bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical gravity wave spectrum in the atmosphere: Strong and weak wave interactions
Authors: Weinstock J.
Published: February 2008

It is pointed out that vertical wave number spectra of gravity waves can be divided into subranges of weak and strong wave interactions and that observed −2.5 to −3 power law spectra may fall within the strong interaction subrange. An elementary criterion is derived to help distinguish between weak and strong waves. It is then shown that the strong wave subrange is predicted by a well‐known theory of Lumley. The relation of theory and observation to breaking gravity waves, turbulence dissipation, and length scales is discussed. The validity of Lumley's theory is summarized, and unresolved questions for future investigations of atmospheric wave spectra are raised.
Middle atmosphere (60–110 km) tidal oscillations at Saskatoon, Canada (52°N, 107°W) during 1983–1984
Authors: Manson A. H. , Meek C. E.
Published: February 2008

Winds have been measured by a medium frequency radar using the spaced antenna method, in the upper middle atmosphere (60–110 km). The mean or prevailing winds are related to the seasonal variation of the semidiurnal and diurnal tide, as inferred at Saskatoon by the amplitude and phase of the 12‐ and 24‐hour wind oscillations. Strong seasonal variations are evidenced, consistent with the importance of antisymmetric tidal modes (2, −3; 2, −5; and 1, −1, respectively). For both tides, winter wavelengths are shorter than summer; the semidiurnal tide dominates in the winter months, but in summer the diurnal tide has its maximum. The fluctuations and trends in winds and tidal oscillations during the stratospheric warming and the spring equinox are discussed in detail.
Properties of lateral electromagnetic fields and their application
Authors: King Ronold W. P. , Owens Margaret , Wu Tai Tsun
Published: February 2008

The formulas for the complete electromagnetic field in region 1 (z ≥ 0, k1) due to a horizontal electric dipole also located in region 1 near its plane boundary with region 2 (z ≤ 0, k2; | k2 |2 « | k1 |2) are reviewed and applied to determine the moment of a magnetic dipole that effectively generates the field in region 2. Use is made of this field to determine the locus of the Poynting vector, the maximum depth of penetration into region 2 of the lateral wave part of the field, and the fraction of power associated with the lateral wave in a specified range. Application is made to the use of horizontal antennas on the seafloor for geophysical exploration of the lithosphere.
Measurements of HF backscatter cross section for striations created by ionospheric heating at different power levels
Authors: Hedberg Å. , Derblom H. , Hamberg G. , Thidé B. , Kopka H. , Stubbe P.
Published: February 2008

Ionospheric field‐aligned irregularities induced by the Tromsø heating facility have been studied by means of an HF backscatter radar. A power law relation between the received echo power and the stepwise changed heater power is found with an exponent varying between 1.5 and 1.9. The radar cross section at 14 MHz is found to be in the order of 35–55 dBsm, and a maximum volume cross section 3.6×10−7 m−1. Examples of three typical recording sequences for three different environmental ionospheric conditions are discussed. During favourable conditions backscatter echoes are observed at the lowest heater ERP of 14 MW. The cross section then saturates at 98 MW ERP and exhibits a pronounced decrease at higher heater ERP. During less favourable conditions the saturation level is not reached even at full power of 280 MW ERP. A correlation seems to exist between variations in the derived Hall current and the variation in cross section of the heater‐induced field‐aligned irregularities.
Limitations and possible improvements of ionospheric models for radio propagation: Effects of sporadic E layers
Authors: Paul Adolf K.
Published: February 2008

Recent angle of arrival measurements of echoes reflected from sporadic E layers show that those layers are frequently tilted in different directions with deviations from the horizontal ranging up to 30°. The observations also indicate that structure sizes of Es are relatively small, on the order of 50 km. Strong tilts of Es can launch HF rays into the whispering gallery mode, and can cause large errors in HF direction finding. They also permit the interpretation of nonblanketing Es as a geometrical effect and not an optical property. The consequences of those findings for HF modelling will be discussed.
The effective recombination coefficient of electrons in the ionosphere between 50 and 150 km
Authors: Gledhill J. A.
Published: February 2008

The literature on values of the effective recombination coefficient α of electrons between heights of 50 and 150 km has been critically examined. It is found that, during auroral events at night, it may be represented by the expression α = 4.30×10−6 exp (−2.42×10−2h) + 8.16×1012 exp(−0.524 h), where α is in cm −3 s−1 and h is in kilometers. During a wide variety of phenomena, including polar cap absorption, sudden phase anomalies, solar proton events and solar flares, the expressions α = 652 exp(−0.234 h) (night) and α = 0.501 exp(−0.165 h) (day) are reasonably representative between 50 and 100 km. No values are available above 100 km. The above values are certainly not reliable to within a factor of 2, and in some cases not even to within an order of magnitude.
Focusing and shadowing of ELF signals
Authors: Field E. C. , Warber C. R. , Joiner R. G.
Published: February 2008

