bluuebunny/crossref_metadata_2025 huggingface dataset:
Regolith variations on Io: Implications for bolometric albedos
Authors: Simonelli Damon P. , Dodd Christopher , Veverka Joseph
Published: February 2004

Global maps of Io produced from two sets of Galileo images (solar phase angles 4°–14° and 71°–86°, respectively) reveal that this satellite's color and albedo patterns change dramatically with phase. At low phase the equatorial band is the brightest, whitest part of Io; that is, it is brighter than the polar regions at all wavelengths. At high phase, however, the equatorial band becomes a dark gray, exhibiting little contrast with the polar regions at violet and green wavelengths and appearing darker than the polar regions at red wavelengths. To quantify these phase‐related changes, we derive global maps of the Henyey‐Greenstein asymmetry factor g that show how the strength of backscattering by Io's regolith varies from region to region. In the green and red, where albedo patterns change most radically with phase, the equatorial band forms a well‐defined unit of strongly backscattering material; material that is more weakly backscattering at these wavelengths, mainly in the polar regions, shows a broad continuum of different g values. In the violet the phase‐related changes in albedo patterns are more subdued, and the scattering units are poorly defined. Using this information on g, we generate a global map showing variations in the bolometric Bond albedo AB, the true “energy balance albedo” that governs insolation‐based surface temperatures. The mean albedo of this map, AB ≈ 0.52, is similar to AB values computed previously for Io, but the distribution of albedos is markedly different. In previous Bond albedo maps the highest AB coincided with the bright equatorial band seen at low phase; the new map, however, more closely resembles Io's high‐phase albedo patterns (i.e., the band of high Bond albedos at the equator is absent). This change in AB patterns has a significant effect on how insolation‐based temperatures vary with latitude on Io; in particular, it increases the satellite's equator‐to‐pole temperature contrast.
A 50‐degree spherical harmonic model of the magnetic field of Mars
Authors: Arkani‐Hamed Jafar
Published: February 2004

This paper presents a 50‐degree and order spherical harmonic model of the magnetic field of Mars derived at 120‐km altitude using the three orthogonal components of the vector magnetic field data acquired by the Mars Global Surveyor within the 80‐ to 200‐km‐altitude range. The downward continued vector magnetic field components to the surface of Mars delineate details of the Martian magnetic field. The strong magnetic anomalies in the southern hemisphere do not show compelling evidence for magnetic lineations similar to those associated with the seafloor spreading on Earth. The anomalies are more or less equidimensional. The magnetic signatures surrounding the impact basins Hellas, Argyre, and Isidis show that the prominent magnetic anomalies are older than the impact events. An average magnetization of ∼2 A/m is estimated for the upper crust surrounding Hellas basin. Four isolated magnetic anomalies are modeled by vertical prisms with circular cross sections of 170‐ to 190‐km radius and 20‐ to 30‐km thickness. The magnetization of the model prisms is 5 A/m more than that of the surroundings.
Icelandic pseudocraters as analogs to some volcanic cones on Mars
Authors: Greeley Ronald , Fagents Sarah A.
Published: February 2004

Pseudocraters are rootless vents formed by the interaction of lava flows with surface or near‐surface water. This interaction can produce mild explosions and the accumulation of scoria and spatter into small constructs. Pseudocraters in several localities in Iceland were examined in the field and compared to similar appearing features observed on Mars. The Icelandic pseudocrater cones in this study range in size from 6 to 70 m in diameter, have summit craters which range from 2 to 28 m in diameter (many cones lack craters entirely), and have flanks that are either concave‐up or convex‐up. The size and spacing of Icelandic pseudocraters might be a function of the availability of water, in which larger, closely spaced features result from efficient lava‐water interaction, as suggested by the environments in which the features formed. Possible Martian pseudocrater cones in Amazonis Planitia range in diameter from 30 to 180 m and have craters 12 to 80 m in diameter. A numerical model for volcanic explosions was adapted to study the formation of pseudocraters under terrestrial and Martian conditions. The results suggest that explosions forming Martian cones require significantly less water (calculated masses are less by a factor of 4 to 16) than those forming Icelandic pseudocraters, despite their larger sizes. This is attributed to the low gravity and atmospheric pressure in the Mars environment and is consistent with the likely lower abundance of water, which might be present as interstitial ice at shallow depths in the regolith. Locations of potential pseudocraters on Mars at latitudes as low as −8°N, imply the presence of crustal ice stores at the time of their formation.
Crossover analysis of Mars Orbiter Laser Altimeter data
Authors: Neumann Gregory A. , Rowlands David D. , Lemoine Frank G. , Smith David E. , Zuber Maria T.
Published: February 2004

