bluuebunny/crossref_metadata_2025 huggingface dataset:
A power law phase screen model for ionospheric scintillation: 1. Weak scatter
Authors: Rino C. L.
Published: February 2008

In this paper the weak scatter scintillation theory is reformulated to show explicitly the ramifications of an arbitrarily large ionospheric outer scale. The measured temporal phase spectrum, for example, is effectively truncated at a fixed frequency corresponding to the detrend time or the length of the data interval over which it is measured (whichever is smaller). As a consequence, the rms phase exhibits a complicated dependence on the relative irregularity drift velocity and the propagation geometry. This effect has not been included in previous analyses. By comparison, intensity scintillation data are intrinsically high‐pass filtered by the diffraction process. By taking advantage of this fact a simple closed form expression for the S4 intensity scintillation index has been derived. The theory is applied to representative data sets from the Wideband satellite. The interpretation of the ionospheric parameters deduced from the analysis is also discussed.
A power law phase screen model for ionospheric scintillation: 2. Strong scatter
Authors: Rino C. L.
Published: February 2008

The multiple‐scatter theory of wave propagation in randomly irregular media is now well developed. Moreover, work in fields such as coherent optics and radio astronomy has stimulated the development of asymptotic formulas that can be used in various scattering regimes. In this paper the power law phase screen model is used to derive the form of the intensity correlation function for strong ionospheric radio wave scattering when the Fresnel radius lies within the power law continuum. The model allows fully for the anisotropy of the scattering medium. Asymptotic formulas valid under conditions of very strong scattering are developed. It is shown that the results depend critically on the power law index such that an important transition in the structure of a strongly scattered field occurs when the three‐dimensional index is greater than 4.
Quantitative determination of thresholds of magnetophosphenes
Authors: Lövsund P. , Öberg P. Å. , Nilsson S. E. G.
Published: February 2008

ELF‐ and transient magnetic fields at moderate flux densities generate visual phenomena called magnetosphosphefles. The aims of this study were 1) to determine threshold values for the appearance of magnetophosphenes as a function of magnetic‐field frequency in normal and colour‐defective volunteers; 2) to study the dependence of the threshold values on the intensity of the background light and its spectral composition; and 3) to investigate the site in the visual system where the magnetophosphenes arise. Quantitative information was obtained about the threshold values of magnetophosphenes both in normal and in colour‐defective volunteers. The phosphene threshold varied with the background light intensity and with its spectral composition.
Operant behavior and rectal temperature of squirrel monkeys during 2.45‐GHz microwave irradiation
Authors: de Lorge John
Published: February 2008

Squirrel monkeys, Saimiri sciureus, were trained to respond on a two‐lever observing‐response task for food pellets. After stable response rates developed, the monkeys were exposed in the far field to 2.45‐GHz microwaves (100% sinusoidally modulated at 120 Hz) at average power densities from 10 to 75 mW cm−2 of incident radiation. Four monkeys were exposed to irradiation for 30 minutes during two‐hour sessions. Three of the monkeys were also exposed for 60 minutes during two‐hour sessions. The animals were restrained in Styrofoam chairs during experimental sessions and exposures occurred in a microwave‐anechoic chamber. The behavior of the monkeys on the observing‐response task was disrupted during 30‐ or 60‐minute exposures to irradiation but only at power densities that were 50 mW cm−2 or higher. This disruption was increasingly evident as power density increased. Under both durations of exposure, behavior was not consistently perturbed until rectal temperatures increased more than 1°C. Rectal temperature was slightly but reliably elevated at 10 mW cm−2, was a monotonie function of the power density, and was markedly increased at power densities between 40 and 50 mW cm−2.
Reversibility of microwave‐induced blood‐brain‐barrier permeability
Authors: Albert Ernest N.
Published: February 2008

Ultrastructural studies have revealed increased permeation of the blood‐brain barrier by protein in Chinese hamsters after exposure to 2450‐MHz microwaves (CW) at 10 mW cm−2. Reversibility of this microwave‐induced permeability in microvasculature of Wistar rats and Chinese hamsters was assessed. The animals were anesthetized and their heads were irradiated in the near field with 2800‐MHz CW microwaves at 10 mW cm−2 for two hours. Horseradish‐peroxidase protein was injected as a tracer to study its transport across capillaries in all regions of the brain. Gross‐ and electron‐microscopic observations indicated that the relative impermeability of the blood‐brain barrier is re‐established within two hours after irradiation. A significant but incomplete increase of permeability was observed one hour after exposure. Preliminary data indicate that the blood‐ocular barrier was similarly affected.
A quantitative method for measuring altered cerebrovascular permeability
Authors: Rapoport S. I. , Ohno K. , Fredericks W. R. , Pettigrew K. D.
Published: February 2008

