bluuebunny/crossref_metadata_2025 huggingface dataset:
The universal digital ionosonde
Authors: Bibl Klaus , Reinisch Bodo W.
Published: February 2008

The digital ionospheric sounding system Digisonde 128PS explores ionospheric structure and dynamics by exploiting all observables of the reflected electromagnetic wave: range, amplitude, phase, Doppler, incidence angle, and wave polarization. The Digisonde operates in two complementary modes of observation: the Ionogram Mode with full range and frequency display but limited resolution in Doppler and incidence angle, and the Doppler‐Drift Mode operating with a limited number of frequency‐range bins but full resolution in Doppler and incidence angle. This technique reduces the volume of magnetically recorded output data to a manageable size. Special digital‐analog hybrid techniques are used for the presentation of the multi‐parameter ionograms and the sky maps.
A boundary‐value problem treatment of an electric dipole in a warm isotropic plasma using the multiple water bag model
Authors: Singh Nagendra
Published: February 2008

A boundary‐value problem for a dipole immersed in a warm isotropic plasma has been solved using the multiple water bag (MWB) and fluid models of the plasma. The MWB model contains information on the electron distribution function, and hence gives results in agreement with the kinetic theory of the warm plasma. Comparing results from the MWB model with those from the fluid model, we find that the latter are correct only when the antenna frequency ω is sufficiently above the plasma frequency ωp. The current distribution and the driving‐point impedance are calculated for dipoles of several lengths. We find that the current distribution is triangular only when ω is well above ωp or when ω < ωl where ωl, is the frequency at which a resonance occurs in the driving point admittance; ωl is less than ωp and decreases with the increasing length of the dipole. The current distribution for frequencies close to ωp is oscillatory, with a complex wavenumber which is close to the wavenumber of the third‐order Landau mode. Only for very short dipoles with lengths less than 10 Debye lengths or so can the current distribution be approximated by a triangular one for all frequencies. The calculated frequency variation of the dipole resistance with frequency based on the MWB model is similar to those measured experimentally; for frequencies just below ωp the theory in this paper predicts a much larger resistance for the dipole than previous theories.
Interpretation of satellite gyroharmonic resonance observations
Authors: Benson Robert F. , Bitoun Jacky
Published: February 2008

An analytical expression is derived that describes the rendezvous conditions between a propagating wave near nƒH and a moving ionospheric sounder antenna, where ƒH is the ambient value of the electron gyrofrequency and n is an integer greater than 1. The agreement between the theoretical predictions and Alouette 1 satellite observations indicates that most of the long duration resonances (those with time durations greater than about 2 msec) can be interpreted in terms of the reception of sounder‐stimulated electrostatic waves that are reflected and returned to the sounder antenna. The reflection is due to the sensitivity of the dispersion relations near nƒH to small changes in the ambient magnetic field
. The n = 2 resonance is the exception because the satellite observations cannot be explained by the analytical expression obtained for the oblique echo theory. The results provide a third technique for obtaining T⊥ information from sounder‐stimulated plasma resonances, where T⊥ is the ambient electron temperature corresponding to electron motions perpendicular to
. The relative variations in T⊥ can be observed to an accuracy of a few percent by recording the relative variations in the nfH resonant time duration if the angles defining the antenna geometry are known to an accuracy of a few degrees.
Wave tilt sounding of multilayered structures
Authors: Warne L. , Jaggard D. L. , Elachi C.
Published: February 2008

The relationship between the wave tilt and the electrical parameters of a multilayered structure is investigated. Particular emphasis is placed on the inverse problem associated with the sounding of planetary surfaces. An inversion technique, based on multifrequency wave tilt, is proposed and demonstrated with several computer models. There is close agreement between the electrical parameters used in the models and the inversion values.
Circularly polarized 2450‐MHz waveguide system for chronic exposure of small animals to microwaves
Authors: Guy Arthur W. , Wallace Jack , McDougall John A.
Published: February 2008

A system developed to expose chronically a large number of small animals to 918‐MHz microwave has been modified to permit exposure at 2450 MHz. Special transmitting and termination transducers were used to retrofit the exposure chambers of the 918‐MHz system. The use of separate cells that consist of cylindrical waveguides, which are excited by circularly polarized, guided waves, provides a relatively constant and easily quantifiable coupling of microwave energy to each animal, regardless of their position, posture, or movement. The VSWR in each cell is sufficiently low that any number of cells can be coupled to a single source through a power splitter without the need for an isolator.
Comparative effects of water‐bath‐ and microwave‐induced hyperthermia on survival of Chinese hamster ovary (CHO) cells
Authors: Livingston Gordon K. , Johnson Curtis C. , Dethlefsen Lyle A.
Published: February 2008