In its first 15 months of continuous operation, the Mars Orbiter Laser Altimeter (MOLA) instrument aboard Mars Global Surveyor ranged to Mars over 330 million times, generating more than 5000 orbital profiles, with a ranging precision of 0.4 m over smooth terrain. The accuracy of the profiles depends on knowledge of the spacecraft position, orientation, and observation time, which are subject to errors. We model these errors via the analysis of over 24 million altimetric crossovers. A quasiperiodic, once per revolution adjustment of the ground tracks as a function of time in three locally orthogonal directions minimizes the altimetric residuals via least‐squares. Using a sparse matrix technique, computational effort scales linearly with the number of crossovers and only marginally with the number of parameters. Orbital errors mainly result from poor modeling of spacecraft thrusting events in the absence of tracking. Seasonal effects, likely due to changing thermal environment, as well as residual miscalibrations, are evident in the pointing solutions. Incorporating multiple parameters per revolution significantly improves crossover residuals, and resolves pointing aberrations during orbital transitions from night to day. Altimetry from the adjusted tracks generates a topographic model whose accuracy is typically better than 1 m vertically with respect to the center of mass of Mars. The centroid position of each MOLA shot is typically accurate to ∼100 m horizontally. Terrain models from accurately located lidar data can be gradient‐shaded to illuminate geological structures with 1 in 1000 slopes that are invisible to cameras. Temporal changes in elevation (e.g., frost deposition/ablation) at decimeter levels may also be assessed using crossovers, but results must be interpreted with caution due to uncertainties in range walk correction.
Lunar Mg suite: Thermobarometry and petrogenesis of parental magmas
Authors: McCallum I. S. , Schwartz J. M.
Published: February 2004

Using compilations of internally consistent thermodynamic data on minerals and advances in quantifying the energetics of crystalline solid solutions, we have recalculated the pressures and temperatures of equilibration of troctolite sample 76535 and a number of spinel cataclasites. The five phase assemblage (olivine + orthopyroxene + clinopyroxene + Cr‐spinel + plagioclase), which occurs in symplectite assemblages in 76535, gives equilibration temperatures from 800°–900°C and pressures from 200 to 250 MPa, indicating a deep (40–50 km) crustal source. Spinel cataclasite assemblages give minimum pressures of ∼100 MPa. Mg suite magmas were apparently intruded throughout the lunar crust early in lunar history. The textures and equilibration temperatures in many Mg suite samples are consistent with subsolidus recrystallization and the Mg‐rich minerals of some refractory troctolites and dunites may be residues of a crustal melting process or products of subsolidus elemental redistribution. The Sm‐Nd age of 4.26±0.06 Ga of troctolite 76535 most likely represents the time of subsolidus recrystallization and/or partial melting. In an attempt to identify the primary source material for the Mg suite magmas, a variety of models were tested using the MELTS and MAGFOX codes. A good fit between mineral compositions and crystallization sequences is obtained by ∼20% batch melting at 0.5 GPa of an average whole Moon composition with an Mg # of 0.85. Melts formed by batch melting at higher pressures and melts formed by polybaric partial fusion over the pressure range from 2.0 to 0.3 GPa were either too poor in Mg and/or crystallized clinopyroxene before orthopyroxene. Mixing with 10–20% urKREEP, which is required to satisfy the REE constraints of the Mg suite, reduces the Mg # of the parental magmas and requires a whole Moon Mg # closer to that of the terrestrial mantle.
Magnetic field of Mars: Summary of results from the aerobraking and mapping orbits
Authors: Acuña M. H. , Connerney J. E. P. , Wasilewski P. , Lin R. P. , Mitchell D. , Anderson K. A. , Carlson C. W. , McFadden J. , Rème H. , Mazelle C. , Vignes D. , Bauer S. J. , Cloutier P. , Ness N. F.
Published: February 2004

The Mars Global Surveyor (MGS) Magnetic Field Investigation was designed to provide fast vector measurements of the ambient magnetic field in the near‐Mars environment and over a wide dynamic range. The fundamental objectives of this investigation were to (1) establish the nature of the magnetic field of Mars; (2) develop appropriate models for its representation; and (3) map the Martian crustal remanent field (if one existed) to a resolution consistent with the spacecraft orbit altitude and ground track separation. Important and complementary objectives were the study of the interaction of Mars with the solar wind and of its ionosphere. The instrumentation is a synergistic combination of a twin‐triaxial, fluxgate magnetometer system and an electron reflectometer. The twin‐magnetometer system allows the real‐time detection of spacecraft‐generated fields, while the electron reflectometer adds remote magnetic field sensing capabilities as well as information about the local electron population. After Mars orbit injection in September 1997 and through the aerobraking (AB) and science‐phasing orbits (SPO) that followed, observations were acquired from more than 1000 elliptical orbits with periapses ranging from 85 to 170 km above Mars' surface. Following injection into the final ∼400 km altitude circular‐mapping orbit, data have been acquired from more than 6000 orbits in the fixed 0200–1400 local time plane. Major results obtained so far by the Magnetometer/Electron Reflectometer (MAG/ER) investigation in the course of the mission include (1) the determination that Mars does not currently possess a magnetic field of internal origin (dynamo), (2) the discovery of linear, strongly magnetized regions in its crust, closely associated with the ancient, cratered terrain of the highlands in the southern hemisphere, and (3) multiple magnetic “cusps” that connect the crustal magnetic sources to the Martian tail and shocked solar wind plasma. The solar wind interaction with Mars is therefore similar in many ways to that at Venus and at an active comet, primarily an ionospheric/atmospheric interaction. A comet‐like “magnetic pileup” region and boundary develop that stand off the solar wind, and mass loading by pickup ions of planetary origin plays an important role in defining interaction regions and overall geometry. This paper focuses primarily on the results obtained by the magnetometer (MAG) portion of the investigation during the MGS aerobraking, science‐phasing, and mapping orbits. A companion paper on this issue summarizes the results obtained from the Electron Reflectometer (ER) sensor.
Paterae on Io: A new type of volcanic caldera?
Authors: Radebaugh Jani , Keszthelyi Laszlo P. , McEwen Alfred S. , Turtle Elizabeth P. , Jaeger Windy , Milazzo Moses
Published: February 2004

