bluuebunny/crossref_metadata_2025 huggingface dataset:
Seasonal variation of aerosols in the Martian atmosphere
Authors: Toigo Anthony D. , Richardson Mark I.
Published: September 2002

Reanalysis of Viking Lander (VL) visible and Viking Orbiter infrared optical depth measurements shows that the visible to infrared ratio of total extinction opacity varies with season. The ratio is near to its previously reported constant value, 2.5, during dust storm periods and higher during northern spring and summer. The increase in ratio is hypothesized to be due to seasonally varying water ice haze, which produces a higher optical depth in the visible than in the infrared. This differs significantly from previous analyses of VL visible opacities which have assumed that only dust contributes to the optical depth measured during the early afternoon. Consequently we suggest that the Martian atmosphere is clearer of dust, especially during northern spring and summer, than previously suggested based upon VL data. We find dust visible optical depths of 0.1–0.4 during the northern spring and summer seasons, compared to previous estimates of 0.4–0.6. We also find that water ice hazes can provide roughly 50% of the total visible opacity in these seasons. For southern spring and summer, dust optical depths are more variable, but generally ≥0.4, with water ice opacity ≤0.1. The data suggest water ice optical depths are slightly higher and peak earlier (Ls = 80°–90°) at VL1 than at VL2 (Ls = 115°–130°). We estimate average northern summer water (daytime minimum) ice masses to be roughly 0.1–0.5 precipitable microns, depending on the assumed particle size distribution and hence 1–5% of the total water column. The observation of significant and previously unrecognized amounts of water ice haze suggests a larger role for water in controlling atmospheric heating rates and the vertical distribution of dust and water vapor than has been widely accepted to date.
Mars south polar spring and summer behavior observed by TES: Seasonal cap evolution controlled by frost grain size
Authors: Kieffer Hugh H. , Titus Timothy N. , Mullins Kevin F. , Christensen Philip R.
Published: September 2002

Thermal Emission Spectrometer (TES) observations of the recession phase of Mars' south polar cap are used to quantitatively map this recession in both thermal and visual appearance. Geographically nonuniform behavior interior to the cap is characterized by defining several small regions which exemplify the range of behavior. For most of the cap, while temperatures remain near the CO2 frost point, albedos slowly increase with the seasonal rise of the Sun, then drop rapidly as frost patches disappear over a period of ∼20 days. A “Cryptic” region remains dark and mottled throughout its cold period. TES observations are compared with first‐order theoretical spectra of solid CO2 frost with admixtures of dust and H2O. The TES spectra indicate that the Cryptic region has much larger grained solid CO2 than the rest of the cap and that the solid CO2 here may be in the form of a slab. The Mountains of Mitchel remain cold and bright well after other areas at comparable latitude, apparently as a result of unusually small size of the CO2 frost grains; we found little evidence for a significant presence of H2O. Although CO2 grain size may be the major difference between these regions, incorporated dust is also required to match the observations; a self‐cleaning process carries away the smaller dust grains. Comparisons with Viking observations indicate little difference in the seasonal cycle 12 Martian years later. The observed radiation balance indicates CO2 sublimation budgets of up to 1250 kg m−2. Regional atmospheric dust is common; localized dust clouds are seen near the edge of the cap prior to the onset of a regional dust storm and interior to the cap during the storm.
Evaluation of models for the formation of chaotic terrain on Europa
Authors: Collins G. C. , Head J. W. , Pappalardo R. T. , Spaun N. A.
Published: September 2002

Chaotic terrain on Europa is interpreted to be the result of the breakup of brittle surface materials over a mobile substrate. Two end‐member models have been proposed for the mobile substrate: a liquid water ocean or solid warm ductile ice. Direct contact between the brittle surface ice and an underlying ocean might be accomplished by melting through Europa's ice shell, but plausible sources of thermal energy are insufficient to locally melt through the ice shell. We calculate that a liquid or partially liquid substrate is probably necessary for tilted blocks to be observed, challenging a solid‐state model. We propose that a model in which chaos areas on Europa formed over bodies of melt or partial melt within the ice shell may be the best match to observational and thermal constraints. Such bodies of melt within the ice shell may be triggered by warm diapirs of relatively clean ice that partially melted an overlying layer of lower melting temperature, such as ice contaminated with salts.
Numerical models of magmatism in convecting mantle with temperature‐dependent viscosity and their implications for Venus and Earth
Authors: Ogawa Masaki
Published: September 2002

