bluuebunny/crossref_metadata_2025 huggingface dataset:
Observations of plasmas associated with the magnetic signature of a plasmoid in the distant magnetotail
Authors: Frank L. A. , Paterson W. R. , Ackerson K. L. , Kokubun S. , Yamamoto T. , Fairfield D. H. , Lepping R. P.
Published: February 2004

The three‐dimensional electron and positive ion velocity distributions were examined for four events for which the magnetic signature of plasmoids occurred at the Geotail spacecraft position in Earth's distant magnetotail. The proton bulk parameters and the electron velocity distributions for one of the events are presented in order to characterize the qualitative character of the set of events. The north‐to‐south turnings of the magnetic field that were taken for evidence of the passage of a plasmoid were often accompanied by a significant strong core of By that was centered on the transition of Bz from north to south. The magnetic signatures thus appeared to be more indicative of flux ropes than of classical plasmoids with weak core fields. The proton and electron bulk velocities in these magnetic structures are not equal, a situation that provides an earthward current. The plasma β was typically one or greater. Counterstreaming of low‐energy electrons was often found on the magnetic field lines around the central magnetic core and suggested the existence of a closed field line topology in these regions. These events appeared to be the tailward flowing debris from acceleration processes occurring in the vicinity of a separator positioned nearer to Earth.
Computer simulation of electrostatic solitary waves: A nonlinear model of broadband electrostatic noise
Authors: Omura Y. , Kojima H. , Matsumoto H.
Published: February 2004

We present simulations of the electrostatic solitary waves (ESW) as observed by GEOTAIL which have been identified as broadband electrostatic noise (BEN) in previous studies. We have found that ESW are generated as a result of the nonlinear coalescence of strong electrostatic waves excited by an electrostatic beam instability. This instability is driven by an electron beam drifting relative to the ions and other electrons drifting with the ions. As a necessary condition for generation of ESW, the ion thermal velocity must be large enough so as to prevent decay of the electrostatic waves to ion acoustic waves. Another condition is that the density of the electron beam drifting relative to the ions must be larger than 30% of the plasma density.
A statistical survey of ELF waves in a geostationary orbit
Authors: Parrot M. , Gaye C. A.
Published: February 2004

GEOS 2 is a geostationary satellite that was launched with a wave experiment. The data provided during its lifetime (∼ 3.5 years) constitute a unique set for the statistical study of electromagnetic and electrostatic waves at L = 6.6. The aim of this paper is to present maps of the average intensity of the electric and magnetic field. The location of the more intense emissions up to 4.6 kHz is shown as a function of local time and frequency. Such emissions are mainly composed of hiss, chorus and electron‐cyclotron harmonic (ECH) waves. Their locations were specified and compared with data from the OGO 5 and SCATHA satellites. In particular, this study highlights the characteristics of chorus emissions with a gap at half the electron gyrofrequency. It is also shown that global magnetic activity only slightly influences wave amplitude.
Dynamics of shear velocity layer with bent magnetic field lines
Authors: Galinsky V. L. , Sonnerup B. U. Ö.
Published: February 2004

A fully three‐dimensional, magnetohydro‐dynamic simulation of velocity‐sheared plasma flow in an ambient transverse magnetic field with bent magnetic field lines has been performed. “Ionospheric‐like” boundary conditions were used for closing field‐aligned currents, the two ionospheres being represented by conducting plates with constant resistivity. Compared to the standard plane 2D case with a uniform transverse magnetic field, the growth rate of the Kelvin‐Helmholtz instability drops significantly as bending increases. Under conditions representative of the Earth's low latitude boundary layer, the instability may be suppressed completely by the magnetic field‐line tension if the field‐line bending is sufficiently strong. For weak bending, a combination of the tearing mode instability and the Kelvin‐Helmholtz instability leads to the formation of localized 3D current/vortex tubes, the ionospheric foot prints of which are possible models of the auroral bright spots observed by the Viking satellite.
Are models of catalytic removal of O3 by HOx accurate? Constraints from in situ measurements of the OH to HO2 ratio
Authors: Cohen R. C. , Wennberg P. O. , Stimpfle R. M. , Koplow J. , Anderson J. G. , Fahey D. W. , Woodbridge E. L. , Keim E. R. , Gao R. , Proffitt M. H. , Loewenstein M. , Chan K. R.
Published: February 2004

