bluuebunny/crossref_metadata_2025 huggingface dataset:
Characteristics of modulated periodic media: 2. Active structures
Authors: Warhola G. T. , Jaggard D. L.
Published: February 2008

We investigate the oscillation characteristics of active modulated periodic slabs such as those used for distributed feedback (DFB) lasers. In particular, we examine the effect of modulation wave number on the threshold gain and oscillation frequency of these structures by employing the vector coupled‐mode approximation. Significant changes in threshold gain are found for large modulation indices and moderate couplings as compared to the unmodulated case. Small changes in the oscillation frequency are also noted. Methods for lowering threshold gain are suggested.
Coupled modes with random propagation constants
Authors: Rowe Harrison E. , Mack Iris M.
Published: February 2008

An exact theory for two coupled modes traveling in the same direction with white random propagation constants is developed. Applications include tolerance studies for integrated optical directional couplers. The expected fields and the expected powers of the two modes are determined versus distance along the coupler, with the ratio of propagation‐constant spectral density B0 to coupling C as a parameter. For small values of B0/C the powers in the two modes behave in a damped oscillatory manner, with distance along the coupler. For large values of B0/C the behavior is monotonic. In all cases, equal power division occurs for sufficiently large distance along the coupler when heat loss is absent. For an optical switch the maximum B0/C is given as a function of the required switch performance by the present results. The dividing line between oscillatory and monotonic behavior of the powers in the two modes versus distance occurs for B0/C = 4. This choice of parameters yields a power divider of minimum length whose performance is insensitive to the dimensions.
Single‐mode propagation in multi‐layer elliptical fiber waveguides
Authors: Rengarajan S. R. , Lewis J. E.
Published: February 2008

Single mode propagation in three‐layer step index elliptical fiber waveguides is investigated theoretically. Previous analysis of the elliptical dielectric tube waveguide is extended to include cladded, W‐type and tube‐type fibers. Numerical results on cutoff characteristics of the first higher‐order mode and phase coefficient, group velocity and power distribution characteristics of the dominant modes are presented over a wide range of parameters. A two‐layer step index elliptical fiber waveguide is presented as a special case. High eccentricity single mode fibers have small modal beat lengths but have small core size and greater power distribution in the claddings. W‐type fiber in the middle range of eccentricities is suggested for single mode single polarization applications.
Noncircular optical waveguides and modal choice
Authors: Yang Shuwen
Published: February 2008

In this paper we investigate the propagation characteristics of light in noncircular dielectric optical waveguides based on the microwave model experiments. It is found that the attenuation of noncircular waveguides is smaller than that of circular guides and the attenuation of the odd modes is smaller than that of the even modes in noncircular dielectric waveguides. Hence it is advantageous to use noncircular optical fibers. Furthermore, it is advantageous to choose the polarization for single mode optical fiber so that a better propagation characteristic is obtained.
Observations of cloud‐produced amplitude scintillation on 19‐ and 28‐GHz earth‐space paths
Authors: Cox D. C. , Arnold H. W. , Hoffman H. H.
Published: February 2008

The amplitudes of satellite signals sometimes scintillate ± several decibels when heavy cumulus clouds pass through the radio path on hot summer days. These scintillations have been measured on a 19‐GHz and 28‐GHz earth‐space path using 7‐m and 0.6‐m antennas at Crawford Hill, New Jersey. Scintillation intensity at 28 GHz is 1.2 times that at 19 GHz, consistent with the ƒ7/12 frequency dependence produced by a thin turbulent layer. The scintillation process is polarization‐independent and has a low‐pass power spectrum with a cutoff frequency of about 0.3 Hz. Rain attenuation often accompanies the more intense scintillation. The mean duration of scintillation‐produced fades is short (about 1.3 s for fades greater than 1 dB), but over 1000 fades of over 1 dB at 28 GHz were observed in two summer months. Because of their weak frequency dependence these short but frequent events could produce repeated outages on 4‐ and 6‐GHz earth‐space links having low attenuation margins.
A comparison of radiative transfer models for predicting the microwave emission from soils
Authors: Schmugge T. J. , Choudhury B. J.
Published: February 2008

