bluuebunny/crossref_metadata_2025 huggingface dataset:
Retrieval of properties for semitransparent clouds from multispectral infrared imagery data
Authors: Lin Xijian , Coakley James A.
Published: February 2008

A method is presented for extracting the properties of clouds which may be semitransparent in the infrared from imagery data at infrared window wavelengths for 50‐ to 100‐km regions that contain single‐layered cloud systems. The scheme utilizes (1) differences in the optical properties of water and ice at the infrared window wavelengths, (2) the nonlinear relationship between emissivity and transmissivity when clouds are semitransparent, and (3) differences in the sensitivity of blackbody emission to temperature at the infrared window wavelengths. A prescription is given for using the constraints on the emitted radiances imposed by a single‐layered cloud system to retrieve the regional‐scale emission of the cloud layer where it becomes opaque and an index related to the effective droplet/ice particle size. Within the region, the prescription provides pixel‐scale retrievals of fractional cloud cover and 11‐μm emissivity (equivalently, liquid/ice water column amount). The retrieval scheme has obvious limitations. First, the scheme is restricted to single‐layered cloud systems. Such systems are frequently found on the 50‐ to 100‐km regional scale. Second, to perform the retrieval, some of the image pixels in the region must be overcast by semitransparent cloud. Third, the effective radius of the hydrometeors must be sufficiently small that the extinction and absorption cross sections at the different wavelengths are significantly different. For combinations of emission at 11 and 12 μm, the hydrometeors must have effective radii of less than 20 μm; for 3.7‐ and 12‐μm emissions, the hydrometeors must have effective radii of less than 60 μm; and for 8‐ and 12‐μm emissions, the hydrometeors must have radii of less than 200 μm (in the case of ice). Fourth, the retrieved effective radius is taken to be only an index of the hydrometeor size within the region being observed. Owing to the typical variability of hydrometeor size within mesoscale regions, the interpretation of emitted radiances as due to either broken cloud or semitransparent cloud is ambiguous. The retrieval scheme is illustrated through application to samples of 1‐ and 4‐km advanced very high resolution radiometer 3.7‐, 11‐, and 12‐μm observations.
Experiments revealing small impact of turbulence on the energy budget of the mesosphere and lower thermosphere
Authors: Lübken F.‐J. , Hillert W. , Lehmacher G. , von Zahn U.
Published: February 2008

We have measured a total of 17 in situ profiles of small‐scale density fluctuations (typical resolution: meters) in the lower thermosphere and upper mesosphere, which are used to derive turbulent parameters, such as the turbulent energy dissipation rate ε, the turbulent diffusion coefficient K, and the mean turbulent velocity wturb. The accuracy of the absolute numbers is unprecedented thanks to the very high spatial resolution and a recently improved data analysis procedure. Concentrating on the 12 flights which were performed during winter conditions at high latitudes (69°N), we find mean energy dissipation rates of 1–2 mW kg−1 in the lower mesosphere (<75 km) and 10–20 mW kg−1 in the upper mesosphere and lower thermosphere (<100 km). The corresponding heating rates are approximately 0.1 and 1 K d−l, respectively. These values are at least 1 order of magnitude smaller than most of the previous measurements and are also significantly smaller than typical values assumed in models. Our observations suggest that the heating effect of turbulence is negligible compared to the most prevailing terms of the heat budget. It can be shown by theoretical considerations involving the turbulent energy budget equation that cooling by turbulent heat conduction is also negligible if ε is small.
Stratospheric OH measurements with a far‐infrared limb observing spectrometer
Authors: Pickett H. M. , Peterson D. B.
Published: February 2008

The Far‐Infrared Limb Observing Spectrometer (FILOS) is an instrument designed to measure the hydroxyl radical (OH) and other chemical species in the upper atmosphere using limb emission in the far‐infrared region of the spectrum. FILOS uses three Fabry‐Perot etalons in series to obtain high spectral resolution near 101 cm−1 and 118 cm−1. The instrument concept, calibration, and atmospheric measurements are discussed.
Rock‐forming metals and Pb in modern Alaskan snow
Authors: Hinkley Todd K.
Published: February 2008

Metal concentrations in annual and subannual increments of snowpack from the accumulation zone of a south central Alaska glacier indicate that the deposition of Pb with and upon snow is decoupled from that of rock dusts. Rock dusts accumulate, apparently as dry deposition, on the topmost, exposed surfaces of snowpacks in spring and summer, whereas Pb does not. Pb concentration is elevated throughout the latest one third of an annual snowpack, whereas that of rock dusts is not. For whole‐year snowpacks, there is a generally sympathetic relationship among concentration of Pb, concentration of rock dust, degree of dominance of rock dusts over ocean solutes, and ferromagnesian character of the rock dusts; however, the fractional abundance of Pb in whole year samples may decrease when rock dust masses become large and/or when rock dusts dominate most strongly over salts. The metal suite chosen to characterize rock dusts and to distinguish them from ocean solutes gives detailed information about rock type of dust source areas and about the nature of the degraded rock products that are taken up, transported, and deposited by the atmosphere. Rock dusts are present at concentrations of only about 300 nanograms (ng) of dust per gram of snow in the Alaskan snowpacks. Concentrations of Pb in the Alaska snow samples are moderate, ranging from 0.1 to 0.3 ng Pb/g snow. This contrasts with larger Pb concentrations of 0.4 to 0.9 ng Pb/g snow in whole‐year snowpack samples from the Sierra Nevada, California; with similar to smaller concentrations from north and south Greenland of about 0.04 ng Pb/g snow or less, and about 0.2 ng Pb/g snow or less, respectively, and with much smaller concentrations from Antarctica, now believed to range from a minimum of about 0.001 to a maximum of 0.005 (or 0.01) ng Pb/g snow.
Mean winds and the semiannual oscillation in the mesosphere and lower thermosphere at Christmas Island
Authors: Palo S. E. , Avery S. K.
Published: February 2008

