bluuebunny/crossref_metadata_2025 huggingface dataset:
Fractal nature of the planetary boundary layer depth in the trade‐wind cumulus regime
Authors: Boers R. , Melfi S. H. , Palm S. P.
Published: February 2004

The fractal nature of the planetary boundary layer depth in the trade wind cumulus regime was explored. The boundary layer depths were derived from discontinuities in airborne lidar backscatter profiles obtained during the Frontal Air‐Sea Interaction EXperiment (FASINEX) in February 1986. Scale‐invariant behaviour was observed over horizontal distances ranging from 50 m to 50 km. The degree of intermittency and non‐stationarity of the signals were calculated using standard multifractal statistical methods.
SO2∶HCl ratios in the plumes from Mt. Etna and Vulcano determined by Fourier Transform Spectroscopy
Authors: Francis Peter , Maciejewski Adam , Oppenheimer Clive , Chaffin Charles , Caltabiano Tommaso
Published: February 2004

Volcanic gases have important climatic and environmental effects, and provide insights into magmatic processes. Direct sampling of volcanic gases is inherently difficult and often hazardous. Here, we report the results of long path measurements of SO2 and HCl from Mt. Etna and Vulcano (Italy) obtained by active mode Fourier Transform InfraRed (FTIR) spectroscopy. Spectra recorded in September 1994 over path lengths of up to 2 km indicate SO2∶HCl ratios of 3–5∶1 for Etna, and 0.7–1.4∶1 for Vulcano, consistent with their different styles of activity. Combined with contemporaneous Correlation Spectrometer (COSPEC) SO2 flux measurements, these ratios indicate an HCl flux for Etna of about 1700 t/d (about 16% of the present global anthropogenic flux) and for Vulcano of about 13 t/d. We also report the first remote spectroscopic detection of volcanic SiF4.
An approximation to improve accuracy in the derivation of surface reflectances from multi‐look satellite radiometers
Authors: Flowerdew Roland J. , Haigh Joanna D.
Published: February 2004

Many applications of Earth‐observing satellite radiometer measurements in the visible and near infra‐red require surface reflectances, necessitating the use of atmospheric correction algorithms. Performing these corrections usually requires the assumption of Lambertian surfaces, a limitation that becomes serious with the new generation of multi‐look satellite radiometers. In this letter, we present and test theoretically a multiple‐channel approximation that does not assume a Lambertian surface, but instead assumes that the wavelength dependence of the shape of the surface bidirectional reflectance factor distribution function is negligible in comparison with the wavelength dependence of the surface and atmospheric scattering properties. The approximation is characterised by a mean ratio, k, that may be retrieved by a suitable atmospheric correction process along with surface reflectances and the important atmospheric parameters, without assuming a canopy or soil model or using “dark” pixels to obtain the atmospheric optical thickness. We test the approximation with a canopy model that includes the hot‐spot effect, and, when using ATSR‐2 configurations, it yields at best an order‐of‐magnitude reduction in the error of the surface reflectance (at the surface) over the Lambertian assumption, and in more stringent conditions, a factor of three improvement. We indicate how the approximation might be implemented in an atmospheric correction algorithm.
Abundances of ammonia and carbon disulfide in the Jovian stratosphere following the impact of comet Shoemaker‐Levy 9
Authors: Atreya S. K. , Edgington S. G. , Trafton L. M. , Caldwell J. J. , Noll K. S. , Weaver H. A.
Published: February 2004

Radiative transfer‐scattering models were used to fit the Hubble Space Telescope UV spectroscopic observations of Jupiter taken 2.5 hours after the impact of fragment G of comet P/Shoemaker‐Levy 9. They yield abundances for NH3 and CS2 of (0.25–1.3) × 1016cm−2 and (0.43–1.1) × 1015cm−2 respectively, at unit optical depth level in the part of the atmosphere above the G‐impact site. This range in the abundances is caused by the uncertainty in the composition of the aerosol material and the atmospheric pressure level where the aerosol unit optical depth in the UV occurs. The upper limits of H2S, PH3, C2H2, and C6H6 at the unit optical depth level are 1.2 × 1016cm−2, 3.3 × 1014cm−2, 1.0 × 1017cm−2, and 3.3 × 1014cm−2, respectively. The abundance of NH3 and the upper end of the CS2 abundance range are now found to be each approximately a factor of two lower than was reported previously [Noll et al., 1995], and H2S is not detected.
On a remarkable similarity between the propagation of whistlers and the bow wave of a ship
Authors: Gurnett Donald A.
Published: February 2004

It is well known that lightning‐generated whistlers propagate along the Earth's magnetic field lines within a cone that at low frequencies makes an angle of 19°28′ with respect to the local magnetic field. This angle turns out to be exactly the same as the half‐angle of the bow wave of a ship in deep water. Both problems are complicated by the fact that the wave propagation is dispersive. In this paper we show that these two seemingly unrelated problems can be understood using the same basic approach, which is to analyze the direction of the group velocity as a function of the wave normal angle. This approach may have applications to other problems of geophysical interest, such as the bow wave generated by the interaction of a large object with a moving plasma.
Possible explanation of the secondary flash and strong flare on IR lightcurves upon impact of Shoemaker‐Levy 9
Authors: Yabe Takashi , Aoki Takayuki , Tajima Masakazu , Xiao Feng , Sasaki Sho , Abe Yutaka , Watanabe Jun‐ichi
Published: February 2004

