bluuebunny/crossref_metadata_2025 huggingface dataset:
Impedance of an ion‐sheathed spherical probe in a warm, isotropic plasma
Authors: Aso Takehiko
Published: February 2008

A self‐consistent analysis of a radio‐frequency impedance of a negatively biased spherical probe in a warm, isotropic plasma is given. Linearized zero‐ and first‐order hydrodynamic and Maxwell equations are numerically solved for a specified probe‐retarding potential, and the influence of a realistic sheath structure upon the resonance behavior is discussed. It is concluded that the usual abrupt‐sheath model can safely be a good approximation for the study of radio frequency properties of a probe immersed in a plasma.
Microwave rotational spectroscopy: A technique for specific pollutant monitoring
Authors: Hrubesh Lawrence W.
Published: February 2008

The properties of microwave rotational spectroscopy that make it a promising technique for gaseous pollutant monitoring are presented. Recent advances in instrumentation and technique that have suggested feasibility of in‐situ monitoring are thoroughly discussed. A laboratory model of a potentially portable Gunn‐diode cavity spectrometer has been interfaced to the atmosphere by way of a multistaged membrane separator. Results of testing this system under controlled experimental conditions as a specific monitor for two common gaseous pollutants are given. Determinations of the optimum sensitivity, specificity, and stability expected for this system are dsciussed along wih considerations for its calibration.
An extreme UV photometer for solar observations from Atmosphere Explorer
Authors: Heath Donald F. , Osantowski John F.
Published: February 2008

A broadband photometer experiment is being prepared for the Atmosphere Explorer‐C, ‐D, and ‐E missions to record the solar irradiance in seven wavelength bands in the range from 40 to 1250 Å. The experiment contains four spiral electron multipliers located behind a moving eight‐position filter wheel. Six of the eight sections of this wheel contain metallic filters used to spectrally isolate the solar irradiance. In addition, three Al2O3 diodes, two with filters, are being used to record the solar irradiance over the full range of orbital altitudes including those at which operation of multipliers with high voltages on open structures would be unsafe. A principal goal of the experiment will be to measure time dependence of the solar irradiance.

Photometric calibration will be based on preflight exposure to a storage‐ring synchrotron light source which has been calibrated by combining synchrotron theory with the absolute values of irradiance at 4000 Å obtained from comparison with the best available standard of irradiance.
The visible‐airglow experiment on Atmosphere Explorer
Authors: Hays P. B. , Carignan G. , Kennedy B. C. , Shepherd G. G. , Walker J. C. G.
Published: February 2008

The visible‐airglow experiment is an airglow photometer designed to measure various thermospheric emission features during the day and night both at low latitudes and in auroras. The photometer has two distinct optical channels, a high‐sensitivity channel with a large field of view and a low‐sensitivity channel with a narrow field of view to resolve small features. The system is protected by a combination attenuator and cathode back‐biasing scheme which allows measurements of maximum sensitivity within a fraction of a second of viewing the sun. This experiment will be a part of the scientific payload on all three Atmosphere Explorer missions.
The Atmosphere Explorer magnetometer experiment
Authors: Armstrong James C. , Zmuda A. J.
Published: February 2008

A three‐axis fluxgate magnetometer/analog‐to‐digital converter system will be included in the Atmosphere Explorer‐D and ‐E payloads. The scientific function of this system (the magnetometer experiment) is to measure magnetic fields due to field‐aligned and ionospheric currents, particularly in the auroral region. These currents relate to the magnetosphere‐ionosphere and solar wind‐ionosphere coupling and represent a considerable source of energy input into the auroral ionosphere. Attempts will also be made to measure the magnetic fields of the equatorial electrojet and other ionospheric current systems, although stray spacecraft fields might preclude these measurements. Each magnetometer axis will be sampled ≈ 8 times per second to a resolution of at least 5γ (1γ = 10−9 Wb m−2). For this experiment, good resolution (≈ 0.007% of the maximum magnetic field to be encountered) and a high sampling rate are much more important than absolute accuracy of the measurements, since systematic errors in absolute accuracy simply shift an already arbitrary baseline level.
Skywave polarization rotation in swept‐frequency sea backscatter
Authors: Barnum James R.
Published: February 2008

A 2.5‐km long slewable broadside receiving‐antenna array was used to study ionospherically propagated backscatter from the Pacific Ocean. Swept‐frequency continuous‐wave recordings revealed a new type of amplitude variation, periodic in both time delay and radio frequency, which caused recordings of time delay versus frequency to resemble large thumbprints. This new phenomenon was seen frequently during the fall and winter and also during spring and summer evenings. It is shown that magnetoionic splitting in the ionosphere (polarization rotation) was responsible for a modulation on the received backscatter signals. This concept was verified experimentally by switching the polarization of the transmitting antenna between vertical and horizontal. Computer ray tracing was used to contruct a synthetic backscatter record displaying lines of signal enhancement due to polarization rotation.

