bluuebunny/crossref_metadata_2025 huggingface dataset:
Alouette‐ISIS radio wave studies of the cleft, the auroral zone, and the main trough and of their associated irregularities
Authors: Mularew D. B.
Published: February 2008

The principal high‐latitude ionospheric features such as the cleft, the nighttime auroral zone, the main trough and their associated irregularities, as observed with the Alouette/ISIS topside sounders, are reviewed. These features can be clearly distinguished in Alouette 1 electron density contour plots as a function of latitude and height below 1000 km. Alouette 2 data recorded near apogee (3000 km) show the location of the light‐ion (H+ and He+) trough as a function of local time and show it to be more closely related to the plasmapause than is the main trough. An attempt is made to clarify the location and terminology of troughlike structures. Regions of small‐scale irregularities (tens of meters) are associated with the high‐latitude ionospheric features. These irregularities are responsible for the presence of aspect‐sensitive spread F on topside ionograms. The most intense spread is found in the cleft region. Spread F regions usually have a sharp equatorward boundary. Some recent unpublished work by the author indicates that the spread F boundary for the nighttime auroral zone tends to be located several degrees equatorward of the keV electron precipitation zone and is probably generated indirectly by this precipitation and convected equatorward. In the main trough region spread is sometimes present and other times absent. Some of the individual small‐scale irregularities responsible for spread F can extend along magnetic‐field lines for hundreds of kilometers.
Electromagnetic scattering by harmonically expanding surfaces and related complex resonances
Authors: Censor Dan
Published: February 2008

A relativistically exact iterative method is developed for scattering of electromagnetic waves by expanding surfaces. In particular, the problems of the expanding cylinder and sphere are computed. It is shown that expanding scatterers respond to harmonic excitation by radiating nonharmonic waves. These waves involve complex frequencies computed here. The method given here involves secular terms in t, therefore its validity for harmonic excitation is limited to early times. However, in the case of impulse excitation and transient scattering this problem is automatically resolved by the fact that the signal is exponentially decaying and the secular terms have no large‐time effect.
Wave instabilities in electron beams traveling through media with a random dielectric constant over a rough surface
Authors: Bass F. G. , Bulgakov A. A. , Pritula G. M. , Khankina S. I. , Yakovenko V. M.
Published: February 2008

A new type of wave instability in beams is studied which takes place in statistically inhomogeneous media. The instability is caused by interaction between the fast and slow waves and eigenoscillations of the medium due to inhomogeneity of the latter.
Ionospheric true height profiles from oblique ionograms
Authors: Reilly Michael H.
Published: February 2008

An improved technique for the reduction of an HF oblique ionogram to an ionospheric true height profile is presented. It is based on a more accurate generalization of the Breit‐Tuve theorem to curved earth and ionosphere geometries. The improvement over previous methods is demonstrated by means of calculations on known cases.
Morphology of phase and intensity scintillations in the auroral oval and polar cap
Authors: Basu Sunanda , Basu Santimay , MacKenzie E. , Whitney H. E.
Published: February 2008

The first long‐term measurements of phase scintillations at high latitudes from a quasi‐stationary Air Force satellite at 250 MHz are reported. The measurements were made from Goose Bay, Labrador covering the corrected geomagnetic latitude range of 64°–72°N and Thule, Greenland covering the corrected geomagnetic latitude range of 85°–89°N. Data from December 1979, March–April 1980 and March–April 1982 are presented from both stations. The sunspot numbers during these specific time periods were approximately the same. The scintillation magnitudes were higher during the vernal equinox at both stations than during the winter solstice. The Goose Bay data showed a well ordered diurnal variation with a nighttime maximum and daytime minimum, whereas the Thule data showed no such ordering. The limited phase scintillation data set from Thule is augmented by intensity scintillation measurements made during 1979–1982 using the same quasi‐stationary satellites. The long‐term Thule data shows that the major variation in scintillation activity is annual with minimum scintillations observed in the local summer, as was determined earlier by Aarons et al. (1981). In contrast to the pronounced annual variation, the dependence of overall scintillation activity on sunspot cycle is not greatly evident during 1979–1982 when the sunspot number varied approximately between 100 and 200. It was found that both Goose Bay and Thule could show phase scintillations of the order of 10 rad (with 82‐s detrend period) even during magnetically quiet times. One such case study, conducted during March 1982, is presented and second‐order parameters such as phase aad intensity spectral strengths and slopes and intensity decorrelation times are discussed for this event at both stations. The use of geostationary satellite data shows the importance of enhanced magnetospheric convection velocities on the observation of large phase scintillation values at both sites. This underscores the need for incorporating irregularity dynamics in the currently developed static models of ionospheric scintillation.
The impact of strong scintillation on space‐based radar design: Clutter rejection
Authors: Knepp Dennis L. , Dana Roger A.
Published: February 2008

