bluuebunny/crossref_metadata_2025 huggingface dataset:
Upper bound on Io's heat flow
Authors: Matson Dennis L. , Johnson Torrence V. , Veeder Glenn J. , Blaney Diana L. , Davies Ashley G.
Published: February 2004

Analysis of the temperatures and areas of Io's thermal anomalies yields an upper bound on the total heat flow. An extended distribution function allows an assessment of the heat flow from undetected, cooler (but larger) anomalies and predicts a limiting temperature of ∼90–95 K for the surface. This value is in agreement with measured Voyager infrared interferometer spectrometer and Galileo Photo‐Polarimeter Radiometer nighttime “minimum temperatures.” In addition, the lack of dependence on both latitude and time of night for these observed temperatures can be explained by cooling lavas on a global scale. We consider the extreme case that Io may be covered completely by lava in various stages of cooling to the exclusion of any thermally passive ‘background’ (excepting the few high mountains). Such a distribution of volcanic thermal anomalies up to the size of Io itself yields the first upper bound for heat flow, 13.5 W m−2. This corresponds to a total global, radiated power of 5.6×1014 W.
Midinfrared spectra of Mercury
Authors: Cooper Bonnie , Potter Andrew , Killen Rosemary , Morgan Thomas
Published: February 2004

Observations of Mercury in the mid infrared (8–12.5 μm region) obtained over a variety of longitudes using the Fourier Transform Spectrometer (FTS) on the McMath‐Pierce 1.5 m solar telescope at Kitt Peak reveal low‐contrast spectra, except in cases where atmospheric noise or difficulties in guiding the telescope caused spurious signals. Although there are features that are suggestive of emissivity maxima (Christiansen features), their proximity to the telluric absorption band, and the noise in the spectra, preclude their unambiguous interpretation. We see little evidence of reststrahlen bands in our data; however, there are strong indications of transparency features. A shallow emissivity minimum occurs at 12 μm in the spectra centered at 80°, 256°, and 266° Hermean longitudes. A minimum occurs at 12.5 μm in the spectra centered at 15°, and a doublet minimum, with one trough at 12.2 μm and a second trough at 12.4–12.6 μm, is seen in the spectra centered at 229°. These features indicate the presence of a fine powder on the Mercurian surface, and their low spectral contrast suggests a significant percentage of agglutinitic material. On the basis of the position of the transparency features, we conclude that our spectra are indicative of intermediate, mafic, and ultramafic rock types. Further specificity is not warranted by the data.
Mineralogy of the last lunar basalts: Results from Clementine
Authors: Staid Matthew I. , Pieters Carlé M.
Published: February 2004

The last major phase of lunar volcanism produced extensive high‐titanium mare deposits on the western nearside which remain unsampled by landing missions. The visible and near‐infrared reflectance properties of these basalts are examined using Clementine multispectral images to better constrain their mineralogy. A much stronger 1 μm ferrous absorption was observed for the western high‐titanium basalts than within earlier maria, suggesting that these last major mare eruptions also may have been the most iron‐rich. These western basalts also have a distinctly long‐wavelength, 1 μm ferrous absorption which was found to be similar for both surface soils and materials excavated from depth, supporting the interpretation of abundant olivine within these deposits. Spectral variation along flows within the Imbrium basin also suggests variations in ilmenite content along previously mapped lava flows as well as increasing olivine content within subsequent eruptions.
Lunar surface thermophysical properties derived from Clementine LWIR and UVVIS images
Authors: Lawson Stefanie L. , Jakosky Bruce M.
Published: February 2004

In an effort to understand the influence of albedo and large‐scale topography on remote lunar surface measurements, we investigate the relationship between measured Clementine long‐wave infrared (LWIR) camera temperatures and ultraviolet‐visible (UVVIS) camera 750‐nm reflectances. We compare the observed temperature‐reflectance relationships with those predicted using a rough‐surface numerical model with varying albedo. The lunar surface response in different highland and mare locations is explored as a function of varying phase angle. At very low phase angles the temperature and reflectance response is primarily governed by the variation in single‐scattering albedo regardless of the presence of topography. As the phase angle increases, the influence of surface roughness grows. Finally, at moderate to high phase angles the effect of surface roughness dominates. In the absence of large‐scale topography the lunar surface temperature and reflectance response at all phase angles is governed by the variation in single‐scattering albedo. LWIR‐measured temperature variations yield local topographic information at high incidence angles that is unavailable via the reflectance, while UVVIS‐measured reflectance variations yield local topographic information at low incidence angles that is unavailable via the temperature.
Martian surface winds: Insensitivity to orbital changes and implications for aeolian processes
Authors: Fenton Lori K. , Richardson Mark I.
Published: February 2004

