bluuebunny/crossref_metadata_2025 huggingface dataset:
Prediction of radar coverage under anomalous propagation condition for a typical coastal site: A case study
Authors: Abdul‐Jauwad Samir Hussain , Khan Pervez Zahir , Halawani Talal Omar
Published: April 2008

In this study a model predicting the electromagnetic wave propagation has been developed. The model is initialized with a known refractivity profile and an initial transmitted field. The troposphere is assumed to be inhomogeneous in height and range. The vertical refractivity profile is computed from the meteorological upper air data. Arbitrary as well as experimentally observed profiles could be used as inputs to the model. Inhomogeneity in range is invoked by injecting refractivity profiles at known locations. A transmitter with a Gaussian beam is assumed to generate the initial field based on the transmitter parameters such as vertical beam width, vertical beam elevation, transmitter frequency, antenna height, and type of polarization. A simulation for the typical site on the coast of Arabian gulf has been chosen for the study. The field strength and radar coverage for different conditions of the typical coastal site prevailing a standard and inhomogeneous surface duct demonstrates the sensitivity of the model with the inhomogeneous and homogeneous atmosphere. Radiated fields are predicted on the basis of the initial field, boundary conditions, and atmospheric conditions considering the effect of refractivity in the case of inhomogeneous surface duct and elevated duct. Results reported in this study demonstrate the trapping of the energy in ducting atmosphere above the minimum trapping frequency with the predicted field by this model.
Propagation measurements and modeling at C band for over‐the‐water, line‐of‐sight propagation links in the Mid‐Atlantic coast
Authors: Goldhirsh Julius , Dockery G. Daniel
Published: April 2008

Fade, radar, and propagation modeling results are described for two nearby over‐the‐water, line‐of‐sight, propagation links operating at 4.7 GHz in the Mid‐Atlantic coast of the United States. During a 5 day “intense experimental period” in August 1989 a focused examination of the propagation mechanisms relating to time series fading was conducted. Cumulative fade distributions associated with both propagation paths are presented for the individual days and the overall 5‐day period. During this period, helicopter measurements of the refractivity structures along the propagation paths, boat measurements of the refractivity structure near the ocean surface at the center of the paths, and radar measurements of the clear atmosphere and ocean backscatter were executed. The refractivity measurements were interfaced with a complex parabolic equation propagation model which incorporated the two‐dimensional structure of the refractive index to yield normalized power density structures as a function of range and height for the geometry of the propagation links. These results were compared with the time series fade measurements and provided clear indications of the mechanisms for fading and nonfading events. Azimuthal and vertical radar scans employing a high‐resolution radar at the NASA Wallops Flight Facility showed excellent correlation between strong backscatter returns from the ocean surface well beyond the standard horizon and ducting events as evidenced by signal fading along the propagation links.
Mapping of overspread targets in radar astronomy
Authors: Hagfors T. , Kofman W.
Published: April 2008

The problem of delay‐Doppler mapping of overspread targets, i.e., targets where the product of delay extent θt and Doppler width Bt is greater than unity is discussed. For a transmitter modulation waveform which is believed to be representative of continuous wave modulation, namely, a random Gaussian noise modulation, the effect of the “self‐noise” and the additive noise on the output signal‐to‐noise ratio is evaluated, and expressions are derived which allows the signal‐to‐noise ratio to be predicted and the quality of the mapping assessed for various types of targets and modulation waveform properties. The problem of the self‐noise is shown to be a serious problem, particularly for targets where the power returned varies greatly with range. A scheme involving a combination of random noise modulation and polarization modulation is developed which will greatly improve the capability to map this type of overspread targets. The particular case of the mapping of Mars with an existing facility is dealt with in detail, and it is shown that a limited amount of mapping with a resolution corresponding to 100 by 100 points on the planet is possible in certain regions.
Theoretical description of the parabolic approximation/Fourier split‐step method of representing electromagnetic propagation in the troposphere
Authors: Kuttler James R. , Dockery G. Daniel
Published: April 2008

A theoretical foundation for the use of the parabolic wave equation/Fourier split‐step method for modeling electromagnetic tropospheric propagation is presented. New procedures are used to derive a scalar Helmholtz equation and to subsequently transform to a rectangular coordinate system without requiring approximations. The assumptions associated with reducing the resulting exact Helmholtz equation to the parabolic wave equation that is used for computations are then described. A similar discussion of the error sources associated with the Fourier split‐step solution technique is provided as well. These discussions provide an important indication of the applicability of the parabolic equation/split‐step method to electromagnetic tropospheric propagation problems. A rigorous method of incorporating an impedance boundary at the Earth's surface in the split‐step algorithm is also presented for the first time. Finally, a few example calculations which demonstrate agreement with other propagation models are provided.
Gigahertz scintillation observations at 22.0° N magnetic latitude in the Indian zone
Authors: Dabas R. S. , Banerjee P. K. , Bhattacharya Sumana , Reddy B. M. , Singh J.
Published: April 2008

