bluuebunny/crossref_metadata_2025 huggingface dataset:
Intercomparison of NO, NO2, NOy, O3, and ROx measurements during the Oxidizing Capacity of the Tropospheric Atmosphere (OCTA) campaign 1993 at Izaña
Authors: Zenker T. , Fischer H. , Nikitas C. , Parchatka U. , Harris G. W. , Mihelcic D. , Müsgen P. , Pätz H. W. , Schultz M. , Volz‐Thomas A. , Schmitt R. , Behmann T. , Weißenmayer M. , Burrows J. P.
Published: February 2004

An informal comparison of NO, NO2, NOy, O3, and ROx measurements obtained by different instruments and techniques at Izaña in 1993 during the European Oxidizing Capacity of the Tropospheric Atmosphere (OCTA) campaign was performed. For O3, two UV instruments agree within 7% (95% cl.) limited by a difference in response of 7.0%±0.2% (95% cl.) which likely was caused by O3 losses in one of the inlet lines. The NO mixing ratios obtained by two NO/O3 Chemiluminescence (CL) instruments range between 0–200 parts per trillion by volume (pptv), except for short periods influenced by traffic pollution. The response of the two CL detectors agrees within 3%±10% (95% cl.). The NOy data, ranging between 100 pptv and several ppbv in plumes, were obtained using two different gold‐CO‐converters and inlet designs with subsequent CL detection of NO. A systematic difference in the slope between the two data series of 1.44±0.05 (95% cl.) was likely caused by NOy, losses in the inlet line of one of the instruments. Three different NO2 data sets were obtained using Tunable Diode Laser Absorption Spectroscopy (TDLAS), a photolytic converter/CL technique (PLC/CL), and the Matrix Isolation Electron Spin Resonance (MIESR) technique. The linear slopes between the data sets of the three methods are consistent with unity at a 95% confidence level, 1.13±0.30 (TDL versus PLC/CL), 0.90±0.47 (TDL versus MIESR), and 1.04+0.34 (PLC/CL versus MIESR). ROx measurements were performed by three different chemical amplifier (CA) designs and the MIESR technique. Using 30‐min averaged values between 13–65 pptv, two CA instruments agree within 25% (95% cl.) with the mean of MIESR (1.01+0.20 and 0.98±0.24, 95% cl.), while the third CA responded low (0.65±0.32, 95% cl.).
Creep of water ices at planetary conditions: A compilation
Authors: Durham William B. , Kirby Stephen H. , Stern Laura A.
Published: February 2004

Many constitutive laws for the flow of ice have been published since the advent of the Voyager explorations of the outer solar system. Conflicting data have occasionally come from different laboratories, and refinement of experimental techniques has led to the publication of laws that supersede earlier ones. In addition, there are unpublished data from ongoing research that also amend the constitutive laws. Here we compile the most current laboratory‐derived flow laws for water ice phases I, II, III, V, and VI, and ice I mixtures with hard particulates. The rheology of interest is mainly that of steady state, and the conditions reviewed are the pressures and temperatures applicable to the surfaces and interiors of icy moons of the outer solar system. Advances in grain‐size‐dependent creep in ices I and II as well as in phase transformations and metastability under differential stress are also included in this compilation. At laboratory strain rates the several ice polymorphs are rheologically distinct in terms of their stress, temperature, and pressure dependencies but, with the exception of ice III, have fairly similar strengths. Hard particulates strengthen ice I significantly only at high particulate volume fractions. Ice III has the potential for significantly affecting mantle dynamics because it is much weaker than the other polymorphs and its region of stability, which may extend metastably well into what is nominally the ice II field, is located near likely geotherms of large icy moons.
Aeolian behavior of dust in a simulated Martian environment
Authors: White Bruce R. , Lacchia B. Michelle , Greeley Ronald , Leach Rod N.
Published: February 2004

The behavior of aeolian dust, particles 1–2 microns (μm) in diameter, was analyzed in a simulated Martian environment. Three main areas have been investigated: (1) characterizing spectral and aeolian properties of Martian particles and natural wind‐blown terrestrial dust in order to identify a suitable surrogate Martian dust, (2) emplacement of the material in the simulated Martian environment, and (3) experimental procedures and testing in the Martian Surface Wind Tunnel (MARSWIT). The first phase of the study involved choosing a terrestrial dust that closely resembled Martian dust. Data accumulated from various sources suggest that Martian dust is derived from the weathering of basaltic parent material, nontronite clay being a good candidate composition. A commercial clay, Carbondale Red Clay (CRC), was determined to be an appropriate surrogate Martian dust. Using a compressed air‐dust ejection system, an air‐entrained dust cloud was generated which settled to cover the test section in the MARSWIT. This method best replicated the natural aerodynamic settling process presumed to exist on Mars. Low‐pressure experiments were performed in the MARSWIT facility located at the NASA Ames Research Center, Moffett Field, California. Two separate wind tunnel floors were used for these experiments; one provided an aerodynamically smooth‐surface flow, and the other was an aerodynamically rough‐surface flow. Initial and momentary particle movement was recorded at friction velocities as low as 2 m/s. However, the “dust” never reached fluid saltation threshold because individual particles less than 10 μm typically do not saltate, but pass into suspension. An estimated dust flux of 3.7×10−7 g/cm2 s was determined for a friction velocity of 2 m/s. This flux could suspend about 10,000 metric tons of dust per second over the surface of Mars. The dust behavior on the smooth surface was markedly different than on the rough surface under Martian conditions. Application of the processes described above to the Martian environment suggests that on rocky surfaces dust suspension would first occur, whereas smooth plains would require higher wind speeds for dust entrainment.
Sedimentary geomorphology of the Mars Pathfinder landing site
Authors: Parker Timothy Jay , Rice James W.
Published: February 2004