Aeolian features observed on the surface of Mars provide insight into current, and potentially past, surface wind systems. In some cases the features are clearly transient and related to the lifting and settling of atmospheric dust. Other features, like dunefields, yardangs, and ventifacts, are more persistent and likely require significant time to form. In this study we analyze the observed directions of selected aeolian features with the aid of the Geophysical Fluid Dynamics Laboratory Mars general circulation model (GCM). Initially, we examine bright and dark streaks which have been observed to form in association with global dust storms. The ability to match these features with Mars GCM wind directions provides an important validation of the model. More important, we are able to define best fit seasons and local times for both types of features which provide the basis for extension and modification of the Veverka et al. [1981] model of bright and dark dust streak formation. In addition these best fit times correspond well with the dark streak “wind storm” model of Magalhães and Young [1995]. The primary focus of this paper is to provide constraints on the range of mechanisms proposed to explain inconsistencies between current wind direction patterns and long‐term wind indicators (for example, the misalignment of rock tail and ventifact orientations at the Mars Pathfinder landing site). Specifically, we assess whether changes in planetary obliquity, precession, or global dust opacity could significantly alter patterns of surface wind directions. In all cases we find the seasonal and annual average wind direction patterns to be highly invariable. While changes in the dust loading (hence the partitioning of solar absorption between the surface and atmosphere) and in the surface latitude of maximum solstitial insolation cause the vigor of the large‐scale circulation to increase (especially the Hadley cell), the spatial patterns of the surface wind orientations remain essentially unchanged. In the case of perihelion during northern summer (opposite of the current perihelion position), the northern summer Hadley cell remains weaker than the southern summer cell despite the strengthened heating in the northern hemisphere. Taken together, these results cast significant doubt on orbital explanations for surface wind changes. It is thus suggested that significant changes in topography (e.g., Tharsis uplift, true polar wander) or climate (e.g., the existence of a significantly thicker atmosphere or an ocean at some point in the past) are more likely explanations for long‐term wind indicators such as the ventifact orientations at the Mars Pathfinder landing site.
Convection in Io's asthenosphere: Redistribution of nonuniform tidal heating by mean flows
Authors: Tackley Paul J.
Published: February 2004

Numerical simulations of convection in a wide domain with spatially varying internal heating intended to mimic tidal dissipation in Io's proposed asthenosphere are performed in order to better understand the convective dynamics and, in particular, to quantify the relationship between surface observables (particularly the distribution of heat flux) and tidal dissipation. Two‐dimensional calculations at high internal heating Rayleigh number RaQ of up to 1010 indicate a mean, domain‐wide flow with superimposed small‐scale instabilities. The mean flow spreads out the tidally‐dissipated heat, resulting in long‐wavelength surface heat flux variations that decrease in proportion to RaQ−0.21.. This scaling is shown to apply to different domain widths and also to three‐dimensional geometry. Powerlaw scalings are also obtained for velocities and temperatures. These mean flow scalings are also derived analytically. Scaling to Io's asthenosphere, long‐wavelength heat flux variations of order several percent are predicted, with temperature variations of order tens of Kelvins and mean flow velocities of 50–5600 m.yr−1.
Global mapping of Martian hematite mineral deposits: Remnants of water‐driven processes on early Mars
Authors: Christensen P. R. , Morris R. V. , Lane M. D. , Bandfield J. L. , Malin M. C.
Published: February 2004

Near‐global (60°S to 60°N) thermal infrared mapping by the Thermal Emission Spectrometer (TES) on Mars Global Surveyor has revealed unique deposits of crystalline gray hematite (α‐Fe2O3) exposed at the Martian surface in Sinus Meridiani, Aram Chaos, and in numerous scattered locations throughout Valles Marineris. The Sinus Meridiani material is an in‐place, rock stratigraphic sedimentary unit characterized by smooth, friable layers composed primarily of basaltic sediments with ∼10–15% crystalline gray hematite. This unit has outliers to the north that appear to have formed by stripping and removal. The hematite within Aram Chaos occurs in a sedimentary layer within a closed basin that was likely formed during the basin infilling and predates the formation of nearby chaos and outflow terrains. This unit appears to be exposed by erosion and may be more extensive beneath the surface. The Valles Marineris occurrences are closely associated with the interior layered deposits and may be in place within the layers or eroded sediments. Overall, crystalline gray hematite is extremely uncommon at the surface, yet in all observed locations it is closely associated with layered, sedimentary units. Here we argue that these hematite deposits have formed by a process involving chemical precipitation from aqueous fluids, under either ambient or hydrothermal conditions. Thus the TES mineralogic data provide evidence that liquid water has been stable at or near the surface, probably for millions of years by analogy with terrestrial iron formations, in specific locations on early Mars.
Recent gullies on Mars and the source of liquid water
Authors: Mellon Michael T. , Phillips Roger J.
Published: February 2004

Geologic features resembling terrestrial water‐carved gullies imply that liquid water has flowed recently on the surface of Mars and challenge our views of the present‐day low‐temperature environment. We evaluate two possible mechanisms for the formation of liquid water under environmental conditions that we expect to have existed on Mars in its recent past. First, we examine the stability of ground ice in the permafrost and the potential for melting near‐surface ground ice (in the top few meters of soil) by solar heating and subsurface conduction. Second, we examine the potential for melting and refreezing of ice at shallow depths due to geothermal heating. We find that near‐surface ground ice does not reach the melting point of water under a range of conditions of soil thermophysical properties, latitudes, obliquities, and surface slopes. The atmosphere remains too dry for the ground ice to melt, even at high obliquity; instead, ice sublimates before reaching melting temperatures. However, the presence of salts in concentrations of 15–40% can adequately lower the melting point to allow melting to occur. We also find that a combination of a global average geothermal heat flux and a thick, low‐conductivity, unconsolidated regolith raises the depth of the melting isotherm to less than a few hundred meters from the surface. Orbitally induced oscillations in the mean annual surface temperature can cause freezing cycles in a confined aquifer at this depth. Freezing pressures generated are adequate to fracture ice‐cemented ground and allow water to escape to the surface, similar to the formation and evolution of terrestrial pingos in shallow permafrost. Both mechanisms are possible; however, the geothermal mechanism is consistent with the observations of the distribution of gullies, while the salty near‐surface ground ice mechanism is not. Further observational tests that can be performed with existing and future spacecraft are suggested.
On the possibility of clathrate hydrates on the Moon
Authors: Duxbury N. S. , Nealson K. H. , Romanovsky V. E.
Published: February 2004