When the receiving antenna beamwidth is broadened from 1/2 to 4° the amplitude variation due to polarization rotation is undetectable, which probably explains why the effect was not positively identified in backscatter using F‐layer propagation taken prior to the availability of the 2.5‐km array. This result is also explained by the use of computer ray tracing. The modulation lines have not been seen to date by the author in recordings of backscatter from the land, probably because of the irregularities of backscattering from land surfaces.
Statistical models for D‐region partial‐reflection experiments
Authors: Shapiro Jeffrey H.
Published: February 2008

A phenomenological model for D‐region partial‐reflection experiments, which includes the effects of turbulent scattering, measurement noise, and coherent returns, is examined. General results for the amplitude and phase‐difference statistics of the ordinary‐ and extraordinary mode signals are obtained. These results are specialized and discussed for cases in which either the measurement noise or the coherent‐signal component is insignificant. A possible criterion for discriminating between turbulence models and a single reflecting‐layer model on the basis of amplitude statistics is obtained.
Pattern recognition for VLF studies
Authors: Jones Dyfrig
Published: February 2008

The pattern‐recognition technique to be described requires the simultaneous recording of a signal on two antennas, for example, orthogonal magnetic or electric sensors; after these signals are cross‐correlated, a fast Fourier transform yields the complex cross‐spectrogram. Provided that the polarization of the waves does not vary rapidly with time, it is possible to reject unwanted noise and to recognize the signal by utilizing the phase information in the cross‐spectrum.

This technique has been tested using computer‐simulated signals, and it has been found that variable frequency signals, for example, whistlers, which are more than 12 db below an incoherent‐noise background and more than 18 db below a coherent‐noise background, are still recognized and are recorded to the exclusion of noise.

The technique is applicable to plasma waves recorded on spacecraft as well as to electro‐magnetic waves. Although the polarizations of ground‐recorded VLF signals are usually more complicated than those in the magnetosphere, the method is still relevant.
Radiating near‐field power density and directivity reduction of tapered circular apertures
Authors: Rabello Marcio M. , Nobili Sergio L. G.
Published: February 2008

For tapered circular aperture with illumination of the form [b + (1 —ξ)v]/(b + 1) expressions for the on‐axis electric‐field intensity, power density, and directivity reduction are developed. Those expressions are presented in terms of the degenerate hypergeometric function. When v is an integer, the use of recurrence relationship permits simple expressions in closed form to be obtained. For special values of v and b, a set of curves is presented. Also, the application of the expressions developed for any other nonuniform illumination involving circular symmetry are briefly presented.
Multiple scattering from random plasma inhomogeneities
Authors: Holway Lowell H.
Published: February 2008

Small‐angle multiple‐scattering is considered from randomly spaced cylindrical plasma striations. It is shown that multiple scattering from several underdense plasma clouds can cause considerable backscatter even though a single undense cloud is capable only of deflecting the ray through fairly small angles. In a case in which the deflection angles are small and many deflections occur before a ray passes through the scattering region, the ray density can be shown to satisfy a Fokker‐Planck transport equation. A diffusion coefficient is obtained for this equation for the case of cylindrical Gaussian striations by means of a numerical ray‐tracing program. The transport equation is solved by a finite‐difference method to obtain scattering patterns and waveform distortion. The methods of solving the transport equation can be applied to the scattering of radio waves, laser pulses, and other radiation propagating in random media.
A theoretical and experimental study of the insulated loop antenna in a dissipative medium
Authors: Smith Glenn S.
Published: February 2008

The performance of a bare antenna as a radiator in a dissipative medium can be significantly altered by placing a dielectric coating around the antenna. For certain antenna types and specific properties of the dissipative medium, the dielectric insulation has been shown to improve the performance of the antenna. In this paper the thin‐wire circular‐loop antenna centered in an insulating spherical cavity and immersed in an infinite homogeneous isotropic dissipative medium is analyzed. A Fourier series solution for the antenna current distribution is derived. The coefficients of the series are a combination of the coefficients for the loop in an infinite dielectric medium, which were determined previously by Wu [1962], and a second term, which is an infinite sum. The properties of the sum are examined, and expressions for the antenna input admittance, impedance, and electromagnetic field in the dissipative medium are obtained. Numerical results are presented for specific antenna sizes and dissipative media. For the special case of a loop with a uniform current distribution in an electrically small sphere the expression for the input impedance is shown to reduce to a solution previously given by Wait and Spies [1964]. The construction of an experimental model for the antenna is discussed, and antenna input impedances and current distributions measured with the model immersed in various salt‐water solutions are compared with theoretical calculations.
Cosmic ray ionization in the D region at sunrise: Evidence from VLF phase measurements
Authors: Abdu M. A. , Ananthakrishnan S. , Krishnan B. A. , Massambani O.
Published: February 2008

