bluuebunny/crossref_metadata_2025 huggingface dataset:
Eruption of lava flows on Europa: Theory and application to Thrace Macula
Authors: Wilson Lionel , Head James W. , Pappalardo Robert T.
Published: February 2004

Europa, on the basis of its spectral characteristics and its apparently young surface age, is likely to be at least partially resurfaced by liquid water volcanic processes; however, convincing evidence for lava flow morphology in Voyager data has not previously been found, and the lower density of liquid water relative to an overlying solid water ice crust suggests that extrusive volcanism could be uncommon. We examine a candidate lava‐flow‐like feature (Thrace Macula) and compare its characteristics with theoretical predictions for the ascent and eruption of water magma on Europa, outlining a range of conditions under which water ice magma could plausibly be erupted to the surface. Thrace Macula (45°S, 171°W) is a dark lobate feature extending NNW to SSE; its northern part is centered in a 140 km diameter arcuate to circular lineament, and within this zone are radial and concentric lineaments and three dark lobes similar in appearance to flow lobes extending away from a central region for distances of 40, 50, and 160 km. The 160 km lobe appears to follow local topographic features, producing lobate margins at subdued structures, broadening locally parallel to structural trends, and terminating against a gray band. Models for the present structure and state of the predominantly H2O exterior of Europa range from solid water ice to a thin ice layer overlying a liquid water layer, and these conditions may even vary as a function of space and time. Ascent and eruption of H2O magma through a less dense water ice crust means that sustained effusive eruptions are very difficult to produce if a globally continuous liquid water (or salt‐water) layer exists [e.g., Crawford and Stevenson, 1988; Kargel, 1991]. For flows as extensive as the Thrace Macula feature, we thus require magma source regions sufficiently localized that excess pressure can be built up in the source region. Given the elastic properties of ice at these source depths, the most likely scenario is intrusion of silicate magma into the base of an ice crust, and melting and mobilization of water magma. With this range of magma source settings, we model plausible surface vent geometries for a water lava flow interpretation of Thrace Macula. Modeling calculations show that flow lobes with the above geometries, and thicknesses on the order of 10 m, could be formed in water eruptions on Europa involving discharge rates of ∼2×105 m3/s through dikes with widths in the range 5–10 m. These discharge rates (about 3 times larger than those of terrestrial flood basalts) should not be unusual for water eruptions on Europa under these conditions and suggest that water volcanic resurfacing could be common on the satellite if source regions meeting these criteria were readily available. The lack of abundant similar dark flows could be due to brightening with time or might imply that conditions for their formation (grounded or nearly solid ice crust) only evolved in the latter part of Europan history.
Venera 8 landing site geology revisited
Authors: Basilevsky Alexander T.
Published: February 2004

Photogeologic mapping of the Magellan images of the Venera 8 site showed that practically all geologic units of this area correlate with the geologic units distinguished by Basilevsky and Head [1995a, b] in many other regions of Venus; however, areal abundance of the units in different areas may significantly differ. In particular, an anomalously high abundance of small volcanic shields was found in the landing site area and especially in the landing circle thus suggesting that the material sampled by the Venera 8 could represent these shields. The formation of shield fields, instead of vast basaltic floods more typical for Venus, is believed to be due to low magma replenishment rates [Crumpler et al., 1997]. These conditions might favor intra‐chamber magma differentiation and/or contamination in the crustal material, thus being a cause of the enrichment of the material sampled by the Venera 8 in K, U, and Th. This conclusion is very tentative, and a possible petrologic link between the presence of shield fields in the landing site and the nontholeiitic character of the sampled material demands further studies. The observed correlation between the high abundance of small volcanic shields in the landing area and the nontholeiitic character of the sampled material implies that other small shield field localities on Venus could be possible sites of high‐K nontholeiitic materials too. The Venera 8 landing site should be considered as one of the potential targets for future missions to Venus designed to study the surface geochemistry of different geologic units.
Metamorphic history of the eucritic crust of 4 Vesta
Authors: Yamaguchi Akira , Taylor G. Jeffrey , Keil Klaus
Published: February 2004

The petrologic properties of eucrites indicate that most formed as rapidly cooled lava flows that were subsequently metamorphosed at >800°C for periods of about 1 Myr. We have modeled the development of the eucritic crust of the eucrite parent body, assumed to be asteroid 4 Vesta, by the eruption and subsequent burial of lava flows (serial magmatism). The burial of the flows causes them to reheat to metamorphic temperatures (>800°C). Provided that the initial thermal gradient in the upper 5 km was steep, as it would be if 26Al was the heat source, our model shows that most eucrites were metamorphosed for periods over 1 Myr in a crust between 15 and 25 km thick, if volcanism lasted only 1 to a few Myr. Longer periods of volcanism do not result in high enough peak metamorphic temperatures to be consistent with eucrite data. The hot lower crust in Vesta provides an environment in which even small (few meters) dikes could have cooled slowly enough to produce cumulate eucrites and possibly diogenites.
Organic degradation under simulated Martian conditions
Authors: Stoker Carol R. , Bullock Mark A.
Published: February 2004

