bluuebunny/crossref_metadata_2025 huggingface dataset:
ISEE 3 observations of traveling compression regions in the Earth's magnetotail
Authors: Slavin J. A. , Smith M. F. , Mazur E. L. , Baker D. N. , Hones E. W. , Iyemori T. , Greenstadt E. W.
Published: February 2008

A traveling compression region (TCR) is a several‐minute long compression of the lobe magnetic field produced by a plasmoid as it moves down the tail. They are generally followed by a longer interval of southward tilting magnetic fields. This study reports the first comprehensive survey of TCRs in the distant magnetotail. A total of 116 TCRs were identified in the ISEE 3 magnetic field observations. Of this population, 37 TCRs were observed to be separated by 30 min or more from any other TCR and are termed “isolated” events. “Paired” events are defined as two TCRs separated by less than 30 min. There were 36 such TCRs corresponding to 18 paired events. “Multiple” events were also observed in which more than two TCRs occurred in a series without a gap between TCRs of more than 30 min. The 11 multiple events identified in this study had an average of about four traveling compression regions each for a total of 43 TCRs. The mean amplitude, ΔB/B, and duration, ΔT, for all TCRs were found to be 7.6% and 158 s, respectively. TCRs occurring as isolated events were the largest (ΔB/B = 8.8% and ΔT = 218 s) and those associated with multiple events were the smallest (ΔB/B = 5.6% and ΔT = 84 s). The mean duration of the period of southward tilting Bz following isolated TCRs was 12.3 min. This time interval was found to be quite similar to the average spacing between TCRs in paired and multiple events, 11.2 and 10.2 min, respectively. TCR amplitude and duration were found to be independent of location within the tail lobes suggesting that the plasmoids which cause the TCRs maintain approximately constant volume and shape as they move down the tail. Mean plasmoid dimensions estimated from TCR duration and amplitude under the assumption of a quasi‐rigid magnetopause are 35 RE (length) × 15 RE (width) × 15 RE (height). Utilizing auroral kilometric radiation, the AL index, Pi 2 pulsations at two ground stations, and energetic particle data from three geosynchronous spacecraft, it is found that over 91% of the TCR events identified in this study followed substorm onsets or intensifications. The number of TCR events identified in this study are consistent with their release in association with a new substorm onset every 4‐6 hrs. The results of this study strongly suggest that the release of plasmoids down the tail near the time of expansion phase onset is an integral step in the substorm process and an important element in the substorm energy budget.
Simulations of driven field line resonances in the Earth's magnetosphere
Authors: Rankin R. , Samson J. C. , Frycz P.
Published: February 2008

Using a compressible, three‐dimensional resistive magnetohydrodynamic (MHD) computer simulation code, we examine the evolution of standing wave field line resonances (FLRs) in the nightside of the Earth's magnetosphere. The MHD code that is used allows for a full nonlinear description and enables us to follow the evolution of FLRs to large amplitude. We take as our MHD driver a source of fast‐mode waves incident from the direction of the magnetopause boundary layer. The ambient density and geomagnetic field are such that the fast mode waves have turning points at radial distances between 8 and 10 Re in the equatorial plane. The fast‐mode angle of incidence is selected such that tunneling of the wave from the turning point to the resonance point leads to resonant mode conversion of energy from congressional waves to shear Alfvén waves. We determine whether kinetic effects or finite electron inertia effects are likely to become important during the nonlinear evolution of the FLRs. For this to occur, the FLRs must narrow to the point where the radial scale size is several ion gyroradii or less or to the point where the equatorial width of the resonance maps to several electron inertia lengths in the polar magnetosphere. It is shown that the resonances do narrow to the point where kinetic effects are likely to be important and that, in contrast to estimates in previously published work indicating that narrowing would take hundreds of wave cycles, this occurs within a few cycles of the driver field, consistent with observations of FLRs in the high‐latitude ionosphere.
The change of Giotto's dynamical state during the P/Grigg‐Skjellerup flyby caused by dust particle impacts
Authors: Pätzold Martin , Bird Michael K. , Edenhofer Peter
Published: February 2008

The Giotto spacecraft was decelerated by impacting cometary dust during its flight through the coma of comet P/Grigg‐Skjellerup on July 10, 1992. The change in radial velocity determined by two‐way Doppler measurements made on July 8 (pre encounter) and July 11 (post encounter) was (−0.4 ± 0.1) mm/s. An abrupt frequency drop of 138 Hz was observed in the one‐way real‐time data during the actual encounter phase at 1530:42.8 UT, 6 s after the expected time of closest approach. This frequency discontinuity is attributed to a disturbance in the spacecraft's radio system and is unrelated to the spacecraft deceleration inferred from the more reliable two‐way Doppler measurements. The spacecraft spin period changed by (−0.8 ± 0.1) ms and a nutation of amplitude (0.10° ± 0.01°) was generated. This relatively large nutation angle, coupled with the very modest deceleration, is most easily generated by a single dust particle. Assuming that the total impacted mass was concentrated in one single large particle, it is possible to determine a range for the total impacted real mass (20‐39 mg), the total change in velocity Δυ (≃1 mm/s), the momentum enhancement factor ϵ (0 to 2.0), and the impact coordinates on the spacecraft. It is deduced that the particle most likely impacted on the upper end of the spacecraft toward the antenna tripod. No definite solution could be found for impacts on the bumper shield.
High‐latitude ground observations of Pc 1/2 micropulsations
Authors: Popecki Mark , Arnoldy Roger , Engebretson M. J. , Cahill L. J.
Published: February 2008

