bluuebunny/crossref_metadata_2025 huggingface dataset:
Carbon isotopic composition of atmospheric CH4: Fossil and biomass burning source strengths
Authors: Quay P. D. , King S. L. , Stutsman J. , Wilbur D. O. , Steele L. P. , Fung I. , Gammon R. H. , Brown T. A. , Farwell G. W. , Grootes P. M. , Schmidt F. H.
Published: April 2008

The 13C/12C of atmospheric methane (CH4) was measured at Point Barrow (71°N, 156°W), Olympic Peninsula (48°N, 126°W), Mauna Loa (19°N, 155°W), and Cape Grim (41°S, 144°E) between 1987 and 1989. The global average δ13CPDB from these measurements (n = 208) was −47.20 ± 0.13%o. The lowest mean annual δ13C value of‐47.61 ± 0.14‰ was measured at Point Barrow with values increasing to ‐47.03 ± 0.14‰ at Cape Grim. The seasonal cycle in the δ13C of CH4 was greatest at Point Barrow, with an amplitude of 0.5‰, and varied inversely with concentration. The isotopic fractionation during CH4 oxidation is calculated to be 0.993 ± 0.002 based on the measured CH4 concentration and δ13C values. The 14C content of atmospheric CH4, measured at monthly intervals at the Olympic Peninsula site between 1987 and 1989, is increasing at 1.4 ± 0.5 pM yr−1, primarily owing to 14CH4 release from nuclear reactors. The global average 14C content of 122 pM for CH4 implies a fossil methane source strength that is 16% of the total source. The global mean δ13C of −47.2‰, when coupled with the 14C results, implies that ∼11% of the total CH4 release rate is derived from biomass burning. These results indicate for a total CH4 source of ∼550 Tg yr−1 that natural gas release accounts for ∼90 Tg yr−1 and biomass burning yields ∼60 Tg yr−1. Preliminary analyses of the δ13C data using a three‐dimensional chemical tracer model indicate that the observed meridional gradients in the annual average δ13C and concentration of CH4 are most closely matched with a CH4 source scenario in which 11% of the CH4 is derived from biomass burning.
Slowing the buildup of fossil CO2 in the atmosphere by iron fertilization: A comment
Authors: Sarmiento J. L.
Published: April 2008

Can the buildup of anthropogenic CO2 in the atmosphere be reduced significantly by fertilizing the surface southern ocean waters with iron? There aremany issues that need to be addressed in answering this question. Is iron scarcity indeed the dominant process limiting biological uptake of carbon, as hasbeen suggested by Martin [e.g., Martin et al., 199O]?
Historical oceanographic events reflected in13C/12C ratio of total organic carbon in laminated Santa Barbara Basin Sediment
Authors: Schimmelmann Arndt , Tegner Mia J.
Published: April 2008

An 1844‐1987 time series of carbon stable isotope ratios from dated sedimentary total organic carbon (TOC) from the center of the Santa Barbara Basin (SBB) is compared with historical climate and oceanographic records. Four isotopically distinct biogeochemical sources of TOC are important: phytoplankton‐derived marine biomass, macroalgal biomass from kelp forests, terrigenous biomass (mainly flushed into the SBB via river discharge), and redeposited fossil organic carbon. The significance of the latter two sources is largely limited to a few unusual flood and oil spill events, whereas the combination of13C‐depleted phytoplankton and13C‐enriched macroalgal biomass appears to be responsible for most of the isotopic variance of the marine coastal biomass as recorded in sedimentary TOC. The isotopic response of marine organic carbon in sediments records strong El Niño‐Southern Oscillation (ENSO) and the frequently associated severe storm and wave events in SBB varved sediments. The plausible major isotopic mechanisms are (1) increased physical liberation of13C‐enriched kelp carbon from locally abundant giant kelp (Macrocystisspp.) forests during times of physical and environmental stress, and (2) decreased productivity of13C‐depleted phytoplankton during ENSO events.
Vertical fluxes of biogenic particles and associated biota in the eastern North Pacific: Implications for biogeochemical cycling and productivity
Authors: Taylor Gordon T. , Karl David M.
Published: April 2008