Postsunset ionospheric scintillation measurements at 4 GHz from the INSAT 1B (74°E) satellite taken during the increasing half of the current solar cycle 22 at Sikandarabad (22.0°N magnetic latitude sub ionospheric) along 149°E geomagnetic meridian in the Indian zone have been analyzed and presented here. Results show that during the low solar activity period, only weak scintillations (peak‐to‐peak fluctuations < 2 dB) were observed mainly during the summer months. As the solar activity increased, occurrence of summer scintillations more or less remained the same, but the intensity and occurrence probability increased substantially in the equinoxes and to some extent in the December solstice also. During October 1989, severe scintillations, with peak‐to‐peak fluctuations exceeding 20 dB were observed at this latitude, which is normally beyond the daytime crest of the equatorial anomaly. The characteristics of scintillations during equinoctial periods of high sunspot year were such that their onsets were mostly abrupt and they developed fully within a few seconds. Simultaneous measurements taken from an equatorial station Chenglepet (5.2°N magnetic latitude) along the same geomagnetic meridian plane during September–October 1989 show that scintillations at Sikandarabad were conditional to their prior occurrence at Chenglepet. As compared to the equatorial location, the onset of scintillations at 22.0°N magnetic latitude was always delayed and also died out first at the higher latitude. Simultaneous observations showed that the intensity of scintillations at the equatorial location never exceeded 5 dB peak‐to‐peak, whereas the intensity of scintillations at 22.0°N magnetic latitude frequently exceeded 10 dB peak‐to‐peak. This is explained on the basis of background ionization and the geometry of the ray paths relevent for these locations. Scintillation activity is in general found to be suppressed during geomagnetic disturbances, but it is observed to be enhanced in the post midnight hours of the same night for those magnetic storms whose recovery phase starts between the midnight and dawn local time sector. Results are also compared with the observations reported from other regions of the world.
Wideband noise observed at ground level in the auroral region
Authors: Benson Robert F. , Desch Michael D.
Published: April 2008

A wideband noise event was detected at ground level from the Andøya Rocket Range in Norway in January 1989. The signals were observed on four commercial communication receivers (tuned to 159, 515, 905, and 1200 kHz), an ionosonde (200‐kHz to 3.5‐MHz interference‐free observations) and a riometer (32.5 MHz). The event, which occurred during a period of magnetic disturbance near magnetic midnight, was the only one observed during nearly 3 weeks of operations. This low frequency‐of‐occurrence is attributed partly to high local noise levels. The ease with which this event was identified on the ionograms produced by the local ionosonde suggests that routine ionosonde recordings should be inspected in search for such events. Such an effort would enhance existing research directed toward developing techniques for identifying quiet communication channels and help to identify the origin and frequency‐of‐occurrence of high‐latitude wideband noise events.
Airborne measurements of incidental radio noise from cities
Authors: Cudak Mark C. , Swenson George W. , Cochran William W.
Published: April 2008

In 1972 and again in 1989–1990 measurements of the incidental radio noise from two cities in central Illinois were made in the range 144–412 MHz by flying over in a light airplane equipped with downward looking antennas. The cities were well resolved by the antennas, so the data could be expressed as brightness temperatures. Care was taken to avoid contamination of the data by radiation from transmitters and the airplane's systems. Variation of brightness with frequency, time of day, and season of the year was determined. Comparison of the 1972 and 1989–1990 data shows no significant changes between epochs.
The effects of electric field and ring current energy increases onFlayer irregularities at auroral and subauroral latitudes
Authors: Aarons J. , Rodger A. S.
Published: April 2008

A model of the dynamics of magnetic storms is proposed which describes the effects of magnetospheric activity on irregularities atFlayer heights for auroral and subauroral latitudes. For high latitudes the initial phase of the storm dynamics first affects auroral latitudes, and then the effects descend to what were subauroral and middle latitudes. The effects weaken as the convective electric field boundary moves equatorward. In the second stage of the storm, i.e., the recovery period, the ring current plays a leading role at subauroral latitudes. The ring current, which has acted as a reservoir for ionospheric and solar wind ions, decays. In this recovery phase the sources of energy producing the irregularities at subauroral latitudes are the ions in the ring current. This new synthesis of storm effects uses illustrations from a number of magnetic storms with observations ranging from December 1971 to March 1989, primarily in years of high solar flux. The data illustrate the concept of two stages of irregularity development. (1) The initial descent and weakening of the effects of the electric field changes and (2) ring current decay which produces the conditions for the generation of the irregularities at subauroral latitudes.
Scintillation near the F layer trough over northern Europe
Authors: Pryse S. E. , Kersley L. , Russell C. D.
Published: April 2008