Measurements of the ratio OH/HO2, NO, O3, ClO, and BrO were obtained at altitudes from 15–20 km and latitudes from 15–60°N. A method is presented for interpreting these simultaneous in situ observations that constrains the rates of chemical transformations that 1) are responsible for over half the ozone removal rate in the lower stratosphere via reactions of HO2 and 2) control the abundance of HO2 through coupling to nitrogen and halogen radicals. The results show our understanding of the chemical reactions controlling the partitioning of OH and HO2 is complete and accurate and that the potential effects of “missing chemistry” are strictly constrained in the region of the atmosphere encompassed by the observations. The analysis demonstrates that the sensitivity of the ratio OH/HO2 to changes in NO is described to within 12% by current models. This reduces by more than a factor of 2 the effect of uncertainty in the coupling of hydrogen and nitrogen radicals on the analysis of the potential effects of perturbations to odd nitrogen in the lower stratosphere.
Electromagnetic weather at 100 km altitude on 3 August 1986
Authors: Feldstein Y. I. , Levitin A. E. , Gromova L. I. , Dremuhina L. A. , Blomberg L. G. , Lindqvist P.‐A. , Marklund G. T.
Published: February 2004

The electromagnetic weather at high altitudes above the Earth's surface is determined by the transport of ionospheric plasma, which in turn is governed by the magnitude as well as the direction of the electric and magnetic fields. Different models [Levitin et al., 1984; Friis‐Christensen et al., 1985; Mishin, 1990] have been proposed that allow an estimation of the electromagnetic parameters of the upper atmosphere, given a knowledge of the magnitude and orientation of the interplanetary magnetic field. Here we use one such model to estimate the global convection pattern and its temporal evolution during a pass of the Swedish satellite Viking over the northern polar cap. The model predictions are shown to agree well with the electric and magnetic fields measured along the satellite trajectory. The good agreement implies that the model can be used to reconstruct, with reasonable confidence, the large‐scale distribution of electric and magnetic fields and their time‐variation in the entire auroral ionosphere.
Scatter and bias in differential PKP travel times and implications for mantle and core phenomena
Authors: Helffrich George , Sacks Selwyn
Published: February 2004

Differential travel times, commonly used to eliminate unwanted near‐source and near‐receiver heterogeneity from travel time studies, can be affected by slabs and hypocentral errors when phase pair slownesses differ greatly. In particular, D″ and core structure studies can be biased if differential PKP times are used. Here we present differential PKP travel time measurements having significant azimuthal travel time anomalies consistent both in size and pattern with a near‐source slab effect and explore other sources of error in differential times.
SeaBeam depth changes associated with recent lava flows, Coaxial Segment, Juan De Fuca Ridge: Evidence for multiple eruptions between 1981–1993
Authors: Chadwick William W. , Embley Robert W. , Fox Christopher G.
Published: February 2004

After a swarm of earthquakes was detected on the CoAxial segment of the Juan de Fuca Ridge in June‐July 1993, the area was resurveyed with SeaBeam multibeam sonar to search for depth changes associated with a submarine volcanic eruption. Quantitative comparison of the 1993 SeaBeam survey with surveys in 1981/82 and 1991 shows one area of seafloor depth change (up to 29 m) between 1991–93 exactly where a pristine lava flow was discovered. In addition, two other depth anomalies (up to 37 m and 20 m) are identified between 1981–91, evidence that other recent eruptions have occurred along this spreading ridge segment.
Ultra‐low frequency magnetic field measurements in southern California during the Northridge Earthquake of 17 January 1994
Authors: Fraser‐Smith A. C. , McGill P. R. , Helliwell R. A. , Villard O. G.
Published: February 2004

Measurements of ultra‐low frequency (ULF) magnetic field fluctuations by two independent monitoring systems in Southern California were in progress during January 1994 when the moderately‐large M6.7 Northridge earthquake occurred on 17 January. Our two measuring systems are located at Table Mountain, on the other side of the San Gabriel mountains and at a distance of 81 km from the epicenter, and at Piñon Flat, south of Palm Springs and at a distance of 206 km from the epicenter. Both systems operated well throughout the month and without interruption due to the earthquake. As a result of the occurrence of a moderate magnetic storm on 11 January, which was followed by a period of enhanced ULF magnetic activity that persisted until after the time of the earthquake, the sensitivity of our measurements throughout California was reduced for roughly a week before the earthquake took place. Nevertheless, no large signals that could be associated with the earthquake were evident at any time, except for the usual co‐seismic shaking response of the detectors. Subsequent removal of the upper atmosphere signals from the Table Mountain measurements, using the measurements from the more distant Piñon Flat location as reference, essentially left no significant residual. Thus, assuming that ULF magnetic fields were produced by the earthquake, their amplitudes were too small to produce obvious increases in the ULF background noise at 81 km from the epicenter, which is in agreement with our earlier estimate of a range of about 100 km for the ULF magnetic field fluctuations observed prior to the M7.1 Loma Prieta earthquake. These results imply that a network of conventional magnetic field detectors spaced less than 100 km apart would be required to detect ULF magnetic field fluctuations prior to earthquakes with magnitudes greater than 7. Under the same conditions, superconducting magnetic field gradiometers could offer greater sensitivity and range.
Production of O(¹D) from photolysis of O3
Authors: Michelsen H. A. , Salawitch R. J. , Wennberg P. O. , Anderson J. G.
Published: February 2004