Previously published data on vertical fluxes of particulate carbon (PC), nitrogen (PN), organisms (MICRO), and extracted adenosine triphosphate (ATP) into screened sediment traps (335 μm) from the VERTEX 5 and ADIOS I programs are reexamined as they relate to biogeochemical cycling and oceanic productivity. The four stations discussed represent an oligotrophic to mesotrophic gradient in total primary production (PT), ranging from 245 to 1141 mg Cm−2 d−1 and a gradient in PC flux from the euphotic zone, ranging from 12 to 164 mg Cm−2 d−1 for particles <335 μm in diameter. Vertical fluxes of PC, PN, MICRO, and ATP decreased as negative power functions of depth with significantly higher depth‐dependent losses for ATP fluxes. The flux of intact biota (free, particle‐associated, and some active “swimmers,” measured microscopically and by extracted ATP) decreased rapidly in the upper 200 m, contributing as much as 52.4% at the most productive station and as little as 1.6% to the flux of PC at oligotrophic stations, remaining relatively constant or increasing slightly (to 3.4 ‐ 9.6% PC flux) between 200 and 2000 m. Multiple regression analyses, expressing fluxes as functions of depth and PT or new production, PN, demonstrated that MICRO and ATP fluxes were more dependent on PT, PN, and depth than bulk PC or PN fluxes. The present analysis illustrates that while sinking particulate organic matter (POM) undergoes rapid attrition in the upper water column, the fluxes of sedimenting biota decrease at even higher rates. Findings support the hypothesis that in oceanic waters, POM sinking from the euphotic zone rapidly becomes a poor habitat for associated microbes, and mechanisms other than remineralization by attached microbes must be invoked to explain observed fluxes and attrition rates. This study also supports the hypothesis that the vertical flux of intact organisms is a more sensitive and less ambiguous record of upper ocean processes than bulk flux measurements of total mass, PC, or PN. Therefore time‐resolved measurement of the flux of biota may be useful in estimating PT and PN in the overlying waters.
Space‐time structure of auroral radio absorption events observed with the imaging riometer at south pole
Authors: Hargreaves J. K. , Detrick D. L. , Rosenberg T. J.
Published: April 2008

An imaging riometer system comprising 49 independent beams has been operating at South Pole station since January 1988. A study of intense, short‐duration events from the premidnight sector has defined their typical shape as elliptical, with axial ratio 2.3 oriented along the local L shell. The space‐time evolution shows rapid intensifications of the moving absorption patches.
Electromagnetic plane wave diffraction by a planar junction of two thin dielectric/ferrite half planes
Authors: Rojas R. G. , Ly H. C. , Pathak P. H.
Published: April 2008

A uniform geometrical theory of diffraction solution is developed for the two‐dimensional problem of high‐frequency plane wave diffraction by a planar junction of two thin dielectric/ferrite half planes. Each material half plane in this two‐part configuration is assumed to be electrically thin so that it can be replaced in the analysis by a generalized resistive boundary condition of 0(t), where t is the corresponding material slab thickness. The solution obtained is based on the Wiener‐Hopf technique, and it is shown that the present boundary value problem can be completely solved by imposing a junction condition at the junction of the two dielectric half planes in addition to the boundary and radiation conditions as well as the usual edge condition. This junction condition can be obtained if the field near the junction is modeled by a quasi‐static solution. The solution developed here can be further specialized to limiting cases for which either of the material half planes can become free space, perfectly conducting, or purely resistive, respectively. Several numerical examples are presented, and it is shown that the present solution reduces to known results and agrees very well with a moment method solution.
Interferometric cross‐spectral studies of mesospheric scattering layers
Authors: Stitt Gary R. , Kudeki Erhan
Published: April 2008

The results of the first simultaneous frequency domain and radar interferometry VHF radar observations of mesospheric scattering layers at Jicamarca are presented. Observations indicate the presence of fine scale structures with vertical extents less than 100–150 m in lower mesospheric layers and the distortion of higher mesospheric layers by gravity wave oscillations. The latter observation suggests the possibility that wind velocity measurements with mesosphere‐stratosphere‐troposphere radars may be non‐Eulerian in the presence of vertically propagating gravity waves. The data also show that gravity wave oscillations may at times significantly enhance the spectral broadening of higher mesospheric radar returns, even at vertical incidence.
Scattering by vertical, perfectly conducting rectangular plate at arbitrary azimuthal angle to incident wave
Authors: Gong J. , Maclean T. S. M.
Published: April 2008

The radiation field diffracted by a vertical, perfectly conducting rectangular plate on a perfectly conducting ground plane, when it is illuminated at any azimuthal angle by a vertical current element at ground level, is derived approximately in closed form. Experimental confirmation of this calculated field on a direct propagation path with different sizes of plate, aligned at different angles to the propagation path, is provided.
Interplanetary magnetic field control of drifts and anisotropy of high‐latitude irregularities
Authors: Basu Sunanda , Basu S. , Costa Emanoel , Bryant C. , Valladares C. E. , Livingston R. C.
Published: April 2008

Recently, much attention has been focused on the control exerted by the north‐south component of the interplanetary magnetic field (IMF) on the nature of large‐scale plasma structures in the polar cap ionosphere. In this paper we investigate whether the above IMF control also extends to the small‐scale irregularities of plasma density associated with the large‐scale structures. For this purpose, we have performed spaced‐receiver scintillation measurements at Thule and Sondrestrom, Greenland, using the 250‐MHz transmissions from quasi‐geostationary polar beacon satellites. Under IMF Bz northward conditions, moderate levels of amplitude (S4 < 0.6) and phase scintillations are observed with highly variable decorrelation times. Spaced‐receiver drifts under this situation show dramatic reversals of the true drift of the diffraction pattern from antisunward to sunward with moderate values of the axial ratio ranging between 4 and 12. For southward Bz we detect, in the central polar cap, a series of large magnitude scintillation (S4 ∼ 1) structures drifting at speeds of the order of 500 m s−1 in the antisunward direction indicating the passage of large‐scale ionization structures in the F region. In these cases the apparent drift speed of the diffraction pattern can only be determined as the pattern on ground is highly anisotropic (axial ratios 15 to 40) which makes it difficult to determine the true drift velocity. However, with suitable orientation of our antenna baseline we find that the apparent drift gives a fair estimate of the actual velocity. We also demonstrate that when these irregularities associated with the ionization patches in the F region transit across the polar cap and are observed at Sondrestrom, in conjunction with underlying E region ionization caused by auroral particle precipitation (as sensed by simultaneous incoherent scatter radar measurements of densities and temperatures), the irregularity anisotropy is much reduced. This reduction of anisotropy is possibly a result of increased cross‐field diffusion due to coupling with the E region having enhanced density. The true drift of the diffraction pattern measured at Sondrestrom on one evening agrees remarkably well with the simultaneous incoherent scatter radar measurements of F region plasma drifts, both sets of drifts showing equatorward and eastward motion varying between 0.5 and 1 km s−1.
Long‐range detection of VLF radiation produced by heating the auroral electrojet
Authors: Barr R. , Stubbe P. , Kopka H.
Published: April 2008