Two general types of numerical models for predicting microwave emission from soils are compared—coherent and noncoherent. In the former, radiation in the soil is treated coherently, and the boundary conditions on the electric fields across the layer boundaries are used to calculate the radiation intensity. In the latter, the radiation is assumed to be noncoherent, and the intensities of the radiation are considered directly. The results from the two approaches may be different because of the effects of interference, which can cause the transmitted intensity at the surface (i.e., emission) to be sometimes higher and sometimes lower for the coherent case than for the noncoherent case, depending on the relative phases of the reflected fields from the lower layers. This coupling between soil layers in the coherent models leads to greater soil moisture sampling depths observed with this type of model, and is the major difference that is found between the two types of models. In noncoherent models, the transmission at the surface is determined by the dielectric constant at the air/soil interface. The subsequent differences in the results are functions of both the frequency of the radiation being considered and the steepness of the moisture (i.e., dielectric constant) gradient near the surface. The calculations were performed at frequencies of 1.4 and 19.4 GHz and for two sets of soil profiles. Little difference was observed between the models at 19.4 GHz; and only at the lower frequency were differences apparent because of the greater soil moisture sampling depth at this frequency. For those situations which have a nonuniform temperature profile a definition for an effective emissivity is given, that is, the ratio of brightness temperature to the effective radiating temperature, the latter being a weighted average temperature over the emitting depth. Measurements from a tower are presented which show excellent agreement with the calculations from the coherent model.
Microwave equatorial scintillation intensity during solar maximum
Authors: Aarons Jules , Whitney Herbert E. , MacKenzie Eileen , Basu Santimay
Published: February 2008

A comparison of scintillation levels at 1.5 GHz made from the Appleton anomaly region of the magnetic equator and from the region close to the magnetic equator (termed the electrojet latitudes) showed increased F region irregularity intensity over the anomaly region during years of high sunspot number. Peak to peak fading greater than 27 dB was noted from Ascension Island (through a dip latitude of 17°) in the anomaly region while only 7–9 dB from Natal, Brazil, and Huancayo, Peru, were noted, the last two paths being close to the magnetic equator. The hypothesis advanced is that the dominant factor responsible for the intense gigahertz scintillation is the traversal of the propagation path through the anomaly region. During years of high sunspot numbers the high levels of ΔN constituting the F region irregularity structure are due to (1) very high electron density in the anomaly region (compared to the electrojet region) and (2) the late appearance of these high electron densities (to 2200 local time) in the anomaly region. The patches or plumes of irregularities seen in the postsunset time period then produce high ΔN; scintillation excursions are proportional to this parameter. The postulation of vertical irregularity sheets in the patches was examined to determine the possibility of this being an important factor in the difference between electrojet and anomaly scintillation levels. Older gigahertz data from the sunspot maximum years 1969–1970 were reanalyzed, and more recent observations from other studies were also reviewed. It was found that through the anomaly region, high scintillation indices were noted at a variety of azimuths of the propagation path rather than just along a path closely aligned with the magnetic meridian. A more complete evaluation of the geometrical factor, which must be of considerable importance in determining the absolute value of the scintillation intensity, awaits further observations.
Use of polarization in electromagnetic inverse scattering
Authors: Boerner Wolfgang‐M.
Published: February 2008

The complete description of electromagnetic scattering processes implies polarization and since an electromagnetic scatterer acts like a polarization transformer we require measurements for the complete description of the target scattering matrices so that the descriptive parameters of a scatterer can be uniquely recovered from the measured field data. For the purpose of introducing the concept of polarization utilization in practice, the radar case is chosen and generalizations to other electromagnetic inverse problems are presented in the conclusions. For example, in radar target discrimination, identification and imaging use of measurement data available over the entire spatial frequency domain of the radar cross section must be made leading to various approximate frequency domain related approaches. Mainly for historical reasons of having had amplitude data available only, in most cases the approximations had been simplified to purely scalar nature; i.e., polarization‐dependent properties were discarded, and the resulting theories are no longer valid or unique. It is the main objective of this analysis to show that because of the vector nature of electromagnetic inverse scattering, theories, if applicable in practice, require incorporation of complete polarization information into their formulation. By applying this approach to existing theories, it is shown that remarkable improvements in fidelity and quality of the reconstructed images are obtained and that indeed there is ample justification for continuing efforts in developing methods and theories of inverse scattering applicable to all those fields of physical sciences where information on the characteristic parameters of a scattering process is to be drawn from remote measurements, be it the electromagnetic vector case or the even more complicated seismic case of s and p wave interactions in elastic media.
Fast tunable active receiving antennas
Authors: Hopf J. F. , Lindenmeier H. K.
Published: February 2008

With mobile RF communication systems, sensitivity and linearity characterized by the dynamic are the main important qualities of the system on the receiving side. Up to now, especially in the low frequency and the short wave frequency range this problem has not been solved satisfactorily with professional passive antenna systems even though heavy filters are used. Considering nonlinear effects and taking the electromagnetic pulse problem (emp) into account, the short selective active receiving antenna is the optimum solution to this problem. A formula is derived for the field strength sensitivity of the selective active rod antenna. Taking the external noise into account, the minimum height of the antenna is evaluated with respect to optimum sensitivity as well as with respect to minimum nonlinear distortion of the amplifier, which means optimum dynamic range. Antenna heights and dynamic range of the selective active antenna are compared to different systems such as active broadband antennas and standard receiving systems with passtive antennas. For sensitivity reasons, capacitive antenna elements must be tuned by inductive means. In combination with a special switching strategy, premagnetized ferrite inductances are appropriate tuning elements. Nonlinear effects in the ferrites, however, are one of the main problems with electrically tuned antennas, which therefore are taken into account as well.
Wave scattering from a periodic dielectric surface for a general angle of incidence
Authors: Chuang S. L. , Kong J. A.
Published: February 2008