We have used a model accounting for absorption by vibrationally and rotationally excited ozone (O3) to examine published measurements of the quantum yield of O(¹D) and O2(¹Δg) from O3 photolysis in the Huggins band (305–325 nm). We conclude that the quantum yield of O(¹D) is 0.2–0.3 for wavelengths between 312 and 320 nm at 298 K; in contrast, the JPL recommended quantum yield of O(¹D) is negligible in this wavelength region. We present a compilation of our results at temperatures and wavelengths relevant to atmospheric calculations. Production of O(¹D) from excited O3 increases calculated concentrations of O(¹D) by up to 40% and OH by 15% for the troposphere and lower stratosphere.
Turbulent magnetic field in the distant magnetotail: Bottom‐up process of plasmoid formation?
Authors: Hoshino M. , Nishida A. , Yamamoto T. , Kokubun S.
Published: February 2004

The GEOTAIL magnetic field observations in the earth's magnetotail are examined in terms of a turbulent magnetic reconnection process. It is found that the observed Fourier power spectral density of the turbulent magnetic fields can be approximated by a power‐law spectrum which changes its slope around a turnover frequency with 0.04 Hz, in spite of the differences among a wide variety of magnetic perturbations observed in the distant magnetotail plasma sheet. The turbulent magnetic field data are classified into two cases (Nishida et al. in this issue): one is accompanied with a plasmoid bipolar signature on Bz magnetic field, and the other without the bipolar signature. The higher frequency regions above the turnover frequency always show similar spectral slope in the cases with and without a bipolar signature in Bz magnetic field component, although the spectral feature in the lower frequency regions exhibits differences in appearance. When there is no bipolar signature on Bz magnetic field, the lower frequency range is almost a flat spectrum. On the other hand, the slope of the lower frequency region becomes steep when a large scale bipolar signature is observed. The observations may indicate that 1) a small‐scale tearing mode vortex with the scale of plasma sheet thickness is excited in the magnetotail, and 2) the large‐scale plasmoid is coalescently formed from the small‐scale tearing mode vortices.
Rapid recovery of anomalous cosmic ray flux at 1 Au in solar cycle 22
Authors: Hasebe N. , Mishima Y. , Fujiki K. , Fujii M. , Kobayashi M. , Doke T. , Kikuchi J. , Hayashi T. , Shino T. , Ito T. , Takashima T. , Yanagimachi T. , Nakamoto A. , Murakami H. , Nagata K. , Kohno T. , Munakata K. , Kato C. , Yanagita S. , Kashiwagi T. , Maezawa K. , Muraki Y. , Nishida A. , Terasawa T. , Wilken B.
Published: February 2004

A new observation of low energy cosmic ray particles with the energies from 4 to 120 MeV/n by the HEP instrument onboard the GEOTAIL satellite orbiting at 1 AU shows a remarkable enhancement of anomalous cosmic ray (ACR) N, O and Ne in the period September 1992–December 1993 before approaching to the solar minimum in solar cycle 22. The ACR fluxes obtained by the GEOTAIL observation are in good agreement with the results earlier obtained from the SAMPEX satellite [Mewaldt et al. 1993a,b]. The ACR carbon is also evident though the C enhancement is small compared with those of N, O and Ne. We confirm the new SAMPEX finding that the ACR fluxes have recovered more rapidly in 1992–1993 than in previous solar cycles and find this tendency continues to the end of 1993.
GEOTAIL observations of electrostatic waves in the lower hyrid frequency range in the plasma sheet boundary layer
Authors: Okada T. , Tsuruda K. , Hayakawa H. , Nakamura M. , Mukai T. , Yamamoto T. , Matsuoka A. , Terasawa T. , Mozer F. S. , Kokubun S. , Matsumoto H.
Published: February 2004

Quasi‐monochromatic waves with an electrostatic nature in the lower hybrid frequency range have been identified in the plasma sheet boundary layer at XGSM=−49Re and −69Re by the wave‐form observations in a band 0–25 Hz from the double probe onboard the GEOTAIL spacecraft. Simultaneous measurements of low energy ions also indicate that the wave coincides with the transition of the ion‐energy from 0.1 keV/e to 10 keV/e.
Crustal velocity structure north of the Mendocino Triple Junction
Authors: Beaudoin Bruce C. , Magee Marian , Benz Harley
Published: February 2004

