes during the storm time.
The study of the effect of solar eclipses on the ionosphere based on satellite beacon observations
Authors: Cohen Elizabeth A.
Published: April 2008

Satellite beacon observations during eclipses have provided much information on the behavior of the ionosphere. A combination of Faraday rotation and differential phase measurements as well as ionosonde data can provide information on the topside and bottomside ionospheric behavior during an eclipse as well as on the production rate. Recent attention has been directed toward a study of the dynamics of the F region during an eclipse and the relation between the amount of depletion in the total electron content and the percent obscuration of the sun. Of particular interest is the time delay from the maximum obscuration to the maximum depletion of the total electron content. Further modeling studies are required in order to reproduce the experimental results. Further observations are also required in order to establish whether TID's are generated following a total solar eclipse as predicted theoretically. In particular, it appears that a more sensitive technique such as differential Doppler rather than Faraday rotation is required. The total solar eclipse of June 11, 1983, with its long totality time of 4–5 min over Indonesia and Papua New Guinea will provide an ideal opportunity for further studies of these unanswered questions.
Effects produced by the ionosphere on radio interferometry
Authors: Spoelstra T. A. T. , Kelder H.
Published: April 2008

Radio astronomical observations with connected element interferometers and very long baseline interferometers are affected by atmospheric (i.e. tropospheric and ionospheric) refraction. If both horizontal and vertical gradients in the ionospheric electron density distribution are combined, it is possible to correct for ionospheric refraction and its variations. Interferometer phase appears to be more sensitive to ionospheric irregularities than, for example, Faraday rotation. Observations made with the Westerbork synthesis radio telescope have been analyzed to improve knowledge of the climatology of ionospheric irregularities. These observations have been combined with differential Doppler measurements using NNSS satellite signals to determine spatial parameters of medium‐scale traveling ionospheric disturbances.
Electron content measurements with geodetic Doppler receivers
Authors: Leitinger R. , Hartmann G. K. , Lohmar F.‐J. , Putz E.
Published: April 2008

Geostationary satellites with radio beacons suitable for ionospheric investigations are becoming scarce. In this situation it is important to plan for other possibilities. Differential Doppler data from the polar‐orbiting Navy Navigation Satellite System (NNSS) have been used successfully from a network of receiving stations in Europe. It would be worthwhile to use differential Doppler on a larger basis. Modern receiving equipment for geodetic purposes based on NNSS signals should be considered as an important source of data for ionospheric research. We try to demonstrate the importance and usefulness of this source, stressing specifically investigations of the major geomagnetic storm which had its sudden commencement on April 11, 1981. During this event we could gain data from four additional “geodetic” stations, which improved considerably the resolution in longitude. Unusual longitudinal differences in storm response of the ionospheric electron content were detected. This effect would have remained unnoticed without the additional receiving stations.
Asymptotic approximations of radiation integrals: Endpoint and double endpoint diffraction
Authors: Kildal Per‐Simon
Published: April 2008

The asymptotic approximation of a radiation integral, in which the integrand has a stationary phase point near one of the integration boundaries, is well known (endpoint diffraction). An alternative formulation of endpoint diffraction is given which is similar to the formulation of edge diffraction in the uniform geometrical theory of diffraction. In many applications the endpoint diffraction solution is the integrand of a new secondary radiation integral over another surface. An asymptotic approximation to one such class of secondary integrals, which represent double‐endpoint diffraction in the direction of a stationary phase caustic, is evaluated explicitly. This explicit expression can be used to obtain simple analytic diffraction corrections to geometric optics solutions, for example, calculations of the aperture efficiencies of dual‐reflector antennas and cylindrical reflector antennas.
Radiation field of a periodic strip grating excited by an aperiodic line source
Authors: Barbosa A. M. , dos Santos A. F. , Figanier J.
Published: April 2008

The excitation by an aperiodic magnetic line source of a periodic strip grating backed by a perfectly conducting plane is studied. The original problem is transformed into a problem of periodic excitation and then solved by reduction to a modified Riemann‐Hilbert problem. The radiation field is calculated and interpreted in terms of the solutions of the modal equation of the structure. The numerical results confirm the possible application of this structure in the design of a leaky‐wave antenna.
Pulse compression techniques with application to HF probing of the mesosphere
Authors: Gonzales Cesar A. , Woodman Ronald F.
Published: April 2008