In this paper we analyze mesospheric wind data collected from August 1988 to November 1989 and January 1991 to December 1991 at Christmas Island (1.95°N, 157.30°W) using the Meteor Echo Detection and Collection system. A brief overview of the data analysis techniques used on these data sets and meteor arrival statistics are presented. The mean winds are computed for 80 to 100 km and compared with the CIRA(86) model of mesospheric equatorial mean winds. Examination of the zonal mean winds and a Fourier decomposition of the zonal wind field shows a strong semiannual oscillation (SAO). To determine the vertical characteristics of the zonal SAO, a least squares fit was computed at each height. The results show a semiannual oscillation that peaks in amplitude at 84 km and decreases with increasing height. The SAO at Christmas Island is compared with data collected at Ascension Island and Kwajalein Island and with the CIRA(86) model. This comparison shows good phase agreement between the sites and some variability in the amplitudes. To fully understand the SAO in the context of the theory presented by Delisi and Dunkerton (1988), the tropospheric mean winds and wind variances are computed to determine the extent of tropospheric gravity wave activity present. Finally, the amplitude and phase of the SAO in temperature are computed indirectly from the Christmas Island wind data and compared to temperature results from SME data. Our results show reasonable agreement below 87 km and a divergence in amplitude near 90 km. One probable reason for this difference is contamination of the SME data from the diurnal tide as noted by Garcia and Clancy (1990).
A possible role of galactic cosmic rays in chlorine activation during polar night
Authors: Müller Rolf , Crutzen Paul J.
Published: February 2008

Observations indicate that extremely low concentrations of HCl prevail in the stratosphere just after the end of polar night in Antarctica. This is mostly explained by the fast heterogeneous reactions of HCl with ClONO2 and HOCl. Just prior to polar night normal gas phase partitioning indicates that HOCl and ClONO2 constitute only about 30% of the inorganic chlorine reservoir, the major portion being HCl. Thus about 60% of the initial HCl would remain (after titration with HOCl and ClONO2) by the end of polar night. We suggest that further processing may be effected through the reaction of N2O5 with HCl on polar stratospheric cloud surfaces, the N2O5 being produced from NO radicals formed by the action of galactic cosmic rays. The efficiency of this process is controlled by the relative rate of the competing heterogeneous reaction of N2O5 with H2O. Moreover, galactic cosmic rays form OH radicals that further augment HCl processing. Through model calculations we show that this mechanism can considerably enhance the conversion of HCl into active chlorine, leading to an almost 50% lower HCl concentration shortly after the end of polar night compared to cases where galactic cosmic rays were neglected.
Numerical investigation of the propagation of the quasi‐two‐day wave into the lower thermosphere
Authors: Hagan M. E. , Forbes J. M. , Vial F.
Published: February 2008

We present results of a series of numerical experiments for January conditions using a linearized spectral model which includes realistic mean winds and dissipation. These experiments were designed to characterize the propagation characteristics of the (3,0) mixed Rossby‐gravity mode through the middle atmosphere and into the lower thermosphere. Our results suggest that the wave magnitude is extremely sensitive to the zonal mean winds assumed in the calculations. In particular, our results suggest that a comparatively weak eastward stratomesospheric jet during northern hemisphere winter can account for a shift in the resonant frequency of the quasi‐two‐day wave response. Further, the inclusion of a realistic lower thermospheric jet in the summer lower thermosphere sets up a reflecting layer and an associated enhancement of the summer mesospheric wave signature. These features are in keeping with meteor and partial reflection drift radar observations.
Venus: Global gravity and topography
Authors: McNamee J. B. , Borderies N. J. , Sjogren W. L.
Published: February 2008

A new gravity field determination that has been produced combines both the Pioneer Venus Orbiter (PVO) and the Magellan Doppler radio data. Comparisons between this estimate, a spherical harmonic model of degree and order 21, and previous models show that significant improvements have been made. Results are displayed as gravity contours overlaying a topographic map. We also calculate a new spherical harmonic model of topography based on Magellan altimetry, with PVO altimetry included where gaps exist in the Magellan data. This model is also of degree and order 21, so in conjunction with the gravity model, Bouguer and isostatic anomaly maps can be produced. These results are very consistent with previous results, but reveal more spatial resolution in the higher latitudes.
Brittle strength of basaltic rock masses with applications to Venus
Authors: Schultz Richard A.
Published: February 2008