The scattering of electromagnetic waves from a periodic dielectric interface is studied for a general angle of incidence. It is called conical diffraction in grating theory for the general case where the incident wave vector is not perpendicular to the ruling direction of the gratings. We adopt the electric and magnetic field components along the row direction as the two unknown scalar functions and reduce the vector nature of the problem to solving simple matrix equations to obtain the scattered fields. The extended boundary condition method is applied with the Fourier series expansion for the surface fields. For the special case of incidence angle perpendicular to the row direction, the results are reduced to those developed before. Numerical results are obtained to show that energy conservation and reciprocity are obeyed for scattering by sinusoidal surfaces for the general case. This serves to check the consistency of the formalism.
Vector forward scattering theorem
Authors: Karam M. A. , Fung A. K.
Published: February 2008

The concept of energy balance is applied to compute the total loss (or extinction) resulting from a partially polarized plane wave impinging upon a scatterer. It is shown that under the far‐zone condition, the application of the stationary phase approximation leads to an expression relating the total loss to the Stokes parameters of the incident wave and the forward scattering amplitudes of the scattered wave. This relation may be referred to as the vector forward scattering theorem. For a linearly polarized incident plane wave, the result reduces correctly to the standard forward scattering theorem in the literature.
Scattering from cylindrical inhomogeneities in a lossy medium
Authors: Peters Leon , Richmond Jack H.
Published: February 2008

Many buried scattering objects of interest take the form of two‐dimensional geometries. This includes scattering from utility lines, tunnels, and geological structures such as fault lines. Radar systems used to detect these objects commonly use antennas that are at least comparable in size to the depth of the target. Thus the target and the antenna do not satisfy far zone conditions, and the radar range equation is not applicable. Consequently, the usual separate analyses of range, antenna, and target are not possible. In a previous paper a method was outlined that permitted the antenna properties and the scattering properties of a two‐dimensional target to be treated separately for the case of a linear electric or magnetic dipole source parallel to the axis of the two‐dimensional scatterer. This involved computing the received voltage for an antenna located at the image position, i.e., at twice the target range, and computing the backscattered fields for an electric or magnetic line source at the position of the transmitting antenna. This model has also been applied approximately to a video pulse radar with an orthogonal dipole antenna system. It would also be applicable to large loop antennas quite commonly used in geophysical explorations, as will be discussed. The primary goal of this paper is to discuss additional scattering analyses that could be used to extend the previous results. The major thrust then is to generate solutions for the scattering attenuation function (SAF) which has the formES/EIorHS/HIwhere theEI(orHI) are the electric (or magnetic) fields of an electric (or magnetic) line source at the image position and theES(orHS) are the respective scattered fields. Eigenfunction solutions have been used to obtain the SAF for circular cylindrical geometries to represent pipes and tunnels. Moment method solutions have been applied to perfectly conducting wires with and without a dielectric sheath. Moment method solutions have been applied to noncircular penetrable bodies using the polarization currents to represent the unknowns. Such solutions were developed for a line source above dikes by Parry and Ward (1971) and Hohman (1975) in the early seventies. Such solutions can be made for frequencies that include several target resonances for potential target identification. These solutions can possibly be extended to include fault lines, joints, etc., provided their electrical properties can be estimated by using some of the concepts involved in the hybrid geometrical theory of diffraction‐moment approach. The methods of the modified geometrical optics could also be applied to obtain scattered fields at higher frequencies. These and other potential approaches will be discussed.
A model for polarization effects in remote probing of clouds
Authors: Weil Herschel , Willis Thomas M.
Published: February 2008

A computer model is described which has been developed to aid in the interpretation of polarization data obtained by electromagnetic remote probing of mixed phase ice crystals and waterdrop clouds. Such clouds can contain many possible crystal forms, most notably thin long cylinders, bullets, and flat plate crystals, and the intent of the measurements is to characterize the ice crystal population by the polarization changes caused by these highly nonspherical shapes. The present work is a partial investigation of the theoretical aspects of this problem using flat circular disks as models of flat plate ice crystals randomly oriented over all rotation angles about axes perpendicular to the flat faces of the crystals. Mixtures of such disks and spherical particles to simulate waterdrops are considered. It is expected to include other crystal shapes in future work. Mie theory is used for the waterdrops, which are modeled as spheres, and a numerical procedure especially designed for dielectric disks is used for the flat plate crystals. Numerical results are presented to illustrate the sensitivity of the different Stokes parameters to ice crystal size and orientation distribution and variables such as the relative number density of ice crystals to waterdrops.
Geometrical optics solution of reflection from arbitrarily curved surface
Authors: Bremmer H. , Lee S. W.
Published: February 2008

