bluuebunny/crossref_metadata_2025 huggingface dataset:
Fourier stratigraphy: Spectral gain adjustment of orbital ice mass models as an aid in dating Late Neogene deep‐sea sediments
Authors: Berger W. H. , Jansen E.
Published: February 2004

Orbital tuning has become an important means of dating deep‐sea sediments of Pliocene‐Pleistocene age. One approach is to generate a template simulating ice mass history based on Milankovitch forcing for comparison with a given δ18O record. To obtain a template that closely mimics a given δ18O record, we propose the following steps: (1) generate a preliminary template based on a simple ice mass model, (2) find the Fourier matrix for this model, (3) find the Fourier matrix for the δ18O record to be tuned (if necessary, adjust preliminary sedimentation rate so that maxima appear at the correct spectral lines), (4) import the spectral power distribution from the matrix of the record into the matrix of the template (while preserving phase of template), and (5) generate the new template from the gain‐adjusted template matrix. The record should now show optimum coherency with the template over the appropriate interval. The efficiency of this method (“Fourier matrix conjugation”) in producing hybrid templates is illustrated using four examples: (1) testing the stacked record of Prell et al. (1986) for constant sedimentation rate, (2) testing the date of 790 ka of the Brunhes/Matuyama boundary in Ontong Java Plateau Ocean Drilling Program Hole 805C (Berger et al., 1993a), (3) testing the age scale for a core from the Norwegian Sea containing meltwater events (Jansen, 1989), and (4) checking the age model of Ocean Drilling Program Site 677 (Shackleton et al., 1990) for internal consistency. The template shows just where the given age models have optimum coherency with the selected orbital model and where in the record discrepancies arise.
How synchronous are neogene marine plankton events?
Authors: Spencer‐Cervato Cinzia , Thierstein Hans R. , Lazarus David B. , Beckmann Jean‐Pierre
Published: February 2004

An electronic supplement of this material may be obtained on adiskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN toAGU's FTP account using ANONYMOUS as the username andGUEST as the password. Go to the right directory by typing CDAPEND. Type LS to see what files are available. Type GET and thename of the file to get it. Finally, type EXIT to leave the system.)(Paper 94PA01456, How synchronous are Neogene marine planktonevents?, by C. Spencer‐Cervato, H. R. Thierstein, D. B. Lazarus, andJ‐P Beckmann). Diskette may be ordered from American GeophysicalUnion, 2000 Florida Avenue, N.W., Washington, DC 20009; $15.00.Payment must accompany order. We analyzed the synchrony and diachrony of commonly used Neogene biostratigraphic events from data published in the Initial Reports of the Deep Sea Drilling Project (DSDP) and in the Proceedings of the Ocean Drilling Program (ODP). On the basis of the combined biostratigraphic and magnetostratigraphic evidence, new Neogene age models were constructed for 35 globally distributed DSDP and ODP holes. Biostratigraphic events from the four major plankton groups (calcareous nannofossils, diatoms, planktonic foraminifera, and radiolarians) were compiled from DSDP and ODP reports. After the elimination of possible sources of error such as stratigraphic hiatuses and reworking of specimens, 124 biostratigraphic events that occurred in at least four holes were analyzed in detail: for each event a biochronologic age estimate was derived by projection of the depth of the event onto the line of correlation of each hole, and from these a global mean age for each event was calculated, together with its standard deviation. Average standard deviations for event ages by fossil group are: calcareous nannofossil first appearance datums (FADs): 0.57 m.y. (21 events), calcareous nannofossil last appearance datums (LADs): 0.60 m.y. (25 events), diatom FADs: 0.57 m.y. (7 events), diatom LADs: 0.85 m.y. (14 events), planktonic foraminifera FADs: 0.88 m.y. (22 events), foraminifera LADs: 0.68 m.y. (16 events), radiolarian FADs: 0.30 m.y. (9 events), radiolarian LADs: 0.31 m.y. (10 events). Since the average sample spacing in the sites used for this analysis is only 0.185 m.y., we have examined the data for true patterns of diachrony and for other biases. Diachrony is more frequent among cosmopolitan than among endemic taxa, thus there is a general trade‐off between the obtainable age precision and the geographic extent of a bioevent. Precision of age calibrations also decreases with increasing age. Although some of these features may be due to investigator bias, they appear in part to be real phenomena, and thus could also provide opportunities for further exploration of important paleobiological processes, such as change in environmental gradients through time, evolutionary adaptation of species populations and migration due to water mass changes.
Mudwaves on the Gardar sediment drift, NE Atlantic
Authors: Manley Patricia L. , Caress David W.
Published: February 2004