We report on laboratory experiments which simulate the breakdown of organic compounds under Martian surface conditions. Chambers containing Mars‐analog soil mixed with the amino acid glycine were evacuated and filled to 100 mbar pressure with a Martian atmosphere gas mixture and then irradiated with a broad spectrum Xe lamp. Headspace gases were periodically withdrawn and analyzed via gas chromatography for the presence of organic gases expected to be decomposition products of the glycine. The quantum efficiency for the decomposition of glycine by light at wavelengths from 2000 to 2400 Å was measured to be 1.46±1.0×10−6 molecules/photon. Scaled to Mars, this represents an organic destruction rate of 2.24±1.2×10−4 g of C m−2 yr−1. We compare this degradation rate with the rate that organic compounds are brought to Mars as a result of meteoritic infall to show that organic compounds are destroyed on Mars at rates far exceeding the rate that they are deposited by meteorites. Thus the fact that no organic compounds were found on Mars by the Viking Lander Gas Chromatograph Mass Spectrometer experiment can be explained without invoking the presence of strong oxidants in the surface soils. The organic destruction rate may be considered as an upper bound for the globally averaged biomass production rate of extant organisms at the surface of Mars. This upper bound is comparable to the slow growing cryptoendolithic microbial communities found in dry Antarctica deserts. Finally, comparing these organic destruction rates to recently reported experiments on the stability of carbonate on the surface of Mars, we find that organic compounds may currently be more stable than calcite.
SO2‐rock interaction on Io 2. Interaction with pure SO2
Authors: Burnett D. S. , Goreva Julia , Epstein S. , Haldemann Susan L. , Johnson Mary L. , Rice Alan
Published: February 2004

A Na‐S mineral on the surface of Io is required to be the source of the famous atomic cloud. SO2 is a confirmed atmospheric and surface constituent, and because of the rapid volcanic resurfacing rate, the SO2 is buried within the crust, where at least occasionally, over many cycles of burial and eruption, it must contact silicate materials at midlevel crustal temperatures. Surface interaction experiments were performed for a wide variety of silicate compositions showing that interaction products of these with SO2 could be observed at 1123 K on laboratory timescales, even in the absence of external redox agents. Not all experiments produced deposits that could be studied by scanning electron microscopy; some required the greater sensitivity of photoelectron spectroscopy (XPS). Characterization of the alteration products by XPS showed that both oxidized and reduced sulfur species were formed, indicating that a disproportionation mechanism producing a sulfate and a reduced S species although smaller amounts of interaction leading to Na2SO3 formation cannot be ruled out. The reduced sulfur species is best explained as elemental S which was independently documented for two compositions. Scanning electron microscopy studies for those compositions where reaction was extensive enough to be observed showed (1) Na2SO4 for a soda‐lime composition, (2) a mixed Na‐Ca‐sulfate liquid and CaSO4 for AbAnDi and a chondrule glass composition, and (3) Fe‐sulfate for a natural obsidian. Infrared spectroscopy for the soda‐lime glass composition showed peaks best explained by Na2SO4. We conclude that SO2 disproportionation as well as direct formation from SO3 under oxidizing conditions can produce Na2SO4 by interaction of SO2 with silicates on Io, but Ca and Fe sulfates may form preferentially in more basaltic compositions. As highly oxidizing conditions may be unlikely for Io, the disproportionation mechanism may be more competitive on Io than it is in laboratory experiments. Very low rates of Na2SO4 production are required to supply the Io atomic cloud, so the interaction processes can be very inefficient.
Filling of flexural moats around large volcanoes on Venus: Implications for volcano structure and global magmatic flux
Authors: McGovern Patrick J. , Solomon Sean C.
Published: February 2004

The absence of topographic moats and concentric normal faulting around large volcanoes on Venus is attributed to filling of the annular flexural depression by lava flows from the central edifice. Large volcanoes on Venus are characterized by prominent, approximately conical edifices surrounded by relatively flat flow aprons. The surfaces of both of these constructional components consist dominantly of radially oriented flows. From analytic plate flexure models, we generate a synthetic stratigraphy for Venus volcanoes from which we calculate the volume of material filling the flexural moats to the level of the flow apron. The total volume of volcano‐associated extrusive lavas, including moat fill, can be an order of magnitude greater than the volume of the edifice alone. Extended to all large volcanoes on Venus, this procedure yields estimates for magmatic flux on Venus comparable to the present terrestrial intraplate extrusive flux, but this scenario is nonetheless consistent with observations of areally limited resurfacing since the most recent global resurfacing event. The absence of flexure‐related tectonic features around most large volcanoes, by the arguments advanced here, is attributable to masking by apron flows and low stresses in the apron moat fill. On Venus, a volcanic edifice and associated moat‐filling material constitute a single structurally coherent unit. Large volcanoes on Venus thus appear to be structurally distinct from those on Earth and Mars.
Isotopic fractionation in low‐energy ion implantation
Authors: Ponganis K. V. , Graf T. , Marti K.
Published: February 2004