A 140‐km‐long refraction/wide‐angle reflection profile recently recorded by Stanford University and the U. S. Geological Survey imaged the subducting Gorda slab beneath northern California. The profile, which is subparallel to the coast from Cape Mendocino northward, indicates that the North American plate is 13‐ to 14‐km‐thick along the coast north of Cape Mendocino. The crust is characterized by relatively uniform, low velocities of ≤6 km/s interpreted as Franciscan rocks. Two strong reflections define the upper and lower boundaries of the subducting Gorda crust. Our data indicate that the subducting Gorda crust thickens northward from Cape Mendocino from 7‐km‐thick just north of Cape Mendocino to 10‐km‐thick 120 km to the north. This change in thickness is coincident with a change in velocity from 6.7 km/s south to 6.2 km/s north. Mantle velocities of 7.7 km/s are observed for offsets greater than c. 80 km. We interpret our model to indicate that the Gorda slab is not imbricated on a crustal scale beneath our profile, that sediments and/or a tectonically thickened oceanic layer 2 are present to the north but not in the vicinity of Cape Mendocino, and that proximity to the Mendocino triple junction affects the way sediments are subducted.
The relationship between interplanetary discontinuities and Alfvén waves: Ulysses observations
Authors: Tsurutani B. T. , Ho C. M. , Smith E. J. , Neugebauer M. , Goldstein B. E. , Mok J. S. , Arballo J. K. , Balogh A. , Southwood D. J. , Feldman W. C.
Published: February 2004

The rate of occurrence of interplanetary discontinuities (ROID) is examined using Ulysses magnetic field and plasma data from 1 to 5 AU radial distance from the Sun and at high heliographic latitudes. We find two regions where the ROID is high: in stream‐stream interaction regions and in Alfvén wave trains. This latter feature is particularly obvious at high latitudes when Ulysses enters a high speed stream associated with a polar coronal hole. These streams are characterized by the presence of continuous, large‐amplitude
Alfvén waves and an extraordinarily high ROID value (∼150 discontinuities/day). In a number of intervals examined, it is found that (rotational) discontinuities are an integral part of the Alfvén waves. The nonlinear Alfvén waves are spherically polarized, i.e., the tip of the perturbation vector resides on the surface of a sphere (a consequence of constant |B|). The slowly rotating part of the wave rotates ∼270° in phase. There is a slight arc in the B1‐B2 hodogram, suggesting an almost linear polarization. The phase rotation associated with the discontinuity is ∼90°, lies in the same plane as the slowly rotating part of the Alfvén wave, and therefore completes the 360° phase rotation. The best description of the overall Alfvén wave plus discontinuity is a spherical, arc‐polarized, phase‐steepened wave.
Influence of layered structure of the lower ionosphere on nonmonotonic spectrum behavior of ELF atmospheric noise
Authors: Molchanov O. A. , Nickolaenko A. P. , Rafalsky V. A. , Yu. Schecotov A. , Hayakawa M.
Published: February 2004

This paper reports on some propagation effects occurring while the ionosphere consists of several layers. A few simple model problems are considered to show in which way this multi‐layer structure can yield a new set of ELF resonance phenomena. The resonances appear in the space between the Earth and layer interfaces or inside the layers and are liable to give rise to peaks or minima in the frequency dependencies of ELF electromagnetic fields both for the cosmic and terrestrial sources. Calculations have been made for the real ionospheric models mid distant Earth's sources have shown that such a mechanism can explain some experimental data including the effect of “Evening Ionosphere Radiation” by Schecotov and Molchanov (1985).
Mechanism of the 1992 Nicaragua Tsunami Earthquake
Authors: Satake Kenji
Published: February 2004

The 1992 Nicaragua earthquake generated larger tsunamis than expected from its surface wave magnitude (Ms 7.2) and is known as a ‘tsunami earthquake’. Seismological studies showed that the duration was very long for its size, about 100 s. Other studies have shown that the seismic moment estimated from tsunamis is an order of magnitude larger than that from seismic waves, even after the long duration is accounted for. Numerical computations of tsunamis from various fault models are made to reconcile this discrepancy. Comparison of calculated waveforms with tide gauge records shows that the fault width is 40 km, much narrower than the aftershock area, and extends only into the upper 10 km of the ocean bottom. Slip amount on the fault is estimated to be 3 m from amplitude comparisons. The fault length is estimated to be 250 km, slightly longer than the aftershock area, from comparison of the tsunami height distribution. The rigidity around the shallow fault may be smaller than that of a standard underthrust fault, and the seismic moment is estimated as 3×1020 Nm, consistent with the seismic observations. A slow rupture on the shallow fault, presumably in the subducted sediments, is responsible to the unusually large tsunami excitation.
Geosat observations of sea level response to barometric pressure forcing
Authors: Hoar Timothy J. , Wilson Clark R.
Published: February 2004