Electromagnetic signals that propagate through strongly disturbed regions of the ionosphere can experience angular scattering that can cause appreciable amplitude and phase scintillation and angle‐of‐arrival fluctuations. This paper considers the clutter rejection performance of a space‐based radar subject to degradation due to signal propagation through a highly disturbed propagation channel. Pertinent characteristics of the disturbed channel and the received radar signal are discussed. The effect of scintillation on the performance of a generic pulse doppler radar that utilizes Dolph‐Chebyshev weighting to reduce clutter sidelobes is investigated. The clutter rejection performance of various doppler filters is given as a function of the decorrelation time of the scintillation. It is shown that this type of clutter rejection technique, which utilizes a filter with constant sidelobes, is robust in strong scintillation.
Multipath effects on the determination of absolute ionospheric time delay from GPS signals
Authors: Bishop G. J. , Klobuchar J. A. , Doherty P. H.
Published: February 2008

Signals from the Global Positioning System (GPS) satellites can be used to make measurements of absolute ionospheric group delay and relative phase advance. These ionospheric parameters have wide application in providing corrections for military systems requiring knowledge of ionospheric time delay and phase scintillation, such as satellite detection radars. Multipath effects from the local environment of the receiving antenna can cause severe contamination of ionospheric group delay measurements made with GPS pseudorandom noise receivers. Measured variations in the multipath effects in some typical clean and reflective environments are presented and shown to be consistent with signal analysis for pseudorandom noise receivers. Measured single reflector multipath is analyzed briefly and shown to correlate well with calculations. Since antennas must frequently be located in environments that are much less than ideal, data analysis procedures were developed for minimizing the impact of multipath on the group delay measurement. These include low‐pass filtering, day‐to‐day correlation, and calibration to relative differential carrier phase advance measurements by using subsets of the absolute ionospheric group delay data. Results of these processes as applied to the test data from various multipath environments are presented.
Remote sensing of high‐latitude ionization with the ISIS 2 auroral scanning photometer
Authors: Vondrak Richard R. , Murphree John S. , Anger C. D.
Published: February 2008

Kilovolt electron precipitation is the primary source of ionization in the high‐latitude nighttime E region. Space‐based measurement, or inference, of the energy distribution of these electrons can be used to specify the distribution of high‐latitude ionization. In certain cases, remote sensing by imaging of auroral luminosities at two appropriate wavelengths can be used to infer both the electron energy flux and the electron energy distribution. To validate the capabilities of deriving ionospheric parameters from such observations, we have compared simultaneous measurements by the Chatanika radar and the ISIS 2 auroral scanning photometer which measures the spatial distribution of auroral emissions at 557.7 nm and 391.4 nm. The ISIS 2 images included a variety of high‐latitude phenomena, such as auroral bands and a detached arc in the evening sector trough. Spatial enhancements in the auroral luminosities were associated with regions of increased E region ionization, provided the luminosities were corrected for earth albedo effects. Quantitative comparisons of the radar measurements and the ionization profiles inferred from the satellite data demonstrate the capabilities of the satellite‐based remote sensing technique.
Field‐aligned electron density irregularities near 500 km: Equator to polar cap topside sounder observations
Authors: Benson Robert F.
Published: February 2008

Striking evidence, in addition to spread F, for field‐aligned electron density irregularities is commonly observed on postsunset Alouette 2 topside sounder ionograms recorded near perigee (500 km). This evidence is provided by distinctive signal returns from sounder‐generated Z mode waves. At low latitudes these waves become guided in wave ducts caused by field‐aligned electron density irregularities and give rise to strong long‐duration echoes. At high latitudes, extending well into the polar cap, transmitted Z mode waves (and stimulated electrostatic waves at the plasma frequency and trapped ordinary mode echoes) produce a series of vertical bars on the ionogram display as the satellite traverses discrete field‐aligned density structures. Within 15° dip latitude of the dip equator, Z mode wave ducts were encountered on about one third of the ionograms sampled; beyond 75° invariant latitude, vertical bar phenomena were encountered on more than three fourths of the ionograms sampled.
Wave‐particle interactions induced by SEPAC on Spacelab 1: Wave observations
Authors: Taylor W. W. L. , Obayashi T. , Kawashima N. , Sasaki S. , Yanagisawa M. , Burch J. L. , Reasoner D. L. , Roberts W. T.
Published: February 2008

Space experiments with particle accelerators (SEPAC) flew on Spacelab 1 in November and December 1983. SEPAC included an accelerator which emitted electrons into the ionospheric plasma with energies up to 5 keV and currents up to 300 mA. The SEPAC equipment also included an energetic plasma generator, a neutral gas generator, and an extensive array of diagnostics. The diagnostics included plasma wave detectors, an energetic electron analyzer, a photometer, a high sensitivity television camera, a Langmuir probe and a pressure gauge. Twenty‐eight experiments were performed during the mission to investigate beam‐plasma interactions, electron beam dynamics, plasma beam propagation, and vehicle charging. The wave‐particle interactions were monitored by the plasma wave instrumentation, by the energetic electron detector and by the optical detectors. All show evidence of wave‐particle interactions, which will be described in this paper.
Radio wave diffraction by a rounded obstacle
Authors: Vogler Lewis E.
Published: February 2008

