bluuebunny/crossref_metadata_2025 huggingface dataset:
Seasonal variations of the thermal structure of the mesopause region at Urbana, IL (40° N, 88° W) and Ft. Collins, CO (41° N, 105° W)
Authors: Senft Daniel C. , Papen George C. , Gardner Chester S. , Yu J. R. , Krueger David A. , She C. Y.
Published: February 2004

Beginning in 1991, Na lidars were used to make routine measurements of the temperature structure between 80 and 105 km altitude during 65 nights at Urbana, IL and 116 nights at Ft. Collins, CO. More than 13,000 temperature profiles representing more than 1100 h of observations were used to characterize the seasonal temperature variations in the upper mesosphere and lower thermosphere. The seasonal behavior is quite similar at the two sites. The mesopause altitude is near 100 km in winter during Nov–Feb and near 86 km in summer during May–Jul. The mesopause temperature varies from 190 K in mid‐winter to 175 K in mid‐summer. The transitions between winter and summer mesopause structures occur rapidly in Mar–Apr and Aug–Sep. The temperature differences between the two sites are less than ±15 K and appear to be the result of atmospheric planetary, tidal, and gravity wave perturbations.
Energetic particle‐induced enhancements of stratospheric nitric acid
Authors: Aikin Arthur C.
Published: February 2004

Inclusion of complete ion chemistry in the calculation of minor species production during energetic particle deposition events leads to significant enhancement in the calculated nitric acid concentration during precipitation. An ionization rate of 1.2×103 cm−3s−1 imposed for 1 day increases HNO3 from 3×105 to 6×107 cm−3 at 50 km. With an ionization rate of 600 cm−3 s−1, the maximum HNO3 is 3×107 cm−3. Calculations which neglect negative ions predict that nitric acid will fall during precipitation events. The decay time for converting HNO3 into odd nitrogen and hydrogen is more than 1 day for equinoctial periods at 70° latitude. Examination of nitric acid data should yield important information on the magnitude and frequency of charged particle events.
Observed correlations between winter‐mean tropospheric and stratospheric circulation anomalies
Authors: Baldwin Mark P. , Cheng Xinhua , Dunkerton Timothy J.
Published: February 2004

It is shown that interannual variability of the northern winter stratospheric flow in 1964‐1993 was closely linked to large‐scale circulation anomalies in the middle troposphere. Of the known tropospheric teleconnection patterns, the one having the strongest relation to the DJF zonal‐mean stratospheric flow was the North Atlantic Oscillation (NAO). Singular value decomposition between the 500 and 50‐hPa geopotential heights produced a 500‐hPa structure containing elements of the NAO pattern, but including an anomaly in eastern Siberia. During this time period, the correlation of NAO‐related modes to the polar lower stratosphere exceeded that of the equatorial quasi‐biennial oscillation.
Ulysses high‐latitude observations of ions accelerated by co‐rotating interaction regions
Authors: Sanderson T. R. , Marsden R. G. , Wenzel K.‐P. , Balogh A. , Forsyth R. J. , Goldstein B. E.
Published: February 2004

We present observations of energetic ions (E∼1 MeV) from the Ulysses spacecraft during its first pass from the ecliptic plane to the southern high‐latitude regions of the heliosphere. At latitudes less than ∼13°S Ulysses was completely immersed in the heliomagnetic streamer belt, and observed a ∼1 MeV proton intensity which showed little evidence of a periodic structure. Between ∼13°S and ∼29°S Ulysses observed one dominant recurrent co‐rotating interaction region, its reverse shocks being mainly responsible for accelerating the ∼1 MeV protons. At ∼29°S the spacecraft left this region and entered the solar wind flow from the polar coronal hole. From ∼29° up to ∼45°S, reverse shocks from this and other interaction regions were still being observed. Accelerated energetic ions, with proton‐to‐alpha ratio signatures consistent with having been accelerated by the reverse shocks of these co‐rotating interacting regions, were still being observed up to latitudes of ∼50°S.
Electric fields derived from electron drift measurements
Authors: Kletzing C. A. , Paschmann G. , Boehm M. H. , Haerendel G. , Sckopke N. , Baumjohann W. , Torbert R. B. , Marklund G. , Lindqvist P.‐A.
Published: February 2004

The first observations of electric fields derived from electron E × B drift measurements aboard the Freja spacecraft are presented. The instrument injects a weak beam of 3 keV electrons and measures the displacement of the returning electrons after one gyroperiod. After removing effects due to beam‐detector geometry and applying an empirical calibration based upon comparison with the computed υ × B electric field induced by the spacecraft motion, good agreement is found when the electron drift measurements are compared with the electric field components measured by the double probe experiment. Examples are presented in which moderately large electric fields are observed near the edges of or adjacent to electron precipitation regions with little or no electric field within.
Investigation of mantle discontinuities from a single deep earthquake
Authors: Estabrook Charles H. , Bock Günter , Kind Rainer
Published: February 2004

Converted phases have been identified in the P‐wave coda of teleseismic seismograms from a single large (Mw 6.8), deep (360 km) earthquake south of Honshu, Japan. Standard array techniques (slantstacking), applied to broadband data from the global broadband network, the German Regional Seismic Network (GRSN) and the Gräfenberg (GRF) array, and synthetic seismogram modeling, reveal that these unusual phases are S to P conversions from under the seismic source. Timing of the converted phase indicates that subduction has had little, if any, effect on the depth of the ‘660 km’ discontinuity. This study shows that upper mantle seismic structure can be constrained from single events using the presently available broadband seismic data and applying array techniques to network data.
Two types of deep seismicity in subducting slabs
Authors: Furukawa Y.
Published: February 2004

