bluuebunny/crossref_metadata_2025 huggingface dataset:
Lightning whistler waves in the high‐latitude magnetosphere
Authors: Holzworth R. H. , Winglee R. M. , Barnum B. H. , Li YaQi , Kelley M. C.
Published: August 2003

Whistler waves caused by lightning are known to penetrate the ionosphere with significant wave amplitudes. Recent rocket experiments have shown that lightning from sources as far as 1000 km from the ionospheric rocket subtrack are easily seen. The spectral energy density of lightning in the ionosphere often has significant power at frequencies near and below 1 kHz. Such low‐frequency whistler waves can propagate all the way to the magnetopause boundary layer if they begin in the cusp or polar cap. Lightning is common over high‐latitude continental land masses in the summer months which places this wave source function well within range of the cusp and polar cap. Ray tracing studies using a global three‐dimensional two‐fluid code are conducted to investigate the propagation of these whistler waves into the high latitude magnetosphere. The estimated, mapped lightning whistler wave amplitude is compared to wave intensity measurements from ISEE and AMPTE and supports the argument that falling tone whistler waves, seen by Geotail near the dayside magnetopause, may have been from intense lightning. It is found that the mapped whistler wave amplitude from lightning is comparable to the in situ wave amplitudes measured in the outer dayside magnetosphere and low latitude boundary layer.
Langmuir waves in a fluctuating solar wind
Authors: Kellogg P. J. , Goetz K. , Monson S. J. , Bale S. D.
Published: August 2003

The Langmuir waves which are resonant with typical type III solar radio burst electrons have a frequency so little above the ambient plasma frequency that they should be strongly affected by known density fluctuations. Some consequences of this observation are worked out, and the expected consequences are demonstrated in the observations of the Langmuir waves from two quite different bursts, of November 4, 1997, and January 19, 1998.
Response of the midtail electric field to enhanced solar wind energy input
Authors: Nakamura R. , Bargatze L. F. , Mukai T. , Nagai T. , Baker K. B. , Hairston M. R. , Reiff P. H. , Petrukovich A. A. , Nozdrachev M. , Troshichev O. A.
Published: August 2003

We study the response of midtail plasma and field parameters to enhanced solar wind electric field input for two substorm intervals on November 22, 1995. The solar wind input signatures were quite different for these two substorms, which had major Pi2 onsets at 1108 and 1502 UT. The solar wind input for the 1108 UT substorm had a short timescale (∼0.5 hour) electric field enhancement up to 1.5 mV/m, whereas the solar wind electric field for the 1502 UT substorm continuously exceeded 1.5 mV/m for ∼2 hours. In association with the expansion phase onsets of both substorms, the electric field fluctuation in the midtail plasma sheet commenced. The electric field disturbances lasted for a time interval close to the length of time, in which the solar wind electric field was enhanced. The midtail plasma sheet electric field responded well to the enhanced solar wind input with a time delay of 45–80 min. The pressure decrease, which started at the onset of both substorms, ended quite differently for these two substorms: the pressure decreased until it approached the quiet time level in the course of the 1108 UT substorm, whereas the pressure dropped below the quiet time level during the expansion phase of the 1502 UT substorm and stayed at this low level until late in the recovery phase. Using polar cap potential drop and the pressure profile in the midtail, we estimate the relationship between the dayside and nightside potential drop. We suggest that the difference in the nightside flux transport rate could control the configuration of the midtail and could explain why the tail pressure responded differently during the expansion and recovery phase of the two substorms.
Flow braking and the substorm current wedge
Authors: Birn J. , Hesse M. , Haerendel G. , Baumjohann W. , Shiokawa K.
Published: August 2003

Recent models of magnetotail activity have associated the braking of earthward flow with dipolarization and the reduction and diversion of cross‐tail current, that is, the signatures of the substorm current wedge. Estimates of the magnitude of the diverted current by Haerendel [1992] and Shiokawa et al. [1997, 1998] tend to be lower than results from computer simulations of magnetotail reconnection and tail collapse [Birn and Hesse, 1996], despite similar underlying models. An analysis of the differences between these estimates on the basis of the simulations gives a more refined picture of the diversion of perpendicular into parallel currents. The inertial currents considered by Haerendel [1992] and Shiokawa et al. [1997] contribute to the initial current reduction and diversion, but the dominant and more permanent contribution stems from the pressure gradient terms, which change in connection with the field collapse and distortion. The major effect results from pressure gradients in the z direction, rather than from the azimuthal gradients [Shiokawa et al., 1998], combined with changes in By and Bz. The reduction of the current density near the equatorial plane is associated with a reduction of the curvature drift which overcompensates changes of the magnetization current and of the gradient B drift current. In contrast to the inertial current effects, the pressure gradient effects persist even after the burst of earthward flow ends.
Low‐speed solar wind from the vicinity of solar active regions
Authors: Kojima M. , Fujiki K. , Ohmi T. , Tokumaru M. , Yokobe A. , Hakamada K.
Published: August 2003