Numerical models are presented for magmatism in a convecting mantle that contains internal heat source. Mantle convection is modeled by a convection of binary eutectic material with Newtonian temperature‐dependent rheology driven by thermal, compositional, and melt buoyancy as well as the buoyancy from the “660‐km” phase transitions. Mantle magmatism is modeled by a gravitationally induced permeable flow of magma generated by pressure release melting through matrix. Magma mostly has the eutectic composition, that is, basaltic composition. The numerical models suggest that magmatism and mantle convection strongly influence each other to form a coupled system when the internal heating is sufficiently strong. Episodic magmatism takes place to chemically differentiate the mantle; basaltic materials occupy deeper part of the lower mantle, while magma residue occupies the uppermost mantle. Compositional buoyancy of the differentiated materials makes mantle convection sluggish except when magmatic activities take place. The style of magmatism and mantle convection depends on the viscosity contrast across the cold thermal boundary layer (TBL) along the top surface boundary. When the viscosity contrast is large, the fluid in the coldest part of top TBL becomes a stagnant lid and a conspicuous lateral heterogeneity arises in the upper mantle. Magmatism is induced by hot uprising diapirs originating in the top of the lower mantle. The diapirs induce convective circulation within the upper mantle when the lower mantle is rather cold but induce flushing event, that is, a massive flow across the 660‐km phase boundary and the resulting vigorous magmatism, when the lower mantle becomes sufficiently hot owing to the internal heating. When the viscosity contrast is moderate, the fluid in the coldest part of top TBL behaves as a moving lid. Both the moving lid and hot uprising diapirs induce magmatic activity; the magmatic activity due to a moving lid resembles ridge volcanism. The mantle of Venus and that of the early Earth are suggested to have been on the regime modeled here, and the magmatic activity by flushing event is compared to the magmatism of volcanic plain formation on Venus.
Elysium Planitia lava flows: Crater count chronology and geological implications
Authors: Hartmann William K. , Berman Daniel C.
Published: September 2002

Mars Global Surveyor (MGS) and Viking images allow analysis of chronological and geological relations among lava flows in southern Elysium Planitia, based on crater populations. MGS and Viking images clearly show morphological features of lava flows with some extremely sparsely cratered young flow units atop somewhat older surfaces. There is no evidence of substantial dust mantling on the young flows, and hence we infer that the crater populations date not sediment accumulation but the lava flows themselves. The youngest flows have some of the lowest crater densities that we have seen on Mars, some <1% of lunar mare values, at crater diameters below about 180 to 500 m. We infer that thin, scattered volcanic flows, some tens of meters thick, have been emplaced atop stratigraphic units within the last 100 Myr, and possibly within the last 10 Myr. Volcanism is thus a continuing process in the recent geologic history of Mars, and this must constrain geophysical models of the planet.
Clementine UVVIS multispectral data and the Apollo 17 landing site: What can we tell and how well?
Authors: Jolliff Bradley L.
Published: September 2002

Multispectral data obtained by the Clementine mission are correlated to compositional and petrographic information for individual sample stations at the Apollo 17 landing site. The soils contain a variety of Fe2+‐bearing phases in different proportions, including low‐ and high‐Ca pyroxene, olivine, ilmenite, and glass. In the mare soils, ilmenite carries a substantial proportion of the Fe2+, and in all soils, glasses, particularly agglutinates, carry nearly as much or more Fe2+ as the silicates. Using an empirical calibration, concentrations of FeO and TiO2 estimated from the ultraviolet‐visible (UVVIS) data correlate well with measured compositions of surface soils, yielding correlation coefficients of 0.94 and 0.95, respectively. The ferrous absorption, however, is weak in the mostly submature to mature Apollo 17 soils, thus spectral curvature is useful only for investigating the mafic‐silicate mineralogy around fresh craters and deposits. The effects of local topography cause significant variations in FeO and TiO2 derived from Clementine UVVIS data but do not impair first‐order geologic interpretations. Parameters derived from the UVVIS data allow quantification of the composition and nature of the soils and, by inference, underlying rock formations and are consistent with the known geologic character of the landing site. The highland regions along this part of the Serenitatis rim are relatively FeO rich, partly as a result of abundant, mafic impact‐melt breccia and partly from the addition of basaltic material in the form of impact debris and pyroclastic deposits. At high elevations, massif soils are rich in a mafic melt‐breccia component, although slopes and areas mixed by small craters reveal a mixture of melt breccia and older highland material of anorthositic‐norite composition. Soils developed on hummocky deposits that postdate the massifs, such as the Sculptured Hills, have a similar range of compositions as the massifs. Elevated plains northeast of the Sculptured Hills are more mafic than the known impact‐melt breccias, suggesting that they may also contain an older volcanic component. Sources of low‐Ti basalt or very low Ti basalt, though not present in the immediate vicinity of the landing site, occur east and northeast of Taurus‐Littrow Valley and are readily identified using the UVVIS data.
Paleo‐sea surface temperature calculations in the equatorial east Atlantic from Mg/Ca ratios in planktic foraminifera: A comparison to sea surface temperature estimates from U37K′, oxygen isotopes, and foraminiferal transfer function
Authors: Nürnberg D. , Müller A. , Schneider R. R.
Published: September 2002