Cerebrovascular permeability to 14C‐sucrose was measured independently of cerebral blood flow in the rat, following unilateral osmotic opening of the blood‐brain barrier by hypertonic arabinose solution. An increase in regional permeability to 14C‐sucrose was correlated with the extent of brain staining by intravascular Evans blue‐albumin, a visual barrier tracer. To obtain the product of capillary permeability and surface area, 14C‐sucrose was injected intravenously, the arterial plasma concentration curve was determined and integrated over a 10‐minute period, and brain parenchymal concentration was calculated after subtracting vascular radioactivity from net tissue radioactivity. Conditions were chosen so that a simple diffusion equation could be applied to exchange between plasma and brain compartments, in the absence of back flux from brain. The method should be of use for studying effects of microwaves on the cerebral vasculature.
On the scattering and reflection mechanisms contributing to clear air radar echoes from the troposphere, stratosphere, and mesophere
Authors: Gage K. S. , Balsley B. B.
Published: February 2008

With the advent of the MST (mesosphere‐stratosphere‐troposphere) radar technique the atmosphere from 0 to 100 km can be observed with a single radar system. Several different mechanisms have been identified as causing the echoes observed by MST radars. Two primary mechanisms are scattering from turbulent irregularities and Fresnel (or partial) reflection from horizontal strata. Two additional mechanisms that apply primarily to the upper mesosphere are meteor scatter and thermal (or incoherent or Thomson) scatter. Below the stratopause the refractive index structure responsible for the echoes arises solely from neutral atmosphere fluctuations; above the stratopause the radar cross section of such fluctuations is enhanced by the presence of free electrons (moreover, in the absence of small‐scale neutral fluctuations the electrons alone can provide the scattered signal). This paper is an attempt to unify the scattering and reflection theories in the light of observational studies.
Spaced antenna VHF radar observations of tropospheric velocities and irregularities
Authors: Vincent R. A. , Röttger J.
Published: February 2008

The application of spaced antenna techniques to VHF radar studies of the motions and structure of irregularities in the clear atmosphere is discussed. Observations made with the 53.5 MHz SOUSY‐VHF radar working at vertical incidence and using spaced antennas for reception confirm previous suggestions that echoes from the troposphere have contributions from specular (i.e., partial) reflection, also called ‘Fresnel reflection,’ as well as random scatter. Measurements of the angular width and spatial coherence show the reflecting regions to be thin and quasi‐horizontal in structure. These reflecting regions also are called sheets. Reflected to scattered power ratios are in the range between 0.2 and 10. The angular spectrum of reflecting/scattering structures is investigated by means of a radar interferometer technique. The angular width and the apparent tilt of the sheets is measured to be a few degrees. It has been possible to measure horizontal velocities by the drift method even on those returns where the angular widths are so narrow that the use of conventional Doppler techniques would be impracticable. The drift velocities agree well with balloon measured winds. It is found that stable, reflecting sheets occur primarily in regions of high dynamic stability. In regions of lower dynamic stability, scattering from turbulent irregularities is observed. In this case the mean dissipation rate, diffusion coefficient, and outer scale of the inertial subrange of turbulence are estimated from measured values of correlation time and Richardson number. The estimates are in accord with other turbulence measurements.
High‐altitude‐resolution stratospheric measurements with the Arecibo 2380‐MHz radar
Authors: Woodman R. F.
Published: February 2008

The 2380‐MHz planetary radar of the Arecibo Observatory and the 30‐m interferometer antenna at Higuialles, Puerto Rico, have been instrumented as a bistatic radar for stratospheric turbulence research studies. The average power of the transmitter is 400 kW, and it uses the 300‐m Arecibo spherical reflector. A vertical resolution of 30 m has been achieved by means of a novel continuous pseudorandom phase coding scheme. Preliminary results obtained with the system are presented. The thickness and internal structure of turbulent stratospheric layers have been resolved for the first time. The potential of the instrument in assessing the role of turbulence in the vertical transport of tracers, contaminants, and momentum is discussed.
The RF spectra of first and subsequent lightning return strokes in the 1‐ to 200‐km range
Authors: Serhan G. I. , Uman M. A. , Childers D. G. , Lin Y. T.
Published: February 2008

We present the first experimental characterization of the frequency spectra of first and subsequent stroke electric fields over a distance range from about 1 km, where the fields are primarily electrostatic, to 200 km, where they are primarily radiation. Spectra are presented to about 700 kHz for lightning within 12 km and to about 300 kHz for lightning at 50 and 200 km. We show that the return stroke ground wave spectrum beyond 50 km has a peak near 4 kHz but that within 10 km the spectrum shows a steady increase with decreasing frequency to 1 kHz. Subsequent strokes beyond 50 km contain more energy above 100 kHz relative to the peak spectral value than do first strokes. Frequency spectra at all ranges fall off roughly as 1/ƒ for frequencies between 5 and 100 kHz, whereas the falloff above 100 kHz is faster as the distance to the stroke increases. From this high‐frequency attenuation we determine an RF conductivity for central Florida of between 0.002 and 0.005 Ω−1/m.
Contribution of water vapor monomer resonances to fluctuations of refraction and absorption for submillimeter through centimeter wavelengths
Authors: Hill R. J. , Clifford S. F.
Published: February 2008

