bluuebunny/crossref_metadata_2025 huggingface dataset:
Anomalous Antarctic ozone during 1992: Evidence for Pinatubo volcanic aerosol effects
Authors: Hofmann D. J. , Oltmans S. J.
Published: February 2008

Unusual stratospheric ozone levels were observed in the Antarctic stratosphere in 1992. The rate of ozone decrease during formation of the springtime ozone hole and the severity of ozone loss in the lower stratosphere were greater in 1992 as compared to previous years. Total ozone reached an all time low of about 105 Dobson units on October 11 at South Pole Station. On this day, the balloon‐borne instrument encountered an apparent ozone void between altitudes of 14 and 18 km. Ozone profiles showed evidence of unusual ozone depletion in autumn, before polar stratospheric cloud existence temperatures were reached. Satellite measurements indicated that the 1992 ozone hole was about 25% larger in geographical extent than in previous years. The possible effects of the eruption of the Pinatubo volcano in the Philippine Islands in 1991 are investigated, and it is concluded that the sulfuric acid droplets, which formed in the stratosphere following the eruption and were trapped in the south polar vortex, are the most likely source of the anomalous Antarctic ozone depletion in 1992.
Multiple steady states in atmospheric chemistry
Authors: Stewart Richard W.
Published: February 2008

The equations describing the distributions and concentrations of trace species are nonlinear and may thus possess more than one solution. Several authors have suggested that the steady‐state equations describing tropospheric and stratospheric chemistry may have multiple solutions, but the existence of such solutions has not been completely demonstrated. This paper develops methods for searching for multiple physical solutions to chemical continuity equations and applies these to subsets of equations describing tropospheric chemistry. The calculations are carried out with a box model and use two basic strategies. The first strategy is a “search” method. This involves fixing model parameters at specified values, choosing a wide range of initial guesses at a solution, and using a Newton‐Raphson technique to determine if different initial points converge to different solutions. The second strategy involves a set of techniques known as homotopy methods. These do not require an initial guess, are globally convergent, and are guaranteed, in principle, to find all solutions of the continuity equations. The first method is efficient but essentially “hit or miss” in the sense that it cannot guarantee that all solutions which may exist will be found. The second method is computationally burdensome but can, in principle, determine all the solutions of a photochemical system. Multiple solutions have been found for models that contain a basic complement of photochemical reactions involving Ox, HOx, NOx, and CH4. In the present calculations, transitions occur between stable branches of a multiple solution set as a control parameter is varied. These transitions are manifestations of hysteresis phenomena in the photochemical system and may be triggered by increasing the NO flux or decreasing the CH4 flux from current mean tropospheric levels.
Carbon disulfide measurements in the atmosphere of the western North Atlantic and the northwestern south Atlantic oceans
Authors: Bandy Alan R. , Thornton Donald C. , Johnson James E.
Published: February 2008

Carbon disulfide (CS2) measurements were made over the western and equatorial North Atlantic Ocean and the northwestern and equatorial South Atlantic Ocean. Carbon disulfide was in the range 0.4–50 pptrv in the atmosphere of the western North Atlantic Ocean. Emissions from anthropogenic sources and wet lands were found to be important although anthropogenic sources were 4–6 times larger than biogenic sources. The flux of CS2 from eastern North America between 30 and 39° latitude was estimated to be 2 × 108g yr−1 of sulfur. The anthropogenic contribution was 1.8 × 108g yr−1 of sulfur whereas the contribution of marshes was 0.2 × 108g yr−1 of sulfur. Sources of CS2 at high latitudes in the northern hemisphere were comparatively weak. Carbon disulfide levels in the western South Atlantic Ocean between −5 and 1° latitude were in the range 0.2–6 pptrv. Most of the CS2 appeared to come from biomass burning in Africa. Carbon disulfide was much higher close to shore suggesting that the South American continent was a significant source although too few data were available to quantify it. On ferry flights from Wallops, Virginia to Natal, Brazil, CS2 levels at the ferry altitude of about 6 km averaged 1.2 pptrv. This background CS2 was adequate to account for all the OCS in the atmosphere.
Evaluation of dynamical parameters with a three‐dimensional mechanistic model of the middle atmosphere
Authors: Kouker W.
Published: February 2008

A three‐dimensional model of the middle atmosphere is introduced. The model is based on the full set of the primitive equations. It is designed to simulate a yearly cycle of the middle atmosphere. Results are presented for the solstice and equinox conditions. The model reproduces the main observed features of the middle atmospheric circulation: the stratospheric‐mesospheric jet streams and the cold summer mesopause region at solstice with reversed zonal wind especially in the summer mesosphere, and the weak westerly circulation at equinox. The parameterized effects of breaking gravity waves in the mesosphere drive the atmosphere out of radiative balance. They lead to a meridional circulation with a one‐cell structure at solstice with upward (downward) motion over the summer (winter) pole and a meridional flow towards the winter hemisphere and a two‐cell structure at equinox with upward motion over the tropics and downward motion over the polar regions. Potential fields are presented for horizontal vector fields. They suggest that the stratospheric circulation can is dominated by horizontally nondivergent flow. This is modified by the results of a more quantitatative view at the interaction of planetary waves on the zonal mean flow, which clearly identifies the essential role of horizontal divergence on the stratospheric circulation. In the mesopause region the forces induced by the breaking gravity waves are of the same order as the horizontal pressure gradient force or the Coriolis force, so that geostrophic theory can no longer be applied here. This is confirmed by a comparison between idealized tracers and the dynamical tracers potential temperature and isentropic potential vorticity (IPV). The IPV is no longer a conservative tracer in the mesopause region.
Evolution of a porous H2O‐CO2‐ice sample in response to irradiation
Authors: Steiner Gerhard , Kömle Norbert I.
Published: February 2008

