bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical maturity of ejecta from large rayed lunar craters
Authors: Grier Jennifer A. , McEwen Alfred S. , Lucey Paul G. , Milazzo Moses , Strom Robert G.
Published: February 2004

Lucey et al. [2000] have developed a methodology for extracting an optical maturity parameter (OMAT) from multispectral Clementine images. The OMAT parameter characterizes the overall maturity of lunar soils and crater ejecta by changes in reflectance spectra. Using these OMAT images, we surveyed large craters (≥20 km diameter) on the Moon that had previously been mapped as possessing or possibly possessing rayed ejecta. We generated average radial profiles of OMAT values for rays of these large craters. From these profiles we classified the craters into three relative age groups: (1) older than Copernicus (inferred age of ∼810 Myr), (2) intermediate, and (3) as young or younger than Tycho (inferred age of ∼109 Myr). We suspect that there is a bias to our classification scheme, such that the OMAT profiles of smaller craters look like that of larger but older craters. Nevertheless, some large craters, such as Eudoxus (67 km) and Aristillus (55 km), are now known from this study to have optically mature ejecta and therefore are suspected to be older than Copernicus (this is consistent with an age of 1.3 Gyr suggested for Aristillus by Ryder et al. [1991]). Such craters were included by McEwen et al. [1997] when estimating the density of craters younger than or contemporaneous with Copernicus. Therefore the case for a modest increase in the cratering rate (in the past 800 Myr versus the previous 2.4 Gyr) indicated from that work has been weakened [Grier and McEwen, 2001]. Given current constraints on dating large and recent lunar craters, we cannot support (or disprove) the hypothesis that there has been a significant increase in the rate of large terrestrial impact events in the past 100–400 Myr.
Compositional gradients across mare‐highland contacts: Importance and geological implication of lateral transport
Authors: Li Lin , Mustard John F.
Published: February 2004

Observations of mixing across mare‐highland contacts using high‐resolution Clementine data allow a new assessment of the relative importance of vertical versus lateral mass transport on the Moon at least at mare‐highland contact. We analyze mare‐highland contacts in the Grimaldi, Orientale, and Fecunditatis basins and Tsiolkovsky crater through image‐based nonlinear spectral mixture modeling of Clementine ultraviolet‐visible (UV‐VIS) light spectrometer multi‐spectral data. The mare in these regions differ in ages, but they are characterized by having simple geological contacts. The symmetric distribution of mare and highland soil constituents across their geological contact indicates that lateral transport is more efficient than deep vertical transport during the formation of the observed mixing zones. Observations across the lunar limb with the Clementine UV‐VIS filters show that there is a stray light component of scattered light across high‐contrast albedo boundaries. However, the magnitude and spatial properties of scattered light do not significantly affect the nature of compositional boundaries determined here. Since repetitive meteorite bombardment governs lateral transport across the geological contacts and is a random process, a stochastic model is developed to describe this lateral transport. Analysis of the power decay law for crater ejecta thickness indicates that the high‐velocity ejecta travels long distances and follows a −3 power decay law. This results in an infinite variance, which requires an anomalous diffusion model since a classical diffusion model would be invalid. Mathematical modeling supports this result where it is shown that classical diffusion produces a relatively poor fit to profiles of material abundance, while the anomalous diffusion model fits the profiles adequately. These results indicate that high‐velocity ejecta dominates the formation of the observed compositional gradients, while the ejecta near the crater rim is relatively less important. On the basis of these ejecta distributions and the assumption of random impact cratering, we derive a relationship that can be used as an index for relative age dating of geological contacts as well as for investigating cratering rates.
Analysis of science team activities during the 1999 Marsokhod Rover Field Experiment: Implications for automated planetary surface exploration
Authors: Thomas Geb , Reagan Mark , Bettis E. Arthur , Cabrol Nathalie , Rathe April
Published: February 2004

This work analyzes the behavior and effectiveness of a science team using the Marsokhod mobile robot to explore the Silver Lake region in the Mojave Desert near Baker, California. The work addresses the manner in which the geologists organized themselves, how they allocated their time in different activities, how they formed and communicated scientific hypotheses, and the frequency with which they requested different types of data from the mission archive during the first 3 days of the mission. Eleven scientists from the NASA Ames Research Center and three of the five scientists who participated from their home institutions were videotaped as they worked throughout the 3‐day experiment. The videotape record indicates that 46% of available person‐hours were consumed in semistructured or formal meetings and that only 1% of their time was spent studying immersive, three‐dimensional virtual reality models of the robot's surroundings. The remainder of their time was spent in unstructured work sessions in groups of two or three. Hypothesis formation and evolution patterns show a meager flow of information from the distributed science team to the on‐site team and a bias against reporting speculative hypotheses. Analysis of the visual imagery received from the robot indicates that acquisition of the large panoramic information leads to high levels of redundancy in the data acquired. The scientists' archive requests indicate that small, specifically requested image targets were the most frequently accessed information. The work suggests alternative organizational structures that would expedite the flow of information within the geologic team. It also advocates emphasizing specific science targets over high‐resolution, stereoscopic, panoramic imaging when programming a mobile robot's onboard cameras.
Chemical information content of lunar thermal and epithermal neutrons
Authors: Feldman W. C. , Lawrence D. J. , Elphic R. C. , Vaniman D. T. , Thomsen D. R. , Barraclough B. L. , Maurice S. , Binder A. B.
Published: February 2004