A temporary phase recovery observed on long‐distance VLF propagation paths at sunrise is shown to be associated with cosmic‐ray ionization. Simultaneous measurements of the phase carried out at Atibaia, Säo Paulo, and Jaguaräo, Rio Grande de SuI, appear to demonstrate a latitudinal effect in this phenomenon.
Hydrogen‐line aperture synthesis at the National Radio Astronomy Observatory: Techniques and data reduction
Authors: Wright M. C. H. , Clark B. G. , Moore C. H. , Coe J.
Published: February 2008

This paper describes the hydrogen‐line interferometer system at the National Radio Astronomy Observatory. The system consists of telescopes, receivers, a data acquisition program, and a reduction program. The reduction program uses a random‐access data base which stores both the interferometer data and the synthesized maps. The calibration, mapping, and display of the data are described in detail
Recursion relations for coefficients in asymptotic expansions of wavefields
Authors: Sherman George C.
Published: February 2008

Recursion relations satisfied by the coefficients in certain asymptotic expansions of monochromatic wavefields are pointed out and discussed. One of the relations provides a simple means for obtaining higher‐order coefficients from a knowledge of the coefficient of the first‐order term, and it yields expressions for the coefficients in a very simple form. The utility of that relation is demonstrated by applying it in Sommerfeld's problem of the radiation of an electromagnetic dipole in the presence of a dissipative half‐space.
Amplification of electromagnetic surface waves supported by a half‐space of a double stream of plasma
Authors: Ha Kee Whan , Seshadri S. R.
Published: February 2008

The characteristics of electromagnetic surface waves supported by a plane interface separating a semi‐infinite region of free space from another semi‐infinite region consisting of a double stream of plasma streaming parallel to the interface are investigated for the case in which the wave is progressing either in or opposite to the direction of the stream. The behavior of the surface wave is quite different from the case in which the plasma is stationary. For the case of waves progressing opposite to the direction of the streams, there are backward‐wave regions as well as regions of instability which occur below the plasma frequency. In the frequency range of instability, the surface wave undergoes traveling‐wave amplification in the direction of the streams. The wave amplification characteristics are examined and the dependence of each on the relative number density of the two streams as well as on the drift velocity is studied. It is found that the maximum amplification rate increases as the drift velocity is increased.
A three‐dimensional model of magnetic merging at the magnetopause
Authors: Hill T. W.
Published: February 2008

The three‐dimensional problem of magnetopause merging has been analyzed using an extension of Alfvén's neutral‐sheet field annihilation model. That model has been generalized for an arbitrary magnitude and direction of the interplanetary magnetic field. The analysis provides a quantitative prediction of the merging rate that is consistent with the observed dependence of geomagnetic activity on the direction and magnitude of the interplanetary magnetic field.
Guiding center motions of magnetosheath protons over the dayside polar cap in a model open magnetosphere
Authors: Boright Arthur L. , Speiser Theodore W.
Published: February 2008

The guiding center equations are solved numerically to follow 200 ev protons with pitch angles near the loss cone from the dayside magnetosheath on the noon meridian to their mirror height in a model open magnetosphere. We examine the effect of magnetic field line curvature and electric field strength upon proton mirror heights. We find that when the magnetic field has dipole‐like curvature along a dayside proton trajectory an increase in the strength of the dawn‐dusk electric field will result in an increase of the proton mirror height. When there exists significant nondipole‐like magnetic field curvature along a dayside proton trajectory an increase in the dawn‐dusk electric field strength can result in lower proton mirror heights. In our model, when the interplanetary magnetic field is directed northward or southward the latitude dependence of changes in proton mirror height resulting from changed dawn‐dusk electric field strength is the same along the noon‐midnight meridian over the northern and southern polar caps, but when the interplanetary magnetic field vector lies in the noon‐midnight meridian plane at an angle to the earth dipole axis, proton mirror height behavior in response to electric field strength changes is different over the northern and southern polar caps. Proton drift perpendicular to the magnetic field but having a component parallel to the electric field can result in kinetic energy changes of up to 25% as a proton moves from the magnetosheath to its mirror height.
Magnetospheric current sheets
Authors: Speiser T. W.
Published: February 2008

A review is presented of theories and observations of current sheets in the magnetosphere, emphasizing the magnetopause and tail current sheets.

Theoretical models of the magnetopause current sheet can be divided into two types: specular reflection and fluid models containing no external magnetic field, and models with an interplanetary/magnetosheath magnetic field. Our understanding of the first type is much better than of the second, although magnetospheric observations indicate that the external magnetic field plays an important role.