We present two ∼270 kyr paleo‐sea surface temperature (SST) records from the Equatorial Divergence and the South Equatorial Current derived from Mg/Ca ratios in the planktic foraminifer Globigerinoides sacculifer. The present study suggests that the magnesium signature of G. sacculifer provides a seasonal SST estimate from the upper ∼50 m of the water column generated during upwelling in austral low‐latitude fall/winter. Common to both down‐core records is a glacial‐interglacial amplitude of ∼3°–3.5°C for the last climatic changes and lower Holocene and glacial oxygen isotope stage 2 temperatures compared with interglacial stage 5.5 and glacial stage 6 temperatures, respectively. The comparison to published SST estimates from alkenones, oxygen isotopes, and foraminiferal transfer function from the same core material pinpoints discrepancies and conformities between methods.
Dansgaard‐Oeschger Cycles and the California Current System: Planktonic foraminiferal response to rapid climate change in Santa Barbara Basin, Ocean Drilling Program Hole 893A
Authors: Hendy Ingrid L. , Kennett James P.
Published: September 2002

High‐resolution planktonic foraminiferal census data from Santa Barbara Basin (Ocean Drilling Program hole 893A) demonstrate major assemblage switches between 25 and 60 ka that were associated with Dansgaard‐Oeschger cycles. Stadials dominated by Neogloboquadrina pachyderma (sinistral), and Globigerinoides glutinata suggest a strong subpolar California Current influence, while interstadials marked by abundant N. pachyderma (dextral) and G. bulloides indicate a relative increase in subtropical countercurrent influence. Modern analog technique and transfer function (F‐20RSC) temperature reconstructions support δ18O evidence of large rapid (70 years or less) sea surface temperature shifts (3° to 5°C) between stadials and interstadials. Changes in the vertical temperature gradient and water column structure (thermocline depth) are recorded by planktonic faunal oscillations suggest bimodal stability in the organization of North Pacific surface ocean circulation. Santa Barbara Basin surface water demonstrates the rapid response of the California Current System to reorganization of North Pacific atmospheric circulation during rapid climate change.

Supporting assemblage data are, available on diskette or via Anonymous FTP from Kosmos.agu.org, Directory APEND (username = anonymous, Password = guest). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009 or by phone at 800‐966‐2481; $5.00. Payment must accompany order.
Geochemical evidence for enhanced productivity during S1 sapropel deposition in the eastern Mediterranean
Authors: Martinez‐Ruiz F. , Kastner M. , Paytan A. , Ortega‐Huertas M. , Bernasconi S. M.
Published: September 2002

The geochemistry of the youngest Mediterranean sapropel layer suggests changes in productivity and water column oxygen conditions during sapropel deposition. The Ba‐enriched interval is broader than the organic‐carbon‐rich interval of this sapropel. We suggest that the Ba‐enriched horizon records the original thickness of the sapropel prior to subsequent partial oxidation. The main carrier of Ba is barite, as microcrystals (0.5–5 µm ) having a morphology characteristic of marine barite, particularly abundant beneath high productivity regions. Ba concentrations do not change at the sapropel layer oxidation front and diagenetic barite crystals are absent, thus the Ba‐enriched layer reflects original oceanic conditions of increased biological productivity during sapropel deposition and not diagenetic Ba remobilization. Paleoredox indicators point to restricted oxygenated bottom water but not to fully anoxic conditions. Detrital elements within this layer indicate a lower eolian terrigenous input, enhanced humidity, and increased precipitation/runoff, thus likely higher nutrient supply.

Supporting table is available on diskette or via Anonymous FTP from kosmos.agu.org, directory APEND (Username = anonymous, Password = Guest). Diskette may be ordered by mail from AGU, 2000 Florida Ave., N. W., Washington, DC 20009 or by phone at 800‐966‐2481; $15.00. Payment must accompany order.
Zinc concentrations in benthic foraminifera reflect seawater chemistry
Authors: Marchitto Thomas M. , Curry William B. , Oppo Delia W.
Published: September 2002

We have measured Zn/Ca ratios in two taxa of Holocene‐aged benthic foraminifera from throughout the world's oceans. Zn/Ca is controlled by bottom water dissolved Zn concentration and, like Cd/Ca and Ba/Ca, by bottom water saturation state with respect to calcite. Measurements on “live‐collected” foraminifera suggest that the saturation effect occurs during growth and is not a postdepositional artifact. Zn/Ca could be a sensitive paleoceanographic tracer because deep water masses have characteristic Zn concentrations that increase about tenfold from the deep North Atlantic to the deep North Pacific. In addition, since Zn/Ca responds to a different range of saturation states than Cd/Ca, the two may be used together to evaluate changes in deep water carbonate ion (CO32−) concentration.
Black shale deposition and faunal overturn in the Devonian Appalachian Basin: Clastic starvation, seasonal water‐column mixing, and efficient biolimiting nutrient recycling
Authors: Murphy Adam E. , Sageman Bradley B. , Hollander David J. , Lyons Timothy W. , Brett Carlton E.
Published: September 2002