Observations of IR lightcurves of Comet Shoemaker‐Levy 9 impacts detected strong thermal emissions at most impact events. During several huge impacts such as C, K, and R events, two precursor flashes were detected before the strong emission (e.g., Graham et al., 1995; Watanabe et al., 1995). Comparison of IR lightcurves with Galileo and HST data suggests that the first flash should correspond to the entry of a comet. At K impact, the secondary flash started rising after 60sec from the first flash and the strong emission started after 350sec from the first flash. To understand the flashes and the strong thermal emission, we have performed direct numerical simulations of break‐up and subsequent explosion processes with highly accurate hydrocode. When the energy deposition length is quite long and is 300 km, two groups of materials are ejected and the two peaks corresponding to the secondary flash and strong thermal flare are obtained in the time evolution of density and temperature whose temporal profile agrees quite well with observations.
Numerical simulations of the Shoemaker‐Levy 9 impact plumes and clouds: A progress report
Authors: Shoemaker Eugene M. , Hassig Paul J. , Roddy David J.
Published: February 2004

Preliminary 2D/3D numerical simulations were carried out for the penetration of 1‐km bodies in the Jovian atmosphere and the subsequent rise and collapse of the erupted plumes. A body that crushed at a stagnation point pressure of 5 kbar produced a plume that rose to 800 km. Evolution of the shape of the calculated plume corresponds rather well to the plumes observed by HST. A crescent‐shaped lobe centered on the “backfire” azimuth was produced by lateral flow during plume collapse. The plumes observed on Jupiter rose about 4 times higher, and their rise and fall times were about twice those in this calculation. Plume height is a sensitive function of the distribution of energy along the entry path; a very low‐strength body will disintegrate higher along the penetration path and will produce a higher plume.
Indication of change in global and regional trends of atmospheric Mercury concentrations
Authors: Slemr F. , Junkermann W. , Schmidt R. W. H. , Sladkovic R.
Published: February 2004

In 1992 we reported an indication of global increase of atmospheric mercury concentrations obtained from the measurements of total gaseous mercury (TGM) over the Atlantic Ocean during five ship cruises between 1977 and 1990 [Slemr and Langer, 1992]. The observed rate of concentration increase and its implications were consistent with previous measurements and the trends derived from analyses of dated peat bog and lake sediment records. To avoid uncertainties from the intermittent character of ship measurements and to obtain more detailed information about this trend, we have continuously monitored TGM concentrations at the summit of the Wank mountain (1780 m a.s.l.) near Garmisch‐Partenkirchen in southern Germany since March 1990. These measurements and the results of another cruise in 1994 indicate a significant change in the trend of global TGM concentration, with a decrease of about 22% in the years between 1990 and 1994. This decrease is most likely the result of reduction in coal consumption and control measures taken in the OECD countries.
A strong dawn/dusk asymmetry in Pc5 pulsation occurrence observed by the DE‐1 satellite
Authors: Nosé M. , Iyemori T. , Sugiura M. , Slavin J. A.
Published: February 2004

Using the magnetic field data obtained by the DE‐1 polar orbiting satellite, statistical characteristics of transverse Pc5 pulsations in the inner magnetosphere are examined. The occurrence distribution is found to have a strong dawn/dusk asymmetry. The occurrence is most frequent in the region around 72° invariant latitude (ILAT) between 0800 and 1000MLT. The distribution shifts to lower ILAT both in the early morning and in the afternoon sectors. Our results are generally consistent with the occurrence distributions obtained by previous studies except for appreciable differences in the local time of the peak occurrence. The dependence of the transverse Pc5 occurrence on solar wind velocity is also investigated. Results show that the higher the solar wind velocity, the more frequent the Pc5 occurrence. This fact suggests that the energy source of these Pc5's is in the Kelvin‐Helmholtz instability (KHI) on the magnetopause. To explain the strong dawn/dusk asymmetry, the dependence of the Pc5 occurrence on the angle between the solar wind velocity and the IMF in the ecliptic plane is investigated. It is found that this angle controls the magnetic local time of Pc5 appearance when the solar wind velocity is small. This dependence may be explained by the low threshold of KHI due to an influence of a quasi‐parallel bow shock.
Analysis of the substorm trigger phase using multiple ground‐based instrumentation
Authors: Kauristie K. , Pulkkinen T. I. , Pellinen R. J. , Janhunen P. , Huuskonen A. , Viljanen A. , Opgenoorth H. J. , Heikkila W. J. , Baker D. N.
Published: February 2004

During the substorm trigger phase, i.e. during the last few minutes before the onset, the slowly and linearly developing growth phase features strengthen exponentially. Often the enhanced auroral activity fades abruptly during a couple of minutes just prior to the breakup. On 5 October 1986, such fading was observed with an exceptionally good coverage of ground‐based instruments (all‐sky‐camera, EISCAT radar and magnetometer cross, riometers and pulsation magnetometer) in northern Finland and Scandinavia. The global activity is characterized with IMF Bz, the Polar Cap index, and AU and AL indices. We present and analyze the local observations in detail and discuss the possible causes for the fading.
A double seismic zone in New Britain and the morphology of the Solomon Plate at intermediate depths
Authors: McGuire Jeffrey J. , Wiens Douglas A.
Published: February 2004