The relation between variation of seismic activity with depth and temperature structure in the subducting slabs in five subduction zones (Kuril, NE‐Japan, Izu‐Bonin, Mariana, and Tonga) is investigated. When the temperature of the core of the slab at depths of 300∼350 km is higher than the kinetic cut‐off temperature of the olivine‐spinel phase transformation, there is no seismicity maximum in the slab below that depth range. When the temperature of the core of the slab at 300∼350 km depths is lower than the kinetic cut‐off temperature, there is a depth range of relatively high seismic activity in the slabs. Moreover, estimated temperature in the depth range of relatively high seismicity is near the kinetic cut‐off temperature. It is inferred that the phase transformation of meta‐stable olivine triggers deep earthquakes and causes high seismic activity below the depths of 300∼350 km in relatively low temperature slabs, when the phase transformation is the cause of deep earthquakes. In relatively high temperature slabs, however, it is considered that the phase transformation occurs in nearly equilibrium conditions and meta‐stable olivine does not exist. Other mechanisms in addition to the phase transformation are required to explain observed deep seismicity especially in relatively high temperature slabs.
Results of the WEGENER/GPS‐92 Campaign
Authors: Springer T. A. , Ambrosius B. A. C. , Noomen R. , Herzberger K.
Published: February 2004

During the International GPS Service for Geodynamics (IGS) Epoch'92 campaign a special sub‐campaign was organized to occupy all WEGENER/MEDLAS SLR sites with Trimble SST GPS receivers. This campaign, WEGENER/GPS‐92, was initiated by the Institute for Applied Geodesy in cooperation with Delft University of Technology. The purpose was to observe the whole SLR network, for the first time, with GPS to establish a connection between the SLR and GPS reference frames. The dataset was analyzed at the Section Space Research and Technology, using the GIPSY software developed at the Jet Propulsion Laboratory and the Bernese GPS software version 3.4. In the analysis a weighted combination of the orbits provided by the IGS was used to model the motion of the GPS satellites. The results show a baseline length repeatability, which is a measure for the precision, in the order of 5 mm. More importantly, the GPS coordinate solution compares very well with the most recent SLR coordinate solution computed at Delft University of Technology. The rms differences are 3.8, 5.2 and 12.5 mm in North, East and Up, respectively, using 14 sites. It is therefore obvious that GPS will play an increasingly important role in the monitoring of the deformation and a further densification of this network. Especially since starting January 1, 1994 the IGS has become operational and combined orbits are being produced and made available with only a few week delay.
Evidence for a constant b‐value above magnitude 0 in the southern San Andreas, San Jacinto and San Miguel Fault Zones, and at the Long Valley Caldera, California
Authors: Abercrombie Rachel E. , Brune James N.
Published: February 2004

The cumulative b‐value (the slope of the Gutenberg — Richter relationship between earthquake occurrence rate and magnitude) is commonly found to be constant (∼1). Network catalogues, however, reveal a decrease at small magnitudes (< 3). Some recent studies have suggested that this decrease in b‐value is not just an artifact of catalogue incompleteness, but that small earthquakes are really not as numerous as a constant b‐value extrapolated from larger events would predict. Brune & Allen [1967] recorded microseismicity in both the San Andreas and San Jacinto fault zones in southern California, calculating that they were complete to magnitude 0. Reyes et al. [1975] performed a similar study of the San Miguel fault zone, Baja California, estimating completeness above magnitude −0.5. We compare the results of these studies with the occurrence rates of earthquakes on the same faults recorded by the Southern California and RESNOR seismic networks. The observed microseismicity rates are in good agreement with the rates predicted by extrapolating the b‐values of the larger earthquakes. In a new microseismicity survey at the Long Valley caldera, Mammoth Lakes, California, microearthquakes were recorded and counted within 24 km of stations CLK and CHS. This study is estimated to be complete above 0.6 M at CLK and 0.9 M at CHS, and the microseismicity rates are found to be in good agreement with the rate predicted by extrapolating the b‐value from the local network recorded seismicity. The constant b‐value in the magnitude range 0 to 5 observed in all four areas considered is consistent with a scale invariant earthquake rupture process.
Split S waveforms observed in northern Taiwan: Implications for crustal anisotropy
Authors: Kuo Ban‐Yuan , Chen Chen‐Chun , Shin Tzay‐Chyn
Published: February 2004

Short‐period stations ENT and NSK in northern Taiwan have frequently recorded split S waveforms from earthquakes in the underlying subduction zone. To determine splitting parameters together with objective, straightforward error estimations, we employ a waveform cross‐correlation method. The error for each measurement is estimated by translating the lower limit of the 95% confidence interval of the cross‐correlation coefficient into azimuth and time. Only measurements with 95% confidence regions excluding zero delay time are accepted as splitting data. We also use an energy minimization method to constrain the solutions with minor splitting. We interpret the splitting as caused by stress‐aligned cracks in the upper crust. The average delay times are 0.03–0.04 s for ENT and 0.07 s for NSK. The fast directions at the two stations are predominantly N‐S, which roughly match the stress trajectories predicted by a model simulating the arc‐continent collision in the Taiwan area. However, at ENT there is a subset of data with fast directions oriented in roughly E‐W. The nearly 90° rotation of the inferred crack orientation occurs in a horizontal distance of 10–15 km in the upper crust. Both the predicted heterogeneous stress regime and the local structural setting in northeastern Taiwan favor a lateral variation with a similar length scale.
Temperature dependent CH3OCl formation in the reaction between CH3O2 and ClO
Authors: Helleis Frank , Crowley John , Moortgat Geert
Published: February 2004