The thermal history and the chemical differentiation of a porous H2O‐CO2‐ice sample is studied for four different examples. In the first two cases, the possible effect of a nonvolatile mantle covering the ice is investigated. In examples 3 and 4 the influence of the average grain size and the Hertz factor, which determine the effective thermal conductivity of the ice, on the sample evolution are investigated. Our computations show that both the amount of heating and the compositional evolution critically depend on the assumed ice material parameters and the presence of a mantle. The results are compared with other theoretical work and relevant experiments, and we found good agreement between experimental results and our theoretical predictions. Finally, some implications for comet nuclei and future spacecraft missions to comets are discussed.
Theoretical modeling of eruption plumes on Venus
Authors: Thornhill G. D.
Published: February 2008

A theoretical model of eruption plume dynamics (Woods, 1988) is used to investigate Plinian eruption columns on Venus. Results for a typical plume, in terms of the temperature, density, velocity, and radius as a function of height are given. The effects of the atmospheric pressure, the volatile content and species, and the eruption temperature on the development of high convecting plumes are discussed, demonstrating the combinations of initial conditions giving rise to either high convective plumes, or low fountains feeding pyroclastic flows. The results show that the development of high convecting plumes on Venus requires an unlikely combination of high volatile contents, high initial temperatures, and high altitude eruptions, leading to the conclusion that explosive eruptions on Venus will generally lead to pyroclastic flows.
Photodissociation in the atmosphere of Mars: Impact of high resolution, temperature‐dependent CO2 cross‐section measurements
Authors: Anbar A. D. , Allen M. , Nair H. A.
Published: February 2008

We have investigated the impact of high resolution, temperature‐dependent CO2 cross‐section measurements (Lewis and Carver, 1983) on calculations of photo dissociation rate coefficients in the Martian atmosphere. Since the cross‐section measurements exhibit large variations on a 10–20 Å scale, the calculations are sensitive to the wavelength resolution. We find that the adoption of 50 Å intervals for the purpose of computational efficiency result in errors in the calculated values for photodissociation of CO2, H2O and O2 which are generally ≤10% but as large as 20% in some instances. These are acceptably small errors, especially considering the uncertainties introduced by the large temperature dependence of the CO2 cross‐section. The inclusion of temperature‐dependent CO2 cross‐sections is shown to lead to a decrease in the diurnally averaged rate of CO2 photodissociation as large as 33% at some altitudes, and increases of as much as 950% and 80% in the photodissociation rate coefficients of H2O and O2, respectively. The actual magnitude of the changes depends on the assumptions used to model the CO2 absorption spectrum at temperatures lower than the available measurements, and at wavelengths longward of 1970 Å. Given our current understanding, these results minimize or even reverse the sense of the CO2 chemical stability problem.
Changes in erosional style on early Mars: External versus internal influences
Authors: Postawko Susan E. , Fanale Fraser P.
Published: February 2008

The conditions which led to the formation of the valley networks on Mars were apparently very different from present‐day conditions. In this paper we investigate the relative importance of higher early surface temperatures versus higher deep regolith temperatures in producing a shallower depth to liquid water in the regolith on Mars. If enough CO2 is in the atmosphere, the surface temperature could be raised to near the freezing point around 3.8 b.y. ago despite a possibly weak early sun, at least in the equatorial regions. On the other hand, it has been argued that higher internal regolith temperatures, associated with a much higher heat flow around 3.8 b.y. ago than at present, could reduce the depth to liquid water sufficiently to account for the change in erosional style with time. In fact, it does not make sense on physical grounds to consider these two mechanisms separately. The effectiveness of both of these mechanisms is dependent on a high early heat flow. In general, we find that if early heat flow were around 100 mW m−2, then the total available CO2 in the atmosphere‐plus‐regolith system becomes critical. If total available CO2 were greater than about 4 bars, then the atmospheric greenhouse effect could play an important role in raising the liquid water level closer to the surface, but only in equatorial regions. On a global basis, condensation of large amounts of atmospheric CO2 may inhibit surface warming. If the total CO2 were 1 bar or less, the atmospheric greenhouse effect raises the surface temperature by only a few degrees. Heat flows greater than 100 mW m−2 can still raise the liquid water level from over 1 km to less than 350 m, enabling substantially more efficient network formation very early in martian history.
Calorimetric studies of the ammonia‐water SYSTEM with application to the outer solar SYSTEM
Authors: Yarger Jeffery , Lunine Jonathan I. , Burke Michael
Published: February 2008