This paper is an extension of and correction to previous results in the diffraction of radio waves by a smooth rounded obstacle. Equations and curves are presented that are applicable to both vertical and horizontal polarization and a wide range of electromagnetic ground constants. The curves provide attenuation predictions in the transition region between knife‐edge and smooth earth diffraction theory.
Radio noise fields generated by corona streamers on a power line above a dissipative earth
Authors: Olsen Robert G. , Rouseff Daniel
Published: February 2008

Formulas are derived for the corona‐generated low‐frequency noise fields near a single wire high‐voltage power line. The theory has been compared to a number of existing theories which are based on quasi‐static or transmission line assumptions. Errors of up to 15 dB in the existing theories have been found.
Radar auroral echo heights as seen by a 398‐MHz phased array radar operated at Homer, Alaska
Authors: Ruohoniemi J. M. , Moorcroft D. R.
Published: February 2008

The first results of an investigation of the altitude characteristics of auroral echoes at 398 MHz are presented. The data were collected over two nights in the spring of 1973 with a phased array radar operated at Homer, Alaska. Heights of maximum backscattered power can be determined with an accuracy approaching 2 km, a temporal resolution as short as 20 s, and a horizontal spatial resolution of 40 km. The spatial distributions of the height of echoing in two premidnight periods are shown to be consistent with a model of scattering from a continuous range of heights between 97 and 117 km; within this interval, the height tends to change in such a way that the echoes come from directions nearly perpendicular to the earth's magnetic field. In a postmidnight period, the echo activity was confined to a region of considerably more restricted (5–10 km) vertical extent. In all cases, the echo region was sharply bounded from below at 96–98 km. The heights may have short‐term (minutes) and small‐scale (< 100 km) structure. Adjacent structures are sometimes observed to differ in height by as much as 10 km.
Observations of vertical motions in the troposphere and lower stratosphere using three closely spaced ST radars
Authors: Ecklund W. L. , Balsley B. B. , Carter D. A. , Riddle A. C. , Crochet M. , Garello R.
Published: February 2008

Three closely spaced vertical clear‐air radar sites were installed and operated in Southern France during the Alpex experiment. This United States‐France cooperative effort was designed to determine atmospheric wave parameters in the troposphere and lower stratosphere. In this report we describe the experiment, give an overview of the data and present vertical wind speed power spectra. We also show spatial coherences obtained between heights and between radar sites.
Comparison observations between the MU radar and the Kyoto meteor radar
Authors: Tsuda Toshitaka , Yamamoto Mamoru , Sato Toru , Kato Susumu , Fukao Shoichiro
Published: February 2008

We have performed a comparison between the MU radar and the Kyoto meteor radar on August 14–15, 1984. The MU radar provided vertical, zonal and meridional wind components in the 65‐ to 85‐km region during daylight hours when echoes from the turbulent scattering were intense. Meteor echoes were also received by the MU radar, and utilized to measure winds at meteor heights. Comparisons among these MU radar measurements and Kyoto meteor radar observations have shown fairly consistent wind profiles at 65‐ to 105‐km altitudes. To interpret differences in the wind fields observed in 2 successive days, it was necessary to consider the behavior of quasi 2‐day oscillation deduced from long‐duration Kyoto meteor radar observations.
Altitude dependence of vertical velocity spectra observed by VHF radar
Authors: Kuo Fu‐Shong , Shen Hung‐Wen , Fu I‐Ju , Chao Jih‐Kwin , Röttger Jürgen , Liu Chao‐Han
Published: February 2008

Vertical velocity data observed by SOUSY VHF radar from May 29 to June 2, 1978, are systematically analyzed. The altitude dependences of frequency spectra and wave number spectra are presented, and their physical implications are given on the basis of existing theoretical models.
Radio spectrographs for observing the planet Jupiter
Authors: Flagg R. S. , Greenman W. B. , Gombola P. P. , Lebo G. R.
Published: February 2008

A variety of techniques and instruments have been developed at the University of Florida to study the decametric radio emissions from the planet Jupiter. Several types of radio spectrograph are being utilized, each offering a unique advantage in speed, resolution or frequency coverage. The individual instruments, as well as their integration into a multiresolution system, are discussed. Representative data are presented.
On caustics related to several common indices of refraction
Authors: Gorman A. D. , Anderson S. P. , Mohindra R. B.
Published: February 2008