A ground‐based survey of Pc 1/2 (0.1‐0.4 Hz) and Pc 1 micropulsations throughout 1986 has provided evidence for the location of the Pc 1/2 source region. Data were taken from three high‐latitude stations, located at South Pole (−75° geomagnetic latitude; 1530 UT local noon), Sondre Stromfjord (+74°, 1330 UT LN) and Siple (−61°, 1700 UT LN). The study revealed a diurnal occurrence pattern for waves in the 0.1‐0.4 Hz band (Pc 1/2) and showed that the pattern was not due to the effects of sunlight on the ionosphere but instead from a postnoon magnetospheric source region. On the basis of the latitudinal occurrence patterns of the waves above and below 0.4 Hz, it is concluded that the waves observed on the ground above 0.4 Hz come primarily from plasmapause latitudes, while the source of the Pc 1/2 lies between the plasmapause and the magnetopause. The estimate of source locations for waves above and below this frequency, combined with the typically sharp upper frequency limit of waves in the 0.1‐0.4 Hz band (Pc 1/2) are interpreted as evidence that He+ ions in the outer magnetosphere influence propagation and possibly wave growth. These results are compared with those of Anderson et al. [1990, 1992a,b], who showed with a spacecraft study that Pc 1 are more commonly observed beyond L = 7 than in regions closer to the Earth. It is concluded that many of the waves above the He+ gyrofrequency from the outer magnetosphere do not always reach the ground. An extensive search for correlations between Pc 1/2 occurrence and solar wind pressure and magnetic field orientation showed no direct connection between solar wind parameters and Pc 1/2 generation. They may instead be amplified by plasma sheet ions that drift sunward on the dusk side of the magnetosphere [Kaye and Kivelson, 1979; Anderson and Hamilton, 1993] and undergo ion‐cyclotron resonance in the afternoon sector. This mechanism is consistent with the diurnal pattern and apparent source location of the Pc 1/2.
On the thermo‐mechanical evolution of continental lithosphere
Authors: Sahagian Dork L. , Holland Steven M.
Published: February 2008

Elastic thickness of continental lithosphere is modified by both mechanical (thickening and thinning) events and thermal evolution. A simple thermo‐mechanical model can account for the processes of tectonic alteration and subsequent thermal reequilibration of lithospheric thermal structure. Mechanical thinning or thickening of the lithosphere directly affects its thickness and flexural rigidity. After a tectonic event, thermal reequilibration proceeds at a rate dependent upon the amount of thinning or thickening and total lithospheric thickness, previous tectonic history, and the heat flux at the base of the lithosphere. Heat flux at the base of the lithosphere is controlled by the efficiency of convective mantle heat delivery. Rapid heat delivery to the base of the lithosphere retards lithospheric thickening, whereas slow conductive heat delivery allows lithospheric thickening at the maximum rate. The relative importance of thermal effects to mechanical alterations of lithospheric elastic thickness depends on the time scale of concern. Very thin lithosphere undergoes rapid thermal evolution such that cooling between tectonic events significantly thickens the lithosphere. For thick lithosphere, conductive heat transport is so slow that only minor changes in thermal structure and no significant changes in elastic thickness take place between tectonic events. Model results with infinite stretching agree with observations of oceanic lithospheric thickness evolution, showing a divergence from a cooling half‐space at about 70 m.y. Model results also agree with independent estimates of continental thermal evolution in the absence of tectonic events. On the longest time scales approaching the age of the Earth, thermal reequilibration is attained, and any tectonic effects are overprinted. The time scale at which the transition between mechanically and thermally dominated elastic thickness evolution occurs depends on the heat transport efficiency of the convecting mantle. For high Nusselt numbers, thermal effects take longer to become important Even for lower Nusselt numbers, mechanical effects still dominate on time scales of 108 years. Crustal radiogenic heat production reduces the equilibrium lithospheric thermal and elastic thickness. It also introduces a dependence of equilibrium thickness on tectonic crustal thinning or thickening. Model results indicate that on time scales of thrust sheet and sedimentary loading in foreland basins, mechanical effects dominate the evolution of elastic thickness except for very thin lithosphere. The model can be applied to the lithosphere beneath foreland basins so that observed basin geometry can be used to constrain tectonic history prior to loading by thrust sheets and basin sediments.
Decoupling of underground explosions by rubble‐filled cavities
Authors: Florence Alexander L. , Miller Steven A. , Keller Carl E.
Published: February 2008

A series of laboratory experiments has been performed to study the decoupling of explosions by placing them in rubble‐filled cavities. The source conditions consisted of a tamped source (for reference), a spherical cavity, and two sizes of cylindrical cavity located at the center of a cylindrical core of granite. The rubble was well‐graded granite aggregate compressed to provide a porosity of 20% (one test at 30%). Measurements consisted of particle velocities at several ranges from which displacements, reduced velocity and displacement potentials, spectra, and radiated kinetic energy were obtained. On the basis of the results of these small‐scale experiments, conclusions are drawn concerning the extent of yield underestimation associated with the rubble‐filled cavities and the dependence of the estimation on the decoupling criterion selected. Underestimation can amount to large factors, and a cavity test is identifiable by means of spectra in which the cavity natural frequency is evident.
Lithospheric extension and magmatism in the Porcupine Basin west of Ireland
Authors: Tate Michael , White Nicky , Conroy John‐Joe
Published: February 2008