Altimeter and sea level pressure data from the Geosat mission are analyzed for evidence of inverted barometer response of sea level to atmospheric pressure forcing. Estimates of the inverted barometer coefficient are given for a variety of geographic regions and time scales using various orbit error removal strategies. There is some sensitivity to the orbit error removal method, but the estimated coefficients show a clear latitudinal dependence and are generally between −0.5 cm/mbar and −0.9 cm/mbar. The southern oceans respond slightly more like an inverted barometer than the northern oceans for similar latitudes. The regression exhibits significant geographic variability, particularly near major circulation features and in the northern hemisphere. The results suggest that the inverted barometer approximation is reasonable over much of the oceans, but that some sea level variability may be correlated with barometric pressure by means other than the inverted barometer effect.
Acidity variations across the cloud drop size spectrum and their influence on rates of atmospheric sulfate production
Authors: Collett Jeffrey L. , Bator Aaron , Rao Xin , Demoz Belay B.
Published: February 2004

Measurements of pH variations within natural cloud drop populations reveal that small drops are often more acidic than large drops. Cloud samples collected from coastal stratus clouds, frontal clouds, and radiation fogs, from heavily polluted and pristine locations, had pH values ranging from below three to more than seven. Differences between small and large cloud drop acidities as large as two pH units were observed, although differences were generally below one pH unit. This chemical heterogeneity can significantly enhance oxidation of sulfur dioxide to sulfate within clouds, relative to oxidation rates predicted from the average cloudwater composition. One‐third of the sampled clouds were estimated to experience an increase of at least 20% in the rate of sulfur oxidation by ozone (8% of the clouds had increases exceeding 100%) as a result of acidity differences between large and small cloud drops. These findings suggest that sulfate production within clouds, a critical component of the global sulfur cycle, may be more rapid than previously thought.
Enhanced velocities and a shear in daytime Esq over Kwajalein and their relationship to 150 km echoes over the dip equator
Authors: Tsunoda Roland T.
Published: February 2004

We present the first evidence of enhanced irregularity‐drift velocities and a velocity shear in the daytime E region at 4.3°N dip latitude. These features, found in radar measurements from Kwajalein during q‐type sporadic E (Esq) events, are interpreted in terms of a polarization electric field that develops in response to the presence of latitudinal gradients in field‐line‐integrated conductivity caused by sporadic‐E layers. Because the E region over Kwajalein connects directly to 150‐km altitude over the magnetic dip equator, we suggest that a similar polarization process is responsible for the so‐called 150‐km echoes that have been observed routinely by the Jicamarca radar.
A new type of auroral radio emission observed at medium frequencies (∼1350–3700 kHz) using ground‐based receivers
Authors: Weatherwax A. T. , LaBelle J. , Trimpi M. L.
Published: February 2004

A new form of broadband emissions (“MF bursts”) appearing between ∼ 1350–3700 kHz has been identified in 8 months of data (October 1993–May 1994) from ground‐based LF/MF/HF receiving systems at Circle Hot Springs (65.5°N, 144.7°W) and Two Rivers (64.9°N, 146.9°W), Alaska. These emissions are often observed simultaneously with 2ƒce and 3ƒce auroral roar emissions, auroral hiss, and absorption of AM broadcast band transmitters. MF bursts display significant amplitude variations on timescales as short as 10 ms, but the variations do not correlate with the fine structures seen in simultaneous auroral hiss and auroral roar. The generation mechanism of these emissions is unknown.
First results from the Freja HF snapshot receiver
Authors: Kintner Paul M. , Bonnell John , Powell Steven , Wahlund Jan‐Erik , Holback Bengt
Published: February 2004

The Freja plasma wave instrument has measured electric field waveforms up to 4 MHz in the auroral ionosphere near 1700 km altitude. The HF snapshot receiver responds to natural signals during every passage through the auroral ionosphere and we have currently identified two kinds of signals: broadband whistler mode emissions with a cut‐off at the plasma frequency, narrow band Langmuir wave emissions at the plasma frequency, and mixtures of both wave emissions. The Langmuir wave emissions are frequently narrow band (Δf/f ⪝ 10−2) and exhibit a variety of modulational features. These Langmuir waves exist up to amplitudes of roughly 1 volt/m. At larger amplitudes (a few V/m and ϵoE²/2nkT ≅10−2) the wave spectra broaden and the waveforms appear to be composed of individual wave packets, each with 5–10 wave periods. The narrow band Langmuir waves appear to be very common and are observed on nearly every auroral zone pass in which precipitating electrons are observed.
Freja observations of a ten‐meter boundary within monoenergetic auroral electron precipitation
Authors: Boehm M. H. , Clemmons J. , Paschmann G.
Published: February 2004