In the Lagrange manifold approach to the asymptotic solution of a differential equation, the caustic curve is determined from the phase of an oscillatory integral. Here we present the phase functions of such integrals for several types of stratified media of geophysical interest.
Wideband measurements of coherence over an HF skywave link and implication for spread‐spectrum communication
Authors: Salous S. , Shearman E. D. R.
Published: February 2008

A wideband frequency‐modulated, continuous‐wave (FMCW) oblique incidence sounder was used in an experiment to measure the coherent bandwidth of a one‐hop F2 layer propagated ionospheric path. The experimental data were collected over a 234‐km link in the United Kingdom by using frequency sweeps of up to 5 MHz repeated at 10‐s intervals. The path had the characteristics of quasi‐longitudinal propagation. The results indicated that the limiting factors to the coherent bandwidth are mainly polarization interference and frequency dispersion. Data on the coherent bandwidth and fade depth as a function of frequency and the time variation of the amplitude frequency response are presented.
Simplified formulas for ELF propagation at shorter distances
Authors: Bannister Peter R.
Published: February 2008

It is the purpose of this article to present new formulas for ELF electric and magnetic dipole propagation in the earth‐ionosphere waveguide. These new formulas extend the results of Wait and Galejs, which are valid for measurement distances greater than approximately three ionospheric reflecting heights, to the quasi‐near field range, which is defined as the range where the measurement distance is greater than an earth wavelength, but much less than free‐space wavelength. These new formulas will be particularly applicable in the range of approximately 20 to 300 km from the source. We will also present plots of the VED and HED (or HMD) radial wave impedance versus distance for both daytime and nighttime propagation conditions. These plots show a substantial variation in the radial wave impedances for distances less than 1,000 kilometers. We will also show that there is a unique distance where the minimum value of the VED radial wave impedance occurs.
Errors in direction finding due to nonvertical lightning channels: Effect of the finite ground conductivity
Authors: Cooray Vernon
Published: February 2008

Errors in direction finding due to nonvertical channel sections over finitely conducting ground have been investigated. The results show that if the direction of the lightning flash is estimated from the initial peak of the magnetic radiation field then, for the conductivity of the ground in the range of 0.01 mho/m to 0.001 mho/m, the propergation errors caused by the inclination of the channel at distances of about 10 km or more would be less than about 1°.
High‐frequency expression for the field in the caustic region of a reflector using Maslov's method
Authors: Hongo Kohei , Ji Yu , Nakajima Eiji
Published: February 2008

High‐frequency solutions for a field generated by a reflector and by a phase transformer are derived using Maslov's method. Derived results are compared with those obtained by other more standard methods when they are available. Agreement is complete for a field generated by a reflector. The results demonstrate that Maslov's method provides an alternative tool for evaluating diffraction images of reflector antennas.
Three‐body scattering produces precipitation signature of special diagnostic value
Authors: Zrnić Dušan S.
Published: February 2008

Examples are given of three‐body scatter signatures produced by high reflectivity cores in thunderstorms. The process consists of (1) scattering by the hydrometeors, (2) backscattering by the ground to the hydrometeors and (3) scattering by the hydrometeors to the radar. When viewed on radar displays, the signatures have an elongated shape, radially aligned behind strong (60 dBZ) reflectivity cores. A radar equation for echo power is developed and predicted r−3 dependence of the echo power on the range between the center of scattering volume and a reflecting ground ring is confirmed by a least squares fit to actual data. It is shown theoretically that mean Doppler shifts associated with three‐body scatter signatures are caused by the vertical and radial components of hydrometeor velocities; this is also verified from least square fits of range profiles of measured Doppler velocities to theoretical curves based on geometrical considerations.
Large‐amplitude changes induced by a seabed in the sub‐LF electromagnetic fields produced in, on, and above the sea by harmonic dipole sources
Authors: Fraser‐Smith A. C. , Bubenik D. M. , Villard O. G.
Published: February 2008

Because its effective electrical conductivity is usually lower than that of seawater, the seabed can influence the amplitude of sub‐LF (frequencies less than 30 kHz) electromagnetic fields generated by electric and magnetic dipole sources located in, on, and above the sea surface. The changes produced in the fields depend heavily on the particular source‐receiver geometry. In general, they tend to be largest when the seawater is electrically shallow (i.e., only a few seawater skin depths δs deep) and when the dipole sources are located within 1δs of the seafloor. However, even when these general tendencies are taken into account, computations we have made of the fields indicate that there are certain configurations where comparatively large changes in the fields can take place due to the influence of the seabed. The horizontal electric field produced on the sea surface by a vertical electric dipole source located on the seafloor, for example, can have its amplitude increased by 5–6 orders of magnitude at horizontal distances of the order of 10–15δs. The purpose of this paper is to describe the form of these comparatively large changes and the conditions under which they can be expected to occur.
Moving random surfaces and correlation analysis
Authors: Burke M. J.
Published: February 2008