Phase velocity transitions in the earth‐ionosphere waveguide can focus or shadow ELF signals and give rise to anomalies less than a megameter in extent. This paper develops a hybrid method to calculate those effects: it uses full‐wave theory to determine local TEM mode parameters, but traces rays to describe the signal's lateral structure. The method is applied to models of the terminator and disturbed polar cap boundary, including one based on the November 22, 1982 solar proton event (SPE). During that event a receiver in the Gulf of Alaska measured a severe signal fade. That fade probably was caused by refraction of the signal away from the polar cap boundary and into the disturbed cap, where the TEM‐mode phase velocity is slowest. Focusing and shadowing, lasting about an hour, are predicted for signals whose great‐circle path grazes the terminator.
Applications of complex image theory
Authors: Bannister Peter R.
Published: February 2008

During the past two decades, complex image theory techniques have successfully been utilized to determine simple form expressions for the electric and magnetic fields produced by antennas located near the earth's surface, for both single‐layered and multilayered earths. It is the purpose of this paper to provide a tutorial overview of complex image theory. Particular attention will be paid to its range of validity. Specific applications will also be discussed.
Radio wave diffraction during the passage of an acoustic shock through a sporadic E layer
Authors: Jacobson Abram R. , Carlos Robert C. , Argo Paul E. , Rickel Dwight G.
Published: February 2008

Bistatic HF Doppler measurements of the ionospheric disturbance arising from a large ground level chemical explosion have revealed a peculiar HF return which begins at large positive Doppler (> + 5Hz) and linearly evolves to large negative Doppler (< ‐ 5 Hz). Various pieces of evidence in our data suggest we observed an HF wave which is diffracted from the intersection of the acoustic shock and an extensive plane of sporadic E ionization at 110 km. From these measurements we estimate the horizontal trace velocity of the locus of intersection.
Lateral wave propagation in a three‐layered medium
Authors: Dunn John M.
Published: February 2008

The effect of an intervening layer on a lateral wave which propagates along the interface between two different electrical media is considered. The lateral wave travels down from a time‐harmonic electric dipole source in the upper region, through the middle region, and parallel to the interface between the middle and lower regions. It is assumed that the magnitude of the wavenumber in the upper medium is much greater than that of the middle, which in turn is much greater than that of the lower. The middle layer affects the lateral wave in two ways. It multiplies the usual lateral wave results by an overall factor which depends on the thickness of the slab and the electrical properties of the three media. It also changes at what distance the transition from intermediate to far field behavior occurs. Typically, the presence of the slab increases the magnitude of the fields near the transmitter, but leads to smaller field strengths in the far field region. The formulas are derived in two ways. The first utilizes an iterative procedure based on impedance boundary conditions and boundary layer theory. The second uses the more traditional steepest descent evaluation of the exact integrals available for the problem.
Spatial correlations in intense ionospheric scintillations: Comparison between numerical computation and observation
Authors: Kumagai Hiroshi
Published: February 2008

The spatial correlations in intense ionospheric scintillations were analyzed by comparing numerical results with observational ones. The observational results were obtained by spaced‐receiver scintillation measurements of VHF satellite radiowave. The numerical computation was made by using the fourth‐order moment equation method with fairly realistic ionospheric irregularity models, in which power‐law irregularities with spectral index 4, both thin and thick slabs, and both isotropic and anisotropic irregularities were considered. Evolution of the S4. index and the transverse correlation function was computed. The numerical result that the transverse correlation distance decreases with the increase in S4. was consistent with that obtained in the observation, which suggests that multiple scattering plays an important role in the intense scintillations observed. The anisotropy of irregularities proved to act as if the density fluctuation increased. This effect, as well as the effect of slab thickness, was evaluated by the total phase fluctuations that the radiowave experienced in the slab. On the basis of the comparison, the irregularity height and electron‐density fluctuation which is necessary to produce a particular strength of scintillation were estimated.
The space‐time intensity correlation function of scintillation due to a deep random phase screen
Authors: Franke S. J.
Published: February 2008

An investigation of the space‐time correlation function of the electromagnetic radiation scattered by a “deep” random phase screen with Gaussian space correlation function is conducted. The model for the statistics of the phase screen allows for random temporal evolution of the screen and for a steady drift, as well. Using this model, an expression for the space‐time intensity correlation function is derived under the Huygens‐Fresnel approximation. The complete space and time intensity correlation function is evaluated numerically for several examples where the time variation is caused purely by random temporal evolution of the screen. The examples are chosen to illustrate the space‐time properties of scattering in the “non‐Gaussian” regime where the field at a point below the screen is influenced by only a few independent scatterers in the screen.
A new analysis technique for estimating zonal irregularity drifts and variability in the equatorial F region using spaced receiver scintillation data
Authors: Vacchione Joseph D. , Franke Steven J. , Yeh Kung C.
Published: February 2008

A new technique for analyzing spaced receiver measurements of equatorial scintillation is applied to VHF scintillation data. The technique is based on a model that includes both propagation effects and the statistical characteristics of scintillation‐producing irregulatities. Nonlinear least squares fitting is used to fit the model to measured auto‐ and cross‐correlation functions of the signal amplitude fading on spaced receivers. The results are compared with mean and “random” drift estimates obtained using the classical type of correlation analysis.
First‐order corrections to reflection and transmission at a curved dielectric interface with emphasis on polarization properties
Authors: Ansorge Harald
Published: February 2008