Simulations of equilibrium thermal and epithermal neutron flux spectra for the Moon have been studied to determine their chemical information content. Most of this information resides in a single parameter, the ratio of macroscopic absorption to energy‐loss cross sections. Although this parameter cannot specify the chemistry of lunar soils uniquely, it provides an index that can be used to quantitatively separate anorthositic from mafic chemistries. Hydrogen abundances can best be derived from measurements of the epithermal neutron flux, which is strongly related to its macroscopic energy‐loss cross section. The ratio of thermal to epithermal neutron flux measured by Lunar Prospector is shown to agree quantitatively with simulations. This agreement is used to show that the highlands terrane on the farside of the Moon is anorthositic, containing high percentages of plagioclase and possibly a range of Mg‐suite minerals with a high Mg/(Mg + Fe) atomic ratio. Our best estimate of the abundance of FeO is 2%, with an upper limit of 5%.
Relative timing of crustal plateau magmatism and tectonism at Tellus Regio, Venus
Authors: Banks Brian K. , Hansen Vicki L.
Published: February 2004

Crustal plateaus, subcontinent‐sized regions of thickened crust that stand 1–4 km above surrounding low‐lying plains, define a major physiographic province of Venus. Thus their mode of emplacement plays a fundamental part in the evolution of the entire planet. Local topographic lows up to several hundred kilometers across filled with radar‐dark volcanic flows, termed intratessera floodlava basins (ITBs), are common within crustal plateaus. ITBs record abundant crustal plateau volcanism and have implications for crustal plateau emplacement and evolution. We ask the fundamental question: Did ITBs develop before, during, or after crustal plateau development? To address this question, we mapped in detail the geology a particular ITB in Tellus Regio to determine temporal relationships between ITB volcanic flows and local crustal plateau tectonism. Mechanical arguments and crosscutting relationships indicate that ITB volcanism occurred during crustal plateau formation. Extending the timing relationships resolved at this basin, we suggest that ITB magmatism played a major role in crustal plateau genesis. The geologic history discerned is consistent with the upwelling model for crustal plateau evolution in which early surface extension and volcanism would be expected in a hotspot environment. Conversely, early widespread extension and volcanism would not be expected if crustal plateaus formed above a mantle downwelling because (1) predicted horizontal and radial convergence would obstruct early surface extension and (2) lower than normal geothermal gradients would impede, rather than enhance, volcanism.
Fragmentation rates of small satellites in the outer solar system
Authors: Colwell Joshua E. , Esposito Larry W. , Bundy Danielle
Published: February 2004

The narrow rings of Uranus and Neptune exist in a system of observed and hypothesized small moons. Catastrophic fragmentation of these moons by comet impact has been proposed as the mode of origin of those rings, and earlier efforts to model the process showed that small moons are destroyed by impact on short timescales, leading to rapid collisional erosion of any primordial satellite system (Colwell and Esposito, 1992). We reexamine the question of impact fragmentation of small satellites in the light of new observational data on the population of Kuiper Belt and Centaur objects that produce the impacting flux and new theoretical and computational studies of catastrophic fragmentation. We find that the impacting flux used by Colwell and Esposito (1992) is consistent with the new observations of Kuiper Belt objects and calculations of their transport into the solar system. However, new fragmentation criteria from modeling of the asteroid belt and hydrocode simulations lengthen the model collisional lifetimes of satellite systems. The observed distribution of rings, dust bands, and moons at Uranus and Neptune suggest a catastrophic disruption model with a relatively weak dependence on target radius.
Results of the Imager for Mars Pathfinder windsock experiment
Authors: Sullivan Robert , Greeley Ronald , Kraft Michael , Wilson Gregory , Golombek Matthew , Herkenhoff Ken , Murphy James , Smith Peter
Published: February 2004