Two models of moving random surfaces with known velocity and known correlation function are described. The random surfaces are formed from an infinite set of plane waves with decorrelation arising from random temporal and spatial changes. For model 1 the random changes are independent of the wave frequency. For model 2 they increase with frequency. It is shown that if there are random spatial changes in the direction of motion, then speeds derived by the full correlation method will be less than the true speed of the surface. If temporal series based on the models are subjected to a high‐pass filter before correlation analysis, the correlation speeds increase in value. Correlation functions from one of these models (model 2) fail the requirements for full correlation analysis because the half widths of the cross‐correlation functions are greater than the half width of the autocorrelation function. However, from a survey of experimental ionospheric drift measurements it is argued that radio interference patterns appear to have the form of this model. For this model, (1) correlation speeds increase with increasing antenna spacing, (2) the correlation ellipse tends to line up with that antenna pair with the greatest spacing, (3) the straight line of the straight‐line method will not be straight, and as stated above, (4) the speeds will be low. These features have been found experimentally and are contrary to the assumptions of the correlation method. It is shown that an alternate analysis, the “similar fades average velocity method,” gives a better estimate of the surface speed than does the correlation method.
General analysis of a narrow slot in a conducting screen between half‐spaces of different electromagnetic properties
Authors: Butler Chalmers M.
Published: February 2008

The electric field in a narrow slot in a conducting screen between two half‐spaces of different electromagnetic properties and the far‐zone diffracted field are determined analytically. Both transverse electric and transverse magnetic excitations are considered, and the slot electric field in each case is obtained by solving the corresponding narrow‐slot integral equation by two methods. In both methods the solutions are expressed as the product of a weight function and a series of Chebyshev polynomials. The series coefficients depend upon an integral of the known excitation in one method, while in the other they depend upon its derivatives. The solution procedure is illustrated by an example in which the excitation is taken to be a uniform plane wave.
Longitudinally varying ducts with guiding to antiguiding transitions
Authors: Felsen Leopold B.
Published: February 2008

Naturally occurring ducting environments (for example, tropospheric elevated or surface ducts) may exhibit continuous longitudinal changes that progressively decrease the guiding capability and eventually destroy it altogether. This circumstance has evident implications for systems employing such ducts as communication or surveillance channels. Recent developments in the theory of propagation in longitudinally varying ducts are reviewed, with emphasis on gradual guiding to antiguiding transitions that convert an initially well‐trapped mode field smoothly (adiabatically) into a leaky mode. The connection between the trapped and leaky mode regimes is provided by a parabolic equation or, in certain cases, by an “intrinsic mode” spectral integral. Numerical implementation for different examples confirms the phenomenology predicted by the analysis and reveals the beaming of initially trapped modal energy into the exterior after the duct disappears. Some consideration is also given to incorporating these concepts into a synthesis of the field excited by a radiating source inside the duct.
Characteristic modes and dipole representations of small apertures
Authors: Wang Taoyun , Mautz Joseph R. , Harrington Roger F.
Published: February 2008

In this paper, characteristic modes of the magnetic current are related to the dipole representation of a small aperture in a conducting plane of infinite extent. Low‐frequency approximations are used to simplify the eigenequations for the first three characteristic currents. It is shown that these three characteristic currents are closely related to the three dipoles of Bethe hole theory. An example of a small circular aperture in an infinite conducting plane is discussed.
Rain rate profiles obtained from the dynamic behavior of the attenuation of microwave radio signals
Authors: Buné P. A. M. , Herben M. H. A. J. , Dijk J.
Published: February 2008

A mathematical model is presented which links the dynamical properties of microwave radio signal attenuation to the spatial rain rate structure of intensive showers. The theoretical relation found between attenuation and its rate of change is verified by radio wave propagation measurements on a satellite‐Earth path at 11.5 GHz.
Electron density and electron content fluctuations in the auroral zone
Authors: Hargreaves J. K. , Burns C. J.
Published: February 2008

High‐latitude observations of electron density by means of incoherent scatter radar have been integrated to give electron content up to about 700 km altitude. The electron content is found to be highly variable on time scales down to a minute, and the variations are large enough to have significant effects on transionospheric radio propagation. In some cases the horizontal velocities of the enhancements, and their temperatures, have been determined. It is likely that more than one source is involved.
Simulations of plasma structure evolution in the high‐latitude ionosphere
Authors: Huba J. D. , Mitchell H. G. , Keskinen M. J. , Fedder J. A. , Satyanarayana P. , Zalesak S. T.
Published: February 2008