The Porcupine Seabight Basin is a north‐south trending, extensional sedimentary basin situated on the continental shelf west of Ireland. It is filled with sedimentary rocks which range in age from Devonian to present‐day. A large amount of geological and geophysical data is now available for this basin, mainly as a result of oil industry exploration. In this paper, information from 23 well logs and from regional seismic reflection surveys is used to determine the lithospheric stretching history of the basin. Subsidence analyses indicate that there was one main phase of stretching which began in late Liassic times (∼ 180 Ma), ending in earliest Cretaceous (∼ 145 Ma). Stretching factors at the northern end of the basin are relatively small (β = 1.1–1.7) but increase rapidly southwards along the axis of the Main Porcupine Basin. In the southern part of the region (the Seabight Basin), β is greater than 6. The simplest way to accommodate this rapid variation in stretching is by clockwise rotation of the Porcupine Ridge through approximately 20°, away from the Irish Shelf. Subsidence‐derived stretching values are in reasonable agreement with those determined previously from crustal thicknesses based on gravity models and deep seismic data (normal incidence and wide‐angle). The existence, location, and inferred age of the Porcupine Median Volcanic Ridge are also consistent with the subsidence‐derived stretching factors. After intermittent Paleocene igneous activity, an anomalous increase in the rate of subsidence occurred in the Eocene, between 55 and 42 Ma. There is little evidence that this rapid increase is caused by lithospheric stretching, and we conclude that it must be associated in some unknown way with melt generated by development of the Iceland hotspot.
Tectonothermal history in the Mattawa Area, Ontario, Canada, deduced from paleomagnetism and 40Ar/ 39Ar dating of a Grenville Dike
Authors: Hyodo Hironobu , York Derek , Dunlop David J.
Published: February 2008

The paleomagnetic ambient temperature has been determined from samples at various distances from a Grenville diabase dike cutting late Proterozoic tonalitic gneiss near Mattawa in the Grenville Province, Canada. Using the 40Ar/39Ar technique, the intrusion age of the dike is estimated to be 570 ± 3 Ma from a 0.5‐mm‐diameter chilled margin chip. The paleo‐ambient temperature of the country rock at the time of dike intrusion, 184° ± 40°C, was calculated from Jaeger's [1964] one‐dimensional heat conduction model. The burial depth of the presently exposed rocks is estimated to be 6.5 ±1.7 km, assuming a surface temperature of 15°C and a geothermal gradient of 26°C/km. This depth at 570 Ma is discordant with the presence of Ordovician shallow‐water (<100 m) limestones about 200 km distant, suggesting that differential uplift may have occurred between the two areas,
Seismic evidence for active magmatic underplating beneath the Basin and Range Province, western United States
Authors: Jarchow Craig M. , Thompson George A. , Catchings Rufus D. , Mooney Walter D.
Published: February 2008

Near‐vertical and wide‐angle seismic reflection data provide evidence for the presence of a magma body at the base of the crust beneath Buena Vista Valley in northwestern Nevada. The seismic response of this hypothesized magma body is characterized by high‐amplitude, near‐vertical P wave reflections and a comparably strong P‐to‐S converted phase. The magma body, referred to here as the Buena Vista Magma Body, is probably a single sill with thickness no greater than 200 m and length no greater than 1.8 km. The melt fraction in the sill is undoubtedly greater than 20–30%, and probably exceeds 50%. Melt composition is unconstrained. Although the age of the Buena Vista Magma Body is difficult to determine precisely, it is probably no older than 500,000 years. This suggests that magmatism in the Basin and Range Province is an ongoing process, despite the relative paucity of volcanic rocks erupted at the surface during the last 6 m.y.
Deformation from 1973 through 1991 in the epicentral area of the 1992 Landers, California, Earthquake (Ms = 7.5)
Authors: Savage J. C. , Lisowski M. , Murray M.
Published: February 2008

Deformation of a 50 × 60 km trilateration network that spans the epicenter of the 1992 Landers earthquake(Ms = 7.5) was measured by seven surveys over the 19 years preceding the earthquake. Three moderate earthquakes (1979 Homestead Valley, Ms = 5.6; 1986 North Palm Springs, Ms = 6.0; and 1992 Joshua Tree, Ms = 6.1) occurred within the network during those 19 years. Here we use geodetic and seismic data to construct a dislocation model for each of the three moderate earthquakes. Coseismic changes due to these three moderate earthquakes as predicted by the dislocation models are then removed from the trilateration data. The residual geodetic changes appear to be uniform in time. We take those changes to represent secular strain accumulation. This strain accumulation clearly shows right‐lateral shear across the San Andreas fault but also suggests a northwest‐southeast extension northeast of the eastern end of the “Big Bend” of the San Andreas fault. The Landers earthquake ruptured along a north‐northwest trend across this region of northwest‐southeast extension.
Effects of salinity and surface tension on microbubble‐mediated sea‐to‐air transfer of surfactants
Authors: Skop Richard A. , Tseng Ruo‐Shan , Brown John W.
Published: February 2008