Results are presented of amplitude and phase scintillation observations made during a two and a half year period at Lerwick in the Shetland Islands, a station in the subauroral zone in the vicinity of the F layer trough and the scintillation boundary. Transmissions from the multisatellite Navy Navigation Satellite System have been used to obtain both latitudinal and temporal coverage for the observations. More than 19,000 satellite passes have been monitored yielding in excess of 750,000 values of S4 and σф for amplitude and phase scintillation, respectively. The paper presents examples of scintillation occurrence as a function of latitude for both long term and diurnal time scales. A dominant feature is the enhanced occurrence of scintillation in response to increased solar activity. This is accompanied by an advance in the scintillation boundary to lower latitudes on a scale comparable to that associated with geomagnetic activity. The position of the premidnight minimum in boundary latitude has moved by some 10 deg in response to the increased solar activity between autumn 1987 and autumn 1989. The paper also discusses the importance of propagation and irregularity geometry to scintillation measurements and by comparison with model studies concludes that the observations are consistent with scintillations arising from irregularities of a field‐aligned rodlike nature with low aspect ratio rather than sheets extended along L shells.
An ionospheric propagation prediction method for low latitudes and mid‐latitudes
Authors: Houpis Harry L. F. , Nickisch L. J.
Published: April 2008

An ionospheric propagation prediction method is proposed that is more responsive than current methods to the actual propagation channel of a radio wave. These other methods typically use an “average” propagation channel. The range of conditions that comprise this “average,” when considered separately, can potentially lead to quite different predictions of the propagation channel relative to the “average” alone. Thus, to the extent that a real‐time trend in the propagation channel can be established, better predictions can be obtained. The proposed method obtains a real‐time trend by employing a nonlinear regression analysis technique coupled together with an appropriately parameterized (empircal) ionosphere model and a ray‐tracing algorithm. To test the effectiveness of this approach, we utilize measurements of group path length and elevation to several orbiting satellites from a dual‐frequency (UHF/VHF) pulsed radar located near the magnetic equator. These data provide the needed statistics for the regression analysis. We then obtain satellite altitude estimations employing our method and compare these estimates to predictions obtained with a standard method. The result is an improved altitude prediction. The proposed method may also be used to obtain a better understanding of ionosphere‐magnetosphere coupling, and in particular, to investigate the influence of geomagnetic activity on the low‐ and mid‐latitude ionosphere. This is accomplished by generating parameter indices from our method and comparing these indices to solar and geomagnetic indices to see if correlations exist. To the extent that such a correlation does exist, a more accurate wave propagation prediction will result, which in turn will provide a more reliable radar tracking performance. We present comparisons between geomagnetic activity (as measured by the Kp three‐hourly range index) and the parameter indices that show a weak correlation.
RF ionization of the lower ionosphere
Authors: Tsang K. , Papadopoulos K. , Drobot A. , Vitello P. , Wallace T. , Shanny R.
Published: April 2008

A comprehensive analysis of the ionization rates of air by RF fields is presented. The analysis relies on a time‐dependent code which treats the electron energization with a Fokker‐Planck type model and the inelastic energy losses with a multiple time scale technique. Derivation of ionization rates for parameters of interest To D region ionospheric by ground‐based RF transmitters with frequency much higher than the electron neutral collision frequency is emphasized. The study provides a physical understanding of the ionization proces and its associated efficiency by combining the computational results with analytic theory. It is shown that for values of quiver energies
« I, where I is the ionization potential, the electron production time corresponds to the electron energization time from energies below 2 eV to 20–25 eV. The analytic expressions derived are consistent with the computational results over 6 orders of magnitude in ionization rates and over 2 orders of magnitude in values of

urn:x-wiley:00486604:media:rds3231:rds3231-math-0002
Power threshold definitions are clarified, and the pitfalls of using fluid descriptions or effective electric field notions are discussed. The paper concludes with an assessment the power requirements for ionization at 70‐km ionospheric altitude with RF in the 100–900 MHz frequency range.
A survey of polar cap F region electron densities measured by the Sondrestrom radar
Authors: Robinson R. M.
Published: April 2008

The Sondrestrom incoherent scatter radar has been used to study F region electron densities in the polar cap. The data were obtained while the radar antenna was scanned in the plane of the magnetic meridian. In this mode, electron densities are measured as a function of altitude and latitude so that ionospheric boundaries can be easily identified. One boundary that is present in much of the data is a vertical wall of electron density between 80 and 200 km altitude that separates the region of aurorally produced electron density from the polar regions where auroral precipitation is absent. On the basis of this criterion we identified more than 500 radar scans during which the radar measured F region electron densities in the polar cap. The data were sorted according to magnetic activity, solar flux, invariant latitude, and magnetic local time. Besides calculating average electron density profiles, we also examined the altitude and latitude variations of the F region peak density. The peak density in the polar cap F region varies by a factor of 4 between solar zenith angles of 85° and 120°. The relative amplitude of large‐scale irregularities is approximately twice as large when the F region is not sunlit. The results of model calculations agree well with the observed local time variations in F region peak density. The highest densities are found in the afternoon local time sector where enhanced plasma from the sunlit hemisphere is convected toward the nightside. The lowest densities are found in the dawn sector where the polar hole is created by a convection cell situated entirely on the nightside. These results demonstrate the importance of solar illumination and convection in determining the properties of the polar F region.
Updated climatological model predictions of ionospheric and HF propagation parameters
Authors: Reilly M. H. , Rhoads F. J. , Goodman J. M. , Singh M.
Published: April 2008