We have investigated the origin of low‐speed winds observed in association with active regions near the equator at times of solar activity minimum. The solar wind velocity distribution on a source surface at 2.5 Rs is derived by interplanetary scintillation tomographic analysis, and compact low‐speed regions in it are investigated in relation to active regions and large‐flux‐expansion regions. We show that although the low‐speed regions tend to be located near active regions, they are more closely associated with large flux expansion from the vicinity of active regions. We find that slow solar wind does not arise from closed magnetic loops above an active region, but instead the low‐speed stream originates from the vicinity of one polarity side of the active region. Therefore the low‐speed stream, unlike the helmet streamer, has a single magnetic polarity. This can explain why compact low‐speed streams are often not associated with a heliospheric current sheet.
Monitoring spatial and temporal variations in the dayside plasmasphere using geomagnetic field line resonances
Authors: Menk F. W. , Orr D. , Clilverd M. A. , Smith A. J. , Waters C. L. , Milling D. K. , Fraser B. J.
Published: August 2003

It is well known that the resonant frequency of geomagnetic field lines is determined by the magnetic field and plasma density. We used cross‐phase and related methods to determine the field line resonance frequency across 2.4≤<L≤4.5 in the Northern Hemisphere at 78°–106° magnetic longitude and centered on L=2.8 in the Southern Hemisphere at 226° magnetic longitude, for several days in October and November 1990. The temporal and spatial variation in plasma mass density was thus determined and compared with VLF whistler measurements of electron densities at similar times and locations. The plasma mass loading was estimated and found to be low, corresponding to 5–10% He+ on the days examined. The plasma mass density is described by a law of the form (R/Req)−p, where p is in the range 3–6 but shows considerable temporal variation, for example, in response to changes in magnetic activity. Other features that were observed include diurnal trends such as the sunrise enhancement in plasma density at low latitudes, latitude‐dependent substorm refilling effects, shelves in the plasma density versus L profile, and a longitudinal asymmetry in plasma density. We can also monitor motion of the plasmapause across the station array. Properties of the resonance were examined, including the resonance size, Q, and damping. Finally, we note the appearance of fine structure in power spectra at these latitudes, suggesting that magnetospheric waveguide or cavity modes may be important in selecting wave frequencies.
Momentum balance of dayside E region neutral winds during geomagnetically quiet summer days
Authors: Maeda Sawako , Fujiwara Hitoshi , Nozawa Satonori
Published: August 2003

An intent of this paper is to study dayside quiet time E region neutral momentum balance in the auroral ionosphere. Neutral wind vectors were obtained by using the European Incoherent Scatter (EISCAT) Common Program 2 (CP‐2) data between 98 and 119 km height from 1100 to 1300 UT (from 1200 to 1400 LT and from 1400 to 1600 MLT) on June 15 and 16, 1993. Individual forcing terms in the neutral momentum equation were determined from the derived neutral winds with the MSISE‐90 model. In order to calculate the velocity change with time including advection, designated as the time‐dependent term, and the viscous force in the neutral momentum equation, a polynomial fitting procedure was applied to perform the vertical and time differentiation of the derived neutral winds. Below 106 km height, the time‐dependent term was greater than the pressure gradient force except for the meridional component at 98 km height on June 15. Between 110 and 116 km height, the pressure gradient force was in the same order of magnitude as that of the Coriolis force and the time‐dependent term was smaller than these two. The neutral wind flow was nearly perpendicular to the pressure gradient force. At 119 km height, the viscous drag was in the same order of magnitude as those of the pressure gradient and Coriolis forces and greater than the Pedersen and Hall drags. The magnitude of ion drag was ∼7% of the Coriolis force at this height level.
Excitation of extraordinary Bernstein waves by a beam of energetic electrons
Authors: Yoon Peter H. , Wu C. S. , Li Y.
Published: August 2003

This paper discusses an instability of extraordinary Bernstein waves, driven by a beam of energetic electrons. The present research is motivated by the study of solar radio emission processes, and its findings may have interesting implications. Specifically, the extraordinary Bernstein waves excited both below and above the plasma frequency by the present instability can, in principle, mode convert to either of the free‐space propagating modes in the presence of magnetic field and/or density inhomogeneities. This paper is devoted to an analysis of the basic properties associated with the present instability over a wide range of various physical parameters.
On the determination of electron polytrope indices within coronal mass ejections in the solar wind
Authors: Gosling J. T.
Published: August 2003

Electron temperature and density often appear to be negatively correlated with one another as a coronal mass ejection, CME, in the solar wind passes over a spacecraft at 1 AU and beyond. It has been suggested that this negative correlation within magnetic clouds, which form a subset of all CMEs, implies that the electron polytropic index for the plasma within the clouds is significantly less than 1.0. We argue that single‐point measurements of density and temperature within clouds do not speak to the issue of the coupled evolution of density and temperature that occurs as the clouds expand out into the heliosphere, and thus do not provide a measure of the polytropic index within them. Moreover, we show that observed electron temperatures within magnetic clouds and CMEs close to the Sun, at 1 AU, and at large heliocentric distances do not agree with predictions based upon electron polytropic indices significantly less than 1.0. We suggest that the negative correlation between electron temperature and density often observed within CMEs and magnetic clouds at 1 AU and beyond simply reflects the presence of structure within the CMEs and the plasma's tendency to achieve local pressure balance as it evolves outward from the Sun. Our results indicate that self‐similar models that require the electron polytrope index to be less than 1.0 in order for a magnetic cloud to expand are physically unrealistic.
Electron temperature anisotropy instabilities: Whistler, electrostatic and z mode
Authors: Gary S. Peter , Cairns Iver H.
Published: August 2003