We show that pulse compression techniques which use sets of more than two complementary sequences can be useful in HF radar probing of the D region of the ionosphere. The sets we propose here have an important advantage over the standard complementary pair of codes: they can eliminate E or F region multiple‐hop echoes which interfere with D region echoes, when relatively short radar interpulse periods are used. Consequently, both altitude resolution and radar sensitivity can be substantially improved over previous experiments by coding the transmitter pulses and by using shorter interpulse periods. Sets of complementary sequences also perform coherent integration of the received signals and can eliminate the instrumental dc bias. The technique has been tested at Arecibo in an experiment in which we measure the spectra of partially reflected D region echoes. In this experiment, sophisticated processing hardware is used to decode, to perform coherent integration, and to compute—simultaneously and on‐line—the spectra of echoes from 256 altitudes in the 0‐ to 230‐km range.
Rocket observations of electron density in the nighttime E region using Faraday rotation
Authors: Smith L. G. , Gilchrist B. E.
Published: April 2008

A rocket radio propagation experiment is described in which the electron density profile of the nighttime E region is obtained with an accuracy of 100 cm−3. The factors limiting the accuracy of the experiment are found to be related to atmospheric and receiver noise and to the use of a magnetometer to determine the spin rate of the rocket. The Fourier analysis used for the frequency measurement may also contribute error under conditions of steep electron density gradients. The accuracy being achieved appears to be adequate for present applications of the experiment.
Radiation of electromagnetic waves from an electron beam antenna in an ionosphere
Authors: Ohnuki Shigeo , Adachi Saburo
Published: April 2008

Wave excitation by using electron beam antennas is planned in the Space Experiments With Particle Accelerators project. This paper discusses the radiation of electromagnetic waves from a density‐modulated electron beam antenna injected along the earth magnetic field into the ionosphere. Strong radiation at the lower hybrid frequency ƒLHR and the higher frequency band (ƒLHR <ƒ ≤ ƒHe/2, where ƒ and ƒHe are frequency and electron cyclotron frequency, respectively) is predicted theoretically for an appropriate beam velocity.
Convection and diffusion modes of induced earth current
Authors: Summers D. M.
Published: April 2008

The physical interpretation which Price [1950] attached to the solutions of the pre‐Maxwell equations is examined and the validity of linear circuit analogies to the problem of geomagnetic induction is questioned. If induced earth currents are solely diffusive, then geomagnetic induction data should be explicable in terms of near‐field scattering theory. However, if the earth's inductive response comprises also a convective current mode similar to a linear circuit, then a more general formulation of the induction problem would be required. It is shown that the internal night‐time Sq field can be attributed more easily to transient induction effects than to the global linear circuits proposed by Ashour and Price [1965].
How edge diffraction couples ground wave modes at a shoreline
Authors: Jones R. M.
Published: April 2008

Ground wave mode coupling coefficients at a discontinuity in ground surface impedance (as at a shoreline) are derived by using the appropriate geometrical theory of diffraction edge—diffraction coefficient. The results agree with previous derivations based on other methods. The discovery that edge diffraction is the mechanism for ground wave mode coupling at a shoreline or other similar discontinuity allows the calculation of ground wave mode coupling by edge diffraction in cases where no easily calculated solution is now available.
Near‐field ray calculation for multiple knife‐edge diffraction
Authors: Whitteker J. H.
Published: April 2008

A new ray approximation for calculating the diffraction attenuation due to several knife edges takes into account the wave‐normal direction of the diffracted wave in the near field, using the Fresnel approximation. The theory is intended for terrestrial radio wave propagation and for other situations where obstructions are nearly collinear. The far‐field form of the wave is adopted, as in asymptotic theories, but the focal line of the diffracted ray pencil coincides with the diffracting edge only in the limit as the field point moves deep into the geometric shadow. In general, the ray passes above the diffracting edges rather than through them. The theory is developed for knife edges only, but may be adaptable to other shapes of diffracting obstruction. In the examples given, the calculated attenuations are much more accurate than if the far‐field approximation is assumed, but less accurate than if the complete Fresnel integration is done. In a comparison with the complete Fresnel integration, the difference turns out to be less than l dB for five or fewer knife edges.
Remote sensing of the refractive index structure parameter via inversion of Tatarski's integral equation for both spherical and plane wave situations
Authors: Barakat Richard , Newsam Garry
Published: April 2008