Suspensions of asychronous CHO cells in the log phase of growth were subjected to a water‐bath or to 2450‐MHz‐microwave hyperthermia at a temperature of 44°C for a period of 60 minutes. Immediately following hyperthermic treatment, the cells were equilibrated at 37°C, counted, and plated out for colony assays, to quantitate survival of cells. Microwave heating was performed by inserting the long dimension of a plastic culture tube, which contained 8×105 cells in 3 ml of media, into a waveguide parallel to the E‐field vector; CW energy was intermittently applied to maintain the suspension at 44±0.1°C. Hyperthermia was induced in a similar sample of cells from the same culture by immersing the culture tubes into a water bath at 44±0.05°C. Temperature‐rise profiles of the two hyperthermic treatments were matched by monitoring suspensions of cells with the Liquid Crystal Fiberoptic temperature probe. Cells subjected to both sources of hyperthermia under comparable conditions showed no significant difference in survival. The Do (slope of the exponential region) of the survival curve was 8.3 minutes for the water‐bath treatment compared with 8.1 minutes for the microwave treatment. Similarly, the Dq (a measure of the survival curve that indexes cell repair) was 23.4 minutes for the water‐bath treatment and 23.7 minutes for the microwave treatment. Intrasample uniformity of heating and accurate thermometry were found to be highly critical to the demonstration of parity.
Numerical computations for a one‐dimensional power law phase screen
Authors: Rino C. L.
Published: February 2008

In this paper, numerical computations of the intensity spectral density function of a wave field scattered by a one‐dimensional, power law phase screen are presented. The computations verify theoretically derived asymptotic results showing that when the power law index is greater than three, the scintillation index saturates at a value larger than unity. Moreover, strong focusing, a local maximum in the scintillation index variation with increasing perturbation strength, only occurs in a one‐dimensional medium when the spectral index is greater than or equal to 3. The application of these results to the interpretation of radiowave scintillation data is discussed.
Reflection and scattering of VHF radar signals from atmospheric refractivity structures
Authors: Röttger J.
Published: February 2008

Very high frequency (VHF) radar echoes received with a quasi‐vertical antenna beam from the troposphere and stratosphere are caused by reflection and scattering. The essential difference between these mechanisms is described by different atmospheric refractivity structures, viz., stable laminae and turbulent irregularities. Several methods for discriminating between reflection and scattering are described and tested experimentally. These involve comparing estimates of the reflection coefficient and the turbulence refractive index structure constant using acceptable models. They also involve the wavelength and volume dependence, as well as the angular distribution of the radar echoes. Doppler spectra, phase and amplitude distributions, temporal and spatial correlation functions, and the dependence between radar echo power and coherence are examined. Diffuse reflection from refractivity laminae, corrugated by turbulence, appears to be an appropriate model for many observations. Finally, the implications of this process for VHF radar investigations of the atmosphere are outlined.
Use of Doppler radar for the measurement of atmospheric turbulence parameters from the intensity of clear‐air echoes
Authors: Gage K. S. , Green J. L. , VanZandt T. E.
Published: February 2008

Doppler radars measure the refractivity turbulence structure constant Cn2 in a straightforward manner. In this paper we explore the relationship between Cn2 and the intensity of atmospheric turbulence as parameterized by the eddy dissipation rate ε. We develop some tentative relations between mean Cn2 and mean ε within the framework of inertial range turbulence theory, taking into account the intermittent nature of atmospheric turbulence. These relations permit the estimation of mean ε from radar observations alone. The analysis is applied to the determination from Sunset radar data of vertical profiles and time‐height contours of mean ε.
Radar measurement of short‐period atmospheric waves and related scattering properties at the altitude of 13–25 km over Jicamarca
Authors: Fukao S. , Wakasugi K. , Kato S.
Published: February 2008

The sensitive Doppler radar (49.92 MHz) at Jicamarca (12.0°S, 76.9°W) is used to investigate short‐period atmospheric waves at the altitudes of 13–25 km. The dominant short‐period waves observed are largely buoyancy, or Brunt‐Väisälä, oscillations, the periods of which are about 5 min in the lower stratosphere and about 11 min in the upper troposphere. The buoyancy oscillations are, on the average, convected with the mean horizontal wind. It is also confirmed that the partial reflection is relatively dominant in these altitudes compared with turbulent scatter. These features are not found around the tropopause, where a vertical shear of horizontal winds of the order of 5 m s−1 km−1 is observed.
The time structure of transionospheric radio wave scintillation
Authors: Rino C. L. , Owen J.
Published: February 2008

The time structure of a representative set of weakly and strongly scintillating transionospheric beacon signals is analyzed. Under conditions of weak scatter, the coherence time of the signal intensity is a monotonic function of the Fresnel radius divided by the effective scan velocity. The shape of the function, however, is controlled by the power law index. Data from a Peruvian station show evidence of a slightly steeper spectral distribution than do data from Kwajalein in the Marshall Islands. Under conditions of strong scattering, the intensity coherence time depends only on the perturbation strength. The strong scatter data show remarkably little dispersion when they are plotted against the perturbation strength. The data are all consistent with a phase spectral slope somewhat less than 3, which is independently verified by using phase scintillation data.
Transmission line model of the interaction of a long metal wire with the ionosphere
Authors: Arnold D. A. , Dobrowolny M.
Published: February 2008