The prediction performance of ionospheric conductivity and electron density (ICED), RADAR C, and Ionospheric Communications Analysis and Predictions Program (IONCAP) climatological ionospheric models is evaluated for different regions and sunspot number inputs. With conventional sunspot numbers, based on the observed solar 10.7‐cm flux, ƒ0F2 prediction errors at Boulder for a particular month are no less than 18%. Use of the U.S. Air Force Global Weather Center (AFGWC) global sunspot number SSNE, extracted from ionosonde data for use with the ICED model, gives 15% errors. A dramatic improvement involves the use of single‐station ionospheric data to update the driving parameters of an ionospheric model for near‐real‐time (NRT) predictions of ƒ0F2 and other ionospheric and HF circuit parameters. For middle latitudes the improvement extends out thousands of kilometers from the update point to points of comparable corrected geomagnetic latitude. At very large distances an update scheme which exploits a strong global dependence on geomagnetic activity shows promise. For the purpose of NRT predictions of the entire electron density profile the use of two distinct effective sunspot numbers for the bottomside, coupled with one or more model parameters for the topside, is suggested for model updates from a single station.
Dyadic Green's functions in a planar stratified, arbitrarily magnetized linear plasma
Authors: Habashy Tarek M. , AIi Sami M. , Kong Jin Au , Grossi Mario D.
Published: April 2008

A general spectral domain formulation to the problem of radiation of arbitrary distribution of sources embedded in a horizontally stratified arbitrarily magnetized linear plasma is presented. The fields are obtained in terms of electric and magnetic type dyadic Green's functions. The formulation is considerably simplified by using the kDB system of coordinates in conjunction with the Fourier transform. The distributional singular behavior of the various dyadic Green's functions in the source region is investigated and taken into account by extracting the delta function singularities. Finally, the fields in any arbitrary layer are obtained in terms of appropriately defined global upward and downward reflection and transmission matrices.
Model studies of the latitudinal extent of the equatorial anomaly during equinoctial conditions
Authors: Klobuchar J. A. , Anderson D. N. , Doherty P. H.
Published: April 2008

The latitudinal extent of the equatorial anomaly has been studied for equinoctial conditions using a theoretical model of the ionosphere which incorporates measured values of vertical E × B drift at the Earth's magnetic equator. Realistic values of neutral winds are also included. The equatorial anomaly region, typically between ±20° magnetic latitude, is that part of the world where the highest values of electron density and total electron content (TEC) normally occur and hence is very important to high‐frequency propagation and to transionospheric propagation effects. During the daytime, upward E × B drift at the magnetic equator drives the ionization across field lines to higher latitudes, causing crests in ionization to occur at approximately ±15° dip latitude. The E × B drift mechanism is explained in detail by Hanson and Moffett (1966). The latitude range over which the anomaly makes a significant difference in values of ƒ0F2 and TEC is calculated as a percent departure from the case with no equatorial electric field. Results from the model studies with different values of realistic electric fields show that the effects of the anomaly can be more highly variable and widespread in latitude and local time than is generally assumed.
VLF radio wave mode conversion for ionospheric depressions
Authors: Wait James R.
Published: April 2008

A simplified analysis is presented for the perturbation produced by a localized depression of the upper boundary of the Earth‐ionosphere waveguide. An earlier two‐dimensional model is generalized to deal with a depression which is limited in extent. Also, allowance is made for the depression to be laterally displaced from the great circle path connecting the transmitter and receiver.
Eigenmodes in a circular waveguide containing an isotropic chiral material
Authors: Hollinger Richard , Varadan Vasundara V. , Varadan Vijay K.
Published: April 2008

In this paper, Maxwell's equations and modified constitutive equations are used to investigate the eigenmodes in a circular waveguide containing an isotropic chiral medium for monochromatic radiation of unspecified wavelength. The dispersion equation for this case is derived analytically, solved numerically, and compared to the nonchiral case and to waveguides containing longitudinally magnetized ferrites. The field components are computed and plotted as a function of the radial coordinate as well as the transverse E‐field lines in the circular cross section. Cutoff frequencies, evanescent modes, polarization characteristics of the fields, and the nondegeneracy of positive and negative mode numbers are discussed and contrasted with those of nonchiral, dielectric filled circular waveguides.
Outage performance analysis of route diversity systems of cellular structure
Authors: Kanellopoulos J. D. , Koukoulas S. G.
Published: April 2008

Cellular diversity systems of rhomboidal structure as a means to overcome rain attenuation problems for frequencies greater than 10 GHz are analyzed and compared to conventional route diversity systems. The proposed model is applicable to systems using diversity hop lengths more than 30 km. Application of the cellular system for regions characterized by different rainfall climates, similar to those observed in Canada and Europe, with ƒ = 11 GHz gives a saving in the number of required repeaters up to about a value of 13% for high‐quality repeaters.
Worst‐month rain attenuation statistics: A new approach
Authors: Crane Robert K.
Published: April 2008