Laboratory measurements of rock deformation in the brittle regime provide constraints on the response of rocks to stress. These values are essential parameters in tectonic models of near‐surface deformation because they influence both the stress state and the conditions for predicting the types and occurrences of brittle structures such as joints and faults. However, additional parameters must be included before the laboratory values can be used to construct brittle strength envelopes for near‐surface materials. The properties of basaltic rock masses provide a more precise estimate of the strengths of basaltic lava flows on the terrestrial planets than other, more widely used approaches (intact rock or frictional strength of a through‐going surface). Rock mass strength is defined by three parameters including unconfined compressive strength of intact basalt and two others related to the degree of fracturing of the material. Experimental results for elevated temperature extend the applicability of these parameters to the near‐surface environment of Venus. Representative values of strength parameters for intact basalt at ambient temperature (20°C)and negligible confining pressure are: Young's modulus, 73 GPa; Poisson's ratio, 0.25; tensile strength, −14 MPa; unconfined compressive strength, 262 MPa; fracture toughness, 1–3 MPa m½ cohesion, 66 MPa; and coefficient of friction, 0.6. At elevated temperature (∼450°C) and zero confining pressure, reference values for the strength of intact basalt are: Young's modulus, 57 GPa; Poisson's ratio, 0.25; unconfined compressive strength, 210 MPa; and fracture toughness, 2–2.8 MPa m½. Corresponding values for a basaltic rock mass that incorporate the weakening effects of scale (but not elevated temperature) are: Deformation modulus, 5–50 GPa; Poisson's ratio, 0.3; tensile strength, −0.2 to −2 MPa; uniaxial compressive strength, 12–63 MPa; cohesion, 0.5–6 MPa. Values of tensile and cohesive strength for the basaltic rock mass are approximately one to two orders of magnitude lower than corresponding values for intact basalt. Temperatures comparable to those at the Venus surface may slightly increase the deformation modulus but decrease the compressive strength of the rock mass. Brittle strength envelopes for the rock mass as a function of depth are typically stronger in both extension and compression than conventional envelopes that assume a simple frictional strength. These results indicate that the strengths of basaltic rocks on planetary surfaces and in the shallow subsurface are significantly different from strength values commonly used in tectonic modeling studies which assume properties of either intact rock samples or single planar shear surfaces.
New perspectives on the Venus nightglow
Authors: Huestis D. L. , Slanger T. G.
Published: February 2008

We have reanalyzed the 190‐ to 250‐nm nightglow spectrum of Venus recorded by the Pioneer Venus Orbiter. This spectrum is known to consist primarily of v′ = 0 features of the NO δ‐ and γ‐bands. However, the summed spectra, taken over many orbits, have weak and previously unidentified bands in the same spectral region, which we interpret as originating from higher vibrational levels of NO(A2Σ+), resulting from collisional relaxation of NO(C2Π)v = 0. The intensity of the airglow is quite variable, with the average number of photon counts in a spectral scan being 3, while the maximum counts observed is 65. We divided the scans into a high‐count group and a low‐count group. The resulting partial‐sum spectra and the wide intensity variations suggest that at least two sources of excitation are operating. The low‐count spectrum looks similar to an N + O afterglow spectrum taken at relatively high pressure, containing a variety of relaxed NO states, and thus appears to have been generated at low altitude. The high‐count spectrum is unrelaxed, and resembles a photoexcitation spectrum of the C2Π (v = 0) state, and thus would reflect N + O recombination at higher altitudes. Of particular interest is a set of three consecutive intense spectra taken on the same orbit, which suggests the presence of a long (1000 km), straight, and narrow (∼5 km) track of secondary NO excitation, possibly caused by a meteor.
The Martian annual atmospheric pressure cycle: Years without great dust storms
Authors: Tillman James E. , Johnson Neal C. , Guttorp Peter , Percival Donald B.
Published: February 2008

A model of the annual cycle of pressure on Mars has been developed for a 2‐year period chosen to include 1 year at Lander 2 and to minimize the effect of great dust storms at the 22°N Lander 1 site. The model was developed by weighted least squares fitting of the Viking Lander pressure measurements to an annual mean, and fundamental and the first four harmonics of the annual cycle. The very close agreement between the two years suggests that an accurate representation of the annual CO2 condensation‐sublimation cycle can be established for such years. The two annual mean pressures are identical to 0.006 mbar out of 7.9 mbar, and the differences in amplitudes for the first five periodic components between the two years range from 0.017 to 0.001 mbar. The phase angles, primarily dependent on solar insolation determined orbital dynamics, differ by −3.0° Ls for the second harmonic (year 1 minus year 2), and drop to ≤ 0.7° for the fundamental and fourth harmonic. Although the slight year‐to‐year differences appear to be real, this model is proposed as a “nominal” Martian annual pressure cycle and applications are suggested. By analogy, the corresponding first year's representation at Lander 2 is also proposed as the “nominal” cycle, although it has not been verified by data from a subsequent year. These models provide a method of removing low frequencies from the annual pressure cycle for spectral analyses of baroclinic, tidal, and normal mode oscillations, and for comparisons of the interannual variability.
Thermal profiles in the auroral regions of Jupiter
Authors: Drossart Pierre , Bézard Bruno , Atreya Sushil K. , Bishop James , Waite J. H. , Boice D.
Published: February 2008