The evolutions of planetary atmospheres and other solar system reservoirs have been affected by a variety of fractionating mechanisms. It has been suggested that one of these mechanisms could be low‐energy ion implantation. Bernatowicz and Hagee [1987] stowed that Kr and Xe implanted at low energy onto tungsten are fractionated by approximately 1% per amu, favoring the heavy isotopes; we confirm these effects. We have extended these studies to Ar and Ne, using a modified Bayard‐Alpert type implanter design of cylindrical symmetry with collector potentials of −40 to −100 V, and observe systematically larger mass dependent isotopic fractionation for argon and neon, ≥3% per amu and ≥4% per amu, respectively. These fractionations scale approximately as Δm/m for all of the noble gases measured, consistent with the findings of Bernatowicz and coworkers. Experimental data at higher energies and predictions by TRIM (Transport of Ions in Matter) code simulations indicate that sticking probabilities may depend upon the mass ratios of projectile and target. Many natural environments for low‐energy ion implantation existed in the early solar nebula, such as in dusty plasmas or in the interaction of the bipolar outflow with small grains or in the wind of the early active Sun with accreting planetesimals. Low‐energy ions provide viable sources for gas loading onto nebular dust grains; the result is isotopic and elemental fractionation of the projectiles.
Optical scattering properties of terrestrial varnished basalts compared with rocks and soils at the Viking Lander sites
Authors: Guinness Edward A. , Arvidson Raymond E. , Clark Ian H. D. , Shepard Michael K.
Published: February 2004

Basalt flows at the Pisgah Volcanic Field, California, are mantled by aeolian silt‐sized deposits. The bright mantle is overlain by a dense pavement of basalt cobbles smoothed by aeolian abrasion and coated with glossy dark gray and red desert varnishes. In contrast, the two Viking landing sites on Mars lack desert pavements, and rocks have glossy surfaces that in some cases are brighter than soils. To pursue further the scattering characteristics of varnished basalt cobbles for comparison to Mars rocks, laboratory measurements of bidirectional reflectance (0.69 μm center wavelength) were made as a function of incidence and emission angle along the principal plane for varnished and fresh basalt surfaces. The reflectance data, fit with a scattering model that includes specular reflections from rough surfaces and volume penetration and reflection, show that the red varnished surfaces have higher single scattering albedoes and higher surface roughness values than the dark gray varnished surfaces (0.56 and 0.42; 21° and 17°, respectively). Fresh cut surfaces have lowest values (0.40 and 6°). Results are consistent with observations made from thin sections for these surfaces and with field observations that show dark gray varnished surfaces are smoothed by exposure to wind abrasion. Dark red varnished surfaces tend to be rougher because they are embedded in the aeolian mantle and develop in a protected environment. Application of the model to atmospherically corrected, radiometrically calibrated Viking Lander images (0.60 to 0.74 μm) shows that many rocks are smoother and have single scattering albedoes higher than soils (9° versus 27°; 0.71 versus 0.63, respectively). Results are consistent with a model in which rocks at the surface of Mars become smoothed by aeolian abrasion and coated with atmospheric dust. Lack of desert pavement on Mars is probably associated with a scarcity of cobbles and a lack of soil forming processes. Similar bright, smooth coatings are expected at other locations on Mars, including the Pathfinder Ares Vallis site.
Radiolarian assemblages from Santa Barbara Basin sediments: Recent interdecadal variability
Authors: Weinheimer A. L. , Cayan D. R.
Published: February 2004

Santa Barbara Basin contains a sedimentary record ideal for high‐resolution paleoclimate studies because of the annual varves and regional‐ to global‐scale climate signals preserved in the sediments [Lange et al., 1990; Kennett and Ingram, 1995], even though it does not lie directly in the path of the California Current. A nearly 100‐year annual time series (1909–1991) of polycystine radiolarian assemblages from Santa Barbara Basin (SBB) sediments was analyzed to the species level. Counts on a replicate SBB core, dated 1870–1987, indicate that results are reproducible and the flux of a few representative species can be extrapolated to estimate fluxes of environmentally sensitive groups. The frequency of species occurrences resembles a lognormal curve and year‐by‐year comparisons of species fluxes revealed only modest changes in the assemblages from 1909–1991, indicating that the assemblages represent a single oceanic province. For paleoceanographic analysis of the radiolarian record, species were combined into groups according to the water mass in which they occur. To test this method, temperature‐sensitive species were identified using t‐ tests. This generated warm and cool classes exhibiting trends in relative flux similar to those of the water mass groups. Both total flux and relative fluxes of water mass groups relate to low‐frequency, decadal‐scale temperature fluctuations, but not strongly to El Niño‐Southern Oscillation events. Generally, fluxes of species from different water masses covary suggesting changing carrying capacities and productivity through time, while the consistent inverse relationship in relative fluxes indicate variability in climate. The subtle, decadal‐scale changes in assemblages, diversity, and increase in percent warm water fauna are consistent with a spin‐down of the California Current System suggested by other records.
Glacial and Holocene stable isotope distributions in the southeastern Indian Ocean
Authors: McCorkle Daniel C. , Heggie David T. , Veeh Herb H.
Published: February 2004