The Imager for Mars Pathfinder (IMP) windsock experiment measured wind speeds at three heights within 1.2 m of the Martian surface during Pathfinder landed operations. These wind data allowed direct measurement of near‐surface wind profiles on Mars for the first time, including determination of aerodynamic roughness length and wind friction speeds. Winds were light during periods of windsock imaging, but data from the strongest breezes indicate aerodynamic roughness length of 3 cm at the landing site, with wind friction speeds reaching 1 m/s. Maximum wind friction speeds were about half of the threshold‐of‐motion friction speeds predicted for loose, fine‐grained materials on smooth Martian terrain and about one third of the threshold‐of‐motion friction speeds predicted for the same size particles over terrain with aerodynamic roughness of 3 cm. Consistent with this, and suggesting that low wind speeds prevailed when the windsock array was not imaged and/or no particles were available for aeolian transport, no wind‐related changes to the surface during mission operations have been recognized. The aerodynamic roughness length reported here implies that proposed deflation of fine particles around the landing site, or activation of duneforms seen by IMP and Sojourner, would require wind speeds >28 m/s at the Pathfinder top windsock height (or >31 m/s at the equivalent Viking wind sensor height of 1.6 m) and wind speeds >45 m/s above 10 m. These wind speeds would cause rock abrasion if a supply of durable particles were available for saltation. Previous analyses indicate that the Pathfinder landing site probably is rockier and rougher than many other plains units on Mars, so aerodynamic roughness length elsewhere probably is less than the 3‐cm value reported for the Pathfinder site.
Pliocene‐Pleistocene ice rafting history and cyclicity in the Nordic Seas during the last 3.5 Myr
Authors: Jansen Eystein , Fronval Torben , Rack Frank , Channell James E. T.
Published: February 2004

A continuous 3.5 Myr IRD record was produced from Ocean Drilling Program (ODP) Site 907. A timescale based on magnetic polarity chrons, oxygen isotope stratigraphy (for the last 1Myr) and orbital tuning was developed. The record documents a stepwise inception of large‐scale glacial cycles in the Nordic Seas region, the first being a marked expansion of the Greenland ice sheet at 3.3 Ma. A second step occurred at 2.74 Ma by an expansion of large scale ice sheets in the Northern Hemisphere. Ice sheet variability around the Nordic Seas was tightly coupled to global ice volume over the past 3.3 Myr. Between 3 and 1 Ma, most of the variance of the IRD signal is in the 41 kyr band, whereas the last 1 Myr is characterized by stronger 100 kyr variance. The Gamma Ray Porosity Evaluator (GRAPE) density record is closely linked with IRD variations and documents sub orbital variability resembling the late Quaternary Heinrich/Bond cycles.
Sea ice as the glacial cycles’ Climate switch: role of seasonal and orbital forcing
Authors: Gildor Hezi , Tziperman Eli
Published: February 2004

A box model of the coupled ocean, atmosphere, sea ice, and land ice climate system is used to study glacial‐interglacial oscillations under seasonally and orbitally varying solar forcing. The dominant 100 kyr oscillation in land ice volume has the familiar sawtooth shape of climate proxy records, and to zeroth order, it does not depend on the seasonal and Milankovitch forcing. The sea ice controls, via its albedo and insulating effects, the atmospheric moisture fluxes and precipitation that enable the land ice sheet growth. This control and the rapid growth and melting of the sea ice allow the sea ice to rapidly switch the climate system from a growing ice sheet phase to a retreating ice sheet phase and to shape the oscillation's sawtooth structure. A specific physical mechanism is proposed by which the insolation changes act as a pacemaker, setting the phase of the oscillation by directly controlling summer melting of ice sheets. This mechanism is shown to induce deglaciations during periods of lower summer insolation. Superimposed on the 100 kyr are the linear Milankovitch‐forced frequencies of 19, 23, and 41 kyr. The transition from 41 kyr glacial cycles to 100 kyr cycles one million years ago may be explained as being due to the activation of the sea ice switch at that time. This would be the case if sea ice extent was more limited during the warmer climate of the early Pleistocene.
Kinetic control of skeletal Sr/Ca in a symbiotic coral: Implications for the paleotemperature proxy
Authors: Cohen Anne L. , Layne Graham D. , Hart Stanley R. , Lobel Phillip S.
Published: February 2004

Modeling of past climates is critically dependent on estimates of past sea surface temperatures (SSTs), for which one of the principal techniques used is the measurement of Sr/Ca ratios in corals [Guilderson et al., 1994; McCulloch et al., 1999; Hughen et al., 1999]. The link between coral Sr/Ca and SST is not well‐understood and there have been a number of discrepant observations [de Villiers et al., 1995; Alibert, 1998]. Corals with symbiotic zooxanthellae are known to show large diurnal fluctuations in calcification rate associated with the photosynthetic activity of their symbionts. Using detailed measurements with the ion microprobe, we compared the Sr/Ca content of discrete daytime and nighttime skeletal structures in the massive hermatypic coral Porties lutea over the course of 1 year and a seasonal temperature range of 4°C. The Sr/Ca content of daytime skeleton is always lower than that of adjacent nighttime skeleton. While the slope of the nighttime Sr/Ca‐SST correlation is close to that seen in inorganic aragonite precipitates, that of the daytime correlation is >4 times as steep. We attribute these differences to the role of photosynthesis in calcification and conclude that bulk Sr/Ca is related principally to daytime calcification rate rather than directly to SST. More reliable estimates of past SST may be arrived at through selective analysis of nighttime skeleton.
Interpretation of climate dynamics from phase space portraits: Is the climate system strange or just different?
Authors: Gipp Michael R.
Published: February 2004

