bluuebunny/crossref_metadata_2025 huggingface dataset:
Energy exchange in early spring over sea ice in the Pacific sector of the Southern Ocean
Authors: Hauser Adrian , Wendler Gerd , Adolphs Ute , Jeffries Martin O.
Published: September 2002

In September‐October 1994, during a cruise to Antarctica, we carried out measurements of the radiation balance and turbulent heat fluxes aboard the R/V Nathaniel B. Palmer. Hourly and daily flux values were calculated. The net radiation and the eddy fluxes showed the expected diurnal variations. On average, the ocean warmed the atmosphere by 30.7 W m−2, an expected value for spring. This flux, which was obtained as the remainder in the heat budget equation, showed larger values at night than during the day. There was a large amount of scatter in the surface albedo when the ship was within the ice pack; low albedo values were associated with young non‐snow‐covered ice and ice with wet snow cover.
Aerosol effects on UV radiation in nonurban regions
Authors: Reuder J. , Schwander H.
Published: September 2002

The effect of variable tropospheric aerosol conditions on UV radiation is analyzed using both model simulations and UV radiation measurements. A sensitivity study of the aerosol effect on UV radiation is performed by numerical modeling with a focus on the dependence on wavelength, solar zenith angle, and detector geometry. Potential day to day variability in atmospheric aerosols produces changes of spectral integrated radiation quantities of the order of 20% to 45%. Equivalent effects are induced by total ozone variations between 40 Dobson units (DU) and 90 DU for those spectral integrals having a high sensitivity in the UV‐B wavelength range. At least 80% of the aerosol effect is caused by typical variations in aerosol optical depth and single scattering albedo. The variation of other aerosol optical properties such as phase function, spectral extinction, and extinction profile is of minor importance. Our results indicate that information on both aerosol optical depth and single scattering albedo is necessary in most cases for an adequate description of aerosol impact on UV radiation. The sensitivity study supplies a tool for the comparison of aerosol effects derived from various UV radiation measurements with respect to spectral weighting and/or detector geometry. The results of the sensitivity study contribute to the discussion of a local aerosol study performed using measurements we made at Hoher Peissenberg, Germany. Two years of ozone and nitrogen dioxide photolysis frequency measurements are analyzed with respect to aerosol‐induced radiation changes in the UV‐B and UV‐A wavelength region. A procedure is introduced to extract the aerosol influence from the impact of other atmospheric parameters, particularly from the ozone amount for investigations in the UV‐B. A comparison of the derived aerosol effect with model simulations shows good agreement.
Interannual variability in net accumulation on the Greenland Ice Sheet: Observations and implications for mass balance measurements
Authors: van der Veen C. J. , Bolzan J. F.
Published: September 2002

Nine 24‐year accumulation records from the Summit region in central Greenland are analyzed to separate the effects of spatial noise and interannual fluctuations on the variability in each core. The study shows that both processes are equally important, with standard deviations of 25 mm water equivalent per year and 24 mm water equivalent per year, respectively. A comparison with estimates of surface roughness based on high‐resolution laser altimetry of the surface indicates that in the studied region the spatial noise can be reliably estimated from surface roughness. The response of the ice‐sheet surface to the interannual fluctuations can be estimated using a simple zero‐dimensional ice‐sheet response model. For the Summit region of central Greenland, a change in surface elevation of ∼20 mm water equivalent per year measured over a 5‐year period, can be attributed with 95% confidence to a trend in climate. This probability decreases rapidly as the observation period is shortened. For intervals greater than ∼5 year, the probability depends only weakly on the measurement interval. This suggests an optimum spacing of ∼5 years between repeat elevation measurements.
Simulation of particle fluxes and cosmogenic nuclide production in the Earth's atmosphere
Authors: Masarik J. , Beer J.
Published: September 2002

A purely physical model for the simulation of cosmic ray particle interactions with the Earth's atmosphere and subsequent production and transport of secondary particles is presented. Neutron and proton spectra as a function of the coordinate in the atmosphere were calculated using a GEANT / MCNP‐based code system. The calculated neutron fluxes are in good agreement with experimental data based on neutron monitor measurements. These fluxes, together with experimental or evaluated cross sections, were used to calculate the production rates of 3H, 7Be, 10Be 14C, and 36Cl. The dependencies of these production rates on solar activity and geomagnetic field intensity were investigated in detail. The obtained production rates agree well with most published experimental and theoretical values. Possible reasons for some differences are discussed.
A detailed evaluation of the stratospheric heat budget: 2. Global radiation balance and diabatic circulations
Authors: Mlynczak Martin G. , Mertens Christopher J. , Garcia Rolando R. , Portmann Robert W.
Published: September 2002