The first landing on Mars in over 20 years will take place July 4, 1997, near the mouth of the Ares Vallis outflow channel located in southeastern Chryse Planitia. Mars Pathfinder, unlike Viking 1, is expected to land on a surface that has a distinct and unambiguous fluvial signature. For safety reasons, the landing site was selected within a broad plains region beyond the mouth of Ares Vallis so as to avoid large topographic obstacles that could pose hazards to the landing. However, this plain is not without its interesting, and in some cases rather problematic, landforms. The 100 km by 200 km landing ellipse contains the following features: (1) primary impact craters, (2) clusters of small secondary craters, (3) streamlined islands, (4) longitudinal grooves, (5) “scabland” or “etched” terrain, (6) pancake‐like shields and dike‐like structures, (7) knobs or buttes, and (8) a previously undetected, subtle undulating or hummocky texture to the plains surface. The nature of these landforms has important bearing on how we will interpret what we see at the scale of the Pathfinder lander once the first images are transmitted to Earth. With the exception of the craters, all of the remaining features described within the Mars Pathfinder landing ellipse can be interpreted as forming as a result of catastrophic flooding from Ares and Tiu Valles into Chryse Planitia, either during the flood itself, or through secondary modification of thick flood deposits after the event.
Small‐scale polygonal features on Mars: Seasonal thermal contraction cracks in permafrost
Authors: Mellon Michael T.
Published: February 2004

A time‐dependent viscoelastic model of thermal stress in Martian ice‐rich permafrost is developed to test the hypothesis that small‐scale polygonal features observed from orbit and by the Viking Lander 2 are the result of thermal contraction cracking, as commonly occurs in terrestrial permafrost. Results indicate that significant tensile stress occurs in Martian ice‐rich permafrost as a result of seasonal cycles in the ground temperature. Using conservative rheological parameters appropriate for ice at low temperatures, tensile stresses poleward of about 20° to 30° latitude easily exceed the tensile strength (assumed to be 2 to 3 MPa) and fractures should readily form. In the equatorial regions, special conditions may allow tensile stresses to approach the tensile strength. These results support a thermal contraction origin of observed small‐scale polygonal features and emphasize the utility of these features as valuable morphological indicators of ground ice.
Mars water abundance: An estimate from D/H ratio in SNC meteorites
Authors: Ghosh Suchita , Mahajan K. K.
Published: February 2004

Recent measurements of water composition in SNC meteorites have shown a high and variable deuterium to hydrogen (D/H) ratio. The highest D/H value is nearly equal to the present‐day value in the Mars atmosphere. This indicates that between 1.3 billion years (Gyr) ago and 180 million years (Myr) ago the Martian atmosphere may have attained the same D/H ratio as it has now. This also provides evidence for an interaction process involving water operating between atmosphere, regolith, and rocks in SNC meteorites. In this paper we have calculated the amount of water at present and at 4.5 Ga via the D/H fractionation process, assuming that water is lost to space as well as to the surface. A surface flux smaller than ∼ 108 cm−2 s−1 operating over the life history of the planet will not have any influence on the size of the fractionated water reservoir. However, we find that substantial surface loss of water will be required in case Mars' exchangeable water inventory has undergone transition from an initial large abundance to a small amount at present. Our results show that water which is lost to the surface would carry the D/H signature of the atmosphere of that period.
Comparison of microwave and infrared measurements of Martian atmospheric temperatures: Implications for short‐term climate variability
Authors: Richardson Mark I.
Published: February 2004

This paper presents the first comparison of simultaneous Viking infrared and ground‐based microwave measurements of the Martian atmosphere. The data are examined in order to investigate a 15–20 K difference between microwave and Viking measurements of mid‐level (10–40 km) air temperature. These data have been used by Clancy et al. [1990] to suggest that the Martian atmosphere is generally cooler and clearer than observed during the Viking era. This study suggests that the 15–20 K difference, which is most apparent during the non‐“dust‐storm” seasons, is not a real temperature difference, but instead results from a disagreement between the measurement techniques. The existence of this instrumental bias implies that the Martian climate has not substantially changed since the Viking era.
Fluctuations of terrestrial and marine biomarkers in the western tropical Pacific during the last 23,300 years
Authors: Ohkouchi N. , Kawamura K. , Taira A.
Published: February 2004