The temperature structure within the northern auroral region of Jupiter is studied by reanalyzing the Voyager 1/infrared interferometer and radiometer spectrometer (IRIS) spectra. The total measured excess infrared auroral zone emission (averaged over the IRIS field of view) in the hydrocarbon bands between 7 and 13 μm is found to be about 208 ergs cm−2 s−1 over an area of about 2 × 1018 cm2 with a resulting power output of 4 × 1013 W. In comparison, the total energy deposition by magnetospheric charged particles has been estimated on the basis of UV observations to range between 1 × 1013 and 4 × 1013 W over a comparable area. The large amount of radiated energy observed in the infrared may imply an additional heat source in the auroral regions (possibly Joule heating). A new set of thermal profiles of Jupiter's high‐latitude upper atmosphere has also been derived. These profiles have a large temperature enhancement in the upper stratosphere and are constrained to reproduce the CH4 emission at 7.7 μm. The emission in the other hydrocarbon bands (C2H2 and C2H6) is found to depend on the depth to which the temperature enhancement extends, which further constrains the thermal profiles. This study shows that a large temperature enhancement in the upper stratosphere and lower thermosphere can explain the observed excess hydrocarbon emission bands; thus smaller variations in hydrocarbon abundances (between the high latitudes and the equatorial and middle latitudes) are required than has been assumed in previous models.
A Search for an Early Holocene CACO3 Preservation Event
Authors: Broecker W. S. , Lao Yong , Klas Mieczyslawa , Clark Elizabeth , Bonani Georges , Ivy Susan , Chen Chin
Published: February 2008

The CO2 record for air bubbles from the Byrd Station ice core suggest a drawdown in the ocean‐atmosphere carbon reservoir during the early Holocene. Such a drawdown would require a corresponding increase in the CO3= ion concentration in the deep sea. We report here the results of a search in Atlantic sediments for evidence that the lysocline showed a corresponding deepening. While both the pteropod and the calcite preservation records we have obtained are consistent with expectation, they are not conclusive.
Glacial/interglacial changes in sediment rain rate in the SW Indian Sector of subantarctic Waters as recorded by 230Th, 231Pa, U, and δ15N
Authors: Francois Roger , Bacon Michael P. , Altabet Mark A. , Labeyrie Laurent D.
Published: February 2008

High‐resolution records of opal, carbonate, and terrigenous fluxes have been obtained from a high‐sedimentation rate core (MD84‐527: 43°50′S; 51°19;′E; 3269 m) by normalization to 230Th. This method estimates paleofluxes to the seafloor on a point‐by‐point basis and distinguishes changes in sediment accumulation due to variations in vertical rain rates from those due to changes in syndepositional sediment redistribution by bottom currents. We also measured sediment δ15N to evaluate the changes in nitrate utilization in the overlying surface waters associated with paleoflux variations. Our results show that opal accumulation rates on the seafloor during the Holocene and stage 3, based on 14C dating, were respectively tenfold and fivefold higher than the vertical rain rates, At this particular location, changes in opal accumulation on the seafloor appear to be mainly controlled by sediment redistribution by bottom currents rather than variations in opal fluxes from the overlying water column. Correction for syndepositional sediment redistribution and the improved time resolution that can be achieved by normalization to 230Th disclose important variations in opal rain rates. We found relatively high but variable opal paleoflux during stage 3, with two maxima centered at 36 and 30 kyr B.P., low opal paleoflux during stage 2 and deglaciation and a pronounced maximum during the early Holocene, We interpret this record as reflecting variations in opal production rates associated with climate‐induced latitudinal migration of the southern ocean frontal system. Sediments deposited during periods of high opal paleoflux also have high authigenic U concentrations, suggesting more reducing conditions in the sediment, and high Pa‐231/Th‐230 ratios, suggesting increased scavenging from the water column. Sediment δ15N is circa 1.5 per mil higher during isotopic stage 2 and deglaciation. The low opal rain rates recorded during that period appear to have been associated with increased nitrate depletion. This suggests that opal paleofluxes do not simply reflect latitudinal migration of the frontal system but also changes in the structure of the upper water column. Increased stratification during isotopic stage 2 and deglaciation could have been produced by a meltwater lid, leading to lower nitrate supply rates to surface waters.
Aftermath of the end‐Cretaceous mass extinction: Possible biogeochemical stabilization of the carbon cycle and climate
Authors: Caldeira Ken , Rampino Michael R.
Published: February 2008

In the aftermath of the Cretaceous/Tertiary (K/T) boundary event (∼65 m.y. ago), pelagic carbonate productivity was greatly reduced for several hundred thousand years. A decrease in carbonate productivity by a factor greater than 3, in the absence of some mechanism to remove excess carbonate from the ocean, should have resulted in the accumulation of carbon and alkalinity in the oceans. This would cause the atmospheric partial pressure of CO2 to fall dramatically and the deep ocean to become fully saturated with respect to calcite. Evidence of such a period of highly calcite‐saturated oceans with low atmospheric pCO2 in the earliest Tertiary is lacking, suggesting that ocean chemistry may have been buffered by some process or processes. Shallow‐water carbonate accumulation rates may depend, in part, on carbonate ion concentrations, and thus shallow‐water carbonate deposition might act to stabilize ocean chemistry in the face of a dramatic reduction in pelagic productivity. In our four‐box ocean model, as the oceanic carbonate ion concentration rises in the face of diminished pelagic carbonate accumulation, the shallow‐water carbonate accumulation rate increases, compensating for the reduction in pelagic carbonate accumulation. These model results indicate that the carbonate‐ion feedback on shallow‐water carbonate sedimentation may have acted to balance oceanic carbon and alkalinity budgets at the K/T boundary, and, furthermore, may have been a primary mechanism maintaining high shallow‐water carbonate accumulation rates prior to the Jurassic onset of widespread pelagic carbonate accumulation.
Evaluation of the astronomically calibrated time scale for the Late Pliocene and Earliest Pleistocene
Authors: Hilgen F. J. , Lourens L. J. , Berger A. , Loutre M. F.
Published: February 2008