We have studied the spatial configuration of earthquakes in the New Britain subduction zone by inverting regional and teleseismic P, PKP, and pP arrival times using a hypocentroidal decomposition method. A previously unreported intermediate depth double seismic zone exists along a small, 70 km long segment of the slab. In this region, centroid moment tensor mechanisms suggest that the upper zone of seismicity shows horizontal, along‐strike compression and the lower zone shows downdip tension. This contrasts with most previously studied double seismic zones, which generally show downdip compression in the upper zone. We propose that along‐strike, rather than downdip compressional stresses exist in the upper seismicity plane due to membrane and lateral bending stresses resulting from the shape of the New Britain trench, which undergoes a 40° change in strike between 148° and 152° longitude. The double seismic zone is bounded by a tear in the slab which accommodates the majority of the change in strike that the slab must undergo. These results suggest that double seismic zones can show very rapid lateral variations controlled by the local tectonics, and that along‐strike membrane and bending stresses resulting from slab morphology can be significant in producing double seismic zones.
On the origin of the long period sawtooth shape of the Late Pleistocene paleoclimate Records: The first derivative of the Earth's orbital eccentricity
Authors: Rial J. A.
Published: February 2004

The first time derivative of the earth's orbital eccentricity function is sawtooth shaped, and closely mimics the ∼100,000 yr cycles of the oxygen isotope δ18O time series (the proxy for global ice volume change) of the last ∼800,000 yr. Conversely, numerical integration of the data's variance closely reproduces the orbital eccentricity function for the same interval. Such long‐lived, strong correlation does not appear to be coincidental, is consistent with the physics of ice sheets and with the time history of their advance and retreat.
The time sequence of SL9/Impacts H and L from infrared observations
Authors: Drossart P. , Encrenaz T. , Lecacheux J. , Colas F. , Lagage P. O.
Published: February 2004

The light curve of the impact sites on Jupiter of the fragments of Shoemaker‐Levy 9 exhibits some characteristic features, which have been retrieved for several impacts, and must therefore correspond to different steps in the release and dissipation of the kinetic energy deposited during the impacts. Accurate geometric calculations are presented, which allow us to estimate the possible altitude of observable events in the light curve. A radiative transfer calculation made at 3.5 µm shows that a hot spot in the atmosphere can indeed be observed at emission angles as large as 95°, at pressure levels of 10–100µbars. Observations taken simultaneously on impact H and L between Pic‐du‐Midi observatory (imaging at 1–2 µm), ESO/NTT (3.5 µm imaging spectroscopy for H only) and NOT (imaging at 10 µm), are analyzed to search for wavelength variations in the light curve. It is found that the time of occurence of the precursor observed just before the main event in the L light curve is shifted by ≈1 minute between 1.2 and 10 µm observations. The observations of the later stage (on impact H) show a similar shoulder in the decreasing part of the light curve. A spatial extension of the emissions could possibly explain the observations.
Reconsideration of the cause of dry air in the southern middle latitude stratosphere
Authors: Mote Philip W.
Published: February 2004

A previous comparison of data from the Halogen Occultation Experiment (HALOE) and of output from the NCAR Community Climate Model (CCM2) in austral spring overstated the effect of dehydration in the southern polar vortex on lower stratospheric water vapor in part due to shortcomings in the HALOE retrieval algorithm and in part to remnants of the initially dry stratosphere in the CCM2 run. A new HALOE retrieval and a longer CCM2 run both show more water vapor than previously, but HALOE still shows less in the southern hemisphere than in the northern in September‐October 1992. A “dried‐air” tracer in the CCM2 suggests that polar dehydration influences middle latitudes only below about 100 hPa, and seasonal variations of HALOE H2O imply no influence north of 50°S.
Water vapour and greenhouse trapping: The role of far infrared absorption
Authors: Sinha Ashok , Harries John E.
Published: February 2004

Few observations have been made of atmospheric absorption across the far infra‐red. Yet water vapour absorption in this spectral region may significantly effect climate. The impact of far infra‐red absorption is assessed by calculating the spectral variation of the total and water vapour greenhouse effects, for the sub‐arctic winter (SAW) and tropical (TRP) standard atmospheres. Although the calculated efficiency of greenhouse trapping peaks outside of the far infra‐red, the low strength there of the Planck function causes relatively small absolute forcings, except in the carbon dioxide and ozone bands. The sensitivity of the normalised greenhouse effect to water vapour concentration is largest in the far infra‐red for the SAW atmosphere, and in the window region for the TRP. The sensitivity differs most between the two atmospheres in the far infra‐red. Maximum water vapour greenhouse trapping arises in the far infra‐red, over the middle/upper troposphere; in the SAW case the contribution from the water vapour continuum is virtually eliminated. Improved spectral observations and simulations at far infra‐red wavelengths thus appear necessary to better understand the contemporary greenhouse effect, and to validate models of climate change.
Cratering experiments on ice: Dependence of crater formation on projectile materials and scaling parameter
Authors: Iijima Yu‐ichi , Kato Manabu , Arakawa Masahiko , Maeno Norikazu , Fujimura Akio , Mizutani Hitoshi
Published: February 2004

We have carried out the cratering experiments on −18°C (255 K) water ice. The projectiles of ice, aluminum alloy, polycarbonate and iron were used to investigate the effects of projectiles' physical properties to crater formation. We found dependencies of the cratering efficiency related to projectile materials on the scaling parameter for strength dominant crater, late‐stage effective energy [Kato et al., 1995]. Shock waves propagating in ice were measured to obtain new Hugoniot data and to consider the effects of the material properties of the projectiles. With analysis of shock wave pulse, the impact duration for ice projectile reduced 20 to 50% from that by one dimensional analyses. On the basis of shock wave measurements, we have modified late‐stage effective energy. With this modified late‐stage effective energy, the experimental results using projectiles with various sizes and material became consistent.
VLF wave localization in the low‐altitude auroral region
Authors: Ergun R. E. , Klementis E. M. , Delory G. T. , McFadden J. P. , Carlson C. W.
Published: February 2004