Atmospheric fluctuations in temperature, humidity, and total pressure cause fluctuations in the absorption and refraction of electromagnetic waves. We consider only the absorption and refraction fluctuations caused by variations in the width and strength of water monomer resonances. We differentiate the absorption coefficient and refractive index with respect to temperature, humidity, and total pressure in order to obtain the coefficients that relate the fluctuations in absorption and refraction to these atmospheric parameters. The resulting coefficients as well as the absorption coefficient and the refractivity are calculated by summing over all water vapor infrared resonances and are displayed graphically. In the limit of low frequencies, these calculated coefficients are in good agreement with the formula for radio wave refractive index. The effects of turbulent fluctuations in total pressure are usually negligible.
Transient electromagnetic field generated by a vertical electric dipole on the surface of a dissipative earth
Authors: Haddad Hussain , Chang David C.
Published: February 2008

The problem of electromagnetic pulse propagation over a dissipative earth surface, excited by a vertical electric dipole located on the earth surface, is investigated. By deforming related integrals in the complex frequency domain it is shown that the scattering contribution of the earth, which is formally expressed in terms of several double infinite integrals, can be efficiently computed from some single finite integrals and is amenable to physical interpretation. Dispersion of electromagnetic waves eminating from an impulse current is presented as a function of both the observation distance and the earth conductivity.
Multimode propagation in anisotropic optical waveguides
Authors: Paul D. K. , Shevgaonkar R. K.
Published: February 2008

Using far from cutoff approximation (2a/λ0≫ 1) we have derived elegant expressions for the characteristic equations and their relevant solutions to determine the guided wave features of an optical waveguide having an axial dielectric anisotropy. Also, the attenuation constant has been calculated following the concept of a complex permittivity in a lossy medium. For TE and TM, as well as hybrid modes studied here, the results computed by the approximate method show very good agreement with those of the exact analytical solutions. The dependence of the power carrying capacity and attenuation on the waveguide parameters highlights the usefulness of the dielectric anisotropy in optical waveguides.
Boundaries of geometrical optics applicability and related problems
Authors: Kravtsov Yu. A. , Orlov Yu. I.
Published: February 2008

On the basis of the Huygens‐Fresnel principle as well as the concept of a Fresnel volume (the part of space essential for diffraction), the following criteria of the ray method applicability are suggested. (1) Parameters of the medium involved and characteristics of the wave (amplitude and phase gradient) should vary just slightly over the cross section of the Fresnel volume. (2) The phase difference of two rays arriving at the same point of space should exceed π. These criteria may be evaluated from the conditions of applicability of the stationary phase method. The effectiveness of the suggested criteria is demonstrated by considering various problems allowing either rigorous or approximate solutions. Moreover, one can formulate generalized criteria for the case of space‐time geometrical optics and for geometrical theory of diffraction as well. And finally, some related problems are considered, the most important one being estimation of the wave field intensity in the vicinity of caustics in terms of the energy conservation law in a ray tube corresponding to the caustical zone width.
Time domain scattering from open thin conducting surfaces
Authors: Bennett C. Leonard , Mieras Harry
Published: February 2008

The computation of transient electromagnetic scattering from open thin surfaces using a time domainEfield integral equation is described. The integral equation is solved by a process of marching in time, eliminating the need for matrix inversion. A Gaussian incident pulse is used, avoiding discontinuities, and yielding the ‘smoothed impulse response’ of the target. Responses are plotted for several target shapes.
Fresnel scattering model for the specular echoes observed by VHF radar
Authors: Gage K. S. , Balsley B. B. , Green J. L.
Published: February 2008

In recent years several groups using VHF radars have reported highly anisotropic echoes originating from stable regions of the troposphere and stratosphere. The echoes are most intense looking vertically, and their occurrence is well correlated with regions of hydrostatic stability. Observations also show that the echoes are usually much stronger when observed with coarse spatial resolution. There is no commonly used scattering or reflection model that can account for these features. In this paper we present a new model to explain the enhanced vertical echoes. The model pertains to scattering from the half‐wavelength Fourier component of horizontally coherent fine structure in the radio refractive index within a region of finite vertical extent. We refer to this echoing mechanism as ‘Fresnel scattering.’ The new model is applied to simulate the magnitude of VHF radar echoes by using radiosonde temperature soundings as input. Good agreement is found between these simulations and observations at the Sunset VHF radar in Colorado.
Analysis of a new corrugated guide feed horn having dual‐depth corrugations
Authors: Zhang Ri‐rong , Wang Han‐li
Published: February 2008