An astronomically calibrated time scale was recently established (Hilgen, 1991a, b) for the Pliocene and earliest Pleistocene, based on the correlation of dominantly precession‐controlled sedimentary cycles (sapropels and CaCO3 cycles) in Mediterranean deep‐sea sections to astronomical solution Ber90 (Berger and Loutre, 1991). This time scale, which differed substantially from the conventional time scales then widely used, is essentially confirmed by a new generation of radiometric dates using the 40Ar/39Ar laser fusion technique. To evaluate this time scale, we here extract both precession‐ and obliquity‐related components from late Pliocene‐earliest Pleistocene climatic proxy records in the Mediterranean and determine coherencies and phase (time) lags between these components and the respective orbital variations. This work is found to support the new time scale, because significant coherencies are found between the astronomically related components in our proxy records and the orbital variations, not only at the main period of precession but also at the main period of obliquity and, because of the eccentricity modulation of precession, in the low‐frequency eccentricity bands of the spectrum. But the calculated time lag between obliquity and 41‐kyr components in the climatic proxy records shows a significant increase of 5.6 (±1) kyr with respect to the late Pleistocene if the phase relation with precession is kept constant. This increase can be explained by a change in the climate response time to obliquity or to precession, by a small uncertainty in the astronomical solution, or by a combination of these factors.
Poststatistic steering wind estimation in the troposphere and lower stratosphere
Authors: Palmer Robert D. , Larsen Miguel F. , Sheppard E. Lamar , Fukao Shoichiro , Yamamoto Mamoru , Tsuda Toshitaka , Kato Susumu
Published: February 2008

Poststatistic steering (PSS) techniques are applied for estimating the wind vector. Since spatially separated receivers can be used to synthesize a radar beam in an arbitrary direction, Doppler beam swinging (DBS) techniques can be used in conjunction with PSS to obtain estimates of the wind vector. Data obtained with the middle and upper atmosphere (MU) radar in Japan are used to implement the technique in the troposphere and lower stratosphere at a VHF wavelength. To obtain an accurate estimate of the beam‐pointing direction, the synthesized two‐way antenna pattern is calculated. Also, the effects of aspect sensitivity on this technique are explored. The mode of operation of the MU radar was alternated between a typical DBS mode and a spatial interferometric (SI) mode, which was used for the PSS technique. Results from the PSS method of obtaining the wind vector are compared to the DBS results, which are assumed to be an accurate representation of the atmospheric wind. This study suggests that aspect sensitivity causes an underestimate of the horizontal wind using PSS. Corrections were made using the aspect sensitivity functions obtained from the DBS experiment and the corrected PSS velocities compare well with the DBS velocities. The vertical velocity estimate using a vertically pointing beam is shown to possess a gravity wave characteristic which is due solely to contamination from the horizontal wind.
Waveguides filled with general biisotropic media
Authors: Koivisto Päivi K. , Tretyakov Sergei A. , Oksanen Markku I.
Published: February 2008

Guided waves in a general class of waveguides with biisotropic filling are studied. The biisotropic medium is the most general isotropic linear medium, that is, nonreciprocal chiral medium. Field equations are derived first for arbitrary waveguide cross sections and then applied for a special circular cross section. The following special boundary impedance conditions have been covered: ideally conducting surface, isotropic, and anisotropic impedance surface and open surface. Some general properties of the eigenwaves are established, and the influence of nonreciprocity together with chirality and losses is discussed. Calculated curves are shown for biisotropic circular waveguides with ideally conducting and isotropic impedance surfaces.
Analytic methods and free‐space dyadic Green's functions
Authors: Weiglhofer Werner S.
Published: February 2008

A number of mathematical techniques are presented which have proven successful in obtaining analytic solutions to the differential equations for the dyadic Green's functions of electromagnetic theory. The emphasis is on infinite‐medium (or free‐space) time‐harmonic solutions throughout, thus putting the focus on the physical medium in which the electromagnetic process takes place. The medium's properties enter Maxwell's equations through the constitutive relations, and a comprehensive listing of dyadic Green's functions for which closed‐form solutions exist, is given. Presently, the list of media contains (achiral) isotropic, biisotropic (including chiral), generally uniaxial, electrically (or magnetically) gyrotropic, diffusive and moving media as well as certain plasmas. A critical evaluation of the achievements, successes, limits, and failures of the analytic techniques is provided, and a prognosis is put forward about the future place of analytic methods within the general context of the search for solutions to electromagnetic field problems.
Ground‐based measurement of gradients in the “wet” radio refractivity of air
Authors: Davis James L. , Elgered Gunnar , Niell Arthur E. , Kuehn Clara E.
Published: February 2008