One of the most important inferences of the Lunar Prospector mission data was the existence of subsurface water ice in the permanently shadowed craters near both lunar poles [Feldman et al., 1998]. We propose and substantiate an alternative explanation that hydrogen can exist in the shallow lunar subsurface in the form of clathrate hydrates: CH4 · 6H2O and/or CO2 · 6H2O.
An improved solution of the gravity field of Mars (GMM‐2B) from Mars Global Surveyor
Authors: Lemoine F. G. , Smith D. E. , Rowlands D. D. , Zuber M. T. , Neumann G. A. , Chinn D. S. , Pavlis D. E.
Published: February 2004

A spherical harmonic solution of the Mars gravity field to degree and order 80, Goddard Mars Model 2B (GMM‐2B), has been developed using X band tracking data of Mars Global Surveyor (MGS) from October 1997 to February 2000 and altimeter crossovers formed from the Mars Orbiter Laser Altimeter (MOLA) data between March and December 1999. During the mapping mission, MGS was located in a near‐polar (92.9° inclination) and near‐circular orbit at a mean altitude of 400 km. The tracking data from this orbit provide a detailed, global, and high resolution view of the gravity field of Mars. Mars gravity solutions are stable to 60×60 even without application of a Kaula power law constraint. The Valles Marineris is resolved distinctly with lows reaching −450 mGals. Olympus Mons and its aureole are both separately resolved, and the volcano has a peak anomaly of 2950 mGals. The global correlation of the GMM‐2B gravity coefficients with MOLA‐derived topography is 0.78 through degree 60, and the correlation remains above 0.6 through degree 62. The global gravity anomaly error predicted from the GMM‐2B error covariance through 60×60 is 11 mGal. The global geoid error from GMM‐2B through 60×60 is 1.8 m. MGS orbit quality using GMM‐2B, as measured by overlapping orbital arcs, is 1 m in the radial direction and 10 m in total position.
Observation of wavelength‐sensitive mass‐independent sulfur isotope effects during SO2 photolysis: Implications for the early atmosphere
Authors: Farquhar James , Savarino Joel , Airieau Sabine , Thiemens Mark H.
Published: February 2004

Mass‐independent isotopic signatures for δ33S, δ34S, and δ36S produced in the photolysis of sulfur dioxide exhibit a strong wavelength dependence. Photolysis experiments with three light sources (ArF excimer laser (193 nm), mercury resonance lamp (184.9 and 253.7 nm), and KrF excimer laser (248 nm)) are presented. Products of sulfur dioxide photolysis undertaken with 193‐nm radiation exhibit characteristics that are similar to sulfur multiple‐isotope data for terrestrial sedimentary rock samples older than 2450 Ma (reported by Farquhar et al. [2000a]), while photolysis experiments undertaken with radiation at other wavelengths (longer than 220 nm and at 184.9 nm) exhibit different characteristics. The spectral window between 190 and 220 nm falls between the Schumann‐Runge bands of oxygen and the Hartley bands of ozone, and its absorption is therefore more sensitive to changes in altitude and atmospheric oxygen content than neighboring wavelengths. These two observations are used to suggest a link between sulfur dioxide photolysis at 193 nm and sulfur isotope anomalies in Archean rocks. This hypothesis includes the suggestion that UV wavelengths shorter than 200 nm penetrated deep in the Earth's atmosphere during the Archean. Potential implications of this hypothesis for the chemistry, composition, and UV absorption of the atmosphere are explored. We also explore the implications of these observations for documentation of bacterial sulfur metabolisms early in Earth's history.
Deep flow variability under apparently stable North Atlantic deep water production during the Last Interglacial of the subtropical NW Atlantic
Authors: Bianchi G. G. , Vautravers M. , Shackleton N. J.
Published: February 2004