An equivalent electric circuit model is used to study the electrodynamic interactions of long orbiting metallic tethers with the ionospheric plasma and, in particular, to derive current and potential profiles along bare metallic tethers. In contrast with other models, this approach is dynamic, enabling both the transient behavior of the wire and its final equilibrium state to be derived. A comparison with the results of other models indicates the advantage of the present approach, especially in those cases where the internal resistance of the tether plays a major role in determining the current and potential distributions.
Total and partial reflection from the end of a parallel‐plate waveguide with an extended dielectric slab
Authors: Chang David C. , Kuester Edward F.
Published: February 2008

This investigation concerns a TEM wave obliquely incident onto the open end of a parallel plate wave guide with a truncated upper plate. The lower plate as well as the dielectric slab between the two plates is assumed to extend out indefinitely in order to support the propagation of at least one surface wave mode. Reflection from the wave guide opening is determined as a function of incident angle via a dual Wiener‐Hopf formulation. It is found that total reflection, i.e., a reflection coefficient with magnitude equal to unity, is possible for incident angles greater than the critical angle associated with the lowest‐order surface wave mode of the grounded dielectric slab. The field external to the parallel plate wave guide in this case becomes completely evanescent in the cross‐sectional plane perpendicular to the plane of incidence, which is determined by the propagation direction of the incident wave and the edge of the upper plate. It is shown that the phase of the reflection coefficient thus obtained can be used to construct the modal equation for the fundamental mode(s) of a wide microstrip.
Theory of a reflection grating with narrow grooves
Authors: Hurd R. A. , Jull E. V.
Published: February 2008

The scattering of a plane H‐polarized wave from a grating composed of narrow grooves in a perfectly conducting surface is investigated theoretically using conformal transformation techniques. The theory is exact in the limit a/d → 0, where a is the slot width and d is the grating period, and remains valid for practical values of a/d. A necessary condition is derived for the elimination of the specularly reflected wave. When only the −1 and 0 order space waves are present, this condition becomes just the well‐known Bragg condition.
Weak coupling theory of optical fiber and film waveguides
Authors: Huang Hung‐chia
Published: February 2008

By adopting appropriate early approximations a weak coupling theory of optical waveguides is developed, which is featured by its particular simplicity in mathematics. Coupling of ideal modes may not be sufficiently weak for the simple theory to apply. Local modes and super‐local modes are in some cases used, preferably, for their successively weakening coupling properties. The said three sets of modes are all related by similar matrices in the sense of Lowey. Problems of periodic coupling and of discrete coupling are also discussed, the former imposing a rather severe restriction on the weak coupling criteria.
Depolarization of radio waves by atmospheric hydrometeors in earth‐space paths: A review
Authors: Cox D. C.
Published: February 2008

Many measurements of earth‐space radio wave depolarization from 10 to 30 GHz have been made in recent years. This review relates the results from these measurements within the framework of theoretical relationships derived from rain and ice depolarization models. The review includes measured statistical relationships between depolarization and rain attenuation and contains cumulative statistics showing the relative frequencies of occurrence of rain and ice depolarization. Depolarization is usually strongest for circular polarizations and for fixed linear polarizations oriented 45° from vertical and horizontal. Depolarization is usually minimized for fixed vertical and horizontal polarizations. For frequencies up to 30 GHz and elevation angles ranging from 10° to 50°, amplitude ratios of ice and rain depolarization are approximately proportional to frequency and to cos2 θ/sin θ, where θ is the path elevation angle.
Occurrence of nighttime VHF scintillations near the equatorial anomaly crest in the Indian sector
Authors: DasGupta A. , Maitra A. , Basu Santimay
Published: February 2008

The behavior of nighttime F region irregularities near the northern crest of the equatorial anomaly in the Indian sector has been investigated by using VHF amplitude scintillation measurements made at Calcutta (27°N dip subionospheric) during the period April 1977 through February 1980. With the increase in solar activity the occurrence of scintillations increases remarkably during the equinoxes and to a lesser extent during the December solstice, while the local summer occurrence shows little change. The observed patterns are assessed in terms of the variation of the F layer height with solar activity and upwelling motion of the depleted flux tubes associated with small scale irregularities. It is shown that the propagation path is more likely to intercept these equatorial irregularities during periods of high solar activity, while during periods of low solar activity the equatorial irregularities lie south of the propagation path.
Formation of elevated refractive layers in the oceanic boundary layer by modification of land air flowing offshore
Authors: Gossard Earl E.
Published: February 2008