We present a detailed evaluation of radiative heating, radiative cooling, net heating, global radiation balance, radiative relaxation times, and diabatic circulations in the stratosphere using temperature and minor constituent data provided by instruments on the Upper Atmosphere Research Satellite (UARS) between 1991 and 1993 and by the limb infrared monitor of the stratosphere (LIMS) instrument which operated on the Nimbus‐7 spacecraft in 1978–1979. Included in the calculations are heating due to absorption of solar radiation from ultraviolet through near‐infrared wavelengths and radiative cooling due to emission by carbon dioxide, water vapor, and ozone from 0 to 3000 cm−1 (∞ ‐ 3.3 μm). Infrared radiative effects of Pinatubo aerosols are also considered in some detail. In general, we find the stratosphere to be in a state of global mean radiative equilibrium on monthly timescales to within the uncertainty of the satellite‐provided measurements. Radiative relaxation times are found to be larger in the lower stratosphere during UARS than LIMS because of the presence of Pinatubo aerosols. The meridional circulations in the upper stratosphere as diagnosed from the calculated fields of net heating are generally stronger in the UARS period than during the LIMS period, while the lower stratosphere meridional circulations are stronger during the LIMS period. A climatology of these calculations is available to the community via a World Wide Web interface described herein.
Potential role of the quasi‐biennial oscillation in the stratosphere‐troposphere exchange as found in water vapor in general circulation model experiments
Authors: Giorgetta Marco A. , Bengtsson Lennart
Published: September 2002

The tropical tropopause is considered to be the main region of upward transport of tropospheric air carrying water vapor and other tracers to the tropical stratosphere. The lower tropical stratosphere is also the region where the quasi‐biennial oscillation (QBO) in the zonal wind is observed. The QBO is positioned in the region where the upward transport of tropospheric tracers to the overworld takes place. Hence the QBO can in principle modulate these transports by its secondary meridional circulation. This modulation is investigated in this study by an analysis of general circulation model (GCM) experiments with an assimilated QBO. The experiments show, first, that the temperature signal of the QBO modifies the specific humidity in the air transported upward and, second, that the secondary meridional circulation modulates the velocity of the upward transport. Thus during the eastward phase of the QBO the upward moving air is moister and the upward velocity is less than during the westward phase of the QBO. It was further found that the QBO period is too short to allow an equilibration of the moisture in the QBO region. This causes a QBO signal of the moisture which is considerably smaller than what could be obtained in the limiting case of indefinitely long QBO phases. This also allows a high sensitivity of the mean moisture over a QBO cycle to the El Niño‐Southern Oscillation (ENSO) phenomena or major tropical volcanic eruptions. The interplay of sporadic volcanic eruptions, ENSO, and QBO can produce low‐frequency variability in the water vapor content of the tropical stratosphere, which renders the isolation of the QBO signal in observational data of water vapor in the equatorial lower stratosphere difficult.
Numerical simulations of local winds over steep orography in the storm over north Norway on October 12, 1996
Authors: Grønås Sigbjørn , Sandvik Anne Dagrun
Published: September 2002

Some power pylons (30 m high) broke down in a storm over north Norway in October 1996. The pylons were situated in a short, narrow valley (3 km long) surrounded by mountains reaching 600–850 m. The pylons fell in a direction opposite to the main southwesterly winds, and it is likely that the damage was caused by wind gusts of more than 50 m s−1. High‐resolution simulations with the nonhydrostatic numerical model MEMO have been made to investigate the local winds. Initial conditions have been taken from successful limited‐area synoptic‐scale predictions of the damaging storm. For upstream southwesterly wind, leeside bluff separation was simulated in the valley. The recirculation filled the valley in a flow organized like a corkscrew down the valley. In the bottom of the valley the recirculation formed a return jet with a maximum wind speed between 17 and 20 m s−1 in the different simulations, and with a considerable component toward the separation line at the ground, downstream from the mountaintops on the southern side of the valley. The axis of the corkscrew was found at 0.4 of the valley height. A veering of the initial upstream wind direction of 20° changed the flow to be steered up the valley, with a shallow recirculation confined to the southern side of the valley only. The turbulence in the recirculation area was found to be large, and it is argued that the wind gusts in the southwesterly case might reach values 3 times the mean wind speed.
Trends in the hydrologic cycle of the Amazon Basin
Authors: Costa Marcos Heil , Foley Jonathan A.
Published: September 2002

Although previous studies have considered the long‐term variability of precipitation and river discharge in the Amazon basin, other components of the hydrologic cycle, such as evapotranspiration and the transport of water vapor, have not received the same attention. This study examines the 20‐year variability of the full hydrologic budget of the Amazon basin, using a 1976–1996 time series from the National Centers for Environmental Protection/National Center for Atmospheric Research reanalyzed meteorological data set. Within this 20‐year record, there is a statistically significant decreasing trend in the atmospheric transport of water vapor both into and out of the Amazon basin. This trend is associated with a general relaxation of the southeasterly trade winds, a weakening of the east‐to‐west pressure gradient, and a warming of the sea surface temperatures in the equatorial South Atlantic region. While the atmospheric transport of water vapor through the Amazon basin has decreased, the internal recycling of precipitation within the basin increased and basin‐wide precipitation, evapotranspiration, and runoff have remained nearly constant. Even though basin‐average precipitation and runoff have remained fairly stable, other components of the Amazon basin's hydrologic cycle have been altered significantly by large‐scale changes in atmospheric circulation.
Dark material in the polar layered deposits and dunes on Mars
Authors: Herkenhoff Ken E. , Vasavada Ashwin R.
Published: September 2002