We have used a ground‐based microwave radiometer, known as a water vapor radiometer, to investigate the local spatial and temporal variation of the wet propagation delay for a site on the west coast of Sweden. The data were obtained from a wide range of azimuths and from elevation angles greater than 23.6° (air mass 2.5). Visual inspection of the data suggested a simple “cosine azimuth” variation, implying that a first‐order gradient model was required. This model was adequate for short time spans up to approximately 15 min, but significant temporal variations in the gradient suggested to us that we include gradient rate terms. The resulting six‐parameter model has proven adequate (rms delay residual ∼1 mm) for up to 30 min of data. Assuming a simple exponential profile for the wet refractivity gradient, the estimated gradient parameters imply average surface wet‐refractivity horizontal gradients of order of 0.1–1 N km−1. These gradients are larger, by 1–2 orders of magnitude, than gradients determined by others by averaging over long (∼100‐km) distances. This result implies that for applications that are sensitive to local gradients, such as wet propagation‐delay models for radio‐interferometric geodetic studies, the use of meteorological data from widely spread stations may be inadequate. The gradient model presented here is inadequate for times longer than about 30 min, even if no gradients are present, because of the complicated stochastic like temporal behavior of the wet atmosphere. When gradients are present, they can change magnitude by ∼50% over 10–15 min. Nevertheless, our ability to fit the radiometer data implies that on timescales <30 min and for elevation angles >23.6°, the local structure of the wet atmosphere can be described with a simple model. (The model is not limited to this range of elevation angles in principle.) The estimated gradient and gradient rate vectors have preferred directions, which indicates a prevailing structure in the three‐dimensional temperature and humidity fields, possibly related to systematic behavior in large‐scale weather systems and/or the local air‐land‐sea interaction at this site.
Sequencing reelfoot extension based on relations from southeast Missouri and interpretations of the interplay between offset preexisting zones of weakness
Authors: Clendenin C. W. , Lowell G. R. , Niewendorp C. A.
Published: February 2008

Late Cambrian stratigraphic relations are used to reconstruct aspects of the structural architecture buried between the St. Francois Mountains and the Mississippi Embayment. Such reconstructions are possible because characteristics of units deposited during early thermal subsidence help document late Proterozoic‐Early Cambrian Catoctin extension in southeast Missouri. New observations of the Lamotte Sandstone and Bonneterre Formation (Dresbachian) show that a mismatched architecture is present and was buried by late Bonneterre Formation (early Franconian) sediments prior to the deposition of the Davis Formation (early‐middle Franconian). A structural map of the region was developed by integrating such new observations with published relations. Analysis of stratigraphic and structural relations reveal that two periods of brittle thinning of the crust resulted from Catoctin extension in southeast Missouri. Although structural architecture is partially masked, these distinct periods of thinning produced (1) a compartmentalized, tilt block terrane characterized by down‐to‐the‐southeast breakaways and (2) a number of linked, convergent halfgrabens. Late Cambrian stratigraphic relations provide evidence that the linked half‐grabens were superimposed asymmetrically on the tilt block terrane. A four‐stage tectonic sequence consisting of early mafic volcanism, two periods of brittle thinning, and late recurrent ultra‐mafic volcanism is interpreted with the application of models of tectonism controlled by preexisting zones of weakness. An interplay between offset, preexisting zones of weakness helps explain the timing of associated magmatism, faulting, and graben development. Changes in structural style resulting from such an interplay suggest that Catoctin extension in southeast Missouri was a protracted episode of polyphase deformation.
Inferred quasi‐steady ionospheric neutral winds and electrical currents at 79° south latitude in austral summer conditions
Authors: Lanzerotti L. J. , Maclennan C. G. , Medford L. V.
Published: February 2008

An analysis of the range of geomagnetic variations as measured at Arrival Heights and South Pole stations in the Antarctic over an 8‐year interval during the present solar cycle shows that there are quasi‐DC level shifts in some of the magnetic components during austral summer conditions when referenced to austral winter. The most dramatic of these level shifts in the signals occurs in the D‐component (west‐east magnetic component) at Arrival Heights (geomagnetic latitude ∼79°S). The onsets of the D‐component shifts occur during a period that lasts one to several days in late austral spring. Such geomagnetic variations would not appear to arise from magnetosphere processes coupling to the ionospheric E‐region. The inferred quasi‐steady ionospheric current is estimated to be of the order of ∼105 amps above Arrival Heights. It is speculated that neutral wind dynamics with quasi‐steady E‐region (∼110–115 km) neutral wind speeds of ∼10 m/sec directed east to west above AH are responsible for generating the austral summer electrical currents in the E‐region.
Experimental Investigation and Modeling of Uranium (VI) Transport Under Variable Chemical Conditions
Authors: Kohler M. , Curtis G. P. , Kent D. B. , Davis J. A.
Published: February 2008