A series of heating experiments was performed on the condensed ammonia‐water system using a differential scanning calorimeter (DSC). The water‐rich samples were cooled quickly to below 130 K, then heated at a variety of rates. Rather than a single peritectic melt at 176 K, expected for the equilibrium system of water ice and ammonia dihydrate, four enthalpic transitions were repeatedly seen in the temperature range 150–176 K. These transitions are generally consistent with the earlier calorimetric results of Van Kasteren (1973), who interpreted the lowest temperature exotherm as crystallization of an amorphous ammonia‐water compound formed during cooling. We propose that both sets of experiments are seeing the crystallization of ammonia monohydrate, which is metastable relative to the dihydrate, followed by partial remelting and crystallization of dihydrate upon further heating. The apparent stability of the monohydrate in the dihydrate equilibrium field implies a potentially complex behavior of ammonia‐water ices in satellites. Possible self‐heating of the mixture by several tens of degrees up to the 170 K eutectic could make mobilization of ammonia‐water liquids in icy satellite interiors energetically easier than previously thought.
Redistribution of subsurface neutrons caused by ground ice on Mars
Authors: Feldman W. C. , Boynton W. V. , Jakosky B. M. , Mellon M. T.
Published: February 2008

As part of an ongoing effort to understand data that will be returned by the Mars Observer Gamma Ray Spectrometer (MOGRS) experiment, we calculate neutron and gamma ray fluxes for simple models describing near‐surface ground ice on Mars. Our goal is to determine the limits in information content of remotely sensed neutrons and gamma rays in delineating the abundance of ground ice and its distribution with depth. Such ice is expected based on simple theoretical calculations of the H2O environment of the martian surface and atmosphere. We find it produces a marked redistribution of neutrons with depth. Neutron‐capture gamma ray production closely follows the flux of thermal neutrons and is likewise redistributed with depth. Calculated effects at Mars mapping orbit are sufficiently large that, if ice is present at the depths and in the quantities expected, the MOGRS should provide a robust detection of its presence. Although the information content of gamma rays and neutrons alone is not sufficient to define the many parameters needed to specify its abundance and emplacement below the surface, their combined use might allow separation of the ice abundance from its vertical distribution within the regolith.
On stable isotopic variation and earliest Paleocene planktonic foraminifera
Authors: D'Hondt Steven , Zachos James C.
Published: February 2008

Extant planktonic foraminifera display positive covariance between δ13C signals and test size. As documented by other studies, primary causes of increased δ13C values with increased test size may include increased reliance on ambient CO2 for calcification at larger test sizes, decreased kinetic fractionation during calcification at larger test sizes, and increased photosymbiotic activity in larger symbiont‐bearing planktonic foraminifera. Planktonic foraminiferal δ18O values also often covary with test size, although the direction of this covariance is taxon dependent. Possible explanations for relationships between δ18O signals and test size include changing habitat depth over ontogeny, correlations between adult test size and environmental conditions, and changing isotopic disequilibrium with size, ontogenetic stage, or photosymbiont density. In order to assess the magnitude and implications of similar size dependence in earliest Paleocene planktonic foraminifera, we measured the stable isotopic signals of multiple size fractions of 10 earliest Paleocene species. All of these taxa exhibit a strong positive correlation between δ13C and test size. The slope and magnitude of this trend varies between species, with Woodringina claytonensis displaying the largest shift (1.1‰ over a 130 µm range in mean sieve size) and Guembelitria cretacea displaying the smallest (0.2‰ over a 38 µm range). By analogy with modern planktonic foraminifera, this general relationship between δ13C and size probably resulted from increased reliance on ambient CO2 for calcification at larger test sizes. The high magnitude of this shift in some taxa may reflect either photosymbiotic enhancement of the general trend or relatively greater changes in the proportions of metabolic and ambient CO2 used for calcification at different test sizes. Failure to account for relationships between test size and δ13C signals can lead to underestimation of early Paleocene surface ocean δ13C values by 1‰ or more. These size‐related δ13C effects provide an alternative explanation for decreases in whole‐rock δ13C values and some decreases in planktonic‐tobenthic foraminiferal δ13C gradients documented at marine K/T boundary sequences. At all size fractions, the 10 Paleocene taxa display a very limited interspecies range of δ18O derived paleotemperatures. Despite this limited range, paleobiogeographic patterns and δ18O signals appear to provide realistic estimates of relative paleodepth and seasonal affinities of earliest Paleocene planktonic foraminiferal species. Earliest Paleocene δ18O and biogeographic data are consistent with a general trend of surface‐to‐deep diversification of microperforate planktonic foraminifera following the K/T boundary. Such a trend may simply result from exploitation of a near‐surface open‐ocean habitat by the epicontinental K/T survivor G. cretacea.
Ice depolarizations on Ka band (20 GHz) satellite‐to‐ground path and correlation with radar observations
Authors: Maekawa Yasuyuki , Chang Nion Sock , Miyazaki Akira
Published: February 2008

