bluuebunny/crossref_metadata_2025 huggingface dataset:
Stimulated electromagnetic emissions from magnetized and inhomogeneous plasma
Authors: Goodman Simon
Published: February 2008

We model the radio emission spectrum for an electromagnetically pumped, magnetized and inhomogeneous plasma and compare the results to observations of upper sideband features of stimulated electromagnetic emissions (SEE) in ionospheric heating experiments when the heater frequency, ω0, is near harmonics of the ionospheric electron cyclotron frequency, Ωe. Our model involves solving the equations of motion for electrons in an external electromagnetic pump wave and geomagnetic field to calculate, for an inhomogeneous plasma, a current which excites electron Bernstein and lower hybrid waves. We reinsert this current into the equations of motion and continue in an iterative manner. The result is a current which depends on density gradients across the geomagnetic field. The current intensifies with decreasing density gradient scale. It has a frequency ω = 2ω0 ‐ pΩe, where p is an integer, is resonant for ω0 ≈ pΩe and for which there is a cutoff in the spectra at about 10 kHz above the pump frequency. Our results agree with observations made in SEE experiments.
Collisionless hydrodynamic shocks in multifluid plasmas: 1. Parallel shocks
Authors: Bürgi Alfred
Published: February 2008

The macrophysics of a single‐fluid plasma at a hydrodynamic shock is characterized completely by the three Rankine‐Hugoniot conditions expressing conservation of particles, momentum flux, and energy flux across the shock. In a multifluid plasma containing an arbitrary number of ion species and electrons the Rankine‐Hugoniot conditions still characterize the behavior of the bulk plasma but not the properties of individual fluid constituents. Conservation laws for individual particle species are derived for parallel collisionless shocks. The shocks are assumed to be nonturbulent; i.e., the particles interact only via the electrostatic field. Given the plasma parameters upstream, the downstream solutions form a single‐parameter family; the free parameter can be taken to be the electrostatic potential jump across the shock.
Wavelength measurement of auroral hiss
Authors: Ergun R. E. , Klementis E. , Carlson C. W. , McFadden J. P. , Clemmons J. H.
Published: February 2008

The average wavelengths of VLF emissions in the auroral ionosphere were measured as a function of frequency using an orthogonal antenna technique. The measurements, made by an auroral sounding rocket, were sensitive to wavelengths between ∼5 m and ∼300 m for wave frequencies from 20 Hz to 10 kHz. The emissions just above the lower hybrid frequency had wavelengths of 12 m or less in many cases, increasing with increasing frequency. The measured wavelengths were such that the VLF emissions were resonant with electrons whose energies were a few hundred eV, whereas the inverted‐V electrons had energies of ∼20 keV. Bursts of low‐energy (<300 eV to ∼3 keV), field‐aligned electron fluxes were observed often during the flight, but no direct correlation between the flux intensity and the VLF intensity or wavelength could be found. The shorter‐than‐expected wavelengths were found in different types of VLF spectra. The steady state VLF spectra generally displayed a typical sharp cutoff below the lower hybrid frequency with a maximum at several times the lower hybrid frequency. These spectra also had structure at hydrogen cyclotron harmonics above the lower hybrid frequency. At other times, 25‐s to 50‐s bursts of lower hybrid emissions were observed. During the periods of the most intense emissions, the spectral peak was very near the lower hybrid frequency while hydrogen Bernstein emissions were seen below the lower hybrid frequency.
The plasma sheet as a shock wave: Geophysical consequences of the hypothesis
Authors: Fridman Sergey V. , Voronov Eugene V.
Published: February 2008

Convection of closed magnetic tubes in the central part of the Earth's magnetospheric tail reduces their length and compresses the plasma contained in them. In a reasonable approximation the problem of plasma compression in a shrinking tube reduces to a classical hydrodynamical problem of a piston being pushed into a cylinder which has contained the gas at rest. In this case the top of the flux tube plays the role of the piston. As the piston is pushed in, a shock wave is known to separate from it. Between the piston and the shock front the gas is compressed and heated, while the gas ahead of the front is undisturbed. Similar processes occur in the plasma contained in the contracting flux tube and are accompanied by the formation in the tail of two shock waves which have separated from the neutral plane in the northward and southward directions, between which the plasma is hotter and denser than in the tail lobes. In the case of stationary magnetospheric convection, shock waves are also stationary. The distinctive plasma characteristics in the region between the shock waves and also the spatial position of this region suggest that in the real magnetosphere this region corresponds to the plasma sheet. We assess the validity of this hypothesis and its effectiveness in describing geophysical effects. We show that the plasma particle energy expected from this model corresponds to experimentally derived values. At the same time, within the framework of the MHD theory with isotropic pressure it is impossible to obtain the required high degree of compression during the transition of the plasma across the shock front (100 times or more). By analyzing the constraints imposed by the laws of conservation of mass, energy, and momentum on plasma parameter jumps in a collisionless shock wave, we find that in order to achieve the observed degree of compression it is sufficient to have a small excess of transverse pressure in the plasma sheet over longitudinal pressure, which is consistent with observations. We have numerically simulated the motion of shock waves in convecting magnetic flux tubes. By postulating the relationship between the shock wave velocity and plasma parameters, we have calculated the form of the shock surface, which we interpret as the plasma sheet boundary. The line of intersection of the shock surface with the ionosphere is taken here as the polar boundary of the auroral oval. Results of calculations agree with the present‐day understanding of the structure of the plasma sheet and of the auroral zone. The applications we consider of this hypothesis lead us to conclude that this approach to describing phenomena associated with the plasma sheet boundary and with the auroral region is promising.
Anisotropy of nonrelativistic particles deduced from analysis of their bulk flow speed
Authors: Tan L. C. , Mason G. M. , Gloeckler G. , Klecker B.
Published: February 2008