A series of experiments were conducted in artificial seawater to study the effects of salinity and surface tension on the bubble‐mediated sea‐to‐air transfer of surfactant materials. The bubble transport of a surfactant tracer (14C‐labeled oleic acid) from seawater to the air‐water interface and, subsequently, to a 10‐cm height above the water surface by bubble bursting was measured. The change in surface tension induced by bubbling was also determined. We have found that the amount of oleic acid transported to the water surface by bubble scavenging increases as the salinity increases. In consequence, so does the quantity of oleic acid ejected into the air. The ratio of the quantity of oleic acid ejected to a height of 10 cm above the water surface to the amount transported to the water surface was constructed. This ratio was found to increase with decreasing surface tension, indicating that the energetics of the bubble bursting process are strongly influenced by the surface tension encountered during bursting.
Structure and transport of the Antarctic Circumpolar Current and Agulhas Return Current at 40°E
Authors: Read J. F. , Pollard R. T.
Published: February 2008

Hydrographic data are presented from a near‐meridional section across the Agulhas Return Current, the Subtropical Front and the Antarctic Circumpolar Current (ACC) in the Southwest Indian sector of the Southern Ocean. The location and characteristics of the fronts are examined and the water masses are defined. The Agulhas Return Current emerges as a strong current as far east as 40°E which forms a marked front that is separate from the Subtropical Front, and can be distinguished by a separate water mass. Comparisons are made with a similar hydrographic section at the Greenwich Meridian and several notable differences are found. The Subantarctic Front at the Greenwich Meridian is a pronounced feature carrying a large proportion of the geostrophic transport of the ACC. On the Southwest Indian section, however, the Subantarctic Front is merged with the Polar Front at 48°S, where the flow associated with the ACC is concentrated. Geostrophic calculations show a large volume transport, 84 to 103 Sverdrup (1 Sv = 106 m3 s−1), associated with the Agulhas Return Current at 40°S. This is quite separate from the transport of the ACC which totals 138 Sv south of the Subantarctic Zone and the Crozet Plateau, or 150 Sv including flow in the Subantarctic Zone. The transport of the ACC out of the South Atlantic sector appears to be higher than the transport into the South Atlantic sector at Drake Passage, with the extra transport most probably carried by upper layer and intermediate waters. Eddies are seen to be an important feature of the Subantarctic Zone where they help to modify the water masses. Antarctic Intermediate Water appears in two forms, a saline (34.4) Indian Ocean variety and a fresh (34.2) Atlantic Ocean variety. The Atlantic variety is freshest at the Polar Front, and occupies the southern half of the Subantarctic Zone. Within the eddies, in the central Subantarctic Zone, there is interleaving between these two forms of Antarctic Intermediate Water. North Atlantic Deep Water is unable to cross the Southwest Indian Ridge but can pass south of the Crozet Plateau in the form of Circumpolar Deep Water, which is slightly more saline (34.75) than at Drake Passage (34.73).
Mesoscale stability of an ocean current in the Bryan‐Cox model
Authors: Ikeda M. , Wood R. A.
Published: February 2008

The Bryan‐Cox primitive equation model is examined in terms of the stability of an ocean current by intercomparison with the other models, taking a focus on different representations (finite difference schemes) of continuous stratification: the Bryan‐Cox level model, in which density is calculated at the same depth as horizontal velocity, versus the other level model with density calculated at the same depth as vertical velocity as well as a layer (isopycnal) model, in which vertical displacement of isopycnals represents density changes. Stability analyses are carried out for highly idealized cases: a basic flow with vertical shear only. The two‐level models are first numerically examined for nongeostrophic cases, whose results are then confirmed with analytical solutions of the quasi‐geostrophic versions. The analytical solutions are extended to the multiple‐level models. The instability of the basic current in the Bryan‐Cox model is shown to be overestimated with low vertical resolution and sensitive to distribution of vertical shear among levels. As the vertical resolution becomes higher, the growth rate rapidly approaches that of the continuous model, while additional unstable modes appear at high wavenumbers (these modes are neutral in the other two models). These deficiencies are related to its representation of stratification in the Bryan‐Cox model. It is suggested that the Bryan‐Cox model should be carefully applied to a study of mesoscale processes.
Pressure‐forced seiches of large amplitude in inlets of the Balearic Islands
Authors: Gomis D. , Monserrat S. , Tintoré J.
Published: February 2008

Large‐amplitude harbor seiches usually occur in summer in the Balearic Islands. A significant correlation between sea level oscillations and atmospheric pressure disturbances has been found, though a proved physical mechanism to account for this atmosphere‐ocean interaction is still missing. Using a flat bottom, shallow water model, we show that a direct coupling between atmospheric pressure and the free mode of an inlet is unlikely but that an oceanic wave of atmospheric origin can act as an intermediate mechanism and adequately force the inlet by resonance. The phase relationship derived from this mechanism is in good agreement with observations, provided the whole spectrum of oceanic waves is in opposite phase to the generating atmospheric disturbance. We also show that the very large oscillations observed at Ciutadella (an elongated, shallow inlet in the west coast of Menorca, Balearic Islands) can be explained in terms of the particular shape of this inlet.
The effects of wave‐induced pore water circulation on the transport of reactive solutes below a rippled sediment bed
Authors: Shum K. T.
Published: February 2008