This paper studies curvature properties of the wave fronts associated with rays emerging from the reflection of a given incident beam against an arbitrarily curved surface. Explicit formulas are given for the principal radii of curvature of the reflected wave front in terms of the corresponding radii of the reflecting surface and the wave front of the incident beam. Some applications of those formulas are considered. In particular, it is shown that the expressions for ‘divergence coefficients,’ as introduced in radio propagation theories, can be obtained from a special case of our general results.
Scattering by a strip with two face impedances at edge‐on incidence
Authors: Tiberio R. , Bessi F. , Manara G. , Pelosi G.
Published: February 2008

A high‐frequency solution for the diffraction from a strip with two arbitrary face impedances, illuminated at edge‐on incidence, is obtained by a spectral extension of the geometrical theory of diffraction. An asymptotic approximation of the solution given by Maliuzhinets for the half plane is used. Uniform expressions for the scattered far field are given for cylindrical and plane wave illuminations. Incidence perpendicular to the edges of the strip is considered in both TE and TM cases. In the case of plane wave illumination the expressions for the field are greatly simplified. Numerical results are presented. In particular for resistive strips, the backscattered field calculated from this solution compares very well with that calculated by other techniques.
Propagation of beam waves through a turbulent layer
Authors: Kiang Yean‐Woei , Liu C. H.
Published: February 2008

The problem of beam waves propagating through a turbulent layer is studied by using both the stochastic and the deterministic approaches. In the statistical approach exact analytical expressions of beam wander and spread are derived for the turbulence with Shkarofsky spectrum by using phase screen model. The beam spread is found to be dependent, among other factors, on the spectral index of the power law spectrum. In the deterministic approach, the beam wave is propagated through a numerically simulated ionospheric bubble. Hot spots in the intensity and fast fluctuations in the phase are found to appear across the beam. Numerical examples are given. Applications to various transionospheric beam wave systems are discussed.
Scattering from a class of randomly rough surfaces
Authors: Brown Gary S.
Published: February 2008

This paper develops new results pertaining to electromagnetic scattering from perfectly conducting random surfaces for which decorrelation does not imply statistical independence. Using an exact theory for the current induced on the surface and the far‐field approximation for the scattered field, it is shown that the incoherent scattered power consists of two parts. The first part corresponds to the so‐called diffuse scattered power. The second part is specular in its angular dependence and is a direct consequence of the fact that the two‐point joint density for the surface height, slopes, etc., does not reduce to the product of the single‐point joint densities for infinite separation distances. Computations for a gently undulating jointly exponentially distributed surface show that the incoherent specular power is equal to coherent scattered power for the Rayleigh parameter near 1. When the Rayleigh parameter is large, the incoherent specular power is significantly larger than the coherent power. The analysis further indicates that scattering measurements provide an ideal way for identifying this class of surfaces.
Time delay variations in HF propagation
Authors: LaBahn R. W. , Rose R. B.
Published: February 2008

An experiment has been established to determine the range of environmentally induced fluctuations in the propagation of HF radio signals. Data are collected with a fully digitized system that is stablized with a cesium beam standard. Initial efforts have been directed toward monitoring at San Diego, California, the time of arrival of WWV transmissions from Fort Collins, Colorado, for the frequencies 5, 10, 15, and 20 MHz. Variations in these arrival times from 25 to 175 μs have been observed depending upon frequency, mode of propagation, time of day, and solar activity.
Doppler measurements with a digital ionosonde: Technique and comparison of results with incoherent scatter data
Authors: Gonzales C. A. , Behnke R. A. , Woodman R. F.
Published: February 2008

A simple technique for measuring the amplitude and Doppler shift of vertically reflected HF echoes has been implemented for the Arecibo digital ionosonde. This technique can easily be adapted to other digital ionosondes controlled by small computers and/or microprocessors to provide the additional Doppler information. By using this technique, we have measured on‐line the echo amplitude and the apparent vertical velocity of the reflecting layers. Furthermore, we have compared these data with simultaneous incoherent scatter measurements of vertical ionization drifts. The ionosonde and incoherent scatter results almost always agrree during the nighttime but do not agree during the daytime. We discuss these results.
CCIR needs in earth‐space radio propagation
Authors: Dougherty H. T.
Published: February 2008

The context and role of the CCIR is briefly summarized. The recent advances in CCIR Study Group 5 texts, relevant for earth‐space systems, are engineering models of propagational effects. They are briefly described, identifying their associated recommendations and reports and specifying the additional experimental data and theoretical studies required.
Propagation effects on radio range and noise in earth‐space telecommunications
Authors: Flock W. L. , Slobin S. D. , Smith E. K.
Published: February 2008