Small‐amplitude (<15 m) mudwaves were mapped on the Gardar Sediment Drift using the Hydrosweep DS multibeam echosounder system on the R/VMaurice Ewing. Improved data processing techniques allowed the swath mapped bathymetry to image features smaller than the nominal resolution (∼14 m) of Hydrosweep. Mudwave heights, wavelengths, and orientations were determined from the Hydrosweep data and in conjunction with 3.5‐kHz profiles, these mudwave fields were analyzed with regard to present models of mudwave formation. Mudwave crest orientation was clockwise to mean bottom flow direction and migration was to the right of measured currents for two areas on the drift. This is consistent with models developed on large‐amplitude mudwaves. Using mudwave orientation and migration direction in a third area, bottom current directions were inferred.
Origin of detrital fluxes in the southeast Indian Ocean during the last climatic cycles
Authors: Bareille Gilles , Grousset Francis E. , Labracherie Monique , Labeyrie Laurent D. , Petit Jean‐Robert
Published: February 2004

Because of a close relationship between detrital flux variations and magnetic susceptibility (MS) flux (MS cm−3 of bulk sediment multiplied by the linear sedimentation rate) variations in the southeast Indian basin of the southern ocean, MS flux profiles have been used to examine the spatial and temporal detrital flux changes in this basin during the last climatic cycle. Results indicate a general increase in detrital material input during the coldest periods, suggesting a widespread phenomenon, at least on the basin scale. Mineralogical data, geochemical data, and 87Sr/86Sr isotopic ratios have been used to determine the origin and transport mechanisms responsible for increased detrital flux during glacial periods. Mineralogical and geochemical data show that these glacial “highs” are due to increases in both Kerguelen‐Crozet volcanic and Antarctic detrital inputs. The 87Sr/86Sr isotopic composition of the >45‐µm fraction indicates that the Kerguelen‐Crozet province contributes to at least 50% of the coarse particule input to the west. This contribution decreases eastward to reach less than 10%. These tracers clearly indicate that the Crozet‐Kerguelen province was a major source region of detrital in the western part of the basin during glacial times. In contrast, material of Antarctic origin is well represented in the whole basin (fine and coarse fractions). Because of the minor amount of coarse particles in the sediments, volcanic particles from Kerguelen and crustal particles from Antarctica have most probably been transported by the Antarctic bottom water current and/or the Circumpolar deepwater current during glacial periods as is the case today. Nevertheless, the presence of coarse particles even in low amount suggests also a transport by ice rafting (sea‐ice and icebergs), originated from both Kerguelen and Antarctic sources. However, the relative importance of both hydrographic and ice‐rafting modes of transport cannot be identified accurately with our data. During low sea level stands (glacial maximum periods), increasing instability and erosion of the continental platform and shallow plateaus could have resulted in a more efficient transfer of crustal and volcano‐detrital material to the Southeast Indian basin. At the same time, extension of the grounded ice shelves over the continental margins and increase in the erosion rate of the Antarctic ice sheet could have induced a greater input of ice rafted detritus (IRD) to southern ocean basins. Enhancement of the circumpolar deepwater current strength might have also carried a more important flux of detrital material from Kerguelen. However, an increase in the bottom water flow is not necessarily required.
Extending the climatic precession curve back into the Late Miocene by signature template comparison
Authors: Benson Richard H. , Hayek Lee‐Ann C. , Hodell David A. , Rakic‐El Bied Kruna
Published: February 2004

The rhythm of sedimentary cycles reflecting the climatic precession signal (19 and 23 kyr components) from the upper Miocene of the Bou Regreg section at Ain el Beida near Rabat, Morocco, had been analyzed to determine the age of the geomagnetic polarity reversals of Subchron 5N1 (C3An.1n). A new method of analysis of cycle “signatures” uses adjusted data series as represented by a signal obtained from grey‐level traces through image‐enhanced photographs of the sediment cycles. The measured, “timeless” signal is reduced and geometrically transformed to a signature template in order to compare its pattern with similar time constrained segments of the target precession signal (retrodicted for 35° N). Conventional statistical and spectral analysis tests are used to regress to the best possible fit. A date estimate of 5.94 Ma for the Chron 5/Gilbert boundary (C3An.1n/C3r), which occurs near the times of the closure of the Rifian Corridor and the beginning of the “drawdown” phase of the Messinian Salinity Crisis, is found to be in close agreement with independent approximations derived from extrapolation of sea floor spreading, radiometric dating, and younger Earth‐orbital tuning ages.
A diagnostic analysis of Heinrich glacial surge events
Authors: Verbitsky Mikhail , Saltzman Barry
Published: February 2004

The episodes of ice‐rafted detritus (IRD) accumulation identified in North Atlantic sediment cores indicate periods of abnormal iceberg production mainly of the Laurentide Ice Sheet. In this paper we develop further a scaling theory of these so‐called Heinrich events, leading to a general criterion for basal melting of an ice sheet which we take as a necessary condition for surging and streaming. This criterion depends not only on the physical properties of ice sheets and the magnitude of the geothermal and viscous heating but also on the history of changes in ice elevation, atmospheric carbon dioxide, and sea surface temperature, the thermal effects of which at the top of an ice sheet are transmitted with a time delay to the basal boundary by thermal advection. We apply this criterion to various past and present ice sheets showing their general vulnerability to surging. In greater detail, we use estimates of the time variations of ice volume (SPECMAP), carbon dioxide (Vostok), and North Atlantic sea surface temperature (site K708‐1) to estimate when basal melting conditions prevailed for the Laurentide Ice Sheet over the past 140,000 years, thereby making it vulnerable to Heinrich‐type surges. A reasonable level of agreement is obtained with the observed record of North Atlantic IRD activity, including the six numbered Heinrich events as well as earlier activity of this kind that occurred when the ice volume was lower.
Cadmium/calcium and carbon isotope reconstructions of the glacial northeast Atlantic Ocean
Authors: Bertram Caroline J. , Elderfield Henry , Shackleton Nicholas J. , MacDonald Jannette A.
Published: February 2004