We present a new method for deducing the anisotropy vector δ′ of nonrelativistic particles in the solar wind frame. To order Vsw/υ, δ′ = −υ−1 [d ln F0(υ)/d ln υ] (VF ‐ Vsw), where υ and F0(υ) are the velocity and the mean velocity phase space distribution function, respectively, of the particles in the spacecraft frame, VF is the bulk flow speed of the energetic particles in the spacecraft frame, and Vsw is the solar wind speed. An illustration of the use of this equation is given for an interval during the upstream period before the April 5, 1979, interplanetary shock.
Electrostatic turbulence in the Earth's central plasma sheet produced by multiple‐ring ion distributions
Authors: Huba J. D. , Chen J. , Anderson R. R.
Published: February 2008

A mechanism to generate a broad spectrum of electrostatic turbulence in the quiet time central plasma sheet (CPS) plasma is discussed. We show theoretically that multiple‐ring ion distributions can generate short‐wavelength (kλde ≲ 1), electrostatic turbulence with frequencies ω ≲ kVj, where Vj is the velocity of the jth ring. Using a set of parameters from measurements made in the CPS, we find that electrostatic turbulence can be generated with wavenumbers 10−2 ≲ kλde ≲ 1.0, and with real frequencies in the range 0 < ωr/ωpi < 10, and linear growth rates γ/ωpi > 0.01 over a broad range of angles relative to the magnetic field (5° < θ ≤ 90°). The fastest growing modes are caused by an ion ring drift instability and occur for nearly parallel propagating modes (5° < θ < 10°) with ωr ≲ ωpi at wavelengths comparable to the electron Debye length. We also find that lower hybrid waves and electron cyclotron waves can be excited for nearly perpendicular propagating modes (θ ≃ 90°). We compare these theoretical results with wave data from ISEE 1 using an ion distribution function exhibiting multiple‐ring structures observed at the same time. The theoretical results in the linear regime are found to be consistent with the wave data. Finally, we discuss these results in relation to nonlinear particle dynamics in the CPS which can generate the multiple‐ring ion distributions.
A comparison of the reduced and approximate systems for the time dependent computation of the polar wind and multiconstituent stellar winds
Authors: Browning G. L. , Holzer T. E.
Published: February 2008

The “reduced” system of equations commonly used to describe the time evolution of the polar wind and multiconstituent stellar winds is derived from the equations for a multispecies plasma with known temperature profiles by assuming that the electron thermal speed approaches infinity. The reduced system is proved to have unbounded growth near the sonic point of the protons for many of the standard parameter cases. For the same parameter cases, however, the unmodified system (from which the reduced system is derived) exhibits growth in some of the Fourier modes, but this growth is bounded. (A physical explanation is provided for the unbounded growth in the reduced system and the bounded growth in the unmodified system.) An alternate system (the “approximate” system) in which the electron thermal speed is slowed down is introduced. The approximate system retains the mathematical behavior of the unmodified system and can be shown to accurately describe the smooth solutions of the unmodified system. The approximate system has a number of other advantages over the reduced system. For example, when the proton speed approaches the electron sound speed, the reduced system becomes inaccurate. Also, for three‐dimensional flows the correct reduced system requires the solution of an elliptic equation, while the approximate system is hyperbolic and only requires a time step approximately 1 order of magnitude less than the reduced system. Numerical solutions from models based on the two systems are compared with each other to illustrate these points.
Accelerated cometary ions observed downstream of the comet Halley bow shock
Authors: Kecskeméty K. , Cravens T. E.
Published: February 2008

Fluxes of energetic ions with energies exceeding 100 keV were observed upstream of the bow shock of comet Halley by the Tünde instrument which was on board the VEGA 1 spacecraft. Downstream of the shock, ion fluxes in the energy range 100 to 800 keV were observed. Cometary ions, such as O+, newly picked up by the solar wind have energies of about 15 keV in the solar wind frame of reference; hence the measured ion fluxes indicate that acceleration processes must have been operating near comet Halley. The measured ion fluxes were transformed into distribution functions in the solar wind frame using a variety of assumptions concerning the energy dependence of the distribution function and the identity of the ion species. The derived distribution function upstream of the shock falls off steeply with energy between 100 and 150 keV, with an effective temperature of about 7 keV or spectral index about −15. The distribution function increases with decreasing cometocentric distance, on average, reaching a maximum at the bow shock. Downstream of the shock, the spectra can be represented in two ways: (1) one population with a single spectral index based on a power law energy dependence or (2) two exponential function populations with different effective temperatures. In the latter case a soft component exists with an effective temperature of about 30 keV for energies less than about 250 keV and a harder component exists for higher energies with an effective temperature of about 100 keV. Downstream of the shock, the ion fluxes decrease with decreasing cometocentric distance. The measured distribution functions are compared with those obtained by similar instruments on Giotto and the International Cometary Explorer as well as with the predictions of several theoretical models that employ different acceleration mechanisms.
Regions of negative Bz in the Tsyganenko 1989 Model Neutral Sheet
Authors: Donovan E. F. , Rostoker G. , Huang C. Y.
Published: February 2008