Deep‐space missions and radio navigation satellite operations place high requirements upon the precision of range and Doppler frequency measurements and may be sensitive to even small increases in radio noise. For paths to geostationary satellites and beyond, the excess range delay due to the ionosphere and plasmasphere is proportional to the total electron content along the path and inversely proportional to frequency squared. The delay for a one‐way path is about 8 m for a total electron content of 1018 el/m2 at a frequency of 2.2 GHz. The one‐way excess range delay due to the dry air of the troposphere on earth‐satellite paths is typically of the order of a few meters and is dependent only on surface pressure for a given elevation angle. The delay due to water vapor, a few tens of centimeters, is responsible for most of the temporal variation in the range delay for clear air. In the navigation of Voyager spacecraft, where two‐way range measurements are made, range is determined with a precision of better than 3 m by the National Aeronautics and Space Administration/Jet Propulsion Laboratory Deep Space Network, made up of stations in Goldstone, California, Madrid, Spain, and Canberra, Australia. System noise temperatures for the Deep Space Network are typically 20–30 K. For such low‐noise systems and for attenuation values up to about 10 dB, the increase in sky noise due to rain and clouds degrades the received signal‐to‐noise ratio more than does the reduction in signal level due to attenuation. Clouds as well as rain contribute significantly to attenuation and sky noise, especially for frequencies greater than 10 GHz. Doppler frequency fluctuations due to the interplanetary plasma can be reduced by using higher frequencies (X or K band rather than S band, for example), but scintillation of tropospheric origin may then become the principal factor limiting the ability to detect gravitational waves.
Microwave noise temperature and attenuation of clouds: Statistics of these effects at various sites in the United States, Alaska, and Hawaii
Authors: Slobin Stephen D.
Published: February 2008

The microwave attenuation and noise temperature effects of clouds can result in serious degradation of telecommunications link performance, especially for low‐noise systems presently used in deep‐space communications. Although cloud effects are generally less than rain effects, the frequent presence of clouds will cause some amount of link degradation a large portion of the time. This paper presents a general review of cloud types and their water particle densities, attenuation and noise temperature calculations, and basic link signal‐to‐noise ratio calculations. Tabular results of calculations for 12 different cloud models are presented for frequencies in the range 10–50 GHz. Curves of average‐year attenuation and noise temperature statistics at frequencies ranging from 10 to 90 GHz, calculated from actual surface and radiosonde observations, are given for 15 climatologically distinct regions in the contiguous United States, Alaska, and Hawaii. Nonuniform sky cover is considered in these calculations.
Centimeter and millimeter wave attenuation and brightness temperature due to atmospheric oxygen and water vapor
Authors: Smith Ernest K.
Published: February 2008

Calculated atmospheric absorption and emission of radiation for several atmospheric conditions and elevation angles are given for frequencies of 1 to 340 GHz. The emission curves are those accepted in 1982 by the International Radio Consultative Committee (CCIR). The absorption values from these calculations are compared with others appearing in the literature or in use elsewhere.
Multiple scattering of electromagnetic waves by rain
Authors: Tsolakis A. , Stutzman W. L.
Published: February 2008

As the operating frequencies of communications systems move higher into the millimeter wave region, the effects of multiple scattering in precipitation media become more significant. In this paper, general formulations are presented for single, first‐order multiple, and complete multiple scattering. Included specifically are distributions of particle size, shape, and orientation angle, as well as variation in the medium density along the direction of wave propagation. Calculations are performed for rain. It is shown that the effects of higher‐order scattering are not noticeable in either attenuation or channel isolation on a dual‐polarized system until frequencies of about 30 GHz are reached. The complete multiple‐scattering formulation presented gives accurate results at high millimeter wave frequencies as well as including realistic medium parameter distributions. Furthermore, it is numerically efficient.
Electromagnetic scattering by open circular waveguides
Authors: Johnson Thomas W. , Moffatt D. L.
Published: February 2008

The calculation of the radar scattering characteristics of open circular waveguides is approached by the Wiener‐Hopf technique. The Wiener‐Hopf factorization functions are calculated by numerical integration, and these results are compared to dc and high‐frequency asymptotic approximations. The exact Wiener‐Hopf solutions for scattering, coupling, and reradiation by semi‐infinite circular waveguides are then computed for representative cases including axial backscatter with no internal obstacles, and backscatter based on the coupling to and radiation from individual waveguide modes. Finally, the high‐frequency asymptotic solution for axial backscatter is transformed into the time domain and compared to the result of numerical transformation of the backscatter results obtained from numerical integration.
Total electron content structure in the Middle East
Authors: Soicher Haim , Houminer Zvi , Shuval Avraham
Published: February 2008

Polarization rotation data recorded from the geostationary satellite SIRIO at Haifa, Israel, have been used to study the structure and variability of the total electron content (TEC) during the maximum phase of the current solar cycle. In addition to the overall diurnal and seasonal variation, the TEC is regularly modulated by the effects of traveling ionospheric disturbances (TID's). The behavior of TID's in terms of their frequency of occurrence, amplitude, period, and time of occurrence is seasonally dependent. Comparison with average behavior of TEC and related parameters observed during the minimum phase indicates the extent of the solar cycle variation.
A phase‐coherent link via synchronous satellite developed for very long baseline radio interferometry
Authors: Knowles S. H. , Waltman W. B. , Yen J. L. , Galt J. , Fort D. N. , Cannon W. H. , Davidson D. , Petrachenko W. , Popelar J.
Published: February 2008