The δ13C and Cd measurements from benthic foraminifera from Biogeochemical Ocean Flux Study (BOFS) northeast Atlantic Ocean sediment cores are presented. The δ13C values in glacial foraminifera are consistent with those from elsewhere in the North Atlantic Ocean. For intermediate water (1000–2000 m water depth), δ13C values were higher at the last glacial maximum than in present North Atlantic Deep Water (NADW), whereas for deep water (>2000 m) they were lower during the glacial maximum. The Cd concentrations of glacial northeast Atlantic intermediate water were lower than those of present NADW. However, deepwater Cd concentrations increased to values between NADW and present Pacific Deep Water (PDW). The δ13C and Cd data are consistent and show that the northeast Atlantic Ocean was strongly stratified with 13C enriched, low Cd intermediate water overlying 13C depleted, high Cd deep water. The glacial water column comprised two different water masses: deep water, similar in character to present Antarctic Bottom Water (AABW), and intermediate water, different in character from both AABW and NADW, and any present intermediate‐depth North Atlantic water. The characteristics of glacial intermediate water were, however, similar to present near‐surface waters in the North Atlantic, which suggests rapid ventilation of the glacial ocean to depths of up to 2000 m by cold, nutrient‐depleted young surface waters.
Late Glacial and Holocene palaeocurrents around Rockall Bank, NE Atlantic Ocean
Authors: Manighetti B. , McCave I. N.
Published: February 2004

Core samples and hydrographic data from 50° to 60°N and 15° to 25°W in the Northeast Atlantic Ocean are used to study changes in current activity from the last glacial to present. We establish a new method to distinguish between the effects of changing bottom current speed and varying size input by drawing upon sediment flux data and detailed grain size analysis. Changes in current speed are recorded by the mean size of the sortable (coarse) noncarbonate silt component, which increases with current vigor by winnowing away of fines. Our method involves the definition of an ideal sortable silt “input function”, recorded at a site accumulating unmodified pelagic flux only. The ideal profile is then compared with records from other sites to determine the history of current speed at a range of water depths. The upper surface of Southern Source Water (SSW) probably shoaled during the glacial, as indicated by the covariation of sortable silt records from sites presently covered by SSW and Lower North Atlantic Deep Water (LNADW). The data suggest that production of LNADW was suppressed during the glacial, increased following the last glacial maximum, and declined at the start of Termination 1A. A second pulse of production occurred immediately before the Younger Dryas event. Intermediate waters were generally faster‐flowing during the glacial and may have expanded their depth domain, such that a single glacial intermediate water mass covered depths from 1100 to more than 2000 m.
Climatically linked carbon isotope variation during the past 430,000 years in Southern Ocean sediments
Authors: Singer Alex J. , Shemesh Aldo
Published: February 2004

We use the isotopic composition of carbon from organic matter enclosed within diatom frustules as a proxy for paleoproductivity and paleo‐dissolved carbon dioxide concentrations in ocean surface waters. A Southern Ocean record from south of the Antarctic Polar Front and spanning 430,000 years of carbon isotopic variation in diatomaceous organic matter is presented for the first time. The most refractory diatomaceous organic matter fraction was extracted and analyzed to avoid problems associated with diagenesis. The results clearly indicate cyclic changes in organic carbon isotopic ratios, with 13C depleted values associated with all of the last five glacial periods, reflecting changes in surface water properties and primary productivity. Changes in dissolved carbon dioxide concentrations are the most probable cause of these cycles, but the possible effect of seawater pH changes cannot be excluded.
Sensitivity of the phosphate‐cadmium‐carbon isotope relation in the ocean to cadmium removal by suboxic sediments
Authors: van Geen Alexander , McCorkle Daniel C. , Klinkhammer Gary P.
Published: February 2004

Reconstructions of past deep ocean nutrient concentrations rely largely on the carbon isotopic composition and cadmium content of shells of benthic foraminifera. When comparing records of these two paleonutrient tracers, a relation similar to the phosphate‐cadmium‐carbon isotope relation in the water column today is generally assumed. We present new water column, sediment, and pore water data from the California continental margin which demonstrate that Cd accumulation is enhanced in suboxic sediments. These results suggest that the oceanic Cd inventory is sensitive to changes in the areal extent and redox intensity of suboxic sediments and thus that the phosphate‐cadmium relation in the ocean could have been different in the past. The magnitude of this effect and its evolution through time is presently hard to quantify because the oceanic Cd budget is poorly constrained.
Grain size distribution and depositional processes of the mineral component of abyssal sediments: Lessons from the North Pacific
Authors: Rea David K. , Hovan Steven A.
Published: February 2004