We point out a disturbing feature of the Tsyganenko (1989) model magnetic field, namely the occurrence of negative Bz in the model neutral sheet. On the basis of observations of Bz in the neutral sheet we conclude that this is an artifact of the model and not a real effect. This feature of the model should be considered when the model is used either to infer mappings from the ionosphere to the vicinity of the neutral sheet or as a tool in theoretical studies. We propose that in the development of future models, it would be useful for the distribution of Bz in the neutral sheet to be imposed as a constraint on the model.
Topological variation in the magnetic field line at the dayside magnetopause
Authors: Wang Zhi , Ashour‐Abdalla Maha
Published: February 2008

This paper uses normal mode analysis to examine topological changes in magnetic field lines at the magnetopause. For a modified Harris sheet, the normal modes include a symmetric solution (tearing mode) and an antisymmetric solution (twisting mode). Both modes form flux ropes. We studied the tearing mode in the presence of shear flow. By modeling the external region plasma with drift kinetic formalism, we found the growth rate of the tearing mode to be reduced by shear flow. On the other hand, the shear flow generates a Kelvin‐Helmholtz instability which makes the magnetic field perturbation a twisting mode structure. When magnetopause parameters were used, it was difficult to induce a tearing mode in the presence of shear flow, though a shear flow twisting mode did grow under these conditions.
A theory for narrow‐banded radio bursts at Uranus: MHD surface waves as an energy driver
Authors: Farrell W. M. , Curtis S. A. , Desch M. D. , Lepping R. P.
Published: February 2008

We describe a possible scenario for the generation of the narrow‐banded radio bursts (n bursts) detected at Uranus by the Voyager 2 planetary radio astronomy experiment. This particular radio emission is suspected to originate within the Uranian northern polar cusp region. In order to account for the emission burstiness which occurs on time scales of hundreds of milliseconds, we propose that ULF magnetic surface turbulence, generated at the frontside magnetopause, propagates down the open/closed field line boundary and mode converts to kinetic Alfvén waves (KAW) deep within the polar cusp. The oscillating KAW potentials then drive a transient electron stream (beam or loss cone) that creates the bursty radio emission. To substantiate these ideas, we show Voyager 2 magnetometer measurements of enhanced ULF magnetic activity at the frontside magnetopause. We then demonstrate analytically that such magnetic turbulence should mode convert deep in the cusp at a radial distance of 3 RU. A condition for mode conversion from magnetic surface waves to KAW is ƒpe/ƒce < 0.2, which is also a condition favorable for the generation of the R‐X mode radio bursts. The fact that a similar plasma condition is required for both processes lends strong support that the active regions for mode conversion and R‐X mode wave generation are one and the same.
High‐frequency spectral falloff of earthquakes, fractal dimension of complex rupture, b value, and the scaling of strength on faults
Authors: Frankel Arthur
Published: February 2008

The high‐frequency falloff ω−γ of earthquake displacement spectra and the b value of aftershock sequences are attributed to the character of spatially varying strength along fault zones. I assume that the high frequency energy of a main shock is produced by a self‐similar distribution of subevents, where the number of subevents with radii greater than R is proportional to R−D, D being the fractal dimension. In this model, an earthquake is composed of a hierarchical set of smaller earthquakes. The static stress drop is parameterized to be proportional to Rη, and strength is assumed to be proportional to static stress drop. I find that a distribution of subevents with D = 2 and stress drop independent of seismic moment (η = 0) produces a main shock with an ω−2 falloff, if the subevent areas fill the rupture area of the main shock. By equating subevents to “islands” of high stress of a random, self‐similar stress field on a fault, I relate D to the scaling of strength on a fault, such that D = 2 − η. Thus D = 2 corresponds to constant stress drop scaling (η = 0) and scale‐invariant fault strength. A self‐similar model of aftershock rupture zones on a fault is used to determine the relationship between the b value, the size distribution of aftershock rupture zones, and the scaling of strength on a fault. The b value for aftershock sequences on a fault is found to equal (3 − 1.5η)/(3 + η). Therefore this model indicates that the typically observed spectral falloffs of ω−2 and b values of 1 can be entirely caused by scale‐invariant strength (η = 0) along fault zones.
Fractal hierarchies of magma transport in Hawaii and critical self‐organization of tremor
Authors: Shaw Herbert R. , Chouet Bernard
Published: February 2008