The usual picture of the development of temperature and humidity boundary layers in a land air mass that moves offshore is shown to be very wrong under one type of Foehn condition in southern California, and it is probable that similar conditions can prevail in widespread areas around the globe, notably the Mediterranean Sea and the monsoonal regions of the Near East and Southeast Asia. A formalism is developed for analyzing the modification that seems to represent the observations satisfactorily, and graphical solutions for radio and optical ducting are given. It is shown that offshore modification can lead to elevated layers rather than to surface based layers, and the height of the layer base is theoretically predicted. Values of evaporation and heat flux into such an air mass are calculated, and the distance offshore at which dew point depression becomes zero is predicted. A method for measuring the downward heat flux in elevated inversion layers is described and results are given.
Diffraction by an infinite set of soft/hard parallel half planes
Authors: Lüneburg E. , Hurd R. A.
Published: February 2008

We consider the diffraction of a plane wave by an infinite set of parallel equidistant half planes. On each plate the total field vanishes on one side, and the normal derivative vanishes on the other side. A closed‐form solution to this new canonical diffraction problem is presented for Bragg angle incidence.
Analysis of electromagnetic wave propagation in multilayered media using dyadic Green's functions
Authors: dos Santos Cavalcante Gervasio Protásio , Rogers David Anthony , Giarola Attílio José
Published: February 2008

The propagation of electromagnetic waves in media with various horizontal layers has been analyzed. The electromagnetic fields outside the source region were obtained from the dyadic Green's functions for the case of media with three and four layers and for horizontal and vertical electric dipoles. These results agree with those obtained using Hertz potentials. One of the major results is a generalization of the dyadic Green's functions for solving the problem of media with N horizontal layers.
Asymptotic approximation of waves due to a dipole on a two‐layer medium
Authors: Chew W. C. , Kong J. A.
Published: February 2008

The TM wave due to a horizontal magnetic dipole is first expanded in terms of a series of integrals, each of which is interpreted as due to an image source. Each of the image source integrals possesses a branch point and a high‐order pole which can be close to the saddle point. To facilitate uniform asymptotic approximation, the branch point is first removed by a transformation, which results in an integral with a high‐order pole near three saddle points. The uniform asymptotic approximation of such an integral is obtained through the use of the generalized Weber's function. The result agrees with numerical integration much better than the geometrical optics approximations (GOA). For the case of TE waves, the pole is far away, and our analysis reduces to the three‐saddle‐point analysis where the behavior of the wave can be approximated by parabolic cylinder functions. We show that in such a case our result agrees better with the experiment than GOA. The uniform asymptotic approximation of the waves extends the domain of validity of the image source representation, which is extremely useful when the layer is thick.
A model to calculate em fields in tropospheric duct environments at frequencies through SHF
Authors: Marcus Sherman W.
Published: February 2008

Free‐space field levels are common in a tropospheric duct environment even at distances well beyond the horizon, when the frequency of propagation is above the minimum trapping frequency of the duct. Thus the likelihood of experiencing such a field enhancement is greater at higher propagation frequencies as long as molecular absorption does not become appreciable. A formulation of the wave‐guide duct problem is described which facilitates numerical solution without encountering the numerical difficulties for higher frequencies reported by previous investigators. Field predictions are shown to compare favorably with measurements for 2.201‐GHz propagation in an elevated duct environment.
A useful decomposition for the Green's functions of cylinders and spheres
Authors: Pozar David M.
Published: February 2008

A method is presented for improving the convergence of the near field evaluation of Green's functions for cylinders and spheres. The slow convergence of the scattered field which occurs when both the field point and source point approach the surface of the scatterer is shown to be due to the presence of an image inside the scatterer, thus giving physical insight into the method. The method involves the subtraction of this image field term by term from the scattered field expression, leaving a series which converges quickly. The image field is restored by using closed form expressions. This special image term is shown to be analogous to the image which occurs in electrostatics problems where a charge is in the presence of a cylinder or sphere.
Results of the VPI&SU COMSTAR Experiment
Authors: Andrews J. H. , Ozbay C. , Pratt T. , Bostian C. W. , Manus E. A. , Gaines J. M. , Marshall R. E. , Stutzman W. L. , Wiley P. H.
Published: February 2008

This paper summarizes annual and cumulative attenuation data, depolarization data, and associated local rain rate distributions obtained with the COMSTAR family of 19.04‐ and 28.56‐GHz satellite beacons during the years 1977–1981. It discusses the relationships between attenuation and rain rate and between attenuation and depolarization, compares measured data on the joint distribution of attenuation and depolarization, and examines the limitations that propagation effects will impose on future 20/30‐GHz satellite communications systems.
Diversity reception of COMSTAR satellite 19/29‐GHz beacons with the Tampa Triad, 1978–1981
Authors: Tang D. D. , Davidson D. , Bloch S. C.
Published: February 2008