The environmental history of the last interglacial period is equivocal, since its climate has been reported to be anything from unstable, with at least one significant cold event, to generally stable and comparable to the Holocene. Here the activity of the southerly Deep Western Boundary Current (DWBC) in the ∼130–110 ka interval is studied at two sites (3481 and 4760 m) in the subtropical NW Atlantic presently bathed by Lower North Atlantic Deep Water (LNADW). Proxies for both bottom water chemistry (δ13C) and near‐bottom flow vigor (
, the mean size of the “sortable silt” fraction) are employed. The δ13C data indicate the interval of peak interglaciation when LNADW bathed both sites studied is followed by the development of a vertical water mass gradient between them, with nutrient‐rich waters of southern origin flooding the deeper parts of the area. After ∼113 ka this gradient disappears as the deeper, low δ13C waters homogeneously invade the whole of the depth range investigated. The
data imply these events are likely to be associated with the vertical migration of the flow axis of the DWBC. Furthermore, they also reveal unexpected fluctuations in near‐bottom flow activity during the period of minimum ice volume which are tentatively ascribed to small changes in the convective activity of the Nordic Seas. Alternatively, DWBC flow fluctuations may be controlled by processes occurring farther downstream from the areas of deep water formation, therefore not requiring changes in the production rates of LNADW. In either case, this series of small fluctuations during the peak of the last interglaciation may ultimately be linked to the insolation‐related evolution of latitudinal sea surface temperature and salinity gradients in the North Atlantic.
Could the Late Permian deep ocean have been anoxic?
Authors: Zhang R. , Follows M. J. , Grotzinger J. P. , Marshall J.
Published: February 2004

Models of ocean circulation and biogeochemical cycles are used to speculate about possible mechanisms of deep‐sea anoxia across the Permo‐Triassic boundary. Two modes of likely Late Permian ocean thermohaline circulation are identified: a vigorous “thermal mode” driven by cooling in southern polar latitudes and a weaker “haline mode” driven by evaporation from the subtropics. We find that the thermal mode, typical of climates such as our own, is unlikely to surpport deep‐sea anoxia. The haline mode, which might exist in warm climates with enhanced hydrological cycles, can lead to significant but periodic depletion of oxygen in the Panthalassic deep ocean. However, owing to the inherent instability of the haline mode, a deep‐sea anoxia persisting for millions of years is not clearly supported by this study, though mechanisms emerge which might be consistent with the observed record. The enhanced hydrological cycle (or reduced oceanic diapycnal mixing) required to obtain the haline mode remain unconstrained for the Late Permian.
Millennial‐scale climate change and oceanic processes in the Late Pliocene and Early Pleistocene
Authors: Mc Intyre Katherine , Delaney Margaret L. , Ravelo A. Christina
Published: February 2004

We generated 200–500 year resolution records of oceanic processes in the North Atlantic (Ocean Drilling Program Site 983, 60°24′N, 23°38′W, 1983 meters water depth) for intervals in the latest Pliocene (1.86–1.93 Ma) and the earliest Pleistocene (1.75–1.83 Ma) in order to examine the linkages between millennial‐scale variations in the ocean and background glacial‐interglacial climate change. Within glacial intervals we find evidence for variations similar to those observed in the late Pleistocene. We find discrete ice‐rafted debris (IRD) events that reoccur every 2–5 kyr. These events are preceded by a short cooling and accompanied by a reorganization of glacial deep waters. The timing of IRD events in the late Pliocene and early Pleistocene intervals is similar to that of Dansgaard‐Oeschger cycles, but we find no IRD events comparable in timing to late Pleistocene Heinrich events. Although interglacial intervals are much more stable, we do find evidence for low‐amplitude variations in deep water properties that reoccur every ∼2 kyr within interglacial intervals. The similarity between our late Pliocene—early Pleistocene records and late Pleistocene records implies that the mechanism driving millennial‐scale variations cannot be uniquely attributed to the strongly nonlinear linkage between climate and insolation and the large ice sheets of the late Pleistocene.
Evidence from authigenic uranium for increased productivity of the glacial subantarctic ocean
Authors: Chase Zanna , Anderson Robert F. , Fleisher Martin Q.
Published: February 2004

Authigenic uranium is precipitated in reducing sediments and therefore responds both to changes in particulate organic carbon flux to the sediment and to changes in the oxygen concentration of bottom waters. By examining a large number of cores over a wide latitudinal and depth range in the Atlantic sector of the Southern Ocean, we hope to distinguish between a predominantly productivity‐driven authigenic U signal and a circulation or sediment‐focusing signal. We find little to no authigenic U in Holocene sediments throughout the South Atlantic Ocean. Glacial sediments north of ∼40°S lack authigenic U, whereas sediments from the Subantarctic Zone show substantial enrichments in authigenic U relative to the Holocene (up to ∼5.3 ppm). The widespread distribution of glacial U enrichment, even in cores with no glacial‐interglacial change in mass accumulation rate, implies that U deposition was not caused by an increased supply of organic carbon via sediment focusing. Authigenic U and organic carbon in a shallow core (∼1000 m) from the Subantarctic region show the same glacial increase found in the deep cores. Because this site was well ventilated during the Last Glacial Maximum, its record provides further evidence that increased glacial productivity of the Subantarctic region contributed to the high concentrations of authigenic U found in the glacial sections of deep cores.
Weathering versus circulation‐controlled changes in radiogenic isotope tracer composition of the Labrador Sea and North Atlantic Deep Water
Authors: von Blanckenburg Friedhelm , Nägler Thomas F.
Published: February 2004