Viking infrared thermal mapping and bistatic radar data suggest that the bulk density of the north polar erg material is much lower than that of the average Martian surface or of dark dunes at lower latitudes. We have derived a thermal inertia of 245–280 J m−2 s−1/2 K−1 (5.9–6.7×10−3 cal cm−2 s−1/2 K−1) for the Proctor dune field and 25–150 J m−2 s−1/2 K−1 (0.6–3.6×10−3 cal cm−2 s−1/2 K−1) for the north polar erg. The uniqueness of the thermophysical properties of the north polar erg material may be due to a unique polar process that has created them. The visible and near‐infrared spectral reflectance of the erg suggests that the dark material may be composed of basalt or ferrous clays. These data are consistent with the dark material being composed of basaltic ash or filamentary sublimate residue (FSR) particles derived from erosion of the layered deposits. Dark dust may be preferentially concentrated at the surface of the layered deposits by the formation of FSR particles upon sublimation of water ice. Further weathering and erosion of these areas of exposed layered deposits may form the dark, saltating material that is found in both polar regions. Dark FSR particles may saltate for great distances before eventually breaking down into dust grains, re‐mixing with the global dust reservoir, and being recycled into the polar layered deposits via atmospheric suspension.
Control of the radiative behavior of the Martian polar caps by surface CO2 ice: Evidence from Mars Global Surveyor measurements
Authors: Hansen Gary B.
Published: September 2002

The seasonal polar caps of Mars are composed primarily of solid CO2, and their growth and decay play a large part in the planet's atmospheric CO2 cycle. The thermodynamic temperature of the caps is ∼145 K, in equilibrium with the ∼6 mbar atmosphere. The optical properties of CO2 ice require that pure snow with a sufficiently small particle size has a high visible/near‐infrared albedo and a low infrared emissivity in parts of the thermal infrared, particularly in the region between 20 and 50 μm. Dust mixed into or on top of the CO2 ice will lower the visible albedo and bring the thermal infrared emissivity closer to 1. Water ice mixed with the CO2 will have little effect in the visible and near‐infrared to 1.4 μm but can raise the 20–50 μm emissivity if the particle size is >50 μm. Observations of both seasonal polar caps during two of the assessment orbits of the Mars Global Surveyor spacecraft are used to show that there is a strong correlation between visible brightness and low 20–50 μm emissivity. The infrared spectra from the Thermal Emission Spectrometer from regions with low 20–50 μm brightness temperature are consistent with surface deposits of CO2 with millimeter‐sized grains and containing varying small amounts of dust, and they are not consistent with the expected signature of water ice or clouds. Large regions of low emissivity in the spring seasonal caps have not been observed previously. They are correlated with visible bright regions which are known to become brighter as the spring progresses. The visible brightness of the south polar deposits examined is inferred from historical observations, while the brightness of the north polar region is determined from Mars Orbiter Laser Altimeter measurements at 1.06 μm. The model of seasonal CO2 caps with bright, low‐emissivity regions agrees with previous visible observations of bright crater rims, streaks, and other bright areas within the polar caps, some of which may evolve in time from dark, high‐emissivity sheet ice to brighter, fractured, lower‐emissivity ice layers. It is also in agreement with models of the CO2 cycle, which require average polar cap emissivities <0.9.
Moments of inertia and rotational stability of Mars: Lithospheric support of subhydrostatic rotational flattening
Authors: Bills Bruce G. , James Thomas S.
Published: September 2002

A revised estimate of the spin axis precession rate of Mars has recently been obtained via analysis of range and range‐rate data from the Viking and Pathfinder landers. When combined with existing estimates of the degree 2 spherical harmonic coefficients of the gravitational field, this yields a complete determination of the inertia tensor of Mars. Despite this progress, there are still numerous unresolved issues related to the internal structure and rotational dynamics of Mars. We compare results of two different approaches to this problem. In one approach, the observed gravitational field is conceptually partitioned into hydrostatic and nonhydrostatic contributions. In the other approach, the input to the system is partitioned into rotational and load components, and the internal structure (density and elastic rigidity) determines the response. We demonstrate that there is an important, and still unresolved trade‐off between lithospheric thickness and the shape of the load component of the gravity field. As the lithospheric thickness is increased, the required load departs more from axial symmetry. The load corresponding to zero lithospheric thickness is nearly symmetric about an equatorial axis, but if the lithospheric thickness is closer to 100 km, the required load is a fully triaxial ellipsoid, with the intermediate moment of inertia halfway between the least and greatest moments. The symmetry of the load has considerable influence on the long‐term rotational stability of Mars.
Reanalysis of Clementine bistatic radar data from the lunar South Pole
Authors: Simpson Richard A. , Tyler G. Leonard
Published: September 2002