The reaction CH3O2 + ClO → CH3OCl + O2 (2b) has been investigated at temperatures of 295, 255, 230 and 215 K. The temperature dependent branching ratio for CH3OCl formation (α2b) is given by α2b = (T/300)−1.65 −0.223exp(411/T). The result at 200 K (ca 0.23) can be combined with our previous determination of the overall rate constant for the reaction between CH3O2 + ClO [Helleis et al, 1993] to calculate a channel specific rate constant for reaction 2b of ca (4.1±1) × 10−13 cm³ molecule−1 s−1 at the temperatures prevalent in the lower polar stratosphere at springtime.
Electrostatic solitary waves (ESW) in the magnetotail: BEN wave forms observed by GEOTAIL
Authors: Matsumoto H. , Kojima H. , Miyatake T. , Omura Y. , Okada M. , Nagano I. , Tsutsui M.
Published: February 2004

Wave forms of BEN (Broadband Electrostatic Noise) in the geomagnetic tail were first detected by the Wave Form Capture receiver on the GEOTAIL spacecraft. The results show that most of the BEN in the plasma sheet boundary layer (PSBL) are not continuous broadband noise but are composed of a series of solitary pulses having a special form which we term “Electrostatic Solitary Waves (ESW)”. A nonlinear BGK potential model is proposed as the generation mechanism for the ESW based upon a simple particle simulation which considers the highly nonlinear evolution of the electron beam instability. The wave forms produced by this simulation are very similar to those observed by GEOTAIL and suggest that the nonlinear dynamics of the electron beam play an essential role in the generation of ESW.
Origin of the sound generated by Strombolian explosions
Authors: Vergniolle S. , Brandeis G.
Published: February 2004

During strombolian eruptions, large bubbles break at the surface of the lava column and produce sound. Acoustic pressure recorded during several explosions on the Eastern vent of Stromboli volcano, shows a pattern consistent between explosions. The well‐marked oscillation contains only very low frequencies (around 7 Hz), and is followed by a signal containing now both higher frequencies and lower frequency (less than 4 Hz). The generation of sound is explained by the vibration (7 Hz) and bursting (higher frequencies) of large bubbles, followed by the drainage of magma down in the conduit (less than 4 Hz). The interpretation suggests that: (1) the bubble vibrates before bursting and (2) the vibration prior to bursting yields more energy than the bursting itself. The radius of bubbles responsible for explosions is determined to be close to 1 meter, in agreement with observations of a few conduit sizes.
The morningside low‐latitude boundary layer as determined from electric and magnetic field measurements on Geotail
Authors: Mozer F. S. , Hayakawa H. , Kokubun S. , Nakamura M. , Okada T. , Yamamoto T. , Tsuruda K.
Published: February 2004

On October 17, 1992, the Geotail satellite crossed the dawnside magnetopause approximately 41 times. At the majority of these crossings, the magnetic field and the normal component of the electric field were larger and the plasma density was smaller in a low‐latitude boundary layer than they were in either the nearby magnetosheath or in the magnetosphere. These results are interpreted in terms of the Kelvin‐Helmholtz instability associated with velocity shear at the magnetopause. Consistent with this interpretation, it is shown that the low‐latitude boundary layer was the region where the flow decreased from its magnetosheath to magnetospheric value. Evidence is presented that the magnetopause was locally oriented within less than 20° of its nominal geometry on these crossings and that it moved with an amplitude of 1500–6000 kilometers. The thickness of the low‐latitude boundary layer averaged 800 kilometers and the electric potential across it averaged 4 kilovolts with a spread of a factor of 2. Thus, the viscous interaction and similar processes are not significant contributors to magnetospheric convection.
Magnetopause encounters in the magnetotail at distances of ∼80 Re
Authors: Williams D. J. , Lui A. T. Y. , McEntire R. W. , Angelopoulos V. , Jacquey C. , Christon S. P. , Frank L. A. , Ackerson K. L. , Paterson W. R. , Kokubun S. , Yamamoto T. , Fairfield D. H.
Published: February 2004

The GEOTAIL satellite experienced several encounters with the magnetopause during the interval 0300 to 0600 hours on 27 October 1992 at a gsm location of (X,Y,Z) ∼(−81,−8,7.5) Re. Energetic ions of both solar wind and ionospheric origin (H+, He++, and O+) were observed streaming in the anti‐solar direction on both the lobe and magnetosheath side of the plasma mantle‐magnetosheath boundary, with maximum fluxes occurring at the boundary. Even though the mantle plasma had penetrated well into the lobe through convective drift, the energetic ions were observed only at the magnetopause and provided a clear signature of the boundary region. Using a measured convective drift velocity and observed energetic particle anisotropies, we estimated that the maximum distance from GEOTAIL that the energetic ions could have been accelerated or made contact with the boundary field lines was <8‐39 Re, implying that both solar wind and ionospheric ions are accelerated and contact the magnetopause many tens of earth radii behind the earth. We further found that the unusual magnetopause positions encountered at this time could be explained in part by solar wind aberration effects. However additional processes (e. g., magnetospheric breathing modes, convoluted tail topologies) are required to fully explain these observations.
Impact of comet Shoemaker‐Levy 9 on Jupiter
Authors: Ahrens Thomas J. , Takata Toshiko , O'Keefe John D. , Orton Glenn S.
Published: February 2004