The reflection and refraction of a high‐frequency electromagnetic field in the presence of an arbitrarily curved dielectric interface is considered. The fields are expanded in asymptotic series of k−1, known as Luneburg‐Kline expansions. Based on a ray method [Lee, 1975], the zero‐ and first‐order terms of k−1 of the reflected and transmitted fields at the interface have been evaluated by Ansorge [1986]. Associated with the fields at the interface, effective surface current densities can be used to evaluate the fields at points away from the interface. This can be done analytically by means of the stationary phase method. In this paper the first‐order terms of the reflected far field in the case of plane wave incidence and the transmitted far field in the case of homogeneous spherical wave incidence are given. The results allow a better description of the polarization state of reflected and transmitted fields then zero‐order terms do alone, which is shown for some typical examples in frequency and time domain.
Ionospheric demodulation of powerful pulsed radio waves: A potential new diagnostic for radars suggested by Tromsø heater results
Authors: Rietveld M. T. , Stubbe P.
Published: February 2008

Pulses of radio waves in the ELF and VLF range can be generated in the auroral ionosphere by the transient conductivity perturbation (in the presence of an ambient electric field) caused by artificial heating using pulsed high‐power HF radio waves. Examples of such waves generated by the Max‐Planck heating facility near Tromsø are briefly reviewed. The waves generated can be used to provide information on the horizontal ionospheric electric field. A model which has successfully explained these results quantitatively is used to predict the amplitudes produced by lower power HF radars (partial reflection experiments) in auroral regions. The model results show ELF/VLF amplitudes which are typically an order of magnitude below those obtained for the Tromsø heater but may nevertheless provide useful ionospheric information as a byproduct of the radar's normal operation.
The effect of thin scattering layers on the determination of wind by Doppler radars
Authors: May Peter Thomas , Fukao Shoichiro , Tsuda Toshitaka , Sato Toru , Kato Susumu
Published: February 2008

It has recently been suggested that the observed tendency for intense turbulence to occur in thin layers in the atmosphere introduces an error in horizontal wind measurements with MST radars, even in the absence of wind shear. This effect, called the finite range volume effect, is a consequence of the tilted pulse volume when using the Doppler technique. A model is shown which illustrates that this effect may be very important in MST radar measurements when wide antenna beam widths are used. These model results are supported by observations performed with the MU radar in Japan, where wind measurements with different beam widths have been performed. The impact of these measurements on MST radar design and possible methods to minimize the error are discussed.
A ground‐based technique for millimeter wave spectroscopic observations of stratospheric trace constituents
Authors: Parrish A. , deZafra R. L. , Solomon P. M. , Barrett J. W.
Published: February 2008

Recent concern over possible long term stratospheric changes caused by the introduction of man‐made compounds has increased the need for instrumentation that can accurately measure stratospheric minor constituents. The technique of radio spectroscopy at millimeter wavelengths was first used to observe rotational transitions of stratospheric ozone nearly two decades ago, but has not been highly developed until recently. We report here on a ground‐based observing technique which employs a millimeter‐wave superheterodyne receiver and multichannel filter spectrometer for measurements of stratospheric constituents that have peak volume mixing ratios that are < 10−9, more than 3 orders of magnitude less than that for ozone. We have used the technique for an extensive program of observations of stratospheric chlorine monoxide and also for observations of other stratospheric trace gases such as 16O3, vibrationally excited 16O3, 18O2 16O, N2O, HO2, and HCN. In the present paper, analysis of the observing technique is given, including the method of calibration and analysis of sources of error. The technique is found to be a reliable means of observing and monitoring important stratospheric trace constituents.
A comparison of mapped and measured total ionospheric electron content using global positioning system and beacon satellite observations
Authors: Lanyi Gabor E. , Roth Titus
Published: February 2008

Total ionospheric electron contents (TEC) were measured by global positioning system (GPS) dual‐frequency receivers developed by the Jet Propulsion Laboratory (JPL). The measurements included P‐code (precise ranging code) and carrier phase data for six GPS satellites during multiple 5‐hour observing sessions. A set of these GPS TEC measurements were mapped from the GPS lines of sight to the line of sight of a Faraday beacon satellite by statistically fitting the TEC data to a simple model of the ionosphere. The mapped GPS TEC values were compared with the Faraday rotation measurements. Because GPS transmitter offsets are different for each satellite and because some GPS receiver offsets were uncalibrated, the sums of the satellite and receiver offsets were estimated simultaneously with the TEC in a least squares procedure. The accuracy of this estimation procedure is evaluated, indicating that the error of the GPS‐determined line of sight TEC can be at or below 1×1016 el/m2. Consequently, the current level of accuracy is comparable to the Faraday rotation technique; however, GPS provides superior sky coverage.
Modeling of F region gravity waves observed during the WAGS campaign: 2. Background wave spectra
Authors: Sheen D. R. , Liu C. H.
Published: February 2008