This paper presents the first evidence of long‐range detection (>1000 km) of calibrated VLF signals resulting from the HF heating of the auroral electrojet, that is, signal detection at a point of direct “line of sight” of the heated patch of ionosphere. Other workers have presented calibrated data from shorter ranges (190–550 km) or claimed the detection of uncalibrated signals at ranges greater than 6000 km, but we believe that no such calibrated and clear signals, like those presented here from a range of greater than 2000 km, have previously been reported. Also, in contrast to earlier long‐range detection experiments we record the “radial” as well as the “azimuthal” magnetic component of the signals and from their ratio obtain the waveguide mode polarization. Observed absolute magnetic field strengths and waveguide polarizations are found to be in line with the predictions of simple waveguide models.
Approximate solutions and error bounds for the wave equation in a stratified ionosphere with turning points
Authors: Nalesso G. F. , Jacobson A. R.
Published: April 2008

In this paper we present a solution of the problem of a plane electromagnetic wave traveling parallel to a constant magnetic field (k‖B0‖z/|z|) in a horizontally stratified ionosphere. We assume, in particular, that the permittivity of the medium can be decomposed as the sum of an unperturbed and a perturbed part and that the unperturbed term is a smooth function of the height z that can be analytically continued in the complex plane and has furthermore a single simple zero (turning point) in a simply connected region D which includes the interval (α, ∞) of the real axis. The method gives approximate solutions valid in the region D and therefore also through the turning point. These solutions are obtained by truncating certain formal (generally divergent) series solutions of the wave equation. Furthermore, a realistic error bound for the remainder term associated with the truncated series is also derived and discussed. This method is successively applied to the case of a linearly varying permittivity of a lossless ionosphere with a superimposed Gaussian perturbing term. The possibility of applying the method when an odd number of turning points are present is also discussed.
Electromagnetic scattering for multilayered sphere: Recursive algorithms
Authors: Wu Z. S. , Wang Y. P.
Published: April 2008

We have proposed more stable and accurate recursing equations and a computing procedure to calculate scattering coefficients for a multilayered sphere. The procedure involves three logarithmic derivatives of Ricatti‐Bessel functions ψn′(z)/ ψn(z), χn′(z)/ χn(z), and ξn ′(z)/ξn (z), as well as the ratio ψn(z)/χn(z). The asymptotic behavior, stability, and accuracy of the procedure and scattering coefficients an and bn are discussed for any complex refractive indices. This procedure can be used for large and small inhomogeneous spherical particles.
Comparison of observed and calculated incoherent scatter spectra from the D region
Authors: Hansen Georg , Hoppe Ulf‐Peter , Turunen Esa , Pollari Päiviö
Published: April 2008

During August 1989, extended twilight and nighttime measurements with the European incoherent scatter (EISCAT) UHF radar were performed under PCA conditions. This provided an excellent data quality in the altitude region of 70 to 90 km throughout the three nights of August 12 to 15, 1989. A sophisticated control program allowed the measurement of the spectral width in the altitude region mentioned. In general, the measured spectral width deviates significantly from model values based on temperatures measured simultaneously by Na lidar combined with CIRA 88 temperature and density values. The observed spectra are up to 2 or 3 times narrower. In our observations the deviation tends to increase with increasing altitude. We also find that earlier spectral width measurements published by other workers are often narrower than current D region theory predicts. The possible reasons for this phenomenon are discussed.
Scattering of an obliquely incident wave by a multilayered elliptical lossy dielectric cylinder
Authors: Kim Chang S. , Yeh C.
Published: April 2008

The scattering problem of obliquely incident waves on a multilayered elliptical cylinder is considered. The cylinder is assumed homogeneous and isotropic but lossy. Both polarization of the incident wave, the E wave, and the H wave are considered. In the theoretical analysis the number of layers are unlimited, but in the numerical analysis we limit the number of layers so we can avoid excessively long computer running time. The numerical results include the bistatic radar cross sections, the specular differential cross sections, and the efficiency factors for absorption, scattering, and extinction. The computed results show the dependence of the scattering on the size, shape, material, and the angle of the incident.
Origin of short‐period (30–300 s) Doppler frequency fluctuations of lower F region reflections in the equatorial electrojet region
Authors: Sastri J. Hanumath , Ramesh K. B. , Somayajulu V. V. , Rao J. V. S. V.
Published: April 2008