Tampa, Florida (28°N, 82.5°W), has about 90 thunderstorm days per year, almost all in summer. These convective events tend to occur in afternoon or early evening. This paper presents results of 19‐GHz downlink rain attenuation diversity studies in Tampa involving site separations of 11, 16, and 20 km and reception at high elevation angle (about 57°), over a period of 29 months, including three rainy seasons. Almost identical long‐term performance with the two larger spacings indicates that for separations above about 15 km, diversity improvement was not sensitive to baseline orientation or length. During a fourth rainy season, using the remaining 29‐GHz beacon, diversity improvement with the 16‐km pair was similar to that predicted by scaling the 19‐GHz results of the previous seasons. Also discussed are the type of attenuation distributions and typical fade durations to be found under persistent convective conditions. For rain climates like Tampa's, site diversity in some form will be required for high‐reliability SHF satellite links. The diversity data may be helpful in designing schemes for resource sharing among numbers of links.
A note on some common diffraction link loss models
Authors: Pogorzelski R. J.
Published: February 2008

A tropospheric propagation path obstructed by transverse obstacles is considered. The obstacles are modeled as perfectly absorbing half planes. Propagation loss relative to the unobstructed path is calculated by means of the method of Epstein and Peterson and the method of Deygout. These results are compared with those predicted by spectral diffraction theory. The comparison is made entirely outside the transition regions surrounding the shadow boundaries, permitting simplification of the spectral theory to the familiar geometrical theory of diffraction. The comparisons are used to explain the apparent superiority of the Deygout method over that of Epstein and Peterson in predicting the link loss.
Measurement of induced electric fields in a phantom model of man
Authors: Chen Kun‐Mu , Rukspollmuang S. , Nyquist D. P.
Published: February 2008

An implantable electric field probe with an interference‐free lead wire system was constructed at a nominal cost for the purpose of measuring the induced electric fields in a phantom model of man irradiated by waves of various frequencies. The probe consisted of a short dipole loaded with a zero‐bias microwave diode, and the interference‐free lead wires were constructed with two series of lumped resistors of 3 k Ω. Characteristics of the probe were checked first by measuring the induced electric fields in irradiated, electrically small cubes filled with phantom material. The measured results were in good agreement with theoretical results obtained from the tensor integral equation method. A phantom model of man was constructed with thin Plexiglas filled with phantom material. The model was irradiated by 500‐ to 3000‐MHz em waves in a microwave anechoic chamber. Induced electric fields were probed over 28 locations in one side of the model. The distribution of the measured electric fields was compared with the distribution of theoretical results obtained numerically from the tensor integral equation methods. A qualitative agreement was obtained between experiment and theory. It was found that the agreement between experiment and theory tended to deteriorate at lower frequencies. The reason for this discrepancy is explained.
An integral equation method for electromagnetic scattering of guided modes by boundary deformations of dielectric slab waveguides
Authors: Morita N.
Published: February 2008

An integral equation method is proposed for analyzing scattering and mode conversion of guided modes by boundary deformations of a dielectric slab waveguide. Integral equations are derived for the tangential electromagnetic fields on the deformed boundaries only, using the integral expressions of fields and the fundamental properties which the integral over a closed path has. Mode conversion coefficients for guided and radiation modes are easily obtained from the solutions of the integral equations. Using the mode conversion coefficients, physical quantities such as radiation loss, reflected mode power, and total scattered power, are calculated. Numerical examples are presented for the two‐dimensional models of the typical boundary deformations in optical fibers caused by a fusion splice.
Scattering cross sections for composite surfaces that cannot be treated as perturbed–physical optics problems
Authors: Bahar Ezekiel , Barrick Donald E.
Published: February 2008

Perturbation and physical optics theories have traditionally been used to derive the scattering cross sections for composite surfaces that can be regarded as small‐scale surface perturbations that ride on filtered, large‐scale surfaces. In this case, perturbation theory accounts for Bragg scattering, while physical optics theory accounts for specular point scattering. However, for a more general class of composite surfaces that cannot be decomposed in such a manner, the perturbed–physical optics approach cannot be used. In these cases, it is shown, using the full wave approach, that the specular scattering associated with a filtered surface (consisting of the larger‐scale spectral components) is strongly modified and that Bragg scattering and specular point scattering begin to blend with each other. Since the full wave solution accounts for Bragg scattering as well as specular point scattering in a self‐consistent manner, it is not necessary to filter (decompose) the composite surface to evaluate the scattering cross sections in the general case. However, filtering the composite surface enhances one's physical insight as to the validity (or lack thereof) of the perturbed‐physical optics decomposition and also facilitates the numerical evaluation of the scattering cross sections.
The Poker Flat, Alaska, MST radar as a meteor radar
Authors: Avery S. K. , Riddle A. C. , Balsley B. B.
Published: February 2008

Meteor echoes received on the mesosphere‐stratosphere‐troposphere radar at Poker Flat, Alaska, have been tested for their usefulness in studying the large‐scale dynamics of the upper mesosphere and lower ionosphere. A selection scheme has been developed to extract meteor echoes from the normally observed turbulence scatter echoes in a composite data set. The selected echoes have height and time distributions that agree with the expected meteor distributions. Wind velocities calculated from the meteor echoes agree well with those calculated from the turbulent scatter echoes.
Radiometric sensing of biological layered media
Authors: Bardati Fernando , Solimini Domenico
Published: February 2008