Paterae, defined by the International Astronomical Union as “irregular crater[s], or complex one[s] with scalloped edges,” are some of the most prominent topographic features on Io. Paterae on Io are unique, yet in some aspects they resemble calderas known and studied on Earth, Mars, and Venus. They have steep walls, flat floors, and arcuate margins and sometimes exhibit nesting, all typical of terrestrial and Martian basalt shield calderas. However, they are much larger, many are irregular in shape, and they typically lack shields. Their great sizes (some >200 km diameter) and lack of associated volcanic edifices beg comparison with terrestrial ash flow calderas; however, there is no convincing evidence on Io for the high‐silica erupted products or dome resurgence associated with this type of caldera. Ionian paterae seem to be linked with the eruption of large amounts of mafic to ultramafic lavas and colorful sulfur‐rich materials that cover the floors and sometimes flow great distances away from patera margins. They are often angular in shape or are found adjacent to mountains or plateaus, indicating tectonic influences on their formation. A database of 417 paterae on Io measured from images with <3.2 km pixel−1 resolution (80% of its surface) reveals that their mean diameter of 41.0 km is close to that for calderas of Mars (47.7 km), is smaller than that for Venus (∼68 km), but dwarfs those for terrestrial basalt shield calderas (6.6 km) and ash flow calderas (18.7 km). Thirteen percent of all paterae are found adjacent to mountains, 42% have straight or irregular margins, and 8% are found atop low shields. Abundant, smaller paterae with more continuously active lava eruptions are found between 25°S and 25°N latitude, whereas fewer and larger paterae are found poleward of these latitudes. Patera distribution shows peaks at 330°W and 150°W longitude, likely related to the direction of greatest tidal massaging by Jupiter. Ionian patera formation may be explained by portions or combinations of models considered for formation of terrestrial calderas, yet their unusual characteristics may require new models with a greater role for tectonic processes.
The mountains of Io: Global and geological perspectives from Voyager and Galileo
Authors: Schenk Paul , Hargitai Henrik , Wilson Ronda , McEwen Alfred , Thomas Peter
Published: February 2004

To search for local and global scale geologic associations that may be related to the internal dynamics of Io, we have completed a global catalog of all mountains and volcanic centers. We have identified 115 mountain structures (covering ∼3% of the surface) and 541 volcanic centers, including paterae (calderas and dark spots) and shield volcanoes. The average length of an Ionian mountain is 157 km, with the longest being 570 km. The mean height of Ionian mountains is 6.3 km, and the highest known mountain is Boösaule Montes (17.5±3 km). Five basic morphologic types of mountains have been identified; mesa, plateau, peak, ridge, and massif. Very few mountains bear any physical similarity to classic volcanic landforms, but many resemble flatiron mountains on Earth and are interpreted as tilted crustal blocks. This would be consistent with the hypothesis that most mountains are thrust blocks formed as a result of compressive stresses built up in the lower crust due to the global subsidence of volcanic layers as they are buried over time [Schenk and Bulmer, 1998]. More than one mechanism may be responsible for all Ionian mountains, however. The proximity of some mountains to paterae may indicate a direct link between some mountains and volcanism, although it is not always clear which came first. In contrast to earlier studies, a pronounced bimodal pattern is observed in the global distribution of both mountains and volcanic centers. The regions of highest areal densities of volcanic centers are near the sub‐ and anti‐Jovian regions, but are offset roughly 90° in longitude from the two regions of greatest concentration of mountains. This anticorrelation may indicate the overprinting of a second stress field on the global compressive stresses due to subsidence. The bimodal distribution of volcanic centers and mountains is consistent with models of asthenospheric tidal heating and internal convection developed by Tackley et al. [2001]. Over regions of mantle upwelling, compressive stresses in the lower crust induced by global subsidence might be reduced, encouraging volcanism and discouraging mountain building. In regions of mantle downwelling, these compressive stresses in the lower crust might be increased, discouraging volcanism and encouraging mountain building. Alternatively, the global pattern may be related to possible (but undocumented) nonsynchronous rotation of Io, which would produce two regions each of compression and extension in the crust. Evidence of layering and of mass wasting, including landslides, block sliding, debris aprons and downslope creep, on Ionian mountains suggests that the crust of Io is essentially a layered stack of partially consolidated volcanic lavas and plume deposits, becoming more consolidated with depth. The lower crust especially may also be ductily deformed, punctuated by volcanic intrusions and faulting at paterae, and broken into blocks, some of which have been uplifted to form mountains.
Density structure of the upper thermosphere of Mars from measurements of air drag on the Mars Global Surveyor spacecraft
Authors: Tracadas Philip W. , Zuber Maria T. , Smith David E. , Lemoine Frank G.
Published: February 2004