Most of the theoretical models of the tail current sheet attempt to arrive at an understanding of the reconnection process or start with the assumption that reconnection is important. Observations are reviewed and implications for the models are discussed.
Electric field and current density profiles in the lower ionosphere
Authors: Gueth K. , Knott K.
Published: February 2008

Results of a rocket flight are reported where electric fields and ion drifts have been measured between 100 and 192 km in the auroral ionosphere. Electric fields were measured by means of a double probe system, while ion drifts were derived from wake measurements. It is found that the electric field vector stays fairly constant over the indicated range in altitude, although a marked difference exists between the upleg and downleg measurements. The ion drift vector changes as expected from collision frequency profiles. By combining the electron drift vector, which is determined by the electric and magnetic field only, and the ion drift vector, it is possible to derive the current density profile for both the upleg and downleg of the trajectory. Again, a pronounced difference in current density profiles exists for upleg and downleg. The profiles obtained are consistent with ground‐based magnetometer observations.
Selective fading and the propagation of radio waves in the magnetoplasma (ionosphere)
Authors: Cutolo M.
Published: February 2008

The technique called “modulated pulse” (the emission of HF pulses of suitable length modulated with high or low audiofrequencies) allows an exhaustive study of the propagation of modulated radio waves in the ionosphere. Using this system it is possible to establish that the diminution or the gain, in regard to that emitted by the transmitting antenna, of the percentage of modulation of the received modulated waves is not due in any way, as some researchers have declared, to the interference in the receiving antenna of rays that have travelled different paths in the ionosphere or to multiple echoes, but should be attributed to other reasons.

The experiments were made with carrier frequencies varying around the cyclotronic frequency and with different modulation frequencies. It was possible to obtain either a resonance curve of demodulation or a resonance curve of overmodulation and it was shown that the selective fading could be obtained with low frequencies as well as with high frequencies of modulation. The most important contribution herein is the determination that the interference does not alter in any way the depth of modulation of a modulated wave provided that the envelopes remain undistorted. If the amplitude of the echo is less than the minimum B, as generally happens, the interference never alters the sinusoidal form of the envelope. If the amplitude is greater the envelope always becomes distorted.
The electron content of the auroral ionosphere during radio absorption events
Authors: Hargreaves J. K. , Lyon G. F.
Published: February 2008

The relations between electron content, scintillation, and radio absorption are studied at an auroral zone station. It is found that during absorption events, indicating particle precipitation, the level of scintillation increases and the electron content increases in winter but is unchanged or slightly decreased in summer. At a given level of scintillation, the absorption is greater on a winter day than on a summer day. Although some of the observations are consistent with the results of other investigations, full explanations are lacking. It seems clear that, in addition to direct ionization production by auroral particles, other mechanisms must be involved. Further studies based on reception of geostationary satellite transmissions are recommended.
A 15.3 GHz satellite‐to‐ground path‐diversity experiment utilizing the ATS‐5 satellite
Authors: Hodge D. B.
Published: February 2008

During several months in 1970 and 1971 the characteristics of a diversity satellite‐to‐ground communication link were measured using the ATS‐5 15.3 GHz downlink. These data were gathered at two ground receiving terminals spaced 4 km apart during 1970 and 8 km apart during 1971 in the vicinity of Columbus, Ohio. These data have subsequently been analyzed to determine the improvement in link performance resulting from the use of space‐diversity. The results of this analysis have shown that substantial improvements in link performance may be gained through the use of space‐diversity on satellite‐to‐ground paths. For example, the durations of fades having depths exceeding 10 db were reduced by more than two orders of magnitude for both the 4 and 8 km site separation distances.
A note on the imperfectly conducting circular‐loop antenna
Authors: Smith Glenn S.
Published: February 2008

The thin‐wire circular‐loop antenna with a distributed impedance loading is analyzed by a simple extension of the Fourier series solution for the perfectly conducting loop antenna. When the loading is a uniform resistance per unit length, the solution obtained is that for the imperfectly conducting loop. The broadband characteristics of the imperfectly conducting loop are examined. Impedances calculated by the Fourier series method are compared with those calculated by Lin [1973] using numerical and differential equation methods. In addition, the effects of the resistive loading on the radiating efficiency and the far‐zone field pattern are investigated.
Some aspects of the design concept of a European incoherent scatter facility in the auroral zone (EISCAT project)
Authors: du Castel F. , Testud J.
Published: February 2008

EISCAT is a project presented by scientists from Finland, France, Germany, Norway, Sweden, and the United Kingdom for establishing a new incoherent scatter facility in the northern part of Scandinavia. The initial constraints and choices introduced by the scientific purpose of high latitude ionospheric studies are recalled and the general characteristics of the facility are described. The facility involves a multistatic system in the UHF range and a monostatic system in the VHF range. Tromsø, Norway, at a geomagnetic latitude corresponding to L = 6.3 at the ground, is the main axis of the measurements. Remote stations are located at Kiruna, Sweden, and Sodankyla, Finland. Some design concepts related to the capabilities of pulse‐coded modulation of a powerful UHF transmitter and others related to new types of receiving antennas are described. The expected performance of the facility is evaluated and compared with desirable performance criteria and with those of other existing or proposed facilities.
Recent improvements in our knowledge of neutral atmosphere structure from satellite drag measurements
Authors: Roemer M.
Published: February 2008