The auroral zone contains a variety of interesting and dynamic plasma processes which have been the subject of intense research over the past two decades. Of particular interest is the subject of high‐latitude ionospheric structure (i.e., electron density irregularities) because of their impact on communications systems. The Naval Research Laboratory has recently developed a two‐dimensional inertial, electrostatic code which has been successfully applied to the development and evolution of ionospheric structure driven by plasma instabilities (e.g., E×B gradient drift, Kelvin‐Helmholtz). This code models the ionosphere and magnetosphere as a set of horizontal two‐dimensional layers which are coupled by the vertical magnetic field lines at high latitudes. It is shown that the development of instability‐generated structure can be strongly dependent on this coupling. For example, the influence of magnetospheric coupling (i.e., inertial effects) on the E×B gradient drift instability is to retard the instability's growth and to isotropize density irregularities. The influence of ionospheric coupling (i.e., collisional effects) on the Kelvin‐Helmholtz instability is to retard its growth and to suppress vortex formation. In this paper we will present the results of numerical simulations of these instabilities and discuss their application to high‐latitude ionospheric structure.
X ray spectrophotometric remote sensing of diffuse auroral ionization
Authors: Vondrak R. R. , Robinson R. M. , Mizera P. F. , Gorney D. J.
Published: February 2008

Auroral electron precipitation produces both E region ionization and bremsstrahlung X rays. In a set of coordinated observations these atmospheric effects of electron precipitation were measured simultaneously by the X ray sensor on the Defense Meteorological Satellite Program F2 spacecraft and by the incoherent scatter radar at Chatanika, Alaska. In all data sets there was good agreement between the locations of the X ray source regions and the regions of enhanced E region ionization. Detailed comparisons were made between the measured and the derived altitude profiles of ionization for two auroral conditions, a premidnight auroral band and a sunlit diffuse aurora. These comparisons validate the ability to derive auroral ionization from satellite‐based X ray measurements.
Frequency and duration of disturbances in the mid‐latitude F region of the ionosphere
Authors: Lambert Sheridan
Published: February 2008

Disturbances of the ionospheric F region at two well‐separated mid‐latitude stations were identified on rapid run ionograms by spread echoes, blackouts, and anomalies in structure or critical frequency. The diurnal variation in the probability of disturbance consistently shows minima around sunrise and sunset. Details such as a brief postsunset enhancement in winter are revealed by the fine time resolution of the data. Disturbance probabilities were lowest in autumn and spring (8% and 9%), while that for winter (33%) exceeded the summer level (20%). Spread F constituted 32% of the events, and disturbances producing sharply defined ionogram signatures 77%, with some overlapping of types. The majority of disturbances were not correlated with geomagnetic phenomena. Disturbances at the two stations were essentially independent (spatial correlation of 0.15). Spread F events were of longer duration (25 min) than sharp disturbances (20 min). TIDs were the exception, with a most probable duration of 80 min, but accounted for only 8.3% of the sharp disturbances. The absence of disturbances in the F region at dusk and especially at dawn coincides with periods of instability in the E region.
A model for the tropospheric excess path length of radio waves from surface meteorological measurements
Authors: Berrada Baby H. , Golé P. , Lavergnat J.
Published: February 2008

Two models are proposed here for the prediction of the excess path length of radio waves from ground‐based measurements of pressure, temperature, and humidity, at zenith and at a given apparent elevation angle. They can be applied to the correction of high‐accuracy range measurements between ground beacons and orbiting satellites. Both of these models use the same formulation for the hydrostatic component of the excess path, ΔLh, with an accuracy better than 5 mm, as compared to measurements of ΔLh. However, two distinct methods have been developed for the wet component ΔLw of the excess path length. The first is based on a theoretical assumption regarding the relative humidity profile and leads to an appropriate accuracy for locations corresponding to standard conditions (≈ 2 cm) with a very small mean deviation (< 5 mm). However, this purely theoretical method must be replaced by a semiempirical one when predictions of ΔLw are made from ground measurements in other types of climates, such as equatorial, tropical, or oceanic climates, because of the much poorer accuracy of ground measurements and greater variability of the humidity profile in such locations (rms deviation > 4 cm, offsets as large as 4 cm). The second model is implemented by the use of an extended radiosonde data base covering the whole world and a 1‐year period. It leads to an rms deviation from measured data comparable to the first method, but to reduced values of the offset, and thus to an overall improvement of the prediction. One advantage of such a method is that it provides a simple global model of the excess path length, with an accuracy comparable to other sophisticated ones.
Scattering by a conducting elliptic cylinder with dielectric coating
Authors: Richmond Jack H.
Published: February 2008

The scattering properties are investigated for a perfectly conducting elliptic cylinder with a homogeneous dielectric coating. In the dielectric coating and in the exterior free space region, the field is expanded in elliptic waves via the Mathieu functions. The Mathieu angular functions are employed as basis and testing functions to enforce the boundary conditions via Galerkin's method at the interface between the dielectric and the free space regions. Numerical results are presented in graphical form for the transverse electric and transverse magnetic polarizations to illustrate the backscattering and bistatic scattering angular patterns as well as the backscatter versus frequency.
Aspect sensitivity of 2.66‐MHz radar returns from the mesosphere
Authors: Adams Gene W. , Brosnahan John W. , Johnson Reese E.
Published: February 2008