Integrated geochemical data suggest that black shale deposition in the Devonian Geneseo Formation of western New York was initiated by the coincidence of siliciclastic starvation and the intensification of seasonal water column stratification and mixing. Once established, however, black shale deposition was maintained through efficient recycling of biolimiting nutrients which enhanced primary productivity. Recycling efficiency was achieved through a positive feedback loop of oscillating benthic redox conditions that enhanced N and P regeneration from sediments, sustained high primary productivity by returning nutrients to the photic zone during mixing, and ensured a downward flux of organic matter that drove or enhanced the episodic development of benthic anoxia during stratification. This feedback was ultimately disrupted by rising siliciclastic influx, which diluted organic matter and restored benthic redox stability. The abrupt overturn of diverse, long‐standing Appalachian basin marine communities may have been the result of trophic resource destabilization during Geneseo deposition.
A hybrid scheme for inverse scattering of electrically large regions
Authors: Rao Bimba , Carin Lawrence
Published: September 2002

An efficient algorithm is presented for two‐dimensional inverse scattering from electrically large regions. The technique is a hybrid combination of the modified‐gradient (MG) method and a beam‐tracing‐based iterative Born method. The beam‐tracing‐based inversion cannot resolve sharp contrasts, but such are subsequently recovered by the MG algorithm. In particular, the hybrid scheme feeds the iterative Born reconstructed profile as the initial guess to the modified‐gradient algorithm. The hybrid scheme has a highly improved convergence rate and also overcomes the limitations of the iterative Born and modified‐gradient schemes, when each is applied alone. Results are presented for several lossless profiles. Issues addressed include the types of profiles that can be successfully imaged, the range of parameters over which the scheme is effective, and performance when noisy data are considered.
Two modes of wave propagation manifested in vertical electric dipole radiation over a sphere
Authors: Houdzoumis Vassilios A.
Published: September 2002

The radiation of a vertical electric dipole over an electrically homogeneous sphere is considered anew, starting with a novel mathematical formulation. Both the dipole and the point of observation are assumed to lie on the spherical interface. The analysis is valid for a sphere whose radius is much larger than the wavelength in the outside region. Contributions to the value of the fields come, on the one hand, from the waves that propagate along the interface and, on the other hand, from the waves that propagate through the sphere by successive reflections.
Nighttime equatorial ionosphere: GPS scintillations and differential carrier phase fluctuations
Authors: Bhattacharyya A. , Beach T. L. , Basu S. , Kintner P. M.
Published: September 2002

The presence of scintillation‐producing irregularities in the nighttime equatorial ionosphere, in the path of Global Positioning System (GPS) signals received at an equatorial station, causes dual‐frequency measurements of the differential carrier phase of GPS L1 and L2 signals to have a contribution from phase scintillations on the two signals. Dual‐frequency data for fluctuations in the total electron content (TEC) along the path of GPS signals to the equatorial station Ancon (1.5° dip), sampled at a rate of 1 Hz, are used to separate this contribution from the slower TEC variations. Rapid fluctuations in the differential carrier phase, usually on timescales < 100 s, which result from diffraction, are seen to follow the pattern of intensity scintillations on the L1 signal. Intensity scintillations are also related to the variations in TEC which arise from density fluctuations associated with ionospheric irregularities. An approximate version of the transport‐of‐intensity equation, based on a phase screen description of the irregularities, suggests that a quantitative measure of intensity scintillations may be provided by the derivative of rate of change of TEC index (DROTI), obtained from the second derivative of TEC. This equation also yields the dependence of the scaling factor between DROTI and S4 on the Fresnel frequency. Comparison of DROTI computed from relative TEC data to corresponding S4 indices indicates that there may be lesser uncertainity in a quantitative relation between the two than between the index ROTI, introduced in recent years, and S4. Power spectral analysis of TEC fluctuations and simultaneous intensity scintillations on L1 signal, recorded at Ancon, does not indicate any simple dependence of the scaling factor between DROTI and S4 on the spectral characteristics.
Low‐frequency induction in a homogeneous sphere by line sources: 2. Line charge excitation and discussion
Authors: Dawson Trevor W.
Published: September 2002

The problem of low‐frequency induction in a sphere by an external line source is amenable to analytic solution by Stevenson's method. This second of two parts develops the complete electromagnetic field solution, through first order in frequency, for (electric) excitation by zeroth‐ and first‐order line charge sources. The solutions can largely be expressed in terms of elementary functions. The result serves as a useful test solution for low‐frequency electromagnetic modeling codes involving nonuniform sources, and in particular for applications involving power lines. The solution is also applicable to a small sphere in the extreme near field of a linear antenna element.
The influence of the protonosphere on GPS observations: Model simulations
Authors: Lunt N. , Kersley L. , Bailey G. J.
Published: September 2002