A new corrugated guide horn having dual‐depth corrugations was proposed by Carpenter and Ghosh in 1980. It is attractive for achieving improved performance over a wide band or a multiband. In this paper a space‐harmonic analysis has been made, the relation between the fields in the adjacent slots of the dual‐depth corrugations has been found, and the eigenvalue equation and the formulae of the equivalent admittance have been established. As an example, the predicted performances of a new multiband horn have been given.
Spectral parameter estimation of CAT radar echoes in the presence of fading clutter
Authors: Sato Toru , Woodman R. F.
Published: February 2008

The 430 MHz, 2 MW radar at the Arecibo Observatory is currently being used as a stratospheric‐troposheric (ST) radar. One of the problems in the automatic analysis of the signals come from the very large amplitude of the ground clutter echoes. The problem is agravated by the fading of these signals, which makes it difficult to discriminate them from the desired stratospheric returns. A parameter estimation technique that parameterizes the first three‐spectral moments corresponding to the desired stratospheric signals as well as a few Taylor series coefficients of the auto‐correlation function of the clutter is presented. The three first moments can be obtained in this manner even in the presence of clutter 50 dB stronger. The technique automatically takes care of instrumental and processing biases. Spectrum aliasing presents no problem. A sequence of fine altitude and high temporal resolution wind profiles is presented, showing the potential of the instrumental and technique for the study of stratospheric dynamics.
Microwave radio meteorology: Diurnal fading distributions
Authors: Schiavone James A.
Published: February 2008

The diurnal variation in the occurrence of fading on a terrestrial line‐of‐sight microwave radio system is studied using a 1‐year data set collected at Palmetto, Georgia. Six‐gigahertz signals transmitted over 31‐ and 57‐km paths are monitored, as well as meteorological parameters at 7.6‐, 38.1‐, and 93.3‐m elevations on the Palmetto receiver tower. Diurnal distributions of fading are compiled, and their changes with fade level, season, and path length are studied; comparisons are also made with published distributions measured elsewhere. It is found that diurnal fading distribution widths can be approximated by the sum of 3.5 hours plus the June 20 sunset‐sunrise time interval for the site latitude. The seasonal variation in daily fading onset and termination times is studied. It is found that 93% of the 20‐dB fading episodes terminate by 2.5 hours after sunrise throughout the year. Observations on all of the data are interpreted in terms of current knowledge of the diurnal evolution of the atmospheric boundary layer.
Binding and release of brain calcium by low‐level electromagnetic fields: A review
Authors: Adey W. R. , Bawin S. M.
Published: February 2008

Evidence has accumulated that sensitivity of brain tissue to specific weak oscillating electromagnetic fields occurs in the absence of significant tissue heating (less than 0.1°C). This review focuses on the ‘windowed’ character of sensitivities of calcium binding and electrical activity in brain tissue to low‐frequency modulation and intensity characteristics of impressed RF fields. ELF fields decrease calcium efflux from isolated chick and cat cerebral tissue by about 15% only in narrow amplitude and frequency ‘windows,’ between 6 and 20 Hz and between 10 and 100 V/m (approximate tissue gradient, 10−7 V/cm). VHF (147 MHz) and UHF (450 MHz) fields increase calcium efflux from isolated chick brain by about 15% when amplitude modulated between 6 and 20 Hz, but only for incident fields in the vicinity of 1.0 mW/cm2. We have now shown that this increased efflux in response to 16‐Hz amplitude‐modulated 450‐MHz, 0.75‐mW/cm2 field exposure is insensitive to variations in calcium concentration from 0 to 4.16 mM in the testing solution but is enhanced by addition of hydrogen ions (0.108 mM 0.1 N HCl) and inhibited in the absence of normal bicarbonate ion levels (2.4 mM). In the presence of lanthanum ions (2.0 mM), which block transmembrane movement of calcium, exposure to these EM fields decreases the 45Ca2 + efflux. Low‐frequency gradients may be transduced in a specific class of extracellular binding sites, normally occupied by calcium ions and susceptible to competitive hydrogen ion binding. Transductive coupling may involve coherent charge states between anionic sites on membrane surface glycoproteins, with longrange cooperative interactions triggered by weak extracellular electric fields. Proton ‘tunneling’ may occur at boundaries between coherent and noncoherent charge zones.
The temporal structure of intensity scintillations near the magnetic equator
Authors: Basu Santimay , Whitney H. E.
Published: February 2008

The temporal structure of weak and strong intensity scintillations observed with a geostationary satellite at 249 MHz near the magnetic equator is studied. It is shown that for weak intensity scintillations, the power spectra exhibit an asymptote immediately beyond the Fresnel frequency, which signifies a power law variation of power spectral density with frequency yielding spectral indices ranging between −3 and −3.5. By the use of simultaneous irregularity drift measurements, it is found that the power law portion of the spectrum for weak scintillations is caused by F region irregularities in the scale length range of about 700 m to 100 m. On the other hand, for strong intensity scintillations, the asymptote signifies a somewhat higher spectral index ranging between −3 and −4.5. Considering both weak and strong scintillations, an average spectral index of −3.5 is obtained which is found to be consistent with the average one‐dimensional in situ spectral index for irregularities with scale lengths less than 1 km. In addition, it is shown that for strong intensity scintillations, a substantial decrease of the correlation time occurs which is dictated not only by the increased strength of scattering but by the increased irregularity drift as well. At 249 MHz the shortest correlation length transverse to the propagation path is found to be about 70 m near the magnetic equator. This is a factor of 5 larger than the corresponding value obtained away from the equator near the crest of the equatorial anomaly at Ascension Island.
Automatic calculation of electron density profiles from digital ionograms: 3. Processing of bottomside ionograms
Authors: Reinisch Bodo W. , Xueqin Huang
Published: February 2008