Three‐dimensional numerical simulations of the impact of Comet Shoemaker‐Levy 9 on Jupiter and the resulting vapor plume expansion were conducted using the Smoothed Particle Hydrodynamics (SPH) method. An icy body with a diameter of 2 km can penetrate to an altitude of ‐350 km (0 km = 1 bar) and most of the incident kinetic energy is transferred to the atmosphere between ‐100 km to ‐250 km. This energy is converted to potential energy of the resulting gas plume. The unconfined plume expands vertically and has a peak radiative power approximately equal to the total radiation from Jupiter's disc. The plume rises a few tens of atmospheric scale heights in ∼10² seconds. The rising plume reaches the altitude of ∼3000 km, but no atmospheric gas is accelerated to the escape velocity (∼60 km/s).
Radial diffusion of relativistic electrons in Neptune's magnetosphere
Authors: Selesnick R. S. , Stone E. C.
Published: February 2004

Measurements of ≳1 MeV electrons in Neptune's magnetosphere from L ≈ 4 to 14 are well represented by solutions of a model radial diffusion equation that includes losses due to absorption by the satellite Proteus. The model provides estimates of the radial diffusion coefficient, which is 5 × 10−8(L/5)8 s−1, and the outer boundary energy spectrum, which is an exponential with an e‐folding energy of 0.1 MeV. The diffusion coefficient is consistent with theoretical estimates based on the assumption that diffusion is driven by atmospheric neutral winds.
Radiometric dating of young MORB using the 40Ar‐39Ar incremental heating method
Authors: Duncan R. A. , Hogan L. G.
Published: February 2004

Determination of reliable crystallization ages by K‐Ar methods for young (<1 Ma), fresh basalts from the seafloor has been frustrated by several effects. The small amounts of radiogenic 40Ar developed over these timescales in such low‐K rocks are difficult to resolve from predominantly atmospheric 40Ar. An additional concern is that mantle‐derived 40Ar may not be totally outgassed when magmas quench at seafloor hydrostatic pressures. We have developed a successful strategy for partially separating atmospheric from radiogenic 40Ar in 40Ar‐39Ar incremental heating experiments on mid‐ocean ridge basalts, from the East Pacific Rise near the Clipperton Fracture Zone. Pre‐heating samples to 400°C removes surficial atmospheric 40Ar, while subsequent heating at 600°–1000°C produces favorable proportions of radiogenic 40Ar. Experiments using “zero‐age” glassy and holocrystalline samples show that mantle‐derived 40Ar is retained only in the outermost few cm of rapidly cooled pillow basalts.
GEOTAIL observation of magnetosonic Pc 3 waves in the dayside magnetosphere
Authors: Takahashi Kazue , Kokubun S. , Matsuoka H. , Shiokawa K. , Yumoto K. , Nakamura M. , Kawano H. , Yamamoto T. , Matsuoka A. , Tsuruda K. , Hayakawa H. , Kojima H. , Matsumoto H.
Published: February 2004

Compressional Pc 3 magnetic pulsations and accompanying electric field oscillations were observed by the GEOTAIL satellite in the dayside magnetosphere on October 17–18, 1992. When interpreted in terms of MHD wave modes, the oscillations in the fields are consistent with fast magnetosonic waves propagating Earthward. This result lends strong support to the view that ULF waves generated near the quasi‐parallel portion of the bow shock propagate into the magnetosphere and are observed as compressional Pc 3 pulsations in the dayside magnetosphere. The Earthward component of Poynting flux associated with the pulsations is ∼ 5 × 10−8 W/m².
Pulsations in seafloor spreading rates and transit time dynamics
Authors: Gaffin Stuart R. , O'Neill Brian C.
Published: February 2004

Correlation studies and theoretical models suggest the major plate driving forces, slab pull and ridge push, are closely linked to seafloor age at subduction zones (= transit time and hereafter denoted τ). Based on this fact it has been proposed that seafloor spreading rate fluctuations should be stably damped because a speedup should lead to rejuvenation of τ and hence a weakening of driving forces. If true, such stability could imply the first‐order cycle in Pacific spreading rates during the Mesozoic‐Cenozoic (“the pulse of the Earth”) was externally forced by mantle dynamics. However we show that a crucial assumption in this stability argument is invalid: increasing spreading rates do not necessarily cause transit time to decrease. The transit time dynamics of seafloor are complex; simple plate models suggest that, rather than stabilizing spreading rate, τ‐dependent plate driving forces may be destabilizing leading to self‐sustained and aperiodic oscillations. These results raise questions about the cause of spreading rate cycles. Most importantly, are the plates themselves, through age dynamics, actively contributing to “the pulse of the Earth” ?
Construction and destruction of volcanic knobs at the Cocos‐Nazca Spreading System near 95°W
Authors: Kleinrock Martin C. , Brooks Benjamin A.
Published: February 2004

Characterization of volcanic knobs observed in Sea Beam bathymetry near the 95°W Galapagos propagator system on the Cocos‐Nazca spreading axis provides insight into volcanic and tectonic processes at propagators and mid‐ocean ridges. Despite evidence suggesting a higher magma supply rate at the propagator axis, crust accreted there contains fewer knobs than crust created at the failing or doomed spreading axes. Fissure‐fed flows rather than seamount construction are more important along the propagator. The process of transferring lithosphere from one plate to another as this ridge offset migrates through a region destroys about half of the preexisting knobs. Data tentatively suggest that on‐axis volcanic cones increase in size and contribution to crustal construction but decrease in abundance with decreasing spreading rate, possibly reflecting the magma plumbing systems in differing thermal regimes.
Altitude distributions of upward flowing ion beams and solitary wave structures in the Viking data
Authors: Mälkki Anssi , Lundin Rickard
Published: February 2004