Localized packets of electrostatic whistler emissions near the lower hybrid frequency have been observed in the low‐attitude auroral region by sounding rockets and the Freja satellite. We present a model for wave localization that requires two ion species, O+ and H+, whereby the minority H+ ions control the localization process. The theoretical basis is that the plasma dispersion is dramatically affected by small changes in the H+ density and that the observed emissions would selectively heat H+. We show observational support for the proposed model.
Using 3D seismic reflection data to find fluid seeps from the Costa Rica Accretionary Prism
Authors: McIntosh Kirk D. , Silver Eli A.
Published: February 2004

Submersible dives planned using 2D and 3D seismic reflection data off Costa Rica successfully found numerous sites of fluid expulsion predominantly along surface fault scarps. These data were used because more typical data, such as deep‐towed side‐scan sonar, bottom photography, and heat flow, were unavailable. Because fluids and fluid pressure distribution profoundly affect the way an accretionary prism deforms, detecting fluid pathways and recovering fluids is a key to its deformation processes. Detailed seismic interpretation of the shallow subsurface identified apparently active faults which are most likely to act as fluid pathways. Due to a quasi inverse relationship between surface dip and reflection amplitude, the amplitude of the seafloor reflection, displayed in map‐view, shows the orientation and extent of surface scarps, many of which appear related to subsurface faults. Because the seafloor amplitude response depends on seafloor morphology as well as near‐surface physical properties, removing the effect of the surface dip, calculated directly from picked surface structure, leaves a residual that may be related to variation in physical properties. Although the search for fluid seeps by submersible dives over the Costa Rica prism was sparse and irregular, the seeps that have been discovered are all on structural highs of the apron/prism boundary.
Diagnosis of the record minimum in Arctic sea ice area during 1990 and associated snow cover extremes
Authors: Serreze Mark C. , Maslanik James A. , Key Jeffrey R. , Kokaly Raymond F. , Robinson David A.
Published: February 2004

The Arctic sea ice cover exhibited its record minimum area during 1990, characterized by extensive ice‐free conditions during August along the Siberian coast. These reductions are consistent with warm, windy conditions in May and continued warmth in June promoting early melt and reductions in ice concentration, followed in August by strong coastal winds forcing a final breakup and retreat of the pack ice. The unusually warm Arctic conditions in 1990 are part of a larger‐scale temperature anomaly pattern, linking the sea ice anomaly to accompanying record minima in Eurasian snow cover.
Unusual PSCs observed by LIDAR in Antarctica
Authors: Stefanutti Leopoldo , Morandi Marco , Del Guasta Massimo , Godin Sophie , David Cristine
Published: February 2004

Polar Stratospheric Cloud (PSC) measurements by ground‐based LIDAR were carried out at Dumont d'Urville, Antarctica, during the years 1989 to 1993. From such measurements it can be seen that there are cases of PSCs that are not consistent with the simplest nitric acid trihydrate (NAT) theories. Several cases of long‐lasting, non‐depolarizing PSCs were detected at temperatures below or close to the NAT freezing threshold, at about 195°K, suggesting the presence of durable supercooled droplets. PSC cases showing depolarizing (frozen) particles well above the NAT expected threshold are also shown. These results seems to be more consistent with recent laboratory and in situ findings, suggesting a close link between sulfate and PSC particles through the HNO3‐H2O‐H2SO4 ternary system. In this framework, non‐depolarizing clouds observed below and close to 195°K would arise from the uptake of HNO3 and water by the background particles before the freezing of the ternary system and the successive growth in nitric‐acid hydrates. Depolarizing “warm” PSCs are also consistent with the laboratory‐observed high melting point of the frozen sulfate core, that remains after NAT evaporation. A problematic PSC case, that cannot be easily explained by this theory is also shown.
Amino acid chronology in the fossil planktonic foraminifera, Pulleniatina obliquiloculata from Pacific Ocean
Authors: Harada Naomi , Handa Nobuhiko
Published: February 2004

This study was undertaken to evaluate the accuracy of amino acid chronology using a first‐order kinetic model and was carried out using a fossil planktonic foraminifera, Pulleniatina obliquiloculata (P. obliquiloculata), in the sediment from the western equatorial Pacific Ocean. The extent of racemization reaction of aspartic acid and glutamic acid and the extent of epimerization reaction of isoleucine were measured and compared with reference ages obtained from 14C dating and the distribution patterns of δ18O. The relationships between the extent of racemization/epimerization reaction of these amino acids and reference ages were linear along with a high correlation coefficient (r = 0.99–0.95), suggesting that amino acid chronology utilizing a first‐order kinetic model could be appropriate for determining geological ages ranging from the present to 106 yBP within the average errors of 4.5, 12.3, and 15.8% for Asp, GIu, and Ile, respectively.
Derivation of temperature anisotropy from shock jump relations: Theory and observations
Authors: Chao J. K. , Zhang X. X. , Song P.
Published: February 2004