Automatic scaling of bottomside Digisonde ionograms gives the E and F region echo traces with high accuracy. Polarization and incidence angle information in the ionogram enables the extraction of the vertical, ordinary polarization echo trace from quiet as well as disturbed ionograms. The scaling algorithm is tested with Digisonde ionograms from Goose Bay, Labrador, which show spread F about 50% of the time. Despite these disturbed conditions, f0 F2 is determined within ½ MHz for 475 ionograrns out of 577 during January 1980. A profile inversion algorithm calculates the electron density profile from the autoscaled h′(ƒ) points. Parabolic profile shapes are assumed for the E region and the valley between the E and the F layer. The F layer is approximated by a single sum of Chebyshev polynomials, and the entire profile is described by a set of 16 numerical values. The CPU time required for ionogram scaling and electron density profile calculation is 14 s on a Cyber 71 computer. In addition to the profile the program outputs all relevant ionogram parameters: critical frequencies, minimum virtual heights, frequency and range spread, average echo amplitudes, and Doppler shifts.
Radio loss in forests using a model with four layered media
Authors: dos Santos Cavalcante Gervásio Protásio , Rogers David Anthony , Giarola Attilio José
Published: February 2008

A model with four layered media is developed for the calculation of the radio loss in forests. In this model two lossy dielectric layers placed over a conducting flat earth are used to represent the crown and the trunk of the trees. Dyadic Green's functions in their eigenfunction expansion forms are used to analyze this model, and the electromagnetic fields are obtained from these functions. The theoretical results of the radio loss for vertical and horizontal radiating dipoles and for typical forests agree with the available experimental data. For this comparison the parameters for the layer representing the crown of the trees were estimated.
Observations of stimulated electromagnetic emissions in ionospheric heating experiments
Authors: Thidé B. , Derblom H. , Hedberg Å. , Kopka H. , Stubbe P.
Published: February 2008

Induced sideband modes, due to stimulated electromagnetic emissions in the ionosphere, have been observed around the frequency of a high‐intensity HF radio wave transmitted from the Heating ionospheric modification facility near Tromsø, Norway. The new observational technique used amounts to analyzing the received HF spectrum directly without utilizing any diagnostic radar. It appears that we have in this way been able to identify the parameteric decay instability and, possibly, the stimulated Brillouin backscattering of the strong HF pump wave. A series of narrow spectral peaks induced in the upper sideband by Heating transmission in extraordinary mode may indicate the excitation of ion Bernstein modes. At times, very strong spectral features, upshifted from the 5.4‐MHz pump by typically 20–45 kHz, are observed. These features are still lacking a convincing theoretical explanation. By monitoring a few fixed frequencies in both the lower and the upper sidebands of the ionospherically reflected modifying wave, we have found that the intensity of the stimulated emissions exhibits a conspicuous long‐term as well as short‐term variability.
Millstone Hill studies of the trough: Boundary between the plasmapause and magnetosphere or not?
Authors: Holt J. M. , Evans J. V. , Wand R. H.
Published: February 2008

Troughs in the auroral ionosphere have been observed in satellite and radar data since 1965. One explanation for the appearance of these troughs is the stagnation theory of trough formation. According to this model, magnetic flux tubes are convected from the dayside to the nightside across the polar cap as a result of the impressed magnetospheric electric field. These tubes return to the dayside by subsequently drifting westward or eastward. Tubes drifting westward must travel faster than earth rotation if they are to reemerge in sunlit regions, and some fail to do so, allowing the ionization in them to fall to very low levels as it recombines. The trough is the locus of flux tubes which spend a long time in darkness due to the competing effects of the convection electric field and earth rotation. Commencing in 1979, a new observing technique was implemented at Millstone Hill in which the 150‐ft antenna is moved slowly under computer control as data collection proceeds. The electron density results obtained with these experiments show that when a trough does form there is a great variation in when and where it is seen and how long it is present. On some nights the trough does not form at all. Furthermore, the appearance of the trough in the data often seems to occur in regions where the electric field is very large, with the ion drift velocity often far exceeding the speed of earth rotation. None of this behavior is entirely consistent with the stagnation theory of trough formation. An alternative explanation is to suppose that troughs are formed in regions of very strong convection (>1 km/s) as a result of the increase in the rate of charge transfer between O+ and O that occurs when the ions are driven through the neutral gas at high speed. According to such a view, “troughs” would be created in localized regions during substorms. The troughs might be created in regions of strong westward flow in the dusk sector. They then could migrate south and eastward depending upon the effects of convection and corotation. In an effort to test this picture we have begun conducting observations through 360° azimuth to see if the ionosphere to the northeast of the radar is the same as that seen later to the northwest. Results are presented for December 21–22, 1980. These do exhibit significant differences, tending to confirm that the trough is not a persistent extended E‐W feature.
Theories of high‐latitude ionospheric irregularities: A review
Authors: Keskinen M. J. , Ossakow S. L.
Published: February 2008