Traditional concepts of spectral analysis are inadequate to characterize the behavior of paleoclimatic data sets because the power spectrum of a nonlinear system is not invariant. Reconstructed phase space portraits of global ice volume proxy data over the past 2100 kyr show that variations in global ice volume during the Pleistocene have resulted from a complex interplay of internal dynamics and insolation forcing. The evolution of global ice volume over this period can be characterized as a series of stepwise transitions between at least five “states” representing stable distributions of ice masses on the globe. Similar stepwise transitions are observed in an ice core temperature record extending over the past 240 kyr. The number of states and their location in phase space are governed by several factors, including continental configurations, distribution of mountain ranges, the number of stable oceanographic circulation configurations, and atmospheric composition. In contrast, phase space portraits of calculated values of Northern Hemisphere insolation and an ice core dust flux record demonstrate chaotic behavior.
Mass anomaly structure of the Earth
Authors: Bowin Carl
Published: February 2004

Gravitational interaction is the weakest among the four known forces in the universe. The particular gravity equipotential field that coincides with sea level is called the geoid, and satellite data have revealed anomalies in its pattern that are puzzling to explain. Dynamic topography solutions have offered the preferred explanation for the past 15 years, but problems remain. An alternative explanation presented here contends that very large mass anomalies lie deep in the Earth, presumably produced by topography at the coremantle boundary (CMB), and only much smaller mass anomalies occur at shallower depths. The principal distinction is whether one is willing to accept the possibility that processes within the Earth's core may generate the CMB topography. An analysis of the South American regional geoid high (spherical harmonic degrees 2–10) indicates that only mass anomalies shallower than 1200 km are its cause. This single positive source is contrary to dynamic topography solutions that would generate negative deflections of both the surface and the CMB in response to the positive subducted mass for the Earth's fourth greatest geoid high. Our alternate explanation is tested utilizing a set of 9274 subduction 5° cube slab bloblet center points, developed from reconstructed plate history, that provide estimates of the locations of material subducted into the Earth's mantle. Two global mass solutions are offered utilizing (1) only those bloblets in the outer 800 km and (2) only those bloblets in the outer 1400 km. Four point masses at 3000 km depth to simulate CMB topography, produced by processes within the core unrelated to mantle dynamic topography, complete the two mass models. Both models show reasonable agreement with patterns and magnitudes of the regional geoid and its component degree parts. The model CMB mass anomalies are only a factor of 3 greater than those initially estimated by gravity to geoid ratios from observed values at geoid anomaly center locations. Difficulties with the dynamic topography solutions, matters of the density of the subducted material, the proper hydrostatic flattening value to use for analyses, and possible causes for the CMB topography are discussed. The subduction mass models are also used to estimate the magnitude of the driving force for plate tectonics (2.78 × 1020 to 3.23 × 1021 N), approximately equivalent to 7.0 × 1012 to 8.1 × 1013 N m−1. These force values are comparable to published slab force estimates based on thermal considerations.
A simulation study of coherent radar imaging
Authors: Yu Tian‐You , Palmer Robert D. , Hysell David L.
Published: February 2004

Coherent radar imaging (CRI) is used in an attempt to overcome the angular resolution limitation of conventional single‐station radars and is used to image the horizontal structure inside the resolution volume. This recently developed technique has been successfully applied to radar observations of the ionosphere as well as the lower atmosphere. However, no statistical analysis of the robustness of the various techniques has been presented to date. In this work, three CRI techniques are reviewed: Fourier‐based, Capon's, and maximum entropy (MaxEnt) methods. The Fourier‐based method is the simplest of the three algorithms but has inherent resolution limitations. Although quite different in nature and performance, both Capon's and MaxEnt methods can be posed as constrained optimization problems. A statistical comparison of performance of the three CRI techniques, using various receiver configurations and two distinct cases of scattering structure, is made using simulated data. The results show that the MaxEnt method exhibits the best performance in the case of aspect‐sensitive scattering with a broad characteristic. In the localized scattering case, however, Capon's method shows superior performance for signals with high signal‐to‐noise ratio (SNR), but MaxEnt method outperforms all methods for low SNR. In general, both Capon's and MaxEnt methods are able to reproduce the gross characteristics of the scattering media under observation.
Comparative analysis of radioholographic methods of processing radio occultation data
Authors: Gorbunov Mikhail E. , Gurvich Alexandre S. , Kornblueh Luis
Published: February 2004

Two methods for the determination of refraction angle profiles from radio occultation measurements in multipath areas are analyzed and compared: (1) the radio‐optic method based on the analysis of the local spatial spectra of the measured wave field and (2) the back propagation of the received wave field to a single‐ray region. The basic limitations of the radio‐optic method are (1) the restriction of the resolution of refraction angle profiles due to the uncertainty relation of refraction angle and impact parameter and (2) diffractive effects in subcaustic areas, where the spatial spectra cannot be interpreted in terms of geometric optical rays. The basic limitation of the back propagation method is related to ray and caustic structures, which may not contain single‐ray areas. It is shown that strong refraction reduces the uncertainties of refraction angle and impact parameter. On the other hand, strong refraction or superrefraction is responsible for complicated caustic structures that cannot be resolved by the back propagation technique. The two methods are thus complementary to each other and can be combined for processing lower tropospheric occultation data. This analysis is corroborated by numerical simulations based on global fields of atmospheric variables from analyses of the European Centre for Medium Range Weather Forecast.
Scattering layer thickness and position estimated by radar frequency domain interferometry: 2. Effects of tilts of the scattering layer or radar beam
Authors: Luce H. , Röttger J. , Crochet M. , Yamamoto M. , Fukao S.
Published: February 2004