In this second paper, we shall concentrate on the “background wave spectra” which were observed during several magnetically quiet days in the Worldwide Atmospheric Gravity Wave Study (WAGS) campaign. The observed parameters used in this study are the ionization density and the line‐of‐sight ion velocity. This provides more information than in most of the previous investigations of traveling ionospheric disturbances, in which only electron density perturbations were used. The data will be analyzed and compared with a theoretical model to yield information about the background gravity wave spectrum in the neutral atmosphere. It will be shown that the neutral vertical velocity spectrum can be modeled as a power law type of spectrum. The overall kinetic energy of this spectrum will be calculated and compared with that associated with the special event observed on a magnetically active day.
A synoptic study of TIDs observed in the United Kingdom during the first WAGS campaign, October 10–18, 1985
Authors: Crowley G. , McCrea I. W.
Published: February 2008

The Leicester University HF Doppler system was operated in the United Kingdom during a 9‐day interval from October 10 to 18, 1985, as a part of the first campaign in the Worldwide Atmospheric Gravity Wave Study (WAGS). Both large‐scale and medium‐scale TIDs were observed, and the tristatic configuration of the Doppler network enabled the horizontal phase trace speeds and propagation azimuths of the waves to be calculated. Geomagnetic conditions ranged from quiet to moderately active. The wave parameters are discussed in the context of different geomagnetic activity levels, and the results are compared with those of previous workers. Clear differences exist in the characteristics of the observed TIDs in progressing from the intervals of lowest to highest geomagnetic activity. Under the conditions of highest geomagnetic activity the data are consistent with the presence of aurorally generated large‐scale waves.
Tethered rocket experiment (Charge 2): Initial results on electrodynamics
Authors: Sasaki S. , Oyama K. I. , Kawashima N. , Obayashi T. , Hirao K. , Raitt W. J. , Myers N. B. , Williamson P. R. , Banks P. M. , Sharp W. F.
Published: February 2008

The tethered mother‐daughter rocket experiment (Charge 2) was carried out by a NASA sounding rocket, Black Brant 9, at White Sands Missile Range in New Mexico in December 1985. It was intended to perform a new type of active experiment in space by applying a high voltage between the two payloads, as well as by injecting an electron beam from the tethered rocket system. An insulated conductive wire connecting the two payloads was deployed up to its maximum length of 426 m during the flight. An electron beam from 0.5 to 48 mA at 1 keV was injected from the mother payload. A voltage up to 500 V was applied between the two payloads with and without the beam injection. This paper describes the initial results on the electrodynamic effects induced by the potential difference between the two payloads. Measurements of the v×B electromotive force and the voltage/current characteristics up to 500 V have been explained by a model in which the ion current to the negatively biased payload effectively limited the tether current. Two kinds of VLF waves were observed when the bias voltage was applied between the two payloads; narrow‐band emission at 2–4 kHz and broadband emission up to 15 kHz, depending on the applied voltage. The characteristic features of these emissions suggest that the lower hybrid instability (modified two‐stream instability) driven by the potential difference between the rocket and the ambient plasma was responsible for the wave generation.
Estimation error of spectral parameters of mesosphere‐stratosphere‐troposphere radars obtained by least squares fitting method and its lower bound
Authors: Yamamoto Mamoru , Sato Toru , May Peter T. , Tsuda Toshitaka , Fukao Shoichiro , Kato Susumu
Published: February 2008

We have calculated the estimation error of parameters of echo power spectra observed by mesosphere‐stratosphere‐troposphere (MST) radars by means of computer simulations for least squares fitting and moment methods. The least squares fitting method is shown to be better than the moment method in the region with low signal‐to‐noise ratio (snr), especially for narrow spectra. However, the estimation error of the fitting method at infinite snr is approximately twice that of the moment method. This has been attributed to the nature of the statistical fluctuation of the power spectral density, which shows a χ2 distribution. For both methods at infinite snr we have derived equations which show the accuracy of the estimates versus observation period and spectral width. When we use the fitting method for the data observed with the MU radar (46.5 MHz), the typical errors of the radial wind velocities are 0.7 and 2.0 m s−1 in the stratosphere and in the mesosphere, respectively. By calculating the logarithm of the spectrum with inifite snr and fitting a parabolic curve to it, the error of the Doppler shift has become approximately 20 times smaller than that of the moment method. It has been shown that this is one technique to achieve the theoretical lower bound of the estimates.
Effects of geomagnetic pulsations on the Doppler shift of HF backscatter radar echoes
Authors: Bourdillon A. , Delloue J. , Parent J.
Published: February 2008