A hierarchical model of magma transport in Hawaii is developed from the seismic records of deep (30–60 km) and intermediate‐depth (5–15 km) harmonic tremor between January 1, 1962, and December 31, 1983. We find two kinds of spatial distributions of magma fractions at depths below 5 km, defined by the fractal dimension D3, where the subscript is the embedding dimension. The first is a focused distribution with D3 = 0.28, and the second is a dispersed distribution with D3 = 1.52. The former dimension reflects conduitlike structures where the magma flow converges toward a summit magma chamber and the fractal dimension tends to zero. The latter dimension reflects multifractal clustering of dendritic fractures where hypocentral domains represent subsets of fractures within spherical domains with an average radius of about 1 km. These geometries constitute a percolation network of clustered intermittent fracture and magma transport. The magma volume of the average fracture is about 2 × 104 m3. A tremor model of magma transport is developed from mass balances of percolation that are proportional to tremor durations. It gives reasonable magma fractions and residence times for a vertical drift velocity of 4 km yr−1 and yields patterns of intermittency that are in accord with singularity analyses of the 22‐year time series record. According to the model, sustained tremor is generated by the relaxation oscillations of the percolation network with a dominant frequency of about 1 Hz to obtain internally consistent values of fracture geometry, fracture opening force, and magma supply rate. Calculated tremor frequencies are higher in fracture networks of small volume in harmony with the observed relation between seismic amplitude and dominant frequency of tremor. Tectonic relaxation times of rock stresses versus magma pressures are in fair agreement with the average length of tremor episodes and average period of tremor intermittencies. These observations suggest that a high degree of self‐organization is characteristic of the nonlinear dynamics of fracture percolation and coupled tremor processes. Logarithms of frequencies (in hertz) of high‐amplitude tremor (1‐s period), mean tremor duration (28‐min period), and mean onset interval (14‐day period) are 0, −3.2, and −6.1, implying broadband maxima in the frequency spectrum of transport at intervals of 103. The next longer period of this sequence, which corresponds to eruptions and shallow intrusions, is about 32 years (10 −9 Hz), comparable to the average eruption intermission of Mauna Loa during the last 150 years (about 20 years). This and other evidence suggest that spatiotemporal universality extends from small to large scales in Hawaiian and other magmatic systems. The apparent universal scaling of frequencies may be more than 15 decades in time (1 s to about 60 m.y.) and 10 decades in length (0.1 mm to 103 km).
High‐temperature deformation of diopside single crystal: 2. Transmission electron microscopy investigation of the defect microstructures
Authors: Ingrin J. , Doukhan N. , Doukhan J. C.
Published: February 2008

The defect microstructures induced by high‐temperature creep in diopside single crystals are reported, based upon investigation by transmission electron microscopy (TEM). The creep experiments as well as the steady state creep laws are reported in a companion paper (Raterron and Jaoul, this issue); they clearly show that two deformation regimes occur, one at temperatures above a critical temperature Tc of 1130°–1140°C and one at temperatures below Tc. At high temperature, diopside is stronger than would be expected by extrapolation of the creep law at lower temperature. TEM observations indicate that the same glide systems {110}1/2〈110〉 are activated above and below this critical temperature; in both cases, evidence for dislocation climb has not been detected. The only noticeable difference between the two deformation regimes is the occurrence in the samples deformed in the higher temperature range of a large and homogeneous density of tiny precipitates of a glassy phase; these precipitates interact with the mobile dislocations. The size and the density of the precipitates increase dramatically with temperature. Just below 1130°–1140°C, a very limited and heterogeneous precipitation seems to occur along the dislocation cores in the form of very tiny precipitates less than 10 nm size. At 1250°C the precipitates reach 0.1 μm diameter. It is suggested that pinning of mobile dislocations by these precipitates is responsible for the hardening effect observed above 1130°–1140°C. Although the volume proportion of glass may be extremely low, its occurrence clearly means that partial melting occurs at temperatures well below the widely accepted melting temperature of this clinopyroxene (1350°C at 1 atm). Such a phenomenon which can be detected directly only by TEM (and indirectly by its consequences on the mechanical properties of the material) might also occur in a number of other silicate minerals.
Application of the global positioning system to crustal deformation measurement: 1. Precision and accuracy
Authors: Larson Kristine M. , Agnew Duncan C.
Published: February 2008