The models for worst‐month analysis are reconsidered. A new procedure is proposed for the estimation of the expected distribution of annual worst‐month statistics from a series of monthly exceedances or from climate zone based rain attenuation models. The new procedure is easy to apply, and the results are consistent with available observations.
The influence of wave groups on the dual‐frequency Doppler spectrum: Experimental and theoretical approach
Authors: Jacobsen Svein , Eltoft Torbjørn
Published: April 2008

Experimental output data from a multifrequency, continuous wave, C band system have been investigated and discussed with reference to physical principles involved in modulation of small‐scale centimetric water waves by larger‐scale wave trains. Attention payed at field and laboratory measurements described in the literature show that the propagation velocity of certain wave components below the spectral peak occasionally differs from predictions of conventional linear theory. A one‐dimensional theoretical analysis of the dual‐frequency microwave response, including Doppler features and modulational effects induced by modulated wave trains, has been developed to throw light on this phenomenon. The theoretical expressions derived predict that wave train modulation should manifest itself as two narrow, Doppler‐shifted peaks in the cross‐product power spectrum generated from two distinct microwave returns. The wave field involved in the modulational process, is modeled as not only consisting of independently propagating Fourier components obeying the linear dispersion relation but also being composed of wave groups of Stokeslike waves, forming independent wave packets. Furthermore, each isolated packet is constructed to be a perturbation solution of the wave equations representing the sea surface boundary conditions. Experimentally, pronounced spectral components were observed with variations in magnitude and Doppler behavior which are consistent with proposed theory, surface truth, and results presented earlier.
HF sideband observations compared to theory
Authors: Huang Z. H. , Gordon W. E. , Noble S. T. , Duncan L. M.
Published: April 2008

The salient features of the sidebands generated on the HF waves in an ionospheric plasma modified by two pump frequencies are presented and compared to theories. Particular attention is focused upon the properties of the nonlinear modulation process by comparing the modulation index theory and the observations. The observations show that the amplitudes of the sidebands are related to the HF power and the difference frequency of the two pump waves. The relationships of the amplitude of the sideband with HF frequency, the scale height, and the temperature of the electrons in the ionosphere are discussed. The modulation index theory shows agreement with the observations over a range of conditions.
Rotation and dichroism associated with microwave propagation in chiral composite samples
Authors: Umari M. H. , Varadan V. V. , Varadan V. K.
Published: April 2008

In this paper we present experimental results on the angle of rotation and the axial ratio (dichroism) associated with the propagation of microwave waves in chiral composite samples (a phenomenon akin to optical rotation and optical dichroism). In our experiments the chiral composite samples tested were made in the form of planar slabs and consisted of low loss dielectric matrix materials in which miniature copper springs of left only, right only, or an equal mixture of left and right handedness were randomly distributed and oriented. The normally incident wave was linearly polarized. In the chiral sample the linearly polarized wave decomposes into left and right circularly polarized waves which propagate with different speeds and different attenuations leading to an elliptically polarized transmitted wave whose orientation ( electromagnetic rotation ) and axial ratio (dichroism) are proportional to the concentration of springs. Rotation and dichroism are shown to be zero in the equichiral samples.
A study of VHF radio wave propagation over a water surface of variable conductivity
Authors: Forget P. , Broche P.
Published: April 2008

The ground wave propagation of VHF radio waves over an inhomogeneous sea surface of variable conductivity and under various sea roughness conditions is studied both theoretically and experimentally. Theoretical simulations predict a high sensitivity of the ground wave attenuation factor upon large variations of the surface conductivity, as can be found in an estuary. This sensitivity can be influenced appreciably by the sea state under high winds in fetch‐limited conditions and under a broader range of wind speeds for a fully developed sea. Results from a VHF radar experiment over a river plume with appropriate in situ documentation are reported here. They validate the theoretical expectations on the influence of the conductivity profile on radio wave propagation. Because of the low winds encountered, the wind/wave influence is evident only on the variations of the radar cross‐section coefficient (linearly related to Bragg wave spectral energy) in fetch‐limited conditions. Also, these variations can be attributed in some cases to the influence of slicks in the river plume. The study suggests that the VHF radar technique may be used for remote sensing of sea surface conductivity.
Electromagnetic plane wave diffraction by a three‐part thin, planar dielectric/magnetic slab
Authors: Rojas R. G. , Ly H. C. , Pathak P. H. , Tiberio R.
Published: April 2008

A uniform asymptotic high‐frequency solution is developed for the plane wave diffraction by a three‐part thin, planar dielectric/magnetic slab. This work is an extension of the previously developed solution for the problem of diffraction by the planar junction of two thin dielectric/magnetic half planes. The multiple interactions up to third order between the edges of the three‐part material slab, which include edge‐diffracted fields, surface wave fields, and two new terms not considered in the majority of papers in the literature, are obtained to yield a uniform far‐zone‐scattered field. The employed formulations are based on the spectrally extended ray method; and the solutions obtained can be specialized to limiting cases for which any part of the material slab can become free space, perfectly conducting, or purely resistive, respectively. Numerical results corresponding to various multiple‐edge planar structures are presented. Some of these numerial examples are compared with the corresponding data based on an available moment method solution; the agreement between these approaches is very good.
Limitations of ionospheric imaging by tomography
Authors: Yeh K. C. , Raymund T. D.
Published: April 2008