The ice depolarization events without appreciable copolarized attenuation are investigated using the CS‐2 beacon signal (19.45 GHz) and the simultaneous radar observations. In these events, increase of instantaneous depolarizations of the beacon signal closely corresponds to appearance of intense radar echoes at the cloud top. Correlation of the time variations between the depolarizations and the radar echoes is, in general, found in the upper half of the ice region. In addition, the large depolarization events are primarily related to the radar echo intensities at 2.0–2.5 km above the bright band. In these heights the ice crystals may have the fastest growth rate around the temperature of −15°C. The impact of ice depolarizations on the satellite‐to‐ground path is shown to depend strongly upon the height and the background temperature at which ice crystals are first produced near the cloud top. This characteristic feature of ice depolarization is also confirmed in the stratus type events with considerable rain attenuation by theoretically subtracting the effects of raindrops from the total observed depolarization.
Controlled millimeter‐wave experiments and numerical simulations on the enhanced backscattering from one‐dimensional very rough surfaces
Authors: Phu Phillip , Ishimaru Akira , Kuga Yasuo
Published: February 2008

We present experimental results on the scattering of electromagnetic waves at millimeter‐wave frequencies from one‐dimensional very rough conducting surfaces with controlled surface roughness statistics. Very rough surfaces are defined as surfaces with rms height and correlation length of the order of a wavelength such that the rms slope is at least unity. It is expected that scattering experiments using these surfaces can provide useful insights since their statistics lie outside the range of validity of the present theories, namely, the Kirchhoff and perturbation theories. Strong backscattering enhancement at different incident angles, both in the transverse electric and transverse magnetic polarizations, are observed experimentally. Numerical calculations based on the exact integral equation method for cylindrical beam wave illumination compare favorably with the experimental results. The agreement between measurements and numerical calculations is good over a wide range of incident angles and for all scattering angles. The close agreement between the experimental results and numerical simulations indicates that this controlled experimental setup can be used to study scattering phenomena from one‐dimensional very rough surfaces with different roughness statistics as well as from two‐dimensional rough surfaces.
Analysis of rocket beacon transmissions for computerized reconstruction of ionospheric densities
Authors: Bernhardt P. A. , Huba J. D. , Chaturvedi P. K. , Fulford J. A. , Forsyth P. A. , Anderson D. N. , Zalesak S. T.
Published: February 2008

Three methods are described to obtain ionospheric electron densities from transionospheric, rocket‐beacon total electron content (TEC) data. First, when the line‐of‐sight from a ground receiver to the rocket beacon is tangent to the flight trajectory, the electron concentration can be obtained by differentiating the TEC with respect to the distance to the rocket. A similar method may be used to obtain the electron‐density profile if the layer is horizontally stratified. Second, TEC data obtained during chemical release experiments may be interpreted with the aid of physical models of the disturbed ionosphere to yield spatial maps of the modified regions. Third, computerized tomography (CT) can be used to analyze TEC data obtained along a chain of ground‐based receivers aligned along the plane of the rocket trajectory. CT analysis of TEC data is used to reconstruct a two‐dimensional image of a simulated equatorial plume. TEC data is computed for a linear chain of nine receivers with adjacent spacings of either 100 or 200 km. The simulation data are analyzed to provide an F region reconstruction on a grid with 15×15 km pixels. Ionospheric rocket tomography may also be applied to rocket‐assisted measurements of amplitude and phase scintillations and airglow intensities.
Azimuth smearing in ocean–synthetic aperture radar image spectra: A study of Hasselmann's closed‐form transformation based on Norwegian Continental Shelf Experiment 1988 synthetic aperture radar data
Authors: Høgda K. A. , Jacobsen S. , Krogstad H. E. , Engen G.
Published: February 2008

The well‐known along‐track resolution loss in synthetic aperture radar (SAR) ocean wave image spectra is investigated comparing simulations based on Hasselmann's nonlinear integral transform and measurements from the Norwegian Continental Shelf Experiment 1988 experiment. In the literature the resolution loss has often been modeled as a low‐pass filter process, described by the rms azimuth shift width σx, acting within a quasi‐linear SAR transformation. Estimates from real data of σx, as a function the range‐to‐platform velocity (R/V) and incidence angle, are compared to Hasselmann's new nonlinear spectral transformation and the widely used quasi‐linear model. Simulations correlated with real wave data show that the quasi‐linear model, with contributions to σx from the entire spectrum, overestimates σx by roughly 30–40%. The conformity between Hasselmann's model and real data is excellent, however. The numerics also indicate that the degree of nonlinearity in Hasselmann's transform is explicitly related to the surface truth parameters' significant wave height and peak wavelength. Furthermore, the spectral bandwidth, including the subresolution part of the ocean wave spectrum, seems to be of minor importance. It is also shown that the resultant smearing (due to imaging nonlinearities) cannot explicitly be separated from the coherent linear velocity bunching part of the transform. This point is especially discussed since it has led previously to some dissension regarding which ocean spectral components are most essential in the smearing process.
A comparison of the magnetic and electric field formulations and their postprocessing functionals in the finite element analysis of passive microwave devices
Authors: Lawrence Ben G. , Cambrell Gregory K.
Published: February 2008