An electron temperature anisotropy T⊥e/T‖e > 1 leads to excitation of three distinct instabilities in collisionless plasmas at frequencies below the electron cyclotron frequency |Ωe| (Here ⊥ and ‖ denote directions relative to the background magnetic field B0.). Linear Vlasov theory is used to study these growing modes, with emphasis on the scaling of the temperature anisotropy at instability threshold. If the electron plasma frequency ωe is greater than |Ωe| and electrons are sufficiently hot, the whistler is the unstable mode with smallest anisotropy threshold; this electromagnetic mode has maximum growth rate at propagation parallel to B0. At ωe > 0.5|Ωe|, an electrostatic electron anisotropy instability can arise at propagation oblique to B0; this mode may have the smallest threshold for sufficiently cool electrons and ωe ∼ |Ωe|. And T⊥e/T‖e > 1 drives the z mode unstable at ωe < |Ωe|; this electromagnetic mode also has maximum growth rate at parallel propagation and is the persistent instability at ωe ≲ 0.5|Ωe|. The results are discussed in connection with observations from the polar and auroral regions of the terrestrial magnetosphere.
Oxygen isotope geochemistry of the second HSDP core
Authors: Wang Zhengrong , Kitchen Nami E. , Eiler John M.
Published: August 2003

Oxygen isotope ratios were measured in olivine phenocrysts (∼1 mm diameter), olivine microphenocrysts (generally ∼100–200 μm diameter), glass, and/or matrix from 89 samples collected from depths down to 3079.7 m in the second, and main, HSDP core (HSDP‐2). Olivine phenocrysts from 11 samples from Mauna Loa and 34 samples from the submarine section of Mauna Kea volcano have δ18O values that are similar to one another (5.11 ± 0.10‰, 1σ, for Mauna Loa; 5.01 ± 0.07‰, for submarine Mauna Kea) and within the range of values typical of olivines from oceanic basalts (δ18O of ∼5.0 to 5.2‰). In contrast, δ18O values of olivine phenocrysts from 20 samples taken from the subaerial section of Mauna Kea volcano (278 to 1037 mbsl) average 4.79 ± 0.13‰. Microphenocrysts in both the subaerial (n = 2) and submarine (n = 24) sections of Mauna Kea are on average ∼0.2‰ lower in δ18O than phenocrysts within the same stratigraphic interval; those in submarine Mauna Kea lavas have an average δ18O of 4.83 ± 0.11‰. Microphenocrysts in submarine Mauna Kea lavas and phencrysts in Mauna Loa lavas are the only population of olivines considered in this study that are typically in oxygen isotope exchange equilibrium with coexisting glass or groundmass. These data confirm the previous observation that the stratigraphic boundary between Mauna Loa and Mauna Kea lavas defines a shift from “normal” to unusually low δ18O values. Significantly, they also document that the distinctive 18O‐depleted character of subaerial Mauna Kea lavas is absent in phenocrysts of submarine Mauna Kea lavas. Several lines of evidence suggest that little if any of the observed variations in δ18O can be attributed to subsolidus alteration or equilibrium fractionations accompanying partial melting or crystallization. Instead, they reflect variable proportions of an 18O‐depleted source component or contaminant from the lithosphere and/or volcanic edifice that is absent in or only a trace constituent of subaerial Mauna Loa lavas, a minor component of submarine Mauna Kea lavas, and a major component of subaerial Mauna Kea lavas. Relationships between the δ18O of phenocrysts, microphenocrysts, and glass or groundmass indicate that this component (when present) was added over the course of crystallization‐differentiation. This process must have taken place in the lithosphere and most likely at depths of between ∼5 and 15 km. We conclude that the low‐δ18O component is either a contaminant from the volcanic edifice that was sampled in increasingly greater proportions as the volcano drifted off the center of the Hawaiian plume or a partial melt of low‐δ18O, hydrothermally altered perdotites in the shallow Pacific lithosphere that increasingly contributed to Mauna Kea lavas near end of the volcano's shield building stage. The first of these alternatives is favored by the difference in δ18O between subaerial and submarine Mauna Kea lavas, whereas the second is favored by systematic differences in radiogenic and trace element composition between higher and lower δ18O lavas.
Slip‐weakening behavior during the propagation of dynamic ruptures obeying rate‐ and state‐dependent friction laws
Authors: Bizzarri Andrea , Cocco Massimo
Published: August 2003

We model the traction evolution and shear stress degradation near the tip of a propagating dynamic rupture by solving the elastodynamic equation for a 2‐D in‐plane fault obeying rate‐ and state‐dependent friction laws and adopting a finite difference numerical method. Modeling results clearly show that our dynamic solution implies a slip dependence of fault friction, as previously observed either in laboratory experiments or in theoretical models. However, the resulting equivalent slip‐weakening distance (d0eq) is different from the length scale parameter (L) characteristic of the rate and state formulation. We demonstrate that the state variable evolution controls the slip acceleration and the absorbed fracture energy. The adopted constitutive parameters a, b, and L affect the traction dependence on slip. We present the results of several numerical simulations, performed after a careful control of the available resolution of the cohesive zone, to unravel the dependence of the equivalent slip‐weakening distance on the constitutive parameters. We also propose analytical relations to interpret our numerical results, which point out that the traction evolution within the cohesive zone cannot be prescribed a priori in the framework of rate‐and‐state constitutive laws. In particular, the yield stress and the kinetic friction level depend on particular slip velocity values characteristic of specific stages of the breakdown process. Finally, we discuss how the adopted evolution law affects the slip‐weakening curve by comparing the simulations performed with a slip and a slowness law. The former yields smaller equivalent slip‐weakening distances than the latter.
Inverse estimation of sea surface heat flux over the equatorial Pacific Ocean: Seasonal cycle
Authors: Yuan Dongliang , Rienecker Michele M.
Published: August 2003