We have reconstructed glacial‐interglacial variations in Cibicidoides wuellerstorfi δ13C and δ18O in the southeastern Indian Ocean using depth transects of cores from off western and southern Australia. We compare these new results with published stable isotope records to constrain glacial‐interglacial changes in Indian Ocean chemistry and circulation. The strong last glacial maximum (LGM) deep water 13C depletions previously observed for the northeastern Indian Ocean did not extend throughout the low‐latitude glacial Indian Ocean. We observe only a small LGM δ13C decrease off southern Australia, in marked contrast to the strong LGM 13C depletions observed in many cores from the Atlantic and western Indian sectors of the Southern Ocean. It is not yet clear whether the very low LGM δ13C values in the deep northern and southwestern Indian Ocean are an artifact of surface ocean productivity or whether they reflect real gradients in deep water chemistry.
Temperatures at the last interglacial simulated by a coupled ocean‐atmosphere climate model
Authors: Montoya Marisa , Crowley Thomas J. , von Storch Hans
Published: February 2004

The last interglacial (Eemian, 125,000 years ago) has generally been considered the warmest time period in the last 200,000 years and thus sometimes been used as a reference for greenhouse projections. Herein we report results from a coupled ocean‐atmosphere climate model of the surface temperature response to changes in the radiative forcing at the last interglacial. Although the model generates the expected summer warming in the northern hemisphere, winter cooling of a comparable magnitude occurs over North Africa and tropical Asia. The global annual mean temperature for the Eemian run is 0.3°C cooler than the control run. Validation of simulated sea surface temperatures (SSTs) against reconstructed SSTs supports this conclusion and also the assumption that the flux correction, fitted for the present state, operates satisfactorily for modest perturbations. Our results imply that contrary to conventional expectations, Eemian global temperatures may already have been reached by the mid 20th century.
Latest Pleistocene ice‐rafting events on the Scotian Margin (eastern Canada) and their relationship to Heinrich events
Authors: Piper David J. W. , Skene Kenneth I.
Published: February 2004

Piston cores from the continental margin off Nova Scotia show up to four discrete intervals of “brick‐red sandy mud,” which are up to 20 cm thick. The ages of these intervals are bracketed by several radiocarbon dates, and three fall in the range 12.5–14.1 ka (radiocarbon years with −0.4 kyr reservoir correction). The youngest dates from ∼10.4 ka, placing it within the Younger Dryas. The distribution of the beds and their petrographic character indicate a source in the Gulf of Saint Lawrence. The grain size of these beds suggests that they comprise a coarse component transported by ice rafting that diminishes distally and a fine component that represents suspension fallout from a surface plume and resulting nepheloid layers. Graded brick‐red beds in some cores were probably redeposited from turbidity currents. The lowermost bed on the Laurentian Fan and East Scotian Rise is immediately overlain by a carbonate‐rich interval that can be identified all around the margin of the Grand Banks. This interval is correlated with detrital carbonate bed DC‐1 in the Labrador Sea and Heinrich event H1 in the North Atlantic. The sequential occurrence of the two beds suggests that they may be a response to the same trigger, probably sea level rise, but that the Gulf of Saint Lawrence source was more easily destabilized.
Systematics of variations in excess Al and Al/Ti in sediments from the central equatorial Pacific
Authors: Timothy David A. , Calvert Stephen E.
Published: February 2004

The proposal that the accumulation rate (AR) of scavenged Al in central equatorial Pacific sediments is controlled by bulk particle flux and therefore can be used as a paleoproduction proxy is tested by examining the relationship between Al and Ti using data on core WEC8803B‐GC51 published by Murray et al. [1993]. The downcore profile of the excess Al AR follows the bulk AR, consistent with the Al‐scavenging hypothesis and is due to a relatively constant abundance of excess Al down core. However, there have been large changes in the aluminosilicate AR to this region, suggesting that changes in the Al/Ti value do not solely trace variability in the delivery rate of scavenged Al. Assuming that the excess Al in this core is not the result of an analytical artifact, we present an alternative to the Al‐scavenging hypothesis that involves authigenic clay mineral formation, or some other process, causing the occurrence of excess Al at a relatively constant abundance of the sediment. This process may well be independent of bulk particle flux or paleoproduction.
Paleocean circulation during the Last Deglaciation: A bipolar seesaw?
Authors: Broecker Wallace S.
Published: February 2004

Hughen et al. [1998] have documented that during the first 200 years of Younger Dryas time the 14C content of atmospheric CO2 increased by ∼50‰ and that during the remainder of this 1200‐year‐duration cold event it steadily declined. The initial increase in 14C/C was likely the result of a reduction in the Atlantic's conveyor circulation. However, were the subsequent radiocarbon decline due to the rejuvenation of this potent heat pump, then it is difficult to understand why the climate conditions in the northern Atlantic basin remained cold throughout the Younger Dryas. Modeling exercises by Stocker and Wright [1996], Mikolajewicz [1998], and Schiller et al. [1998] show that if the conveyor is terminated, the transfer of radiocarbon into the deep sea shifts to the Southern Ocean, thereby stabilizing the atmospheric 14C/C ratio. Paleoclimatic evidence from the Antarctic continent suggests that this model‐based scenario might have been played out in the real world. While the Younger Dryas cooling has been documented in many places around the world, including New Zealand [Denton and Hendy, 1994], Sowers and Bender [1995], using their 18O in O2‐based correlation between the ice core 18O in ice records for Antarctica and Greenland, have demonstrated that in Antarctica the Younger Dryas was a time of maximum warming. The point of this paper is that the steep rise in 18O rise in Antarctic ice which commenced close to the onset of the Younger Dryas might have been caused by heat released to the atmosphere in response to an increase in deep‐sea ventilation in the Southern Ocean.
Application of vector potential theory to spontaneous potential computation
Authors: Zhang Geng Ji , Wang Gong Li
Published: February 2004