Observational results on the density in the thermosphere and lower exosphere, i.e., within the altitude range from about 150 to about 1000 km, are discussed in this paper. Most observational results on total gas density were obtained from orbital drag and more recently also from in‐situ drag analysis. The primary parameter measured is atmospheric density with temperature as a secondary structural parameter deduced with the help of theory and/or atmospheric models. Both the merits and shortcomings of the drag analysis method are outlined in view of a comparison of temperature deduced from total density and kinetic gas temperature measured by incoherent scatter. Recent improvements of our knowledge of the known density variations are presented.
F‐layer irregularities as determined by backscatter studies at 19 MHz over half of a solar cycle
Authors: Basu Sunanda , Vesprini Robert L. , Aarons Jules
Published: February 2008

A continuous series of backscatter observations at 19 MHz made at Plum Island, Massachusetts, 56°N invariant latitude, over nearly half a solar cycle is utilized to derive occurrence characteristics of ground backscatter propagated by means of F‐layer reflection, 1F, as well as those of aspect‐sensitive field‐aligned echoes from the F layer, FAE(F). Neither field‐aligned E‐layer echoes nor ground backscatter propagated by means of sporadic E were found to produce any noticeable blanketing of the radar signal for this frequency and radar location. The 1F propagation showed marked sunspot cycle and azimuthal dependence and was strongest in winter and weakest in summer.

Under quiet magnetic conditions FAE(F) was primarily a sunset phenomenon, occurring usually at delays of 7 to 10 msec, with a marked sunspot cycle dependence. Some daytime activity was observed at longer delays of 20 to 22 msec which could be explained in terms of F‐layer field alignment after one intermediate ground reflection. The occurrence of FAE(F) during magnetic storms showed a seasonal variation in agreement with the well‐known storm behavior of f0F2 at these latitudes. In general, however, the occurrence of FAE(F) increased monotonically with increasing magnetic activity until the threshold of KFr = 4 was reached, beyond which the depletion in the F‐layer background ionization caused it to decrease, with complete cut‐off occurring irrespective of season during very disturbed conditions, KFr ≥ 7.

With the aid of ray traces, the occurrence statistics of both 1F and FAE(F) are interpreted and discussed in terms of the geometry of the probing radar, the various factors controlling F‐layer propagation, and the extent of the F‐layer irregularity region.
Application of ionospheric predictions to HF propagation in three dimensions
Authors: Beckwith R. I. , Bailey A. D. , Rao N. Narayana
Published: February 2008

The application of ionospheric predictions is extended to three‐dimensional HF propagation by using a simplified ray‐tracing technique involving an ionospheric model consisting of an equivalent spherical reflection surface eccentric with the earth. Predicted ray‐path deviations from the great‐circle plane and other propagation characteristics are investigated for a medium range midlatitude path by using the NOAA prediction coefficients. The predicted monthly mean diurnal behavior of the azimuthal angle of arrival of the HF signal is compared with experimental data. It is found that the observed occurrence of wave‐interference effects is strongly correlated with the predicted occurrence of multiple mode propagation and that the trend of the diurnal variation of the predicted azimuthal angle of arrival for the dominant mode is consistent with the average diurnal trend of the experimental observations.
Theory of wave propagation along a thin wire inside a rectangular waveguide
Authors: Mahmoud Samir F. , Wait James R.
Published: February 2008

A modal equation for the propagation constants along a thin wire located inside a rectangular waveguide is derived. The regions external to the two horizontal broad walls are homogeneous lossy dielectrics. To facilitate the analysis, the two narrow vertical walls are assumed to be either perfect electric or perfect magnetic reflectors. Special cases that have been treated earlier [Wait, 1972] are recovered.
Measurements of rain‐induced polarization rotation at 30.9 GHz
Authors: Semplak R. A.
Published: February 2008

An experiment to determine the rotation of linearly polarized millimeter waves by rain is described; the results obtained at a frequency of 30.9 GHz, over a 2.6‐km propagation path in New Jersey, during the spring‐fall period of 1971, are discussed. Cumulative distributions of rain‐induced rotation of the vertically polarized electric vector, and of the associated cross‐polarized component, are presented. The polarization rotation shows a strong relationship with the cross‐path wind. For 0.01% of the time, during the measuring period, the rotation of the electric vector exceeds four degrees, equivalent to a cross component down 23 dB; for 0.001% of the measuring period, the latter value was about 18 dB.
Measurement of the nonlinear impedance of a satellite‐borne, electric dipole antenna
Authors: Koons H. C. , McPherson D. A.
Published: February 2008

The nonlinear impedance of a satellite‐borne, electric dipole antenna was measured at ELF/VLF at drive voltages between 10 mv and 100 v peak‐to‐peak. The magnitude of the impedance is in general agreement with the predictions of theoretical models of plasma‐clad antennas. At low voltages the phase angle of the impedance is in agreement with linear sheath models, i.e., at low densities the current leads the voltage and as the density increases the phase angle approaches zero. However, at high drive voltages the current lags the voltage. Above 10 v p‐p the phase angle often exceeds +90 deg.
Prediction of azimuthal angle of arrival of HF waves during the sunrise period
Authors: Rao N. Narayana , Beckwith R. I.
Published: February 2008