The retrieval of temperature profiles in layered living tissues from microwave radiometric measurements is considered. The brightness temperature of a layered biological structure is expressed as a function of the thermal distribution in the tissues for different microwave frequencies, angles of observation, and polarization of the emitted waves. A “coherent” approach to the radiative transfer problem is followed to obtain the weighting functions in closed form for use in the inverse problem. Finally, the extraction of the thermal profiles from brightness data sets by use of Kalman filtering is discussed, with particular reference to the detectability of deep thermal anomalies.
East‐west plasma bubble irregularity motion determined from spaced VHF polarimeters: Implications on velocity shear in the zonal F region bulk plasma motion
Authors: Abdu M. A. , Kantor I. J. , Batista I. S. , de Paula E. R.
Published: February 2008

VHF electronic polarimeters were used to monitor Faraday rotation angle and amplitude of geostationary satellite (GOES 3) beacon signals, simultaneously at two stations, Cachoeira Paulista (22°41'S, 45°W) and São José dos Campos (23°12'S, 45°55'W), separated by 110 km exactly in the magnetic east‐west direction in Brazil. The analysis of the data for a few months in the 1981–1982 period, carried out in this paper, shows TEC fluctuations correlated with range type spread F events in the local ionograms and amplitudes scintillation in the satellite beacon signals, thereby identifying themselves as plasma bubbles and associated structures in the equatorial ionosphere. The spaced polarimeter system has permitted determination of the east‐west velocities of these plasma bubble structures whose nighttime variation pattern is in excellent agreement with those of the eastward bulk plasma measured by Jicamarca radar and with those of the plasma bubble irregularities obtained from different radio and optical measurements carried out in the equatorial and low‐latitude regions. There exist, however, significant differences in the magnitude of the velocities in the different measurements, our results over low‐latitude locations showing highest values of all. These differences are interpreted as suggesting the possibility of a latitudinal variation in the plasma zonal velocities, consistent with the already known existence of vertical shears in the bulk plasma zonal flow over the equator.
Simultaneous excitation of large‐scale geomagnetic field fluctuations and plasma density irregularities by powerful radio waves
Authors: Lee M. C. , Kuo S. P.
Published: February 2008

We elaborate upon the physical mechanism of thermal filamentation instability of radio waves whose frequencies can be as low as in the VLF band and as high as in the SHF band. This instability can excite large‐scale magnetic and plasma density fluctuations simultaneously in the ionosphere and magnetosphere. We comment on relevant experiments in terms of this instability and other mechanisms.
Inference of rain rate profile and path‐integrated rain rate by an airborne microwave rain scatterometer
Authors: Fujita Masaharu , Okamoto Ken'ichi , Yoshikado Shin , Nakamura Kenji
Published: February 2008

A new estimation algorithm of rain rate by a dual‐frequency radar is proposed, and its performance is evaluated by a computer simulation technique. Data on an airborne microwave rain scatterometer are analyzed by the present algorithm to infer rain rate profiles. The results are compared with those measured by a ground‐based rain radar to demonstrate the potential of an airborne active sensor and the feasibility of a space‐borne active sensor for remote sensing of rain. The total attenuation method for the inference of rain rates is evaluated by comparing the radar echo powers backscattered from the ocean surface with the round‐trip attenuations calculated by the inferred rain rate profiles seen from the aircraft to the ocean surface. Good correlation exists between the above two quantities; however, the measured attenuations are greater than those predicted. Further investigations into the microwave scattering characteristics of the ocean surface, especially with the presence of rain, are necessary for practical applications of the method.
Collision frequency measurements in the high‐latitude E region with EISCAT
Authors: Fla T. , Kirkwood S. , Schlegel K.
Published: February 2008

Measurements of the ion‐neutral collision frequencies with the EISCAT UHF incoherent scatter radar in the high‐latitude, lower E region (90–105 km) show that for a time period of 2 weeks (at the beginning of February 1984) under different geophysical conditions the collision frequencies change little and compare well with a model calculated from the CIRA (1972) neutral atmosphere. When the ion and electron temperatures are approximately equal, it is confirmed that the measured collision frequency is consistent with the induced dipole collision frequency and an atmosphere in static equilibrium. One event at another time where collision frequencies increase by a factor of 1.5–2 over a period of 1½ hr could be due to neutral density enhancements from the passage of a large‐amplitude, long‐period gravity wave.
ZDR measurement considerations for a fast scan capability radar
Authors: Sachidananda M. , Zrnic' D. S.
Published: February 2008