Geological reconstructions and general circulation models suggest that the onset of both Northern Hemisphere glaciation, 2.7 Myr ago, and convection of Labrador Sea Water (LSW) were caused by the closure of the Panama Gateway ∼ 4.5 Myr ago. Time series data that have been obtained from studies of ferromanganese crusts from the northwestern Atlantic suggest that radiogenic isotopes of intermediate ocean residence time (Pb and Nd) can serve as suitable tracers to reconstruct these events. However, it has been unclear until now as to whether the changes that have been observed in isotope composition at this time are the result of increased thermohaline circulation or due to the effects of increased glacial weathering. In this paper we adopt a box model approach to demonstrate that the shifts in radiogenic isotope compositions are unlikely to be due to changes in convection in LSW but can be explained in terms of increases of erosion levels due to the glaciation of Greenland and Canada. Furthermore, we provide experimental evidence for the incongruent release of a labile fraction of strongly radiogenic Pb and nonradiogenic Nd from continental detritus eroding into the Labrador Sea. This can be attributed to the glacial weathering of old continents and accounts for the paradox that one of the areas of the world most deficient in radiogenic Pb should provide such a rich supply of radiogenic Pb to the oceans. An important general conclusion is that the compositions of radiogenic isotopes in seawater are not always a reflection of their continental sources. Perhaps more importantly, the transition from chemical weathering to mechanical erosion is likely to result in significant variations in radiogenic tracers in seawater.
Nitrogen isotope evidence for water mass denitrification during the Early Toarcian (Jurassic) oceanic anoxic event
Authors: Jenkyns Hugh C. , Gröcke Darren R. , Hesselbo Stephen P.
Published: February 2004

Bulk sedimentary nitrogen isotope (δ15Ntot) data have been generated from Lower Jurassic black, carbon‐rich shales in the British Isles and northern Italy deposited during the early Toarcian oceanic anoxic event. A pronounced positive δ15Ntot excursion through the exaratum Subzone of the falciferum Zone (defined by characteristic ammonites in the British Isles) broadly correlates with a relative maximum in weight percent total organic carbon and, in some sections, with a negative δ13Corg excursion. Upwelling of a deoxygenated water mass that had undergone partial denitrification is the likely explanation for relative enrichment of δ15Ntot, and parallels may be drawn with Quaternary sediments of the Arabian Sea, Gulf of California, and northwest Mexican margin. The development of Early Toarcian suboxic water masses and consequent partial denitrification is attributed to increases in organic productivity. Approximately coincident phenomena include the following: a relative climatic optimum, realignment of major oceanic current systems, and a possible release of methane gas hydrates from continental margin sediments early in the history of the oceanic anoxic event.
Terrigenous input and paleoproductivity in the Southern Ocean
Authors: Latimer Jennifer C. , Filippelli Gabriel M.
Published: February 2004

The geochemistry of several sites along the Polar Front Zone of the Southern Ocean revealed significant changes in terrigenous input and productivity on glacial timescales. Our records indicate concentrations and fluxes of terrigenous elements (i.e., Fe, Al, and Ti) are low during interglacials and increase tenfold to twentyfold during glacials. Phosphorus accumulation, reactive P concentrations, and P/Ti ratios exhibit similar trends as the terrigenous components, suggesting increased export production during glacials. In all cores, elemental ratios remain relatively constant, indicating terrigenous sources at each site have not likely varied over the past ∼250 kyr. Iron accumulation rates for these sites are 10 times higher during glacials than interglacials and exceed the estimated regional eolian fluxes by ∼50 times. The source of this terrigenous material is thus likely to be hemipelagic rather than direct eolian input or ice rafting, and we speculate Fe upwelled from this source may have stimulated glacial productivity in the Southern Ocean.
Glacial cycles: Toward a new paradigm
Authors: Paillard Didier
Published: February 2004

The largest environmental changes in the recent geological history of the Earth are undoubtedly the successions of glacial and interglacial times. It has been clearly demonstrated that changes in the orbital parameters of our planet have a crucial role in these cycles. Nevertheless, several problems in classical astronomical theory of paleoclimate have indeed been identified: (1) The main cyclicity in the paleoclimatic record is close to 100,000 years, but there is no significant orbitally induced changes in the radiative forcing of the Earth in this frequency range (the “100‐kyr problem”); (2) the most prominent glacial‐interglacial transition occurs at a time of minimal orbital variations (the “stage 11 problem); and (3) at ∼0.8 Ma a change from a 41‐kyr dominant periodicity to a 100‐kyr periodicity occurred without major changes in orbital forcing or in the Earth's configuration (the “late Pleistocene transition problem”). Additionally, the traditional view states that the climate system changes slowly and continuously together with the slow evolution of the large continental ice sheets, whereas recent high‐resolution data from ice and marine sediment cores do not support such a gradual scenario. Most of the temperature rise at the last termination occurred over a few decades in the Northern Hemisphere, indicating a major and abrupt reorganization of the ocean‐atmosphere system. Similarly, huge iceberg discharges during glacial times, known as Heinrich events, clearly demonstrate that ice sheet changes may also be sometimes quite abrupt. In light of these recent paleoclimatic data the Earth climate system appears much more unstable and seems to jump abruptly between different quasi steady states. Using the concept of thresholds, this new paradigm can be easily integrated into classical astronomical theory and compared with recent observational evidence. If the ice sheet changes are, by definition, the central phenomenon of glacial‐interglacial cycles, other components of the climate system (atmospheric CO2 concentration, Southern Ocean productivity, or global deep‐ocean circulation) may play an even more fundamental role in these climatic cycles.
Persistent layers of enhanced C2N in the lower stratosphere from VHF radar observations
Authors: Nastrom G. D. , Eaton F. D.
Published: February 2004