Upward flowing ion beams are a typical feature observed in the auroral acceleration region. Observations show that a significant part of the upward field‐aligned acceleration of the ions takes place at altitudes between 5000 and 10,000 km. Another frequently observed feature in these regions are solitary waves (or weak double layers, as they are often called). These solitary structures have been observed practically only inside ion beams. In this letter we compare the occurrences of ion beams and solitary waves as a function of altitude. It is shown that there is a clear correlation between the occurrence probabilities of these two phenomena. This suggests that some of the acceleration is due to the solitary waves. Alternative interpretations are that the ion beams, accelerated by some other mechanism, are the cause of the SWs, or that these two phenomena are independent consequences of the same physical process.
Attenuation of P‐waves in oceanic crust: Multiple scattering from observed heterogeneities
Authors: Goldberg D. , Yin C.‐S.
Published: February 2004

We extend a model for seismic attenuation that includes scattering and intrinsic components, Q−1 = Qs−1 + Qi−1, over five decades in experimental wavelength. Core images and downhole logs from Ocean Drilling Program Hole 735B near the Atlantis II Fracture Zone in the Indian Ocean were used to construct vertical profiles of heterogeneity and to compute a spectrum of Qs−1 using a multiple scattering model. Measurements of velocity dispersion suggest that Qi−1 is small compared to the measured attenuation Q−1. Measurements of Q−1 made at this site by laboratory, sonic log and VSP experiments are compared to the computed Qs−1 spectrum. Peaks in the Qs−1 spectrum occur at six major correlation lengths scales (0.006 m, 0.02 m, 0.16 m, 0.63 m, 8.0 m, and 25.0 m) which can be attributed to formation heterogeneity. Compared to typical seismic estimates of Q−1 in oceanic Layer 3, the measurements in Hole 735B are high, but can be explained by large Qs−1 values in shallow, fractured ocean crust. Based on this comparison, we conclude that the proximity of the experimental wavelength in band‐limited studies to the length scale of heterogeneity is critical in the measurement of Q−1.
Iberian Triassic paleomagnetism revisited: Intraplate block rotations versus polar wandering
Authors: Parés Josep M. , Dinarès‐Turell Jaume
Published: February 2004

A paleomagnetic study of over 220 samples, obtained from 21 sites from Triassic redbeds in the Algarve (S Portugal) and La Meseta (SE Spain), yields two paleomagnetic poles at lat = 62°N,lon = 202°E, A95 = 9 and lat=54°N, lon=178°E, A95=7 respectively. The Algarve paleopole reflects a rotation associated with the Alpine evolution of the Iberian Atlantic margin. A comparison of the La Meseta pole with other data indicates a progressive clockwise rotation of the Eurasian Plate from the Late Paleozoic to Middle Triassic, followed by rapid drift until the Upper Triassic. This late Triassic drift event may be related to the opening and spreading of the Tethys towards the west.
Tracer‐tracer relationships and lower stratospheric dynamics: CO2 and N2O correlations during SPADE
Authors: Boering Kristie A. , Daube Bruce C. , Wofsy Steven C. , Loewenstein Max , Podolske James R. , Keim Eric R.
Published: February 2004

Simultaneous measurements of CO2 and N2O from the NASA ER‐2 aircraft during SPADE deployments in November 1992, April/May 1993, and October 1993 provide new information on transport rates in the lower stratosphere. The tropospheric seasonal cycle in CO2, superimposed on the long‐term trend, is observed to propagate into the stratosphere. The compact correlations observed between CO2 and N2O indicate that meridional transport is sufficiently rapid to create a uniform set of relationships over the northern hemisphere up to at least 21 km even though CO2 changes significantly on a time scale of 8 to 12 weeks. The observed seasonal dependence of the correlations indicates that vertical transport above 20 km is slower in northern summer than in winter and slow throughout the year between 19 km and the tropopause. The inferred amplitude of the seasonal CO2 oscillation in the stratosphere, viewed relative to N2O, places constraints on the mean latitude for air entering the stratosphere.
Lithospheric thickness and mantle/lithosphere density contrast beneath Beta Regio, Venus
Authors: Moore William B. , Schubert Gerald
Published: February 2004

The spatial variation of the geoid/topography ratio over the large Venusian volcanic highland Beta Regio is suggestive of thermal compensation, i.e., support of the highland's topography by lithospheric thinning. Both the thickness of the lithosphere and the density contrast at its base can be inferred from a quadratic regression of suitably filtered (600 km < wavelength < 4000 km) geoid vs. topography data. The regression yields a mean lithospheric thickness of 270 km and a density contrast of magnitude 2.5% to 3.0%. Simple isostatic balance of the long‐wavelength topography at Beta Regio requires thinning of the lithosphere by 50–60% beneath the rise.
Grain contact adhesion hysteresis: A mechanism for attenuation of seismic waves
Authors: Sharma Mukul M. , Tutuncu Azra N.
Published: February 2004