The effects of geomagnetic pulsations on the Doppler shifts of HF backscatter radar echoes are studied. Three mechanisms are considered: (1) variation of the phase refractive index under the influence of the varying magnetic component, (2) modulation of the plasma density, and (3) electrodynamic drift of the ionosphere. It is shown that the electrodynamic drift is probably the most efficient mechanism to account for the measured Doppler shifts. In this model the Doppler shift on F propagation modes is sensitive to the eastward electric field of the wave, while E propagation modes are sensitive to the northward and eastward components with weights depending on the reflection height. Pi 2 and Pc 3 effects on the Doppler shift of backscattered echoes are presented in the experimental part. In one example of continuous pulsations it is found that the coherence of ground magnetic variations and HF echo Doppler shifts is high near specific frequencies in the spectra. The coherence of radar measurements in resolution cells about 100 km apart is nearly unity and the cross‐phase spectra show very small phase differences. An example of the effects of Pi 2 pulsations on F and Es propagation modes is presented.
An aerostat‐supported ELF/VLF transmitter
Authors: Field E. C. , Kies L. R. , Bannister P. R. , Ingram R. F. , Hopkins W. D. , Roberts M. A.
Published: February 2008

A demonstration of an aerostat‐supported extremely low frequency/very low frequency (ELF/VLF) transmitting antenna was performed. At ELF the vertical electric dipole (VED) antenna radiated at least 100,000 times more power than would a horizontal electric dipole having the same moment. That efficiency was achieved with an altitude of 12,500 feet (3810 m). Calculations show that the radiated power will increase as the fourth power of aerostat altitude. The tether antenna exhibited a corona onset voltage of 180 kV and was resistant to the degrading effects of ELF corona. Prolonged in‐corona operation is therefore possible. The antenna was continuously tuned, despite changes in height and capacitance caused by the aerostat flight dynamics. The huge 300‐H ELF tuning inductor posed no problem. Enhanced VED moments were achieved at ELF by operation at voltages up to 260 kV, 40% above the corona onset voltage. At VLF the antenna emulated a monopole that had a radiation efficiency greater than 90%. The measured bandwidths were large: 1.5 kHz at 23 kHz and 3.5 kHz at 34 kHz. The antenna height exceeded one‐quarter wavelength at VLF, so the antenna could be tuned capacitively and required relatively low base voltages. At both VLF and ELF the measured fields agreed closely with predictions.
Sheath wave propagation in a magnetoplasma
Authors: Laurin J.‐J. , Morin G. A. , Balmain K. G.
Published: February 2008

A theory is presented for propagation of electromagnetic waves concentrated in the vacuum‐gap sheath region separating a plane conductor from a cold magnetoplasma. The applied static magnetic field is parallel to the conductor and in the direction of wave propagation. The theory predicts propagation at all frequencies between zero and the upper‐hybrid frequency divided by √2. Experimental results are reported in which resonances caused by the wave reflection at the end of a thin cylindrical antenna aligned with the magnetic field are used to determine the sheath‐wave dispersion relation. The effect of the antenna bias, plasma density and antenna diameter on the dispersion relation are studied experimentally and the results are compared with the plane‐surface theory. In spite of the difference between the theoretical and experimental geometries, qualitative agreement is noted with respect to variations in magnetic field, plasma density, sheath thickness and antenna radius. The prediction that the presence of an applied magnetic field raises the upper sheath wave cutoff frequency has been confirmed experimentally, as well as the prediction that the presence of the sheath allows propagation above the cyclotron frequency. In particular, both theory and experiment show the existence near the cyclotron frequency of a special frequency at which the sheath‐wave wave number is independent of the plasma density.
Parametric excitation of long‐wavelength collisional modes
Authors: Stenflo L.
Published: February 2008

The dispersion relation, which describes the nonlinear coupling of a large‐amplitude electromagnetic pump wave with low‐frequency collisional modes in the high‐latitude ionosphere, is derived. In particular, we point out that the nonlinearities in the electron‐neutral collision term are important for the long‐wavelength part of the spectrum. This fact has been overlooked by previous authors.
Natural and man‐made emissions at 1.0–5.6 MHz measured between 10 and 18 RE
Authors: LaBelle J. , Treumann R. A. , Boehm M. H. , Gewecke K.
Published: February 2008

A survey has been undertaken of the 1.0‐ to 5.6‐MHz electric field spectrum measured with the Active Magnetosphere Particle Tracer Explorers/Ion Release Module spacecraft at distances of 15–18 RE(100,000–120,000 km) from the Earth. Data from one full year of operation were considered in order to obtain complete local time coverage at the ecliptic plane. The data were further binned according to the presence or absence of intense auroral kilometric radiation (AKR) emissions at lower frequencies, in order to resolve emissions processes or interference sources that might be related to the level of AKR. For the times of intense AKR, the principal features found in this survey are (1) the occurrence statistics of the intense AKR with local time generally confirm previous AKR surveys using, for example, the ISEE satellite; and (2) the only 1‐ to 5.6‐MHz emissions unique to the times of intense AKR are the occasional occurrences of signals at the first harmonic of the AKR. The latter observation seems likely to be natural rather than artificial, and it is interesting because so few examples of harmonic AKR have previously been published that the effect has been controversial. For the times of weak or absent AKR, the results of this survey may be summarized as follows: for times of roughly 0800–1600 LT (dayside), a nearly constant level was recorded which at 1–3.2 MHz was equal to the galactic background level, and at 3.2–5.6 MHz was above the galactic backgound level and probably was due to spacecraft‐generated noise; at other local times, the same nearly constant level was sometimes recorded, but frequently bursts of noise at 3–5.6 MHz were observed. These bursts seem more likely to represent ionospheric breakthrough of man‐made radiation rather than natural emissions. A comparison with previous satellite observations implies that the terrestrial background level in this frequency range may have increased considerably between 1970 and 1985.
The Chung‐Li VHF radar: Technical layout and a summary of initial results
Authors: Röttger J. , Liu C. H. , Chao J. K. , Chen A. J. , Chu Y. H. , Fu I.‐J. , Huang C. M. , Kiang Y. W. , Kuo F. S. , Lin C. H. , Pan C. J.
Published: February 2008