We have measured radar returns from the mesosphere by operating a 10‐antenna phase‐coherent 2.66‐MHz radar as an imaging Doppler interferometer. We use the resulting three‐dimensional description of the scattering to characterize the aspect sensitivity of the returns for 2 hours of data taken one spring equinox afternoon. The returned power distribution is generally circularly symmetric, so that it can be described at each altitude in terms of its variation with zenith angle, or, more accurately, with the angle relative to the center of power of the distribution. The horizontal displacement from zenith of the center of power versus altitude suggests a gravity wave‐like variation with a 13‐km vertical wavelength and may provide an alternative to wind profiles as a way to observe propagating gravity and tidal waves. We correct the received power for antenna gain and distance and display the dependence of scattered power on the angle measured from the center of power to describe the angular dependence of the scattering processes. These results show that the scattering generally consists of two components : an anisotropic (preferentially normal) component and an isotropic component. The anisotropic component dominates within about 6° of the center of power, while the isotropic component is seen beyond 6°. The angular width of the scattering increases with altitude; above 85 km the scattering becomes almost completely isotropic.
Pulse propagation on a horizontal wire above ground: Far‐zone radiated fields
Authors: Dudley Donald G. , Casey Kendall F.
Published: February 2008

A transient current pulse propagating on a semi‐infinite wire above ground is considered. A quasi‐transverse electromagnetic approximation to the modal equation for the propagation wave number is used to obtain the time history of the pulse on the wire. Pulse amplitude decay and degradation of rise time as a function of distance along the wire are demonstrated. The classical Sommerfeld expressions for the field of an infinitesimal horizontal current element over ground are integrated over the wire to give the frequency domain fields. Saddle point integration techniques yield the far‐zone fields in closed form for polar angles away from grazing. A numerical inverse Fourier transform yields the transient response. Representative results for the far‐zone fields in the frequency and time domains are presented.
On the frequency dependence of ELF/VLF waves produced by modulated ionospheric heating
Authors: Rietveld M. T. , Stubbe P. , Kopka H.
Published: February 2008

We present results of the local field strength and, for the first time, apparent source height as a function of frequency from 200 to 6600 Hz of ELF/VLF waves produced in the auroral D region through modulated RF heating using the ionospheric heating facility near Tromsø, Norway. These data show, besides the normal amplitude peaks near 2, 4, and 6 kHz, the occasional presence of peaks at intermediate frequencies, especially in the right‐hand circularly polarized mode. A comprehensive model has been developed to explain the data, using a full wave solution of the ELF/VLF waves produced by the self‐consistently calculated source currents, and correcting for the effects of finite source current extent and multiple Earth‐ionosphere reflections. The model can reproduce the amplitudes and source heights for typical sweeps. The irregular peaks occasionally seen can also be reproduced by invoking interference between the downcoming wave and a partially reflected whistler mode wave from sharp electron density gradients in the ionosphere such as sporadic E layers. The model seems to work well above about 600 Hz, but below that it increasingly underestimates the measured amplitudes.
Beyond the Kirchhoff approximation
Authors: Rodriguez Ernesto
Published: February 2008

The three most successful models for describing scattering from random rough surfaces are the Kirchhoff approximation (KA), the small perturbation method (SPM) and the two‐scale roughness (or composite roughness) surface scattering (TSR) models. In this paper it will be shown how these three models can be derived rigorously from one perturbation expansion based on the extinction theorem for scalar waves scattering from perfectly rigid surfaces. It will also be shown how corrections to the Kirchhoff approximation proportional to the surface curvature and higher order derivatives may be obtained. Using these results, the scattering cross section will be derived for various surface models.
Automatic scaling of digital ionograms
Authors: Fox Matthew W. , Blundell Craig
Published: February 2008

A new system has been devised to automatically scale digital ordinary‐ray ionograms. The system has been developed for use at mid‐latitudes and has been trained on a full set of ionograms that are typical of the region. This paper describes the stages adopted in forming, recognizing, and scaling traces from the various ionospheric layers. Examples of the output available with this system and an error analysis of autoscaled parameters are presented as evidence of the system's capabilities.
Equatorial plasma bubble rise velocities in the Indian sector determined from multistation scintillation observations
Authors: Dabas R. S. , Reddy B. M.
Published: February 2008