In the companion paper (part 1), theoretical studies on the dual frequency domain interferometry (FDI) technique have been presented. Two possible causes of biases in the layer thickness and position estimations by FDI have been considered: the limited extent of the scattering structure in the horizontal plane and the advection of this structure by the wind. In the present work, we study the effects of the tilts of the scattering layer from horizontal. It is shown that in case of large tilt angles, substantial biases on position and thickness can occur. The model, first developed by Liu and Pan [1993] but more extensively described in this paper, can also be used for a prediction of the variations of the FDI coherence with the zenith angle and their relation to the anisotropy of the scatterers. Some preliminary observations of the zenith angle dependence of the FDI coherence and echo power obtained with the middle and upper atmosphere (MU) radar from the vertical up to 28° off zenith with a step of 2° are shown and discussed. In principle, comparisons between the observed power and coherence variations with those given by the model could give more information on the structures that contribute around and far from the zenith.
Real‐time meteor entrance speed determinations made with interferometric meteor radars
Authors: Hocking W. K.
Published: February 2004

Previously developed methods for determination of meteor entrance speeds into the Earth's atmosphere are extended and unified and applied to a real‐time radar system. A Fourier transform approach, using a rescaled time axis, is employed, permitting entrance speed determinations with accuracies of the order of 0.5 km s−1. After the method is described, it is demonstrated by using it to determine the entrance speeds of several major streams, including the Zeta Perseids, the Arietids, the Beta Taurids, the June Bootids, meteors from within the Piscids complex, and the Geminids. Values are consistent with previous optical measurements, but in some cases the radar measurements have higher accuracy. The possibility of detection of space debris, or perhaps dust in Earth orbit, is also considered.
Comparison of ion densities measured in the topside ionosphere at low latitudes and midlatitudes with calculations of ionospheric models over a full solar cycle
Authors: Rich Frederick J. , Sultan Peter J.
Published: February 2004

Measurements of total ion density at 840 km altitude at all latitudes in four local time sectors are available from 1987 to the present from spacecraft of the Defense Meteorological Satellite Program (DMSP). Thus all phases of the past solar cycle are represented in the data set. We present comparisons of the measurements with values obtained from climatology models of the ionosphere. The models examined are the International Reference Ionosphere (IRI) model, the Parameterized Ionospheric Model (PIM) and the Reilley‐ICED‐Bent‐ Gallagher (RIBG) model. We show that all of the models reproduce some of the features of the observed topside ionosphere, but none of the models match all of the observations.
Observation of long‐duration gyroharmonic resonances: A refutation of the short‐duration explanation for interpreting the anomalous URAP sounder spectra observed in the Io torus
Authors: Canu Patrick
Published: February 2004

The resonance spectra collected by the Ulysses Unified Radio and Plasma Wave relaxation sounder in the Jovian magnetosphere at the outskirts of the Io torus exhibit significant differences from those gathered from earthbound spacecraft. The main deviation is the lack of resonances at the harmonics of the electron gyrofrequency. Extrapolated from statistics of gyroharmonics observed by ionospheric topside sounders, recent works have suggested that the duration of the gyroharmonics can be too short to be detected by URAP, because of the large velocity of Ulysses in the torus. Actual magnetospheric sounder observations performed by GEOS and ISEE 1 spacecraft are used here to verify this explanation. The gyroharmonic resonances triggered by magnetospheric sounders appear quite different with respect to those observed by ionospheric topside sounder; particularly, they have a longer duration than those derived by these recent studies. Their characteristics give further arguments in favor of their interpretation as oblique echoes. The gyroharmonic resonances are also observed in the large‐velocity plasma encountered in the Earth's magnetosheath. The earthbound observations refute, then, qualitatively and quantitatively the explanation of the absence of harmonics because of their too short duration and are a strong support in the interpretation of URAP observations as large frequency shifts induced on the gyroharmonics by the Ulysses velocity.
Multiresolution time domain analysis of scattering from a rough dielectric surface
Authors: Dogaru Traian , Carin Lawrence
Published: February 2004

The multiresolution time domain (MRTD) algorithm is applied for modeling scattering from a rough dielectric surface. We formulate the Haar MRTD model for an arbitrary interface between two dielectric media. The advantages of this formulation, as compared with the classic finite difference time domain (FDTD), are demonstrated on various types of surfaces. A detailed error analysis is performed for two versions of the MRTD algorithm and for the staircase FDTD model, at different discretization rates.
Propagation model for signal fluctuations on transionospheric radio links
Authors: Gherm Vadim E. , Zernov Nikolay N. , Radicella Sandro M. , Strangeways Hal J.
Published: February 2004