The potential of imaging large ionospheric structures by applying tomographic techniques has only been proposed recently and has already received interest and increasing attention by workers in ionospheric research. In the combined case of an idealized plane geometry and complete and continuous data extending to ±∞ the inversion is exact and unique. This is no longer the case if any of the idealized conditions are removed. Specifically, we investigate the limitations arising from limited angles, sampled data of finite receiving aperture and circular geometry. Under these various nonidealized but more realistic conditions, fidelity of the reconstructed image is investigated by computing the response to an impulse source function. The deterioration of the reconstructed image, the sensitivity to the position in the image plane, and the possible existence of ghost images are all numerically investigated and illustrated. Possible methods of improvement are suggested.
The Fecal Pellet fraction of biogeochemical particle fluxes to the deep sea
Authors: Pilskaln Cynthia H. , Honjo Susumu
Published: April 2008

Fecal pellets produced by suspension‐feeding crustacean zooplankton, specifically copepods and euphausids, have frequently been cited as an important mode of large particle transport in the open ocean. The objectives of the present study were to determine the various biogeochemical fluxes provided by pelagic crustacean fecal pellets, to examine such fluxes as a function of depth and variable levels of surface water productivity, and to assess the overall fecal pellet contribution to oceanic particle fluxes as measured with sediment traps. Pellet subsamples were obtained from particulate samples collected at depths between 389 and 5068 m by moored PARFLUX sediment traps deployed for up to 12 months at three tropical‐subtropical open ocean localities. The sites were located over the East Hawaii Abyssal Plain (P site), over the Demerara Abyssal Plain (E site), and in the Pacific Panama Basin (PB site). Fecal pellet flux and chemical composition were found to vary significantly on a geographic scale as a function of productivity levels in the surface waters. The total carbonate, organic carbon, opaline silica, and lithogenic fluxes provided by pellets at the oligotrophic P1site were 1–2 orders of magnitude less than that measured at the eutrophic station in Panama Basin. The pellet data show that contrary to previous assumptions, these biogenic aggregates are responsible for no more than 5% of the total mass flux of oceanic particulate material. Despite the fact that at all trap depths, large numbers of intact pellets were collected which displayed minimal effects of dissolution and microbial degradation, fecal pellets contributed an average of only 1–10%, 0.5–5%, 1–3%, and 0.5–4% to the total measured mass fluxes of organic, carbonate, opaline silica, and lithogenic material, respectively. However, the pellets showed elevated C/N ratios (9–14) as well as high organic content (representing up to 50% of the individual pellet weight), suggesting that they constitute an important source of organic carbon for the deep‐sea benthos. Rapid remineralization of the organic‐rich pellets must occur at the deep‐sea sediment/water interface, as these biogenic aggregates were completely absent from all site core tops.
Production, respiration, and the isotope geochemistry of O2 in the upper water column
Authors: Bender Michael L. , Grande Karen D.
Published: April 2008

The isotopic composition (or 18O/16O ratio) of O2 produced by photosynthesis and the isotopic composition of O2 consumed during respiration are different from each other as well as from the isotopic composition of ambient dissolved O2. In a closed body of water in which photosynthesis and respiration are co‐occurring, the 18O/16O of dissolved O2 will be a function of both the rate of respiration and the gross rate of photosynthesis. Therefore gross primary production can be calculated if one determines both net O2 production and the18O/16O of dissolved O2. As a test of the feasibility of this approach, we present data on the 18O/16O ratio of dissolved O2, along with ancillary parameters, in the top 200 m of waters in the North Pacific gyre in August and September 1985. The results are shown to reflect isotope fractionation during photosynthesis and respiration. We outline the additional information needed before natural 18O/16O variations can be used to constrain quantitatively the gross production.
Methane emission from natural wetlands: Global distribution, area, and environmental characteristics of sources
Authors: Matthews Elaine , Fung Inez
Published: April 2008

A global data base of wetlands at 1° resolution has been developed from the integration of three independent global, digital sources: (1) vegetation, (2) soil properties and (3) fractional inundation in each 1° cell. The integration yielded a global distribution of wetland sites identified with in situ ecological and environmental characteristics. The wetland sites have been classed into five major wetland groups on the basis of environmental characteristics governing methane emissions. The global wetland area derived in this study is ∼5.3 × 1012m2, approximately twice the wetland area previously used in methane‐emission studies. Methane emission was calculated using methane fluxes for the major wetland groups, and simple assumptions about the duration of the methane production season. The annual methane emission from wetlands is ∼110 Tg, well within the range of previous estimates (11‐300 Tg). Tropical/subtropical peat‐poor swamps from 20°N‐30°S account from ∼30% of the global wetland area and ∼25% of the total methane emission. About 60% of the total emission comes from peat‐rich bogs concentrated from 50°‐7O°N, suggesting that the highly seasonal emission from these ecosystems is the major contributor to the large annual oscillations observed in atmospheric methane concentrations at these latitudes.
Long core study on natural and anthropogenic acidification of Huzenbachersee, Black Forest, Federal Republic of Germany
Authors: Steinberg Christian , Arzet Klaus , Krause‐Dellin Dieter , Sanides Silvia , Frenzel Burkhard
Published: April 2008