The accuracy of the Global Positioning System (GPS) satellite navigation system can be degraded by propagation effects on ray paths through the ionized atmosphere. The bulk of the plasma resides in the F layer, and models of total electron content have been developed to compensate for the effects of this ionization. However, use of the GPS system involves long ray paths through the tenuous hydrogen‐based plasma of the protonosphere, and little is known about the electron content in this region. In the present study, the Sheffield University plasmasphere ionosphere model has been used to determine the electron content on the protonospheric section of GPS ray paths. Results are presented for stations at midlatitudes in the European and American sectors at both extremes of the solar cycle. The results are discussed in terms of the geometry of the flux tubes and the known behavior of the plasma.
On the choice of basis functions to model surface electric current densities in computational electromagnetics
Authors: Gürel Levent , Sertel Kubilay , Şendur Ibrahim KurŞat
Published: September 2002

Basis functions that are used to model surface electric current densities in the electric field integral equations of computational electromagnetics are analyzed with respect to how well they model the charge distribution, in addition to the current. This analysis is carried out with the help of the topological properties of open and closed surfaces meshed into networks of triangles and quadrangles. The need for current basis functions to properly model the charge distribution is demonstrated by several examples. In some of these examples, the basis functions seem to be perfectly legitimate when only the current distribution is considered, but they fail to deliver a correct solution of the electromagnetic problem, since they are not capable of properly modeling the charge distribution on some surfaces. Although the idea of proper modeling of the charge distribution by the current basis functions is easy to accept and can even be claimed well known, the contrary uses encountered in the literature have been the motivation behind the investigation reported in this paper.
Probe experiment characterizing 30‐MHz radio wave scatter in the high‐latitude ionosphere
Authors: Nishino M. , Gorokhov N. , Tanaka Y. , Yamagishi H. , Hansen T.
Published: September 2002

A probe experiment, consisting of radio links between a common 30‐MHz transmitter located at Murmansk, Russia, and two receivers used as the imaging riometer (two‐dimensional 64 multiple‐beam antenna) located at Ny Ålesund, Svalbard, and Tjornes, Iceland, was carried out to characterize wave scatter in the high‐latitude ionosphere. They are nearly aligned with and perpendicular to the geomagnetic meridian, respectively. In experiments conducted in March–April 1994, the 30‐MHz probe signals were identified at nighttime more frequently than during the day at both receiver stations during periods of increased geomagnetic activity near the path midpoints, indicating that a relationship between the propagation path and the location of the auroral oval controls signal identification. For the nighttime propagation paths within or crossing through the auroral oval, duty cycles of the probe signals were roughly correlated with increases in geomagnetic activity. Their arrival directions showed a spread with a dominant power on the low elevation and a normal distribution in azimuth. These results indicate that the probe signals are characterized as nonmeteoric “auroral E” scatter caused by irregular, large‐scale profiles of electron density enhancements at the lower edge of the ionosphere. However, on 2 days of weak geomagnetic activity, strong probe signals with bursty behavior were identified by an extremely high duty cycle (∼98%) for the nighttime meridian path only, and their arrival directions showed an isotropic spread in azimuth. Such nonmeteoric probe signals are characterized as “coherent” scatter caused by small‐scale (∼5 m) field‐aligned irregularities in electron density in the E region ionosphere, related to “sporadic E” occurrence.
Performance and uses of a refurbished 30‐m former satellite communications antenna: The Georgia Tech Woodbury Research Facility
Authors: DeBoer David R. , Steffes Paul G.
Published: September 2002

A class of large satellite communication antennas built in the 1970s for commercial purposes comprises a potential set of large antennas useful for education, research, satellite communications, or radio astronomy upon upgrade. Many of these facilities were abandoned as the advent of low‐noise electronics obviated the need for such large antennas with their associated maintenance costs. Although many have sat idle and decaying over the intervening years, these facilities remain a potential resource for research and education. One such facility located south of Atlanta near the town of Woodbury, Georgia, was acquired for use by the Georgia Institute of Technology. This facility consists of a pair of 30‐m antennas built in the mid‐1970s that were used by AT&T through the early 1980s for satellite communications. Initially as a set of small‐scale student projects and later as a full‐scale project funded by the SETI Institute, one of the 30‐m antennas has recently been fully renovated and used for radio astronomy and the search for extraterrestrial intelligence (SETI). This paper discusses the process and results of refurbishing the Woodbury facility.
New physical effects when leakage on printed‐circuit lines occurs into more than one surface wave
Authors: Tsuji Mikio , Shigesawa Hiroshi , Oliner Arthur A.
Published: September 2002