The role of pore water flow in solute transport below a rippled sediment bed in a coastal environment is investigated. Based on the velocity field induced by the passage of gravity waves, a steady‐state transport equation is solved numerically over a range of field conditions. A consumption term governed by first‐order kinetics, a constant diffusivity and a constant boundary concentration slightly above the sediment‐water interface are assumed in the two‐dimensional continuum model. Significant spatial variations in concentration are found when there is an appreciable pore water flow. The concentration below a ripple trough can be many times higher than that at the same depth below a crest, and the concentration gradient in the horizontal can be of the same order of magnitude as that in the vertical. At high advective transport, a region of uniform concentration is found immediately below the ripple surface away from the crests. The rate of consumption over the sediment layer is compared with the corresponding rate in the absence of advective transport. Their ratio increases monotonously with the ripple slope and the ratio of the magnitudes of advective to diffusive transport and can reach up to 5 even for a relatively mild ripple slope of 0.1. As the magnitude of advective transport depends on the prevalent wave conditions, the rate of consumption and the distribution of solute concentration can vary appreciably with time. These results suggest that hydrodynamic conditions, sediment characteristics, and surface topography should be integral parts of field measurements of the distribution and flux of solutes in coastal sediments.
Nonlinear and dissipative dynamics in the connection region between western boundary currents and equatorial currents
Authors: Springer Scott R. , Kawase Mitsuhiro
Published: February 2008

The roles of nonlinearity and dissipation in connecting a western boundary current to an equatorial current are explored with a 1½ ‐layer numerical model. A permanent western boundary current ‐ equatorial current system is maintained between a mass source located near the western boundary several deformation radii from the equator and a sponge layer located along the eastern boundary. When a no‐slip boundary condition is used, the western boundary current separates within two deformation radii after crossing the equator to feed the eastward equatorial current. Because of equatorial scalings, flow within the connection region between these two currents is nearly nondivergent, and similarities with barotropic models of western boundary current extensions are emphasized by potential vorticity analysis. A series of experiments explores the relative importance of inertia to dissipation, measured by the Reynolds number, by varying the forcing strength. For small Reynolds numbers advection of relative vorticity out of the western boundary layer eastward results in a meandering equatorial current, which is interpreted as a damped, stationary Rossby wave. At higher Reynolds numbers, the boundary current is susceptible to horizontal shear instability, resulting in eddies which translate along the western boundary. At yet higher Reynolds numbers, eddy variability is not confined to the western boundary. Disturbances are advected out of the boundary layer, producing large (300 km diameter) eddies in the connection region. Variability also radiates eastward along the equator as free Yanai wave packets with periods between 30 and 70 days. Even in the presence of energetic eddies, net flow across the equator is due mostly to vorticity dissipation in the western boundary current. The effect of different boundary conditions is also examined. When a free‐slip rather than no‐slip boundary condition is used, the boundary current overshoots well outside the equatorial waveguide, but all flow returns to the equatorial region in an offshore countercurrent, and the solution is otherwise similar to solutions with a no‐slip boundary condition.
Gravitational settling of particles and bubbles in homogeneous turbulence
Authors: Fung J. C. H.
Published: February 2008

I investigate whether, in general, the average settling velocity of a small particle/bubble subject to a square drag force in homogeneous, stationary turbulence with a Kolmogorov spectrum differs from that in still fluid and, in particular, the role played by the structures of the flow field. I study particle or bubble trajectories in a simple flow structure. Statistics have been computed for the motion of small particles/bubbles settling under gravity with a Gaussian random velocity generated by Fourier modes. As a main result it is shown that homogeneous turbulence does cause a net decrease of settling velocity in the case VT/u' ∼ O(1).
Diurnal restratification and turbulence in the oceanic surface mixed layer: 1. Observations
Authors: Brainerd K. E. , Gregg M. C.
Published: February 2008

By repeated microstructure profiling during PATCHEX, we observed 11 cycles of daytime stratification and turbulent decay within the surface mixed layer during a period of generally light winds. Absorption of solar radiation strongly stratified water near the surface, but also weakly stratified all depths in the remnant mixed layer below the diurnal thermocline. Advection due to relaxation of horizontal density gradients also routinely contributed to remnant layer restratification. Between 0.2 and 0.4 MPa stratification increased linearly with time from (N2) ≈ 5 × 10−7 s−2 at the end of nighttime convection to (N2) ≈ 3.5 × 10−6 s−2 by the end of the day. Throughout the day we observed high dissipation rates (〈ε〉 > 10−6 W kg−1) in the near‐surface zone, which extended down to 0.07–0.10 MPa. Within the remnant layer, dissipation remained nearly constant for the first half hour after the end of convective forcing and then decayed by a factor of 40 in 4 hours, reaching 5 × 10−10 W kg−1; throughout the decay, overturning scales were limited by the growing stratification. Within the seasonal thermocline there was a daily cycle in ε, with nightly maxima of about 3 × 10−8 W kg−1 and daytime minima around 2 × 10−9 W kg−1. The mean turbulent heat flux within the seasonal thermocline was comparable in magnitude to the mean heat flux through the surface.
Bragg reflection of infragravity waves by sandbars
Authors: Liu Philip L.‐F. , Cho Yong‐Sik
Published: February 2008