Using a waveguide T junction and an orthogonal mode transducer as illustrative examples, the admittance matrix elements for a passive microwave device are computed by three‐dimensional finite element analysis from the magnetic field formulation having natural Neumann boundary conditions and from the electric field formulation having enforced Dirichlet boundary conditions. The results from these two formulations are compared using both linear and bilinear postprocessing functionals.
New integral equation method for CAD of open waveguide bends
Authors: Tanaka Kazuo , Tanaka Masahiro , Tashima Hisamitsu , Ootera Hiroki , Yoshino Yoshikatsu
Published: February 2008

New boundary integral equation method which was previously proposed have been examined in detail and extended to the computer‐aided design (CAD) of asymmetrical dielectric slab waveguide bends. Many numerical results of symmetric waveguide discontinuities and asymmetric waveguide bends confirm the validity of the new method. They show that all numerical results satisfy the energy conservation law within an accuracy of 1% and indicate that the new integral equation method is very suitable for the basic theory or a solver of CAD software for various open dielectric waveguide circuits.
Radar cross section computation of inhomogeneous scatterers using edge‐based finite element methods in frequency and time domains
Authors: Mahadevan K. , Mittra R.
Published: February 2008

This paper presents a flexible, body‐conforming finite element modeling technique, using Whitney's edge and face basis functions, for efficient computation of radar cross section (RCS) of inhomogeneous scatterers, both in the frequency and time domains. The vector absorbing boundary conditions, originally developed for a spherical absorbing boundary, are generalized for planar and cylindrical boundaries. The computational resources required for the proposed frequency and time domain techniques for RCS computation are compared with those needed in the finite difference time domain (FDTD) and the nonorthogonal FDTD methods. Illustrative numerical results, that demonstrate the accuracy of the technique, are presented.
Electromagnetic fluctuations for anisotropic media and the generalized Kirchhoff's law
Authors: Yueh Simon H. , Kwok R.
Published: February 2008

In this paper the polarimetric emission parameters for anisotropic media are derived using the generalized Kirchhoff's law for media with a uniform temperature and the fluctuation‐dissipation theory for media with a temperature profile. Both finite‐size objects and half‐space media are considered. When the object has a uniform temperature across its body, the Kirchhoff's law, based on the condition of energy conservation in thermal equilibrium is generalized to obtain the emission parameters of an anisotropic medium, which can be interpreted as the absorptivity or the absorption cross section of the complementary object with a permittivity that is the transpose of the original object. When the medium has a nonuniform temperature distribution, the fluctuation‐dissipation theory is applied for deriving the covariances between vector components of the thermal currents and, consequently, the covariances of the polarizations of electric fields radiated by the thermal currents. To verify the formulas derived from the fluctuation‐dissipation theory, we let the temperature of the object be a constant and show that the results reduce to those obtained from the generalized Kirchhoff's law.
Condition for random distribution of many dielectric spheres to be random for a coherent field
Authors: Tateiba Mitsuo , Nanbu Yukihisa
Published: February 2008

The condition for many dielectric spheres to be randomly distributed in view of expression of a coherent field is given in an analytic form. The condition is shown graphically for changing the parameters concerning spheres. The numerical results yield a quick computer simulation of a random distribution and may be useful for precise calculation of the effective wavenumber of a coherent field in a random distribution of dielectric spheres.
Polarimetric remote sensing of geophysical medium structures
Authors: Nghiem S. V. , Yueh S. H. , Kwok R. , Nguyen D. T.
Published: February 2008

Polarimetric remote sensing of structures in geophysical media is studied in this paper based on their symmetry properties. Orientations of spheroidal scatterers described by spherical, uniform, planophile, plagiothile, erectophile, and extremophile distributions are considered to derive their polarimetric backscattering characteristics. These distributions can be identified from the observed scattering coefficients by comparison with theoretical symmetry calculations. A new parameter is defined to study scattering structures in geophysical media. Experimental observations from polarimetric data acquired by the Jet Propulsion Laboratory airborne synthetic aperture radar over forests, sea ice, and sea surface are presented to illustrate the use of symmetry properties. For forests, the coniferous forest in Mount Shasta area and mixed forests near Presque Isle show evidence of the centrical symmetry at C band. In sea ice from the Beaufort Sea, multiyear sea ice has a cross‐polarized ratio e close to e0, calculated from symmetry, due to the randomness in the scattering structure. For first‐year sea ice, e is much smaller than e0 as a result of preferential alignment of the columnar structure of the ice. From polarimetric data of a sea surface in the Bering sea, it is observed that e and e0 are increasing with incident angle and e is greater than e0 at L band because of the directional feature of sea surface waves. Use of symmetry properties of geophysical media for polarimetric radar calibration is also suggested.
Tunneling modes in ionospheric waveguides
Authors: Platt I. G. , Dyson P. L.
Published: February 2008

The importance of tunneling rays in ionospheric structures is examined by considering VHF transequatorial propagation in field‐aligned equatorial plasma bubbles. It is found that tunneling rays can cause a significant decrease in the amount of VHF power capable of being guided by bubbles, therefore indicating that allowance should be made for tunneling modes when modeling this sort of propagation. While the power loss associated with tunneling rays decreases with a decrease of frequency, it increases with a decrease in the radius of curvature of a guide's cross section. Consequently, the effect of these modes on HF ducting within guides of small cross section, similar to those observed on topside ionograms, is also investigated. Results from this analysis also indicate a substantial amount of power loss due to tunneling modes, which must be accounted for in any detailed analysis of experimental HF ducting results.
Comparative observations of short‐period gravity waves (10–100 min) in the mesosphere in 1989 by Saskatoon MF radar (52°N), Canada and the MU radar (35°N), Japan
Authors: Nakamura T. , Tsuda T. , Fukao S. , Kato S. , Manson A. H. , Meek C. E.
Published: February 2008