Systematic time differences observed in the onset of postsunset VHF scintillations, recorded simultaneously at a meridian chain of stations in the Indian sector, have been analyzed in order to determine the plasma bubble rise velocities over the magnetic equator. The method is based on the assumption that the postsunset scintillations up to several degrees on either side of the magnetic equator are generated by the equatorial plasma bubble and associated irregularities which are strongly field aligned. As the plasma bubble rises in the equatorial ionosphere, after its generation in the bottomside of the F layer, the low‐latitude extremities of the bubble propagate away from the equator in such a way that the upper height limit of the irregularities defines the latitudinal limit of the scintillation occurrence associated with the given event. From the relative time delays observed in the onset of postsunset scintillations at a chain of different latitude stations situated nearly along a common meridian plane and the altitudinal differences over the magnetic equator between their respective field lines corresponding to the F region heights, we have determined the average bubble rise velocities between three altitudinal slabs. The velocities so determined are found to vary from about 128 to 416 m/s between the altitudes of 450 and 550 km, 38 to 327 m/s between 550 and 1140 km, and 15 to 200 m/s between 1140 and 1270 km, which shows that the bubble rise velocities decrease with altitude. The results are found to be comparable to the values measured by VHF radar and satellite techniques. The calculated values of bubble rise velocity during the early postsunset hours are found to have good correlation with the equatorial F layer vertical drift velocities. Also, as compared to the results derived by using spaced ionosondes, present results should yield the plasma bubble rise velocities with better precision because of better spatial and temporal resolution. Thus the present technique provides an effective and rather simple way of investigating the equatorial ionospheric dynamics.
A new meteor echo detection and collection system: Christmas Island mesospheric wind measurements
Authors: Avery S. K. , Avery J. P. , Valentic T. A. , Palo S. E. , Leary M. J. , Obert R. L.
Published: February 2008

A second generation meteor echo detection and collection system has been installed on the Christmas Island, Republic of Kiribati, stratosphere‐troposphere radar system. Data from the first 6 months of operation have yielded mean wind and tidal climatologies that are compared with theoretical models.
Scattering by randomly irregular narrow strips of constant width
Authors: Eftimiu Cornel
Published: February 2008

Scattering of plane electromagnetic waves by randomly irregular narrow strips is considered in the case when the constant width is much smaller than the wavelength and the randomly irregular strip is a straight line on the average. The randomness is treated in second order of perturbation theory. The domain of validity of perturbation theory is extended by using Padé approximants. The results obtained are also used to establish the connection with earlier work based on a different approach.
Semidiurnal variations in the time scale of irregularities near the Antarctic summer mesopause
Authors: Fraser G. J. , Khan Usman
Published: February 2008

Measurements from the MF spaced antenna mode radar at Scott Base (78°S, 167°E) include an estimate of the autocorrelation time scale (T0.5) of the scatterers, corrected for advection by the mean wind. Summer observations (December 1983) show a semidiurnal variation of T0.5. At heights of 87–95 km the minimum values of T0.5 occur when the zonal component of the semidiurnal tide is at its westward maximum. If it is assumed that decreasing values of T0.5 are associated with increasing turbulence, then these results are consistent with those of Balsley et al. (1983), who proposed that enhanced turbulence is generated in summer by low‐frequency gravity waves and tidal components above the polar mesopause.
Chaotic features of the atmospheric motions near the tropopause revealed by MU radar observation: Case study
Authors: Yang Peicai , Lu Da‐ren , Li Wei , Wu Beiying , Fukao Shoichiro , Yamamoto Mamoru , Tsuda Toshitaka , Kato Susumu
Published: February 2008

On the basis of the data of the upper wind measurements made with middle and upper atmosphere (MU) radar, some dynamical characteristics of the atmospheric motions occurring near the tropopause are analyzed. The results show that the investigated atmospheric systems are chaotic, their correlation dimensionalities lie between 4 and 6, and their order‐2 entropies lie between 0.008 and 0.015 (bit/min). They present the minimum number of the essential variables necessary to simulate the dynamics of the systems and the mean predictable time scale of the attractors, respectively. These are very useful for establishing a model atmosphere, designing the observation schema, and forecasting by means of MU radar.
New quasi‐complementary code sets for atmospheric radar applications
Authors: Rastogi Prabhat K. , Sobolewski Gregory
Published: February 2008

The basics of using low‐autocorrelation binary sequences in high‐resolution atmospheric radar experiments are briefly reviewed. An exact cancellation of correlation side lobes with repeated complementary code pairs is often not attained due to transmitter nonlinearities and due to poor coherence of atmospheric targets. Sulzer and Woodman (1984) have recently introduced quasi‐complementary code sets (QCCS) of low‐autocorrelation binary sequences, obtained by an extensive computer search, for which the side lobes nearly cancel on accumulation over the set. Improved performance of QCCS over complementary pairs has been confirmed in experiments with the Arecibo UHF radar. We report here an exhaustive computer search for a base set of 32‐bit binary sequences with correlation side lobes at level 3. These codes are then combined in optimal QCCS of several even set sizes with a residual correlation side lobe level of 2, or 6 dB better than for a sample QCCS of 48 codes given by Sulzer and Woodman. A similar search for 64‐bit codes, using a partial base set of ∼4000 codes with correlation side lobes at level 5 and 6, gives QCCS with a residual correlation side lobe level of 6.