Using a multiple‐scale perturbation method, we derive a set of governing equations describing the transformation of long‐wave and short‐wave components in a wave group. These equations are derived from Boussinesq equations with the assumption that the length scale of wave group modulation is of the same order of magnitude as that of the bottom variation and is much longer than the length scale for the carrier (short) waves. Therefore the reflection of carrier waves by the topographical variation is small and neglected. Numerical examples are presented to show that long waves associated with a wave group can be reflected reasonantly by a field of periodic sandbars.
Rainfall, total water, ice water, and water vapor over sea from polarized microwave simulations and Special Sensor Microwave/Imager data
Authors: Bauer Peter , Schluessel Peter
Published: February 2008

A radiative‐transport model has been developed to study the emitted radiation from variable cloud and rain systems at the frequencies of the special sensor microwave imager (SSM/I). The model is based on the matrix operator method and takes into account multiple scattering as well as polarization. The simulations are based on a multivariate approach using globally distributed atmospheric profiles of pressure, temperature, humidity, and different cloud types in multiple layers. The inhomogeneous vertical distributions of water and ice in clouds and rain are modeled by bulk analytical formulae. Fractional coverages of cloud and rain clusters are incorporated into the analysis to account for the low spatial resolution of microwave radiometers, which decreases the correlations between the brightness temperatures and the microphysical parameters. Retrieval algorithms are developed for the water vapor, total water, and ice paths as well as for surface rain rates. By the exclusion of strong convective conditions, the retrieval of ice paths is found to be problematic owing to the strong gradient of scattering efficiency with sizes. The water vapor paths are also derived for cloudy and raining conditions with limited spatial coverages. The theoretical standard errors of the retrievals lie in the range of 0.1–0.2 g/cm2 for vapor paths of 0–7 g/cm2, 0.02–0.03 g/cm2 for total water paths of 0–1.2 g/cm2, 0.015 g/cm2 for ice paths of 0–0.25 g/cm2, and 1.3 mm/h for rain rates of 0–30 mm/h. The algorithms are applied to global sets of SSM/I data for August 1987. The resulting fields of monthly rain amounts are compared to those of satellite infrared measurements, a surface climatology, and a general circulation model (GCM) experiment. The results are found to be mainly consistent with the climatology, whereas the IR data and the GCM experiment produce questionable inconsistencies. The water vapor path estimations are compared to colocated radiosonde measurements showing deviations below 0.78 g/cm2 including cloudy and rainy situations, whereas our total water path estimations show the best results for inhomogeneous scenes compared to those calculated by algorithms taken from the literature.
Stratospheric aerosol optical depths, 1850–1990
Authors: Sato Makiko , Hansen James E. , McCormick M. Patrick , Pollack James B.
Published: February 2008

A global stratospheric aerosol database employed for climate simulations is described. For the period 1883–1990, aerosol optical depths are estimated from optical extinction data, whose quality increases with time over that period. For the period 1850–1882, aerosol optical depths are more crudely estimated from volcanological evidence for the volume of ejecta from major known volcanoes. The data set is available over Internet.
Propagation of electromagnetic waves parallel to the magnetic field in the nightside Venus ionosphere
Authors: Huba J. D. , Rowland H. L.
Published: February 2008

A theoretical and numerical analysis of the propagation of electromagnetic waves in the nightside Venus ionosphere is presented. The special case of propagation parallel to the magnetic field is considered. The model assumes a source of electromagnetic radiation in the Venus atmosphere, such as produced by lightning, and specifically addresses wave propagation for realistic ionospheric parameters in the altitude range z = 130 – 160 km at the four frequencies detectable by the Pioneer Venus Orbiter Electric Field Detector (OEFD): 100 Hz, 730 Hz, 5.4 kHz, and 30 kHz. The results are summarized as follows. The ƒ = 100 Hz and 730 Hz waves can propagate as whistler waves, provided ƒ < ƒce, where ƒce is the electron cyclotron frequency, and provided that the ionospheric electron density is sufficiently small that the waves are not strongly attenuated by collisional effects (i.e., Pedersen conductivity). The attenuation length scale λ0 associated with the Pedersen conductivity is λ0 ∼ 2(c/ωpe)[Ωe3/ω (νen + νei)2]½; for parameters typical of the nightside Venus ionosphere above 140 km, electron‐ion collisions are more frequent than electron‐neutral collisions and are therefore responsible for the attenuation of the waves. A parameterization of the wave intensities and Poynting flux as a function of magnetic field and peak electron density is presented. The waves are found to propagate most easily for high magnetic field and low electron density conditions (i.e., ionospheric holes); this result is consistent with observational data. The incident Poynting flux at the bottom of the ionosphere (z ≃ 130 km) is estimated to be S ≃ 0.1 W/m2 for the 100 Hz waves. The ƒ = 5.4 kHz and 30 kHz cannot propagate through the nightside Venus ionosphere, as expected, because ƒpe> ƒ> ƒce.
Source characteristics of Jovian hectometric radio emissions
Authors: Reiner M. J. , Fainberg J. , Stone R. G.
Published: February 2008