Seasonal climatologies of persistent layers of enhanced refractive index structure parameter C2N were developed for the lower stratosphere from VHF radar observations at White Sands Missile Range, New Mexico, for the period January 1991 to September 1996. Knowledge of the nature of enhanced refractivity layers is of high interest to the atmospheric sciences, propagation, and remote sensing communities. The layers reported have C2N enhanced at least 7 dB above the background continuously for at least 11 hours and migrate vertically no more than one radar range gate (150 m) over 1 hour. The cumulative frequency of the lengths (11–37 hours) of the 259 persistent layers identified shows that 25% of the layers last over 17 hours. Comparisons of profiles of wind speeds, variances of the wind components, vertical shear of the horizontal wind, Doppler spectral width, temperature, Brunt‐Vaisala frequency, and Richardon's number for times with and without persistent layers at 17 km show that wind speed at 5.6 km in addition to spectral width, wind shear, and vertical velocity variances at 17 km are stronger during enhanced layer episodes than during nonlayer periods. Possible sources for the persistent layers are suggested, and the shortcomings of each hypothesis are discussed. Several case studies of radiosonde ascents during persistent layers give no obvious indication of the source of these layers.
Experimental investigation on rectangular plate antenna with Ω‐shaped slot
Authors: Chen Zhi Ning
Published: February 2004

In this paper, a single‐layer single‐element plate antenna with an Ω‐shaped slot is presented for broadening impedance bandwidth and suppressing cross‐polarization levels. The plate is basically suspended above a ground plane and fed by a coaxial probe. A thin Ω‐shaped slot is cut at the middle of the plate. The investigations on impedance and radiation characteristics are experimentally conducted. The measured results show that the achieved impedance bandwidths for VSWR = 2:1 reach up to the order of 20% and the improved radiation patterns are stable across the bandwidths. The studies have demonstrated three important points. The first is that the spacing of ∼ 0.1 operating wavelength between the plate and the ground plane is essential to broaden the impedance bandwidth. The second is that the slot effectively compensates for the large inductance arising from the long feeding probe. The last is that the proper selections of the slot shape and the aspect ratio of the rectangular plate are important to depress the high cross‐polarization levels.
Correcting atmospheric path variations in millimeter wavelength very long baseline interferometry using a scanning water vapor spectrometer
Authors: Tahmoush David A. , Rogers Alan E. E.
Published: February 2004

The coherence of very long baseline interferometry (VLBI) at millimeter wavelengths can be improved through the use of a water vapor radiometer (WVR) to estimate the atmospheric path length variations along the line of sight to each antenna. Measurements of the water vapor emission spectrum using a simple scanning WVR have been used to correct the interferometer phases, thereby lowering the rms phase variation and increasing the coherence time. The average improvement in the coherent fringe amplitude of 400 s VLBI scans at 3 mm wavelength was 20% with a general trend of a greater improvement for the scans with larger phase variations. Sampling of the spectrum from 18 to 26 GHz allows separation of the water vapor emission from instrumental effects and from the emission of water droplets in clouds. The observed line profiles favor the Van Vleck‐Weisskopf line shape.
Validation of line‐of‐sight water vapor measurements with GPS
Authors: Braun John , Rocken Christian , Ware Randolph
Published: February 2004

We present a direct comparison of nonisotropic, integrated water vapor measurements between a ground‐based Global Positioning System (GPS) receiver and a water vapor radiometer (WVR). These line‐of‐sight water vapor observations are made in the straight line path between a ground station and a GPS satellite. GPS double‐difference observations are processed, and the residual line‐of‐sight water vapor delays are extracted from the double‐difference residuals. These water vapor delays contain the nonisotropic component of the integrated water vapor signal. The isotropic component is represented by the zenith precipitable water vapor measurement and can be scaled to a specific elevation angle based on a mapping function. The GPS observations are corrected for station‐dependent errors using site‐specific multipath maps. The resulting measurements are validated using a WVR which pointed in the direction of the observed satellites. The double‐difference technique used to make these water vapor observations does not depend on accurate satellite clock estimates. Therefore it is especially well suited for near‐real‐time application in weather prediction and allows for sensing atmospheric structure that is below the noise level of current satellite and receiver clock errors. This paper describes the analysis technique and provides precision estimates for the GPS‐measured nonisotropic water vapor as a function of elevation angle for use with data assimilation systems.
Nonturbulent layers in polar summer mesosphere: 1. Detection of sharp gradients using wavelet analysis
Authors: Alcala C. M. , Kelley M. C. , Ulwick J. C.
Published: February 2004