The MHD Rankine‐Hugoniot (RH) relations for shock waves in a collisionless plasma with bi‐Maxwellian distribution functions are considered. By introducing the pressure anisotropy in the RH relations, the number of unknowns (a total of 9) becomes one more than the total number of conservation equations. However, it is possible to use the observed quantities on both sides of a shock to study the anisotropy changes across the shock. The relationship of the anisotropy change across the shock is derived as a function of the ratio of magnetic fields m(= B′/B), the shock normal angle θBn and the plasma betas β and β′ (primes are downstream values). Since for low‐β, quasi‐perpendicular (Q‐⟂) laminar shocks, m and θBn can be determined fairly accurate from observations, the reliability of the anisotropy change deduced is mostly dependent on the accuracy of the measurements β and β′. The flow velocity and shock speed which consist of large uncertainties do not enter into our derivations. Other physical parameters such as the various Mach numbers, and the jumps of all other quantities across the shock can be calculated. We have applied the results to six low‐β Q‐⟂, laminar bow shock crossings with temperature anisotropy measured in the magnetosheath. It is found from these test cases that the predicted pressure anisotropy agrees well with the observed one in the magnetosheath.
The feasibility of normal mode constraints on higher degree structures
Authors: Ritzwoller Michael H. , Resovsky Joseph S.
Published: February 2004

Case studies of four multiplets (υs‐sensitive 1S7 and 1S8 and υp‐sensitive 5S5 and 5S6) are considered to test the accuracy of normal mode constraints on aspherical structure at degrees above 2 (4,6,8). Analyses of misfit, along‐branch consistency, and consistency with existing mantle models argue for the feasibility of constraints on higher structural degrees. Preliminary interpretation indicates a relative insensitivity of data misfits to dlnυp/dlnυs at degrees 6 and 8 in the lower mantle (perhaps due to inaccuracies in current δυs models), and provides weak evidence of some decorrelation between δυs and δυp in the middle and lower mantle.
Runaway electrons as a source of red sprites in the mesosphere
Authors: Bell Timothy F. , Pasko Victor P. , Inan Umran S.
Published: February 2004

Large quasi‐electrostatic (QE) fields above thunderclouds [Pasko et al., 1995] produce an upward traveling beam of ∼1 MeV runaway electrons which may contribute to the production of optical emissions above thunderclouds referred to as Red Sprites. Results of a one dimensional computer simulation model suggest that the runaway electrons can produce optical emissions similar in intensity and spectra to those observed in Red Sprites, but only for large QE fields produced by positive CG discharges lowering 250 C or more to ground from an altitude of at least 10 km. Differences in predicted optical spectra from that of other mechanisms suggest that the runaway electron mechanism can be readily tested by high resolution spectral measurements of Red Sprites.
Three‐dimensional magnetospheric equilibrium with isotropic pressure
Authors: Cheng C. Z.
Published: February 2004

In the absence of the toroidal flux, two coupled quasi two‐dimensional elliptic equilibrium equations have been derived to describe self‐consistent three‐dimensional quasi‐static magnetospheric equilibria with isotropic pressure in an optimal (ψ,α,χ) flux coordinate system, where ψ is the magnetic flux function, χ is a generalized poloidal angle, α = ϕ‐δ(ψ,α,χ), ϕ is the toroidal angle, δ(ψ,α,χ) is periodic in ϕ, and the magnetic field is represented as B = ∇ψ × ∇α. A three‐dimensional magnetospheric equilibrium code, the MAG‐3D code, has been developed by employing an iterative metric method. The MAG‐3D code is the first self‐consistent three‐dimensional magnetospheric equilibrium code. The main difference between the three‐dimensional and the two‐dimensional axisymmetric solutions is that the field‐aligned current and the toroidal magnetic field are finite for the three‐dimensional case, but vanish for the two‐dimensional axisymmetric case. The toroidal magnetic field gives rise to a geodesic magnetic field curvature and thus a magnetic drift parallel to the pressure gradient direction, which gives rise to the field‐aligned current.
Membrane strain rates in the subducting plate beneath South America
Authors: Creager Kenneth C. , Chiao Ling‐Yun , Winchester John P. , Engdahl E. Robert
Published: February 2004

Geometric constraints associated with the concave oceanward bend in the trench and Andes Mountains, known as the Bolivian Orocline, force along‐strike compression in the subducting slab. Our calculations of membrane strain rates, in an assumed continuous slab, demonstrate that this can be accommodated by either 10% along‐strike compressive strain, or by geometric buckling. In nearly all other subduction zones exhibiting a similar bend in the trench, the compression is accommodated by slab‐arching landward of the trench bend. The slab geometry is markedly different in South America. Two regions of nearly flat‐lying slab north and south of the Bolivian Orocline result in the 150‐km slab depth contour being much straighter than the trench profile. Our calculations show that this geometry helps to relieve along‐strike compression. Along the deeper part of the subduction zone, the along‐strike compression seems to be accommodated by pronounced along‐strike buckling. We infer the approximate geometry of this buckle, in an otherwise aseismic region, from the great Mw 8.3 Bolivian deep earthquake on June 9, 1994, and its aftershocks. The deep slab appears to have a nearly east‐west strike in the vicinity of this earthquake, whereas the strike is more nearly north‐south to the north and south. The focal mechanisms of deep earthquakes exhibit consistent down‐dip compression and along‐strike null axes, tracking the contortions in slab geometry. However, our calculations indicate that the particle paths themselves are not down dip everywhere. Instead, where the deep‐slab strike is oriented nearly east‐west, the particle flow is more nearly along strike than down dip. In general this is not a major problem, but if the slab is anchored in the lower mantle, and moving very slowly, this produces large deformation rates near the base of the upper mantle where the slab is assumed to move at near plate rates. This increased, complex deformation in the deep slab may contribute to the conditions for very large earthquakes in two ways. First, this may dramatically increase deformation rates, and secondly, the increased deformation may lead to anomalous advective thickening of a proposed overdriven olivine wedge. This provides a mechanism for locally thickening the olivine wedge and allowing transformational faulting to occur over a much larger volume than allowed for normal subduction.
Comment on “Nonlinear studies of coronal heating by the resonant absorption of Alfvén waves” by Ofman et al.
Authors: Hollweg Joseph V.
Published: February 2004