Direct confirmation that low‐frequency Jovian hectometric (HOM) radio emissions centered near 0° central meridian longitude consist of distinct, oppositely polarized northern and southern beams has been achieved using data from the Unified Radio and Plasma Wave (URAP) experiment on the Ulysses spacecraft during the Ulysses‐Jupiter encounter in early February 1992. Distinct northern and southern beams were observed in the frequency range from ∼300 kHz to 1 MHz for at least eight Jovian rotations during the Ulysses inbound pass at distances from 100 to 40 RJ. The radiation from the two magnetic hemispheres was measured from different Jovigraphic longitudes and magnetic (or centrifugal) latitudes. Observed temporal variations in the radio intensities, with time scales on the order of 30 min, may result either from longitudinal variations of the HOM sources or from longitudinal density variations in the Io plasma torus. Using the URAP direction‐finding capabilities and assuming a tilted dipole planetary magnetic field model, the three‐dimensional HOM source locations, the L shell through these source locations, and the beam opening angles were independently deduced. The HOM sources were found to originate at ∼3 RJ and on low L shells (L ∼ 4 to 6), with beam opening angles ranging from 10° to 50°.
Diffraction by a half plane with third‐order transition conditions
Authors: Senior Thomas B. A.
Published: February 2008

A plane wave is incident on a semiinfinite thin dielectric layer simulated using third‐order transition conditions. The unique solution for the total field is obtained and illustrated in the case of a homogeneous layer.
Combined random rough surface and volume scattering based on Monte Carlo simulations of solutions of Maxwell's equations
Authors: Pak Kyung , Tsang Leung , Li Li , Chan Chi H.
Published: February 2008

The scattering of electromagnetic plane waves by one‐dimensional lossy dielectric random rough surfaces containing densely distributed two‐dimensional dielectric scatterers is studied. The finite element formulation for random rough surface scattering is extended to include random volume scattering in the lower half‐space. An advantage of this method is that the scattering by the discrete scatterers below the dielectric rough surface can be taken into account with only a small increase in computational complexity. The Monte Carlo simulation results show that the presence of volume scattering can have significant effects on the backscattering intensity and bistatic scattering cross section depending on the statistics of the rough surface, the permittivity of the lower half‐space, and properties of the discrete scatterers such as sizes, permittivities, and concentrations.
The nature of the spectral gap between bound and leaky solutions when dielectric loss is present in printed‐circuit lines
Authors: Shigesawa H. , Tsuji M. , Oliner A. A.
Published: February 2008

For lossless printed‐circuit transmission lines, it is known that a spectral gap is present at the transition in frequency between bound and leaky modal solutions, and that the solutions are nonphysical within that gap. This paper examines the corresponding modal behavior when the dielectric substrate material is lossy and reports several new interesting and important results. The new features become particularly clear when the data are displayed on the steepest‐descent plane, which itself changes when loss is present and contains a new narrow spectral region that vanishes when the dielectric becomes lossless. Among the other new results we find that the leaky solution reverts to a bound solution at still higher frequencies, in contrast to the lossless case, and that the modal pole solutions are captured by the steepest‐descent path at all frequencies, so that an explicit spectral gap is not found in the lossy case examined here.
Mid‐Cretaceous strike‐slip and contractional fault zones in the western intermontane terrane, Washington, and their relation to the North Cascades‐Southeastern Coast Belt Orogen
Authors: Hurlow Hugh A.
Published: February 2008

Structural and geochronologic data from the 111–114 Ma Okanogan Range batholith in north central Washington are used to characterize the timing and style of deformation during the early stages of the mid‐to Late Cretaceous North Cascades‐southeastern Coast Belt (NC‐SECB) orogen. The Pasayten fault zone bounds Jurassic‐Cretaceous sedimentary and volcanic rocks of the Methow basin to the west against predominantly Mesozoic igneous rocks of the Intermontane terrane to the east. The Pasayten fault zone accommodated intrusion of the western units of the Okanogan Range batholith during high‐angle slip, then approximately 20 km of left‐lateral strike‐slip at 109–95 Ma. South of the Pasayten fault zone, the Red Shirt and Methow River thrust zones accommodated (1) west vergent contraction at amphibolite grade that began by 113 Ma and ended by 112 Ma, based on new U‐Pb zircon dates for deformed and undeformed granitoids, and (2) northwest vergent contraction in the Methow River thrust zone followed by west vergent contraction in the Red Shirt thrust zone, both at greenschist grade prior to 104 Ma. These contractional displacements thrust the Methow basin 10–20 km southeastward beneath the western Okanogan Range batholith. Deformation in the NC‐SECB thus began by earliest Albian time, earlier than previously thought, and was characterized by coeval sinistral transcurrent and west vergent contractional faulting. Sinistral strike‐slip in the Pasayten fault zone may have reflected the pre‐100 Ma sense of oblique plate convergence or relatively minor southward tectonic escape of the adjacent Methow basin during contractional deformation. The slip history of the Pasayten fault zone is undocumented between approximately 80 Ma and 60 Ma when, on the basis of paleomagnetic data, some workers have proposed that the Methow basin moved 1700 km northward to its present position relative to the Intermontane terrane. However, significant Late Cretaceous to Early Eocene dextral slip on the Pasayten fault zone is unlikely because the western intrusive units of the Okanogan Range batholith continue without disruption across its southeastward trend.
Solution of the flow and mass transport equations by means of spectral elements
Authors: Goblet P. , Cordier E.
Published: February 2008