Just as scalar potential theory can be applied to magnetostatic field, where vector potential theory is the dominating theory, so may vector potential theory be used in steady current field computations, dominated by scalar potential theory. The introduction of vector potential theory simplifies the computation of spontaneous potential (SP) and brings about the innovation of concept. Finite element method is applied to the computation based upon both vector potential theory and scalar potential theory. Their results agree very well with each other. SP correction charts computed on vector potential agree with existing ones.
An examination of the wind‐driven effect on the drift of precipitation particles using the Chung‐Li VHF radar
Authors: Chu Yen‐Hsyang , Chen Tsai‐Yuan , Lin Tszer‐Hong
Published: February 2004

One of the essential assumptions made in estimating the terminal velocity and the drop‐size distribution of hydrometeor from the precipitation Doppler spectrum observed by VHF/UHF radar (or wind profiler) is that all of the precipitation particles are moving together at the same velocity as the ambient wind. However, this may not be the case. In this paper, taking advantage of the capability of simultaneously measuring the motions of precipitation particles and the clear‐air turbulence with a VHF radar, we develop a method to examine the wind‐driven effect on the drift of precipitation particles using the Chung‐Li VHF radar. By appropriately setting the radar parameters and steering the radar beams, experimental results show that the drift velocity of the precipitation particles in response to the drag force exerted by the background atmosphere is not the same as the ambient wind velocity, approximately 83% of the latter in this case. This feature indicates that the precipitation particles are not moving with the background wind, causing inaccurate estimations of the terminal velocity and the drop‐size distribution of hydrometeor if this effect is not taken into account.
Radar observations of updrafting and downdrafting plasma depletions associated with the equatorial spread F
Authors: Rao P. B. , Patra A. K. , Chandrasekhar Sarma T. V. , Krishna Murthy B. V. , Subba Rao K. S. V. , Hari S. S.
Published: February 2004

Radar observations at VHF on equatorial spread F (ESF) made at Gadanki (13.5°N, 79.2°E; magnetic latitude 6.3°N) and Trivandrum (8.5°N, 77°E; magnetic laitude 0.3°N) are presented in the form of height‐time maps of signal intensity and Doppler velocity. The peak signal intensities are found to be 30–40 dB above the noise level at Gadanki and, normalized to the same system sensitivity, about 6 dB higher at Trivandrum. The discrete plasma structures and the phase velocities of the 3m irregularities observed at Gadanki are well correlated to that observed at Trivandrum at the height linked by the same flux tube. The height‐time‐intensity maps show both updrafting and downdrafting of the plasma structures; the downdrafting observed on one occasion at Gadanki is somewhat unusual in that it extends down to the E region. The Doppler velocities observed at Gadanki show that the highest values are encountered in the rising plumes with the upward velocities ranging from 100 to 300 ms−1. The velocities are predominantly downward in the bottomside F region, particularly during the later phase of the ESF development. On occasion, the downdrafting is observed well into the topside, reaching as high as 550 km, which emphasizes the influence of electric field being extended to a greater height as pointed out by Anderson and Haerendel [1979] in their model based on flux tube integrated quantities. The downdrafting velocities range from 20 to 100 m s−1. These values being mostly well above the background plasma drift velocity, the downdrafting structures are to be regarded as plasma depletions.
Comparison of ionospheric total electron content from the Navy Navigation Satellite System and the GPS
Authors: Ciraolo Luigi , Spalla Paolo
Published: February 2004

About 25,000 total electron content (TEC) values obtained in 2 years from Navy Navigation Satellite System (NNSS) differential Doppler were compared with the corresponding TECs obtained from the Global Positioning System (GPS). The method by which the comparison is effected is also described. The resulting statistics are very effective in showing a good agreement between the two techniques. The GPS TEC results are statistically larger than NNSS TEC, namely, 3±1 TEC units; the lower quartile of the differences is always greater than zero.
Measurements of melting layer attenuation at X‐band frequencies
Authors: Bellon Aldo , Zawadzki Isztar , Fabry Frédéric
Published: February 2004