The transport of adsorbing and complexing metal ions in porous media was investigated with a series of batch and column experiments and with reactive solute transport modeling. Pulses of solutions containing U(VI) were pumped through columns filled with quartz grains, and the breakthrough of U(VI) was studied as a function of variable solution composition (pH, total U(VI) concentration, total fluoride concentration, and pH‐buffering capacity). Decreasing pH and the formation of nonadsorbing aqueous complexes with fluoride increased U(VI) mobility. A transport simulation with surface complexation model (SCM) parameters estimated from batch experiments was able to predict U(VI) retardation in the column experiments within 30%. SCM parameters were also estimated directly from transport data, using the results of three column experiments collected at different pH and U(VI) pulse concentrations. SCM formulations of varying complexity (multiple surface types and reaction stoichiometries) were tested to examine the trade‐off between model simplicity and goodness of fit to breakthrough. A two‐site model (weak‐ and strong‐binding sites) with three surface complexation reactions fit these transport data well. With this reaction set the model was able to predict (1) the effects of fluoride complexation on U(VI) retardation at two different pH values and (2) the effects of temporal variability of pH on U(VI) transport caused by low pH buffering. The results illustrate the utility of the SCM approach in modeling the transport of adsorbing inorganic solutes under variable chemical conditions.
The superdense plasma sheet: Plasmaspheric origin, solar wind origin, or ionospheric origin?
Authors: Borovsky Joseph E. , Thomsen Michelle F. , McComas David J.
Published: February 2008

A few times per month, the density of the plasma sheet is several times higher than its usual density. Such superdense plasma sheet intervals are observed both in the midtail region and at geosynchronous orbit. Typically at geosynchronous orbit, a superdense plasma sheet occurs on the first day of a geomagnetic storm and lasts about 12–18 hours. The occurrences of superdense plasma sheets are found to be related to a distinct pattern of Kp: Kp rising after it has been low for an extended period. The occurrences are also associated with high‐density solar wind. Three sources for the material of the superdense plasma sheet are explored: (1) the outer plasmasphere, which is stripped away and drawn into the dayside neutral line when Kp increases, wherein it joins the lobe and eventually joins the plasma sheet; (2) high‐density solar‐wind, which may have its entry into the plasma sheet controlled by the solar‐wind magnetic field; and (3) ionospheric outflow, which is known to be Kp dependent. The occurrence of a superdense plasma sheet has several consequences: it adds to the intensity of the ring current, it may alter the dynamics of the magnetotail and the nature of substorms, and it may provide an enhanced source population for storm‐time energetic particles.
Near‐Earth Asteroid Rendezvous: Mission overview
Authors: Cheng A. F. , Santo A. G. , Heeres K. J. , Landshof J. A. , Farquhar R. W. , Gold R. E. , Lee S. C.
Published: February 2008

The Near‐Earth Asteroid Rendezvous (NEAR) mission, the first launch of NASA's Discovery Program, will be the first mission to orbit an asteroid. NEAR will make the first comprehensive scientific measurements of an asteroid's surface composition, geology, physical properties, and internal structure. NEAR launched successfully on February 17, 1996, aboard a Delta 11–7925. It will orbit the 20‐km‐diameter near‐Earth asteroid 433 Eros for about 1 year, at a minimum orbit radius of about 35 km from the center of the asteroid. The NEAR is a solar‐powered, three‐axis stabilized spacecraft with a launch mass including propellant of 805 kg. NEAR uses X band telemetry to the NASA Deep Space Network, with the data rates at Eros up to 8.8 kbits/s using a 34‐m High EFficiency (HEF) dish, and up to 26.5 kbits/s using a 70‐m dish. A solid‐state recorder is accommodated with a memory capacity of 1.8 Gbytes. Attitude control is to 1.7 mrad, line‐of‐sight pointing stability is within 50 μrad over 1 s, and post processing attitude knowledge is within 50 μrad. NEAR accommodates 56 kg of instruments and provides them with 84 W. The instruments are a multispectral imager (MSI), a near‐infrared spectrograph (NIS), an X ray/gamma ray spectrometer (XRS/GRS), a magnetometer (MAG), and a laser rangefinder (NLR), while a radio science (RS) investigation uses the coherent X band transponder. NEAR will make a flyby of the C‐type asteroid 253 Mathilde in June 1997 and will rendezvous with 433 Eros in February 1999. It will execute an initial slow flyby of Eros, with a flyby speed of 5 m/s and a closest approach distance of 500 km. Subsequently, its orbit will be lowered to 35 km. The NEAR Mission Operations Center and the Science Data Center are at the Johns Hopkins Applied Physics Laboratory. The Science Data Center will maintain the entire NEAR data set on‐line, and data from all instruments can be accessed by every member of the NEAR Science Team. Data, including images, are released over the Internet as soon as they are validated.
Biogenic particle fluxes in the equatorial Pacific: Evidence for both high and low productivity during the 1982‐1983 El Niño
Authors: Dymond Jack , Collier Robert
Published: February 2008