Grain size analyses of the mineral component of abyssal Pacific surface sediments show two distinctly different size distributions that can be associated with sediments that are dominantly eolian and dominantly hemipelagic, respectively. In the range of 1 to 30 µm, eolian grains show a distinct size mode at about 2 µm (9Φ) and little material coarser than about 16 µm (6Φ). Hemipelagic grains are characterized by a much flatter size distribution that has a broad size mode near 2 µm (9Φ) and important amounts of material coarser than 16 µm (6Φ). This ability to distinguish between hemipelagic and eolian grains will obviate some long‐standing concerns in the interpretation of deep‐sea sediments. Examples of the utility of this technique are given for the North Pacific and South Atlantic.
The Earthquake Prediction Experiment at Parkfield, California
Authors: Roeloffs Evelyn , Langbein John
Published: February 2004

Since 1985, a focused earthquake prediction experiment has been in progress along the San Andreas fault near the town of Parkfield in central California. Parkfield has experienced six moderate earthquakes since 1857 at average intervals of 22 years, the most recent a magnitude 6 event in 1966. The probability of another moderate earthquake soon appears high, but studies assigning it a 95% chance of occurring before 1993 now appear to have been over‐simplified. The identification of a Parkfield fault “segment” was initially based on geometric features in the surface trace of the San Andreas fault, but more recent microearthquake studies have demonstrated that those features do not extend to seismogenic depths. On the other hand, geodetic measurements are consistent with the existence of a “locked” patch on the fault beneath Parkfield that has presently accumulated a slip deficit equal to the slip in the 1966 earthquake. A magnitude 4.7 earthquake in October 1992 brought the Parkfield experiment to its highest level of alert, with a 72‐hour public warning that there was a 37% chance of a magnitude 6 event. However, this warning proved to be a false alarm. Most data collected at Parkfield indicate that strain is accumulating at a constant rate on this part of the San Andreas fault, but some interesting departures from this behavior have been recorded. Here we outline the scientific arguments bearing on when the next Parkfield earthquake is likely to occur and summarize geophysical observations to date.
Approximate integral‐operator methods for estimating the natural frequencies of coupled objects
Authors: Ross John E. , Rothwell Edward J. , Nyquist Dennis P. , Chen Kun‐Mu
Published: February 2004

Of central importance to the singularity expansion method (SEM) for analysis of conducting objects are the frequencies and currents for the natural modes of the object. When there is more than one object in proximity, the direct calculation of these quantities becomes difficult, and it is useful to have approximate methods to simplify the computations. This paper presents two approximate methods for calculating the natural frequencies for a group of coupled objects. The first method is a perturbational one, leading to a transcendental equation for the natural frequencies. It assumes weak coupling between objects so that the natural mode current distribution is only slightly perturbed from that existing on an isolated object. The second method is also perturbational and further assumes that the Green's function can be approximated by leading terms of a Taylor series expansion about the natural‐mode frequencies of isolated objects Results are shown for the case of coupled thin wires and compared to more rigorous moment‐method solutions as well as experimental results.
Model for the prediction of the differential rain attenuation between a terrestrial line‐of‐sight radio relay system and a low‐angle satellite path based on the two‐dimensional lognormal distribution
Authors: Kanellopoulos J. D. , Kossidas C.
Published: February 2004

The main propagation effect on interference of a terrestrial line‐of‐sight radio relay system from a low‐angle satellite path is the differential rain attenuation. In the present paper, a predictive method for the differential rain attenuation statistics is proposed which is based on a modified model convective raincell structure by taking into account the conventional limits of the raincell size. The assumption that the point rainfall statistics follows a lognormal form is also adopted. The results of the predictive procedure are applied to a particular problem of terrestrial line‐of‐sight system interference from a low‐angle broadcasting satellite service in the 22.5–23 GHz band. The sensitivity of the various parameters affecting the differential attenuation is also examined. As a general remark, the conditional differential rain probability is found to be significantly dependent upon the rainfall characteristics of the climatic zone and the parameters of the spatial rainfall structure. Finally, a comparison of the present predictive procedure with a semiempirical developed model is treated and gives a quite reasonable agreement.
Practical strategies for the solution of large‐scale electromagnetic inverse problems
Authors: Oldenburg Douglas W.
Published: February 2004