A previously reported method of application of ionospheric predictions to HF propagation in three dimensions is employed for an investigation of the monthly mean predicted behavior of the azimuthal angle of arrival of HF signals during the sunrise period for a medium range, midlatitude path oriented 27° east from north. A comparison of the prediction model results with experimental data collected over a five‐day period each month for twelve months shows that the use of the prediction model offers a significant overall improvement in reducing the errors due to the azimuthal angle‐of‐arrival deviations from the great circle azimuth. The factors contributing to the discrepancies between the prediction model results and experimental observations are examined.
High latitude ionospheric irregularity model
Authors: Pope J. H.
Published: February 2008

Modifications were made to the global scintillation model that was developed by Fremouw (Stanford Research Institute), to better represent recent high latitude scintillation data. These modifications were made primarily in the high latitude term of the model and include a provision for variation in the position of the scintillation boundary with respect to magnetic activity. The resulting model is compared with published curves of scintillation activity with respect to latitude and is found to produce a considerably better agreement with those curves than the original model. A modification of the model is made to represent the finding that the northern and southern polar regions of the scintillation activity are not symmetrical with respect to each other.
Multifrequency coherences of short‐term fluctuations of line‐of‐sight signals—Electromagnetic and acoustic
Authors: Mandics P. A. , Harp J. C. , Lee R. W. , Waterman A. T.
Published: February 2008

Spherical wave weak‐scattering propagation theory is employed to show that the resolution of multifrequency atmospheric probing techniques is limited to inhomogeneity scale sizes that are larger than a certain cutoff scale size lc. The magnitude of lc depends on the frequency separation and the path geometry. Inhomogeneities smaller than lc cannot be resolved and inevitably contribute to measurement errors. Propagation experiments performed at radio and acoustic wavelengths confirm the existence of the predicted cutoff and are in good agreement with the theory. An example is given to illustrate how this cutoff limits the ultimate accuracy of a radio‐optical comparison measurement aimed at determining path‐averaged water vapor variations.
Dispersive motions in the equatorial ionosphere
Authors: Misra R. K.
Published: February 2008

The results of the dispersion analysis of the spaced receiver records of fading for an equatorial station in the Indian zone are discussed. It is shown that the apparent drift speed increases almost linearly with the frequency of fading in all the cases studied for both the E and F regions. The dispersion of drift speed is larger for the F region than for the E region in general. Furthermore, the F region shows a larger dispersion during the daytime whereas it is of the same order in the day and nighttimes for the E region. The drift speed corresponding to the highest Fourier frequency present in a particular record is found to be close to the apparent drift speed Va as determined from the shift of the peak of cross correllograms while the total frequency spread of the speed is linearly related to the random drift speed Vc derived by the full correlation analysis.

Several possible causes of dispersion in the ionosphere such as gravity waves, independently moving reflecting screens, height gradient of the horizontal drift and size of irregularities, the dispersive nature of the vertical drift velocity, and the variability of the drift during the course of record are discussed. Interference of the signals reflected from two or more highly aspect‐sensitive areas in the ionosphere is suggested as a possible mechanism resulting in the positive and linear dispersion observed on the ground at an equatorial station.
Ionospheric modification theory: Past, present, and future
Authors: Meltz G. , Perkins F. W.
Published: February 2008

Ionospheric modification theory is reviewed and the principal phenomena are discussed in the context of present understanding. One fundamental advance has been the recognition that differential heating in collisional plasmas due to the beating of high frequency waves can generate low frequency density fluctuations more effectively than ponderomotive forces. This thermal coupling between the disturbing wave and density fluctuation plays a central role in producing field‐aligned structure. Future applications of ionospheric modification in transequatorial scatter communications and D‐, E‐, and F‐region aeronomy are also discussed.
Field‐aligned instability due to stimulated scattering of intense radio waves from diffusion quasi‐modes
Authors: Goldman Martin V.
Published: February 2008

A field‐aligned instability is found when intense radio waves undergo stimulated scattering off low frequency quasi‐modes related to parallel diffusion. This instability may arise during ionospheric modification. The wavelength of the unstable field‐aligned diffusion mode is between 75 and 200 m, in agreement with the experimental findings of Bowhill [1974]. The growth time is at least a second, and the instability should extend over an altitude range on the order of 100 km.
A unified theory on radiation of a vertical electric dipole above a dissipative earth
Authors: Chang David C. , Fisher Richard J.
Published: February 2008

Analytical expressions are derived for the electromagnetic field associated with a vertical electric dipole (VED) above a dissipative earth. The scattered field is shown to consist of a direct contribution from a perfect image source, and a correction due to the finite conductivity of earth which is expressible in terms of an incomplete Hankel function. The resultant expression readily reduces to the well‐known asymptotic solutions in both the space‐wave and ground‐wave regions. Validity of the space‐wave expression in the far zone is found to be dependent upon the observation angle as well as the refractive index of earth. In the near field, alternative approximate formulas in closed form are derived from the incomplete Hankel function expression for different elevation angles. These formulas are shown to yield accurate results whenever the distance from the image source is greater than about 0.2λc where λc is the effective wavelength (or skin depth) of earth. For distance smaller than 0.2λc, however, an analytical expression based upon a quasistatic approximation is generally applicable and has been discussed elsewhere.
Oblique resonances in the ionosphere
Authors: Gonfalone A.
Published: February 2008