An assimilation of satellite‐derived sea surface temperature (SST) observations into the Poseidon quasi‐isopycnal ocean model is conducted to study the seasonal cycle of surface thermodynamics of the equatorial Pacific Ocean and to estimate the sea surface heat flux inversely from the SST observations. A simplified approach in the use of the thermodynamic submodel that separates the momentum and the temperature fields in the variational assimilation procedure proves successful in improving the model simulation of the observed SST. From a comparison with several existing bulk estimates, it is seen that the seasonal cycle of sea surface heat flux has been captured well by the inversion except in the eastern equatorial Pacific, where the optimized surface heat flux shows an upward flux during the boreal summer/fall season when the cold tongue SST starts to decrease dramatically. This spurious upward surface heat flux is inconsistent with the bulk estimates and is determined to be due to model deficiencies in calculating vertical turbulent diffusion. The SST data assimilation is shown to have improved the model simulation of horizontal temperature advection significantly. A comparison of the inferred heat budget is made with an analysis by Wang and McPhaden [1999] from TAO mooring observations.
Vertical distributions of dust and sea‐salt aerosols over Puerto Rico during PRIDE measured from a light aircraft
Authors: Maring H. , Savioe D. L. , Izaguirre M. A. , Custals L. , Reid J. S.
Published: August 2003

We developed the Light Aircraft Aerosol Package (LAAP) to make measurements of aerosol properties up to ∼4000 m asl. The LAAP reliably measured, with empirical and theoretical corrections, aerosol microphysical and chemical properties. During the Puerto Rico Dust Experiment (PRIDE), June–July 2000, we measured aerosol properties over the tropical North Atlantic. On the basis of aerosol filter samples and measured aerosol size distributions, we (1) modeled sea‐salt aerosol concentration with altitude, characterizing the rapid decrease in sea‐salt concentration with altitude; (2) found no evidence of sea salt above the marine boundary layer (MBL); and (3) observed mineral dust in the Saharan Air Layer (SAL) to exceed 170 μg m−3. Substantial variability in the vertical distribution and small differences in the size distribution of dust between the MBL and SAL suggested dust properties were more dependent on the separate transport histories than the interaction between the MBL and SAL near Puerto Rico. Relatively constant dust size distributions with altitude suggested either (1) no relofting of dust aerosols once they get into the “deposition layer” or (2) dry deposition removes dust particles independent of particle size. Dust size distributions measured at low (∼25%) and high (∼93%) relative humidity were indistinguishable. A simple model calculation suggests dust particles over the tropical North Atlantic during summer increase their geometric diameter <6% for a relative humidity change from 25 to 95%. Thus optical properties of those dust particles should not change for relative humidities <95%.
Retrieval of microphysical, geometrical, and radiative properties of marine stratocumulus from remote sensing
Authors: Schüller Lothar , Brenguier Jean‐Louis , Pawlowska Hanna
Published: August 2003

Radiative transfer calculations in boundary layer stratocumulus are performed and used to retrieve cloud properties from remotely measured reflected radiances in the visible and near‐infrared. Calculations based on the vertically uniform plane‐parallel cloud model have been currently used for the retrieval of cloud optical thickness and droplet effective radius. They were extended in a previous work to the retrieval of cloud droplet number concentration and cloud geometrical thickness, by assuming a vertically stratified cloud model, with prescribed droplet spectrum. The retrieved values of cloud droplet number concentration though were substantially underestimated. Improvements are presented here with a more realistic parameterization of the droplet size distribution, based on the theory of droplet growth by vapor diffusion. The retrieval technique is applied to eight case studies of the second Aerosol Characterization Experiment and the retrieved values are compared to estimates derived from in situ cloud microphysical measurements. The technique provides accurate retrievals of cloud droplet number concentration, with no bias. The retrieval of cloud geometrical thickness though is systematically overestimated compared to that directly measured. This bias partly reflects the difference of cloud sampling between the two strategies. Remote sensing measurements, which are continuous, are capable of detecting the thickest cells of the cloud layer within the field of view of the radiometer, while discontinuous in situ vertical profiles provide a statistical average of the geometrical thickness. Part of the bias could also reflect three‐dimensional effects of the radiative transfer in spatially heterogeneous clouds, which are not accounted for by the plane‐parallel models. The data are then processed for deriving statistics of the retrieved values of cloud droplet number concentration, optical thickness, and liquid water path, for validation of large‐scale parameterizations of the aerosol indirect effect.
Law of mass action in the Arctic lower stratospheric polar vortex January–March 2000: ClO scaling and the calculation of ozone loss rates in a turbulent fractal medium
Authors: Tuck Adrian F. , Hovde Susan J. , Gao Ru‐Shan , Richard Erik C.
Published: August 2003