We analyzed the wind variances of gravity waves during 10–100‐min periods observed in the mesosphere at 67–80 km altitude with the MU radar at Kyoto (35°N, 136°E) and the MF radar at Saskatoon (52°N, 107°W) in 1989. In order to compare the results quantitatively we applied similar data analysis procedures to the data sets which were obtained with different observation techniques. We first calculated frequency spectra of wind velocity, then estimated variances,
,
, and
by integrating the spectra. The frequency spectra of the horizontal wind u′ and υ′ generally showed a spectral slope of about −5/3 at both locations. The spectral slope of w′ at Saskatoon ranged from −2/3 to −1 in winter, and −1/3 to −1/2 in other seasons, while it became nearly zero in May and September at both locations. The slope for the MU radar observations was less steep and sometimes became +0.15–0.3 in the frequency region lower than 50–70 day−1 (periods more than 20–30 min). The variance,
,
, and
, showed semiannual variation with a large maximum in summer, a secondary peak in winter, and minima in September and April. The annual average of the ratios of
,
, and
between the MU and Saskatoon MF radars was 1.6, 1.5, and 1.6. This result is interpreted as suggesting that the variance of gravity waves with periods of 10–100 min at 35°N is stronger than that at 52°N, even if the possible biases are taken into account. The variances due to gravity waves are more conspicuously enhanced in summer at Kyoto than at Saskatoon.
Thin‐phase screen estimates of TID effects on midlatitude transionospheric radio paths
Authors: Reilly Michael H.
Published: February 2008

The thin‐phase screen model for ionospheric irregularity perturbations to transionospheric radio propagation is redefined. It is argued that the phase screen normal should be along the line of sight (LOS) between a receiver on the ground and a space transmitter, rather than in the zenith direction at the point of intersection with the LOS, which is traditional. The model is applied to a calculation of TID strength thresholds for the occurrence of multipath and scintillation. The results are in sharp disagreement with the traditional model, which predicts thresholds lower by an order of magnitude in typical cases. Midlatitude observations of TID strengths are reviewed, and it is found that multipath thresholds can be exceeded under one or more favorable circumstances, which include frequencies below about 100 MHz, low elevation angles, winter, night, atmospheric gravity wave velocity near the magnetic field direction and away from parallel with the LOS, and low solar activity.
On the derivation of an improved parameter configuration for the Dynasonde
Authors: Tsai L.‐C. , Berkey F. T. , Stiles G. S.
Published: February 2008

The inherent precision and ambiguity in the measurement of ionospherically reflected echoes by digital ionosondes, which utilize interferometric receiving arrays, depends on the transmitted pulse set pattern, the receiving array configuration, and the data analysis scheme. Building on earlier work carried out for the National Oceanic and Atmospheric Administration HF radar [Grubb, 1979] by Pitteway and Wright [1992], we use six phase parameters (Φo, Φx, Φy, Φl, Φp, and Φf) to derive echo location, Doppler shift, and wave polarization. We have applied the method of least squares to determine the precision and a “zero‐freedom” technique to derive the ambiguity associated with each of the phase parameters. Three criteria can be specified which lead to an optimum design of the system parameters: (1) the phase parameters must have aliasing values equal to 2π; (2) the relative confidence limit factors of the derived phase parameters should be as small as possible; and (3) there must be no aliasing of echo location. By varying the array configuration, frequency pattern, the number of pulses per pulse set, and the number of parallel receivers and receiving dipoles, various designs have been analyzed, and an improved configuration has been obtained. We have shown that various arrangements of the four‐pulse, two‐receiver configuration can reduce the sum of the relative uncertainties in the phase parameters found for the WERPOL array by up to 33% and improve the ambiguity in each of the derived phase parameters to 2π.
Proper and improper dominant mode solutions for a stripline with an air gap
Authors: Nghiem David , Williams Jeffery T. , Jackson David R. , Oliner Arthur A.
Published: February 2008

An investigation of the various proper and improper dominant mode solutions that may exist on a stripline with an air gap above the conducting strip is reported. The modes are called “dominant,” even though they sometimes do not propagate down to zero frequency, because the strip current has the same form as that of the conventional stripline dominant mode. The improper modes, which are not well known in this context, do not exist on stripline with a homogeneous substrate but may exist when the substrate is made inhomogeneous (for example by the presence of an air gap). These modes, whose fields increase to infinity transversely, are present in addition to, and independently of, the proper dominant mode, which has bounded fields. Depending on the frequency and geometrical parameters, the improper modes may be either leaky modes, which are physical and have a complex propagation constant, or improper real modes, which are nonphysical. Numerical results are presented that describe the properties of both the proper and improper modes and how they change with frequency and air gap size.
Radar studies of lightning‐induced plasmas with potential applications to radio communications and space surveillance
Authors: Dalkir Y. R. , Lee M. C.
Published: February 2008