Measurements of phase path P of lower F region reflections at normal incidence at Kodaikanal (10°14′N, 77° 28′E, dip 3.0°N) revealed the ubiquitous presence of 30–300 s quasi‐sinusoidal variations in the time rate of change of phase path, P (Doppler frequency shift) during day time. A study is made of the influence of the irregularities in the equatorial electroject on the P fluctuations using simultaneous observations of F region phase path at Kodaikanal and of equatorial electrojet with the VHF backscatter radar at Thumba (08° 29′N, 76° 56′E, dip 0.9°S). It is shown that the spectral content of the Doppler fluctuations (quantified in terms of variance, σ2 computed from P time series synthesized through FFT−1 (fast Fourier transfrom) in the chosen period bands, 30–300 s/30–120 s of the FFT of original P time series) bears a significant positive linear relationship to the horizontal phase velocity (Vp) of electrojet irregularities (3‐m scale size)on a hourly basis. This result is in consonance with our earlier findings (Sastri et al., 1990) of a significant linear relationship of σ2 to the electrojet strength (estimated from H field data) and a practical cessation of the P fluctuations at times of disappearance of Esq on ionograms (partial/complete counterelectrojet). The present work substantiates the interpretation that the short‐period Doppler frequency fluctuations are due to phase path changes imposed on lower F region reflections by the refractive index variations associated with the convective motions of plasma density irregularities (type I and II) in the daytime equatorial electrojet.
Watox‐86 overview and western North Atlantic Ocean S and N atmospheric budgets
Authors: Galloway James N. , Whelpdale Douglas M.
Published: April 2008

This paper is an overview of an intensive period of data collection performed as part of the Western Atlantic Ocean Experiment (WATOX) during early 1986. The objective of WATOX‐86 was to determine the flux and fate of a variety of atmospheric materials that are advected eastward from North America to the atmosphere of the western North Atlantic Ocean (WNAO). WATOX‐86 used two National Oceanic and Atmospheric Administration aircraft, a twin engine King Air and a four engine WP‐3D, to collect gases and aerosols off the east coast of North America from Florida to Newfoundland. The WP‐3D was based at McGuire Air Force Base, New Jersey, during the first 10 days of January 1986. The King Air was based at Hanscom Field, Massachusetts, during periods in January and February 1986 and on Bermuda during May 1986. Measurements were made of a variety of gases including SO2, NOx, NOy, peroxyacetyl nitrate, CO, O3, C2−C6 hydrocarbons, N2O and dimethyl sulfide. In addition, aerosols were collected and analyzed for black carbon, SO4=, NO3−, NH4+ and trace metals. The individual papers of this special issue provide details on the results and analysis of the individual measurement programs. This paper provides a brief summary of each program. During WATOX‐86, there were large regions of the WNAO atmosphere that had elevated concentrations of the measured gas and aerosol species. The elevated concentrations were the result of transport of the products of anthropogenic activities from North America to the WNAO atmosphere. The high degree of correlation among the concentrations of the different species indicated that these materials were transported in “clouds” of polluted air that covered thousands of square kilometers. The WATOX‐86 data on sulfur and nitrogen oxides are analyzed in the context of atmospheric budgets for WNAO. Approximately 25 to 35% of the sulfur and 15 to 25% of the nitrogen oxides emitted to the eastern North American atmosphere are advected eastward to the WNAO atmosphere. Of this material, about half of the sulfur and most of the nitrogen oxides are deposited to the WNAO. For both sulfur and nitrogen oxides, wet and dry deposition are of approximately equal importance.
Deep basement seismic reflection profiling of the Purcell Anticlinorium using a land air gun source
Authors: Cook Frederick A.
Published: April 2008

Seismic reflection profiling in the Purcell anticlinorium of southern British Columbia, Canada, using a land air gun source, has revealed reflections from depths of at least 15–20 km. Interpretations of these data, in conjunction with related geological and geophysical information, confirm the allochthonous structure of the Purcell anticlinorium and suggest that autochthonous North American basement at 15–20 km depth beneath the anticlinorium has a westward dip of 15°–20°. The thickness of the crust above the North American basement surface may be accommodated by structural repetition of Purcell strata, by thrust slices of North American basement rocks, or by both. The geometry observed on the reflection data implies that the Purcell anticlinorium is cored by a crustal‐scale thrust anticline (hanging wall anticline). Further, the data demonstrate that land air guns are effective sources for crustal reflection work.
Tectonic rotations within the Rio Grande Rift: Evidence from paleomagnetic studies
Authors: Brown Laurie L. , Golombek Matthew P.
Published: April 2008