The attenuation at X band caused by the melting layer of stratiform precipitation is derived by comparing long‐term average reflectivity profiles from collocated X‐band and UHF vertically pointing radars. The total attenuation caused by the bright band is equivalent to the attenuation across approximately 10 km of rain that is present below it. This bright band attenuation is still 3–5 times the “rain equivalent” attenuation, the latter being obtained by integrating the specific attenuation deduced from the entire reflectivity profile across the bright band as if the reflectivity were due to rain. This suggests that in most stratiform rain cases, the total attenuation due to the melting layer exceeds that of the rain below it.
Exact eigenvalue equations of modes in elliptical fibers of step‐index profile
Authors: Do‐Nhat Tam , Alhargan Fayez A.
Published: February 2004

The exact eigenvalue equations of modes in elliptical fibers of step‐index profile are not available in the literature. By using a rigorous method, an elliptical fiber mode of step‐index profile is expressed in terms of the even and odd Mathieu wave functions of the first kind in its core and in terms of their decaying even and odd modes in its cladding. Then, by matching the tangential components of the total electromagnetic fields across the core‐cladding boundary and using the orthogonalities of the circumferential Mathieu functions, the eigenvalue equations are derived in closed forms. Results are shown for the propagation characteristics of some low orders of the elliptic modes.
Extended uniform geometrical theory of diffraction solution for the radiation by antennas located close to an arbitrary, smooth, perfectly conducting, convex surface
Authors: Chou Hsi‐Tseng , Pathak Prabhakar H. , Hsu Mimi
Published: February 2004

In this paper, the uniform geometrical theory of diffraction (UTD) based field expressions developed previously [Pathak et al., 1981] for predicting the radiation from antennas which are flush mounted directly on an arbitrary, smooth, perfectly conducting, convex surface are extended to allow the antennas to be raised slightly above the convex surface. This extension thus allows an efficient ray analysis of many practical antenna structures such as blade antennas or pod‐mounted antennas on convex surfaces or even antennas near convex cylindrical masts, etc. This extended UTD (EUTD) is developed such that it employs the same ray paths and retains the same simplicity and efficiency as the earlier UTD radiation solution [Pathak et al., 1981]. Furthermore, if the antenna is moved directly onto the surface, then the EUTD solution developed here automatically reduces to the earlier UTD solution. Numerical results based on EUTD are presented for antennas close to smooth, curved surfaces; in some of these cases, impressed sources of radiation are oriented so that they would produce no radiation if they were moved directly onto the surface.
Comparison of tomographic and conventional methods for resolution of the constant of integration
Authors: Page David L. , Austen Jeffrey R.
Published: February 2004

Computerized ionospheric tomography (CIT) is an imaging technique that produces a two‐dimensional image of the electron density in the ionosphere. This technique uses total electron content (TEC) measurements from multiple ground stations as input data to computerized tomography (CT) algorithms. Unfortunately, the TEC data suffer from ambiguities that are introduced during the measurement process. Ionosphere researchers model these ambiguities as an unknown constant which is added to the true TEC data. This is known as the constant of integration problem. Traditional methods for resolving this problem, such as the single‐ and two‐station methods, are based on ionospheric models and may perform poorly when the true ionospheric structure differs greatly from the model. This paper introduces a new method to estimate the unknown constants. The method, called the reconstruction residuals method, does not make use of ionospheric models but is based on the correlation between the unknown constants and the residual errors from a CIT reconstruction. Simulation results using this method are compared with the traditional estimation methods. Results from the new method compare favorably with results from the traditional methods.
Ionospheric specification from GPS data and the RIBG ionospheric propagation model
Authors: Reilly Michael H. , Singh Malkiat
Published: February 2004

Global Positioning System (GPS) data for phase delay differential between Ll and L2 frequencies are analyzed by a model in which the combined effect of hardware and phase ambiguity terms is represented by a constant for each receiver‐satellite path and the ionospheric contribution is represented by the RIBG model (ray trace through the combined ICED (ionospheric conductivity and electron density), Bent, and Gallagher ionospheric models). RIBG contains a detailed global empirical climatological ionospheric model of the electron density versus height profile (EDP) from 80 km up to the plasmapause, which is combined with a full three‐dimensional ray trace propagation model. Discrete inverse theory (DIT) is used to fit the driving parameters of RIBG to about 2 hours worth of data from a single GPS receiver at various geographical locations. Validity is demonstrated by the ability of the GPS‐updated RIBG model to predict independent measurements of (1) vertical total electron content (TEC) over the ocean out to substantial distances from a GPS receiver, often in excess of 2000 km at midlatitudes, and (2) the electron density versus height profile (EDP). Performance variation with magnetic latitude is noted. Flexibility in RIBG to adjust different driving parameters for different parts of the EDP, in order to obtain a best fit of GPS data, benefits the EDP prediction. Implications of this approach for GPS navigation and other radio system applications are discussed.
Multipath EM scattering from breaking ocean waves at grazing incidence
Authors: West James C. , Sletten Mark A.
Published: February 2004