We present measurements of the density of the Martian atmosphere between 170‐ and 180‐km altitude above the high northern latitudes over a 6‐month period in 1998, when the solar cycle was beginning to rise out of its activity minimum. These measurements were made from the observed orbital decay of the Mars Global Surveyor (MGS) spacecraft during its Science Phasing Orbits (SPO) (April to September 1998) using X band Doppler tracking observations. The densities that we retrieve are comparable to model values given by Culp and Stewart [1984], Stewart [1987], Mars‐GRAM 3.7 [Justus et al., 1996], and recent Mars Thermospheric Global Circulation Model (MTGCM) simulations [Bougher et al., 2000]. However, the SPO period can be divided into two time periods (separated at Ls ≈ 355°∼0°) that are characterized by significantly different orbit‐to‐orbit variability that is not predicted by these earlier models. The first time period corresponds to the time during which the MGS orbit perifocus moved toward the north pole while the local solar time was 1000–1100; during this period, orbit‐to‐orbit variability is 50–70%, and our average measured density at 175 km is 0.018±0.007 kg km−3 (between 67° and 72°N and Ls = 315° to 320°). The second time period corresponds to the time during which the orbit perifocus moved south from the north pole and the local time was 1700–1730; during this period, orbit‐to‐orbit variability is 40–20%, and our average measured density at 175 km is 0.024±0.004 kg km−3 (between 62° and 69°N and Ls = 17° to 28°). For both time periods the observed latitudinal gradient of density on a constant altitude surface exhibited a factor of 3–4 decrease between 60° and 90°N. This gradient is comparable to that expected by the polar vortex (high‐latitude wind) effect modeled by the MTGCM for solar medium conditions at southern summer solstice [Bougher et al., 2000]. A southern hemisphere dust storm that the MGS Thermal Emission Spectrometer (TES) observed at Ls = 309° is distinguishable in our data set as a 100% rise in density at 180 km above the 70° northern latitudes 7 days later (Ls = 313°).
Estimation of volcanic eruption conditions for a large flank event on Elysium Mons, Mars
Authors: Wilson Lionel , Mouginis‐Mark Peter J.
Published: February 2004

Mars Orbiter Camera image (5.23 m/pixel) and topographic data from the Mars Orbiter Laser Altimeter data are used to analyze a channel on the upper northern flank of the Elysium Mons volcano, Mars. We show that this channel is best explained as the product of the thermal erosion of the preexisting surface by a high‐speed, turbulent lava flow fed from a fissure eruption. We deduce the volume eruption rate (∼3400–13,000 m3/s dense rock equivalent), the total volume of erupted lava (∼29–39 km3 dense rock equivalent), and the duration of the activity (40–130 days). Perhaps significantly, the erupted lava volume is essentially identical, given the uncertainties in the analysis, to the volume of the observed caldera (∼31 km3), which has implications for volume‐limited eruptions on Mars and potentially for the unusual steep upper slopes of the volcano. The eruption that produced the channel appears to have been typical for Elysium Mons and implies that the other lava channels on the upper flank of the volcano may also have taken ∼1–4 months to have formed.
Growth mechanism and additional constraints on FeNi metal condensation in the solar nebula
Authors: Meibom Anders , Petaev Mikhail I. , Krot Alexander N. , Keil Klaus , Wood John A.
Published: February 2004

Chemically zoned FeNi metal grains in the metal‐rich chondrites QUE 94411 and Hammadah al Hamra 237 formed by gas‐solid condensation in the temperature range from ∼1500 to 1400 K during highly energetic thermal events in the solar nebula. We observe a linear correlation between the apparent diameter of the zoned FeNi metal grains and their inferred condensation temperature interval, which indicates that the grain growth rate was essentially constant. This lends strong support for a kinetic “hit‐and‐stick” growth model that yields growth timescales of ∼20–85 hours and gas cooling rates of ∼1–2 K h−1 for six representative zoned metal grains studied in QUE 94411. In the core regions of the zoned metal grains the Ni concentration is systematically lower than the thermodynamically predicted values, suggesting that solid‐state diffusion played an important role in shaping the zoning profiles. Combined with existing data, our observations provide a set of constraints on the physics and chemistry of large‐scale, high‐temperature processes in the earliest solar nebula, which present astrophysicists with profound challenges.
Late Miocene‐early Pliocene biogenic bloom: Evidence from low‐productivity regions of the Indian and Atlantic Oceans
Authors: Hermoyian Casey S. , Owen Robert M.
Published: February 2004

Studies of the late Miocene‐early Pliocene biogenic bloom typically have focused on high‐productivity areas in the Indian and Pacific Oceans in order to achieve high resolution samples. Thus there is a paucity of information concerning whether the Atlantic Ocean, in general or low‐productivity regions in all three basins experienced this bloom. This study measured the phosphorus mass accumulation rate (PMAR). in five cores from low‐productivity regions of the Atlantic and Indian Oceans. All cores exhibit a peak in productivity ∼4–5.5 Ma, coincident with the Indo‐Pacific bloom. This suggests that nutrients were not shifted away from low‐productivity regions nor out of the Atlantic Ocean. Instead, it appears that the bloom was caused by an overall increase in nutrient flux into the world oceans. Four of the cores record the bloom's PMAR peak as bimodal, indicating a pulsed increase in phosphorus to the oceans. This suggests that there may have been multiple causes of the biogenic bloom.
Numerical paleoceanographic study of the Early Jurassic Transcontinental Laurasian Seaway
Authors: Bjerrum Christian J. , Surlyk Finn , Callomon John H. , Slingerland Rudy L.
Published: February 2004