This paper discusses the possibility of incorporating differential reflectivity (ZDR) measurement in Doppler radars having fast scan capability, without compromising the accuracy of the spectral moment estimates. The presently established technique of ZDR measurement using alternate sampling of horizontally and vertically polarized signals (H and V) has long acquisition time and reduced maximum unambiguous velocity. It is shown that ZDR acquisition time can be reduced significantly by simultaneously sampling H and V signals and range averaging, which does not compromise spectral moments estimation capability and volume update time. An analysis of variance of ZDR suggests that accurate estimation of ZDR at scan rates of 18° s−1 is possible only if correlation between simultaneously received horizontally and vertically polarized echoes is better than 0.995. A theoretical investigation of all the factors that contribute to the decorrelation established that this indeed is the case for rain media. A scheme for ZDR measurement is suggested which uses +45° and −45° polarized transmissions alternately and simultaneous reception of H and V signals to compensate for bias error due to propagation. Further, a method for nearly eliminating the bias error due to receiver mismatch is suggested.
A 408‐MHz aperture synthesis radio telescope
Authors: Veidt B. G. , Landecker T. L. , Vaneldik J. F. , Dewdney P. E. , Routledge D.
Published: February 2008

A 408‐MHz earth rotation synthesis telescope is described, which has been constructed as a second frequency channel on an existing 1420‐MHz telescope. Both frequencies are received simultaneously. The angular resolution at 408 MHz is 3.5 arc minutes, and the field of view is 7°. Sensitivity is 3 mJy/beam area (1 Jy = 10−26W m−2 Hz−1). A dual‐frequency, prime focus feed receives both hands of circular polarization at each frequency. The signal transmission system is designed so that the signal phase from its 408‐MHz input to its 30‐MHz output is entirely unaffected by changes in the length of the single coaxial cable which joins each antenna to the central processor. The functions of phase rotation, signal delay equalization, and correlation are performed in a digital signal processor, using microcomputer software rather than hardware. The quadrature output is also produced by computation in real time, using a band‐limited form of the Hilbert transform. Some observational results are presented.
Observations of frontal zone and tropopause structures with a VHF Doppler radar and radiosondes
Authors: Larsen M. F. , Röttger J.
Published: February 2008

Since March 1977 the SOUSY (Sounding System) VHF radar has been used to make a series of frontal passage observations in the spring and fall. The radar operates at a wavelength of 5.6 m and thus is sensitive to the temperature structure of the atmosphere. It has already been shown that VHF radars can be used to determine the location of the tropopause, for instance. We present data from five separate experiments showing that the reflectivity data can also be used to determine the location of tropopause breaks and frontal boundaries between the surface and the tropopause. In each case, we compare the reflectivities measured with the radar and the potential refractivity gradient calculated using radiosonde data from a nearby station. There is good agreement between the calculated and observed values, although the radar data show more detailed structure due to the much higher time resolution.
On observation of gravity wave spectra in the atmosphere by using MST radars
Authors: Scheffler A. O. , Liu C. H.
Published: February 2008

Recently, gravity waves and two‐dimensional turbulence have been proposed as possible causes for the observed mesoscale wind fluctuations in the atmosphere. In order to use wind velocity data measured by MST radars to investigate atmospheric gravity wave spectra, it is important to apply the correct relation between the observed wind fluctuation spectrum and the wave spectrum. In this paper, based on acoustic‐gravity wave theory and radar observation principle, this relationship is quantitatively derived for the general case of acoustic‐gravity waves. Examples are given to show how the theoretical results can be used to interpret the observational data as well as to derive parameters for the model wave spectrum. From the theoretical results, tests for the consistency of the gravity wave model for atmospheric wind fluctuations will be proposed.
A model for gravity wave spectra observed by Doppler sounding systems
Authors: VanZandt T. E.
Published: February 2008

It has been proposed that mesoscale fluctuations of wind and temperature in the free atmosphere are due to internal gravity (buoyancy) waves. Critical tests of this hypothesis must involve the comparison of models based on the theory of gravity waves with suitable measurements. The MST radar technique is particularly attractive for this purpose, because it can measure several independent power spectra simultaneously. However, because of the complexities of the geometry of the MST radar experiment, which measures the radial velocity as a function of radial range and time, and the particular geometry of buoyancy waves, the relation between the observed spectra and the usual description of buoyancy wave spectra is not simple. In this paper a model for MST radar spectra is developed following the formalism presented by Pinkel. Expressions for the one‐dimensional spectra of radial velocity versus frequency and versus radial wave number are presented. Their dependence on the parameters of the gravity wave spectrum and on the experimental parameters, radar zenith angle χ and averaging time Tave, are described and the conditions for critical tests of the gravity wave hypothesis are discussed. This model applies to any monostatic Doppler sounding system, including MST radar, Doppler lidar and Doppler sodar in the atmosphere, and Doppler sonar in the ocean.
The kinetic energy density in the troposphere, stratosphere and mesosphere: A preliminary study using the Poker Flat MST radar in Alaska
Authors: Balsley B. B. , Garello R.
Published: February 2008

Continuous observations of the wind field obtained using the MST radar at Poker Flat, Alaska, over the past 5 years enable a determination of kinetic energy density (Joules per m−3) height profiles in the troposphere, stratosphere and mesosphere. Three specific frequency intervals of the horizontal wind velocity spectrum are processed to yield height profiles of kinetic energy density corresponding roughly to periods of gravity waves, tides and planetary waves (although the wind fluctuations do not necessarily correspond to wavelike processes). Preliminary results of this analysis are presented here for both summer and winter periods.
Differential propagation phase shift and rainfall rate estimation
Authors: Sachidananda M. , Zrnic D. S.
Published: February 2008