The pH history of Huzenbachersee in the northern Black Forest has been studied by using diatom records of a long core and a short core. The results reveal that the pH of the lake has decreased by 1 unit since the end of the ice age until circa 1800, owing to natural long‐term acidification. The impact of man in the last 200 years, such as catchment afforestation and especially acid deposition, have caused additional dramatic pH declines in a very much shorter period of time.
A comparison of atmospheric inputs and deep‐ocean particle fluxes for the Sargasso Sea
Authors: Jickells T. D. , Church T. M. , Deuser W. G.
Published: April 2008

Trace metal distributions in ocean waters are dominated by internal recycling processes. However, fluxes out of the water column must balance inputs if steady state is to be maintained. We use deep‐sea particle fluxes at 3200 m in the Sargasso Sea as measured by sediment traps to define the flux to the sediments beneath the Sargasso Sea and compare this flux to the atmospheric input for a group of trace metals. Despite the uncertainties associated with the comparison, several important features are revealed. For elements predominantly associated with crustal particles (Al, Mn, Fe, and V), lateral advection provides a significant part of the deep‐sea particle flux. For Cd, Cu, Ni, Pb, and Zn this lateral component is minor, and atmospheric inputs to the Sargasso Sea exceed deep‐sea particle fluxes. These excesses appear to be consistent with calculations of the export of these metals from the Sargasso Sea arising from ocean boundary scavenging (Pb) and release from decomposing organic matter in the southward flowing North Atlantic Deep Water (Cd, Cu, Ni, and Zn). In the atmospheric deposition itself, wet deposition exceeds dry deposition for Cd, Cu, Mn, Ni, Pb, and Zn, while wet and dry deposition are of similar magnitudes for V, Fe, and Al.
Oceanic primary production estimates from measurements of spectral irradiance and pigment concentrations
Authors: Bidigare Robert R. , Smith R. C. , Baker K. S. , Marra J.
Published: April 2008

A major objective of biological oceanography today is to quantify the mean and the variance of phytoplankton production on a global basis. Synoptic satellite sensing of the world's ocean is essential to this effort which requires contemporaneous sea surface data to provide complete water column information. Toward these objectives we present a spectrally dependent bio‐optical model for the computation of in situ phytoplankton production. Using this model we show that in situ phytoplankton production can be accurately estimated from measurements of incident spectral irradiance and phytoplankton pigment concentrations. We also present estimates of photosynthetically absorbed radiation as a function of wavelength for a natural phytoplankton population. These complete spectral data provide insight into the wavelength‐dependent utilization of radiant energy by phytoplankton and the influence of phytoplankton on the optical properties of the water column. We show that the model can be used for shipboard observations and that it may be especially useful for predicting production rates from data provided by untended buoys.
The ocean as a net heterotrophic system: Implications From the carbon biogeochemical cycle
Authors: Smith S. V. , Mackenzie F. T.
Published: April 2008

The global ocean apparently consumes more organic carbon than it produces. The excess heterotrophy probably occurs in the nearshore zone. This nearshore heterotrophy has significant implications with respect to processes such as organic matter transport from the nearshore zone to the adjacent open ocean, nutrient limitation of primary production, and the role of the coastal zone as a short‐term sink for anthropogenic CO2.
Mercury in surface waters of the open ocean
Authors: Gill Gary A. , Fitzgerald William F.
Published: April 2008

Mercury was determined in samples of surface seawater and rainfall from coastal and open ocean areas of the northwest Atlantic and Pacific Oceans. Mercury concentrations in surface seawater and open ocean rainfall ranged from 0.5 to 11 pM and 6 to 130 pM, respectively. The pluvial flux of Hg to the open ocean was estimated at 1.7 × 109 g/yr, using mean concentrations of Hg in rainfall and annual rainfall volume estimates for the major ocean basins. Fluvial input, while difficult to reliably assess, was a less important (about 0.18 × 109 g/yr) source of Hg to the ocean. The mean residence time of Hg in the surface mixed layer (to 75 m) was calculated at 4–7 years, which is similar to other very reactive elements such as Al, Pb and Bi. Surface seawater Hg distributions are markedly influenced by atmospheric sources. Due to an elevated supply of Hg to the northwest Atlantic by rain, higher Hg values were observed in surface seawater in the northwest Atlantic Ocean (4.0 ± 0.7 pM) than at low latitudes in the central North Pacific Ocean (2.1 ± 0.4 pM). Surface seawater Hg concentrations between coastal New England and the Sargasso Sea do not show the large offshore concentration gradient typical of elements which have relatively large fluvial or coastal sources. Rather, the Hg distribution in this region was similar to that obtained for the atmospherically derived constituents Pb and 210Pb. Surface seawater Hg measurements in the central Pacific Ocean along 160°W between 20°N and 20°S showed a depression (>60%) in the equatorial upwelling area which coincided with the transect region exhibiting low 234Th/238U activity ratios. This relationship implies that Hg has enhanced scavenging and removal from surface seawater in biologically productive oceanic zones. In contrast to the surface distribution in the northwest Atlantic, a pronounced on‐shore gradient in sea surface Hg concentration was observed in the Tasman Sea. This may have resulted from the upwelling of water high in Hg content into the near coastal zone.
WATOX meteorological overview for January 1986 WP‐3D aircraft intensive
Authors: Stunder Barbara J. B. , Artz Richard S. , Rolph Glenn D.
Published: April 2008