In this paper we assess the precision and accuracy of interstation vectors determined using the Global Positioning System (GPS) satellites. These vectors were between stations in California separated by 50–450 km. Using data from tracking the seven block I satellites in campaigns from 1986 through 1989, we examine the precision of GPS measurements over time scales of a several days and a few years. We characterize GPS precision by constant and length dependent terms. The north‐south component of the interstation vectors has a short‐term precision of 1.9 mm+0.6 parts in 108; the east‐west component shows a similar precision at the shortest distances, 2.1 mm, with a larger length dependence, 1.3 parts in 108. The vertical precision has a mean value of 17 mm, with no clear length dependence. For long‐term precision, we examine interstation vectors measured over a period of 2.2 to 2.7 years. When we include the recent results of Davis et al. (1989) for distances less than 50 km, we can describe long‐term GPS precision for baselines less than 450 km in length as 3.4 mm+1.2 parts in 108, 5.2mm+2.8 parts in 108, 11.7mm+13 parts in 108 in the north‐south, east‐west, and vertical components. Accuracy has been determined by comparing GPS baseline estimates with those derived from very long baseline interferometry (VLBI). A comparison of eight interstation vectors shows differences ranging from 5 to 30 mm between the mean GPS and mean VLBI estimates in the horizontal components and less than 80 mm in the vertical. A large portion of the horizontal differences can be explained by local survey errors at two sites in California. A comparison which suffers less from such errors is between the rates of change of the baselines. The horizontal rates estimated from over 4 years of VLBI data agree with those determined with 1–2 years of GPS data to within one standard deviation. In the vertical, both GPS and VLBI find insignificant vertical motion.
The effect of topography on the state of stress in the crust: Application to the site of the Cajon Pass Scientific Drilling Project
Authors: Liu Lanbo , Zoback Mark D.
Published: February 2008

This paper describes a generalized approximate method for assessing the effect of topography on the state of stress at depth in the crust and applies it to the site of the Cajon Pass Scientific Drilling Project. While previous studies have addressed idealized topography with regular shapes, we develop a three‐dimensional (3‐D) method for arbitrary surface topography. The analysis leads to four sets of elastic boundary value problems that are solved by convolutional methods based on the Green's functions of Boussinesq's problem and Cerruti's problem. The effect of topography on the state of stress can be divided into two parts by the linear superposition theorem. The first part is the effect on gravitational stress in the crust. In this case the effect of the topography can be expressed by the summation of the zero‐order vertical load and the first‐order horizontal shear at the base of the relief. The second part is the effect of topography on tectonic stress. In this case the effect of topography involves only the first‐order horizontal shear. The total effect of the topography on the state of stress in the crust can be obtained by adding the effect on gravitational stress with the effect on tectonic stress. The stresses caused by a simple case of 3‐D topography are calculated with this approach to serve as illustration. The identity of this study with previous studies (McTigue and Mei, 1981; Savage et al., 1985) for the same two‐dimensional case demonstrates the correctness of this method. In applying this technique to the Cajon Pass Scientific Drilling Project, we considered stresses induced by the topography associated with the San Gabriel and the San Bernadino mountains as a function of depth at the site. The regional topography induces stresses of several megapascals at the drilling site which decrease rapidly with depth. The induced maximum horizontal compressive stress is approximately in the N‐S direction. If we assume that in the absence of topography there is little right‐lateral shear stress acting on the San Andreas fault at depth (a weak fault assumption), the regional topography tends to cause a slight increase of right‐lateral shear on the San Andreas fault at a shallow depth in the crust. On the other hand, if we assume the strong fault model, the topography has no effect on the sense of shear on the San Andreas fault at all. Therefore it is clear that the small amount of left‐lateral shear stress on the San Andreas fault observed in the upper 3.5 km at the Cajon Pass site is not associated with the topographic effect.
Lithospheric breakup in three dimensions: Necking of a work‐hardening plastic plate
Authors: Agnon Amotz , Eidelman Amir
Published: February 2008

Lithospheric breakup is, in general, oblique to the principal strain axes, with shortening in one horizontal axis (eh < 0). Local necking of the plate, including the mantle part of the lithosphere, is necessary for the creation of a divergent plate boundary. The necking develops as a mechanical instability that decreases the resistance of the plate to extension despite the intrinsic hardening of the material. We model the flow that precedes breakup stressing two aspects that have been overlooked before: (1) three‐dimensional flow, and (2) strain dependent strength of the lithosphere. The model mimics spatial and temporal features of the most extensively documented rift systems. The neck is oblique to the direction of principal extension, and the transition from diffuse stretching to local necking is abrupt at 5–10% strain. The critical strain for local necking in the model appears to be independent of the intrinsic strength. Therefore various rheological layers (brittle upper crust, ductile lower crust, and stiffer but hotter mantle lithosphere) may neck concomitantly to form a rift.
Dispersion dependence for Rayleigh‐like vibrations in a low‐density channel
Authors: Markushevich V. M. , Nyland Edo
Published: February 2008

Although it is in principle possible to compute dispersion curves for a buried low‐density channel by numerical means, the interpretation of such results is fraught with uncertainty relating to numerical approximations, inaccuracies, and a poorly developed theoretical framework for Rayleigh waves. It is thus interesting to investigate by analytical techniques some simple cases. One of these is a study of the Rayleigh wave in an idealization of a low‐density channel. Such a channel has at least two stationary vibration points and numerous modes with complex wave number as well as the usual four propagating Rayleigh modes. The stationary points are interesting because they correspond to resonant vibrations in an unbounded medium. The algebra required to investigate even this idealized case of a layer bounded above and below by high‐density plates and overlying an elastic half‐space is tedious. It can be automated quite effectively with Mathematica. This algebraic manipulation program also provides simple effective graphics tools to study the qualitative behavior of the results.
Thermoelastic stress in oceanic lithosphere due to hotspot reheating
Authors: Zhu Anning , Wiens Douglas A.
Published: February 2008