The polar mesosphere is characterized by a very high radar backscattering coefficient. Both radar and in situ rocket spectra indicate that two qualitatively different scattering mechanisms are operating: one turbulent and one edge‐like. Fourier analysis methods applied to in situ data have been very useful in showing that spectra are steeper than the canonical −5/3 turbulent form. Such steep spectra accompany the edge‐like structures. However, since Fourier methods assume that the energy is distributed throughout a given window, they are not very useful for describing a finite number of sharp edges in a data set, which is the normal situation. Here we use the techniques of wavelet analysis to partition a rocket data set taken during a polar mesosphere summer echo event characterized by the sounding system radar in Norway. We find that the edges can be either isolated in space or collocated with turbulence and, likewise, that turbulent regions can be found without steep edges. Our partitioning verifies previous notions concerning the interpretation of shallow and steep spectra. The first clear evidence for theoretically predicted viscous‐convective and viscous‐diffusive subranges is also presented. In a companion paper [Alcala and Kelley, this issue] we use the well‐known scattering properties of our wavelet basis functions to calculate the scattering from the region sampled by the rocket probe in a manner that accounts for multiple edges in the same scattering volume.
Modeling of a cylindrical wire antenna with flat end caps using a rigorous moment method technique
Authors: Huffman J. A. , Werner D. H.
Published: February 2004

With recent capabilities for accurately and efficiently modeling electrically thicker wire antennas, questions concerning the necessity of including end caps in the model have resurfaced. The finite charge on the ends, which can significantly affect the current distribution, as well as improve convergence and stability of the solution, has been characterized in the past by several approximate models. However, a more in‐depth analysis of a wire antenna with end caps is necessary for completely characterizing the end charge density and understanding its effects on the near‐field behavior and the current distribution of the wire. Such a treatment is especially important for moderately thick wires or resonant structures. A rigorous method of moments formulation is developed in this paper for the analysis of cylindrical wire antennas with flat end caps. Circumferentially invariant axially directed trigonometric current basis functions are assumed on the cylindrical surface, and pulse charge density basis functions are assumed on the end caps. In addition, the formulation introduces a ring charge at the outer boundary of the top and bottom end caps to account for the edge discontinuity. Exact analysis techniques are employed to obtain the solution to the electric field integral equation.
Lifetime of a depression in the plasma density over Jicamarca produced by space shuttle exhaust in the ionosphere
Authors: Bernhardt P. A. , Huba J. D. , Kudeki E. , Woodman R. F. , Condori L. , Villanueva F.
Published: February 2004

When the space shuttle orbiting maneuver subsystem (OMS) engines burn in the ionosphere, a plasma density depression, or “hole,” is produced. Charge exchange between the exhaust molecules and the ambient O+ ions yields molecular ion beams that eventually recombine with electrons. The resulting plasma hole in the ionosphere can be studied with ground‐based, incoherent scatter radars (ISRs). This type of ionospheric modification is being studied during the Shuttle Ionospheric Modification with Pulsed Localized Exhaust (SIMPLEX) series of experiments over ISR systems located around the globe. The SIMPLEX 1 experiment occurred over Jicamarca, Peru, in the afternoon on October 4, 1997, during shuttle mission STS 86. An electron density depression was produced at 359 km altitude at the midpoint of a magnetic field line. The experiment was scheduled when there were no zonal drifts of the plasma so the modified field line remained fixed over the 50 MHz Jicamarca radar. The density depression was filled in by plasma flowing along the magnetic field line with a time constant of 4.5 min. The density perturbation had completely vanished 20 min after the engine burn. The experimental measurements were compared with two models: (1) SAMI2, a fully numerical model of the F region, and (2) an analytic representation of field‐aligned transport by ambipolar diffusion. The computed recovery time from each model is much longer than the observed recovery time. The theory of ambipolar diffusion currently used in ionospheric models seems to be inadequate to describe the SIMPLEX 1 observations. Several possible sources for this discrepancy are discussed. The SIMPLEX 1 active experiment is shown to have the potential for testing selected processes in ionospheric models.
An attempt to model the influence of the trough on HF communication by using neural networks
Authors: Tulunay Yurdanur , Tulunay Ersin , Senalp Erdem Turker
Published: February 2004

Trough is an interesting phenomenon in characterizing the behavior of the ionosphere, especially during disturbed conditions. The subject, which was introduced around the 1970s, is still attracting attention, especially during recent years. In HF communication, in particular, over the midlatitude ionospheric regions the electron density trough exhibits a phenomenon of abrupt gradients of electron densities in space and time which are directly reflected to foF2. Thus the performances of HF communications are directly affected. In this work an attempt has been made for the modeling to quantify the influence of the ionospheric midlatitude electron density trough on the ionospheric critical frequency foF2 by using neural networks. Data sets are used from the ground stations that include observations in the trough region. It has been demonstrated that the neural‐net based approaches are promising in modeling of the ionospheric processes. Data generated by using statistical relationships are used to train the neural network. Then the trained neural network is used to forecast the ionospheric critical frequency, foF2, values 1 hour in advance for the cases when the probability of influence of the trough is high. Preliminary results will be presented to discuss the suitability of the neural‐network‐based approach in the modeling of complex processes such as the influence of the trough on foF2. The basic contributions of this work are 1) generation and organization of significant data for teaching complex processes, 2) neural‐network‐based modeling of a highly complex nonlinear process such as the influence of the trough on foF2 forecasting, and 3) general demonstration of learning capability by calculating cross correlations and general demonstration of reaching a proper operating point by calculating errors (that is, during the optimization process the neural network reaches the global minimum by using the gradient descent method).
An integrated tectonothermal model for the evolution of the High Himalaya in western Zanskar with constraints from thermobarometry and metamorphic modeling
Authors: Walker C. B. , Searle M. P. , Waters D. J.
Published: February 2004