On April 9, 1994, the Clementine spacecraft high‐gain antenna was aimed toward the Moon's surface, and the resulting 13‐cm wavelength radio echoes were received on Earth. Using these data, we have found that the lunar surface generally follows a Lambertian bistatic scattering function σ0 = KD cosθi cosθs with KD∼0.003 for the opposite (expected) sense of circular polarization and KD∼0.001 for the same (unexpected) sense. But there are important deviations (of up to 50% in some parts of the echo spectrum) from this simple form. Based on an earlier analysis of these same data, Nozette et al. [1996] claimed detection of an enhancement in echoes with the same sense circular polarization from regions near the South Pole in a near‐backscatter geometry. Such behavior would be consistent with presence of perhaps large quantities of water ice near the pole. We have been unable to reproduce that result. Although we find weak suggestions of enhanced echoes at the time of South Pole backscatter, similar features are present at earlier and later times, adjacent frequencies, and in the opposite circular polarization. If enhanced backscatter is present, it is not unique to the South Pole; if not unique to the pole, then ice appears less likely as an explanation for the enhancement.
Field examination of the oceanic carbonate ion effect on stable isotopes in planktonic foraminifera
Authors: Russell Ann D. , Spero Howard J.
Published: September 2002

We determined the δ18O and δ13C of individual Globigerinoides ruber and Pulleniatina obliquiloculata from sediment traps located from 5°N to 12°S along 140°W in the Pacific Ocean to evaluate the effects of varying [CO3=] on shell δ18O and δ13C. Variations in the offset between shell δ13C and δ13CDIC (Δδ13Cs‐DIC) are attributed to differences in [CO3=], temperature, and shell size between sample sites. When Δδ13Cs‐DIC of G. ruber was corrected for variations in [CO3=] using the experimental slope of Bijma et al. [1998], the residual Δδ13Cs‐DIC was correlated with mixed layer temperature (+0.10±0.04‰ °C−1). The slope of this temperature effect is consistent with experimental results. In P. obliquiloculata, Δδ13Cs‐DIC and temperature were strongly anticorrelated (−0.14±0.03‰ C−1). We are unable to separate the influences of [CO3=] and temperature in this species without independent experimental data. Correcting for [CO3=] variability on δ18Os of G. ruber improves the accuracy of estimated sea surface temperatures.
Core Top 14C Ages as a Function of Latitude and Water Depth on the Ontong‐Java Plateau
Authors: Broecker Wallace S. , Clark Elizabeth , McCorkle Daniel C. , Hajdas Irka , Bonani Georges
Published: September 2002

Radiocarbon measurements on core tops from the Ontong‐Java plateau confirm a previous finding by Berger and Killingley [1982] that at any given water depth, cores taken on the equator have higher accumulation rates and younger core top ages than their off‐equator counterparts. Further, these new results fortify the conclusion by Broecker et al. [1991] that the increase in core top radiocarbon age with water depth rules out homogeneous dissolution within the pore waters as the dominant mechanism. Either most of the dissolution must occur prior to burial or it must occur during the first pass through the respiration‐CO2‐rich upper pore waters after which the calcite grains become armored against further dissolution. A puzzling aspect of this new data set is that despite the sizable difference in accumulation rate, the extent of dissolution as measured by either the CaCO3 content or the ratio of CaCO3 in the >150‐µm size fraction to that in the < 63‐µm fraction is no different off than on the equator. In order to reconcile the results of this study with those obtained by Hales and Emerson [1996] using in situ electrodes, it is necessary to call upon calcite armoring.
High‐pressure experiments and the phase diagram of lower mantle and core materials
Authors: Boehler R.
Published: September 2002

The interpretation of seismic data and computer modeling requires increased accuracy in relevant material properties in order to improve our knowledge of the structure and dynamics of the Earth's deep interior. To obtain such properties, a complementary method to classic shock compression experiments and theoretical calculations is the use of laser‐heated diamond cells, which are now producing accurate data on phase diagrams, equations of state, and melting. Data on one of the most important measurements, the melting temperatures of iron at very high pressure, are now converging. Two other issues linking core properties to those of iron are investigated in the diamond cell: One is the melting point depression of iron in the presence of light elements, and the other is the structure of iron at the conditions of the inner core. First measurements on eutectic systems indicate a significant decrease in the melting point depression with increasing pressure, which is in contrast to previous predictions. X‐ray diffraction measurements at simultaneously high pressure and high temperature have improved significantly with the installation of high‐pressure “beam lines” at synchrotron facilities, and structural measurements on iron are in progress. Considerable efforts have been made to develop new techniques to heat minerals at the conditions of the deep mantle in the diamond cell and to measure their phase relations reliably. Even measurements of the melting behavior of realistic rock compositions at high pressure, previously considered to be impossible in the diamond cell, have been reported. The extrapolated solidus of the lower mantle intersects the geotherm at the core‐mantle boundary, which may explain the seismically observed ultra low velocity zone. The diamond cell has great potential for future development and broad application, as new measurements on high‐pressure geochemistry at deep mantle and core conditions have opened a new field of research. There are, however, strict experimental requirements for obtaining reliable data, which are summarized in the present paper. Results from recent measurements of melting temperatures and phase diagrams of lower mantle and core materials at very high pressure are reviewed.
In situ measurement of reflection and diffraction coefficients of UHF radio waves on buildings using a ring array
Authors: Rossi J.‐P. , Wiart J. , Eynard F.
Published: September 2002