We investigate the effect of hotspot reheating on the intraplate stress field by modeling the three‐dimensional thermal stress field produced by nonuniform temperature changes in an elastic plate. Temperature perturbations are calculated assuming that the lithosphere is heated by a source in the lower part of the thermal lithosphere. A thermal stress model for the elastic lithosphere is calculated by superposing the stress fields resulting from temperature changes in small individual elements. The stress in an elastic plate resulting from a temperature change in each small element is expressed as an infinite series, wherein each term is a source or an image modified from a closed‐form half‐space solution. We apply the thermal stress solution to midplate swells in oceanic lithosphere with various thermal structures and plate velocities. Our results predict a stress field with a maximum deviatoric stress on the order of 100 MPa (1 kbar) covering a broad area around the hotspot plume. The predicted principal stress orientations show a complicated geographical pattern, with horizontal extension perpendicular to the hotspot track at shallow depths and compression along the track near the bottom of the elastic lithosphere. Although stress data near oceanic swells are limited, the source parameters of intraplate earthquakes near several hotspots are consistent with the thermal stress model. These results indicate that thermal stress due to reheating may be an important contributor to stress fields near hotspots in old oceanic lithosphere.
A rifted margin origin for the crescent basalts and related rocks in the northern Coast Range Volcanic Province, Washington and British Columbia
Authors: Babcock R. S. , Burmester R. F. , Engebretson D. C. , Warnock A. , Clark K. P.
Published: February 2008

The remarkable early to middle Eocene volcanic sequence of the Crescent Formation exposed on the Olympic Peninsula consists predominantly of tholeiitic to minor transitional alkaline basalts with sparse sedimentary interbeds. A composite section measured in the vicinity of the Dosewallips River includes 8.4 km of pillowed to massive submarine basalts overlain by 7.8 km of subaerial flows. An upper limit of about 48 Ma on the age of the Crescent basalts is indicated by faunal assemblages in sediments interbedded with the uppermost flows in the sequence and a circa 50 Ma 40Ar/39Ar age on a leucogabbro from the presumably correlative Bremerton Igneous Complex. Stratigraphically controlled samples collected from throughout the Crescent basalt sequence show that two distinctly different chemical types exist. The lower part of the sequence originated from a relatively depleted mantle course resembling normal (N) to enriched (E)‐MORB. The upper flows have a chemistry resembling E‐MORB to oceanic island tholeiites. This difference could be due to either variable metasomatism of a single source domain, or influx of a separate enriched‐mantle source component during the extrusion of the upper part of the sequence. Paleomagnetic measurements indicate that the Crescent basalts have not been significantly rotated, nor translated northwards since their extrusion. Paleotectonic reconstructions show that formation of the Crescent basalts and the Coast Range volcanic province as a whole coincided with a marked increase in the velocity of oblique convergence of the Kula plate with North America at about 60 Ma. Other geologic, geochemical, and paleomagnetic data are consistent with the interpretation that extrusion occurred in a basin or series of basins formed by a rift system along the continental margin of North America. Rifting might have been initiated by the influence of a hotspot, an increase in the rate of oblique convergence, or the kinematic effects of the Kula‐Farallon ridge as it migrated along the margin. If extrusion is related to the passage of the triple junction, then the Coast Ranges can be considered to be an important tectonic marker for early to middle Eocene plate reconstructions.
Slow‐time modulation of finite‐depth nonlinear water waves: Relation to longshore current oscillations
Authors: Shemer L. , Dodd N. , Thornton E. B.
Published: February 2008

Nonlinear dynamics of steep waves in water of finite depth are analyzed. The most unstable Benjamin‐Feir sidebands are found for a number of values of carrier wave steepness and water depth. The long‐time evolution of a three‐wave system, consisting of the carrier and the two most unstable sidebands, is then studied. Such a wave system undergoes periodic recurrence. It is shown that the slow‐time modulation leads to a corresponding periodic variation in the radiation stress when the propagation directions of the sidebands differ from that of the carrier. The variation of the radiation stress, in turn, results in slow‐time modulation of longshore current.
An examination of the “full‐wave” method for rough surface scattering in the case of small roughness
Authors: Thorsos Eric I. , Winebrenner Dale P.
Published: February 2008