It is known that the bound mode on many printed‐circuit transmission lines becomes leaky above some critical frequency and that the leakage occurs in the form of a surface wave on the surrounding substrate. At still higher frequencies the leakage can occur into more than one surface wave. Depending on the structure and the geometric parameters, the physical phenomena can take on different forms. This paper first describes the various forms that we have found in our study and then focuses on the one that involves an unusually wide spectral gap at the onset of the additional surface wave. The wide‐spectral‐gap case is examined in more detail because its features differ in important ways from those of the usual spectral gap. Theoretical calculations and measured results are presented which describe these new features.
A new strategy to reduce the truncation error in near‐field/far‐field transformations
Authors: Bucci O. M. , D'Elia G. , Migliore M. D.
Published: September 2002

A new strategy to reduce the far‐field reconstruction error due to the finite size of the measurement region in planar and cylindrical near‐field/far‐field transformation techniques is presented. The approach exploits the (spatial) band‐limitation property of the radiated field in order to estimate the field outside the measurement area. A number of different estimation algorithms are investigated, with reference to both the accuracy of the estimation and the required computational effort. Numerical and experimental results show that with a small computational effort, it is possible to obtain a significant improvement of the reconstructed far‐field pattern with respect to that obtained without near‐field extrapolation.
Regional analytical model of ionospheric total electron content: Monthly mean and standard deviation
Authors: Gulyaeva T. L.
Published: September 2002

Simple analytical expansion of monthly mean vertical ionospheric electron content (TEC) and its standard deviation (SD) is made over the American continent. TEC Faraday rotation observations at four receiving sites are used around the 75°W meridian. Hourly values for each day of 1981 (high solar activity) and 1986 (low solar activity) linearly interpolated for any given sunspot number Rz allow us to use simple trigonometric functions F(t,s,g) for analytical modeling of TEC and SD. Contributions due to local time t, season‐month s, geomagnetic latitude g are separated based on four nodes for t, s, g defined as 90°× n (n = 0, 1, 2, 3). Comparison of the present regional analytical model with International Reference Ionosphere global TEC predictions is presented. Application of the model for estimates of the monthly probability of occurrence of TEC disturbances is illustrated. The same approach can be adapted to other geographic area and/or data sources by relevant replacement of the input database, for example, using Global Positioning System ‐ Global Navigation Satellite System observations of TEC including contributions of the ionosphere and plasmasphere.
Scattering properties of a statistically rough interface inside a multilayered medium
Authors: Yarovoy A. G. , de Jongh R. V. , Ligthart L. P.
Published: September 2002

In this paper, electromagnetic wave scattering from a slightly rough interface inside a stratified medium is considered. A three‐layer model for the medium is chosen, which allows us to investigate both possible cases of the medium's stratification: above and below the rough interface. It also is a simple but realistic model for the ground, in which the upper layer is thought to be vegetation or snow, while the middle layer and the homogeneous half‐space below it represent the ground itself. The vegetation‐ground interface is considered to be rough with statistically homogeneous corrugation, while the two other interfaces are flat. The backscatter coefficients for vertically and horizontally polarized waves are found in the framework of the small perturbation method combined with the Green function formalism. Such an approach allows one to consider arbitrary layers and, in this respect, go far beyond the radiative transfer theory. The influence of the stratification of the medium above and below the rough interface on the scattered field is analyzed numerically. Qualitatively new angular and frequency dependencies of backscattering are found. It is shown that in the case of small ohmic losses stratification of a medium can cause the deviation of backscattering up to 20 dB.
Study of a dielectric‐loaded slot antenna fed by a finite‐width substrate microstrip line
Authors: Hui H. T. , Yung Edward K. N.
Published: September 2002

A dielectric‐loaded slot antenna fed by a finite‐width substrate microstrip line is studied theoretically and experimentally by using the spatial dyadic Green's functions. Emphasis is placed on the analysis of the new feeding method, the finite‐width substrate microstrip line, which is a versatile microstrip structure and can be made as a stand‐alone device. The dielectric load is a hemisphere placed over the slot. The volume equivalence theorem is used to establish the theoretical model of the microstrip line. A simple excitation method for the microstrip line is introduced and modified to fit our problem. The spatial domain multilayered dielectric sphere dyadic Green's functions are used to obtain the scattered electromagnetic fields from the hemispherical dielectric load. The microstrip‐line current, the microstrip‐substrate electric field, and the equivalent magnetic current on the slot are obtained and used to calculate the impedance and the radiation pattern of the antenna. Computed impedances of the unloaded slot antenna and the loaded slot antenna show good agreements with either published data or measurement values. Analysis shows that the permittivities of the microstrip substrate have a significant effect on the impedance level of the antenna. It is found that electrically dense (high permittivities) substrates can give better impedance‐matching between the feeding microstrip line and the slot antenna. The reduction of the antenna impedance by using a thinner substrate has also been demonstrated in our investigation.
Specific absorption rate distributions in a multilayered spheroidal human head model exposed to mobile dipoles
Authors: Kang X. K. , Li L. W. , Leong M. S. , Kooi P. S.
Published: September 2002