During the January 1986 Western Atlantic Ocean Experiment (WATOX), two research aircraft (the WP‐3D and King Air) flew a series of flights off the east coast of North America, with the purpose of determining chemical and air fluxes off North America. An overview of the meteorological conditions during the WP‐3D flights is presented. Surface and 850‐mbar synoptic charts, isobaric air parcel back trajectories, and vertical cross sections of potential temperature and water vapor mixing ratio are also presented. The surface synoptic conditions were typically characterized by low pressure near Newfoundland and high pressure over the southeastern United States, causing generally northwesterly winds north of about 38°N latitude and more variable winds farther south. In general, the flights were characterized by a well‐defined marine boundary layer capped by a layer of stratus clouds. Wind climatologies suggest that the meteorology during the four flights was typical of much of January and possibly much of the winter.
Calculation of air flux values from WATOX 1986 WP‐3D aircraft observations
Authors: Artz Richard S. , Stunder Barbara J. B.
Published: April 2008

During the Western Atlantic Ocean Experiment 1986, the National Oceanic and Atmospheric Administration WP‐3D aircraft collected meteorological data and samples of atmospheric gases and aerosols during approximately 40 hours of flying time on January 4, 6, 8, and 9, 1986. This research effort attempted to quantify the fluxes of materials off the Atlantic coast following periods of frontal passage. The meteorological data are averaged into 15‐min periods and grouped into boundary layer and free tropospheric sections. Areas of clouds and precipitation were not included in the analysis. Air mass fluxes are calculated and air mass flux cross sections are presented using ambient temperature and pressure values. The range of boundary layer flux values was calculated to be from approximately ‐4 kg/(m2/s) indicating onshore flow, to about 25 kg/(m2/s); free tropospheric flux values ranged from approximately 5 kg/(m2/s) to 30 kg/(m2/s). The uncertainty of the air flux data was estimated to be less than 20%.
Sulfur dioxide over the western Atlantic Ocean
Authors: Thornton D. C. , Bandy A. R. , Driedger A. R.
Published: April 2008

We have made in‐flight determinations of SO2 in clouds, the cloud‐free boundary layer, and the free troposphere in a wintertime marine environment . The salient feature of the SO2 vertical distribution was the consistently low SO2 levels (0.14 parts per billion by volume (ppbv) mean; 0.065 ppbv median) in the free troposphere compared to the boundary/mixed layers (1.83 ppbv mean; 1.02 ppbv median). In some instances SO2 in the clouds that capped the boundary layer was as high as in the air below the clouds. The estimated SO2 fluxes were 2.1 Tg S−1 for the boundary layer, 0.21 Tg S yr−1 for the free troposphere between 850 and 700 hPa, 0.36 Tg S yr−1 for the free troposphere between 700 and 500 hPa.
The flux of reactive nitrogen compounds from eastern North America to the western Atlantic Ocean
Authors: Luke Winston T. , Dickerson Russell R.
Published: April 2008

This paper describes the measurement of reactive nitrogen compounds as part of the 1986 Western Atlantic Ocean Experiment (WATOX). Species measured include NO and NOY (NOY = NO + NO2 + NO3 + HNO3 + 2N2O5 + HONO + PAN + R‐ONOx + fine NO3, where PAN is peroxyacetyl nitrate). A ferrous sulfate converter, which was subsequently determined to respond to PAN, was used to measure NOx' (NOx' = NO + NO2 + ˜0.7 PAN). Although concentrations of NOY reached several parts per billion in offshore urban plumes, average levels of NOY in the planetary boundary layer (PBL) resemble those measured at rural sites along the east coast. Free tropospheric concentrations of NOY were near continental background levels observed in previous studies. Concentrations of NO were typically below the detection limit of 20 parts per trillion in the free troposphere, with higher concentrations detected in the PBL. By combining the odd nitrogen data set with calculations of ambient air mass fluxes observed on each flight, we calculate an instantaneous gross N flux through the area from 31.5 to 44.1°N and 0 to 3000 m to be 18.5 kg N/s. The dependence of the magnitude of this flux upon specific meteorological conditions is discussed. For comparative purposes, we also combine our odd nitrogen data set with climatological wind data for the eastern seaboard to estimate an annual gross N flux of 0.5 Tg N/yr, significantly less than previous estimates.