We consider the effects of power law scaling in the 1999–2000 Arctic lower stratospheric vortex from the point of view of the law of mass action and its application to the chemical kinetics of ozone loss embedded in a turbulent, macroscopic, fractal medium. The ER‐2 observations of ClO obey power law scaling; the exponent varies with time in a manner shown to be consistent with the scaling of NOy and O3, via the influences of polar stratospheric clouds and actinic solar radiation. While the microscopic rate coefficient for ClO three‐body recombination to the dimer applies as measured to three‐dimensional volumes in which the sole transport mechanism is molecular diffusion, this cannot be true in the 2.56‐dimensional space in which macroscopically fluctuating ClO reacts in the lower stratosphere. We show that the rate of loss of ozone via the ClO dimer mechanism is proportional to [ClO]2.20 in late January/early February and to [ClO]2.55 in March. Chemical ozone loss had already occurred by the date of the first flight, 20000120.
Reanalysis of tropospheric CO trends: Effects of the 1997–1998 wildfires
Authors: Novelli P. C. , Masarie K. A. , Lang P. M. , Hall B. D. , Myers R. C. , Elkins J. W.
Published: August 2003

For the past decade NOAA/CMDL has measured tropospheric carbon monoxide from a global network of sampling sites. The resulting data set provides an internally consistent picture of CO in the lower troposphere that is used to study its distribution, trends and budget. All measurements were referenced to the so‐called CMDL Reference Scale (WMO 88), which was based on two sets of primary standards produced at CMDL during the late 1980s and early 1990s. A long‐term downward trend in tropospheric CO during the 1990s, overlaid with shorter periods of increase and decrease, was indicated from the air measurements. Primary standards prepared in 1999 and 2000 suggested that the scale had drifted upward over time, and that mixing ratios determined in field samples were underestimated. We have applied a time dependent correction to our CO measurements based upon four sets of primary standards. In this paper, we describe the revision of the CO scale and our atmospheric measurements. A reanalysis of tropospheric trends through 2001 was based on the revised global data set. The results support previous reports of a decline in tropospheric CO. This decrease is now found largely confined to the Northern Hemisphere, where dramatic reductions in fossil fuel emissions have reportedly occurred. In contrast, no significant trend is determined in the Southern Hemisphere between 1991 and 2001. Globally averaged CO exhibits large interannual variability, primarily reflecting year to year changes in emissions from biomass burning. Dramatic enhancements of tropospheric CO in 1997 and 1998 resulted from exceptionally widespread wildfires which provided a strong pulse of CO to the atmosphere. In years of extensive boreal biomass burning, fire emissions can perturb CO levels over regional and global scales, disturbing oxidation/reduction chemistry in the troposphere.
Carbon and hydrogen isotopic compositions of stratospheric methane: 2. Two‐dimensional model results and implications for kinetic isotope effects
Authors: McCarthy M. C. , Boering K. A. , Rice A. L. , Tyler S. C. , Connell P. , Atlas E.
Published: August 2003

New high‐precision measurements of the carbon and hydrogen isotopic compositions of stratospheric CH4 made on whole air samples collected aboard the NASA ER‐2 aircraft are compared with results from the Lawrence Livermore National Laboratory 2‐D model. Model runs incorporating sets of experimentally determined kinetic isotope effects (KIEs) for the reactions of CH4 with each of the oxidants OH, O(1D), and Cl are examined with the goals of determining (1) how well the 2‐D model can reproduce the observations for both the carbon and hydrogen isotopic compositions, (2) what factors are responsible for the observed increase in the apparent isotopic fractionation factors with decreasing methane mixing ratios, and (3) how sensitive the modeled isotopic compositions are to various experimentally determined KIEs. Bound by estimates of the effects of uncertainties in model chemistry and transport on isotopic compositions, we then examine the constraints the ER‐2 observations place on values for the KIEs. For the carbon KIE for reaction of CH4 with O(1D), for example, the analysis of model results and observations favors the larger of the experimental values, 1.013, over a value of 1.001. These analyses also suggest that intercomparisons of results from different models using a given set of KIEs may be useful as a new diagnostic of model‐model differences in integrated chemistry and transport.
Measurement and modeling of the Saharan dust radiative impact: Overview of the Saharan Dust Experiment (SHADE)
Authors: Tanré D. , Haywood J. , Pelon J. , Léon J. F. , Chatenet B. , Formenti P. , Francis P. , Goloub P. , Highwood E. J. , Myhre G.
Published: August 2003

Aerosols are known to be important in determining Earth's radiative balance. Dust aerosols are of particular interest since, in addition to their scattering and absorbing properties that affect the solar radiation, they also perturb the terrestrial radiation. Recent studies have shown that a significant proportion of mineral dust in the atmosphere may be of anthropogenic origin, and therefore they may have an important role in climate change by exerting a significant radiative forcing. However, the optical and radiative properties of dust are not yet very well‐determined, and even the sign of the resulting forcing is still questionable. The Saharan Dust Experiment (SHADE) was designed to better determine the parameters that are relevant for computing the direct radiative effect. Two aircraft combining in situ and remote sensing instruments were coordinated with satellite overpasses and ground‐based observations during the experiment, which was based in the Cape Verde area during the period 19–29 September 2000. These in situ and remotely sensed data provide new valuable information on the microphysical, optical properties, and radiative effects of a large mineral dust outbreak. In addition, a global chemical transport model was used for assessing the radiative impact of these events, which are shown to be important on regional and global scales.
Transverse‐beam incoherent scatter radar measurements of F region plasma densities at Jicamarca
Authors: Feng Zhaomei , Kudeki Erhan , Woodman Ronald F. , Chau Jorge
Published: August 2003