Nonlinear inverse problems in electromagnetics are typically solved by dividing the Earth into cells of constant conductivity, linearizing the equations about a current model, computing the sensitivities, and then solving an optimization problem to obtain an updated estimate of the conductivity. In principle, this procedure can be implemented for any size problem, but in practice the computations involved may be too large for the available computing hardware. In electromagnetics this is currently the situation irrespective of whether the interpreter has access to a workstation or a supercomputer. In addition to the demands imposed by the need to compute the predicted responses from a specified model (i.e., invoking a forward mapping) there are two computational roadblocks encountered when solving an inverse problem: (1) calculation of the sensitivity matrix and (2) solution of the resultant large system of equations. If either of these operations cannot be carried out in reasonable time then an alternate strategy is required. Such strategies include generalized subspace methods, conjugate gradient methods, or approximate inverse mapping (AIM) procedures. The theoretical foundations and computational details of these strategies are explored in this paper with the ultimate goal that the inversionist, after assessing his/her computing power and knowing the time required to perform forward modeling, can generate a methodology by which to solve the problem. The methodologies are compared quantitatively by considering an archetypal inversion problem in electromagnetics, the inversion of dc potential data to recover the electrical conductivity.
A Gibbs random field‐based active electromagnetic method for noninvasive diagnostics in biomedical applications
Authors: Caorsi S. , Gragnani G. L. , Medicina S. , Pastorino M. , Pinto G. A.
Published: February 2004

A new approach to the reconstruction of dielectric properties of unknown biological bodies is proposed, which is developed starting from an integral equation formulation of the electromagnetic inverse scattering problem. A Gibbs Random Field model is used to develop a stochastic approach based on the Bayes theorem. An iterative solution is obtained by the simulated annealing technique. The method integrates the important features of the spatial domain method with the possibility of taking into account a priori information on the scatterers under test. No large ill‐conditioned matrix inversions are required. A two‐dimensional formulation is considered, involving cylindrical scatterers illuminated by transverse magnetic‐wave incident fields. A multi‐illumination multiview reconstruction process is also performed. Some results on the reconstruction of strong scatterers are reported.
Some numerical methods for dosimetry: Extremely low frequencies to microwave frequencies
Authors: Gandhi Om P.
Published: February 2004

This paper describes some of the numerical methods that have been developed for calculations of induced electric fields, current densities, and specific absorption rates for anatomically based heterogeneous models of the human body with increasingly finer resolutions. These methods, namely, the impedance method and the finite difference time domain (FDTD) method, have been used for dosimetric calculations for a number of bioelectromagnetic problems for whole‐body or partial‐body exposures, for far‐field or near‐field sources, and for CW or transient fields. The paper gives detailed calculations for some recent applications such as currents induced in the user's body by the electromagnetic fields (emfs) of electric blankets using the impedance method, coupling of an ultrawideband pulse using the frequency‐dependent FDTD method incorporating dispersive properties of the various tissues, and specific absorption rate distributions in the head for emfs of cellular telephones. Because of accurate modeling of tissue heterogeneities and shapes, these methods are likely to play an increasing role in emerging technologies with bioelectromagnetic concerns.
Electroporation in cells and tissues: A biophysical phenomenon due to electromagnetic fields
Authors: Weaver James C.
Published: February 2004

The effect of “strong” electromagnetic fields on cells and tissue can be dramatic but not necessarily harmful. The essentially universal biophysical phenomenon of “electroporation” occurs if an applied field causes the cell transmembrane voltage to reach about 0.5–1 V in a time of microseconds to milliseconds. Ordinarily the cell membrane is a formidable barrier to the transport of ions and charged molecules. However, electroporation results in a large increase in transmembrane conductance, which is believed to be caused by ion transport through temporary membrane openings (“pores”). This high‐conductance state limits the transmembrane voltage and thereby protects the membrane. A large increase in molecular transport generally occurs for the same conditions and allows polar molecules to be introduced into cells. Similar enhanced molecular transport can be caused in living tissues. Not only cell membranes, but also cell layers or even the stratum corneum of human skin can be temporarily altered by the electrical creation of aqueous pathways. The mechanism of electroporation is partially understood, in that the electrical and mechanical behavior of artificial planar bilayer membranes can be described quantitatively by a theoretical model based on transient aqueous pores. More complex behavior in cell membranes may be due to both the complicated shapes of cell membranes and the additional participation of metastable pores and interactions with cell structures. In the case of tissues the situation is even more complex and has only recently begun to be studied but has the prospect of providing a new approach to transporting polar molecules across tissue barriers.
An extension of the physical theory of diffraction concept for aperture radiation problems
Authors: Altintaş Ayhan , Büyükdura O. Merih , Pathak Prabhakar H.
Published: February 2004

A correction to the Kirchhoff‐Huygens approximation in the format of a diffraction coefficient is derived for an aperture terminated by a half plane. As in the physical theory of diffraction (PTD), this is achieved by considering the end point contribution to the aperture integral. It is seen that when the aperture is taken as conformal with the surface of the half plane, the conventional PTD result is obtained.
Comparison of homogeneous and heterogeneous tissue models for coil optimization in neural stimulation
Authors: D'Inzeo G. , Esselle K. P. , Pisa S. , Stuchly M. A.
Published: February 2004