The problem of radio backscatter from lightning‐induced plasmas is briefly reviewed. The predicted wavelength dependence of the backscattered power is compared to observations made with the MIT S band, C band and the Millstone UHF radar. The analysis is extended to a pancakelike plasma created through single radar beam heating of the atmosphere. The artificial plasmas are modeled as perfectly conducting, rough plasma disks, with random surface inhomogeneities described by either a Gaussian or power law type spectrum. Both monostatic and bistatic radar geometries are considered. These may be used in diagnostic and space surveillance/radio communication applications, respectively. The wavelength dependence of the radio reflectivity of a plasma pancake with Gaussian type surface roughness is found to be different from that of a plasma with a power law type surface roughness in the backscatter radar geometry. In the radio communication and space surveillance applications, the radio reflectivity may be independent of the incident wavelength, if the surface density inhomogeneities are characterized by an azimuthally symmetric power law type spectrum. By contrast, a wavelength dependence is expected for the Gaussian type spectrum.
Lithospheric stresses associated with nontransform offsets of the Mid‐Atlantic Ridge: Implications from a finite element analysis
Authors: Grindlay N. R. , Fox P. J.
Published: February 2008

Nontransform offsets are a fundamental aspect of the offset geometry exhibited along the mid‐oceanic ridge system, independent of spreading rate. Along the slow/intermediate opening (<40 mm/y full rate) Mid‐Atlantic Ridge these offsets of the ridge axis range in length from less than 10 km to approximately 30 km and vary in age offset from 0.5 to 2.0 m.y. The variable morphotectonic geometries associated with these discontinuities indicate that horizontal shear strains are accommodated by both extensional and strike‐slip tectonism and that the geometries are unstable in time. In many cases, there appears to be an evolutionary relationship between transform fault boundaries and nontransform offsets as the result of prolonged differential asymmetric spreading between adjoining ridge segments. The finite element method is used to study the complex stress field associated with these small‐offset discontinuities of ridges with slow (30 mm/y) and fast (100 mm/y) total opening rates. A plane stress plate model examines the variation in the horizontal tectonic stress field produced by offsets with different lengths and changes in the ratio of a ridge‐normal tensile stress resisting plate separation to a shear stress resisting relative plate motion along the discontinuity. The predicted fault patterns based on the calculated stress field are compared with seafloor observations in terms of the morphotectonic patterns and evolution of nontransform offsets. For a slow spreading rate, the analysis shows that all structural geometries observed can be modeled by a range of offset lengths (5, 10, 20, 30, and 40 km) and by a ridge‐normal stress 3 to 5 times greater than the discontinuity shear stress. These findings suggest that nontransform offsets are zones of mechanical weakness relative to the surrounding lithosphere. An offset length between 10 and 20 km is predicted to be the threshold length for maintaining a transform fault geometry. As inferred from ridge axis morphology, there seems to be a strong link between the magnitude of the stress ratio and the time varying magmatic activity along and between ridge segments. While our models are consistent with a weak discontinuity shear stress relative to the ridge‐normal stress to explain the geometries of nontransform offsets of slow‐spreading centers, a weaker ridge‐normal stress to discontinuity shear stress most closely models the development of an overlapping spreading center geometry, the distinctive geometry of nontransform offsets of spreading centers opening at fast rates. This difference is attributed to magma supply along‐axis, relatively continuous for fast‐spreading centers and intermittent for slow‐spreading centers, and a preexisting zone of mechanical weakness linked to the evolution of nontransform offsets from transform faults on slow‐spreading centers.
Crustal structure and reflectivity of the Swiss Alps from three‐dimensional seismic modeling: 1. Helvetic Nappes
Authors: Stäuble M. , Pfiffner O. A. , Smithson S. B.
Published: February 2008

Forward modeling based on combined geologic surface data, borehole and laboratory data on velocities, and seismic reflection data aid in understanding the seismic response in a transect through complex Alpine nappe structures. The synthetic seismic response from the model compares well with the observed data. The results suggest, however, that strong three‐dimensional effects result in out‐of‐plane reflections and diffractions, as well as defocussing and scattering of seismic rays, thus rendering the use of two‐dimensional migration techniques questionable. The model shows an unexpected rapid change in internal structure of the Helvetic nappes between the seismic line and outcrops located 6–8 km farther west. Moreover, the basement‐cover interface on the northern flank of the Aar Massif, a basement uplift with a heave of 8 km, does not rise regularly in the hanging wall of a single major thrust fault, but rises in a series of steps due to several thrusts and folds. The northernmost structures appear to be fault‐related open folds, whereas tight fold structures appear to dominate in the south.
Oxidants in the marine troposphere: H2O2 and O3 over the western Atlantic Ocean
Authors: Ray John D. , Van Valin Charles C. , Luria Menachem , Boatman Joe F.
Published: February 2008