Models for the self‐ and cross‐correlation of orthogonal polarized quasi‐transverse incoherent scatter returns are presented. The models are used to estimate the F region plasma densities with dual‐polarization transverse‐beam incoherent scatter data collected at the Jicamarca Radio Observatory using a technique introduced by Kudeki et al. [2003]. The cross‐correlation model takes into account magneto‐ionic propagation effects associated with quasi‐transverse O‐ and X‐modes in an inhomogeneous ionosphere. The phase of the cross‐correlation provides the information to convert the measured power data into ionospheric density estimates via a least squares fitting procedure.
Application of a radially viewed inductively coupled plasma‐optical emission spectrophotometer to simultaneous measurement of Mg/Ca, Sr/Ca, and Mn/Ca ratios in marine biogenic carbonates
Authors: Wara M. W. , Anderson L. D. , Schellenberg S. A. , Franks R. , Ravelo A. C. , Delaney M. L.
Published: August 2003

We developed and applied an analytical system for the simultaneous determination of magnesium/calcium, strontium/calcium, and manganese/calcium ratios in biogenic carbonates using a Perkin‐Elmer 4300 Optima DV inductively coupled plasma‐optical emission spectrophotometer in radial mode. Intensity ratio responses to calibration standard elemental ratios were linear over suitable working ranges. We evaluated instrumental response using solution replicates, and we analyzed planktonic foraminifera in multiple runs. Ca concentrations were kept in a relatively narrow range, typically ∼0.6–2.4 mM Ca, although we found no resolvable Ca matrix effect over a much wider range. Each analysis required a minimum sample volume of 300 μl and took 3.5 min instrument time. An analytical run of 30 samples plus standards, blanks, and control materials typically took ∼5–6 hours. Reproducibility (interrun precision, 1s %RSD) for solutions was 1.2%, 1.5%, and 1.2% for Mg/Ca, Sr/Ca, and Mn/Ca ratios, far smaller than reproducibility for foraminiferal replicates over multiple analytical runs. There was no independent reference material for assessing accuracy. Detection limits were equivalent for an 0.5 mM Ca solution to 0.058 mmol/mol for Mg/Ca, 0.015 mmol/mol for Sr/Ca, and ∼0.005–0.020 mmol/mol for Mn/Ca. Precautions included limiting length of time the peristaltic tubing for the autosampler was used, avoiding Mg contamination from plastic ware, and screening out results from samples with very low Ca concentrations (<0.25 mM Ca).
Retrieval of mass and sizes of particles in sandstorms using two MODIS IR bands: A case study of April 7, 2001 sandstorm in China
Authors: Gu Yingxin , Rose William I. , Bluth Gregg J. S.
Published: August 2003

A thermal infrared remote sensing retrieval method developed by Wen and Rose [1994], which retrieves particle sizes, optical depth, and total masses of silicate particles in the volcanic cloud, was applied to an April 07, 2001 sandstorm over northern China, using MODIS. Results indicate that the area of the dust cloud observed was 1.34 million km2, the mean particle radius of the dust was 1.44 μm, and the mean optical depth at 11 μm was 0.79. The mean burden of dust was approximately 4.8 tons/km2 and the main portion of the dust storm on April 07, 2001 contained 6.5 million tons of dust. The results are supported by both independent remote sensing data (TOMS) and in situ data for a similar event in 1998. This paper demonstrates that Wen and Rose's retrieval method could be successfully applied to past and future sandstorm events using IR channels of AVHRR, GOES or MODIS.
Geomagnetic moment instability between 0.6 and 1.3 Ma from cosmonuclide evidence
Authors: Carcaillet Julien T. , Thouveny Nicolas , Bourlès Didier L.
Published: August 2003

The reliability of paleomagnetic records as proxies of the geomagnetic field intensity is still a matter of controversy since volcanic materials hardly provide continuous records, and marine sediments are suspected to carry a remanence biased by post‐depositional realignments and/or by overprints. Such long standing debate emphasizes the need for the development of methods independent from paleomagnetism to decipher geomagnetic intensity variations. High resolution measurements of authigenic 10Be/9Be along with a detailed sedimentary record of directional and relative paleointensity variations evidence, over the 0.6–1.3 Ma time interval, frequent and recurrent excursions or short events in the late Matuyama and the early Brunhes epochs, among which two Brunhes‐Matuyama reversal precursors and an intra‐Jaramillo excursion. The results of this study confirm the idea of a highly unstable geomagnetic field as suggested by paleomagnetic evidences.
Electronic structures of two-phase microstructures α and β in heavily Pb-doped Bi2Sr2CaCu2Oy single crystals investigated by scanning tunneling microscopy/spectroscopy
Authors: Kinoda G. , Hasegawa T. , Nakao S. , Hanaguri T. , Kitazawa K. , Shimizu K. , Shimoyama J. , Kishio K.
Published: August 2003