A new diagnostic tool for space plasmas is presented. The oblique resonance excited by an antenna in a magnetoplasma has been detected in the ionosphere with a rocket experiment. The instrument is simple and consists of a fixed frequency transmitter and a receiver. The resonance cone and its fine structure are used to deduce the plasma density and the electron temperature. The possibility of measuring field‐aligned currents is briefly discussed.
Computed scintillation spectra for strong turbulence
Authors: Marians Marilyn
Published: February 2008

Spectra of intensity fluctuations resulting from the propagation of a plane wave through a thin but arbitrarily strong layer of refractive index variations are computed using the theory in a companion paper [Rumsey, 1975]. A power law spectrum with index α = 3 was assumed for the turbulence or refractive index spectrum. The intensity spectra were found to duplicate the Born approximation for weak scattering and the high frequency approximation for strong scattering. The scintillation index, or the rms intensity fluctuation, saturates at unity for a plane wave. The scintillation index has been computed for sources of various sizes. The results of the computation are in remarkable agreement with the results of radio star scintillation measurements.
Effects of layered turbulence on oblique waves
Authors: Taylor Leonard S.
Published: February 2008

Using the method of smooth perturbations, expressions are derived for the spectra of phase and log‐amplitude fluctuations for plane waves obliquely incident on layered media in which the turbulence strength varies smoothly from layer to layer. The effects of anisotropic turbulence are included.
A narrow beam antenna system using an air column having a central conductor and surrounded by an anisotropic plasma column
Authors: Joshi N. K. , Verma J. S.
Published: February 2008

The radiation pattern of a magnetic ring source of electromagnetic waves in an air column having a central conductor along its axis and surrounded by an annular axially magnetized plasma column is studied. This configuration gives rise to a radiation peak or peaks near, before, or beyond the critical angle depending upon the value of the magnetic field. The direction of the radiation peak depends upon the inner and outer diameters of the annular plasma column, the diameter of the central conductor, the plasma density, and the magnetic field. The direction of the radiation peak is independent of the diameter of the ring source. For very high magnetic fields the peak shifts toward the end fire direction. Based upon these results it is proposed to develop an electronically scannable narrow beam plasma antenna system.
Silicon ions below 100 km: A case for SiO2+
Authors: Goldberg Richard A.
Published: February 2008

Silicon ions are normally detected at altitudes above 100 km and within sporadic‐E layers. Traces have rarely been observed within the more permanent metallic layer near 93 km. This is surprising since silicon is an important constituent of chondritic meteorites, which ablate material in this region to provide a primary source of the metallic species observed there. In this work, we present evidence that Si+ ions form SiO2+ at the lower altitudes, and exist in this ionic state prior to recombination. A rocket launched from El Arenosillo, Spain, on 3 July 1972, at 0743 LMT (χ = 57.1°), during the predicted period of the β Taurids meteor shower, passed through a continuous belt of metallic ions that began near 85 km, ended near 115 km, and exhibited an order of magnitude increase in the form of a layer near 114 km. Si+ was measured in and below the ledge down to 103 km. It showed a rapid decrease below this height. A comparison of the isotopic ratio for nickel, [60Ni]/[58Ni], with the measured ions shows too large a ratio for the ions below 100 km, suggesting contribution to 60+ from other sources. A prime candidate is SiO2+, since Si+ is observed to decrease in the region where the ratio becomes too large. No evidence for SiO+ (44+) is present in the data, indicating that Si+ forms SiO2+ either directly or through SiO+ in a short‐lived state. Radiative association is offered as a primary mechanism for SiO2+ production and yields a calculated reaction rate of 2×10−16 cm3 sec−1.
Electric field measurements in the auroral E region
Authors: Mahon H. P. , Smiddy M. , Sagalyn R. C.
Published: February 2008

Dipole electric field, positive ion and electron densities and temperatures, vehicle potential, and plasma sheath measurements have been made in the auroral E region by means of rockets flown from Fort Churchill, Canada. These results are described and compared over the altitude region 100 to 165 km. On a rocket flight launched on 10 December 1969 during very quiet conditions, adjacent to a stable, low intensity auroral arc, the plasma density and temperatures are found to be high and the electric fields large and steady. Electric field components of the order of −17 mv m−1 to +6 mv m−1 were measured along the Earth's magnetic field. The plasma results indicate that these fields may be contributing to enhanced electron temperatures. On a flight of 9 March 1970 during a large magnetic storm with widespread auroral activity, lower plasma densities and temperatures and much smaller and more erratic electric fields were observed with no significant component parallel to the magnetic field.
Concluding remarks, session 3: Neutral and ion chemistry
Authors: Monro P. E.
Published: February 2008

When one begins to summarize the conference proceedings dealing with the neutral and ion chemistry of the E region, one becomes forcefully aware of the spectacular advances that have been made in this area during the past decade. This conference has well demonstrated that these advances are continuing, and that there is yet a wide range of problems to be overcome. Both these aspects will be discussed briefly below.