We examine the accuracy of Bahar's “full‐wave” method for rough surface scattering in the case of small roughness. The specific problem examined is scattering from one‐dimensional rough surfaces assuming the Dirichlet boundary condition. This corresponds to scattering of horizontally polarized electromagnetic waves from perfectly conducting one‐dimensional surfaces. By one dimensional surfaces we mean surfaces with constant height in the direction perpendicular to the incident wave direction. Our main result is that full‐wave predictions for the normalized bistatic radar cross section are found to disagree substantially with first‐order perturbation theory when the latter is known to be accurate through comparison with exact integral equation studies. This disagrees with Bahar's conclusions. The full‐wave predictions for the normalized radar cross section were computed using a Monte Carlo method and also by averaging formally; the two methods show excellent agreement. For the examples discussed in the paper, the full‐wave prediction agrees closely with the Kirchhoff approximation result, though the Kirchhoff approximation is not accurate away from the specular direction for these cases. In fact, for the special case of backscattering with shadowing neglected, the full‐wave predictions are shown to agree exactly with the Kirchhoff approximation results, independent of surface roughness properties. We examine Bahar's analytic argument for the reduction of full‐wave theory to perturbation theory and show that it is based on an inconsistent expansion in small surface height. When the small height expansion is done consistently, the full‐wave prediction does not reduce to first‐order perturbation theory. Finally, we consider the computational procedure given by Bahar to reduce the numerical effort in evaluating full‐wave theory. We find that numerical results obtained via this procedure approximately coincide with numerical results from first‐order perturbation theory for sufficiently small surface heights and slopes. This may at first seem confusing in light of our other findings. However, we show that certain simplifying assumptions in Bahar's computational procedure change the numerical results significantly from those computed from full‐wave theory without such assumptions. Thus these assumptions are not acceptable approximations for full‐wave theory, and results based on them should not be considered valid results of full‐wave theory.
A mesoscale ocean feature study using synthetic aperture radar imagery in the Labrador Ice Margin Experiment: 1989
Authors: Ikeda M. , Livingstone C. E. , Peterson I.
Published: February 2008

Four Synthetic Aperture Radar (SAR) images were collected during the Labrador Ice Margin Experiment (LIMEX) in March 1989 to study mesoscale ocean features over the Newfoundland shelf. Ice floe movement is detected by displacement in the first two images collected on consecutive days. Streamfunction maps of ocean currents are drawn using an objective method after direct wind‐driven components are removed. The general flow pattern shows the inshore branch of the Labrador Current to be isolated from the seaward flow over the northern edge of the Grand Banks, in agreement with the mean circulation detected in NOAA satellite images for 3 years by the same method. Ice distribution in the SAR images implies mesoscale features much richer than those in the objectively drawn flow field. It is difficult to detect ice movement in the images with two day separation, because no persistent ice feature is identified in the study area. It is suggested, during the ERS‐1 ice mission, to take airborne SAR images one day before or after a satellite pass for ocean current detection from ice floe movement.
Response of baroclinic waves to nonuniform thermal forcing at the lower boundary
Authors: Buzyna G. , Kung R. , Pfeffer R. L.
Published: February 2008

A series of laboratory experiments in a rotating differentially heated annulus have been conducted to determine the effect of azimuthal thermal forcing at the lower boundary, of the kind representative of continent/ocean thermal contrasts, on the forcing of standing waves and the modulation of free baroclinic waves. An azimuthal wave number 2 thermal forcing at the lower boundary induces a wave number 2 in the flow field, which interacts with transient baroclinie waves. The induced wave number 2 in the flow field is a climatologically standing wave, with its phase dependent on the intensity of forcing. An analysis of the velocity data reveals that the trough of the time‐averaged stream field is located at the “continent/ocean” boundary, on the downstream side of the “continent,” and that the variance of the transient waves peaks approximately in the middle of the “continent”.
Dynamics of the Coastal Transition Zone jet: 1. Linear stability analysis
Authors: Pierce Stephen D. , Allen J. S. , Walstad Leonard J.
Published: February 2008

The linear stability of a coastal transition zone (CTZ) jet is analyzed using a six‐layer quasi‐geostrophic model with observed basic state velocity profiles. The velocity profiles are obtained from objectively analyzed hydrographic and acoustic doppler data from the 1987 CTZ pilot experiment. Along‐jet perturbation wave‐lengths of 260–265 km are found to be the most unstable, with e‐folding growth periods of 7–11 days and along‐jet phase speeds of 4–8 km/d downstream. Energy transformation terms and energy budgets are discussed. Both barotropic and baroclinic instability processes are important.
Computation of ripple wave parameters: A comparison of methods
Authors: Fernandez D. M. , Vesecky J. F. , Napolitano D. J. , Khuri‐Yakub B. T. , Mann J. A.
Published: February 2008

Remote sensing of oceanic and atmospheric quantities, such as the waveheight spectrum and wind velocity, depend largely on the interaction of electromagnetic radar waves with ocean surface waves in the short gravity and capillary range. The propagation of such ripple waves upon the ocean surface is strongly affected by the presence of surface films which constitute the ocean microlayer. In order to understand the impact of surface films on oceanic and atmospheric remote sensing, it is necessary to have an accurate and, if possible, convenient model relating film properties with the propagation and damping of ripple waves. This study examines two different methods of obtaining the damping coefficient of short gravity and capillary waves. One method, developed by Bock and Mann, utilizes numerical techniques based on the modified Levich characteristic equation to accurately yield complex roots governing the propagation of Laplace transverse and Marangoni longitudinal waves on the air‐sea interface. The other method, developed by Cini and Lombardini, involves finding an approximate analytical solution of the Levich equations using a perturbational technique. A comparison of the two methods yields tolerance ranges of various parameters, including frequency, modulus of surface dilational viscosity, and modulus of surface dilational elasticity, over which the two methods agree to 5–10%. To make our comparison concrete, we apply both methods to a specific surface film sample collected by Garret. Both theoretical results are compared to experimental results.
Reflectance model for quantifying chlorophyll a in the presence of productivity degradation products
Authors: Carder K. L. , Hawes S. K. , Baker K. A. , Smith R. C. , Steward R. G. , Mitchell B. G.
Published: February 2008