A sediment core collected from Caroline Basin, western tropical Pacific was analyzed for lipid class compounds (aliphatic hydrocarbons, long‐chain alkenones, fatty alcohols, sterols, and fatty acids) to reconstruct changes in paleoenvironments during the last 23.3 kyr. Around the boundary between the glacial and deglacial periods (19 ka), mass accumulation rates of terrestrial biomarkers, C25‐C35 n‐alkanes, C24‐C28 fatty alcohols, and C23‐C34 fatty acids, were found to decrease significantly and stayed in low levels during the deglaciation, suggesting a reduction of atmospheric transport of continental materials during that time. In the same period, mass accumulation rates of C17‐C20 n‐alkanes, pristane, cholesterol, and dinosterol which are thought to be mainly derived from marine organisms also decreased, suggesting a significant drop of marine biological productivity. The decreased biological productivity in the western tropical Pacific may be caused by a reduced supply of nutrients from upwelling which is associated with decreased wind velocity and/or caused by a shift of upwelling zone.
Sedimentary response to ocean gateway circulation changes
Authors: Heinze Christoph , Crowley Thomas J.
Published: February 2004

Previous modeling studies suggested that changes in ocean gateways may have exerted a dramatic influence on the ocean circulation. In this pilot study we extend those results to examining the potential ramifications of circulation changes on the sedimentary record. A version of the Hamburg carbon cycle/sediment model is used in these sensitivity experiments. Results indicate that internal reorganization of the ocean circulation can potentially cause very large regional changes in lysocline depth (1500–3000 m) and opal deposition. These shifts are sometimes comparable in magnitude to those imposed by changes in external forcing (e.g., climate, sea level, and weathering). Comparisons of the model response with the geologic record indicate some significant levels of first‐order agreement. This exercise suggests that opportunities now exist for physically based modeling of past sediment responses to circulation and climate changes.
Late Quaternary variations of sea surface salinity and temperature in the western tropical Atlantic: Evidence from δ18O of Globigerinoides sacculifer
Authors: Dürkoop Anke , Hale Walter , Mulitza Stefan , Pätzold Jürgen , Wefer Gerold
Published: February 2004

In order to reconstruct hydrographic changes during glacial‐interglacial cycles for a transequatorial transect we analyzed oxygen isotopes of Globigerinoides sacculifer (without sac‐like chamber) and abundances of Globorotalia truncatulinoides (dextral) from FS Meteor cores GeoB 2204‐2 (Brazilian continental slope) and GeoB 1523‐1 (Ceara Rise). Δδ18O values of G. sacculifer between the two cores were calculated. Modern Δδ18O (G. sacculifer) is ∼0.2‰ between the two core positions, reflecting differences in sea surface salinity (SSS). Higher SSS at GeoB 1523‐1 (Ceara Rise) is the result of increased precipitation in the region of the Intertropical Convergence Zone. During glacials the δ18O records from the two cores converge to the same absolute value, resulting in Δδ18O values of around 0‰. Maximum abundances of G. truncatulinoides (dex) correlate with minimum Δδ18O, suggesting a possible increase of SSS at GeoB 1523‐1 during stages 2, 3, 4, and 6, which is related to a glacial weakening of the tropical Hadley Cell [Gates, 1976]. Variations in tropical sea surface temperatures are assumed to be low [Climate: Long‐Range Investigation, Mapping, and Prediction (CLIMAP), 1981].
A 400 kyr record of combustion oxygen demand in the western equatorial Pacific: Evidence for a precessionally forced climate response
Authors: Perks Helen M. , Keeling Ralph F.
Published: February 2004

We have developed a combustion analysis technique for sediments which measures the amount of O2 consumed by the reduced species. We have measured this quantity, which we call “combustion oxygen demand (COD),” on a carbonate‐rich sediment core from the Ontong‐Java Plateau in the western equatorial Pacific back to marine oxygen isotope stage 11. The precision of the COD technique is ±6.3 µmol O2 g−1, which corresponds to ∼±0.0076% wt Corg, assuming oxidation of organic carbon dominates the signal. The COD time series is characterized by values which are about twice as high during glacials as during interglacials, the largest shift occurring from 401 µmol O2 g−1 in midstage 6 to 144 µmol O2 g−1 at 5e, and is coherent with the oxygen isotope curve of Globigerinoides sacculifer in the same core at the Milankovitch frequencies of 100 and 41 kyr. Pronounced variations in the 19–23 kyr band suggest that the climate of the western equatorial Pacific is sensitive to precessional forcing, a response not apparent from other records obtained in this region.
Volcano geodesy: The search for magma reservoirs and the formation of eruptive vents
Authors: Dvorak John J. , Dzurisin Daniel
Published: February 2004