In stimulation of peripheral nerves by strong pulsed magnetic fields, it is desirable to produce a well‐focused activating function. Previous analyses and an experiment have indicated that quadruple coils provide many advantages. An analytical solution has been obtained for a homogeneous cylindrical volume conductor, and a finite difference numerical solution for a realistic heterogeneous model of a human forearm. In both models it has been assumed that nerves are straight and long, and therefore the activating function is an induced field derivative in the neuron direction. The magnitudes of the activating function for various coil geometries are compared for the two models.
Attenuation and scintillation of radio waves in the Earth's atmosphere from radio occultation experiments on satellite‐to‐satellite links
Authors: Yakovlev O. I. , Matyugov S. S. , Vilkov I. A.
Published: February 2004

A theoretical analysis of refractive loss of radio waves by the Earth's atmosphere in radio occultation measurements along the satellite‐to‐satellite line for various altitude profiles of the refractive index is given. Experimental results for refractive loss on the orbital spacecraft ‐ geostationary satellite link are presented. Theoretical calculations are compared with experimental data, and a conclusion is drawn that the signal amplitude during radio occultation is strongly dependent on the layered structure of the refractive index profile. Amplitude scintillations of centimeter (λ1 = 2 cm ) and decimeter (λ2 = 32 cm ) radio waves used in radio occultation experiments are described. Dependences of the rms value of the amplitude scintillations versus the minimum altitude of the ray line for the two above wavelength bands are presented. The frequency spectra of the log‐amplitude scintillations are analyzed together with the dependence of the rms amplitude on the wavelength. Experimental data are compared to the theory of scintillations in a turbulent atmosphere, and the altitude model of the structure constant of refractivity fluctuations is determined.
A full finite difference time domain implementation for radio wave propagation in a plasma
Authors: Young J. L.
Published: February 2004

A full finite difference time domain methodology is developed for electromagnetic wave propagation in a plasma. The finite difference grid is consistent with central difference approximation of the curl, divergence and gradient operators that appear in the joint equations of Euler and Maxwell, and the coupling effects between the fluid velocity and the electric field. To accomplish the time advancement, the central difference approximation is invoked for the time derivatives and leapfrog concepts are employed. The resulting difference equations converge to the exact equations, provided that the developed stability requirement is satisfied. Finally, numerical results are provided and compared with the inverse fast Fourier transform results of closed‐form, frequency domain solutions for the half space problem; the agreement between solutions is shown to be excellent.
Radiotelephone with reduced electromagnetic field in human head
Authors: King Ronold W. P.
Published: February 2004

The quarter‐wave monopole base driven over a circular ground plane with a finite radius has applications in over‐the‐horizon radar and on surveillance aircraft. A new use, for which the analysis is given in this paper, is as an over‐the‐head‐mounted antenna for cellular telephones. With this design, the electromagnetic field in the head and the associated specific absorption rate of electromagnetic energy are greatly reduced when compared with the conventional hand‐held transceiver. A complete analysis is carried out of the electromagnetic field on the surface of the head and throughout its interior when the head is modeled as a cylinder with the electrical properties of the brain enclosed in a wall with the thickness and electrical properties of the skull. Graphs and tables are provided that give the field in the air on the surface of the head and in the skull and brain. The far field is also determined. The results are compared with those obtained with the hand‐held radiotelephone (King, 1995).
On the response of polarimetric synthetic aperture radar signatures at 24‐cm wavelength to sea ice thickness in Arctic leads
Authors: Winebrenner D. P. , Farmer L. D. , Joughin I. R.
Published: February 2004

We investigate observationally and theoretically the response of polarimetric backscattering at 24‐cm wavelength to the thickness of Arctic sea ice in leads and first‐year ice features. We employ backscattering data acquired by the Jet Propulsion Laboratory airborne synthetic aperture radar (SAR) during March 1988 in the Beaufort Sea, together with nearly simultaneous passive microwave imagery acquired by the U.S. Navy Ka band radiometric mapping system. We find that 24‐cm copolar ratios and copolar phases vary strongly with apparent ice thickness. We observe copolar phase shifts between −10° and −50° (relative to multiyear ice phases) for new ice features in the imagery, as well as positive copolar phases in a first‐year ice feature. Copolar ratios also vary with apparent thickness, from values larger than those expected theoretically for seawater to values slightly lower than those expected for thick ice. We derive a signature model based on scattering from a rough air/sea ice interface with realistic vertical profiles of brine volume and relative permittivity beneath. Model predictions for copolar ratios and phases show ice thickness‐dependent variations consistent with those observed. We present simulation results showing that plausible ice thickness variations between pixels in a multilook average diminish, but do not eliminate, the signature response to thickness. This suggests that direct thickness estimation of sea ice in leads may be possible using polarimetric SAR at wavelengths of 24 cm or longer.
On radar interferometric techniques in the situation of volume scatter
Authors: Briggs B. H.
Published: February 2004

A volume scatter situation is assumed, and results expected from radar interferometric techniques are derived. It is shown that fictitious scattering points will appear, with the property that the points for different Doppler frequencies lie along a straight line. This line is not generally in the direction of the wind; only if the scatter is isotropic in azimuth will the line coincide with the wind direction. In the presence of turbulence the effective scattering points still lie along a straight line. However, the scattering point for a particular Doppler frequency will be closer to the zenith than it would be in the absence of turbulence. The results apply if smoothed cross spectra are used. If the cross spectra are not smoothed, the expected effects are harder to predict.
Closed form solutions to Green's functions in microstrip problems with a thin substrate
Authors: Hoorfar Ahmad , Chang David C.
Published: February 2004