Measurements of tropospheric H2O2 and O3 concentrations were made over the Atlantic Ocean near the U.S. east coast (southeast of Newport News, Virginia) and near Bermuda during July 1988 with an instrumented aircraft. Oxidant concentrations were high (O3 up to 100 parts per billion by volume (ppbv) and H2O2 up to 2.5 ppbv) near the U.S. east coast at low altitude but decreased rapidly with increasing offshore distance to concentrations comparable to those measured near Bermuda. O3 concentrations observed near Bermuda at low altitude were 14–22 ppbv, comparable to those reported for remote subtropical marine locations. O3 concentrations increased with altitude (up to 2.6 km); high‐altitude concentrations were 30–80 ppbv. H2O2 concentrations near Bermuda varied by a factor of 2, 0.7–1.5 ppbv, from day to day, but were nearly constant at all altitudes during individual flights. Because a high‐pressure system centered over the western Atlantic Ocean limited eastward advection of continental air toward Bermuda, these concentrations are assumed to be typical of mid‐latitude marine air during summer.
Distributions of chlorophyll and primary productivity in relation to water column structure in the eastern North Atlantic Ocean
Authors: Frazel Denis W. , Berberian G.
Published: February 2008

Latitudinal variations in the megascale (103 km) distribution of biological properties are described in relation to water column structure between 60° and 7°N in the eastern North Atlantic Ocean. Stations were occupied along a meridional transect of stations at 20°W in August‐September, 1988, during the third leg of the National Oceanic and Atmospheric Administration Global Change Expedition. An additional transect to the south (38°N to 7°N) was occupied to extend the total range of latitudinal observations. Chlorophyll a concentrations were highest in the northern latitudes (<2.51 mg m−3) , decreasing to >0.2 mg m−3 in the vicintiy of the subtropical gyre, south of 40°N. The nitracline was associated with a shoaling of the pycnocline in the northern latitudes. At 7°N, high chlorophyll concentrations (approximately 0.5 mg m−3), and enhanced primary productivity (375.5 mg C m−2 d−1) were associated with a lens of fresh Amazon River Water. Primary productivity rates were variable throughout the transect, ranging from 646.10 mg C m−2 d−1 to 138.26 mg C m−2 d−1. Productivity maxima were located south of Iceland, at 46°N (646.10 mg C m−2 d−1) and in the vicinity of the Azores Front at 35°N (259.85 mg C m−2 d−1). Latitudinal distributions of primary productivity corresponded closely to a model of productivity along a transect at 40°N by Yentsch (1990).
Sulfur dioxide over the western North Atlantic Ocean during GCE/CASE/WATOX
Authors: Luria M. , Van Valin C. C. , Gunter R. L. , Wellman D. L. , Keene W. C. , Galloway J. N. , Sievering H. , Boatman J. F.
Published: February 2008

Sulfur dioxide (SO2) was measured with a pulsed fluorescence analyzer and filter pack system from the instrumented NOAA King Air aircraft. The measurements were part of the GCE/CASE/WATOX project in July 1988 and included a series of eight flights of 4‐hour duration each. During the first four flights the atmosphere was sampled over the Atlantic Ocean within 250 km east of Newport News, Virginia; during the remaining four flights the atmosphere in the vicinity of Bermuda was sampled. SO2 data were collected at two elevations: about 750 mbar in the free troposphere (FT), and in the lower part of the boundary layer (BL) at about 1000 mbar. Average BL concentrations were almost 10 times greater near the east coast than near Bermuda (500±800 pptv versus 60±40 pptv). FT concentrations near the east coast also were greater than near Bermuda but by a lesser factor (110±50 pptv versus 70±40 pptv). The data from the continuous analyzer compared favorably with the data obtained from filter samples collected concurrently. SO2 data from this study are somewhat lower (factor of 0.6‐0.9) than previous measurements at both sites. Average concentrations of SO2 measured near Bermuda (60 pptv) were in the range of those observed in the remote marine atmosphere, although higher SO2 concentrations (up to 450 pptv) were also detected on occasion. The SO2 removal rate over the western Atlantic Ocean, near the east coast, is estimated empirically at 0.07±0.03 hour−1. Analyses of the known removal mechanisms suggested that the main removal mechanism is dry deposition.
Venus ionosphere: An interpretation of Mariner 10 observations
Authors: Bauer S. J. , Hartle R. E.
Published: February 2008

The dayside ionosphere of Venus observed by Mariner 10 may be understood in terms of a dynamic interaction with the solar wind which results in a compressed topside above an “F2 ledge” consisting of O+ and a dynamically unaffected F1 layer corresponding to a neutral temperature T≃380°K and consisting of O2+ and CO2+. The top of the upper ledge appears to be an ionopause caused by solar wind scavenging of He+, representing a solar‐wind obstacle consistent with the bow shock observations.
Determination of the external geomagnetic field from intensity measurements
Authors: Backus George E.
Published: February 2008

Suppose a closed, bounded region M contains all the sources (currents and magnets) of the geomagnetic field B , and M′ , the rest of space, is connected (any two points in M′ can be joined by a smooth curve). Suppose that the total intensity of B is known in some open region contained entirely inside M′. Then the vector field B is uniquely determined up to sign in all of M′. The result is less trivial than it seems, being false in two dimensions. Since the method of proof involves analytic continuation, computations based on it will probably be numerically unstable.