Ofman et al. [1994] claim to have simulated the nonlinear development of the resonant absorption of Alfvén waves. We argue that linear resonant absorption does not occur in the configuration they studied and we suggest that phase mixing (refraction) is the dominant linear effect in their simulations. We point out, however, that their demonstrtion of the Kelvin‐Helmholz instability of refracting Alfvén waves may have important implications for solar coronal heating.
Mapping thermospheric winds in the auroral zone
Authors: Conde M. , Smith R. W.
Published: February 2004

A new all‐sky imaging Fabry‐Perot (ASIFP) spectrometer has been developed for ground‐based mapping of upper atmospheric wind and temperature fields in the auroral zone. Although several other ASIFP spectrometers exist for atmospheric studies [Rees et al., 1984; Sekar et al., 1993; Biondi et al., 1995] these instruments have all operated with etalons of fixed optical gap, a method potentially subject to errors in the presence of auroral intensity gradients. In this instrument the etalon plate spacing is scanned periodically over one order of interference and each photon detected is assigned to a wavelength interval which is determined from both its arrival location on the detector and the etalon plate spacing prevailing at the detection time. Spectra accumulated this way are not distorted by spatial intensity gradients. Preliminary λ630 nm observations were made during the winter of 1994/95 from Poker Flat Research Range, Alaska. To illustrate some of the features we have observed in this study we present line‐of‐sight wind estimates derived for the night of December 7, 1994. The background wind matches averages presented previously by Sica et al. [1986] and is consistent with winds driven principally by momentum deposition from ionospheric plasma convection through ion‐drag. Smaller scale curvature and divergence features are also discernable and are discussed.
Limits on deceleration and asymmetry of solar wind speed
Authors: Gazis P. R.
Published: February 2004

It has been suggested by Richardson et al. [1995] that the solar wind shows signs of deceleration due to mass‐loading by interstellar pickup ions. They based their conclusion on observations from the Voyager 2 and IMP 8 spacecraft. Observations from the Pioneer Venus Orbiter (PVO), IMP 8, Voyager 2, and Pioneer 10 spacecraft are compared to look for this deceleration. While these comparisons suggest the possibility of deceleration at Voyager 2, the magnitude of this deceleration appears to be significantly less than the deceleration reported by Richardson et al. [1995]. Some of this difference could be explained by a north‐south asymmetry in solar wind speed during solar cycle 21. The implications of these results are discussed, along with possibilities for further investigations.
Line strength measurements of trans‐HONO near 1255 cm−1 by tunable diode laser spectrometry
Authors: Becker K. H. , Kleffmann J. , Kurtenbach R. , Wiesen P. , Febo A. , Gherardi M. , Sparapani R.
Published: February 2004

Infrared absorption spectra and line strengths in the ν3 H‐O‐N bending mode of trans‐HONO near 1255 cm−1 have been measured with a tunable diode laser spectrometer. High purity HONO (>98%) was produced by a reaction of gaseous HCl with solid NaNO2. Absolute HONO concentrations were determined either by ion chromatography or by a NOx chemiluminescence analyser.

Line strengths of the strongest lines observed were found to be in the range of (7–12) × 10−21 cm under near Doppler‐limited conditions and differ significantly from literature data. In addition, the dependence of the line width on the total air pressure was investigated in the range 0.–25 Torr, and also the possibility of measuring HONO atmospheric mixing ratios with a tunable diode laser was assessed.
Oxygen outflow in the Martian magnetotail
Authors: Kallio E. , Koskinen H. , Barabash S. , Nairn C. M. C. , Schwingenschuh K.
Published: February 2004

Characteristic features of the oxygen ion (O+) outflow in the nightside of Mars have been studied. The data set used includes all high telemetry O+ energy spectra observations made by the ASPERA particle instrument onboard the Phobos 2 spacecraft. Oxygen outflow within the optical shadow of Mars was detected on all orbits, but no clear structure was found when the ions were organised according to the cross‐flow component of the interplanetary magnetic field (IMF). Oxygen ions in the tail outside the shadow were, in contrast, detected only on a few orbits and were related to fluctuating magnetosheath magnetic and electric fields with non‐Parker spiral IMF orientation. The analysis indicates that O+ ions within the optical shadow from a “steady”, ever persisting outflow feature. The high energy O+ ions in the tail outside the optical shadow are in contrast possibly a manifestation of the dynamic nature of the Martian tail, although effects associated to an asymmetric tail can not be fully excluded.
Effects of frictional ion heating and soft‐electron precipitation on high‐latitude F‐region upflows
Authors: Liu Chao , Horwitz J. L. , Richards P. G.
Published: February 2004