A new numerical approach is proposed to solve the equations of flow and mass transport. This approach relies on a finite‐element type formulation using basis functions of arbitrary order (spectral elements). An appropriate choice of the nodal points and the integration points leads to a very effective computation of the element matrices. The transport equation is solved by combining a spectral approximation for the dispersion term with a backtracking algorithm for the advection term, which is aided by the most precise interpolation possible with the spectral method. The formulation is shown to yield an accurate description of very steep fronts with reasonable computer resources.
Dimensions of Pu'u O'o lava flows on Mars
Authors: Glaze Lori S. , Baloga Stephen M.
Published: February 2008

Longitudinal thickness profiles are simulated for lava flows on Mars erupted under conditions identical to those of Episodes 2 and 18 of the Pu'u O'o eruption at Kilauea volcano, Hawaii. Three separate approaches are used to estimate the change in the Newtonian viscosity along the length of these flows. The first approach assumes that all parts of the flow are in uniform motion and the volumetric flow rate is conserved at all stations along the flow path. The second approach assumes the same Newtonian flow model but takes advantage of independently acquired advance rate data. The third approach uses a recent model for the conservation of lava volume. This model apportions the flow rate into an active, advancing component and a separate component that is stationary (e.g., levees, stagnant zones, stationary margins). Viscosity changes for all three approaches are compared, with new insights into the rheologic changes in the Pu'u O'o flows. The three approaches to flow emplacement are used as a basis for scaling the terrestrial flow thickness profiles to account for the difference in Mars gravity. For the same volumetric effusion rate and identical topography, explicit scaling for the lower gravity affects the initial flow thickness and advance rate. More subtle is the implicit influence of the lower gravity on the viscosity change along the flow path. Differences in viscosity change estimated by all three models are magnified when applied to lava flows on Mars. To illustrate the sensitivity of relative viscosity estimates on the assumed flow rate model, we also estimate viscosity increases for lava flows at Ascraeus Mons, Elysium Planitia, and Alba Patera. We show that the third model consistently predicts significantly greater increases in viscosity and must be considered when analyzing channelized flows where significant volumes of material have been lost to stagnant margins during emplacement.
In‐flight intercomparisons of some aircraft meteorological and chemical measurement techniques
Authors: Boatman J. F. , Wellman D. L. , Schnell R. C. , Busness K. M. , Luria M. , Van Valin C.
Published: February 2008

A Beechcraft King Air and a Lockheed WP‐3D (P‐3) Orion aircraft were flown side by side on January 8, 1986 near Boston, Massachusetts, to compare the scientific measurements being made using the two platforms. The King Air was also flown in formation with a Beechcraft Queen Air and Douglas DC‐3 on March 1, 1986 near Raleigh, North Carolina, for the same purpose. This paper summarizes the results of these comparison flights and discusses the scientific measurements made by the aircraft. The accuracies and response times of the aircraft instrumentation are estimated.
Investigation of particulate matter by size and composition during WATOX, January 1986
Authors: Cahill Thomas A.
Published: February 2008

Samples of fine (Dp <2.5 μm) particulate matter were taken during the Western Atlantic Ocean Experiment (WATOX) flights of January 1986 and at three remote continental sites in Great Smoky Mountains, Shenandoah, and Acadia National Parks. Samples were analyzed for mass, hydrogen (an organic matter tracer), sulfur, silicon (a soil tracer), potassium (a smoke tracer), zinc (presumed largely anthropogenic in this size mode), and other elements heavier than sodium. The results show that the WATOX period was typical of mean winter aerosol conditions, 1983‐1986, and that, for most species, the continental sites were quite similar in concentrations. These data were compared with WATOX aircraft data of January 4, 6, 8, and 9 in order to evaluate aerosol transport and transformations. The soil and smoke tracers, Si and K, averaged about 15% of remote continental sites, while S and H averaged 30% of the continental sites. The higher values for these reactive species could reflect either transport from the urban seaboard sources or gas to particle transformations during transport. Zinc averaged 35% of continental values, with a high degree of variability from flight to flight, a possible consequence of seaboard industrial activity.
Measurements of H2O2 during WATOX‐86
Authors: Heikes B. G. , Walega J. G. , Kok G. L. , Lind J. A. , Lazrus A. L.
Published: February 2008

Measurements of gas phase H2O2 were made on all Western Atlantic Ocean Experiment 1986 (WATOX‐86) flights aboard the National Oceanic and Atmospheric Administration (NOAA) WP‐3D aircraft. These were some of the first airborne real‐time H2O2 measurements made in winter. Operation of the instru‐ ment was limited to altitude < 3.1 km with a detection limit, determined by interference considerations, of 0.05 parts per billion by volume (ppbv), 10% calibration accuracy and 0.03‐ppbv precision. Experimental measurements showed the mean H2O2 to be 0.12 ppbv (standard deviation = 0.07, maximum = 1.2 ppbv). Vertical structure was observed with maximum H2O2 above the cloud‐capped marine boundary layer. Boundary layer H2O2 was typically at or below the detection limit.