The electromagnetic scattering from a breaking water wave with near grazing illumination has been examined using both experimental measurement and numerical calculation. The scattering from a metal model of a breaking wave of 1.2 m wavelength was characterized at grazing angles ranging from 0° to 25° and at frequencies ranging from 6 GHz to 12 GHz. Strong multipath interference is observed in the vertical polarization (VV) backscatter and much weaker interference is seen in the horizontal polarization (HH) backscatter. Scattering from the wave profile both with and without the breaking plume shows that the plume is the dominant scattering feature. Numerical calculation of the scattering from modified surface profiles shows that the interference is due to multipath scattering from the plume and front face of the wave. HH interference is substantially smaller than that of VV because the scatter from the plume in the multipath direction (toward the front face) is greatly reduced owing to the conducting surface boundary conditions. The relative levels of the plume and expected distributed‐surface scattering are consistent with the wave‐tank measurement of horizontally polarized sea spikes from breaking waves of this scale.
Current on an array of narrow strips at a planar interface for transverse magnetic excitation
Authors: Martin Anthony Q. , Butler Chalmers M.
Published: February 2004

An analysis is presented of a planar array of narrow conducting strips residing at the interface between two semi‐infinite half‐spaces and excited by an incident plane wave that is transverse magnetic to the strip axes. The analysis is based upon an integral equation formulation in which the kernels of the coupled equations reduce from Sommerfeld‐type integrals to closed‐form expressions when the permeabilities of the two half‐spaces are equal. The restriction that the strips be narrow relative to wavelength in either medium results in an approximate expression for the self‐coupling of each strip. The use of special basis functions to expand the current on each narrow strip leads to a finite‐term power‐series expansion for the self‐coupling of each strip. Expansion of the mutual coupling between narrow strips and of the excitation in series of simple powers allows the formation of a matrix equation that is solved for the expansion coefficients. Induced currents are presented for arrays of various size. The method presented is very efficient for the analysis of arrays consisting of a large number of narrow strips.
A long‐term investigation of the HF communication channel over middle‐ and high‐latitude paths
Authors: Goodman John , Ballard John , Sharp Eugene
Published: February 2004

A study of HF communication link and network availability has been carried out using frequency modulated continuous wave (FMCW) swept‐frequency sounders operating in an oblique‐incidence configuration. The sounder deployment enabled 28 middle‐ and high‐latitude paths to be evaluated; the time period of the study was from December 1994 until the summer of 1996. Propagation conditions including ionospheric mode information, maximum observable frequencies, signal‐to‐noise ratios, and channel availabilities for digital data communication were derived and archived. The objective of the study was to ascertain the efficacy of HF data link communication for a proposed aeronautical‐mobile service. Data were used to simulate the propagation environment which would be experienced in actual operation and to evaluate the value of path and frequency diversity in overcoming various propagation effects. It has been concluded that path and frequency diversity will lead to channel availabilities approaching nearly 100% and that a practical engineering solution was possible. These positive results are achievable if and only if dynamic frequency management methods are invoked. The paper outlines the nature of a real‐time system, which is based upon a terrestrial FMCW swept‐frequency sounder constellation and by which communication nowcasts and short‐term forecasts are developed to drive a dynamic frequency management system.
Statistical studies of an effective sunspot number
Authors: Secan J. A. , Wilkinson P. J.
Published: February 2004

Two decades ago, the U.S. Air Force Air Weather Service space forecasting group began generating what was termed an effective sunspot number (SSNe) by fitting a model of the critical frequency of the F2 layer (ƒoF2) to observedƒoF2 values. Initially a preprocessing step in a larger analysis package, this parameter has taken on a life of its own and is now used in various applications for both forecasts and specification of the globalƒoF2 field. This paper describes the various ways in which this parameter is calculated, investigates the behavior of this parameter over solar cycle 21 (1976 through 1986), and compares it with other solar‐ionospheric indices, including R12, IF2, IG, and the Ionospheric Prediction Service (IPS) T index.
Variability of monthly time fraction of excess of atmospheric propagation parameters
Authors: Mawira A.
Published: February 2004

This paper investigates the variability of the monthly time fraction of excess of atmospheric propagation parameters. In the first part of the paper a theoretical investigation is carried out regarding the relationship between the variability of monthly time fraction of excess vis‐a‐vis the variability of the annual‐worst‐month time fraction of excess. It is shown that the tail of the distribution of the annual‐worst‐month time fraction of excess can be derived from the average distribution of the monthly time fraction of excess without requiring detailed knowledge of the seasonal behavior of the propagation. In the second part of the paper the modeling of the average distribution of the monthy time fraction of excess (the parent distribution) is investigated using some 92 site years of data (from slant path attenuation and transhorizon measurements). Three competing models were investigated: the conditional exponential, the conditional lognormal, and the shifted gamma distribution. For the testing of the suitability of the model a novel data pooling method is applied. The tests results show the shifted gamma model as the best model, followed by the conditional exponential model and then the conditional lognormal model.
An optical remote sensing technique for determining nighttime F region electron density
Authors: Dymond K. F. , Thonnard S. E. , McCoy R. P. , Thomas R. J.
Published: February 2004