A high‐precision, phase coherent link between remote interferometer stations has been developed and has been utilized in a transcontinental radio interferometer to assess the feasibility of improved astrometric and geodetic measurements. A dual‐tone transmission method effectively cancels most phase‐degrading effects. An experimental Allen variance curve indicates that stability of 1 × 10−13 was achieved over a 1‐hour period; the slope of the Allen variance curve was close to − 1 for all time intervals up to 24 hours, indicating that the link is truly phase stable at frequencies of less than 1 GHz. Changes in the link path were measured with an accuracy of 300 ps, corresponding to 10 cm, over periods of several hours.
Mutual coherence functions and intensities of backscattered signals in a turbulent medium
Authors: Yeh K. C.
Published: February 2008

The effect of a turbulent medium on the backscattered signal is investigated. Since the backscattered signal is traveling in the same medium as the forward incident signal, coherence between these oppositely propagating waves is expected. This coherence can be treated by using a reciprocity theorem applied to the propagator. In this paper the mutual coherence functions and the aperture‐averaged intensities are calculated under the saturated regime. Because of the presence of turbulence, changes from the respective free space value in both intensity and beam size can occur. For example, enhancement in the backscattered intensity is most likely to occur for narrow beams, while a large reduction in intensity is expected in the limit of small irregularities. In general, the backscattered intensity and beam size depend on the interplay of four scales: the irregularity size, the size of the scattering body, the size of the electromagnetic beam, and the Fresnel size. Formulas have been derived to show this dependence.
Controlled stimulation of VLF emissions from Siple Station, Antarctica
Authors: Helliwell R. A.
Published: February 2008

Coherent VLF (1.8–7 kHz) signals injected into the magnetosphere from Siple Station (L = 4.2), Antarctica, are amplified (30 dB) and trigger intense, narrowband whistler mode emissions. The mechanism of growth is believed to be wave‐induced phase bunching of cyclotron resonant electrons in an interaction region near the equatorial region. A key parameter in these experiments is the spectral purity of the transmitted signal. If the width of the signal spectrum exceeds ∼ 10 Hz, there is significant loss of gain. In one experiment (f ≈ 3 kHz), two equal‐amplitude signals are transmitted with a frequency spectrum of Δf. When Δf = 20 Hz, the gain is reduced as much as 20 dB below its single‐frequency value. For f ≥ 100 Hz, the two signals tend to behave independently of one another, indicating that they interact with different electron populations. Other effects of interest include entrainment of strong free‐running emissions by a relatively weak Siple signal and the frequent generation of sidebands (2–100 Hz) on Siple signals that have reached saturation. Future work at Siple Station will emphasize frequencies below 4 kHz in order to raise the energies (to > 10 keV) of the precipitating electrons and hence make them more readily detectable by ground‐based techniques. To achieve this goal, the dipole length will be doubled (to 43 km), which is expected to increase the maximum radiated power at 2.5 kHz by 7 dB. Later, it is planned to install a circularly polarized crossed dipole that will double the effective radiated power and permit new experiments on the polarization properties of the ionosphere and magnetosphere at VLF.
Summary of results obtained with the Tromsø heating facility
Authors: Stubbe P. , Kopka H.
Published: February 2008

It is the purpose of this paper to briefly summarize the results of the ionospheric modification experiments performed at Tromsø, Norway, from 1980 up to mid‐1982.
Direct measurements of ion composition with EISCAT in the high‐latitude F1 region
Authors: Lathuillere C. , Lejeune G. , Kofman W.
Published: February 2008

We present a method for obtaining O+ relative abundance directly from the autocorrelation functions measured by EISCAT; no assumption is made regarding the temperatures. Results of O+ relative abundance and temperatures obtained simultaneously are shown for a 20‐hour period on May 9, 1982, and the time variations of O+ relative abundance are compared to previous work.
Mesospheric radar echoes at Poker Flat, Alaska: Evidence for seasonally dependent generation mechanisms
Authors: Balsley B. B. , Ecklund W. L. , Fritts D. C.
Published: February 2008

The long‐term character of VHF mesospheric echoes obtained by using the Poker Flat MST radar in Alaska show the following characteristics: (1) The winter time echoes are relatively weak and are only present in the height range 55–80 km, while (2) the summertime echoes are much stronger, with the maximum echo intensity centered at 86 km. The evidence currently available indicates that the winter time echoes arise, from the nonlinear breakup of tropospherically generated gravity waves. The stronger summertime echoes, on the other hand, appear to arise in situ as a result of the combined effects of the steep summertime vertical temperature gradient that exists at these latitudes and the horizontal wind shears.
Observations of gravity waves in the auroral zone
Authors: Bertin F. , Kofman W. , Lejeune G.
Published: February 2008