We have performed scanning tunneling microscopy/spectroscopy (STM/STS) of a high-Jc superconductor, heavily Pb-doped Pb0.6Bi1.4Sr2CaCu2Oy, at low temperatures below Tc. The obtained STM images showed a lamella-type phase separation into Pb-poor and Pb-rich regions. At 4.3 K, values of energy gap Δ, deduced from STS spectra, showed considerable inhomogeneity in the range of 20–80 meV, while Δ variations across the phase boundaries were less significant. At 63 K, in contrast, we found that Δ altered sharply at the boundaries particularly in the annealed samples, resulting from the suppression of electronic inhomogeneity in each phase. This suggests that the phase boundaries act as strong pinning centers at higher temperatures.
Phase states of a non-Heisenberg ferromagnet with complex single-ion anisotropy
Authors: Fridman Ya. A. , Kosmachev O. A. , Eingorn B. L.
Published: August 2003

The phase states and spectra of coupled magnetoelastic waves of a non-Heisenberg ferromagnet with complex single-ion anisotropy are investigated. The influence of inclined anisotropy on the phase states and spectra of the system is determined. The phase diagrams of the system are constructed for different relationships among the material constants.
Propagation of heat pulses in disordered media
Authors: Salamatov E. I.
Published: August 2003

Processes of heat pulse propagation in nonconducting disordered systems containing thermal-phonon trapping centers (two-level systems) are investigated at low temperatures. The main parameters of the model studied, containing two different two-level systems, are the heat capacity of each of the subsystems and the relaxation time for the inelastic scattering of phonons on the two-level systems. The influence of the interaction of phonons with the two-level centers on the time of arrival of the heat pulse at the bolometer is calculated for different densities of centers. The theoretical calculations are compared with the experimental data on the propagation of slightly nonequilibrium phonons in solid solutions of rare-earth yttrium–aluminum garnets and in two-phase ceramics at helium temperatures.
Analysis of ultrathin SiO2 interface layers in chemical vapor deposition of Al2O3 on Si by in situ scanning transmission electron microscopy
Authors: Klie R. F. , Browning N. D. , Chowdhuri A. Roy , Takoudis C. G.
Published: August 2003

The development of Al2O3 as an alternative gate dielectric for microelectronic applications depends on the ability to grow a high-quality nanoscale thin film that forms an atomically abrupt interface with Si. Here, the combination of in situ Z-contrast imaging, electron energy loss spectroscopy and x-ray photoelectron spectroscopy of amorphous Al2O3 films grown by metalorganic chemical vapor deposition shows that excess oxygen incorporated into the film routinely reacts with the Si substrate to form an amorphous SiO2 interface layer during postdeposition annealing. The intrinsic oxygen-rich environment of all films grown by such techniques and the necessity of postdeposition processing in device applications implies that control and optimization of the SiO2 interface layers could be of utmost interest for high-κ dielectric stacked structures.
Growth of vertically aligned carbon nanofibers by low-pressure inductively coupled plasma-enhanced chemical vapor deposition
Authors: Caughman J. B. O. , Baylor L. R. , Guillorn M. A. , Merkulov V. I. , Lowndes D. H. , Allard L. F.
Published: August 2003

Vertically aligned carbon nanofibers (VACNFs) have been grown using a low-pressure, plasma-enhanced, chemical vapor deposition process. The nanofibers are grown from a nickel catalyst that can be patterned to form arrays of individual, isolated VACNFs. The fibers are grown at pressures below 100 mTorr, using an inductively coupled plasma source with a radio-frequency bias on the sample substrate to allow for independent control of the ion energies. Plasma conditions are related to growth results by comparing optical emission from the plasma to the physical structure of the nanofibers. We find that the ratio of etching species in the plasma to depositing species is critical to the final shape of the carbon structures that are formed.
Tuning of the intersubband emission below the longitudinal optical phonon energy in GaAs/AlGaAs quantum cascade emitters
Authors: Dhillon S. S. , Davies A. G. , Beere H. E. , Linfield E. H. , Ritchie D. A. , Arnone D. D.
Published: August 2003

We present a study of far-infrared intersubband electroluminescence from a number of GaAs–Al0.15Ga0.85As quantum-cascade emitters with emission energies below the LO phonon energy. A range of samples with emission energies between 13 and 21 meV were investigated. A systematic decrease in the normalized emission intensity with increasing intersubband separation was observed. The possible mechanisms such as the grating coupling efficiency and the role of scattering processes are discussed.
Noise and photoconductive gain in InAs quantum-dot infrared photodetectors
Authors: Ye Zhengmao , Campbell Joe C. , Chen Zhonghui , Kim Eui-Tae , Madhukar Anupam
Published: August 2003

We report noise characteristics, carrier capture probability, and photoconductive gain of InAs quantum-dot infrared photodetectors with unintentionally doped active regions. At 77 K, a photoconductive gain of 750 was observed at a bias of 0.7 V. The high gain is a result of the low carrier capture probability: p=0.0012.
Generation of coherent terahertz radiation with multifrequency modes in a Fibonacci optical superlattice
Authors: Qin Yi-qiang , Su Hong , Tang Sing-hai
Published: August 2003