The particular characteristics of the VHF mesosphere‐stratosphere‐troposphere (MST) radar and wind profiler system, operated at the National Central University in Chung‐Li, Taiwan, Republic of China, are described. These are the dual‐mode applications of the Doppler and the spaced antenna technique, where the latter is used in the conventional mode to determine the wind velocity as well as in the novel interferometer mode. A brief outline of the scientific rationale and the technical layout of this particular configuration of the Chung‐Li VHF radar is given. A generally applicable method for system performance checks by deducing principle radar data parameters before the data analysis has been developed and is briefly described. Receiver system and antenna calibration measurements with celestial radio sources are reported and prove the appropriateness of the antenna system. Radar reflectivity profiles, measured with vertical as well as with off‐vertical antenna beams, are presented and discussed. We also present height‐time‐reflectivity plots for quiet and disturbed weather conditions and reflect on their peculiarities. We show that the wind profiles measured with the conventional Doppler method and the spaced antenna method are in good agreement with radiosonde measurements. The spatial correlation parameters, deduced with the spaced antenna method, are consistent with results from other radars. We also discuss measurement capabilities to deduce the tropopause height with the Chung‐Li VHF radar. We also describe echo power variations due to focusing from reflectivity structures modulated by gravity waves and a brief case study of Kelvin‐Helmholtz instability. We finally outline future research directions which could be performed with the Chung‐Li VHF radar.
A study of the characteristics of VHF radar echo power in the Taiwan area
Authors: Chu Y. H. , Hsu T. S. , Chen L. H. , Chao J. K. , Liu C. H. , Röttger J.
Published: February 2008

In this paper the mean gradient of the generalized potential refractive index, denoted by M, calculated from the meteorological data observed at Pan‐Chiao rawinsonde station and the vertical echo power measured with the Chung‐Li VHF radar are studied. It is found that the ratio of wet term to dry term in M varies from 20 dB at 1 km to −10 dB at 10 km, where 0 dB is occurred at 8 km. This indicates that the magnitude of M is dominated by the contribution of humidity below 8 km. The correlation between echo power and M2 is good. The slope of the regression line relating the echo power and M2 is found to change for different weather conditions. Furthermore, the echo power profiles during the period of typhoon passage are analyzed. The variation of tropopause height and the maximum radar detectable height during the typhoon passage will be studied.
Frequency domain interferometry studies of mesospheric layers at Jicamarca
Authors: Kudeki Erhan , Stitt Gary R.
Published: February 2008

Equatorial mesospheric scattering layer widths and motions were measured using the high‐resolution frequency domain interferometry technique at Jicamarca. Layer widths as small as ∼200 m were identified, as well as oscillatory vertical layer motions with 1–1.5 km peak‐to‐peak amplitudes. Layers are shown to oscillate with the vertical component of simultaneously observed gravity wave perturbation velocities. There is some suggestion in the data that lower mesospheric layers are more horizontally structured and patchy than higher mesospheric layers.
On the role of parametric instability of internal gravity waves in atmospheric radar observations
Authors: Klostermeyer J.
Published: February 2008

In the presence of a monochromatic finite‐amplitude internal gravity wave, many properties of tropospheric and mesospheric radar observations can be explained consistently in terms of parametric instability. At small primary wave amplitudes, the fastest growing instability modes can be divided into two classes. In one class, the instability modes have scales comparable to that of the primary wave leading to a broadening of the wave number spectrum. The second class consists of small‐scale instability modes with a continuous wave number spectrum so that the primary wave can be “seen” by radars even in the absence of conventional instability mechanisms such as shear or static instability. The small‐scale instability modes generated by long‐period primary waves propagate almost vertically and form extended layers moving with the phase velocity of the primary wave. Thus, in accordance with observational results, the signal power of backscattered radar signals is strongest at near vertical antenna beam directions and shows a layered structure. The small‐scale instability modes further satisfy Taylor's frozen turbulence field hypothesis so that the Doppler shift of scattered radar signals yields the space and time dependent fluid velocity of the primary wave. At sufficiently large primary wave amplitudes, there is in particular an isolated fast growing instability mode which has a frequency close to the mean Väisälä‐Brunt frequency and is probably due to shear instability. It may give rise to a cascade of successive instability modes that can explain organized substructures in layers of enhanced tropospheric UHF radar returns and the frequent occurrence of short‐period evanescent waves in mesospheric VHF radar observations which cannot be attributed to Kelvin‐Helmholtz instability of the mean shear flow.
High‐power radiating facility at the HIPAS Observatory
Authors: Wong A. Y. , Carroll J. , Dickman R. , Harrison W. , Huhn W. , Lum B. , McCarrick M. , Santoru J. , Schock C. , Wong G. , Wuerker R. F.
Published: February 2008