A ∼2‐ms transition in auroral electron precipitation observed by the TESP electron spectrometer on Freja is described. The lack of significant pitch‐angle dependent velocity dispersion in the observations implies that the transition is a ∼10‐m spatial, rather than temporal, boundary. There is no evidence of associated changes in the accelerating potential. Either the source population, or the mechanism allowing entry of the source population to the acceleration region, must provide for the sharp observed transitions. The energy fluxes are such that the regions of stronger precipitation are marginally visible arcs.
In‐situ stress fields and focal mechanism solutions in central France
Authors: Scotti Oona , Cornet François H.
Published: February 2004

Focal mechanisms of 31 Central France earthquakes (mb= 2.9‐4.4) were evaluated to determine if slip is compatible with stress field determinations conducted at four nearby sites. The coherent picture that results indicates that the regional stress fields can be inferred by linear extrapolation of in‐situ measurements performed between 400–1000 m depths. Furthermore, the results show that in spite of rather constant principal stress directions from one site to the next (100 km apart), principal stress amplitudes vary significantly. For the domain investigated (200×200 km²), the size of the homogeneous stress field regions appears to be fairly small (about 100×100 km²). In addition, within these regions, important local stress perturbations as identified by extremely inconsistent focal mechanisms, are present. Thus only by integrating different data types is it possible to map properly the regional stress field. This mapping provides means to identify the homogeneous regions as well as to highlight zones of local stress perturbations, most likely indicative of active faulting.
In situ measurements of the NO2/NO ratio for testing atmospheric photochemical models
Authors: Jaeglé L. , Webster C. R. , May R. D. , Fahey D. W. , Woodbridge E. L. , Keim E. R. , Gao R. S. , Proffitt M. H. , Stimpfle R. M. , Salawitch R. J. , Wofsy S. C. , Pfister L.
Published: February 2004

Simultaneous in situ measurements of NO2, NO, O3, ClO, pressure and temperature have been made for the first time, presenting a unique opportunity to test our current understanding of the photochemistry of the lower stratosphere. Data were collected from several flights of the ER‐2 aircraft at mid‐latitudes in May 1993 during NASA's Stratospheric Photochemistry, Aerosols and Dynamics Expedition (SPADE). The daytime ratio of NO2/NO remains fairly constant at 19 km with a typical value of 0.68 and standard deviation of ± 0.17. The ratio observations are compared with simple steady‐state calculations based on laboratory‐measured reaction rates and modeled NO2 photolysis rates. At each measurement point the daytime NO2/NO with its measurement uncertainty overlap the results of steady‐state calculations and associated uncertainty. However, over all the ER‐2 flights examined, the model systematically overestimates the ratio by 40% on average. Possible sources of error are examined in both model and measurements. It is shown that more accurate laboratory determinations of the NO + O3 reaction rate and of the NO2 cross‐sections in the 200–220 K temperature range characteristic of the lower stratosphere would allow for a more robust test of our knowledge of NOx photochemistry by reducing significant sources of uncertainties in the interpretation of stratospheric measurements. The present measurements are compared with earlier observations of the ratio at higher altitudes.
Can a charge‐exchange induced density rise at the heliopause explain the frequency drift of the 3 kHz Voyager signal?
Authors: Czechowski Andrzej , Grzedzielski Stanislaw
Published: February 2004

A physical model for the recently advanced interpretation of the 3 kHz data [Gurnett, Kurth et al., 1993] is proposed. Basing on the results of the flow dynamical model of the heliosheath [Czechowski and Grzedzielski, 1993, 1994] we conclude that the post‐shock solar wind (SW) plasma, cooled down by charge‐exchange with neutral hydrogen of LISM origin, should form a high density layer on the inner side of the surface of the heliopause. The model allows us to estimate the plasma density profile and the size of the layer. These we use as input for MHD calculation of (1) a transient shock moving through this layer towards the heliopause and (2) of resulting 2ωp (frequency) drift of VLF emissions generated at the shock. For the heliopause distance L= 150‐180 A.U. our model reproduces both the time duration of the emissions and their typical frequency drifts.
Extensive lidar observations of the Pinatubo aerosol layers at Tsukuba (36.1°N), Naha (26.2°N), Japan and Lauder (45.0°S), New Zealand
Authors: Uchino O. , Nagai T. , Fujimoto T. , Matthews W. A. , Orange J.
Published: February 2004