We investigate theoretically the generation of coherent narrow-band terahertz (THz) radiation with multifrequency modes in an optical superlattice produced by quasiperiodically poled ferroelectric crystal. The superlattice consists of two building blocks A and B, in which each containing a pair of antiparallel 180° ferroelectric domains, arranged as a Fibonacci sequence. Second-order nonlinear optical rectification of femtosecond pulses gives rise to a THz wave form whose multifrequency modes are determined by the quasiperiodic structure properties. The relationship between multifrequency modes terahertz generation and Fibonacci structure parameters is also analyzed and studied.
Formation energy of Stone–Wales defects in carbon nanotubes
Authors: Zhou L. G. , Shi San-Qiang
Published: August 2003

A Stone–Wales (SW) defect is a dipole of 5–7 ring pair in a hexagonal network, which is one of the most important defective structures in carbon nanotubes (CNTs) that will affect mechanical, chemical, and electronic properties of CNTs. Using the extended Hückel method, we calculated the formation energy of SW defects in carbon nanotubes. The formation energy of SW defects was then fitted to a simple formula as a function of the tube radius and the orientation of a SW defect in the tube. This result provides a convenient tool for the study of thermodynamics and kinetics of SW defects, as well as the interaction of SW defects with other types of defects in CNTs.
Defects in m-face GaN films grown by halide vapor phase epitaxy on LiAlO2
Authors: Vanfleet R. R. , Simmons J. A. , Maruska H. P. , Hill D. W. , Chou M. M. C. , Chai B. H.
Published: August 2003

Free-standing wafers (50 mm diameter) of GaN were grown by halide vapor phase epitaxy on lattice-matched γ-LiAlO2. We report a transmission electron microscopy study of defects and defect densities in these wafers. The growth direction is [101_0]. Stacking faults in the basal plane are seen when viewing the specimen in the [12_10] direction with an average spacing of less than 100 nm. Convergent beam electron diffraction measurements show no switch in the polarity and thus the faults are proposed to be ABABACAC changes in the stacking. Threading dislocations are found to have a correlated arrangement with a density of 3×108 cm−2 when viewing the [12_10] direction and widely varying (depending upon location) when viewing in the [0001] direction. These dislocations act as “seeds” for postgrowth surface features that directly exhibit the correlated nature of these threading dislocations.
Ion-induced chemical vapor deposition of copper films with nanocellular microstructures
Authors: Ross F. , Thompson C. V. , Chiang T. , Sawin H. H.
Published: August 2003

Ion-induced chemical vapor deposition using a broad ion beam has been used to deposit nanocellular films. These films consist of closely packed 10–20 nm diameter copper rods separated by a carbonaceous residue, and growing in a direction normal to the substrate surface to lengths equal to the film thickness. The effects of ion flux, ion energy, and substrate temperature on rod spacing were investigated. A growth mechanism analogous to that leading to cellular structures during solidification from alloy melts is proposed and qualitatively described. Films with nanocellular structures are expected to have useful, highly anisotropic properties.
Photorefractive lithography with synchrotron light in poly(methyl methacrylate)
Authors: Andreas B. , Peithmann K. , Soergel E. , Buse K.
Published: August 2003

Synchrotron light changes the refractive index of poly(methyl methacrylate) (PMMA). Refractive-index enhancements as well as reductions depending on dose and energy of the radiation used can be observed. This effect allows the manufacturing of, for example integrated-optical components, as is demonstrated by realization of waveguides in PMMA.
Measurements of electron emission uniformity from cathodes in high-voltage electron beam diodes
Authors: Blaugrund A. E.
Published: August 2003

Spatial uniformity of electron emission from velvet cathodes has been measured by observing the distribution of electrons hitting the anode in a planar high-voltage diode. It is shown that unless all electrons move parallel to the diode axis, the electron distribution measured at the anode does not represent the cathode emission pattern. Here, parallel trajectories were achieved by applying a strong, uniform axial magnetic field. A 5 ns gated ICCD camera recorded light from a fast anode scintillator. The emission from two brands of velvet was found to be mostly concentrated in small spots with density of about 55/cm2. The pixel-to-pixel standard deviation of the emission amounted to at least 16%.
Electrostrictive behavior of poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene)
Authors: Garrett J. T. , Roland C. M , Petchsuk A. , Chung T. C.
Published: August 2003

The mechanism underlying the large electric-field-induced strains in terpolymers of vinylidene fluoride, trifluoroethylene, and chlorotrifluoroethylene was investigated. The electrostrictive strain increased by an order of magnitude with increasing temperature, up to the Curie transition, and was essentially invariant to temperature thereafter. Infrared absorption spectra, obtained as a function of both temperature and electric field strength, revealed no change in the crystal phase structure for electric fields sufficient to induce longitudinal strains of ∼1%. Thus, the electrostriction observed herein is not due to crystal phase conversion. The Maxwell strain was also negligible under all conditions, because of the terpolymer’s high mechanical modulus (10 to 100 MPa). The mechanical properties exhibit an anomalous change in behavior near the Curie transition, whose origin is unclear.
Enhanced room-temperature piezoconductance of metal–semiconductor hybrid structures
Authors: Rowe A. C. H. , Hines D. R. , Solin S. A.
Published: August 2003

Metal–semiconductor hybrids (MSHs) are found to exhibit enhanced room-temperature piezoconductance in the presence of uniaxial tensile strain. The magnitude of the enhanced piezoconductance is more than five times greater than that of the homogeneous semiconductor alone and is strongly dependent on both the location and properties of the metal–semiconductor interface. MSHs may be useful in determining the electrical properties of low-resistance metal contacts on semiconductors.