Some consideration will, first of all, be given to the neutral chemistry at E‐region heights although it cannot be separated realistically from the ion chemistry. Most of the proceedings in this area were devoted to the chemistry of odd nitrogen and it now seems clear that the major problems are associated with uncertainties related to the production and loss of N(2D).
Concluding remarks, session 4: Atmospheric dynamics
Authors: Geller Marvin A.
Published: February 2008

Meteorologists used to criticize those who described themselves as aeronomers or ionospheric physicists, pointing out their neglect of neutral air motions in calculating ionospheric behavior. This situation has changed dramatically in the last few years. At this meeting, the effects of neutral air motions were considered either implicitly or explicitly in over one third of the papers presented. Some of the topics that explicitly touched on neutral atmosphere dynamics are described in the following paragraphs.
Concluding remarks, session 5: Models, electric fields; session 6: Electric currents, late contributions
Authors: Holzer T. E.
Published: February 2008

These two sessions, which cover the topics of E‐region models, electric fields, and electric currents, underscore a major difficulty necessarily encountered in studies of the E region. This difficulty arises from the fact that the E region of the ionosphere is just one part of a very large system, all of whose component parts are strongly coupled electrodynamically and hydrodynamically. Before describing a general approach to overcome this difficulty, let me briefly discuss some of the important coupling processes in the overall system.

As the solar wind flows past the earth, it exerts a tangential stress at the magnetopause boundary. Information of this stress is propagated by hydromagnetic waves to the polar cap ionosphere, where an electric potential distribution is established that is consistent with the plasma motions required by the applied tangential stress. The resultant electric field pattern covers not only the polar cap region, but extends to the auroral zone and, to some extent, to middle and low latitudes. The electric fields imposed on the ionosphere equatorward of the polar cap are mapped into the closed field line regions of the magnetosphere, thus establishing a plasma convection pattern in the magnetosphere. The actual electric field pattern established is strongly affected by the magnetospheric plasma population, ionospheric conductivity, and neutral thermospheric winds. In fact, due to magnetospheric plasma effects, the electric field is largely shielded (on time scales of several hours or more) from the middle and low latitude regions. Nevertheless, relatively steady solar‐wind‐generated electric fields of a few mv m−1, along with much larger transient fields, probably are imposed at middle and low latitudes.
LF and VLF phase antinomies during solar X‐ray flares
Authors: Perona Giovanni E.
Published: February 2008

During solar X‐ray flares the phases of VLF and LF signals reflected from the ionosphere at grazing incidence often show an opposite behavior, the so‐called phase antinomy. Two years of data have been analyzed to show the main characteristics of this phenomenon and, in particular, its seasonal dependence. An explanation of such behavior is sought in terms of changes in the phase of the ionospheric reflection coefficient. Only simple models of electron density distributions with height are numerically analyzed, namely, perturbed exponentials.
Parametric instability of the sheath‐plasma resonance
Authors: Stenzel R. , Kim H. C. , Wong A. Y.
Published: February 2008

A large amplitude RF pump signal is applied to a wire antenna in a magnetic‐field‐free plasma. When the pump frequency ω0 corresponds to twice the sheath‐plasma resonance frequency ωr, and the pump amplitude exceeds a threshold value, spontaneous excitation of the signal at ωr = ω0/2 is observed. The process is explained as a periodic variation of the sheath thickness causing the sheath‐plasma resonance to become parametrically unstable. The RF field pressure can induce density relaxation oscillations near the ion plasma frequency which are observed to modulate the instability spectrum strongly.
Worldwide variation in atmospheric noise intensities with sunspot number: An in‐depth look at the 20–24 hour seasonal time block
Authors: Joglekar P. J. , Sathiamurthy T. S.
Published: February 2008

Comparisons of the variation of atmospheric radio noise intensities for 20–24 hr to sunspot numbers have been completed. Statistical dependence between the noise intensities and sunspot numbers was found for different seasons at a number of frequencies for many locations in the global network of ARN‐2 noise recorders. The noise intensities generally tended to decrease with sunspot number in the range from 50 kHz to 5 MHz, which is presumed to be due to increases in residual ionospheric absorption during nighttime. At frequencies greater than 5 MHz, noise intensities increased with sunspot number in many cases, which would be expected from our present knowledge of ionospheric behavior in the HF range. By convention, CCIR treats year‐to‐year variation in the noise intensities as random and includes them in the prediction uncertainty σ Fam (for which one value is given at a frequency for a seasonal time block for all locations) in system performance evaluation. An error analysis on a global basis shows that a large portion of the year‐to‐year variability is due to sunspot variation. This suggests the possibility of improved noise estimates.