We propose a new mechanism for attenuation of seismic waves in rocks. It is shown that grain contact adhesion hysteresis leads to energy dissipation without having to invoke frictional sliding at grain contacts. Adhesion hysteresis gives rise to frequency independent attenuation that increases with strain amplitude for large strains and decreases with overburden stress. We can obtain qualitatively correct trends that show the effect of fluid saturation, pore structure and grain size through simple models for granular sedimentary rocks.
Acoustic detection of a seafloor spreading episode on the Juan de Fuca Ridge using military hydrophone arrays
Authors: Fox Christopher G. , Radford W. Eddie , Dziak Robert P. , Lau Tai‐Kwan , Matsumoto Haruyoshi , Schreiner Anthony E.
Published: February 2004

Until recently, no practical method has been available to continuously monitor seismicity of seafloor spreading centers. The availability of the U.S. Navy's SOund SUrveillance System (SOSUS) for environmental research has allowed the continuous monitoring of low‐level seismicity of the Juan de Fuca Ridge in the northeast Pacific. On June 22, 1993, NOAA installed a prototype system at U.S. Naval Facility Whidbey Island to allow real‐time acoustic monitoring of the Juan de Fuca Ridge using SOSUS. On June 26, 2145 GMT, a burst of low‐level seismic activity, with accompanying harmonic tremor, was observed and subsequently located near 46°15′N, 129°53′W, on the spreading axis of the Juan de Fuca Ridge. Over the following 2 days, the activity migrated to the NNE along the spreading axis with the final locus of activity near 46°31.5′N, 129°35′W. The nature of the seismicity was interpreted to represent a lateral dike injection with the possibility of eruption on the seafloor. Based on this interpretation, a response effort was initiated by U.S. and Canadian research vessels, and both warm water plumes and fresh lavas were subsequently identified at the site.
Latitude variations of stratospheric trace gases
Authors: Toon G. C. , Blavier J.‐F. , Szeto J. T.
Published: February 2004

We present vertical column abundances of H2O, N2O, HNO3, NO2, O3, HF, HCl, and ClNO3 determined from solar absorption spectra measured by the JPL MkIV interferometer from the NASA DC‐8 aircraft. These observations, taken in 1987 and 1992, covered latitudes ranging from 85°S to 85°N. Although most gases display latitude symmetry, large asymmetries in H2O, HNO3, and O3 are apparent, which can be ascribed to processes enhanced by the colder Antarctic winter temperatures.
A detailed examination of a X‐line region in the distant tail: ISEE‐3 observations of jet flow and Bz reversals and a pair of slow shocks
Authors: Ho C. M. , Tsurutani B. T. , Smith E. J. , Feldman W. C.
Published: February 2004

We report an observation of Petschek‐type magnetic reconnection at a distant neutral line (X = −230 Re, July 8, 1983) with a full set of signatures of the magnetic merging process. These features include a reversal of plasma flows from earthward to tailward, a pair of slow shocks and the magnetic field X‐type line. These two slow shocks are shown to satisfy the shock criteria used by Feldman et al. [1987]. The spacecraft first crosses a slow shock to enter the earthward flowing plasmasheet with velocity of about 440 km/s. The embedded magnetic field has a positive Bz component. The spacecraft next enters a region of tailward plasma flow with speed ∼ 670 km/s and an embedded negative Bz, indicating entry into the plasmasheet tailward of the X‐line. These observed velocities are comparable to calculated velocities based on Rankine‐Hugoniot conservation relationships. The spacecraft subsequently returns into the south tail lobe by crossing another slow shock. Coplanarity analyses show that the two slow shocks have orientations consistent with that predicted by the Petschek reconnection model. We note that this event occurs during northward interplanetary magnetic fields. Thus, a magnetic stress built‐up in the distant tail may be responsible for this reconnection process.
Magnetopause characteristics during a four‐hour interval of multiple crossings observed with GEOTAIL
Authors: Kawano H. , Kokubun S. , Yamamoto T. , Tsuruda K. , Hayakawa H. , Nakamura M. , Okada T. , Matsuoka A. , Nishida A.
Published: February 2004

This paper presents multiple magnetopause crossings observed with Geotail on October 17, 1992. On this day Geotail skimmed the magnetopause in the dawn flank sector (the spacecraft had a speed of ∼0.7 km/s normal to the nominal magnetopause) and crossed the magnetopause more than 45 times in four hours. When the magnetopause crossings were observed, the interplanetary magnetic field was southerly directed. For the large number of crossings we have examined the magnetopause normal directions by using the minimum variance analysis technique, and as a result we have confirmed in a statistical sense a clear inbound/outbound dependence of the magnetopause orientation consistent with the tailward‐moving recurrent bumps in the magnetopause. In addition, there is an implication that the northward (southward) flow along the magnetopause occured for the inward (outward) directed magnetic field component normal to the magnetopause.
Dependence of earthquake size distributions on convergence rates at subduction zones
Authors: McCaffrey Robert
Published: February 2004

The correlation of numbers of thrust earthquakes of moment magnitude 7 or greater in this century at subduction zones with convergence rate results from a combination of lower recurrence intervals for earthquakes of a given size where slip rates are high and a peak in the global distribution of subduction zone convergence rates at high values (55 to 90 mm/yr). Hence, physical mechanisms related to convergence rate, such as plate interface force, slab pull, or thermal effects, are not required to explain the distribution of large earthquakes with convergence rate. The seismic coupling coefficient ranges from 10% to 100% at subduction zone segments where convergence is faster than 45 mm/yr but does not correlate with rate. The coefficient is generally orders of magnitude lower at rates below 40 mm/yr which may be due to long recurrence intervals and a short sampling period (94 years).
A detailed gravity field over the Reykjanes Ridge from Seasat, Geosat, ERS‐1 and TOPEX/POSEIDON altimetry and shipborne gravity
Authors: Hwang Cheinway , Parsons Barry , Strange Theresa , Bingham Andrew
Published: February 2004