Routine geodetic measurements are made at only a few dozen of the world's 600 or so active volcanoes, even though these measurements have proven to be a reliable precursor of eruptions. The pattern and rate of surface displacement reveal the depth and rate of pressure increase within shallow magma reservoirs. This process has been demonstrated clearly at Kilauea and Mauna Loa, Hawaii; Long Valley caldera, California; Campi Flegrei caldera, Italy; Rabaul caldera, Papua New Guinea; and Aira caldera and nearby Sakurajima, Japan. Slower and lesser amounts of surface displacement at Yellowstone caldera, Wyoming, are attributed to changes in a hydrothermal system that overlies a crustal magma body. The vertical and horizontal dimensions of eruptive fissures, as well as the amount of widening, have been determined at Kilauea, Hawaii; Etna, Italy; Tolbachik, Kamchatka; Krafla, Iceland; and Asal‐Ghoubbet, Djibouti, the last a segment of the East Africa Rift Zone. Continuously recording instruments, such as tiltmeters, extensometers, and dilatometers, have recorded horizontal and upward growth of eruptive fissures, which grew at rates of hundreds of meters per hour, at Kilauea; Izu‐Oshima, Japan; Teishi Knoll seamount, Japan; and Piton de la Fournaise, Réunion Island. In addition, such instruments have recorded the hour or less of slight ground movement that preceded small explosive eruptions at Sakurajima and presumed sudden gas emissions at Galeras, Colombia. The use of satellite geodesy, in particular the Global Positioning System, offers the possibility of revealing changes in surface strain both local to a volcano and over a broad region that includes the volcano.
Selected science highlights from the first 5 years of the Upper Atmosphere Research Satellite (UARS) Program
Authors: Dessler A. E. , Burrage M. D. , Grooss J. ‐U. , Holton J. R. , Lean J. L. , Massie S. T. , Schoeberl M. R. , Douglass A. R. , Jackman C. H.
Published: February 2004

We present scientific highlights from the Upper Atmosphere Research Satellite (UARS) program obtained during the first 5 years of the program. The UARS measures the chemical composition and meteorological state of the middle and upper atmosphere as well as the solar irradiance. The comprehensive suite of UARS geophysical parameters, their near‐global coverage, and the multiyear time span have yielded new insights into the processes affecting our atmosphere. In this article we select seven scientific issues, including stratospheric ozone chemistry, the dynamics of the middle and upper atmosphere, and solar irradiance variations, and discuss the significant impact that the UARS observations have had on our understanding of each issue.
Analysis of direction‐of‐arrival aliasing for MF/HF Doppler‐sorted interferometry measurements of ionospheric drift
Authors: Parkinson M. L. , Dyson P. L. , Smith P. R.
Published: February 2004

The determination of accurate direction of arrival (DOA) of echoes is paramount when performing Doppler‐sorted interferometry (DSI) at MF/HF to measure the drift of ionospheric plasma. Important factors affecting the accuracy include inadequate signal‐to‐noise ratio (SNR) and coherency of ionospheric echoes. As the SNR deteriorates, there is an increase in the errors in the phases measured at the individual antennas of an array, and this leads to ambiguities in DOA equivalent to directional aliasing. In this paper we model this effect, the effects of Doppler‐frequency aliasing, and the finite resolution of Doppler spectra. We thereby show the importance for vertical incident sounders of rejecting echoes with large off‐vertical angles, and the choice of appropriate signal‐processing parameters for the accurate estimation of ionospheric motions. As an example, we consider the situation for vertical incident ionosondes and show that a low SNR can produce spurious echoes at relatively large zenith angles, which can, however, be avoided or minimized by the suitable choice of operating parameters.
A relationship between horizontal flow gradients, in‐beam incidence angles, and vertical velocities
Authors: Larsen M. F. , Palmer R. D.
Published: February 2004

Interferometric radar measurements using vertically pointing beams generally show that the dominant scatter is coming from small off‐vertical angles within the beam. At longer wavelengths, such as those in the VHF frequency range, the effects are often attributed to aspect‐sensitive layers inclined at small angles with respect to horizontal. We show that off‐vertical in‐beam incidence angles can also be produced by gradients in the flow due to divergence and vorticity, for example, irrespective of whether aspect‐sensitivity effects are present. Since the divergence is intrinsically linked to the vertical velocity through the mass continuity equation, our results imply that vertical velocity measurements will always be biased to some extent if the off‐vertical incidence angles are not taken into account explicitly by making use of interferometric or extended variable azimuth display (EVAD) measurement techniques to correct for those biases.
Diffraction by a sinusoidal phase screen
Authors: Beach Theodore L. , Lovelace Richard V. E.
Published: February 2004

In order to develop computer simulations of diffraction patterns produced by scintillations, it is important to verify the simulations with examples of diffraction patterns that may be calculated analytically. One such example is classical knife‐edge diffraction from a conducting half plane. Here we present another test case based on the one‐dimensional sinusoidal phase screen first considered by A. Hewish. Using Huygens‐Fresnel diffraction theory, we derive expressions, as a function of sinusoid scale size (Λ) and maximum phase deviation (ϕ0) for (1) the intensity, (2) the horizontal wave number spectrum of the intensity pattern, and (3) the modulation index, seen by an observer situated below a one‐dimensional phase screen with an incident plane wave. Then we compare the analytic results with a computer simulation. Provided that the frequency resolution of the simulation is adequate, the simulation agrees quite well with the analytic results. Additionally, diffraction from a sinusoidal phase screen provides a case in which the influence of increasing distance from the screen on the modulation index (deviation of intensity from its mean) may be explored analytically for both weak and strong scattering.
A new parametric approach for wind profiling with Doppler Radar
Authors: Le Foll Gwenaëlle , Larzabal Pascal , Clergeot Henri , Petitdidier Monique
Published: February 2004