The electron density, ion temperature, and velocity measured on November 18, 1981, from 1100 to 1700 UT by EISCAT facilities exhibited clearly periodic behavior. We present an attempt to interpret this feature as a manifestation of gravity waves. November 18 was a day of high magnetic activity, and it was difficult to separate completely the local effects from those induced by the gravity waves. The observed waves had a period of about 75–80 min and vertical wavelength of the order of 800 km. They were possibly generated in the auroral oval, about 1000 km from Tromsø.
Pole and residue extraction from measured data in the frequency domain using multiple data sets
Authors: Ksienski David A.
Published: February 2008

When the singularity expansion method is applied to measured data, the accuracy of the results is often severely limited by the noise and clutter present in the measurement. An effective method of reducing these contaminations is repeating and averaging the measurements. The effectiveness of this approach is related to the number of measurements which are combined. The present paper develops a method which expands the set of measurements which may be combined from the set of identical measurements to the set of all measurements made on the target. The measurements are combined using an optimal weighting scheme to provide a superior estimate for a specified pole. An algorithm is described which increases the accuracy of the estimated pole locations and accurately computes the residues associated with these new pole locations. The algorithm is used to extract a single pole and residue, and additional poles and residues may be obtained through iterative application of the algorithm.
High‐frequency ray paths in ionospheric layers with horizontal gradients
Authors: Davies K. , Rush C. M.
Published: February 2008

The results of a study to determine the effects of horizontal gradients in the F2 region electron density on short‐distance HF propagation paths (0–1000 km) are presented. Ray paths were obtained by employing specific models to represent the horizontal and vertical variations of the F2 region. These models permitted ray path characteristics to be expressed by algebraic relationships. The changes in ray path characteristics with height of reflection, frequency, and azimuth are presented for gradients typically encountered on short‐distance HF propagation circuits over Europe.
VHF‐UHF interferometric measurements, applications to lightning discharge mapping
Authors: Richard P. , Auffray G.
Published: February 2008

A VHF‐UHF imaging system for applications to the reconstruction, with a high temporal resolution, of the two‐dimensional spatial structure of lightning has been implemented. The interferometric technique used for obtaining high resolution and low ambiguities is presented, and the influence of source and system parameters on the location accuracy, ambiguity removal, and reconstruction of source structure is discussed.
Multiple phase‐screen simulation of HF wave propagation in the turbulent stratified ionosphere
Authors: Kiang Yean‐Woei , Liu C. H.
Published: February 2008

HF wave propagation in the turbulent stratified ionosphere is investigated using the multiple phase‐screen simulation method. For waves incident along the axis of the background stratification, generalized parabolic equations for the modified complex amplitude u are derived which are applicable even in the strong scattering case. Based on these parabolic equations, several one‐dimensional random phase screens are numerically generated to represent the effect of the random irregularities on the wave. For plane wave incidence, various statistical properties of the field, such as correlation, power spectrum, scintillation index, etc., can be directly computed using spatial averaging under the ergodicity assumption. The simulation method is also used to examine the effect of the velocity profile on the drift pattern on the ground.
Electron density irregularities observed on DE‐2
Authors: Hoegy W. R. , Curtis S. A. , Brace L. H.
Published: February 2008

Observations of electron density irregularities have been made with the Langmuir probe (LANG) on DE‐2. The DE‐2 LANG data were examined for irregularities with scale sizes of 30 to 170 km. Such irregularities were found at all longitudes in the polar cap and auroral oval with stronger fluctuations in the oval. Night time equatorial passes having local times near 1900 or 2400 LT and occurring in an 80 day wide band about equinox were examined for irregularity occurrence. A definite longitude pattern was found in the data from several hundred orbits which showed an eastward shift at later local times. The equatorial irregularity occurrence pattern found in the LANG data is consistent with earlier in situ and remote observations of irregularities and spread F. In fact, the combined data set was found to closely follow the season‐longitude pattern determined by the condition of solar terminator alignment with magnetic field lines. Tsunoda (1985) first showed this correlation with scintillation data.
Water vapor‐absorption line shape comparison using the 22‐GHz line: The Van Vleck‐Weisskopf shape affirmed
Authors: Hill R. J.
Published: February 2008

A test for correctness of line‐shape theories is devised for the near‐line‐center shape. The test is insensitive to variation in any parameter such as line strength, line width, water vapor continuum absorption, or oxygen absorption. The absorption data for the 22‐GHz water vapor line obtained by Becker and Autler clearly agree with the absorption line shape derived by Van Vleck and Weisskopf, and their data rule out the Gross and full Lorentz line shapes. Properties of the three line shapes are discussed. Further experimentation is suggested.
The measurement of the ocean wave directional spectrum from HF radar Doppler spectra
Authors: Wyatt Lucy
Published: February 2008