UCLA's radio frequency ionospheric heater, 25 miles (40 km) east of Fairbanks, Alaska, is described. The heater consists of eight crossed dipole antennas arranged in a circular pattern to give a gain of 18.4 dB over isotropic at 2.85 MHz (˜ 2nd electron cyclotron harmonic). At 1.2 MW total radiated power the array has a calculated equivalent radiating power of 84 MW. Each antenna is driven by a 150‐KW transmitter, originally from the Platteville heater. The eight transmitter antennas are managed by a personal computer which controls power, modulation, and beam steering. Methods of tuning the antennas, to achieve either right (O mode) or left (X mode) circular polarized radiated beams, are described. The heater is powered by two 1500‐horsepower diesel electric generators. It can be operated throughout the year over −30°C to 40°C ambient temperature extremes. Future improvements include the construction of an even larger 840 × 840 m (24 × 24 antenna) array with a gain of 37.5 dBi.
Continuous measurement of stimulated electromagnetic emission spectra from HF excited ionospheric turbulence
Authors: Armstrong W. T. , Massey R. , Argo P. , Carlos R. , Riggin D. , Cheung P. Y. , McCarrick M. , Stanley J. , Wong A. Y.
Published: February 2008

Stimulated electromagnetic emissions of the auroral ionosphere in the vicinity of Fairbanks, Alaska, have been observed during high‐power HF heating experiments. Asymmetric spectra around the heater frequency with dominant lower sidebands are observed under conditions of very long density scale lengths corresponding to f0F1 transition of the heater frequency. Continuous recording of these spectra indicate their strong dependence on controlled cycling of the HF power density and weak dependence on ionospheric fading of the HF wave. Possible physical mechanisms for generating these spectra include stimulated backscattering of the HF pump wave from parametric instabilities and cavitons.
On linear conversion in a magnetized plasma
Authors: Mjølhus Einar
Published: February 2008

The theory of linear conversion of high‐frequency electromagnetic waves into electrostatic waves in a magnetized plasma is briefly reviewed, with emphasis on its possible role in experiments on the interaction between electromagnetic waves and plasmas, in the ionosphere as well as in laboratories. New results on the transition from magnetized to unmagnetized plasmas are given.
West Antarctica: Problem child of Gondwanaland
Authors: Dalziel Ian W. D. , Elliot David H.
Published: February 2008

The evolution of West (Lesser) Antarctica and its relation to East (Greater) Antarctica have major implications for global plate interactions, paleoclimate, and paleobiogeography, as well as Gondwanaland reconstruction. Analyses of marine geophysical data still lead to seemingly unacceptable overlap between the Antarctic Peninsula and the South American continent or else to geologically questionable relationships. A review of the relevant geological and geophysical data indicates that the problem lies in microplate movement and crustal thinning within West Antarctica during Gondwanaland breakup in the late Mesozoic and Cenozoic. The available data allow a range of possible reconstructions with West Antarctica subdivided into several discrete or semidiscrete microplates. Final solution of this problem requires additional geological and, particularly, geophysical data from West Antarctica as a whole, and the Weddell Sea‐Ross Sea embayment in particular. Meantime, it seems inadvisable to use the present continental outline of Antarctica on the Pacific side of the Transantarctic Mountains in reconstructing Gondwanaland.
Focal mechanisms and locations of earthquakes in the vicinity of the 1975 Kalapana Earthquake aftershock zone 1970–1979: Implications for tectonics of the south flank of Kilauea Volcano, Island of Hawaii
Authors: Crosson R. S. , Endo E. T.
Published: February 2008

The November 29, 1975, Kalapana earthquake of magnitude 7.1 on the south flank of Kilauea Volcano, Hawaii, represents a major rift tectonic event. It is generally understood that the earthquake resulted from movement on a nearly horizontal fault plane, with the crustal block south of Kilauea's east rift zone moving south‐southeastward up to several meters. Well‐recorded earthquakes on the south flank of Kilauea from 1970 to 1979 were analyzed and focal mechanisms determined to obtain a better understanding of the details of south flank tectonics. Included are a large number of earthquakes from the aftershock sequence of the 1975 earthquake. Hypocenters are distributed in a subplanar zone that dips 2° to 3° to the west at a depth of about 8–9 km depth. We interpret this pattern to coincide with a portion of the slip plane of the Kalapana earthquake and to be consistent with the notion of the slip plane occurring in a weak layer at the top of the old oceanic crust. Focal mechanisms are remarkably constant throughout this time period, both before and after the 1975 event, and closely agree with the mainshock mechanism. P axis azimuths and slip vectors for the low‐angle fault planes are normal to the middle and lower east rift zones of Kilauea. There is suggestion of a change in the orientation of P axes from shallower to steeper plunge at the time of the Kalapana earthquake. Such a change is consistent with a general reduction in the horizontal compressional stress normal to the rift zone at the time of the large earthquake. No other evidence of precursory reorientation of the stress field was found and the precise predictability of this event remains in doubt. However, a general model of the tectonics of the volcanic rift system is now available that satisfies a number of geological and geophysical observations.
Tectonic stresses in the lithosphere
Authors: Fleitout Luce , Froidevaux Claude
Published: February 2008