A brief review of theories of naturally occuring high latitude E and F region ionospheric irregularities and structure is presented. Emphasis is placed on irregularity source mechanisms. Recent theoretical results are stressed.
Stimulated scattering by collisional modes in the ionosphere
Authors: Stenflo L.
Published: February 2008

We point out that it ought to be possible to observe stimulated scattering by the two‐stream Farley‐Buneman and the gradient drift modes in the ionosphere with, for example, the radar facilities in northern Scandinavia.
An HF phased‐array radar for studying small‐scale structure in the high‐latitude ionosphere
Authors: Greenwald R. A. , Baker K. B. , Hutchins R. A. , Hanuise C.
Published: February 2008

Since October 1983, a new coherent backscatter radar has been in operation at Goose Bay, Labrador, for the purpose of studying small‐scale electron density structure in the high‐latitude ionosphere. This radar operates over a frequency band that extends from 8 to 20 MHz, and it uses an electronically phased array of 16 log‐periodic antennas for both transmission and reception. The radar transmits a seven‐pulse pattern that enables one to determine 17‐lag complex autocorrelation functions of the backscattered signals as a function of range and azimuth. In this paper we present a complete description of the radar including explanations of the operation of the phasing matrix, the techniques of data acquisition and analysis as implemented in the radar microcomputer, and the possible on‐line and automatic operating modes that may be instituted. We also present examples of some of the initial results that we have obtained with the radar during the afternoon and late evening hours. These examples include images of the two‐dimensional distribution of small‐scale structure and of their associated mean Doppler motion. We also present examples of F region Doppler spectra derived from the complex autocorrelation functions. These Doppler spectra show interesting differences from those of high‐latitude E region irregularities.
Large scale structures from topside sounding
Authors: Gross S. H.
Published: February 2008

ISIS 2 orbits at an altitude of about 1400 km and sounds the ionosphere in the range from 0.1 MHz to 20 MHz. On analyzing a sequence of ionograms measured on April 26, 1971, covering a region from about −73° to 88° in geographic latitude along a trajectory that is mostly close to 80°W longitude, at about 1000 UT or about 5 a.m. local time, one finds large‐scale structures in the spatial fluctuations of the electron density above the F peak. The structural features are analyzed by using isoelectronic contours, separating out trends in the data to obtain the spatial fluctuations in the altitude for various levels of electron density. The altitude variations for the various electron density levels strongly correlate, and spatial fluctuation waves are found on the order of 2000 km in scale size. Another example is shown for February 20, 1975, and is for an orbit that is in near conjunction with the Atmosphere Explorer C (AE‐C) satellite. AE‐C is at a much lower altitude, about 250 km, and measures, among many parameters, in situ ionization density. Comparison of the structures observed by ISIS 2 and AE‐C in the electron density fluctuations relates features above and below the F peak; the same spatial wave system is found in the data of both satellites with scale sizes from about 600 km to more than 2000 km.
Standing wave pattern of HF radio waves in the ionospheric reflection region: 1. General Formulas
Authors: Lundborg Bengt , Thidé Bo
Published: February 2008

General formulas for the accurate computing of the field strength near the reflection point of an electromagnetic wave impinging vertically upon a horizontally stratified ionosphere are derived by means of a uniform approximation valid throughout the whole reflection region. Smoothly varying profiles of general shape but with the gross features determined by the presence of one or two transition points are assumed. Coupling and nonlinear effects are not explicitly included. Since the uniform approximation method is analytic in its essence, it is in many respects more versatile than existing brute force numerical methods.
The determination of representative ionospheric conductivity parameters for ELF propagation in the earth‐ionosphere waveguide
Authors: Bannister Peter R.
Published: February 2008

In this article, we use experimentally determined values of the ELF effective attenuation rate and excitation factor, along with a modified theory of Greifinger and Greifinger, to establish representative ionospheric conductivity parameters for each propagation path over which ELF measurements were made from 1966 to 1982. Both daytime and nighttime (ambient and disturbed) propagation conditions will be considered. The representative ionospheric parameters include the reflection height h0, inverse scale height β, and reference height H.
The MU radar with an active phased array system: 1. Antenna and power amplifiers
Authors: Fukao Shoichiro , Sato Toru , Tsuda Toshitaka , Kato Susumu , Wakasugi Koichiro , Makihira Tsuneichi
Published: February 2008