The method of least‐squares collocation (LSC) was used to construct a detailed gravity field over the Reykjanes Ridge using Seasat, Geosat, ERS‐1, TOPEX/POSEIDON altimeter data and shipborne gravity data. Mean altimeter‐derived geoid gradients, along with their standard deviations, were used in order to avoid cross‐over adjustment of sea surface heights. The ship data were adjusted to a satellite‐only gravity field using a quadratic polynomial in time and then merged with the satellite data to produce a combined gravity field. The necessary covariance functions were derived using the law of covariance propagation. In a test case, a comparison between a combined gravity field and ship data not included in the calculation yielded an rms difference of 2.31 mgals. The combined gravity field contains a wealth of short‐wavelength information and clearly reveals the detailed tectonic structures associated with the Reykjanes Ridge.
Fate of event hydrothermal plumes on the Juan de Fuca Ridge
Authors: Cannon Glenn A. , Pashinski David J. , Stanley Tamara J.
Published: February 2004

Current meter moorings were deployed in August 1993 at the megaplume site of the June 1993 seismic event on the CoAxial segment of the Juan de Fuca Ridge and were recovered in April 1994. Record‐averaged flow was about 2 cm/s across axis, which is sufficiently large to advect plumes off axis in a few days. During July two megaplumes (1800 m level) disappeared between repeated CTD tows. A deeper plume (2150 m) had a relatively strong signature during August and extended southeastward in agreement with the observed flow. A CTD tow along the east flank of Axial Volcano did not locate remnants of the megaplumes but did find deeper plumes which could have come from off‐axis advection from the CoAxial sources followed by southward advection along the east side of ridge.
Initial results of the rapid response to the 1993 CoAxial event: Relationships between hydrothermal and volcanic processes
Authors: Embley Robert W. , Chadwick William W. , Jonasson Ian R. , Butterfield David A. , Baker Edward T.
Published: February 2004

Between June 26 and July 10, 1993, swarms of “T‐wave” events occurred over a 40‐km portion of the CoAxial segment on the northern Juan de Fuca Ridge. A rapid response utilizing a CTD/rosette/chemical scanner and a remotely operated vehicle occurred in the month following the T‐wave swarms. The pattern of T‐wave events and water‐column anomalies (including several event plumes) are remarkably coincident. The only known eruptive area is at the northern swarm area, where a very fresh pillow lava ridge was discovered, mapped, and sampled with the remotely operated vehicle ROPOS. A vent area about 22 km south of the lava flow was emitting large quantities of bacterially generated floccular material. The temporal pattern of T‐wave events and the coincidence between the T‐wave swarms, the young lava flows, and hydrothermal plumes suggests that there is a close analogy between this activity and lateral dike injections such as have been closely monitored at Icelandic central volcanoes.
Interannual variations in the Earth's polar motion for 1963–1991: Comparison with atmospheric angular momentum over 1980–1991
Authors: Abarca del Rio R. , Cazenave A.
Published: February 2004

Analysis of interannual variations in polar motion for the period 1963–1991 reveals two prominent components, a quasi biennial component and a lower frequency component of ∼ 4–6 yr period. These frequencies, previously reported in the length of day, are possibly related to the El Niño‐Southern Oscillation and Quasi Biennial Oscillation of the coupled ocean‐atmosphere system. Over the overlapping period (1980–1991), we compared the geodetically‐derived polar motion excitation with equatorial atmospheric angular momentum (AAM). At the biennial frequency polar motion and AAM are well correlated, when the pressure excitation is estimated assuming a non‐inverted barometer response of the oceans. However AAM alone is unable to explain the total observed polar motion. An oceanic contribution associated with the dynamical response of the oceans to pressure‐derived wind stress is a likely candidate to explain the missing signal. At longer periods (4–6‐yr), the east components of polar motion and AAM vary coherently. No clear conclusion can be drawn however for the north components because of large uncertainties reported in the corresponding AAM data at low frequency.
Dilatancy in hydraulically isolated faults and the suppression of instability
Authors: Lockner David A. , Byerlee James D.
Published: February 2004

Laboratory studies have shown that consolidated fault gouge dilates in response to an increase in the rate of shearing. During the period of accelerating slip that characterizes the onset of an instability, or earthquake, on a fault containing consolidated, fluid‐saturated gouge, the accompanying dilatancy will result in a reduction in pore fluid pressure and an increase in effective normal stress. We report laboratory observations of a 3% drop in pore pressure per decade increase in sliding rate of quartz gouge between granite fault surfaces. This dilatancy strengthening is greater than intrinsic rate‐weakening observed for dry gouge and can play an important role in earthquake nucleation.
Imaging a slow bilateral rupture with broadband seismic waves: The September 2, 1992 Nicaraguan tsunami earthquake
Authors: Velasco Aaron A. , Ammon Charles J. , Lay Thorne , Zhang Jiajun
Published: February 2004