A thin substrate approximation is used to derive closed form solutions for the Green's functions Ge and Gm appearing in the mixed potential integral equation modeling of planar microstrip structures. The expression for Gm is given in terms of the contributions from a source and its image, while that of Ge is derived in terms of the so‐called quasi‐dynamic contribution plus a “correction” term. Excellent agreements with the exact numerical integration of the corresponding Sommerfeld integrals for a wide range of parameters are obtained. In particular, it is demonstrated that the addition of the correction term is needed in order to accurately predict the sharp dip and its associated phase reversal that appear in the magnitude of Ge in the near field.
VHF radar sea scatter and propagation at grazing angles less than 1°
Authors: Ott R. H. , Pollock M. A. , Dinger R. J. , Tice T. E.
Published: February 2004

Measurements of low grazing angle radar sea backscatter and multipath in the frequency range from 30 to 300 MHz, along with the supporting theoretical modeling, have been lacking because of the practical difficulties associated with fielding radars in this band. An ultrawideband (UWB) radar that radiates waveforms encompassing this band has recently been installed at a seaside location near the Naval Research and Development facility, Pt. Loma, San Diego, California. Propagation path loss measurements were performed at 40, 90, 140, and 190 MHz. Broadband clutter measurements were performed over the frequency band 200 to 300 MHz. The theoretical model is based on an integral equation solution for the surface fields. The surface fields are obtained using a new mathematical derivation of the integral equation for the surface fields (Ott, 1992). This new integral equation extends the frequency range of the former algorithm, WAGNER, to the VHF band. The surface fields are then integrated to obtain either the forward scatter (multipath) or the backscatter (clutter).
A characteristic mode solution of the parallel plate‐fed slot antenna
Authors: Kabalan Karim Y. , El‐Hajj Ali
Published: February 2004

In this paper the characteristic mode theory is used for the computation of the receive and transmit mode aperture field of the parallel plate‐fed antenna. In the transmit case the input region has the form of a waveguide carrying a single mode wave with electromagnetic characteristics that are different from those of the output region. The output region is considered to be free space. In the receive case the structure is excited by an incident wave which can be either a transverse electric (TE) or a transverse magnetic (TM) plane wave.
A metal strip grating dihedral for polarimetric calibration
Authors: Fujita Masaharu , Masuda Toshihiro
Published: February 2004

This paper deals with a metal strip grating (MSG) dihedral as a polarization selective target for polarimetric SAR calibration. An experiment was carried out for the radar cross section (RCS) characteristics for polarizations both parallel and perpendicular to the gratings and for polarization isolation ratio (PIR) characteristics by using a prototype MSG dihedral manufactured in our laboratory. Although the prototype includes many manufacturing errors in the grating and dihedral structures, the RCS decrease was only 1.1 dB relative to the theoretical prediction, and a PIR of 44.9 dB was attained. The results show promising characteristics of an MSG dihedral as a target for polarimetric calibration.
General formulation for the electromagnetic response of a source within a concentrically layered region
Authors: Wait James R.
Published: February 2004

A matrix formulation is performed for a linear current‐carrying source located within a cased borehole valid for any frequency. The surrounding formation could be any number of concentric homogeneous regions. It is shown that induced polarization phenomena at the casing‐formation interface can also be modeled as a limiting special case.
Electromagnetic scattering and depolarization across rough surfaces: Full wave analysis
Authors: Bahar Ezekiel , Huang Guorong , Lee Bom Son
Published: February 2004

Full wave solutions are derived for vertically and horizontally polarized waves diffusely scattered across an interface that is two‐dimensionally rough separating two different propagating media. Since the normal to the rough surface is not restricted to the reference plane of incidence, the waves are depolarized upon scattering; and the single scattered radiation fields are expressed as integrals of a surface element transmission scattering matrix that also accounts for coupling between the vertically and horizontally polarized waves. The integrations are over the rough surface area as well as the complete two‐dimensional wave spectra of the radiation fields. The full wave solutions satisfy the duality and reciprocity relationships in electromagnetic theory, and the surface element scattering matrix is invariant to coordinate transformations. It is shown that in the high‐frequency limit the full wave solutions reduce to the physical optics solutions, while in the low‐frequency limit (for small mean square heights and slopes) the full wave solutions reduce to Rice's (1951) small perturbation solutions. Thus, the full wave solution accounts for specular point scattering as well as diffuse, Bragg‐type scattering in a unified, self‐consistent manner. It is therefore not necessary to use hybrid, perturbation and physical optics approaches (based on two‐scale models of composite surfaces with large and small roughness scales) to determine the like‐ and cross‐polarized fields scattered across the rough surface.
Spatial diversity in meteor scatter radio links
Authors: Merrill Steven C. , Desourdis Robert I.
Published: February 2004