The MU (middle and upper atmosphere) radar of Japan is a 46.5‐MHz pulse‐modulated monostatic Doppler radar with an active phased array system. The nominal beam width is 3.6° and the peak radiation power is 1 MW with maximum average power of 50 kW. The system is composed of 475 crossed three‐subelement yagi antennas and an equivalent number of solid state power amplifiers (transmitter‐receiver modules). Each yagi antenna is driven by a transmitter‐receiver module with peak output power of 2.4 kW. This system configuration enables very fast and almost continuous beam steering that has not been realized by other mesosphere‐stratosphere‐troposphere radars. Also, a variety of sophisticated operations are made feasible by dividing the antenna array into several independent subarrays. A brief description of the system, particularly its antenna and power amplifiers, is presented herein.
Convective and dynamical instabilities due to gravity wave motions in the lower and middle atmosphere: Theory and observations
Authors: Fritts David C. , Rastogi Prabhat K.
Published: February 2008

Dynamical and convective instabilities are two mechanisms that contribute significantly to the dissipation of larger‐scale motions and the generation of turbulence in the middle atmosphere. The former are normally due to enhanced velocity shears and/or a local minimum of the static stability either in the mean flow or associated with low‐frequency wave motions. The most common dynamical instability is the Kelvin‐Helmholtz (KH) instability which is often manifested in the atmosphere as a series of KH billows. Convective instabilities occur where the lapse rate becomes superadiabatic through the action of gravity waves and appear to predominate for high‐frequency wave motions. This paper reviews the theory and the observational evidence for both types of instabilities in the lower and middle atmosphere.
Evolution of atmospheric spectrum by processes of wave‐wave interaction
Authors: Yeh K. C. , Liu C. H.
Published: February 2008

Previous work has shown that a primary gravity wave of sufficient amplitude will always decay through resonant wave‐wave interactions with two secondary gravity waves. The interaction among the resonant trio can be reasonably rapid and may be an important process responsible for energy exchange among gravity waves of different wavelengths in the atmosphere. By taking statistical ensemble averages to the interaction equations we obtain an hierarchy of moment equations, the closure condition of which can be effected by making a random phase approximation. The obtained kinetic equation is Boltzmann‐like and describes the time evolution of wave action. The nonlinear kinetic equation is impossible to solve in general except numerically, but the equation is drastically simplified in three limiting cases identified as elastic scattering, parametric subharmonic instability, and induced diffusion processes. Through elastic scattering, an upgoing wave is scattered into a downgoing wave by interacting resonantly with a vertical shear. This process is thus responsible for making the atmospheric spectrum vertically symmetric if it is not so initially. The parametric subharmonic instability process is responsible for transferring energetic large‐scale waves to small‐scale waves at one‐half the frequency. The induced diffusion process is responsible for time evolution of small‐scale waves by a three‐wave process involving two nearly identical waves of small scales interacting resonantly with a large‐scale vertical shear. The rapidity with which these three processes take place in the atmosphere and their implication in the atmospheric spectrum are investigated and discussed.
Observations of mesospheric wind velocities: 2. Cross sections of power spectral density for 48–8 hours, 8–1 hours, and 1 hour to 10 min over 60–110 km for 1981
Authors: Meek C. E. , Reid I. M. , Manson A. H.
Published: February 2008

Cross sections of power spectral density for 1981 between 60–110 km for the 48‐ to 8 hour and 8‐ to 1‐hour‐period bands, and of mean square velocity components in the 10‐ to 60‐min‐period band, are compared to cross sections of mean wind. Individual spectra agree well in form and amplitude to similar spectra for other sites when averaged over the entire year, but there are substantial variations in power spectral density in each band over the year. In the 10‐min to 8‐hour‐period band, minima in spectral densities follow the zero line of mean zonal velocity very closely, indicating strong gravity wave mean flow interactions. This, and the variation in gravity wave intensity in general, suggests that gravity wave momentum deposition plays an important role in balancing the heat and momentum budgets of the upper middle atmosphere.
Radar comparison of 2.66‐MHz and 40.92‐MHz signals scattered from the mesosphere
Authors: Royrvik O.
Published: February 2008

Comparisons have been made between 2.66‐MHz and 40.92‐MHz radar signals simultaneously scattered from the mesosphere. Echoes were generally received from the same height ranges, indicating that the same scattering mechanism operates at both radar frequencies. A comparison of the observed radar cross sections was made for data obtained during the spring of 1984. Indications are that these echoes are due to scattering from turbulent irregularities in the refractive index. It has been possible to estimate the inner scale of turbulence from these data. The inner scale was found to be close to that estimated from turbulence theory. A comparison of the reflection coefficients indicates that a specular reflection process cannot account for the relative signal strength observed in these data.
A modified Fresnel scattering model for the parameterization of Fresnel returns
Authors: Gage K. S. , Ecklund W. L. , Balsley B. B.
Published: February 2008