Paleomagnetic studies on Miocene‐Pliocene volcanic rocks from the Espanola basin of the Rio Grande rift, New Mexico, reveal directions discordant from the expected mean direction for North America. The Paliza Canyon Formation, Tschicoma Formation, and Lobato Basalt, all sampled in the Jemez Mountains west of the Pajarito fault zone, have mean declinations east of the expected mean. The Cerros del Rio volcanics, lying east of the Pajarito fault zone, have a westerly declination. Combined with published data on the Santa Fe Group sediments east of the fault zone, and the Valles Rhyolite, west of the fault zone, distinct rotations of the two areas are evident. The western block has rotated clockwise 12°, while the eastern block shows 16° of counter‐clockwise motion. Differential rotations of 25°–30° are calculated between the two blocks; 4°/m.y. is the minimum differential rotation for the past 5 m.y. Geologic explanations for these rotations include the opening of the Rio Grande rift in response to clockwise rotation of the Colorado Plateau and significant left slip along the Rio Grande rift.
Late Quaternary paleomagnetic field secular variation from two Minnesota Lakes
Authors: Lund Steve P. , Banerjee Subir K.
Published: April 2008

Detailed paleomagnetic records from the wet sediments of two Minnesota lakes are the basis for estimating the local secular variation of the earth's magnetic field during late Quaternary time. Results from Lake St. Croix cover the interval 0‐9500 years B.P., and results from Kylen Lake cover the interval 4000–14,000 years B.P. Paleomagnetic data were recovered from two replicate cores in each lake by sampling the sediments at 3‐ to 10‐cm intervals. Analysis of the potential errors in the results indicates that all cores were correctly oriented in the vertical plane but that some azimuthal offsets of individual core segments did occur. Various methods for correcting the occasional azimuthal offsets are described. The final paleomagnetic data sets from each lake show a high degree of internal consistency. These results also show close correspondence (1) between the two lakes, (2) with the known historic magnetic field variation, and (3) with paleomagnetic results from dated archeological material and lava flows from the western United States. Rock magnetic analysis of the lake sediments indicates that the natural remanent magnetization is the result of a physical remanence acquisition process (detrital remanent magnetization) active at or near the sediment/water interface. There is no evidence for a systematic inclination error in the results. Statistical analysis of vector time series determined from the paleomagnetic data indicates that both the field‐vector and virtual geomagnetic pole (VGP) distributions are non‐Fisherian and essentially axial‐dipolar when averaged over 10,000 years. Spectral analysis of the scalar inclination and declination suggests that the field behavior is separable into a dominant low‐frequency dipolelike waveform (period of ∼9500 years) and a higher‐frequency band irregular waveform (“fundamental” period of ∼2400 years with multiples at 1200, 800, and 600 years) that may represent the nondipole field. This irregular waveform is interpreted as resulting from zonal drift of the nondipole field past Minnesota on the basis of waveform morphology and VGP circularity. Zonal drift of the nondipole field plus a low‐frequency dipole variation can account for almost all of the observed paleomagnetic field variation.
The role of Earth tides in the balance of tidal energy
Authors: Platzman George W.
Published: April 2008

The main role of earth tides in the planetary balance of tidal energy is to dispose elastically of energy transmitted to the solid earth from the ocean. This is accomplished by negative correlations between body tide and load tide transports and forces. The correlations arise from ocean dissipation. They are illustrated here in a model that has an elastic lithosphere and a rigid mantle.
Radiogenic rare gases and the evolutionary history of the depleted mantle
Authors: Fisher David E.
Published: April 2008

U, Th/He, K/Ar, and U/Xe ages of the depleted mantle source region of oceanic basalts are much less than the age of the earth, showing clearly that the midocean ridge basalt mantle source was not completely degassed and decoupled from the atmosphere early in earth history. Rather, extensive degassing has continued up to at least the last few hundred million years. A stable, layered mantle is not suggested by these data.
Excitation of a buried magmatic pipe: A seismic source model for volcanic tremor
Authors: Chouet Bernard
Published: April 2008

Recent observations of seismic events at various volcanoes suggest that harmonic tremor results from the sustained occurrence of so‐called long‐period or low‐frequency events. Accordingly, we can view the long‐period volcanic event as the elementary process of tremor and interpret it as the impulse response of the tremor‐generating system. We present a seismic model in which the source of tremor is the acoustic resonance of a fluid‐filled volcanic pipe triggered by excess gas pressure. The model consists of three elements, namely, a triggering mechanism, a resonator, and a radiator. For simplicity, we assume a hemispherical trigger, cylindrial resonator, and circular radiator set in a vertical configuration with the trigger capping the top of the pipe and the disk‐shaped radiator shutting off its bottom. Considering the simple case of a source buried in a homogeneous half space, we then apply the discrete wave number method to obtain a complete representation of the ground motion response at near and intermediate distances. The results demonstrate that the displacement attributed to the pipe dominates the near‐field motion, while that due to the disk is representative of the intermediate and far fields. The trigger itself has a smaller contribution, mainly limited to the field in the proximity of the source. The characteristics displayed by the free surface response evolve from a strong impulsive signature in the immediate vicinity of the epicenter to a well‐developed harmonic wave train dominated by Rayleigh waves at larger distances. No clear shear arrival can be detected in the synthetic seismograms. The displacement spectrum reflects the organ‐pipe modes of the conduit, and the bandwidth associated with the dominant spectral peak of motion is controlled by the combined losses due to viscous attenuation in the fluid and elastic radiation into the solid. In the case of the cylindrical magma column considered, the radiation loss is proportional to the square of the pipe radius, while the loss related to viscous damping is inversely proportional to the same factor, indicating that the relative importance of the two loss mechanisms is critically dependent on the geometry of the magma reservoir. The relative importance of the pipe and disk elements, likewise, is a function of the conduit cross section. This suggests the possibility of determining the geometry of the source as well as the radiation loss and in situ magma viscosity from a comparison of near‐ and far‐field observations.
Impact cratering history of the Saturnian satellites
Authors: Plescia J. B. , Boyce J. M.
Published: April 2008