In this paper, we propose a new approach for wind profile extraction with Doppler radar. To perform this, we first focus on the analysis and modeling of VHF or UHF waves backscattered by clear‐air turbulence. A physical description of the backscattered wave is given. This description involves a spectral model that includes a parametric profile of the Doppler spectrum. A parametric approach of the wind profile can be easily generated. The sounding volume is divided into slabs whose thickness is consistent with that of the expected homogeneous turbulent layer. The echo spectrum of each slab is supposed Gaussian. Thus, for the range gate, the backscattered spectrum is a priori non‐Gaussian, since it is weighted by a nonconstant reflectivity. This represents a more realistic assumption than the classical ones. The realistic temporal model thereby obtained can be used in simulation, which provides a valable tool for testing the extraction algorithm. An original recursive fitting, in terms of maximum likelihood, between the experimentally recorded spectrum and the parametric candidate spectrum is described and implemented as a second‐order, steepest‐descent algorithm. This optimization problem is solved in a weighted fashion on the entire gate simultaneously. The regularized parametric method, described in this paper, is a way to minimize some of the drawbacks encountered with traditional methods. Simulations reveal good statistical performance compared with traditional methods. The algorithm is then tested on real data. To achieve this, original methods are proposed for noise suppression and clutter removal.
Wind estimation errors of the spaced antenna technique studied with simulations and observations: A case study for the MU radar
Authors: Tahara Yukihiro , Yamamoto Mamoru , Fukao Shoichiro
Published: February 2004

We studied random error of horizontal wind velocity estimated by the spaced antenna (SA) technique by means of computer simulations with one spatial dimension. Complex time series of data were generated for the no‐noise case and were processed through the full correlation analysis (FCA) to estimate horizontal wind velocities. The simulation parameters were chosen to simulate typical observations with the middle and upper atmosphere (MU) radar. For simplicity we assumed isotropic scattering in the simulations. Comparing performances of the SA and Doppler techniques, we found that the estimation error of the SA winds is larger than that of the Doppler winds in most cases. For large wind velocities, however, the SA technique showed better performance than the Doppler technique. By using the MU radar, we conducted simultaneous multibeam/interferometry observations to evaluate the error of wind estimation from real data. Performance of the SA winds obtained from the observations was seen to be similar to that obtained from the computer simulations.
Rates of scattering occurrence in routine HF radar observations during solar cycle maximum
Authors: Ruohoniemi J. M. , Greenwald R. A.
Published: February 2004

The HF radars of the Super Dual Auroral Radar Network (SuperDARN) provide continuous monitoring of the high‐latitude ionosphere over large areas in both the northern and southern hemispheres. The HF technique generates estimates of the E×B convective drift of ionospheric plasma when suitable small‐scale (∼10 m) irregularities are present and viewed under favorable magnetic aspect conditions (radar k vector ⊥ to B). In this paper we report on the statistics of HF scattering occurrence. The study period encompasses 5.5 years (1988 to mid‐1993) of routine observations with the HF radar located at Goose Bay, Labrador, Canada, and corresponds to the most recent period of solar cycle maximum. The data were carefully filtered for those instances when high‐confidence F region velocity measurements were obtained within 1°×3° magnetic latitude/longitude regions and 12‐min UT intervals. For average conditions the rates at which scatter occurred at particular values of invariant latitude ranged from <10% to >40% with MLT. The probability of making velocity measurements somewhere in the radar field of view varied from ∼80% on the nightside to ∼45% on the noon meridian. When scatter was detected, the average latitudinal coverage varied from a minimum of 4° Λ on the dayside to over 6° Λ on the nightside. There were significant dependencies on Kp and season. The highest occurrence rates (>60%) were obtained on the nightside for quiet conditions and in the afternoon for disturbed conditions. Winter was the most active season. HF scattering was limited by the equatorward boundary of the Feldstein oval but extended poleward of the oval into the polar cap. Scattering in the noon sector was associated with the aggregate cusp/mantle/low‐latitude boundary layer region. The scattering activity was notably suppressed in the morning sector under disturbed conditions and in the noon and afternoon sectors in summer. We discuss the geophysical factors that influence the likelihood of generating HF scatter. These results have value for the design of experiments involving the SuperDARN HF radars.
Influence of suprathermal electrons upon the transfer impedance of a quadrupolar probe in a plasma
Authors: Grard Réjean
Published: February 2004

Measuring the coupling impedance of two dipoles in a plasma is a diagnostic technique which has been used with success for several decades in space and in the laboratory. The plasma density and temperature are directly derived from the transfer impedance in a thermalized plasma, but the interpretation of the measurements is not as straightforward in the presence of suprathermal electrons. The purpose of this analytical work is to evaluate the transfer impedance of a quadrupolar array in the presence of two electron populations with different densities and temperatures. Such plasma mixtures are encountered not only in planetary environments but also in the close vicinity of cometary nuclei. It is concluded that it is possible to characterize the properties of the thermal electrons with a reasonable accuracy (≃10%) using an array with dimensions about 10 times the Debye length, even in the presence of a suprathermal population with a temperature 50–100 times larger and a comparable density.
Equatorial bubbles observed at the north and south anomaly crests: Dependence on season, local time, and dip latitude
Authors: Whalen J. A.
Published: February 2004