The complex phase method has been further extended to the problem of electromagnetic (EM) field scintillations on Earth‐satellite GPS paths of propagation. The numerical and analytic technique based on the method has been developed to characterize the transionospheric channel of propagation. The effects of additional range errors due to the ionospheric electron density fluctuations in space and time have been studied taking into account the ray bending due to the inhomogeneous background ionosphere and the diffraction on local random ionospheric inhomogeneities. In the method developed, the impact of the Earth's magnetic field is accounted for by the anisotropic spatial spectrum of the ionospheric turbulence with different outer scales along and across the magnetic field lines. The variances of the EM field phase (yielding range errors) and level (log amplitude) fluctuations have been calculated for different models of the background ionospheres characterized by different height electron density profiles and total electron content. The conditions of the saturated regime of propagation, which will likely result in the degradation of a GPS navigation system, have been discussed. In addition, the scattering function of the GPS transionospheric channel of propagation has been constructed and simulated for a wideband signal.
Instability problem of the electric field antennas on the Polar spacecraft
Authors: Kolesnikova E. , Béghin C.
Published: February 2004

This paper is an application of the surface charge distribution method to the modeling of the electric antennas installed on board the Polar spacecraft in order to identify the plasma conditions leading to the instability of the sphere‐preamplifier‐stub‐guard system. First we present an analytic approach, which allows us to understand the physical mechanism and to define the conditions of the instability. We then show the results of the numerical modeling for the more common types of instabilities observed in flight. The latter, which we call type‐1 oscillations, are observed solely in the low‐L plasmasphere region. The modeling predicts that the oscillations can occur in a weakly magnetized Maxwellian plasma in the upper hybrid range (ƒp<ƒ<ƒt) when the Debye length lies between well‐defined limits. The frequency modulation of the oscillations in this range versus the spin angle of the antennas with respect to the Earth's magnetic field is well explained by the model. The type‐2 oscillations are observed occasionally and occur always in the exterior cusp, at large L values. They are most likely to be associated with high‐density clouds of solar wind streaming plasma entering into that region. Our analytic modeling indeed predicts that the instability conditions can be satisfied when the antennas are crossing a 10 eV electron flow, with a density of ∼100 cm−3 and a bulk velocity of the order of 200 km s−1.
More total electron content climatology from TOPEX/Poseidon measurements
Authors: Codrescu Mihail V. , Beierle Kathryn L. , Fuller‐Rowell Tim J. , Palo Scott E. , Zhang Xiaoli
Published: February 2004

We have used the TOPEX satellite data set to compare climatologies of total electron content (TEC) measurements from the dual‐frequency altimeter with the Doppler orbitography and radiopositioning integrated by satellite (DORIS) TEC results and with the international reference ionosphere (IRI) and Bent model predictions corresponding to the satellite measurements. We have used the TOPEX measurements from launch in 1992 through 1997 to build a database that includes time, geographic and geomagnetic coordinates of the measurement, geomagnetic indices (Kp, previous Kp, hemispheric power, and integral of hemispheric power over the previous 36 hours), solar index (F 10.7), TOPEX and DORIS TEC measurements, and empirical model (IRI and Bent) results corresponding to the TOPEX measurements. We have binned the measurements in a magnetic local time, magnetic latitude coordinate system to produce global maps of TEC. We present climatological differences between TOPEX TEC and DORIS measurements as well as between TOPEX and the two empirical models. The maps were constructed using all appropriate TOPEX TEC data, DORIS TEC measurements, and the corresponding Bent and IRI model results for solar minimum conditions.
Assimilation Ionosphere Model: Development and testing with Combined Ionospheric Campaign Caribbean measurements
Authors: Sojka J. J. , Thompson D. C. , Schunk R. W. , Bullett T. W. , Makela J. J.
Published: February 2004

Assimilation Ionosphere Model (AIM) is a physics‐based, global, ionospheric specification model that is currently under development. It assimilates a diverse set of real‐time (or near‐real‐time) measurements, such as ionograms, GPS slant total electron content (TEC), and in situ plasma measurements. This study focuses on a middle latitude ionosonde assimilation capability in both local and regional forms. The models described are capable of using theƒ0F2 and hmF2 from ionograms to generate either a local or a regional distribution of the induced plasma drift. This induced drift is usually caused by the meridional neutral wind. Results from a local model (AIM1.03L) and a regional model (AIM1.03R) are presented and compared with the international reference ionosphere (IRI) climatological predictions as well as GPS slant TEC measurements. Results from year‐long studies during solar maximum show that the accuracy of the AIM1.03L model is about a factor of 2 better than that of IRI. An initial month‐long regional study is also presented, and the results are almost as good. A study is also carried out using observations taken during the Combined Ionospheric Campaign (CIC) held in November, 1997, in the Caribbean. The digisonde located at Ramey Solar Observatory is used to drive the AIM1.03L model, and the predicted GPS slant TECs are compared to those observed by a GPS receiver located at St. Croix. This study confirms that this first step in preparing a weather‐sensitive ionospheric representation is superior to a climatological representation. This sets the stage for the development of full assimilation of GPS TEC, in situ density measurements, etc., and it is anticipated that the AIM1.03L‐R ionospheric representation will provide an accurate ionospheric specification.
Polar region ionospheric effects on UHF radar track acquisition
Authors: Reinking J. Todd , Knepp Dennis L. , Hausman Mark A.
Published: February 2004