Since the explosive volcanic eruption of Mt. Pinatubo on June 15, 1991, extensive lidar observations at a wavelength of 532 nm have been made at Tsukuba and Naha in the northern hemisphere and Lauder in the southern hemisphere. The first increase of the aerosols from the Pinatubo eruption was observed at 15.7 km over Tsukuba on June 28, 1991. From fall 1991, the Pinatubo aerosol particles were effectively transported from the tropical region into the northern mid‐latitude. The maximum scattering ratios observed were 14.1 at 22.7 km on November 29, 1991 at Tsukuba and 22.5 at 25.9 km on December 4, 1991 at Naha respectively. The Lauder measurements started in November 1992, too late to pick up the Pinatubo maximum. The maximum values of integrated backscattering coefficient (IBC) above the tropopause were 5.4×10−3 sr−1 over Naha on November 18, 1991 and 7.1×10−3 sr−1 over Tsukuba on February 22, 1992 respectively. The IBC over Tsukuba showed clear seasonal variation with a maximum in winter and early spring and a minimum in summer. The IBC over Tsukuba in winter of 92/93 decreased only about 26 percent compared with the IBC in 91/92 winter, but the IBC in winter of 93/94 decreased largely. The IBC over Lauder was significantly larger than those over Tsukuba and Naha from June 1993 to August 1993.
Windblown sand on Venus: The effect of high atmospheric density
Authors: Williams Steven H. , Greeley Ronald
Published: February 2004

The high density of the venusian atmosphere significantly affects aeolian saltation transport and may also allow rapid alterations in the radar backscatter cross‐section of the surface. Saltation flux experiments were conducted under terrestrial and simulated venusian conditions to assess the effect of atmospheric density on aeolian transport of sediment. Total lane flux, the vertical distribution of saltating particles, and the average horizontal component of particle speed were measured and used to estimate the volumetric concentration of particles above the surface. Results show that so many particles are set into motion when wind speeds exceed a critical value that mid‐air collisions are a common result, a condition termed “choked” saltation. When choking occurs, there is a reduction in the rate at which the total saltation flux increases with increasing wind speed at least in part due to a reduction in the mean horizontal speed of the particles. Choked saltation on Venus may result in a reduction of the local aeolian erosion rate, the obliteration of aeolian bedforms, and rapid alteration of surface radar backscatter characteristics.
The diurnal variation of hydrogen, nitrogen, and chlorine radicals: Implications for the heterogeneous production of HNO2
Authors: Salawitch R. J. , Wofsy S. C. , Wennberg P. O. , Cohen R. C. , Anderson J. G. , Fahey D. W. , Gao R. S. , Keim E. R. , Woodbridge E. L. , Stimpfle R. M. , Koplow J. P. , Kohn D. W. , Webster C. R. , May R. D. , Pfister L. , Gottlieb E. W. , Michelsen H. A. , Yue G. K. , Prather M. J. , Wilson J. C. , Brock C. A. , Jonsson H. H. , Dye J. E. , Baumgardner D. , Proffitt M. H. , Loewenstein M. , Podolske J. R. , Elkins J. W. , Dutton G. S. , Hintsa E. J. , Dessler A. E. , Weinstock E. M. , Kelly K. K. , Boering K. A. , Daube B. C. , Chan K. R. , Bowen S. W.
Published: February 2004

In situ measurements of hydrogen, nitrogen, and chlorine radicals obtained through sunrise and sunset in the lower stratosphere during SPADE are compared to results from a photochemical model constrained by observed concentrations of radical precursors and environmental conditions. Models allowing for heterogeneous hydrolysis of N2O5 on sulfate aerosols agree with measured concentrations of NO, NO2, and ClO throughout the day, but fail to account for high concentrations of OH and HO2 observed near sunrise and sunset. The morning burst of [OH] and [HO2] coincides with the rise of [NO] from photolysis of NO2, suggesting a new source of HOx that photolyzes in the near UV (350 to 400 nm) spectral region. A model that allows for the heterogeneous production of HNO2 results in an excellent simulation of the diurnal variations of [OH] and [HO2].
Stratospheric aerosol growth and HNO3 gas phase depletion from coupled HNO3 and water uptake by liquid particles
Authors: Carslaw K. S. , Luo B. P. , Clegg S. L. , Peter Th. , Brimblecombe P. , Crutzen P. J.
Published: February 2004