Equatorial bubbles, which cause scintillation that disrupts transionospheric RF propagation, are difficult to study because they are unpredictable and extremely variable. This work undertakes to study them by continuous observation throughout a year at solar maximum using ionospheric sounders operating at sites in the western hemisphere in both north and south Appleton anomaly crests. Here direct observation in an extensive case study has found that bubbles can be identified when passing over a sounder near the Appleton anomaly crest by the appearance of range spread F (RSF) that is so severe that it prevents evaluation of ƒoF2. Using this obscuring spread F as a signature, bubbles are recorded in the hourly tabulations of ƒoF2 on 37 days a total of 104 times by three anomaly sounders at dip latitudes of 16° and 20.3° in the north and 19.9° in the south. At each location, occurrence is maximum near the equinoxes, zero in July with a minimum in December, and maximum between 2200 and 2400 LT. Interpretation of the signature is supported by its consistency in both season and local time with bubbles observed by others. Overall, occurrence decreases by a factor of 0.38 between 16° and 20° dip latitude, apparently symmetrically about the dip equator. Whereas RSF at the anomaly always results from bubbles, RSF at the equator does not, the most evident period being near December when bubbles have a minimum but equatorial RSF has a maximum, occurring nearly every day following sunset. This nonbubble equatorial RSF apparently results from bottomside spread F.
An effect of receiver noise on the measurement of Doppler spectral parameters
Authors: Gordon William B.
Published: February 2004

We consider the Doppler power spectra calculated from radar precipitation echos. It is shown that receiver noise can produce a spurious linear realtion between the spectral width and radar range. This effect occurs at all elevation angles and disappears when the effects of receiver noise are eliminated by means of a thresholding technique. A simple model explains why this effect occurs when the signal spectrum is sufficiently narrowband and the variation of radar reflectivity isnot too great over the range interval of interest. Examples are given from data taken with a coherent X‐band radar.
Comparisons of ionospheric total electron contents made at Boulder, Colorado, using the Global Positioning System
Authors: Conkright Raymond O. , Davies Kenneth , Musman Steven
Published: February 2004

We discuss two different methods of deriving ionosphere total electron content (TEC) from observations of Global Positioning Satellites (GPS) at Boulder, Colorado. In the first method, data from the National Institute of Standards and Technology Ionospheric Monitoring System (NIMS) are examined for the period of January 1994 to January 1995. In the second method, carrier phase models are constructed for four days in January 1995 and at two stations near Boulder. Both methods are compared with Faraday rotation values. Smoothed NIMS observations are an adequate representation for smoothed TEC variation during the day. Carrier phase models show good agreement with Faraday rotation and consistency between nearby receivers.
Faraday rotation in a cold, inhomogeneous magnetoplasma: A numerical comparison of ray and full wave analyses
Authors: Barnes Rod I.
Published: February 2004

A numerical full wave analysis is used to determine the spatial variation of electromagnetic fields of a radio wave in a stratified ionosphere. It is shown that the Faraday rotation may be accumulated as the analysis proceeds by breaking the electric field into constituent characteristic modes and accumulating phase. This has been achieved for vertical, oblique, reflective, and transionospheric propagation. Comparison of the full wave analysis with simpler methods of calculating the rotation, such as ray tracing and very high frequency approximations, permits the determination of the conditions under which these simpler methods break down. It is found that ray tracing agrees very closely with the full wave analysis in most situations and always so for transionospheric propagation. Some discrepancy between these methods appears when reflecting from severe vertical gradients where the full wave analysis is known to be required in accurately estimating the transmission coefficients. For reflective propagation at oblique incidence, small departures in the methods are also evident. Both methods converge to the high‐frequency approximation as the radio frequency increases above the peak plasma frequency.
Electromagnetic scattering from bounded or infinite subsurface bodies
Authors: Saillard M. , Toso G.
Published: February 2004

The direct scattering problem from a stack of two rough interfaces or from a buried object with a rough interface above it is addressed. The two boundaries are separating three homogeneous and isotropic media. The profiles are assumed to be invariant in one (and the same) direction, and the probing field is time harmonic. The problem is solved with a rigorous integral method leading to a single boundary integral equation. This equation is converted into a linear system of equations, thanks to a boundary finite element method. To improve the accuracy, especially when the two boundaries are very close to each other, the singularity of the kernel is isolated and its integration is performed analytically. As shown through several examples, this versatile tool is well suited for the investigation of physical phenomena, especially the competition between surface and subsurface scattering.
Total electron content over the Pan‐American longitudes: March‐April 1994
Authors: Doherty Patricia H. , Anderson David N. , Klobuchar John A.
Published: February 2004