The analysis of electromagnetic (EM) energy deposition inside the human head close to mobile handsets has been a hot topic in recent years. This paper presents the computation of the EM field distribution inside a multilayered prolate spheroidal model of a human head due to the radiation from a mobile dipole, using spheroidal vector wave functions. The coupled unknown coefficients were determined from a square matrix equation system, and the specific absorption rate (SAR) distributions inside the spheroidal head models were calculated. The various SAR distributions versus the distance between the source and the head at different operating frequencies and antenna inclination angles are presented and discussed.
A programmable riometer for Earth and Mars
Authors: Fry C. D. , Rosenberg T. J. , Lutz L. , Detrick D. L. , Weatherwax A. T. , Knouse E. , Breden H. , Giganti J.
Published: September 2002

We have developed a miniature radio receiver designed to operate as a relative ionospheric opacity meter (riometer). This project was funded by NASA as an enabling technology for future planetary radio science missions. We sought to reduce the instrument's size, mass, and power so that it would be practical for a Mars lander or rover mission. A recent study by our group indicates that a riometer might work well on Mars and offers a potentially rich science return. The technology also has immediate terrestrial applications. For example, the University of Maryland operates a chain of imaging riometers at the Automatic Geophysical Observatories (AGOs) in Antarctica. Our riometer includes features that are desirable for extended autonomous operation such as those with AGOs: low power consumption, wide dynamic range and linearity, computer command and data interface, and the ability to be remotely reconfigured. The receiver design provides significant improvements over previous implementations used in riometers. The high degree of system linearity, combined with a digital feedback loop (including a low‐duty calibration cycle), allows more time for viewing the radio sky. We implemented several of the receiver subsystems in a field‐programmable gate array, including the receiver detector, the control logic, and the data acquisition and processing blocks. Considerable efforts were made to eliminate or minimize RF noise and spurious emissions generated by the receiver's digital circuitry. Results of laboratory and field tests are presented and discussed.
Frontal part of the northern Apennines fold and thrust belt in the Romagna‐Marche area (Italy): Shallow and deep structural styles
Authors: Coward Mike P. , De Donatis Mauro , Mazzoli Stefano , Paltrinieri Werter , Wezel Forese‐Carlo
Published: September 2002

In this study, surface geological data resulting from a detailed field survey, including structural and biostratigraphic analysis, have been integrated with subsurface (seismic lines and well logs) data in order to reconstruct the tectonic evolution of the external zones of the northern Italian Apennines in the Romagna‐Marche foothills and Adriatic Sea areas. This integrated analysis shows: (1) a late Messinian to lower Pleistocene progression of structural development from the hinterland to the foreland of the studied sector of the thrust belt; (2) relatively limited (≤20%), southward increasing, amounts of shortening (obtained by the construction of line‐length balanced and restored geological cross sections); (3) a regional deformation style characterized by the presence of backthrusts associated with most foreland‐vergent thrust ramps, leading to quasi‐symmetric uplift and a low critical taper for the wedge, typical of foreland fold and thrust belts with a weak basal decollement (Triassic anhydrites in the present case); (4) an important influence of basement faulting which, despite a general basement‐cover decoupling, appears to control stress localization in the latter, producing linkage of basement and cover stuctures in a combination of thin‐ and thick‐skinned tectonic styles; and (5) contrasting structural styles characterizing deep features, as imaged by seismic reflection profiles, and shallow ones. Deep stuctures consist of growth anticlines bounded by major thrust ramps and back limb back thrusts, separated by broader, open synclines, both involving a Mesozoic‐Paleogene, mainly carbonate, passive margin succession. In the crestal zones of major anticlines, shallow structures, affecting Neogene terrigenous foredeep sediments, show a complex pattern of upright to recumbent folds (of tens to hundreds of meters wavelength) related to minor thrusts and backthrusts. Deformation of the Mesozoic‐Paleogene multilayer appears to be dominated by thrust propagation in the cores of early formed anticlines developed by buckling instabilities. The overlying Neogene deposits are detached from the carbonate substratum along the base of the foredeep succession. Bedding‐parallel slip occurring along this detachment level appears to be accommodated by the complex structures in the crests of major anticlines, where the thrusts lramp to the surface cutting up section. Complex shallow structures, interpreted to accommodate at shallow structural levels the deep deformation, would therefore represent a geometrical requirement for maintaining strain compatibility across the shallow detachment level located at the base of the foredeep succession.
Palinspastic restoration of a post‐Taconian successor basin deformed within a transpressive regime, northern Appalachians
Authors: Kirkwood Donna
Published: September 2002