Remote determination of the refractive index structure parameter from experimental data by Tatarski's integral equation requires numerical inversion of a compact linear operator. Such problems are known to be ill posed; the consequent ill conditioning of the inversions has led to a large degree of uncertainty in reported reconstructions. In this paper we use the singular value decomposition of a compact operator to define a measure of the maximum amount of recoverable information in such an inversion, terming it the essential dimension of the operator. We propose the use of filtered singular value decomposition as the numerical algorithm that will recover most of the information and minimize uncertainty. A detailed study of the operators appearing in determination of horizontal (wave front is spherically symmetric) and vertical (wave front is plane) profiles of the atmosphere is under‐taken to determine their essential dimensions by both analysis and computation. The results indicate that for typical parameter values, both operators have small essential dimensions, with vertical profiles being harder to reconstruct than horizontal profiles.
The radio power reflected from rough and undulating ionospheric surfaces
Authors: Whitehead J. D. , From W. R. , Smith L. G.
Published: April 2008

It is shown for both rough and undulating surfaces that the mean radio power reflected by the ionosphere averaged over a sufficiently long time is exactly the same as for a smooth flat surface at the same height provided the sounder is equally sensitive for echoes from all directions. When making radio wave absorption measurements under spread conditions the total integrated power over the whole time the direct echoes are being received must be used but the distance attenuation factor must be calculated from the time of arrival of the first echo.
Maps of ƒ0F2 derived from observations and theoretical data
Authors: Rush Charles M. , PoKempner Margo , Anderson David N. , Perry Jane , Stewart Frank G. , Reasoner Rita
Published: April 2008

Observations of the F2 region critical frequency, ƒ0F2, and values determined from the time‐dependent continuity equation for ions and electrons in the ionosphere have been used to develop a new set of numerical coefficients to represent the global variation of ƒ0F2. Like those in earlier investigations, the new coefficients permit monthly median hourly values of ƒ0F2 to be obtained at any location around the globe for any month of the year and solar activity level. Comparisons between ƒ0F2 determined using older sets of numerical coefficients and ƒ0F2 determined using the new set of coefficients are given along with a description of how well each set of coefficients specifies and predicts the observed variations in the F2 region critical frequency.
Ionospheric electron density irregularities observed by satellite‐to‐satellite, dual‐frequency, low‐low doppler tracking link
Authors: Estes Robert D. , Grossi Mario D.
Published: April 2008

A low‐low, satellite‐to‐satellite, dual‐frequency, Doppler tracking experiment was performed by the Harvard‐Smithsonian Center for Astrophysics on the occasion of the 1975 Apollo‐Soyuz Test Project. The data are analyzed here for irregularities in electron density at the altitude of 212 km. The differential Doppler data with the relative motion term removed are integrated to obtain a representation of the electron density variation along the satellite path. Well‐known large‐scale features such as the equatorial geomagnetic anomaly and day/night ionization level differences are clearly observed in the integrated data. The larger crest of the morning geomagnetic anomaly is seen to occur in the southern (winter) hemisphere in agreement with previous observations. In addition, a sharp peak in the electron density at the day‐to‐night transition point is observed in two consecutive revolutions. This effect may be due to the previously postulated atmospheric shock wave generated by supersonic motion of the terminator.
Topology and geometry of the electromagnetic field
Authors: Engl Walter L.
Published: April 2008

A separation of topology and geometry which is well known in network theory may be reached in electrodynamics as well by using adequate differential forms instead of classical vectors; this, moreover, yields a better insight into the physical nature of the field quantities. Differential forms are objects which can be integrated over curves, surfaces, etc., without reference to geometrical structures. With Ē and
as even 1‐ and 2‐forms, respectively,
and
as odd 1‐ and 2‐forms, respectively, Maxwell's equations become purely topological constraints. The metric properties of space then appear only in the constituent relations which become an isomorphism mapping the even multiforms (measures of intensity) Ē,
onto the odd multiforms (measures of quantity)
,
. Transition from threedimensional electrodynamics to the theory of special relativity can be done in a quite natural way.
Conformal time domain finite difference method
Authors: Mei Kenneth K. , Cangellaris Andreas , Angelakos Diogenes J.
Published: April 2008