Various types of observables (earthquake focal mechanisms, in situ measurements and geological deformations) give information about the large scale lithospheric stress field. The latter has often been explained by postulating appropriate forces acting at the edges and beneath the plates. This approach ignores the role of mass heterogeneities within the lithosphere. Here we analyze the effect of both boundary and internal forces on the stress pattern and show that both contributions are of comparable magnitude. The presence of internal sources makes the problem three‐dimensional. We show however that it can be reduced to a two‐dimensional plane stress formulation, whereby the edge forces are expressed by the ‘non hydrostatic stresses’ and the basal shear is increased by the addition of a term proportional to the gradient of the mean vertical stress. For the oceanic lithosphere we derive a compression that increases with age. The comparison with geophysical observables yields an upper bound of a few bars on the magniude of the basal drag. For the continents we infer the existence of an underlying upper mantle somewhat denser than under oceans.
Structural analysis of recent and active faults and regional state of stress in the epicentral area of the 1978 Thessaloniki earthquakes (northern Greece)
Authors: Mercier Jacques‐Louis , Carey‐Gailhardis Evelyne , Mouyaris Nicolas , Simeakis Konstantinos , Roundoyannis Theodora , Anghelidhis Christos
Published: February 2008

During spring and summer 1978, Macedonia (Greece) suffered damaging seismic activity. During the two main shocks which occurred on May 23 (Mb = 5.7) and June 20 (Mb = 6.1), seismic open cracks and faults were formed in the epicentral area, 25 km NNE from Thessaloniki. They have been mapped and their kinematics analyzed. These studies have shown that they result from the reactivation of existing faults of Pleistocene, Recent, and historical age and that the crust is stretching along a N‐S to NNE‐SSW direction in that part of the North Aegean. Principal stress directions have been computed from the slip vectors measured on recent neotectonic faults and seismic cracks and faults, this computation utilizing a simple mechanical model (homogeneous deformation through the addition of small and independent displacements of rigid blocks in a highly fractured body). A striking feature of this analysis is that the tensional directions σ3 deduced from seismic faulting and from recent neotectonic faulting are nearly the same. They are compatible with the regional tension axis deduced from focal mechanisms of North Aegean and with the focal mechanisms of the 1978 Thessaloniki main shocks. They are also compatible with in situ stress measurements in this region. Studies of superficial seismic faulting strongly support previous assumptions proposed from aftershock distribution: the 1978 Thessaloniki earthquakes resulted from reactivation of several different faults situated within the same NE dipping fault zone of Upper Eocene‐Lower Oligocene age (and older) separating the Serbo‐Macedonian Massif from the Peonias (East Vardar) zone.
Relationships between plate motions and Late Cretaceous to Paleogene magmatism in southwestern Alaska
Authors: Wallace Wesley K. , Engebretson David C.
Published: February 2008

Four major Late Cretaceous to Paleogene magmatic belts have been recognized in southwestern Alaska, and tentative boundaries have been drawn for each. The belts indicate minimum ages by which the tectonostratigraphic terranes they overprint must have been linked together, though there has been significant postmagmatic tectonism. The Kuskokwim Mountains belt (73–60 m.y. B.P.), the Alaska Range belt (74–55 m,y. B.P.), and the Gulf of Alaska belt (62–57 m.y. B.P.) all may have been related to a period of rapid north‐northeastward motion of the KuIa plate from about 74 to 56 m.y. B.P. The angle of convergence was low with respect to the present orientation of the belts, suggesting the possibility of postmagmatic rotation of the belts. A higher angle of convergence may have existed with respect to the Alaska Range belt, a probable arc, if postmagmatic rotation resulted from 300–400 km of right‐lateral displacement on the Denali fault. The Gulf of Alaska belt is in an anomalous forearc position with respect to the Alaska Range belt, but probably formed by a convergence‐related mechanism. The Kuskokwim Mountains belt is in an apparent back arc position, but the mechanism of its origin is uncertain. It may have been part of an unusually broad Alaska Range arc or, less likely, it may have been a separate arc. Magmatic quiescence occurred throughout south‐western Alaska during a period of plate reorganization from about 56 to 43 m.y. B.P., a period which began with extremely rapid northward motion of the KuIa plate. Magmatism resumed when the present period of rather slow northwestward Pacific plate motion began at about 43 m.y. B.P. The Aleutian arc probably formed at this time, and has been episodically active in about the same location ever since.