The impact crater flux history of the Saturnian system has been estimated based on observed crater density data from the satellite surfaces, cratering data for the terrestrial planets, and theory. The proposed cratering flux has an history similar to that for the inner solar system: an initial very high cratering rate which exponentially decayed during the first few hundred million years followed by a more constant but lower cratering rate to the present. The dominant source of crater‐forming projectiles for the Saturnian satellites is suggested to be external to the Saturn system. The modeled present cratering rate at Iapetus is about the same order of magnitude as on the earth. Crater ages derived from the model suggest that most surfaces, the heavily cratered terrains, are >3.5 b.y. old. However, some surfaces have distinctly younger ages, <3.5 b.y., even considering the statistical uncertainties. The plains units on Dione and Enceladus formed over several hundred to billions of years beyond the end of the heavy bombardment.
Global map of eolian features on Mars
Authors: Ward A. W. , Doyle K. B. , Helm P. J. , Weisman M. K. , Witbeck N. E.
Published: April 2008

Ten basic categories of eolian features on Mars were identified from a survey of Mariner 9 and Viking orbiter images. The ten features mapped are (1) light streaks (including frost streaks), (2) dark streaks, (3) sand sheets or splotches, (4) barchari dunes, (5) transverse dunes, (6) crescentic dunes, (7) anomalous dunes, (8) yardangs, (9) wind grooves, and (10) deflation pits. The features were mapped in groups, not as individual landforms, and recorded according to their geographic positions and orientations on maps of 1:12.5 million or 1:25 million scale. In the north polar region, light and dark streaks indicate winds from the west and northwest. Frost streaks show northeast and northwest winds. Barchan dunes show southwest and west winds. Transverse dunes show east and west winds. Local erosional features show winds from the northwest or southeast and northeast or southwest. In the middle and low northern latitudes, streaks show northeast winds; yardangs and deflation pits show mostly easterly and northeasterly winds. In the low southern latitudes, light streaks and dunes record northwest and northeast winds; dark streaks record southeast winds. In high southern latitudes, most streaks and dunes record southeast and east winds. In the south polar region, light and frost streaks record southwest and northwest winds, whereas dark streaks and transverse dunes show southeast winds. The patterns recorded for ephemeral features conform to global conditions of strong southern spring and summer (northern fall and winter) wind circulation. Erosional features in bedrock indicate long‐term and perhaps ancient wind trends, whereas depositional features may record relatively more recent winds. Wind directions indicated by yardangs, pits, and other erosional features seldom correspond to those shown by streaks. These erosional features indicating winds of different directions than those that formed the streaks may have been carved when the effective regional winds on Mars were different due to relative hemisphere insolation differences, which are related to obliquity variations. Alternatively, many yardangs and pits may be largely controlled by structure and carved along surface structural patterns or features that channeled both ancient and present‐day winds. Deflation pits in the mantled terrain may contain the best record of ancient wind trends. These pits seem to have stratigraphically related orientations. Different stratigraphic units composing the mantle contain wind erosion pits that possibly record large‐scale wind direction changes because of long‐term changes in the obliquity of the Mars.
Storm time changes of geomagnetic field at Magsat altitudes (325–550 km) and their comparison with changes at ground locations
Authors: Kane R. P. , Trivedi N. B.
Published: April 2008

The values of H, X, Y, and Z at MAGSAT altitudes were first expressed as residuals ΔH, ΔX, ΔY, and ΔZ after subtracting model values HMD, XMD, YMD, and ZMD. The storm time variations of ΔH showed that ΔH (dusk) was larger (negative) than ΔH (dawn) and occurred earlier, indicating a sort of hysteresis effect. Effects at MAGSAT altitudes were roughly the same (10% accuracy) as at ground, indicating that these effects were mostly of magnetospheric origin. The ΔY component also showed large storm time changes. The latitudinal distribution of storm time ΔH showed north‐south asymmetries varying in nature as the storm progressed. It seems that the central plane of the storm time magnetospheric ring current undergoes latitudinal meanderings during the course of the storm.
Antisunward space current below the Magsat level during magnetic storms and its possible connection with partial ring current in the magnetosphere
Authors: Suzuki Akira , Yanagisawa Masahisa , Fukushima Naoshi
Published: April 2008

It is possible to calculate, with the Magsat data, the total amount of electric current passing through the plane enclosed by the Magsat orbit, from the line integration of the observed magnetic field component tangential to the Magsat orbit, i.e., a direct application of Ampère's theorem. After eliminating the spurious effect arising from the earth's rotation under the Magsat orbit, it is concluded that the space current of a few million amp. flows antisunward below the Magsat level during magnetic storms, whereas such a space current is almost absent on quiet condition. The depression in the horizontal intensity of geomagnetic field in low latitudes is noticeably greater on the duskside than on the dawnside, especially at the developing stages of magnetic storms. Combining these two observations, we may conclude that 10–20% of the westward ring (or shell) current in the dusk magnetosphere will close its current circuit with antisunward field‐aligned current in the magnetosphere connected with a net horizontal current flowing in the ionosphere.
Magnetic field disturbances over auroral arcs observed from Spitsbergen
Authors: Lanchester B. S. , Wallis D. D.
Published: April 2008