This paper presents a modified Fresnel scatter model and compares the revised model with observations from the Poker Flat, Alaska, radar, the sounding system (SOUSY) radar, and the Jicamarca radar. The modifications to the original model have been made to better account for the pulse width dependence and height dependence of backscattered power observed at vertical incidence at lower VHF. Vertical profiles of backscattered power calculated using the revised model and routine radiosonde data show good agreement with observed backscattered power profiles. Relative comparisons of backscattered power using climatological data for the model agree fairly well with observed backscattered power profiles from Poker Flat, Jicamarca, and SOUSY.
Reduction of ambiguities in HF radar results through a revised receiving antenna array and sounding pattern
Authors: Jarvis M. J. , Dudeney J. R.
Published: February 2008

The reconfiguration of both the receiver dipole array and the pulse set used by the National Oceanic and Atmospheric Administration (NOAA) HF radar enables all ambiguity in polarization to be removed. Further, it enables the aliasing of Doppler velocities to be reduced by a factor of 2, which allows more flexibility in the pulse repetition rate used and permits lower velocities to be detected above the noise. The revised four‐dipole array is L‐shaped, with a crossed dipole at the corner. The addition of two extra dipoles provides the facility for unambiguous “sky map” echo location.
Empirical models of the electron concentration of the ionosphere and their value for radio communications purposes
Authors: Dudeney John R. , Kressman Richard I.
Published: February 2008

The availability of representative empirical models of the variation of the electron concentration with height in the ionosphere is one critical element in the development of practical schemes for providing reliable HF radio propagation predictions. The degree to which a specific model succeeds must be judged against three criteria: (1) quality of profile match: the model must reproduce the range of N(h) profiles encountered in practice; this match should be maintained in the first and second derivatives with height; (2) availability and simplicity of the external data required for profile specification: input data which consist of only those parameters routinely scaled from ionograms and available from CCIR predictions best fit this criterion; (3) numerical complexity: preferably the mathematical algorithms describing the model should be as simple as possible. These criteria conflict so that schemes have been developed optimized with respect to one or two of them at the expense of the other(s), the choice being driven by the application. In this paper, four such schemes are reviewed and contrasted in the light of these criteria and usage in practice. It is concluded that the Booker (1977) “skeleton” profile approach is best for simulating a known profile for precision radio wave studies, while the Dudeney (1978) technique provides good overall performance for general purpose communications predictions. The performance of the scheme currently recommended by the CCIR (Bradley and Dudeney, 1973a) cannot match these two, and should be superceded except where the need for mathematical simplicity is of overriding importance. The International Reference Ionosphere profile is found to have significant disadvantages for communications work with respect to all three criteria.
Computation of Hertzian dipole radiation in stratified uniaxial anisotropic media
Authors: Kwon Young S. , Wang Johnson J. H.
Published: February 2008

Solutions to the problem of radiation of Hertzian dipoles in stratified uniaxially anisotropic media are obtained by decomposing a general wave field into transverse electric and transverse magnetic modes. With the use of primary excitation fields due to a Hertzian dipole embedded in an anisotropic medium, the electromagnetic fields in stratified anisotropic media are derived by applying the appropriate boundary conditions. General expressions for fields due to vertical electric, vertical magnetic, horizontal electric, and horizontal magnetic dipoles are presented. Numerical computations were carried out for the case of a vertical electric Hertzian dipole in a uniaxially anisotropic substrate backed by a ground plane.
The effect of the dynamic wet troposphere on radio interferometric measurements
Authors: Treuhaft R. N. , Lanyi G. E.
Published: February 2008

Calculations using a statistical model of water vapor fluctuations yield the effect of the dynamic wet troposphere on radio interferometric measurements. The statistical model arises from two primary assumptions: (1) the spatial structure of refractivity fluctuations can be closely approximated by elementary (Kolmogorov) turbulence theory, and (2) temporal fluctuations are caused by spatial patterns which are moved over a site by the wind. The consequences of these assumptions are outlined for the very long baseline interferometry (VLBI) delay and delay rate observables. For example, at 20° elevations in midlatitudes, the wet troposphere induces about 2 cm of delay fluctuation for two‐station, 3‐hour observations. At 20° elevations for 200‐s time intervals, water vapor induces approximately 1.5 × 10−13 s/s of delay rate fluctuation for two‐station observations, which corresponds to an Allan standard deviation of 1.2 × 10−13 s/s. The statistical model suggests that delay rate measurement error is dominated by water vapor fluctuations for most VLBI experimental situations. Water vapor induced VLBI parameter errors and correlations are calculated as a function of the delay observable errors. For example, intercontinental baseline length parameter errors due to water vapor fluctuations are of the order of 2–3 cm. The above physical assumptions also lead to a proposed method for including the water vapor fluctuations in the parameter estimation procedure, which is used to extract baseline and source information from the VLBI observables.