It is well known that small‐scale variations in electron density in the natural ionosphere can disturb electromagnetic signals. These electron‐density irregularities can cause rapid variations in the amplitude and phase of propagating signals, referred to as scintillation. UHF radars that operate in the polar region can experience scintillation anytime of the day or night. The occurrence of polar region scintillation is a complicated function of the physics of the ionosphere and the geometry of the line of sight. The severity of the scintillation is a strong function of time in the 11 year solar cycle and the geomagnetic activity (as quantified by Kp). This paper considers the detection performance during track acquisition of UHF radars that must perform a wide‐volume search while operating during various levels of scintillation disturbance that are likely to occur during the 11 year solar cycle. Under conditions of strong fading, ionospheric scintillation is the primary source of amplitude and phase variations, dominating the effect of target radar cross section fluctuations. The impact on the performance of the commonly used two‐step sequential detection process for track acquisition can be improved by considering ionospheric scintillation in the radar design. The cost of ignoring ionospheric scintillation in the design process is demonstrated through examples. In the examples the design parameters for a two‐step sequential detector are found using a constrained optimization procedure for cases with and without ionospheric scintillation. Cases are also shown where mismatched (to the prevailing ionospheric conditions) parameter sets are used instead of the optimal parameters.
An introduction to Wide Area Augmentation System and its predicted performance
Authors: El‐Arini M. Bakry , Poor Walter , Lejeune Roland , Conker Robert , Fernow James , Markin Kelly
Published: February 2004

The Federal Aviation Administration is developing a Wide Area Augmentation System (WAAS) to GPS that will broadcast clock, ephemeris, and ionospheric corrections. Aviation user equipment will apply the corrections to GPS measurements and also convert error bounds into the position domain. Flight operations can be conducted using WAAS guidance only when the horizontal, and in some cases also the vertical, position error bound is less than a threshold that depends on the phase of flight. The expected fraction of time that a given flight operation can be conducted is termed its availability. Geomagnetic storms sometimes are accompanied by large spatial and temporal gradients in ionospheric delay that result in an increase in ionospheric and position error bounds, which reduces availability. The paper estimates availability of precision approach operations under various scenarios.
GPS normalization and preliminary modeling results of total electron content during a midlatitude space weather event
Authors: Makela Jonathan J. , Kelley Michael C. , Sojka Jan J. , Pi Xiaoqing , Mannucci Anthony J.
Published: February 2004

On November 22–23, 1997, a geomagnetic storm occurred during a period of excellent viewing conditions over the Arecibo Observatory in Puerto Rico. Here we explore the total electron content (TEC) registered by Global Positioning System (GPS) receivers located close to the Cornell All‐Sky Imager (CASI) at the Arecibo Observatory. The storm began with the equatorward surge of a very high (100% increase) TEC enhancement stretching for many hours of local time on the dayside. At dusk the TEC over the Caribbean remained elevated with levels equal to the noontime monthly averages. During the event the TEC was highly structured and clearly correlated with high and low airglow emission levels. In one fortuitous instance a common ionospheric penetration point (15 km apart), shared by two GPS satellites viewed from two receiving stations, registered an 8 TEC unit difference during the active period. We show that a GPS station can be calibrated using the pseudorange method and a reliable data‐driven technique during quiet conditions and still have absolute TEC capability within 2 TEC units (RMS) 5 days later. We compare the observations to a climatological model which, although reasonable for quiet times, is very poor during the storm period. We also present an independent evaluation of the GPS TEC. This study is an initial step toward quality control of this database, needed before it is used in an assimilation model.
Prediction of the degradation of the carrier‐to‐noise plus total interference ratio applied to frequency reuse satellite systems suffering from differential rain attenuation and cross polarization
Authors: Livieratos S. N. , Kanellopoulos J. D.
Published: February 2004

Interference effects are of utmost importance to the reliable design of satellite communication systems. In this paper, the differential rain attenuation from an adjacent Earth‐space system as well as the rain cross polarization (valid only for frequency reuse systems) are considered as the dominant interference sources. In particular, a method to predict the carrier‐to‐interference ratio statistics, used for noise dominant systems, is extended to include the general case when the calculation of the carrier‐to‐noise plus total interference ratio (CNIDR), in the presence of rain fading conditions, is needed. In this way, the total outage time of the interfered system can be calculated. The method is again based on a model of convective rain cells and on the lognormal assumption for point rainfall rate statistics. The numerical results examine the influence of various parameters upon the total availability performance. The deduced conclusions will be very useful toward the optimum design of modern satellite systems where frequency and orbital congestion is expected to be a serious problem.
Geochronology of the Proterozoic basement of southwesternmost North America, and the origin and evolution of the Mojave crustal province
Authors: Barth Andrew P. , Wooden Joseph L. , Coleman Drew S. , Fanning C. Mark
Published: February 2004