Sediment traps deployed at two sites in the equatorial Pacific during and following the 1982‐1983 El Niño southern oscillation (ENSO) demonstrate the biological effects of this event. Biogenic particle fluxes for a site 1° north of the equator were at least a factor of 2 lower during a 3 month period of intense ENSO influence compared to fluxes recorded at any other time during a 28‐month period beginning in December 1982. These low particle fluxes reflect the expected decrease in primary production in response to the ENSO event. Surprisingly, the biogenic particle fluxes measured at the second site, 11°N, were anomalously high during this same ENSO‐affected period. The apparent increase in productivity north of the equator seems to be a consequence of enhanced flow of the North Equatorial Countercurrent (NECC) and the associated doming of the shallow thermocline. The observed temporal variability in magnitude of the biogenic particle flux was accompanied by compositional changes in the biogenic components. The period of high carbon flux at the 11°N site was a time of exceptional opal flux, while the particle flux during the period of low carbon flux observed at 1°N during the ENSO was depleted in opal. These patterns appear to reflect the dominance of diatom productivity relative to coccolithophorid productivity during conditions of greater nutrient availability.
Sources of atmospheric methane in the south Florida environment
Authors: Harriss Robert C. , Sebacher Daniel I. , Bartlett Karen B. , Bartlett David S. , Crill Patrick M.
Published: February 2008

Direct measurement of methane (CH4) flux from wetland ecosystems of south Florida demonstrates that freshwater wet prairies and inundated sawgrass marsh are the dominant sources of atmospheric CH4 in the region. Fluctuations in soil moisture are an important environmental factor controlling both seasonal and interannual fluctuations in CH4 emissions from undisturbed wetlands. Land use estimates for 1900 and 1973 were used to calculate regional CH4 flux. Human settlement in south Florida has modified wetland sources of CH4, reducing the natural prairies and marsh sources by 37%. During the same period, impoundments and disturbed wetlands were created which produce CH4 at rates approximately 50% higher than the natural wetlands they replaced. Preliminary estimates of urban and ruminant sources of CH4 based on extrapolation from literature data indicate these sources may now contribute approximately 23% of the total regional source. We estimate that the integrated effects of urban and agricultural development in south Florida between 1900 and 1973 resulted in a 26% enhancement in CH4 flux to the troposphere.
Carbon isotopic composition of methane in Florida Everglades soils and fractionation during its transport to the troposphere
Authors: Chanton Jeffrey P. , Pauly George G. , Martens Christopher S. , Blair Neal E. , Dacey John W. H.
Published: February 2008

The δ13C stable carbon isotopic composition of methane collected in bubbles from the submerged soils of specific environments within the Everglades wetland in southern Florida, United States, varied from −70‰ to −63‰ across the system while organic carbon in the soils and dominant plants varied from −28‰ to −25‰. A methane isotopic budget based upon the soil bubble isotope data and published methane flux measurements predicted a flux of isotopic composition −65‰, a value 5‐10‰ more depleted in 13C than the isotopic composition of methane emanating to the atmosphere. Emergent aquatic plants, which are known to be active methane transporters between soil and atmosphere in this ecosystem, were found to transport methane of δ13C content up to 12‰ different from the δ13C content of the soil methane bubble reservoir. Methane 14C content at one site was determined to be 108.6% modern (Δ14C = 83 ± 10‰).
Carbon and hydrogen isotope fractionation resulting from anaerobic methane oxidation
Authors: Alperin M. J. , Reeburgh W. S. , Whiticar M. J.
Published: February 2008

Methane oxidation in the anoxic sediments of Skan Bay, Alaska resulted in fractionation of carbon and hydrogen isotopes in methane. Isotope fractionation factors were estimated by fitting methane concentration, δ13C‐CH4, and δD‐CH4 data with depth distributions predicted by an open system, steady state model. Assuming that molecular diffusion coefficients for 12CH4, 13CH4, and12CH3D are identical, the predicted fractionation factors were 1.0088±0.0013 and 1.157±0.023 for carbon and hydrogen isotopes, respectively. If aqueous diffusion coefficients for the different isotopic species of methane differ significantly, the predicted fractionation factors are larger by an amount proportional to the diffusion isotope effect.
Methane flux and stable hydrogen and carbon isotope composition of sedimentary methane from the Florida Everglades
Authors: Burke Roger A. , Barber Timothy R. , Sackett William M.
Published: February 2008

Methane flux and the stable isotopic composition of sedimentary methane were measured at four locations in the Florida Everglades system. Individual estimates of methane flux ranged over more than 3 orders of magnitude, from about 0.001 to 2.6 g CH4 m−2 d−1. Significant interstation differences in total methane flux were also observed and are judged most likely attributable to differences in the size and spacing of emergent aquatic vegetation, and possibly differences in the type (i.e., vascular plant versus algal) of organic matter incorporated into the sediments. On the basis of measurements presented here and by other investigators, the Everglades system appears to be a relatively weak source of atmospheric methane, probably contributing less than 0.5 Tg CH4 yr−1. Emergent aquatic plants appear to be capable of indirectly affecting the stable isotopic composition of sedimentary methane by stimulating methane oxidation via root aeration. A significant positive correlation between δD‐CH4 and δ13C‐CH4 was observed for samples collected from sediments covered by tall, dense stands of emergent plants. In contrast, a significant negative correlation between the δD and δ13C of sedimentary methane was observed for samples collected at an open water site where ebullition dominated methane transfer to the atmosphere. The mean δ13C of sedimentary methane samples measured in the Everglades system (mean δ13C =−61.7‰, s.d. = 3.6‰, n = 51) is not significantly different from the estimated average δ13C of all natural sources (−58.3‰). The mean δD of Everglades sedimentary methane (mean δ D = −293‰, s.d. = 14‰, n = 50) appears to be slightly less D‐depleted than the estimated average methane (δD =−360 ± 30‰) from all sources.