The forces governing marine circulation of a meridional transcontinental seaway is explored with the Princeton Ocean Model. The Jurassic Laurasian Seaway, which connected the low‐latitude Tethys Ocean with the Arctic Sea is modeled quantitatively. The global ocean is found to have a profound influence on seaway dynamics. A north‐south density difference and hence sea level difference of the global ocean was probably the main factor in forcing the seaway flow. When the Tethys waters were the denser water, the net seaway flow was southward, and conversely, it was northward for denser Arctic waters. Marine bioprovincial boundaries and sediment data indicate that the seaway probably was dominated by Boreal faunal groups and reduced salinities several times in the Jurassic. The model results suggest that this can be explained by southward flowing seaway currents, which may have been related to an oceanic thermohaline circulation where no northern high‐latitude deep convection occurred.
An osmium isotope excursion associated with the Late Paleocene thermal maximum: Evidence of intensified chemical weathering
Authors: Ravizza G. , Norris R. N. , Blusztajn J. , Aubry M. ‐P.
Published: February 2004

In the latest Paleocene an abrupt shift to more negative δ13C values has been documented at numerous marine and terrestrial sites [Bralower et al., 1997; Cramer et al., 1999; Kaiho et al., 1996; Kennett and Stott, 1991; Koch et al., 1992; Stott et al., 1996; Thomas and Shackleton, 1996; Zachos et al., 1993]. This carbon isotope event (CIE) is coincident with oxygen isotope data that indicate warming of surface waters at high latitudes of nearly 4°–6°C [Kennett and Stott, 1991] and more moderate warming in the subtropics [Thomas et al., 1999]. Here we report 187Os/188Os isotope records from the North Atlantic and Indian Oceans which demonstrate a >10% increase in the 187Os/188Os ratio of seawater coincident with the late Paleocene CIE. This excursion to higher 187Os/188Os ratios is consistent with a global increase in weathering rates. The inference of increased chemical weathering during this interval of unusual warmth is significant because it provides empirical evidence supporting the operation of a feedback between chemical weathering rates and warm global climate, which acts to stabilize Earth's climate [Walker et al., 1981]. Estimates of the duration of late Paleocene CIE [Bains et al., 1999; Bralower et al., 1997; Norris and Röhl, 1999; Röhl et al., 2000] in conjunction with the Os isotope data imply that intensified chemical weathering in response to warm, humid climates can occur on timescales of 104–105 years. This interpretation requires that the late Paleocene thermal maximum Os isotope excursion be produced mainly by increased Os flux to the ocean rather than a transient excursion to higher 187Os/188Os ratios in river runoff. Although we argue that the former is more likely than the latter, we cannot rule out significant changes in the 187Os/188Os ratio of rivers.
Numerical models of diagenesis, sediment properties, and pore fluid chemistry on a paleoceanographic transect: Blake Nose, Ocean Drilling Program Leg 171B
Authors: Rudnicki Mark D. , Wilson Paul A. , Anderson William T.
Published: February 2004

A critical issue in many paleoceanographic studies is the potential of diagenetic processes to compromise the fidelity of geochemical proxies (e.g., δ18O) in deep‐sea carbonate sediments and microfossils. A recently established strategy for dealing with this issue is to drill depth transects in anomalously shallowly buried and therefore presumably well‐preserved target sediments. Here we report the results of an experiment designed to test the validity of this approach. We apply a simple one‐dimensional numerical model to complex pore water chemical profiles (for Ca, Mg, Sr, 87Sr/86Sr, and δ18O) observed in a shallowly buried paleoceanographic transect of paleogene and Cretaceous age drilled on Blake Nose during Ocean Drilling Program Leg 171B. We show that it is possible to reproduce these complex pore water profiles simply by invoking (1) a deep‐seated zone of chemical reaction well below the recovered sections and (2) minimal chemical alteration within the overlying recovered sediments but significant restriction of advective flow by embedded low‐porosity layers. These results suggest that the sediments recovered during Ocean Drilling Program Leg 171B can be considered to be virtually chemically inert at their present burial depth (their carbonate recrystallization histories having been largely limited to events during early, ∼10 Ma timescale, burial. These findings support the strategy of selecting unusually shallowly buried sections for paleoceanographic transect drilling. Ironically, however, the very attraction of targeting such sections (the lack of overlying sediments) dictates that it is not possible to use pore water chemistry to constrain the early diagenetic histories of such sections because of the diffusion of the pore water signal through the sediment‐water interface. Diffusional loss may also account for the absence of pore water chemical anomalies associated with the ooze‐chalk transition at Blake Nose. However, our work suggests that traditional interpretations of this boundary as the inferred locus of maximum rates of carbonate recrystallization require reevaluation.
Variations in atmospheric CO2 during glacial cycles from an inverse ocean modeling perspective
Authors: LeGrand P. , Alverson K.
Published: February 2004