This paper presents two methods for computing differential propagation phase shift (ϕDP) using time series data form a coherent radar with alternate switching between two linear but orthogonal polarizations. An analysis of the statistical error in ϕDP estimate shows that ϕDP can be estimated with less than 0.5° standard error using time and range averaging. To evaluate the usefulness of ϕDP for estimating rainfall rate (R) vis‐a‐vis the ZDR method, a discussion on the sensitivity of R to standard errors in ϕDP as well as ZH and ZDR is presented. It is shown that the relation, differential propagation phase constant Δϕ versus R, is relatively insensitive to drop size distribution (DSD) variations and thus can yield more accurate R estimate, even when a direct relationship between Δϕ and R is assumed. However, standard error in Δϕ estimate causes large inaccuracies of R at low values (R < 50 mmh−1), thus limiting its usefulness to higher rainfall rates. It is also shown that Δϕ can be used as a third remote measurable in conjunction with ZH and ZDR to determine a three parameter gamma DSD. Another use of Δϕ is likely to be in hydrometeor type identification, especially hail in severe storms.
Irregularities in the 80–100 km region: A photographic approach
Authors: Armstrong E. B.
Published: February 2008

A technique is described for obtaining photographs of the patterns produced by the infrared hydroxyl airglow emission in the night sky by using exposure times of less than l min, and some examples of such patterns are shown. A photometer capable of obtaining the temperature of the emitting region at 3‐min intervals is also described, and an example of a temperature run on a dynamically disturbed night, with accompanying pattern photographs, is presented. Some references to work on the influence of gravity waves upon the hydroxyl airglow intensity are given and briefly discussed in relation to the present observational results.
A numerical study of waves reflected from a turbulent ionosphere
Authors: Wagen J.‐F. , Yeh K. C.
Published: February 2008

The phase screen‐diffraction layer method is a powerful tool to study the signal scintillation of a wave propagating in a turbulent, stratified medium. Under the forward scattering approximation, the complex amplitude is shown to satisfy a parabolic equation which describes effects arising from phase changes due to irregularities and diffraction due to phase mixing. Below the turning point, these two effects can be computed sequentially. Stepping in altitude, phase changes are imbeded into each phase screen; diffraction between phase screens is accomplished using FFT techniques. This method is equivalent to the split‐step algorithm known in ocean acoustics but generalized to the case of oblique incidence. Near the turning point, the diffraction effects are assumed negligible due to the small vertical thickness of the considered region. The deterministic part of the wave fields is taken to be proportional to the Airy functions. This allows a more accurate evaluation of the phase change near the turning point than the WKB solutions. The coupling between the ascending and descending wave is discussed. The simulation model is described and, as an example, results for a linearly stratified turbulent ionosphere are given, including statistics of the reflected wave such as power spectrum, scintillation index and spatial correlation.
The dyadic Green's function as an inverse operator
Authors: Collin R. E.
Published: February 2008

It is shown that the integral of the scalar product of the current density with the electric field dyadic Green's function can be carried out without difficulty by allowing the observation point to be an arbitrarily located point within a small spherical volume and using an appropriate representation for the Green's dyadic function obtained from the eigenfunction expansion. On the basis of this result it is established that the integral operator using the Green's dyadic function is the correct inverse operator for the vector wave equation provided the integration is taken over the whole volume (the support of the current density).
Improved algorithms for the determination of specific attenuation at sea level by dry air and water vapor, in the frequency range 1–350 GHz
Authors: Gibbins C. J.
Published: February 2008

This paper presents new algorithms for the determination of specific attenuation by dry air and water vapor, at sea level, in the frequency range from 1 to 350 GHz. The algorithms are shown to be an improvement on those recommended by the International Radio Consultative Committee (CCIR), by including the effects of high‐frequency rotational transitions in oxygen, pressure‐induced nitrogen absorption (important for ƒ > 150 GHz) and a quadratic dependence of attenuation on water‐vapor density. Simple scaling factors are given to include the effects of temperature dependence of attenuation, over the range −20°C to 40°C.
Comparison of Fourier and maximum entropy techniques for high‐resolution scattering studies
Authors: Walton Eric K.
Published: February 2008

In this paper, the problem of the high‐accuracy estimation of the location of specular scattering centers on electromagnetic scatterers is considered. Two techniques are compared. The first technique utilizes a Gaussian‐window, Gaussian‐fit approach. The second technique utilizes maximum entropy spectral estimation. It is shown that an improvement over the classical accuracy of the Fourier transform of a factor of 10 or better is possible for widely spaced scatterers utilizing either technique. For cosely spaced specular scatterers, the maximum entropy algorithm can provide information well beyond the resolution of the Fourier transform.