An experimental campaign to measure diurnal changes in total electron content (TEC) over the wide latitude range from approximately 50°N to 40°S was carried out from March 28 through April 11, 1994, by monitoring the differential carrier phase from the U.S. Navy Navigation Satellite System using a chain of ground stations aligned along the approximate 70°W longitude meridian. This Pan‐American campaign was conducted primarily to study the day‐to‐day variability of the equatorial anomaly region. The experimental plan included using the received values of TEC from the chain of stations to construct profiles of electron density versus latitude using tomographic reconstruction techniques and, then, to compare these reconstructions against a theoretical model of the low‐latitude ionosphere. The diurnal changes in TEC along this latitude chain of stations showed a high degree of variability from day to day, especially during a magnetic storm which occurred near the beginning of the campaign. The equatorial anomaly in TEC showed large changes in character in the two hemispheres, as well as differences in magnitude from day to day. The latitudinal gradients of TEC, especially in the lower midlatitudes, also showed large differences between magnetic storm and quiet conditions. Comparisons of the TEC data with the theoretical model illustrate the sensitivity of the model calculations to changes in magnetic E×B drift and serves to validate the strong influence that these drifts have on the formation and the strength of the equatorial anomaly regions.
Influence of coupled environmental features on propagation characteristics for an RF and IR multisensor radar system
Authors: Goldhirsh Julius
Published: February 2004

We address the question, Given a multifrequency system comprising an RF radar and an IR sensor, what is the probability of “good multisensor propagation”? We define good multisensor propagation as one in which there is no fog and/or the RF propagation conditions are at the same time superrefractive. In addressing this question, we define “good IR propagation” as that occurring in the absence of fog, and “good RF propagation” as the condition in which the propagation is superrefractive. Data were analyzed from 981 radiosondes launched during 1973 and 1974 by the U.S. Air Force at three Iranian coastal locations on and in the vicinity of the Persian Gulf. Statistics were derived which demonstrate that 98% of the time, good multisensor propagation will exist, with 99% likelihood during the summer and 94% during the winter. Ancillary statistics associated with the likelihood of occurrence of the following environmental and propagation features were derived: the potential for fog, good RF propagation, temperature inversions, extreme subrefraction, and ducting.
Coherent radar imaging: Signal processing and statistical properties
Authors: Woodman Ronald F.
Published: February 2004

The recently developed technique for imaging radar scattering irregularities has opened a great scientific potential for ionospheric and atmospheric coherent radars. These images are obtained by processing the diffraction pattern of the backscattered electromagnetic field at a finite number of sampling points on the ground. In this paper, we review the mathematical relationship between the statistical covariance of these samples, (

†), and that of the radiating object field to be imaged, (

†), in a self‐contained and comprehensive way. It is shown that these matrices are related in a linear way by (

†) = aM(FF†)M†a*, where M is a discrete Fourier transform operator and a is a matrix operator representing the discrete and limited sampling of the field. The image, or brightness distribution, is the diagonal of (FF†). The equation can be linearly inverted only in special cases. In most cases, inversion algorithms which make use of a priori information or maximum entropy constraints must be used. A naive (biased) “image” can be estimated in a manner analogous to an optical camera by simply applying an inverse DFT operator to the sampled field
and evaluating the average power of the elements of the resulting vector
. Such a transformation can be obtained either digitally or in an analog way. For the latter we can use a Butler matrix consisting of properly interconnected transmission lines. The case of radar targets in the near field is included as a new contribution. This case involves an additional matrix operator b, which is an analog of an optical lens used to compensate for the curvature of the phase fronts of the backscattered field. This “focusing” can be done after the statistics have been obtained. The formalism is derived for brightness distributions representing total powers. However, the derived expressions have been extended to include “color” images for each of the frequency components of the sampled time series. The frequency filtering is achieved by estimating spectra and cross spectra of the sample time series, in lieu of the power and cross correlations used in the derivation.
Tropical Rainfall Measuring Mission microwave imaging capabilities for the observation of rain clouds
Authors: Bauer P. , Bennartz R.
Published: February 2004

Brightness temperature simulations were used to assess the information loss due to the coarse spatial sampling of the Tropical Rainfall Measuring Mission (TRMM) microwave imager (TMI). While the high‐resolution simulation of a three‐dimensional rain cloud clearly identifies the 10.7‐GHz channel to be superior for surface rainfall measurements, its comparably poor spatial resolution reduces the dynamic range of observable brightness temperatures and associated area‐averaged rain rates considerably. This problem can be partly overcome by applying deconvolution techniques as demonstrated by Robinson et al. [1992]. When the sensitivity of resolution enhancement to noise amplification is investigated, an optimum configuration of key parameters was identified. Using an 11×11 matrix of neighboring pixels and noise reduction parameter of γ = 0.5° provides a 42×26 km resolution, compared with the original resolution of 63×38 km. Thus a noise amplification by a factor of 3 is introduced. Although this method does not match the resolution of the 19.35‐GHz channel, the dynamic range of brightness temperatures and area‐averaged rain rates could be increased by a factor of 2.7 and 2.2, respectively. Further resolution enhancement leads to noise levels which seem unacceptable with respect to the retrieval of geophysical parameters.
Confidence interval estimation for VHF Doppler radar measurements of wind velocities
Authors: Astin Ivan
Published: February 2004