Meteor burst communication links rely on radiowave scattering from ionized trails left in the wake of meteors. Because of the significant path loss, on the order of 50 dB, associated with the trail‐scattering process, many means have been sought to improve link performance. Adaptive beam control algorithms and variable burst rate modems have been developed to achieve better use of the meteor burst (MB) channel. In addition to these techniques it is the contention of this paper that link performance may be further improved by making use of the spatial diversity inherent in meteor trails. Given the limited geographic extent of trail‐scattered signal footprints, adequately spaced receive sites can use different propagation paths associated with independent meteor trails from a single transmit site. The assumption here is that these receive sites are linked by a local communications network. The effective performance from the transmit site to any of the receive sites is significantly improved. Of course, this improvement is a function of receive site spacing and the performance of the short‐range network. This paper describes the meteor scatter phenomena that impact spatial diversity, details an approach for predicting the associated link performance improvement, and presents a sensitivity study based on computer model predictions.
Delay, Doppler, and amplitude characteristics of HF signals received over a 1300‐km transauroral sky wave channel
Authors: Wagner L. S. , Goldstein J. A. , Rupar M. A. , Kennedy E. J.
Published: February 2004

Channel probe observations of propagation conditions along a 1294‐km transauroral path between Sondrestrom, Greenland, and Keflavik, Iceland, were made during the period from March 13 to April 2, 1992. The midpoint of this path was located at a corrected geomagnetic latitude of 72°. The objective of these measurements was to supplement the existing data base describing propagation conditions on the HF transauroral channel with data pertaining to a period around the time of solar maximum. Received signals for this path fell into three distinct groups depending on their amplitude and delay and Doppler spread characteristics. These are (1) strong, specularly reflected ionospheric returns characteristic of a quiescent daytime ionospheric channel during magnetically quiet conditions; (2) strong specular multipath signals reflected from horizontal gradients of electron density and regularly encountered at night; and (3) weak scatter returns that are also a persistent nighttime phenomenon. The scatter returns are usually observed at delays exceeding those anticipated for the one‐hop return and, very often, at frequencies that are well above the MUF for the great circle propagation path. The multipath and scatter returns exhibit large delay and Doppler spreads indicative of spatially extensive distributions of drifting and randomly moving irregularities. Two measurement events are discussed which illustrate these conclusions: a noontime measurement with Kp = 3, and a midnight measurement with Kp = 2. The noontime measurement exhibited a scatter return from an isolated irregularity region in addition to the usual ionospheric reflected signals. A simple irregularity drift model produced delay and Doppler shift curves that were consistent with those observed for the scatter component of the received signal and supported a hypothesis of an irregularity region drift speed of 1200 m s−1 parallel to the great circle propagation path.
Ionosphericƒ0F2 values and their gradients at European longitudes
Authors: Soicher H. , Gorman F. J. , Tsedilina E. E. , Weitsman O. V.
Published: February 2004

Ionospheric ƒ0F2 values were deduced from vertical ionograms taken near the maximum of the current solar cycle at three European locations during 1990–1991: Dourbes, Belgium (50.1°N, 4.6°E); Roquetes, Tarragona, Spain (40.8°N, 0.3°E); and Rome, Italy (41.9°N, 12.5°). The three stations form a nearly equilateral triangle with a base distance of ∼1100 km. Diurnal, day‐to‐day, as well as seasonal variations are evident for the parameter and its gradients. Cross‐correlation analysis was performed to assess the predictability of the ionospheric variability at one locale based on real‐time measurement at another.
The sunspot cycle and “auroral” F layer irregularities
Authors: Aarons J. , Kersley L. , Rodger A. S.
Published: February 2004

The use of the word “aurora” for many different observations at high latitudes has limited the concepts involved; this is particularly true for F region irregularities. Observations setting the position of the auroral oval (Starkov and Fel'dshtein, 1970) were made using primarily the 555.7‐nm green line, which is emitted predominantly at E layer heights. These observations have shown that the change in position of the auroral oval for low values of Kp as a function of sunspot cycle is of the order of 1° to 2° between sunspot maximum and sunspot minimum. However, irregularities in the F region show much larger solar cycle variations in the locations of the equatorward boundary, typically 10°. A review of scintillation data indicates that at a given auroral latitude, the scintillation activity increases with sunspot number. In addition, for a constant scintillation intensity, the equatorward boundary moves to lower latitudes as sunspot maximum is approached. We review existing spread F studies and show that for quiet geomagnetic conditions, there is lower occurrence during years of low sunspot numbers than during years of high sunspot numbers. However, the spread F index, related to Δ ƒ/ƒ0F2, is higher during years of low sunspot number than during years of high sunspot number. We demonstrate that this apparent dichotomy can be reconciled by using a new method of normalizing the spread F index by the maximum electron concentration of the F layer. We briefly discuss the possible explanations for the observed solar cycle variations of irregularity occurrence in terms of the absolute values and gradients of electron concentration and the E region conductivity.