The Proterozoic Baldwin gneiss in the central Transverse Ranges of southern California, a part of the Mojave crustal province, is composed of quartzofeldspathic gneiss and schist, augen and granitic gneiss, trondhjemite gneiss, and minor quartzite, amphibolite, metagabbro, and metapyroxenite. Sensitive high resolution ion microprobe (SHRIMP) data indicate that augen and granitic gneisses comprise a magmatic arc intrusive suite emplaced between 1783 ± 12 and 1675 ± 19 Ma, adjacent to or through thinned Archean crust. High U/Th rims on zircons in most samples suggest an early metamorphic event at ∼1741 Ma, but peak amphibolite facies metamorphism and penetrative, west vergent deformation occurred after 1675 Ma. The Baldwin gneiss is part of a regional allochthon emplaced by west vergent deformation over a Proterozoic shelf‐slope sequence (Joshua Tree terrane). We hypothesize that emplacement of this regional allochthon occurred during a late Early or Middle Proterozoic arc‐continent collision along the western margin of Laurentia.
Paleomagnetically defined rotations from the Precordillera of northern Chile: Evidence of localized in situ fault‐controlled rotations
Authors: Randall Darren E. , Tomlinson Andrew J. , Taylor Graeme K.
Published: February 2004

A paleomagnetic study of Paleocene, Cretaceous, and Jurassic units in the Precordillera of northern Chile (26°S to 27°S) demonstrate a complex pattern of crustal rotations. The region was selected to investigate the pattern of deformation associated with the Sierra Castillo‐Agua Amarga Fault and its associated structural subdomains, which form part of the Domeyko Fault System, the fault system that controls much of the structure of the Chilean Precordillera. Paleocene lavas from the center of the study area pass a fold test, indicating the primary nature of the remanence in these rocks. A second group of lavas from the south has a similar formation mean inclination after tilt correction for a uniform dip, which coupled with the presence of reversals, suggests that these lavas also carry a primary remanence. The data indicate clockwise rotations of ∼35° and 42° respectively, for the two sampling localities. Three geographically separate areas of Lower Cretaceous red beds also reveal primary remanences based on the presence of sites with reversed polarity at two of the localities and an inclination‐only fold test of the three locality mean directions for this formation. These sampling areas yield remanence directions that reveal 30° of counterclockwise and 31° of clockwise rotation as well as an area without statistically significant rotation. These data include the first well‐constrained counterclockwise rotation to be recovered from rocks in the southern central Andes, although mapping suggests the block involved is of limited geographic extent. Jurassic strata failed to yield any reliable results. Overall, the data indicate a greater variability in the rotation pattern than has previously been observed in most comparable areas south of the Arica Deflection. The variation cannot be explained by large‐scale rotation mechanisms, such as oroclinal bending or domino‐style block rotation by widely separated transandean faults. Instead, the rotations are consistent spatially, and temporally and in their sense and magnitude with the Eocene transpressional deformation associated with the Domeyko Fault System; the deformational event that generated most of the important structures in the studied area. The data emphasize the significance of local tectonics in controlling rotations in this part of the south central Andes and the importance of understanding the local structure of regions sampled for paleomagnetic study.
Comparison of recent Indian Ocean anthropogenic CO2 estimates with a historical approach
Authors: Sabine Christopher L. , Feely Richard A.
Published: February 2004

This work compares the classic Chen and Millero [1979] approach for estimating anthropogenic CO2 from ocean carbon measurements with the more recent ΔC* technique used by Sabine et al. [1999] to estimate anthropogenic CO2 concentrations in the Indian Ocean. Application of the Chen technique to the WOCE/JGOFS Indian Ocean data set gives a total anthropogenic CO2 inventory that is essentially the same as the ΔC* inventory, but there are substantial differences in the distributions within the water column. Some of these differences result from details of the application of the techniques such as the choice of which equation to use for the preformed alkalinity concentration or the choice of stoichiometric ratio to use for the biological correction. More significant differences, however, result from two fundamental differences in the techniques. One fundamental difference between the two techniques is that changes in the properties of the subsurface waters are referenced to a single deep water value in the Chen approach instead of the multiple reference points from the isopycnal analysis used in the ΔC* approach. The second fundamental difference is in the estimation of the preindustrial TCO2 distribution. Many of the differences examined have counteracting effects that may result in a total anthropogenic CO2 inventory that is similar for the two techniques. However, this similarity does not imply that both approaches are right. Comparison with global carbon models and other measurement‐based techniques do not clearly demonstrate that one technique is better than another. However, given the additional constraints of the transient tracers and the isopycnal analysis, we believe that the ΔC* technique provides a more robust estimate.