The lack of reliable estimates of the reflection and diffraction coefficients of radio waves by urban structures is one of the main barriers to deterministic modeling of the UHF channel. In complex environments such as urban areas, the reception of multiple echoes of the transmitted signal generally prevents direct in situ measurements of these coefficients. For the present study, the contributions of the different paths to the measured received power were separated by coupling the channel impulse response sounding with the determination of the direction of arrival (DOA). From experimental data we were able to isolate the field component generated by reflection or diffraction on buildings by both delay and angle of incidence. We deduced the corresponding reflection and diffraction coefficients. The wave DOA was determined using a ring array. The angle of arrival patterns were extracted by two different methods, namely the Wiener inversion method and the beam‐forming method. The coefficients deduced by these two techniques are shown to be very close. These results are also similar to those obtained without the help of DOA separation when delay resolution makes them available. We also show the validity of geometrical rays to represent reflection and diffraction of decimetric waves on the kind of building concerned.
Further examples of seasonal variations of ELF radio propagation parameters
Authors: Bannister Peter R.
Published: September 2002

In this paper we use experimentally determined values of effective attenuation rate, excitation factor, and relative phase velocity, along with the theoretical expressions derived by C. and P. Greifinger, to establish the seasonal variation of representative ionospheric conductivity parameters. These parameters include the reflection heights h0 and h1 (or hE), inverse scale height β, and reference height H. The basis for this analysis is provided by the 1990–1992 76‐Hz field strength measurements taken at four land‐based ELF monitoring sites established by the U.S. Navy. The source for these measurements was the U.S. Navy's dual‐antenna transmitting system (WTF/MTF). The main conclusion of this paper is that the summertime and January nighttime attenuation rates are substantially lower than during other times of the year. This nighttime attenuation rate decrease appears to be mainly due to an increase in the inverse scale height β, rather than to an increase in the reflection heights h0 and hE.
Design of doubly periodic filter and polarizer structures using a hybridized genetic algorithm
Authors: Weile D. S. , Michielssen E.
Published: September 2002

A hybrid genetic algorithm/local optimization technique for designing cascaded planar periodic structures such as frequency selective surfaces is developed. The genetic algorithm optimizes the shape and size of the periodic metallization in each of the screens as well as the thickness and dielectric constant of intervening dielectric layers in order to achieve a desired frequency response characteristic. Careful local optimization to fine‐tune the dielectric layer thicknesses is shown to accelerate both the spectral analysis of the cascades and the convergence of the stochastic search. Results are presented for the design of a satellite subreflector with a 3:1 ratio of stop band center to passband center, a circular polarizer with an axial ratio of less than 1 dB between 47 and 64 GHz, and a twist polarizer with a better than −1‐dB reflection into the cross‐polarized field component for 47–65 GHz.
Microwave Gaussian beam splitting with a variable split angle by using a chiral lens
Authors: Tariq Salim , Mahmoud Samir F. , Laghari Mohammad S.
Published: September 2002

In this paper we present analysis and design of a chiral lens that is used to split a linearly polarized Gaussian beam into right‐circularly and left‐circularly polarized beams traveling in different directions. It will be shown that split beams remain Gaussian if the curvature of the lens is large in comparison with the beam width. Moreover, the split angle can be varied by changing the point of beam incidence on the curved surface of the lens. Parameters like the beam width and cross‐polarization ratio for the split beams are derived and computed.
Equatorial spread F effects on an HF path: Doppier spread, spatial coherence, and frequency coherence
Authors: Fitzgerald T. Joseph , Argo Paul E. , Carlos Robert C.
Published: September 2002

In August 1990 we participated in the Equatorial Ionospheric Studies sounding rocket campaign near Kwajalein Atoll in the equatorial Pacific region. The campaign included measurements of plasma density using rocket probes and coherent and incoherent scatter radar. During the campaign we fielded high‐frequency ionospheric sounders over a bistatic path between Maloelap Atoll and Bikini Atoll in the Marshall Islands. The distance between the transmitters and receivers was 700 km; the ionospheric‐reflection region was at 10.18°N, 168.40°E, near the magnetic equator. We made three types of measurements: Doppler spread and spatial coherence for a single‐frequency CW path; frequency coherence of multiple CW paths; and Doppler spread and time‐delay spread for a 60‐kHz bandwidth path. We obtained such data over a period of 2 weeks for approximately 2 hours each evening; during this period spread F was common. Fifty percent of the evenings showed Doppler spread of greater than 6 Hz at the −10 dB level (relative to the peak signal power) and greater than 15 Hz at the −30 dB level. Forty percent of the evenings showed spatial coherence distance of less than 180 m in the direction normal to the bistatic path; 40% of the evenings showed spatial coherence of less than 75 m in the direction parallel to the path. Seventy‐five percent of the evenings showed coherence bandwidths of less than 1.5 kHz.
Optimal truncation criterion for application of singular value decomposition to ionospheric tomography
Authors: Zhou Chucai , Fremouw E. J. , Sahr J. D.
Published: September 2002