Observed ionospheric F‐region upflows associated with convection‐driven frictional ion heating and soft electron precipitation at high latitudes are modeled with a dynamic ionospheric fluid code. Precipitating soft (≤ 1 keV) auroral electrons are effective in rapidly enhancing the F‐region ionization and electron temperature, which leads to a strong upward plasma expansion. It is shown, for example, that an electron flux of 1 erg cm−2s−1 with a characteristic energy of 150 eV can produce a 109cm−2s−1 O+ outflow at altitudes of 700–800 km. The more widely‐recognized convection ion heating is indicated to be significant but somewhat smaller effects on the upflows. We have performed comparisons with published HILAT and DE‐2 observations. Using a latitudinal distribution of ionospheric flux tubes with “inputs” of the observed average precipitating electron energies, energy fluxes and convection drift velocities, we find satisfactory agreement with latitudinal profiles of ion upflow velocities, densities, fluxes, and ion and electron temperatures. Therefore, we suggest that the combined effects of soft electron precipitation and frictional ion heating may be identified as the principal drivers of these upflows.
High resolution horizontal wave number spectra of mesospheric wave perturbations observed during the 21 October triangular flight of ALOHA‐93
Authors: Gardner Chester S. , Qian Jun , Coble Mark R. , Papen George C. , Swenson Gary R.
Published: February 2004

On 21 October 1993 during ALOHA‐93 the NCAR Electra flew a triangular path to the northwest of Maui. To improve the horizontal resolution of the Na lidar and OH imager, the aircraft cruising speed was reduced to an average of ∼135 m/s. The lidar and imager observations are used to study the high horizontal wave number (k) region of mesospheric wave perturbation spectra. The relative atmospheric density spectrum inferred from the Na lidar data is well approximated by the k−1.3 power‐law over the complete wave number range from k = 2π/(400 km) to 2π/(2 km). The observed spectrum shows no evidence of the index change near k = m* predicted by the separable gravity wave model of Gardner et al. [1993], where m* is the characteristic vertical wave number. The OH intensity spectra computed across and along the flight track, as well as the Na abundance spectrum, are inconsistent with both the separable gravity wave model [Gardner et al., 1993] and scale‐independent diffusive filtering theory [Gardner, 1994]. Existing models of Na abundance and OH intensity fluctuations are based on linear perturbation theory and may not be valid because they do not include nonlinear effects.
Anomalous nitrate concentrations in polar ice cores—Do they result from solar particle injections into the polar atmosphere?
Authors: Zeller Edward J. , Dreschhoff Gisela A. M.
Published: February 2004

Nitrate concentrations and electrical conductivity in an ice core from central Greenland have been measured simultaneously on 1.5 cm sections along the entire length of the 122 m core. This method of micro‐resolution (time resolution is 1 week‐1 month) provides for statistically significant determinations of rapid variations in nitrate fallout from the polar stratosphere superimposed on a background from all other sources. The seasonal NO3− background variations and the volcanic signal contained in the electrical conductivity data provide accurate dating of the ice core. This type of micro‐resolution nitrate measurement appears to be useful as a tool to investigate the possible sources of polar nitrate anomalies such as solar proton events.
Potential gross primary productivity of terrestrial vegetation from 1982–1990
Authors: Myneni Ranga B. , Los Sietse O. , Asrar Ghassem
Published: February 2004

Monthly mean global green leaf area and photosynthetically active radiation estimated from observations made from several geostationary and polar orbiting satellites were used to estimate potential, (i.e., unstressed) gross primary productivity of terrestrial vegetation from 1982 to 1990. Mean annual gross primary productivity was estimated to be about 179.9±1.6 Pg C (1 Pg = 1015 g). Grasses and xerophytic vegetation on all continents exhibited high interannual variability. The evolution of monthly gross primary productivity anomalies was similar to atmospheric CO2 growth rate during 1982–1990. This was observed both globally and for the 30°–90°N latitudional band, but not at other latitudes. From this, we may conclude that total ecosystem respiration and GPP respond similarly but disproportionately to climate, such that the difference between the two resembles the opposite of the observed atmospheric CO2 growth rate.
Control of tropical instability waves in the Pacific
Authors: Allen M. R. , Lawrence S. P. , Murray M. J. , Mutlow C. T. , Stockdale T. N. , Llewellyn‐Jones D. T. , Anderson D. L. T.
Published: February 2004

Westward‐propagating waves with periods of 20–30 days and wavelengths of ∼ 1,100km are a prominent feature of sea‐surface temperatures (SSTs) in the equatorial Pacific and Atlantic Oceans. They have been attributed to instabilities due to current shear. We compare SST observations from the spaceborne Along Track Scanning Radiometer (ATSR) and TOGA‐TAO moored buoys with SSTs from a model of the tropical Pacific forced with observed daily windstress data. The phases of the strongest “Tropical Instability Waves” (TIWs) in the model are in closer correspondence with those observed than we would expect if these waves simply developed from infinitesimal disturbances (in which case their phases would be arbitrary). If we filter out the intraseasonal component of the windstress, all phase‐correspondence is lost. We conclude that the phases of these waves are not arbitrary, but partially determined by the intraseasonal winds. The subsurface evolution of the model suggests a possible control mechanism is through interaction with remotely‐forced subsurface Kelvin and Rossby waves. This is supported by an experiment which shows how zonal wind bursts in the west Pacific can modify the TIW field, but other mechanisms, such as local feedbacks, are also possible.
Stochastic variability at the air‐sea interface on decadal timescales
Authors: Power Scott B. , Tseitkin F. , Dix M. , Kleeman R. , Colman R. , Holland D.
Published: February 2004