An inverse ocean box modeling approach is used to address the question of what may have caused decreased atmospheric CO2 concentration during glacial periods. The inverse procedure seeks solutions that are consistent, within prescribed uncertainties, with both available paleo observations and box model conservation equations while relaxing traditional assumptions such as exact steady state and precise prescription of uncertain model parameters. Decreased ventilation of Southern Ocean deep water, decreased Southern Ocean air‐sea gas exchange, and enhanced high‐latitude biological pumping are all shown to be individually capable of explaining available paleodata constraints provided that significant calcium carbonate compensation is allowed. None of the scenarios require more than a very minor (order 1°C) glacial reduction in low and midlatitude sea surface temperature although scenarios with larger changes are equally plausible. One explanation for the fairly wide range of plausible solutions is that most paleodata directly constrain the inventory of paleotracers but only indirectly constrain their fluxes. Because the various scenarios that have been proposed to explain pCO2 levels during the last glacial maximum are distinguished primarily by different fluxes, the data do not allow one to confidently chose between them.
The dynamics of ocean heat transport variability
Authors: Jayne Steven R. , Marotzke Jochem
Published: February 2004

The north‐south heat transport is the prime manifestation of the ocean's role in global climate, but understanding of its variability has been fragmentary owing to uncertainties in observational analyses, limitations in models, and the lack of a convincing mechanism. We review the dynamics of global ocean heat transport variability, with an emphasis on timescales from monthly to interannual. We synthesize relatively simple dynamical ideas and show that together they explain heat transport variability in a state‐of‐the‐art, high‐resolution ocean general circulation model. Globally, the cross‐equatorial seasonal heat transport fluctuations are close to ±3 × 1015 W, the same amplitude as the cross‐equatorial seasonal atmospheric energy transport. The variability is concentrated within 20° of the equator and dominated by the annual cycle. The majority of the variability is due to wind‐induced current fluctuations in which the time‐varying wind drives Ekman layer mass transports that are compensated by depth‐independent return flows. The temperature difference between the mass transports gives rise to the time‐dependent heat transport. It is found that in the heat budget the divergence of the time‐varying heat transport is largely balanced by changes in heat storage. Despite the Ekman transport's strong impact on the time‐dependent heat transport, the largely depth‐independent character of its associated meridional overturning stream function means that it does not affect estimates of the time‐mean heat transport made by one‐time hydrographic surveys. Away from the tropics the heat transport variability associated with the depth‐independent gyre and depth‐dependent circulations is much weaker than the Ekman variability. The non‐Ekman contributions can amount to a 0.2–0.4 × 1015 W standard deviation in the heat transport estimated from a one‐time hydrographic survey.
Snow on Antarctic sea ice
Authors: Massom Robert A. , Eicken Hajo , Hass Christian , Jeffries Martin O. , Drinkwater Mark R. , Sturm Matthew , Worby Anthony P. , Wu Xingren , Lytle Victoria I. , Ushio Shuki , Morris Kim , Reid Phillip A. , Warren Stephen G. , Allison Ian
Published: February 2004

Snow on Antarctic sea ice plays a complex and highly variable role in air‐sea‐ice interaction processes and the Earth's climate system. Using data collected mostly during the past 10 years, this paper reviews the following topics: snow thickness and snow type and their geographical and seasonal variations; snow grain size, density, and salinity; frequency of occurrence of slush; thermal conductivity, snow surface temperature, and temperature gradients within snow; and the effect of snow thickness on albedo. Major findings include large regional and seasonal differences in snow properties and thicknesses; the consequences of thicker snow and thinner ice in the Antarctic relative to the Arctic (e.g., the importance of flooding and snow‐ice formation); the potential impact of increasing snowfall resulting from global climate change; lower observed values of snow thermal conductivity than those typically used in models; periodic large‐scale melt in winter; and the contrast in summer melt processes between the Arctic and the Antarctic. Both climate modeling and remote sensing would benefit by taking account of the differences between the two polar regions.
Diurnal and seasonal variability of turbulence parameters observed with Indian mesosphere‐stratosphere‐troposphere radar
Authors: Rao D. Narayana , Rao T. Narayana , Venkataratnam M. , Thulasiraman S. , Rao S. V. B. , Srinivasulu P. , Rao P. B.
Published: February 2004

Seasonal and diurnal variation of turbulence parameters such as refractivity structure constant Cn2 and eddy dissipation rate ε is presented using the data collected with the Indian mesosphere‐stratosphere‐troposphere (MST) radar over 3 years. The log Cn2 values estimated from signal‐to‐noise ratio (SNR) are found to be in the range of −17 to −19 m−2/3 in the height range of 7.5–21 km. Monthly mean values of log Cn2 show a maximum variation below 12 km with a magnitude of 12–15 dB during the course of annual cycle. A maximum variability of ∼7–10 dB is observed below 12 km in seasonal mean values of log Cn2. The diurnal variation of log Cn2 at different heights is also given. Cn2 is found to be more in the region of strong shears, generally observed in the boundaries of jet streams. Different methods for the estimation of eddy dissipation rate and their limitations are discussed. For the present study, spectral width method is used after correcting the observed spectral width from beam and shear broadening effects. The observed median log ε is on the order of −3 to −4 m2 s−3. Monthly variation of log ε is found to be ∼5–7 dB. Below 10 km the magnitude of ε is more in the postmonsoon than that observed in other seasons. The interannual variation of ε is less in winter than in other seasons. The diurnal variation of log ε is found to be small in postmonsoon at most of the heights. To facilitate a comparison with the other results, we have estimated the eddy diffusivity K and the inner and outer scales of turbulence. The observed values of Cn2, ε,K, and the inner and outer scales of turbulence are largely consistent with the results available in the literature.
K factor statistics for subrefraction in the mid‐Atlantic coast of the United States
Authors: Goldhirsh Julius , Dockery G. Daniel
Published: February 2004