In this paper, we present a generic method to solve the subspace‐oriented estimation problem. We have optimized our approach by taking account of the a priori and a posteriori covariances of both the data and the model parameters in general linear inverse notations. In our work, singular value decomposition (SVD) was employed to provide a robust optimal solution. In computation of the generalized matrix inverse, a very simple truncation criterion on the singular value (SV) spectrum was set up which guarantees the minimal variance of the estimate. This algorithm based on SVD produces an optimal estimate independent of computing resources. Specifically, we applied this method to studies of ionospheric tomography by inverting total electron content (TEC), which may be measured by means of satellite beacons. We processed two simulated cases. The residual variances of the a posteriori covariances of the model parameters were used as the measure to evaluate the uncertainties of the estimates. Our examples indicate that this algorithm can resolve about 60% of the a priori variance while achieving a significant decrease of the computation time by truncation of the SV spectrum.
Currents induced in the human body by ELF electric fields inside houses located under power lines
Authors: Chen Hsing‐Yi , Peng Chen‐Kuen , Lin Der‐Phone
Published: September 2002

The moment method and the image technique are used to study currents induced in human bodies standing under 345‐kV power lines with and without house shielding. The maximum induced current in a human body without house shielding is found to be 68.9 μA. The maximum induced current in a human body with a cement‐house shielding is found to be 4.36 × 10−11 A. In this paper the electric fields induced on human body surfaces are also presented. It is found that there is an electric field enhancement factor at the top of the head.
Aspect sensitivity of VHF radar echoes observed in the middle and upper troposphere during the passage of a cut‐off low
Authors: Campistron Bernard , Pointin Yves B. , Lohou Fabienne , Pagès Jean‐Pierre
Published: September 2002

The aspect sensitivity of VHF radar signals is related to the enhancement of the echo power and the narrowing of the spectral width at vertical incidence relative to the off‐zenith directions. The strong aspect sensitivity is usually associated with stable atmospheric layers. In this paper the aspect sensitivity measured between an altitude of 4 and 15 km during the passage of a tropopause folding linked to a cut‐off low is analyzed and discussed. The study mainly uses data acquired during a 2‐day period with a three‐beam VHF radar with relatively broad beam widths (6.8°). The angular dependence of the echoes is investigated using the power ratio of vertical to oblique beam and the spectral width obtained at vertical incidence. These two diagnostic parameters provided a nearly identical depiction of the stratospheric descent associated with the upper frontal zone. However, a discrepancy is observed between the location of the stratospheric subsidence and the tropopause height offered by the radar and that deduced from the meteorological analysis produced by an operational forecasting numerical model. Owing to the aspect sensitivity of the echoes, an underestimate amounting to 20% was found in the horizontal wind measurement. Finally, a comparison of simultaneous observations of two closely spaced VHF radars (100 km) has shown a good spatial coherence of the aspect sensitivity structures.
Phenomena observed by HF long‐distance diagnostic tools in the HF modified auroral ionosphere during magnetospheric substorm
Authors: Blagoveshchenskaya N. F. , Kornienko V. A. , Brekke A. , Rietveld M. T. , Kosch M. , Borisova T. D. , Krylosov M. V.
Published: September 2002

The HF long‐distance diagnostic tools located in St. Petersburg combined with multi‐instrument observations at Tromsø have been operated in conjunction with the ionospheric heating facility near Tromsø to examine the heater‐induced phenomena in the auroral ionosphere. The distinctive feature of the heating experiment was the reflection of the pump wave from auroral sporadic Es‐layer. During substorm expansion, strong stimulated electromagnetic emission (SEE) at the third harmonic of the downshifted maximum frequency was found. It is believed that SEE is accompanied by excitation of the VLF waves penetrating into magnetosphere and stimulating the precipitation of the energetic electrons (10–40 keV) of ∼1‐min duration due to a cyclotron resonant interaction of the natural precipitating electrons (1–10 keV) with the heater‐induced whistler waves in the magnetosphere. In the recovery phase of the auroral substorm, emissions at the heater second harmonic frequency were accompanied by excitation of AFAI in the Es layer with the large decay time up to several minutes after the heater turn‐off. The possible explanation for these emissions can be found from the linear conversion process when the pump wave of O mode may directly convert to electrostatic waves in the region of the HF plasma resonances. The large decay time may result from electromagnetic instability growth in the heater‐modified ionosphere at the downward field‐aligned currents.
The relationship between E region electron density and the power of auroral coherent echoes at 45 MHz
Authors: Williams P. J. S. , Jones B. , Kustov A. V. , Uspensky M. V.
Published: September 2002