Multiple integrations of an atmospheric general circulation model (AGCM) exhibit differences in the decadal mean of the surface heat flux, Q. The differences (typically 2–3 Wm−2) can be as large as 10 Wm−2 in places, which are shown to produce sea‐surface temperature (SST) changes up to approximately 0.5°C in a hybrid coupled atmosphere/ocean/sea‐ice model or HCM. This underscores a significant mechanism by which long‐lived SST anomalies can arise, independent of any internal ocean variability, for which there is very little predictive capability. Consequently, AGCM integrations using prescribed SSTs provide upper bounds on the predictability of atmospheric variability. The HCM is used to estimate the magnitude of the drift in the climatology of coupled models if flux adjustments are based on AGCM integrations of decadal duration.

The random interannual fluctuations in Q averaged over the ocean are most closely associated with fluctuations in surface latent heating and long wave radiation to space. The same fluctuations are not associated with land surface variability.

The impact of variability in other fluxes on SST in the HCM was also analysed. Changes due to wind‐stress were approximately half those due to heating, while changes due to freshwater forcing were relatively unimportant except at polar latitudes.
Tides in emission rate and temperature from the O2 nightglow over Bear Lake Observatory
Authors: Wiens R. H. , Zhang S. P. , Peterson R. N. , Shepherd G. G.
Published: February 2004

The mesopause oxygen rotational temperature imager, MORTI, was operated at Bear Lake Observatory (41.9°N, 111.4°W) during the period November, 1991 to May, 1993. Fluctuations of long period in both emission rate and temperature of the O2 Atmospheric (0–1) nightglow layer are evident that seem related to the diurnal and semidiurnal tides. The data show a dominant semidiurnal tidal mode in January, but a dominant diurnal component at the spring equinox. Other long‐period fluctuations also appear for which a linear tidal explanation does not seem applicable.
Lateral variations in D″ below the Caribbean
Authors: Kendall J. ‐M. , Nangini C.
Published: February 2004

Broad‐band data from the Canadian National Seismograph Network (CNSN) shows evidence for a laterally‐varying S‐wave discontinuity in the lowermost mantle beneath the Caribbean. The magnitude of the velocity contrast across this D″ discontinuity varies inversely with its distance from the core‐mantle boundary (CMB). Clear evidence for a 2.75% increase in shear velocity 250 km above the CMB is observed in a region 10°N and 60–85°W; a thicker D″ layer (290 km) with a less pronounced topside discontinuity (2.45%) is interpreted in the region 80°–90°W and 20°N; and there is no evidence for a D″ discontinuity in the region 65°–80°W and 25°N. Considerable 3D structure within the D″ region is suggested by travel‐time scatter in ScS arrivals and S arrivals which turn within D″ . Transverse‐component SKS signals due to upper‐mantle anisotropy can be confused with D″ signals; however, we show that false interpretations of D″ structure can be mitigated through the careful analysis of travel‐time moveout and low‐pass filtering.
Applicability of the Reynolds Equation for modeling fluid flow between rough surfaces
Authors: Brown Stephen R. , Stockman Harlan W. , Reeves Sally J.
Published: February 2004

Predictions of the Reynolds equation for flow between rough‐walled surfaces have been compared to a more exact calculation of Navier‐Stokes flow based on a lattice‐gas automaton method. Two‐dimensional channels were constructed with an idealized sinusoidal roughness on each wall. Flow in the channels was studied by both methods for various amplitude to wavelength ratios of the roughness, surface separations, relative alignment or phase of the sinusoids, and Reynolds numbers. The Reynolds equation overestimates fluid velocity as the surfaces are placed together or the amplitude of the roughness increases relative to its wavelength.
Comparison of ALOHA‐93, ANLC‐93 and ALOHA‐90 observations of the hydroxyl rotational temperature and gravity wave activity
Authors: Lowe R. P. , Turnbull D. N.
Published: February 2004

Hydroxyl airglow measurements during the ALOHA‐93, ALOHA‐90 and ANLC‐93 campaigns are used to evaluate features of the temperature variation of the mesopause region. Each campaign shows a persistent pattern in the averaged local time variation of temperature with peak‐to‐peak amplitudes of 5 K to 15 K, suggestive of the presence of tidal modulation. The nightly temperature variance, after detrending to remove low frequency variations due to tides, is shown to be significantly less during the Hawaiian campaigns than during the ANLC‐93 and other campaigns at sites where significant orographic excitation of waves might be expected.
Evidence of HF‐driven wave interactions in the ionospheric focused heating experiment
Authors: Rodriguez P. , Siefring C. L. , Haas D. G. , Bernhardt P. A. , Baumback M. M.
Published: February 2004

The Ionospheric Focused Heating rocket experiment created an ionospheric hole above the Arecibo HF radiowave heater by the release of an electron‐attachment chemical. The hole served as a focusing lens for HF waves at 5.1 MHz and produced various plasma wave effects measured by onboard diagnostic instruments. In this report, we present evidence for the occurrence of plasma wave interactions driven by the HF pump at altitudes where the pump frequency is near the electron plasma frequency and one‐half the electron plasma frequency. These interaction regions are observed on the upper side of the ionospheric hole. The data suggest that the frequency and wavenumber matching requirements for coupled waves are met in these regions.