The conventional time domain finite difference (TDFD) method uses right rectangular meshes in space. The result is that curved object surfaces are approximated by steps. This paper considers finite difference meshes, which are not restricted to any shape, thus capable of conforming to the object surfaces. Other advantages of this type of meshes include the easier handling of the approximate radiation conditions.
A time domain energy theorem for scattering of plane electromagnetic waves
Authors: de Hoop A. T.
Published: April 2008

A time domain energy theorem for the scattering of plane electromagnetic waves by an obstacle of bounded extent is derived. It is the counterpart in the time domain of the “optical theorem” or the “extinction cross section theorem” in the frequency domain. No assumptions as to the electromagnetic behavior of the obstacle need to be made; so, the obstacle may be electromagnetically nonlinear and/or time variant (a kind of behavior that is excluded in the frequency domain result). As to the wave motion, three different kinds of time behavior are distinguished: (1) transient, (2) periodic, and (3) perpetuating, but with finite mean power flow density. For all three cases the total energy (case 1) or the time‐averaged power (cases 2 and 3) that is both absorbed and scattered by the obstacle is related to a certain time interaction integral of the incident plane wave and the spherical‐wave amplitude of the scattered wave in the far‐field region, when observed in the direction of propagation of the incident wave. The practical implications of the energy theorem are briefly indicated.
Electromagnetic heating of layered biological systems with nonlinear thermoregulatory properties
Authors: Caorsi S.
Published: April 2008

This paper deals with the electromagnetic heating of biological bodies that have been modeled by homogeneous plane layers and exposed to plane waves. The thermoregulatory properties of such biological layers are assumed to be nonlinear functions of the local temperature, and the cooling of their external surfaces is taken into account. The space‐time thermal response is obtained by using a numerical program based on the method of finite differences.
Synthesis of hexagonal and square arrays using discrete convolution
Authors: Shelton J. P. , Laxpati S. R.
Published: April 2008

A discrete convolution procedure is applied to the synthesis of radiation patterns with prescribed sidelobe levels. This procedure begins with a target polynomial, such as Chebyshev or Taylor, and determines the array element amplitudes by a simple, closed‐form, noniterative computational method. Symmetrical hexagonal arrays of 3n2 + 3n + 1 elements, where n is the number of rings in the array, and symmetrical square arrys of (2m + 1)2 elements are analyzed. Examples are presented, and this procedure is compared with other synthesis methods.
Ray invariants, plane wave spectra, and adiabatic modes for tapered dielectric waveguides
Authors: Arnold J. M. , Felsen L. B.
Published: April 2008

In nonseparable problems resulting from the analysis of wave propagation in longitudinally varying waveguides, such as a wedge‐shaped taper, singularities appear in both ray and coupled mode treatments at the local normal mode cutoff transition. A uniformization of the local normal (adiabatic) mode is proposed, using plane wave spectra, which effectively resolves this difficulty.
Propagation along azimuthally magnetized, ferrite‐loaded circular guide
Authors: Ivanov K. P.
Published: April 2008

An analytical technique using confluent hypergeometric functions is developed and applied to calculate and interpret the non‐reciprocal phase characteristics of a circular cylindrical guide, loaded with azimuthally magnetized latching ferrite, propagating angular symmetric transverse electric mode.
Toward a stable marching‐on‐in‐time method for two‐dimensional transient electromagnetic scattering problems
Authors: Tijhuis A. G.
Published: April 2008

The transient scattering of two‐dimensional electromagnetic fields by an obstacle of finite extent is investigated with the aid of the time domain integral equation technique. In solving such equations with the marching‐on‐in‐time method, numerical instabilities form a major problem. These instabilities can be attributed to errors in the discretization of the source type integrals that occur in the equations. In this paper, we formulate two so‐called stability criteria for such a discretization that, if they are met, guarantee that the instability can be controlled by reducing the discretization step. With the aid of these criteria, we analyze the solution of two two‐dimensional electromagnetic scattering problems, namely the scattering of a pulsed plane wave by a perfectly conducting and an inhomogeneous, lossy dielectric cylinder. Numerical results are presented and discussed.