Data taken during joint observations of the European incoherent scatter (EISCAT) radar and the Karmaselga auroral coherent radar (46.5 MHz) are combined to investigate the dependence of the coherent echo power upon the maximum electron density in the E region. For electron densities in the range (1–3)×1011 m−3, an increase in the echo power with the electron density was found. A few coherent echoes were observed for electron densities larger than 3×1011 m−3, but when such echoes occurred, their power was small even though the ionospheric electric field was strong, more than 30 mV/m. These measurements agree reasonably well with the model of Uspensky and Williams [1988] which takes into account ionospheric refraction and assumes that auroral coherent echo is a superposition of scattering from a thick layer of electrojet irregularities. Results of the comparison indicate that electron density at altitudes around 110 km can be roughly estimated solely from coherent radar data.
An active, deep marine strike‐slip basin along the North Anatolian fault in Turkey
Authors: Okay Aral I. , Demirbağ Emin , Kurt Hülya , Okay Nilgün , Kuşçu İsmail
Published: September 2002

The Tekirdağ depression within the Marmara Sea in the Mediterranean region is an active, rhomb‐shaped strike‐slip basin along the North Anatolian fault with a basin floor at a water depth of −1150 m. New multichannel seismic reflection data and on‐land geological studies indicate that the basin is forming along a releasing bend of the strike‐slip fault and is filled with syntransform sediments of Pliocene‐Quaternary age. The basin is bounded on one side by the North Anatolian fault and on the other side by a subparallel normal fault, which forms the steep submarine slope. In cross section the basin is strongly asymmetric with the thickness of the syntransform strata increasing from a few tens of meters on the submarine slope to over 2.5 km adjacent to the North Anatolian fault. Seismic sections also show that the slope‐forming normal fault connects at depth to the North Anatolian fault, implying that the basin is completely detached from its substratum. The whole structure can be envisaged as a huge, rather flat, negative flower structure. The releasing bend of the North Anatolian fault, responsible for the formation of the basin, is flanked by a constraining bend. Along the constraining bend, the syntransform strata are being underthrust, implying a recent change in the direction of the regional displacement vector. This thrusting is responsible for the uplift of the submarine slope to a height of 924 m, possibly by a mechanism of elastic rebound. Regional geology suggests that most of the syntransform strata are lacustrine with only the topmost few hundred meters consisting of deep marine clays. The anomalous present depth of the Tekirdağ depression is due to reduced Quaternary sedimentation coupled with high rates of displacement along the North Anatolian fault, which amounts to 20 mm/yr in the Marmara Sea region.
History and tectonic implications of low‐angle detachment faults and orogen parallel extension, Picentini Mountains, Southern Apennines fold and thrust belt, Italy
Authors: Ferranti Luigi , Oldow John S.
Published: September 2002

Late Miocene to Pliocene movement on low‐angle extensional faults within the internal Southern Apennines orogenic belt was superposed on an earlier, Miocene imbricate thrust stack. The low‐angle faults formed within the interior of the belt during orogen parallel extension as thrust imbrication continued in the foreland. Extreme tectonic thinning defines discrete structural domains of hyperextension which are linked by a complex system of extensional and transcurrent faults. Some of the best examples of hyperextension structures in the Southern Apennines are exposed in the Picentini Mountains. In this area, detailed mapping, fault‐kinematic analysis, and excellent stratigraphie control contributed to the construction of restorable cross sections and forward models of deformation. With these constraints, it is possible to document extensional displacement on shallowly dipping supercrustal faults whose orientation is a primary feature and is not due to later tilting. During movement, upper plate rocks were disarticulated by listric and planar normal faults that soled into a ramp‐flat detachment system. The depth of the basal detachment increased in the direction of upper‐plate motion and ranged from 5 to 10 km. Displacement on the low‐angle detachment was accompanied by block tilting in the upper plate assemblage, incisement and excisement of the upper and lower plate rocks as fault trajectories changed through time, and the progressive cataclasis of hanging wall and footwall assemblages. Preexisting thrusts were only locally reactivated during extension, and faults emanating from the underlying decollement systems cross the imbricated thrust sheets at moderate to high angles. Longitudinal extension resulted in thinning of the thrust stack to less than half the original thickness and had a cumulative magnitude of between 200 and 250% (beta>2).
Recent deformation in the Turan and South Kazakh platforms, western central Asia, and its relation to Arabia‐Asia and India‐Asia collisions
Authors: Thomas Jean‐Charles , Grasso Jean‐Robert , Bossu Rémy , Martinod Joseph , Nurtaev Bakhthiar
Published: September 2002

In this study, we investigate the recent and active tectonics of the Turan and South Kazakh platforms in western central Asia. This area, which has been considered stable during the Tertiary, was struck in its eastern part (Gazli and Kyzyl‐Kum) by three earthquakes of magnitude Ms ∼7.0 between 1976 and 1984. From structural observations we show that in the Kyzyl‐Kum, Cenozoic deformation on structures oriented NW–SE is dominated by dextral strike‐slip motion on a flower structure involving reactivated Late Paleozoic faults. This deformation is compatible with the stress pattern induced by the Pamir/Tien Shan collision zone. At the scale of the Turan‐South Kazakh platform, comparison of repeated leveling measurements gives mean uplift rates of 5 mm yr−1 for most of the area south of the 44° latitude including the Kyzyl‐Kum. These observations indicate that the Turan‐South Kazakh platform has been actively deforming since recent times (Pleistocene) in response to the collision of both India and Arabia with Asia. Earthquake data also reveal that propagation of deformation from the Kopet Dagh and the Pamir/Tien Shan collision zones into the Turan‐South Kazakh platform is accommodated aseismically except in the Kyzyl‐Kum. This might reflect differences in the deformation style and nature of the upper crust in the Kyzyl‐Kum and north of the Kopet Dagh respectively. We propose that the Gazli earthquakes highlight the northwestward continuation, within the Turan‐South Kazakh platform and along reactivated older structures, of the deformation related to the Pamir indentation and to a lesser degree to the Arabia‐Asia convergence.
Crustal thermal regime prior to, during, and after rifting: A geochronological and modeling study of the Mesozoic South Alpine rifted margin
Authors: Bertotti G. , Seward D. , Wijbrans J. , ter Voorde M. , Hurford A. J.
Published: September 2002