Statistics of “equivalent K factor” are derived for the mid‐Atlantic coast of the United States. These statistics are associated with subrefraction using 3 years of near‐continuous line‐of‐sight link signal measurements at 4.7 GHz in the mid‐Atlantic coast of the United States. Probabilities are derived for a family of threshold levels of K factors ranging from 0.6 to 1.0 which are sustained over durations exceeding 1–5 hours. An analytical model is derived that characterizes this family of curves with excellent accuracy. It is, for example, demonstrated that for 6% of the average year the K factor is smaller than 1 for durations greater than 1 hour in the mid‐Atlantic coast of the United States. Caveats that should be considered in applying these results are reviewed. Monthly probabilities of K factor statistics reveal that the winter and early spring months dominate. For example, during February of year 1 and December of years 2 and 3 the K factors were smaller than 0.8 for periods exceeding 2 hours at the probabilities of 11.5%, 12.0%, and 11.5%, respectively. This result is consistent with the fact that cold water conditions and warm overlying moist air are more prevalent during the winter season. These conditions represent the correct meteorological ingredients for extreme subrefraction to occur. Annually, the K factor was smaller than 0.8 with durations greater than 1 hour at a probability of 3.2%.
An interesting phenomenon in the finite difference time domain simulation of a Gaussian pulse propagation in a waveguide
Authors: Dou W. B. , Yung Edward K. N.
Published: February 2004

In this paper, an interesting phenomenon is revealed in the finite difference time domain (FDTD) simulation of a Gaussian pulse propagation in a rectangular waveguide. It is found that when a Gaussian pulse is used in a rectangular waveguide, an electric field peak will occur at the cutoff frequency of the dominant mode of the waveguide even when the FDTD simulation is implemented in the absence of nonlinear material. It appears that the spectrum power at the zero frequency of the Gaussian pulse is transformed into the spectrum power at the cutoff frequency of the dominant mode. The cause is investigated, and an explanation based on the Poynting theorem is presented.
Comparison of VLF sferics intensities at Halley, Antarctica, with tropical lightning and temperature
Authors: Watkins N. W. , Bharmal N. A. , Clilverd M. A. , Smith A. J.
Published: February 2004

We compare measurements for 1998 of VLF sferics‐generated noise power at Halley, Antarctica, with the predictions of the long wave noise prediction (LNP) noise model. We compare the highest‐ranked predicted sources of radio noise for Halley from LNP with the observed lightning flash rates in January and July 1998 observed by the Lightning Imaging Sensor (LIS) instrument on the Tropical Rainfall Measurement Mission (TRMM). In both cases we find better agreement between LNP and the measurements in southern winter than those in southern summer. We compare an anomaly time series derived from VLF power with an analogous temperature anomaly series. We find evidence of a change in measured power in 1998 compared to previous years, suggesting that the tropical temperature signal of the strong El Niño of 1997–1998 may be detectable in VLF power measurements at Halley. Assuming that such an event and a single temperature station can be used to calibrate VLF measurements as a “tropical thermometer”, we confirm previous estimates that long‐term changes in tropical noise measured at Halley are consistent with a tropical temperature rise of the order of ∼ 1°C over 25 years.
Incoherent scatter experiments at Jicamarca using alternating codes
Authors: Hysell D. L.
Published: February 2004

An incoherent scatter experiment using alternating coded pulses has been implemented at Jicamarca. The experiment is intended to facilitate aeronomy research in the topside and protonosphere. By virtue of utilizing the full duty cycle capabilities of the radar transmitter, the experiment has improved sensitivity over the conventional double pulse experiment and permits multiple parameter estimation at protonospheric altitudes. Error analysis, including the estimation of the complete error covariance matrix for the alternating coded pulses, is discussed. Example density, temperature, and composition data illustrate the capabilities of the new experiment.
On the Laws‐Parsons distribution of raindrop sizes
Authors: de Wolf David A.
Published: February 2004

A new analytical function is given for the distribution density of raindrop diameters implied by the measured Laws‐Parsons distribution of the volume fraction of water in rain. It yields results that differ importantly from those obtained via the Marshall‐Palmer exponential function of drop diameter in the millimeter range of radar wavelengths.
Low‐frequency electric field induced in a spherical cell during nuclear and cytoplasmic division
Authors: King Ronold W. P.
Published: February 2004

The electric fields induced in the interior and in the membrane of a spherical cell when exposed to an incident low‐frequency electric field are determined before, during, and after the cell divides into two daughter cells. It is shown that when the incident electric field is along the axis of division, the induced field in the cell increases in magnitude by a factor greater than 3, and that it decreases to the value in the parent cell when the daughter cells are separated by a distance between centers that exceeds 4 times the radius of the cells. Possible biological effects of the induced electric field at the several stages of division are mentioned but not determined.
Concepts, observations, and simulation of refractive index turbulence in the lower atmosphere
Authors: Wyngaard John C. , Seaman Nelson , Kimmel Shari J. , Otte Martin , Di Xaio , Gilbert Kenneth E.
Published: February 2004