Linear model theory is used to find exact confidence intervals for wind components from radial wind speeds observed using a VHF Doppler radar. The theory is also used to evaluate exact confidence intervals for wind direction. However, exact confidence intervals for wind speeds can only be found where the errors in the two estimated horizontal wind speed components are equal. Thus, in the general case, a Taylor expansion has to be used to give approximate confidence intervals for wind speeds. It is shown that irrespective of the wind vector, the mean confidence interval for the wind components, or the confidence volume of the wind vector, can be minimized by using radar beams with the largest off‐vertical angles. However, there is no optimal sounding strategy for the measurement of wind speed or direction that can be defined before making any measurements.
Multiple aircraft microwave observations of storms over the western Pacific Ocean
Authors: Wang J. R. , Zhan J. , Racette P.
Published: February 2004

During the second half (January–February 1993) of the intensive observation phase of the Tropical Ocean Global Atmosphere/Coupled Ocean‐Atmosphere Response Experiment (TOGA/COARE), NASA deployed both DC‐8 and ER‐2 research aircraft in Townsville, Australia. A number of flights were made over the western Pacific Ocean with the objective of studying strong equatorial convection and clouds. A host of microwave radiometers operating in the frequency range of 10–220 GHz were placed on board both aircraft. Many other instruments were also present in both aircraft; among them was a 13.8‐GHz radar in the DC‐8 aircraft that provides reflectivity profiles of atmospheric hydrometeors essential for the present study. Observations were made by both aircraft on weather systems of convection as well as of cloud radiation. Some of these observations were conducted with both aircraft flying over the same regions at near coincidence but different altitudes. The nearly concurrent microwave radiometric measurements at comparable frequencies from the two aircraft at the altitudes of about 11 and 20 km provide a unique means to examine the properties of hydrometeors associated with storms. Three well‐coordinated DC‐8 and ER‐2 aircraft flights over storms are described below. All of the microwave radiometers and the 13.8‐GHz radar functioned normally during these flights. Some radiometers on the ER‐2 and the DC‐8 aircraft have comparable frequencies; signatures acquired by these radiometers from these flights are analyzed in detail. It is shown that storm‐associated brightness temperature depressions observed at frequencies ≤90 GHz from two different altitudes are quite comparable. At high frequencies ≥150 GHz the brightness temperature depressions observed at 20‐km altitude are significantly larger than those observed at 11‐km altitude in some portions of the storms. This suggests the presence of hydrometeors in the 11‐ to 20‐km altitude region that effectively scatter radiation at frequencies ≥150 GHz.
Microwave imaging of the brightness temperature distribution of extended areas in the near and far field using two‐dimensional aperture synthesis with high spatial resolution
Authors: Peichl Markus , Suess H. , Suess M. , Kern S.
Published: February 2004

Aperture synthesis is an attractive alternative to conventional passive microwave imaging systems for remote sensing applications. Whenever a high spatial resolution is desired or the imaging process should work like an optical camera or when microwave remote sensing in the near field has to be distance adaptive, conventional systems cause a lot of problems. However, aperture synthesis for practical applications requires a lot of single receivers and correlators. This can cause other problems because of the large number of possible imperfections. To get an estimation of such effects, the imaging expressions for the aperture synthesis principle are developed for the near‐ and far‐field conditions of a real system. To support the theory and to get an estimation of the feasibility of this imaging method, a simple experimental system is described and experimental results of high spatial resolution for the near and far field are shown. For the near‐field case a special reconstruction algorithm was developed and is described theoretically and validated by the experiments. A discussion of the temperature resolution of aperture synthesis concludes the paper.
Tomography of the ionosphere: Four‐dimensional simulations
Authors: Howe Bruce M. , Runciman Kay , Secan James A.
Published: February 2004

Using a four‐dimensional stochastic model of ionosphere perturbations, simulations are made of a tomography system based on data from the Global Positioning System and a low Earth‐orbiting satellite. The perturbations are departures from a simple time‐independent reference state. The spatial structure is parameterized in terms of empirical orthogonal functions (EOFs) in the vertical and spherical harmonics in the horizontal. The horizontal covariance structure is specified by variance and correlation length scales as functions of latitude and longitude. Time dependence is modeled as a first‐order Markov process with a 6‐hour timescale and white‐noise forcing. A Sun‐fixed coordinate system is used so that ionospheric features are more nearly steady in time. A Kaiman filter is used to objectively assimilate the simulated data into the simple time‐dependent model. In addition to solving for the three‐dimensional electron density field at each time step, the procedure solves for instrumental biases. The simulations show that the fractions of resolved variance for vertical EOF modes 1, 2, and 3 are 0.99, 0.93, and 0.73, respectively. The resolution of the vertically integrated total electron content is 0.99.
Series expansions for the electric and magnetic fields produced by a line current source
Authors: Pirjola Risto J.
Published: February 2004

The electric and magnetic fields that an overhead infinitely long line current produces at the surface of the Earth can be expressed as inverse Fourier integrals over a horizontal wave number. In the quasi‐static approximation acceptable in most applications and assuming a uniform Earth, expressions in terms of the Neumann and Struve functions are also possible. These are further developed in this paper, resulting in new infinite series expansions, not presented before, for the electric and magnetic fields. An application of the series expansions will remove the need to calculate the standard integral expressions, and performing exact numerical computations will be straightforward.