Restoration of transpressional orogens requires a three‐dimensional analysis of the strain state in order to take into account both the pure shear and simple shear components of the deformation and assess their relative contribution throughout the deformation history. Restoration of the post‐Taconian successor basin, the Gaspé Belt located within the external part of the Acadian orogen in the Canadian Appalachians, was performed by using a step‐by‐step method based on the successive removal of strain increments. The pure shear component contributed 70% of the total strain during the first stages of the deformation whereas the simple shear component contributed 30% of the total strain during the last stages. Three retrodeformation steps are defined, starting with restoration of the last deformation stage and working back to the first deformation stage. The first step is to restore slip along Acadian strike‐slip faults. The second step is to restore shortening related to the horizontal extension during the Acadian regional simple shear event. Finally, the third step is to restore vertical extension related to the pure shear event by removing slip along the reverse faults, unfolding and removing internal strain associated with cleavage development. Retrodeformation of the simple shear component of the transpressive deformation during the first two steps is performed on a horizontal view of the Gaspé Belt basin which contains the horizontal extension direction and which is perpendicular to the Acadian NW–SE directed compression. Likewise, restoration of the pure shear component is performed in vertical sections which contain the vertical extension and which are perpendicular to the Acadian NW–SE directed compression. The palinspastic map presented here is a much more appropriate base map for the basin and allows for a more realistic paleogeographic interpretation of the Gaspé Belt.
Distribution of displacement on and evolution of a young transform fault system: The northern San Andreas fault system, California
Authors: Wakabayashi John
Published: September 2002

This paper presents a working model for the spatial and temporal distribution of dextral slip on the northern San Andreas fault system of coastal California, based primarily on field relations in the San Francisco Bay area, and offers insight into the evolution of a young transform fault system. A fundamental difference between this and previous models of slip distribution is that this model assigns negligible slip to the Pilarcitos fault, which has been suggested to have 120 to 250 +km of post early‐Miocene dextral slip, in previous models. Because separation on the San Francisco Peninsula reach of the San Andreas fault is about 25 km, and displacement on the San Andreas fault in central California is 310 to 320 km, more than 250 km of late Cenozoic dextral slip must be accommodated east of the San Francisco Peninsula. The distribution of this dextral slip is constrained by offset late Cenozoic and basement units. Slip distribution, combined with ages of offset features and plate boundary kinematic constraints, show that the distribution of slip rates on groups of faults along the transform boundary has changed in an irregular fashion through time, in contrast to existing models that propose progressive eastward migration of active faulting in the San Andreas system. In addition to the shifting patterns of displacement, a migrating transition zone connecting the eastern faults of the strike‐slip system to the Mendocino triple junction may have resulted in distributed dextral faulting and shortening in the northernmost and youngest part of the transform fault system.
Plate deformation at depth under northern California: Slab gap or stretched slab?
Authors: ten Brink Uri S. , Shimizu Nobumichi , Molzer Philipp C.
Published: September 2002

Plate kinematic interpretations for northern California predict a gap in the underlying subducted slab caused by the northward migration of the Pacific‐North America‐Juan de Fuca triple junction. However, large‐scale decompression melting and asthenospheric upwelling to the base of the overlying plate within the postulated gap are not supported by geophysical and geochemical observations. We suggest a model for the interaction between the three plates which is compatible with the observations. In this “slab stretch” model the Juan de Fuca plate under coastal northern California deforms by stretching and thinning to fill the geometrical gap formed in the wake of the northward migrating Mendocino triple junction. The stretching is in response to boundary forces acting on the plate. The thinning results in an elevated geothermal gradient, which may be roughly equivalent to a 4 Ma oceanic lithosphere, still much cooler than that inferred by the slab gap model. We show that reequilibration of this geothermal gradient under 20–30 km thick overlying plate can explain the minor Neogene volcanic activity, its chemical composition, and the heat flow. In contrast to northern California, geochemical and geophysical consequences of a “true” slab gap can be observed in the California Inner Continental Borderland offshore Los Angeles, where local asthenospheric upwelling probably took place during the Miocene as a result of horizontal extension and rotation of the overlying plate. The elevated heat flow in central California can be explained by thermal reequilibration of the stalled Monterey microplate under the Coast Ranges, rather than by a slab gap or viscous shear heating in the mantle.
Slip partitioning and deformation cycles close to major faults in southern California: Evidence from small‐scale faults
Authors: Ghisetti Francesca
Published: September 2002

Geometry and kinematics of small‐scale faults adjacent to large seismically active faults have been analyzed for a large region of southern California. The sampled population of small‐scale faults is scattered in orientation and slip but possesses distinct trends. Two important characteristics are, first, the paucity of small‐scale faults that mimic orientations and mechanisms of the adjacent master faults, and, second, the large variation in slip mechanisms of fault planes oriented E‐W and N‐S. In order to test the similarity of the sampled fault population to present‐day deformations, data on geometry and slip of the small‐scale faults have been used for stress inversions. These results are compared with those derived from focal mechanisms of low‐magnitude earthquakes in the same region. Stress orientations derived from small‐scale faults are spatially heterogeneous, but horizontal axes of stress maintain N‐S and E‐W trends over the entire region. The similarities between stress inversions from small faults and seismicity support the conclusion that most of the sampled faults are recent and formed during cyclical events of fracturing in a relatively strong crust adjacent to weak master faults.