We present an integrated model for the tectonothermal evolution of the High Himalaya in NW Zanskar based on detailed field mapping, petrographic and microstructural analysis, thermobarometric techniques, and metamorphic modeling. Metasedimentary lithologies in the Suru valley can be correlated with the Palaeozoic‐Mesozoic Tethyan shelf sediments along the north Indian continental margin in Kashmir and Ladakh, and metaigneous amphibolites correlate with Permian rift‐related igneous units. Subsequent to India‐Asia collision at ca. 54 Ma, crustal thickening of Indian plate rocks resulted in a polyphase deformational and metamorphic history. The large‐scale structure of the area is that of kilometer‐scale, SW vergent recumbent folds that have been folded by structurally lower, later domes such as the Suru Dome. Prograde M1 metamorphism reached a maximum of kyanite grade and is believed to be synkinematic to postkinematic with respect to the formation of the large folds. Thermobarometric analysis indicates that peak conditions relating to this Harrovian event between 33 and 28 Ma were 9.5‐10.5 kbar and 620°–650°C. A later metamorphic event (M2) associated with doming throughout the Zanskar Himalaya and crustal anatexis in the sillimanite + K‐feldspar‐grade core of the High Himalaya caused reequilibration of deeper Suru Dome rocks to slightly lower pressures (4.5–7 kbar). Metamorphic modeling, involving phase diagram construction and pressure‐temperature (P‐T) path determination, suggests that metamorphic garnets grew under conditions of heating and burial along moderate slopes in P‐T space. Rapid exhumation of the High Himalayan Crystallines between the Main Central Thrust and the Zanskar Shear Zone occurred during or immediately after peak M2 metamorphism (21.5–19.5 Ma).
Plate boundary readjustment in oblique convergence: Example of the Neogene of Hispaniola, Greater Antilles
Authors: Pubellier Manuel , Mauffret Alain , Leroy Sylvie , Vila Jean Marie , Amilcar Helliot
Published: February 2004

The Haitian fold‐and‐thrust belt is the major mountain belt of Haiti (western part of Hispaniola, Greater Antilles) and resembles a compressive restraining bend between the two major faults which have driven the opening of the Cayman Basin since the Eocene. During the rifting stage, from the middle to the late Eocene, this area underwent an extensional evolution with fissural volcanic activity along NE‐SW tilted blocks. The Haitian fold‐and‐thrust belt was constructed from the Early Miocene until the Present by stacking sedimentary units into a collisional wedge perpendicular to the tilted blocks, which was propagating to the southwest. During the construction of the wedge, piggyback basins were formed and progressively uplifted. During the late Neogene, convergence is localized in the Cul‐de‐Sac‐Enriquillo trough where the active front proceeds southward onto the Beata ridge. In this area, Miocene to Quaternary wrench structures of the lower plate, like the Southern Peninsula and the Enriquillo‐Plantain Garden faults, are reactivated as normal faults, owing to the loading of the fold‐and‐thrust belt.
Tectonics of SE China: New insights from the Lushan massif (Jiangxi Province)
Authors: Lin Wei , Faure Michel , Monié Patrick , Schärer Urs , Zhang Liangsheng , Sun Yan
Published: February 2004

In south China the Lushan massif forms a topographic high of the South China Block south of the Qinling‐Dabie belt. The Lushan massif consists of two main lithotectonic units separated by a major tectonic contact: a Neoproterozoic (upper Sinian)‐Paleozoic unit comprising primarily unmetamorphosed sandstones overlies a Paleoproterozoic unit mainly composed of low‐pressure, high‐temperature gneisses and micaschists. Both units are cut by Cretaceous granitic intrusions. Three primary tectono‐metamorphic and magmatic events are recognized. The eastern part of the Lushan massif is cut by a NNE‐SSW trending ductile normal fault (D3 deformation) coeval to the emplacement of a 100–110 Ma leucogranite dated by 40Ar/39Ar laserprobe on biotite and muscovite. D2 deformation is responsible for the formation of a decakilometer‐scale NE‐SW trending upright anticline characterized by NE‐SW stretching and NW‐SE shortening. The age of this folding event is defined by a 127±1 (2σ) Ma U/Pb titanite date obtained for a syntectonic granodiorite and 40Ar/39Ar ages of 133 Ma for amphibole. This Cretaceous age also corresponds to the 40Ar/39Ar ages of 126 Ma found on syntectonic muscovites at the base of the Sinian unit. An older deformation event, D1, characterized by a top‐to‐the‐NW extensional decollement of the Sinian‐Paleozoic series above Proterozoic metamorphic rocks is related to the Triassic tectonics of the Dabieshan. Lastly, in the lower part of Sinian rocks, the occurrence of kyanite cataclased during D1 documents an older, poorly preserved, late Paleozoic‐early Mesozoic tectonometamorphic event (Dx) related to a blind thrust in the continental crust of the South China Block in the southern foreland of the Dabieshan.