The spatio‐temporal rupture history of the September 2, 1992 Nicaragua tsunami earthquake (Ms=7.2; Mw=7.6) is analyzed using long‐period (157–288 s) Rayleigh and Love wave spectral inversions and an empirical Green function analysis of very broadband (10–250 s) body and surface waves. The event has a rupture duration exceeding 110 s with a slow (0.6–2.3 km/s) rupture velocity, and involves shallow dipping (6°–10°) thrust faulting at a shallow depth (≤ 10 km), with the latter being primarily responsible for the strong tsunami excitation. Two subevents dominate the source radiation, resulting from a slow asymmetric bilateral, 100–160 km long rupture. Long‐period surface wave directivity reveals a predominant rupture azimuth of 140° ± 30°, consistent with asymmetric extension of the bilateral rupture at an azimuth of 125° ± 30° resolved by the source time function analysis. An inverse Radon transform of the source time functions confirms the asymmetric bilateral rupture characteristics.
Variations in the FUV dayglow after intense auroral activity
Authors: Craven J. D. , Nicholas A. C. , Frank L. A. , Strickland D. J. , Immel T. J.
Published: February 2004

A localized ∼55% decrease is observed in the brightness of Earth's FUV dayglow in the morning sector at 130.4 nm after an interval of intense geomagnetic activity. This large decrease is interpreted as being the consequence of auroral‐associated heating which reduces the thermospheric column density of O relative to that of N2. Spatial extent of the observed decrease exceeds 107 km² at the −25% level, and the depth of the decrease lessens during more than two hours of observations. These remote observations provide the first instantaneous, two‐dimensional measurement of the large‐scale spatial extent of such a change in thermospheric composition.
Accuracy of O+−O collision cross‐section deduced from ionosphere‐thermosphere observations
Authors: Reddy C. A. , Hoegy W. R. , Pesnell W. D. , Mayr H. G. , Hines C. O.
Published: February 2004

Aeronomic observations applied to the empirical derivation of the ion‐neutral collision cross‐section—a basic parameter governing the mutual interactions between the neutral and plasma components in the Earth's upper atmosphere—have given values considerably larger than those derived from theory. The empirical scheme uses the plasma velocities obtained with the Incoherent Scatter Radar and the neutral winds obtained with the Fabry‐Perot Interferometer. It is shown here that such an analysis overestimates the collision cross‐section due to the effects of errors in the observables. The effect may be sufficiently large to bring about agreement with theory, and calls for a re‐analysis of the aeronomic data using methods which minimize the bias caused by measurement errors.
Magnetic properties of biogenic soil greigite (Fe3S4)
Authors: Fassbinder Jörg W. E. , Stanjek Helge
Published: February 2004

Magnetic measurements were performed on soil samples, which contained bacterial greigite. The greigite‐containing soil horizon is characterized by SIRM/χ > 70 kA m−1 and by Q‐factors >5. Alternating field (AF) demagnetization curves of the NRM gave stable directions. According to IRM acquisition and alternating field demagnetization of the IRM and ARM, the greigite in bulk soil samples shows single domain behaviour. This SD behaviour is also confirmed by hysteresis and by frequency‐dependent susceptibility measurements. The thermomagnetic behaviour was checked on extracted greigite. The magnetic parameters of soil greigite are compared with parameters of sedimentary greigite and of bacterial magnetite.
Empirical Green's function study of the January 17, 1994 Northridge, California earthquake
Authors: Dreger D. S.
Published: February 2004

The January 17, 1994 Northridge main‐shock was analyzed using an empirical Green's function approach to estimate the spatial and temporal distribution of fault slip. Far‐field source time functions obtained from seismograms recorded by the Berkeley Digital Seismic Network (BDSN) and TERRAscope stations reveal the distributed nature of the source and the presence of a second large subevent. The location of the fault beneath the San Fernando valley as well as the updip directivity of the rupture contributed to the locally heavy damage due to this earthquake.
Observations of manganese and iron at the CoAxial Seafloor Eruption Site, Juan de Fuca Ridge
Authors: Massoth G. J. , Baker E. T , Feely R. A. , Butterfield D. A. , Embley R. E. , Lupton J. E. , Thomson R. E. , Cannon G. A.
Published: February 2004

Magmatic injection during June/July 1993 on the CoAxial segment of the Juan de Fuca Ridge resulted in an axial lava eruption, an overlying chronic‐style hydrothermal plume, and at least three megaplume‐like event plumes. A comprehensive view of Mn and Fe liberated as hydrothermal discharge during this volcano‐hydrothermal event was obtained as part of a rapid field response that included continuous in situ measurements of Mn and Fe. Distribution maps for Mn, Fe and temperature anomaly within the chronic and event plumes reveal significant thermochemical diversity that we ascribe to variable extraction efficiencies for heat and metals by source fluids, and to plume age differences. The Mn/heat data suggest that diffuse fluids could be the primary source of the chronic plume at northern CoAxial segment but make only a minor contribution to event plumes. Measurements of ephemeral Fe(II) were used to estimate that the event plumes were ∼2, ∼4, and >6–8 days old when first sampled, corresponding to formation times during peak seismic activity. The inventories of Mn and Fe within the three CoAxial event plumes were similar to values previously reported for megaplumes.
Mass‐loading asymmetry in upstream region near Mars
Authors: Dubinin E. , Obod D. , Pedersen A. , Grard R.
Published: February 2004

The measurements carried out by the electric field probe onboard the PHOBOS‐2 spacecraft reveal that the mass‐loading taking place in the solar wind far upstream of the Martian bow shock is dependent upon the orientation of the interplanetary magnetic field. The asymmetry of mass‐loading is caused by H+ pickup ions originating from the extended cold hydrogen Martian exosphere and reflected from the bow shock. The distribution of pickup protons in the upstream region is analysed in a test particle approach, which seems to be confirmed by the Phobos data.