Marine colored dissolved organic matter (CDOM) (also gilvin or yellow substance) absorbs light at an exponentially decreasing rate as a function of wavelength. From 410 nm to about 640 nm, particulate phytoplankton degradation products including pheopigments, detritus, and bacteria have absorption curves that are similar in shape to that of CDOM. In coastal areas and areas downstream from upwelling regions, these constituents of seawater often absorb much more light than do living phytoplankton, leading to errors in satellite‐derived chlorophyll estimates as high as 133%. Proposed NASA sensors for the 1990s will have spectral channels as low as 412 nm, permitting the development of algorithms that can separate the absorption effects of CDOM and other phytoplankton degradation products from those due to biologically viable pigments. A reflectance model has been developed to estimate chlorophyll a concentrations in the presence of CDOM, pheopigments, detritus, and bacteria. Nomograms and lookup tables have been generated to describe the effects of different mixtures of chlorophyll a and these degradation products on the R(412) : R(443) and R(443) : R(565) remote‐sensing reflectance or irradiance reflectance ratios. These are used to simulate the accuracy of potential ocean color satellite algorithms, assuming that atmospheric effects have been removed. For the California Current upwelling and offshore regions, with chlorophyll a ≤ 1.3 mg m−3, the average error for chlorophyll a retrievals derived from irradiance reflectance data for degradation product‐rich areas was reduced from ±61% to ±23% by application of an algorithm using two reflectance ratios (R(412) : R(443) and R(443) : R(565)) rather than the commonly used algorithm applying a single reflectance ratio (R(443) : R(565)).
A simple treatment of very low frequency wind‐driven sea level variations
Authors: Mertz Gordon
Published: February 2008

It is shown that very low frequency sea level variations can be calculated from the sum of the steric or density effect and the wind‐driven setup derived from barotropic dynamics, provided that a weak slope criterion is met. This result is shown to be consistent with a recent two‐layer formulation for wind‐driven shelf motions.
High‐wavenumber shear and temperature structure during the Patches Experiment
Authors: Marmorino G. O. , Trump C. L.
Published: February 2008

Acoustic Doppler current profiler (ADCP) measurements made during the Patches Experiment (PATCHEX) in the Pacific Ocean are examined for the occurrence of high‐wavenumber shear (vertical scales of >2 m) which is presumed to lead to patches of small‐scale waves and turbulent mixing. Shear is found to be dominated by near‐inertial waves having upward energy propagation and ∼30‐m vertical wavelengths over the depth range ∼80–180 m. The predominance of upgoing waves is unexpected and supports the RiNo float measurements made by Kunze et al. (1990a) at 180 to 200‐m depth. Profiles of rms shear are qualitatively similar to those measured by Gregg and Sanford (1988) with the multi‐scale profiler (MSP); and the fraction of time shear exceeds a threshold based on Gregg's (1989) scaling is about 10%, similar to the fraction of active turbulence seen in MSP profiles of dissipation rate. Towed thermistor chain data show infrequent patches of short (∼15 m) waveforms which may be Kelvin‐Helmholtz shear instabilities (as assumed by Kunze et al.). The data support previous suggestions that near‐inertial waves are important sources of turbulent mixing in the thermocline.
Trace gas exchange above the floor of a deciduous forest: 1. Evaporation and CO2 efflux
Authors: Baldocchi Dennis D. , Meyers Tilden P.
Published: February 2008

The eddy correlation method has great potential for directly measuring trace gas fluxes at the floor of a forest canopy, but a thorough validation study has not been yet conducted. Another appeal of the eddy correlation method is its ability to study processes that regulate and modulate gas exchange between the soil/litter complex and the atmosphere that cannot be probed with chambers. In this paper we report on eddy correlation measurements of water vapor, sensible heat, and carbon dioxide exchange that were made at the floor of a deciduous forest. The validity of the eddy correlation method to measure the emission of water vapor and CO2 from a deciduous forest floor is demonstrated by our ability to close the surface energy budget during periods that meet the requirements of the technique. Water vapor fluxes from a dry forest floor are strongly influenced by large‐scale turbulent events that penetrate deep into the canopy. The frequency of these turbulent events prevents equilibrium evaporation rates from being achieved because the dynamic time constant for water vapor exchange is longer. Consequently, maximal evaporation rates are capped to rates defined by the product of the driving potential of the atmosphere and the surface conductance. On the other hand, evaporation from a wet forest floor proceeds at rates reaching or exceeding equilibrium evaporation and are highly correlated with static pressure fluctuations. CO2 efflux rates are governed by litter and soil temperature, as expected. But we also find a significant correlation between static pressure fluctuations and soil/litter CO2 exchange rates.