Coincident two‐station optical auroral data and magnetic field measurements from the polar‐orbiting satellite Magsat have allowed the comparison of the positions of small‐scale disturbances in the magnetic field with the positions of optical forms over Spitsbergen. For one discrete arc seen at 0715 UT on January 15, 1980, we have found at the position of intercept between the satellite and arc a narrow sheet of enhanced current density embedded in a broader region of lower current density which appears to mark the poleward boundary of the upward current region (region 2). Other simultaneous optical and magnetic field measurements made at 1500 UT on January 19, 1980, indicate that multiple arcs were embedded in the broad region of upflowing current with a bright arc close to the boundary between the auroral oval and polar cap.
A comparison of satellite and observatory estimates of geomagnetic secular variation
Authors: Barraclough D. R.
Published: April 2008

A comparison is made between secular variation values computed at the positions of 168 magnetic observatories from two spherical harmonic models based almost entirely on the 7‐month spread of Magsat data and observed secular variation values derived from the observatory annual mean values. The two models differ from each other (rms difference in Z = 27.8 nT/yr) and from the observed values (rms differences in Z of 22.1 and 30.6 nT/yr). It appears that a time span of more than 7 months is needed to produce accurate models of the secular variation from satellite data.
Magsat vertical field anomalies above 40°N from spherical cap harmonic analysis
Authors: Haines G. V.
Published: April 2008

A new technique, the method of spherical cap harmonic analysis, has provided the means to construct an analytical model of the vertical field above 40°N from Magsat data. With this technique, proper consideration can be taken of altitude variation and of the source‐free constraint that the vertical field be the vertical derivative of a potential function. Maps can be produced at any altitude. Here they are given at the lower and upper limits of the Magsat satellite, 6700 and 6900 km radial distance or approximately 335 and 535 km geodetic altitude. A novel decimation procedure, applied to NASA “Investigator B” data screened for low disturbance levels, results in an approximately uniform distribution in area, about 1 point per (150 km)2. Each of the 5087 data points represents an average over 5 s of time or 37 km of satellite track. The harmonic model, of maximum index 22, includes only those basis functions that are necessary for fitting a potential which need not be differentiable in colatitude and that are therefore completely orthogonal over the spherical cap. Although there are 276 coefficients in the theoretical model, only 190 were found to be statistically significant. The resulting scatter, or standard error of estimate, was 3.6 nT.
The physical basis ofmb:Msand variable frequency magnitude methods for earthquake/explosion discrimination
Authors: Stevens Jeffry L. , Day Steven M.
Published: April 2008

We use recent results from deterministic numerical modeling of earthquakes and explosions to develop an improved physical basis for earthquake/explosion discrimination. We use six three‐dimensional finite difference simulations of earthquakes for the earthquake source models and four empirical and four numerical models for explosions in different materials. Four factors contribute to the success of themb:Msdiscriminant: source spectra, focal mechanism, near‐source elastic properties, and interference of ρp. Spectral differences contribute about 0.5 magnitude unit to the separation between earthquake and explosion populations for large events, but this difference vanishes for events withmb< 4.5. The difference in focal mechanism (quadrupole versus monopole) contributes about 0.35 magnitude unit on the average but also introduces a scatter of ±0.5 magnitude unit into the earthquake population. Differences in near‐source elastic properties tend to increase the separation, contributing a factor of approximately (ρα3)½(ρ is density and α isPwave speed) to the ratio of surface wave amplitude to body wave amplitude. Interference of ρpimproves discrimination for large, deeply buried explosions but may degrade or preventmb:Msdiscrimination of small, shallow explosions. If biases due to regional variations in propagation path are absent, or corrected for, we predict that themb:Msmethod will successfully discriminate most earthquakes from explosions, but there will be some overlap between populations at small magnitudes. The variable frequency magnitude (VFM) discriminant is based on spectral differences betweenPwaves produced by earthquakes and explosions. Theoretical separation of earthquake and explosion populations occurs for events withmb> 4.0, using a frequency band of 0.5–3.0 Hz. For small events the VFM discriminant is complementary tomb:Msin several ways: VFM is improved by ρpinterference, whilemb:Msis degraded; VFM is more effective for hard rock explosions, whilemb:Mswill preferentially discriminate explosions in low‐velocity materials; and VFM is insensitive to fault orientation. VFM is more sensitive to noise and regional attenuation differences thanmb:Ms. Simultaneous use of both methods should allow improved discrimination of small events.
Permeability versus depth in the upper oceanic crust: In situ measurements in DSDP hole 504B, eastern equatorial Pacific
Authors: Anderson Roger N. , Zoback Mark D. , Hickman Stephen H. , Newmark Robin L.
Published: April 2008