A model‐fitting technique to extract the ocean long wave directional spectrum from the Doppler spectrum of HF backscatter from the sea surface is described here. This technique is an extension of one developed by Lipa and Barrick [1982] and extends the ocean wave frequency range that can be measured using radar frequencies in the lower HF range (6‐18 MHz). Results of the application of this technique to a wide variety of simulated radar spectra are presented which demonstrate some of the weaknesses of the method as well as its strengths. The method predicts accurate estimates of the longwave amplitude spectrum as long as all wave components are propagating at an angle which is not perpendicular to the radar beam. When there are wave components perpendicular to the beam, this direction is estimated with very good accuracy, but there is an associated overprediction in amplitude. The solutions indicate that substantial improvements in accuracy are possible if two radars are used to survey the same patch of sea from two different directions.
Effects of incoherent scattering on attenuation and depolarization of millimeter and optical waves due to hydrometeors
Authors: Oguchi Tomohiro
Published: February 2008

Numerical solutions for the radiative transfer equation are obtained in millimeter and optical wave regions by the use of extended spherical harmonics method to investigate the importance of incoherent effect on propagation of electromagnetic waves in hydrometeors. Detailed directional patterns of incoherent intensities are obtained in a very wide range of distances which covers from the first‐order multiple scattering region to the diffusion region. Solutions in the case of linearly polarized incident wave are also obtained. The difference of the multiple scattering effects between millimeter and optical waves, the difference of the multiple scattering effects between linear and circular polarizations, etc., are discussed.
A radar technique for high range resolution incoherent scatter autocorrelation function measurements utilizing the full average power of klystron radars
Authors: Sulzer Michael P.
Published: February 2008

Incoherent scatter radar measurements with high range resolution have been made at Arecibo Observatory using a new technique, called the coded long pulse technique. This technique is similar to the multiple pulse technique in its uses, and in the way in which random clutter from unwanted ranges sets a lower limit on the noise level. It differs in that the full average power can be obtained from a radar that uses klystrons in the output stage. A description of the new technique is given, and the two techniques are compared. Some data showing the range variation of the HF induced enhanced plasma line are presented, and the superiority of the new technique for most E region measurements is demonstrated. Computational algorithms are discussed, and it is shown that in many cases the amount of computation required is not significantly more than is required by the multiple pulse technique.
The PROUST radar: First results
Authors: Bertin F. , Cremieu A. , Glass M. , Massebeuf M. , Ney R. , Petitdidier M.
Published: February 2008

The stratosphere‐troposphere radar PROUST (prototype de radar pour l'observation en UHF de la stratosphère et troposphère) is located at Saint Santin, France (44°39′N; 2°12′E), a place devoted to the French ionospheric incoherent scatter facility whose frequency is 935 MHz. The use of this equipment as stratosphere‐troposphere radar has required important modifications of the transmitting and receiving systems, which have been planned in several steps. In a first one, the radar works in a bistatic mode with a 600‐m height resolution, allowing measurements in the altitude range 3–10 km. The first results presented here have been obtained with this configuration during two campaigns. In the first one, observations can be interpreted in terms of lee waves; in the second, an intense turbulence activity associated with a high‐altitude temperature inversion is observed. In this last case, the oscillations observed in the vertical wind are consistent with trapped gravity waves generated by a decaying turbulence in a stratified medium. In a second step, now operating, a 30‐m height resolution has been implemented and will allow a better understanding of these phenomena. In its final version, the PROUST radar will operate in a monostatic mode that will permit to extend the observations up to 15 km.
Summary of ELF propagation variations at mid and high latitudes during the November/December 1982 and February 1984 solar proton events
Authors: Katan J. R. , Bannister P. R.
Published: February 2008

This paper reports the results of the first concerted effort to study the effects of solar proton events (SPEs) on ELF propagation using a manmade source (i.e., the Wisconsin Transmitter Facility (WTF)). The major result is the observation of substantially increased effective attenuation rates along high altitude paths during SPEs. The largest values of inferred attenuation rate during SPE disturbed propagation conditions were for the WTF/Gulf of Alaska path (2.51 dB/Mm at night, and 2.66 dB/Mm during the day). In fact, during the 23 November 1982 minimum nighttime field strength period of 0630 to 0830 GMT, the effective attenuation rate for this path exceeded 3 dB/Mm. These attenuation rates are substantially higher than those measured during ambient propagation conditions on WTF/east‐and‐northeast paths (∼1.0 dB/Mm at night, and ∼ 1.25 dB/Mm during the day). The probable reason for the higher effective attenuation rates observed on the WTF/Gulf of Alaska path is that this path is nearly tangent to the exterior boundary of the disturbed polar cap. That geometry causes lateral refraction of the transverse electromagnetic (TEM) mode and thus shadow zones.