We present a technique for using the measured variations of ultraviolet emissions produced by radiative recombination at 911 and 1356 Å to determine the nighttime altitude distribution of F region O+ ions and electrons. The algorithm uses an iterative scheme based on discrete inverse theory to determine the best fit to the data. We present the results of simulations that demonstrate the convergence properties of the algorithm and the fidelity with which it reproduces the input ionosphere. The algorithm was tested against more realistic simulated “data” generated using the international reference ionosphere (IRI‐90) [Bilitza, 1990]. The algorithm accurately retrieved the nighttime F region electron density at midlatitudes (±25°–65°N) over a wide range of solar and geomagnetic activity and local time.
Monitoring of boreal forests with multitemporal special sensor microwave imager data
Authors: Kurvonen Lauri , Pulliainen Jouni , Hallikainen Martti
Published: February 2004

The feasibility of multitemporal special sensor microwave imager (SSM/I) data for monitoring boreal forests was evaluated. The parameters of interest were forest coverage fraction and forest stem volume (biomass). The employed test sites covered almost the whole of Finland. Two measurement periods were used: July through September in 1993 and January through February in 1994. The apparent emissivities of various land cover types were determined with the mixed pixel approach under summer and winter conditions. The aim was to define the dominating factors on emissivity under winter and summer conditions. The mixed pixel approach was tested for the estimation of forest coverage fraction. The results with multitemporal SSM/I data show that the pixel‐wise fractions of water, nonforested, and forested area can be estimated with rms errors of around 10 percent units. The correlation between the estimates and the ground truth was over 0.85. A new inversion method for stem volume estimation was presented. At SSM/I frequencies the forest canopy and the snow‐covered ground dominate the emissivity behavior under winter conditions in the boreal forest zone. The method is based on the fact that the emissivity of forest canopy is close to 1, while that of dry snow cover is relatively low. The results with the method showed promising accuracies when wintertime SSM/I data were employed. The rms error was from 13 to 19 m3/ha per pixel (25 km by 25 km), which was 15–16% of the mean stem volume. In the test area the stem volume ranged from 40 to 160 m3/ha per pixel.
Diffraction by a right‐angled resistive wedge
Authors: Demetrescu C. , Constantinou C. C. , Mehler M. J.
Published: February 2004

In this paper the electromagnetic diffraction by a right‐angled resistive wedge made up by a resistive sheet and a perfectly conducting half plane is presented. The resistive wedge is normally illuminated by an E‐plane wave. The Sommerfeld‐Maliuzhinets method is used to represent the fields in the interior and exterior regions of the resistive wedge by means of two spectral functions. Four coupled functional equations are obtained for the two unknown spectral functions which can be reduced to a fourth‐order difference equation with 2π‐periodic coefficients. The solution of this difference equation is constructed using the Fourier transform method. The high‐frequency approximation of the Sommerfeld integral is obtained with the corresponding uniform diffracted field. Representative curves of the uniform total field are presented.
Recent advances in radar instrumentation and techniques for studies of the mesosphere, stratosphere, and troposphere
Authors: Hocking W. K.
Published: February 2004

The last few years have seen several significant advances in mesosphere‐stratosphere‐troposphere (MST) radar studies. This is true with respect to studies ranging from MF through VHF and up to UHF. These advances have been in each of the areas of equipment, techniques, and general radar theory. In this short paper we will highlight some of these advances and discuss their significance in the longer‐term application of MST radar techniques. We will concentrate primarily on discussion about instrumentation and the raw products (powers, radial velocities, and spectral widths) produced by the systems, with some reference to the nature of the radio wave scattering entities. Quantities which are derived from the measurements of these raw quantities like gravity wave fluxes, tidal studies, and so forth will not generally be considered in any detail. Sample advances include new methods of data analysis (both on‐line and post‐collection), lightning detection, rainfall measurement, and temperature determination. Other interesting applications include simultaneous application of MST techniques with other procedures such as in radio acoustic sounding (RASS) and the artificial periodic inhomogeneity (API) method. Areas of advance in terms of understanding scattering mechanisms include new insights into the controversy about the nature of aspect‐sensitive scattering (specular reflection compared with anisotropic turbulence) and improvements and refinements in our measurements of turbulence with these radars. Radars working at frequencies other than those mentioned, especially L and S band (1215–1710 and 1710–2700 MHz respectively), have also been used with good success, and these will be discussed where appropriate. The paper will concentrate on radars which can employ clear‐air scattering; we will not consider in much detail systems which rely predominantly on precipitation and other scattering targets embedded in the air.
Rain rate distributions for China from hourly rain gauge data
Authors: Li Li , Zhu Yuan‐jing , Zhao Bolin
Published: February 2004

Rain rate cumulative lognormal and Gamma distributions are calculated from hourly rain gauge data collected in 1991 at 333 locations in China. The Gamma distribution better represents the statistical rain rate distribution. So 1‐min cumulative distribution to every gauge station is derived from the piecewise regression results of Gamma distribution through half‐empirical conversion coefficients. We suggest the following actions: (1) In south China, there should be an approximately 9°E×5°N area of rain climatic zone K centered at about 112°E, 26°N inside International Telecommunication Union(ITU)‐R designed zone M. (2) A geostationary communication satellite for China should be placed over eastern China (between 100°E and 125°E) in order to generally reduce the rain attenuation of Earth‐satellite links all over China, based on rain rate distributions characteristic of China. Our data treatment procedure and results presented in this paper may be practical and useful in engineering design.