Advances in computers and in computational techniques now allow the calculation of electromagnetic (EM) wave propagation through simulated refractive index turbulence in the lower atmosphere. Such applications call for instantaneous turbulence fields, not turbulence statistics, the traditional focus of the turbulence community. We clarify their important differences and review what is known about key statistics of refractive index turbulence. We discuss the calculation of EM propagation with a parabolic equation model that uses composite refractive index fields, the larger scales being calculated with a dynamical mesoscale model and the smaller scales being calculated through large‐eddy simulation. The locally, instantaneously sharp top of the atmospheric boundary layer can have a profound effect on forward scatter of EM waves. This top appears to be even sharper than is revealed by conventional measurements, particularly in the convective boundary layer.
First observations of HF heater‐produced airglow at the High Frequency Active Auroral Research Program facility: Thermal excitation and spatial structuring
Authors: Pedersen Todd R. , Carlson Herbert C.
Published: February 2004

HF heater‐produced artificial airglow emissions at 630.0 nm were detected at the High Frequency Active Auroral Research Program (HAARP) ionospheric research facility near Gakona, Alaska (62.39° N 145.15° W), for the first time in March 1999. HF excitation was identified by virtue of two measurements: A region of emissions enhanced 50‐60 R above the background of ∼100 R was observed in the approximate region illuminated by the HF heater beam; and the rise and decay of the airglow followed the HF transmitter on/off cycle with time constants of approximately 80 s and 50 s, respectively. The observations were made in close proximity to the natural aurora, which gradually moved southward over the site during the experiment and eventually overwhelmed the much weaker artificial airglow. Significant structure in the airglow region was observed, including an overall equatorward displacement relative to the nominal transmitter beam, preferential occurrence of intensity maxima along the overhead field line up to and including magnetic zenith, and periods of elongation of the airglow region along the magnetic meridian and later parallel to the encroaching auroral zone. We discuss potential sources of this structuring of the emission shape including auroral plasma transport and neutral winds. Transmitter frequencies of 3.1–3.3 MHz matched the ionospheric plasma frequency near the F region peak at high altitudes of 300–350 km. We identify the primary mechanism behind excitation of the 630.0 nm airglow as thermal electron excitation on the basis of this evidence, along with thermal balance arguments, further substantiated by the observed asymmetry in rise and decay times and a lack of detectable emissions at 557.7 nm.
A technique for using measured ionospheric density gradients and GPS occultations for inferring the nighttime ionospheric electron density
Authors: Dymond K. F. , Thomas R. J.
Published: February 2004

We present a technique for using the measured variations of ultraviolet emissions produced by radiative recombination at 911 or 1356 Å in conjunction with occultations of Global Positioning System (GPS) satellites observed from a low Earth‐orbiting satellite to infer the two‐dimensional structure of the nighttime ionosphere. We present the results of simulations that demonstrate the applicability and accuracy of the algorithm and observational concept. Our technique uses UV nadir radiances to infer the electron density gradients and total electron content measurements at the limb from an onboard GPS receiver to infer the altitude versus latitude structure of the ionosphere. To test our technique, we simulated “data” in a region of high gradient using the international reference ionosphere (IRI‐90; Bilitza [1990]) to generate the ionosphere. In the simulation the satellite was assumed to be in a polar orbit at 750 km, the proposed altitude for the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) satellites. We simulated the limb total electron content (TEC) and the nadir radiances and superimposed noise on these “data.” These data were then inverted using a newly developed algorithm that assumes a constant altitude variation of the electron density with latitude but scales this profile shape by the peak electron density, which varies with latitude. We find that our technique more accurately determines the electron density than direct inversion of the TEC data using Abel inversion techniques.
Bimodality in the climatological topside electron and exospheric temperature distributions at Arecibo
Authors: Meléndez‐Alvira D. J. , Picone J. M. , Kelley O. A. , Zhou Q. , Sulzer M. P.
Published: February 2004

We report on the local time, occurrence frequency, and solar activity variations of the measured electron temperature and the ratio of the electron to ion temperatures at 589 km, and the inferred exospheric neutral temperatures. The World Day measurements at Arecibo, Puerto Rico, were made between 1985 and 1995, spanning both solar maximum and minimum conditions during predominantly quiet geomagnetic conditions. Histograms of the electron and exospheric temperatures reveal bimodal distributions. The electron temperature bimodality is primarily due to the sharp difference between daytime and nighttime temperatures whereas the exospheric temperature bimodality is primarily due to the difference between high and low solar activity. The nighttime exosphere and the electrons at 589 km are hotter by up to 400 K during high solar activity than during low solar activity. The daytime electrons at 589 km are, however, colder after 0800 LT during high solar activity than during low solar activity. The daytime ratio of electron to ion temperatures is also lower at high solar activity but not until after 1000 LT on average. Unlike the electron temperature at 589 km, the distribution of the electron to ion temperatures is unimodal.