In situ permeabilities measured within the upper kilometer of oceanic crust in hole 504B on the south flank of the Costa Rica Rift decrease exponentially from 10−13 to 10−14 m2 in layer 2A (the upper 150 m) to 10−15 to 10−17 m2 in layer 2B (150–550 m into basement), and to 10−17 m2 and lower in layer 2C (deeper than 550 m). We estimate the permeability (k) versus depth (z) to vary as k(z) = 0.11e(−z/50) × 10−12 m2. If this permeability versus depth function is representative of the oceanic crust in general, then hydrothermal convection would be vertically stratified, with the most vigorous circulation confined to the shallowest pillow basalt layers of the crust. The use of a full suite of geophysical logs (including borehole televiewer imagery; multichannel, full waveform sonic; nuclear; and variable spacing electrical resistivity logs) allows the characterization of the scale and interconnectedness of fractures, and the degree of infilling of these fractures by alteration minerals. At hole 504B, we observe a relationship between permeability and fracture porosity, determined from the geophysical logs. If fractures with similar aspect ratios are encountered at other oceanic sites, then prediction of the permeability of the oceanic crust to within an order of magnitude is possible from geophysical logs. At hole 395A in the Atlantic, for example, in situ permeability measurements yielded values similar to those predicted by the logging relationships established at hole 504B.
Thermoluminescence measurements on shock‐metamorphosed sandstone and dolomite from Meteor Crater, Arizona: 2. Thermoluminescence age of meteor crater
Authors: Sutton S. R.
Published: April 2008

Thermoluminescence (TL) dating measurements were performed on shock metamorphosed rocks from Meteor Crater, Arizona. IL ages were obtained for eight specimens, four Coconino sandstone and four Kaibab dolomite, which experienced sufficient shock heating during the impact to reset their preimpact TL. Postimpact radiation doses were determined for each shocked fragment from thermoluminescence measurements on quartz mineral separates. Effective radiation dose rates were calculated from concentrations of uranium, thorium, potassium, and rubidium in the rocks and their burial environment and the distributions of these elements as determined by fission track and microprobe analyses. Cosmic radiation dose rates were estimated by altitude scaling of sea level data and were corroborated by TL dosimeter measurements at the crater. The mean TL ages for the two rock types were reasonably concordant, 50,400±2900 and 46,000±3100 years for sandstones and dolomites, respectively. The overall mean age, 49,000±3000 years, is significantly older than the 25,000‐year age estimate based on the inferred correlation of the oldest pluvial episode at the crater with radiometrically dated δ18O maxima.
Gravity and lithospheric stress on the terrestrial planets with reference to the Tharsis Region of Mars
Authors: Sleep Norman H. , Phillips Roger J.
Published: April 2008

An analytical theory for isostatic compensation in one‐plate planets, including membrane stresses in the lithosphere, self‐gravitation, and rotational ellipticity, is applied to Mars. Interpretation of the results depends on whether the outer radial fractures around Tharsis are old or young features. If the outer fractures are old, Mars is inferred to be basically isostatic and the Tharsis region is inferred to have thin crust over lower‐density lithospheric mantle. For an average crustal thickness of 50 km and a lithospheric thickness of 450 km, the computed crust is essentially absent near the center of Tharsis. Calculations indicate that a pronounced directionality of the stress field should occur only as the radial fractures around Tharsis, in agreement with observations. The greater extent of radial graben early in Martian history is attributed to general thermal expansion of the planetary interior which adds extension to the strains produced by the isostatic stresses. Conversely, later planet‐wide contraction enhances circumferential thrusts around Tharsis. For planet‐wide expansion and isostatic stresses to interact in this way, the strain rates from each process must be comparable. This implies an average lithospheric viscosity of 1028 P or 1027 Pa s. Long wavelength loads, such as those associated with extrusive volcanism, produce strains similar to those associated with expansion of the interior. These flexural strains may have been a significant, but not dominant, contribution to Martian tectonics earlier in the planet's history when load‐forming processes were active. That is, some of the older fractures may have been produced by the relaxation of flexure at that time. It is likely that most of the stresses associated with flexure have now relaxed and the present stresses, crustal thicknesses, and lithospheric densities are similar to those predicted assuming isostasy. In contrast, if the outer radial fractures are young, a couple of kilometers of the volcanic load on Tharsis is probably supported by flexure. As in the model for older radial fractures, the lithospheric mantle is less dense beneath Tharsis and the relaxation time for flexure in the lithosphere is quite long. The flexurally supported loads probably formed late in the planet's history, and the compensation was probably more nearly isostatic before that time.
Horizontal deformation in the Mojave Desert near Barstow, California, 1979–1983
Authors: King N. E.
Published: April 2008

The Barstow trilateration network spans late Cenozoic right‐lateral strike‐slip faults in the Mojave Desert block in southern California. The network has been surveyed four times between 1979 and 1984. A marginally significant increase of 0.47±0.18 μrad in fault‐parallel right‐lateral shear strain occurred in 1980. This increase may reflect random error in measurement since more than half of it is due to changes in only three lines, while other lines favorably oriented to detect shear show no change. The overall rate of fault‐parallel shear is 0.06±0.05 μrad /yr, a rate which does not differ significantly from zero. The maximum shear strain rate is 0.08±0.05 μrad /yr, oriented N21°W±17°.