An approximately 15‐km‐thick segment of the Mesozoic passive continental margin of Adria was involved in Alpine shortening and is now exposed in the Lake Como‐Lake Lugano area (southern Alps, northern Italy and southern Switzerland). The segment comprises the Monte Generoso basin, a deep (∼9 km) extensional basin controlled by the activity of the Lugano‐Val Grande normal fault. The paleosurface and Mesozoic geometric relations are well preserved so that the kinematics of fault blocks prior to, during, and after extension are well constrained. Rb/Sr, 40Ar/39Ar and zircon fission track data from basement rocks underlying at various the Generoso basin depths, provide independent estimates on the thermal evolution of the crustal segment during and after rifting. Most Rb/Sr and 40Ar/39Ar ages are contemporaneous with the initial stages of rifting (Late Triassic to Early Jurassic) documenting (1) strongly altered thermal conditions before the onset of extension and (2) cooling throughout rifting. Thermal gradients decreased from >60°C km−1 at the onset of extension to ∼20°C km−1 shortly after breakup. Zircon fission track ages are Jurassic to Early Cretaceous providing evidence for continued cooling until the beginning of Alpine shortening. Thermokinematic numerical modeling of the considered crustal segment provides a full, two‐dimensional description of synrift and postrift evolution. Synthetic time versus temperature curves are extracted from the model and compared with those derived from absolute ages.
Paleomagnetic data from the Caborca terrane, Mexico: Implications for Cordilleran tectonics and the Mojave‐Sonora megashear hypothesis
Authors: Garza Roberto S. Molina , Geissman John W.
Published: September 2002

Two ancient magnetizations have been isolated in rocks of the Caborca terrane, northwest Mexico. The characteristic magnetizations of Neoproterozoic and Paleozoic miogeoclinal shelf‐strata, arc‐derived Lower Jurassic marine strata, and Jurassic volcanic and volcaniclastic rocks are of dual polarity and east‐northeast declination (or south‐south‐west) and shallow inclination. Magnetizations in Neoproterozoic and Paleozoic miogeoclinal strata are interpreted as secondary (J*) and to be of similar age to those observed in Lower and Middle Jurassic rocks. Remanence acquisition is bracketed between about 190 and 160 Ma. The overall mean (D=15.0°, I=8.5°; n=38 sites; six localities; k=19.1, α95=5.5°) suggests a moderate to large clockwise rotation of 12 to 50° (depending on reference direction assumed) of the Caborca terrane, and rocks of the Sonoran segment of the Cordilleran volcanic arc, with respect to the North America craton. When compared with expected inclinations, observed values are not anomalously steep, arguing against statistically significant southward latitudinal displacement of the Caborca block after remanence acquisition. Late Cretaceous intrusions yield primary, dual‐polarity steep inclination “K” magnetizations (D=341.4°, I=52.3°; n=10 sites; five localities; k=38.3, α95=7.9°) and have locally remagnetized Neoproterozoic and Jurassic strata. When present, secondary (K*) magnetizations in Neoproterozoic strata are of higher coercivity and higher unblocking temperature than the characteristic (J*) magnetization. Importantly, the regional internal consistency of data for Late Cretaceous intrusions suggests that effects of Tertiary tilt or rotation about a vertical axis over the broad region sampled (∼5000 km2) are not substantial. Late Cretaceous primary (K) magnetizations and secondary (K*) magnetizations yield a combined mean of D=348.1°, I=50.7° (N=10 localities; 47 sites; k=53.5, α95=6.7°), indicating at most small (<∼10°) clockwise rotation of the Caborca region with respect to the craton. Permissible post‐Late Cretaceous latitudinal displacement is near or below the detection limit of paleomagnetism (<∼300 km). Limited data from Lower Cretaceous strata of the Bisbee Group (D=339.9°, I=47.9°; n=4 sites) suggest that the modest clockwise rotations inferred on the basis of J* magnetizations in Jurassic and older strata occurred in Jurassic time. Together, the lack of evidence for southward displacement, yet evidence for statistically significant clockwise rotation, and the overall similarity of Jurassic magnetizations in the Cordilleran arc with those of the Caborca block, despite the fact that some of them are clearly secondary, are not consistent with the Mojave‐Sonora megashear hypothesis of Late Jurassic left‐lateral strike‐slip motion of the crust of northern Mexico.