Aqueous sulphuric acid droplets, which constitute the background stratospheric aerosol, strongly absorb HNO3 and HCl under cold conditions. A thermodynamic model is used to predict partitioning of HNO3, HCl and H2O between gas and aerosol phases, and show that a 50‐fold increase in aerosol volume, observed in the Arctic stratosphere as temperature approached the frost point (188.9 K), can be explained in terms of uptake of HNO3 and H2O by liquid aerosols. Calculated degrees of saturation of the droplets with respect to solid hydrates, taking into account the reduction in vapour phase HNO3, suggest that the droplets remain liquid to the frost point. Near this temperature, they can yield larger aerosol volumes than would have been the case for solid NAT (HNO3•3H2O) particles. The depletion of gas phase HNO3 into enhanced volumes of liquid aerosols resulting from volcanic eruptions may hamper NAT formation.
Altimeter observations of the Peru‐Chile countercurrent
Authors: Strub P. Ted , Mesias Jorge M. , James Corinne
Published: February 2004

Data from Geosat and TOPEX altimeters are used to infer the structure of the Peru‐Chile Countercurrent, a jet that flows from at least as far north as 10°S (historical data suggests 7°S) to 35°–40°S, maintaining its position between approximately 100–300 km offshore. Although the annual mean current cannot be determined from altimeter observations, the nearly antisymmetric patterns in spring and fall, combined with historical observations, suggest that the countercurrent is poleward at most times and is maximum in spring and minimum in fall. Previous studies have linked the offshore countercurrent at 7°S to the Equatorial Undercurrent west of the Galapagos Islands, suggesting that the countercurrent is part of a continuous flow that extends from the western equatorial Pacific to the region off Chile between 35°–40°S.
Predictions of vertical uplift caused by changing polar ice volumes on a viscoelastic Earth
Authors: Wahr John , DaZhong Han , Trupin Andrew
Published: February 2004

Measurements of crustal uplift from bedrock around the edges of Antarctica or Greenland could help constrain the mass balance of those ice caps. Present‐day changes in ice could cause vertical displacement rates of several mm/yr around Antarctica and up to 10–15 mm/yr around Greenland. Horizontal displacement rates are likely to be about 1/3 the vertical rates. The viscoelastic response of the earth to past changes in ice could cause uplift rates that are several times larger. By measuring both gravity and vertical displacements, it is possible to remove the viscoelastic effects, so that the observations can be used to constrain present‐day thickness changes.
Equations of state of α, ϵ and liquid iron and Iron's melting curve—thermodynamic calculations
Authors: Chen Guangqing , Ahrens Thomas J.
Published: February 2004

The melting curve between ϵ and liquid iron (100GPa< P <300GPa) has been derived by computing Gibbs free energies at high pressures and high temperatures from equations of state of the α, ϵ and liquid phases. The most uncertainty lies in the equation of state (EOS) of the ϵ phase. By comparing the calculations to experimental data, the internal thermodynamic consistency of the three phases are examined. The best fits to the melting curves of Boehler [1993] and Williams et al. [1987] can be obtained with lower bulk moduli than determined by static compression. Using available equations of state of the iron phases, our calculations indicate that if sub‐solidus iron is of the ϵ phase, Boehler's melting curve is thermodynamically more consistent than Williams et al.'s. The problem is complicated by the possible existence of a new phase between the ϵ and the liquid fields.
The importance of new chemical sources for the hot oxygen geocorona
Authors: Gérard J. C. , Richards P. G. , Shematovich V. I. , Bisikalo D. V.
Published: February 2004

Exothermic reactions involving metastable neutrals and ions were recently proposed as sources of hot oxygen atoms in addition to the classical O2+ and NO+ dissociative recombination. The Boltzmann equations for thermal and nonthermal populations of O atoms are solved with a Monte Carlo stochastic simulation method. It is shown that the calculated energy distribution functions of O atoms are significantly in nonequilibrium in the transition region between the thermosphere and the exosphere. It is found that the inclusion of additional sources leads to stronger disturbances of the energy distribution function and, as a consequence, increases the nonthermal fraction of hot O atoms. The variation of the vertical distribution of hot O between solar maximum and minimum conditions is also evaluated and shows good agreement with the available experimental evidence.
Tidal influence on O(¹S) Airglow emission rate distributions at the geographic equator as observed by WINDII
Authors: Shepherd G. G. , McLandress C. , Solheim B. H.
Published: February 2004

WINDII, the Wind Imaging Interferometer on the Upper Atmosphere Research Satellite, observes winds, temperatures and emission rates in the upper mesosphere and thermosphere. In this paper we report on nighttime observations of the vertical distribution of the O(¹S) 557.7 nm emission near the geographic equator for March/April, 1993. The airglow volume emission rate distribution is found to be strongly dependent on local time. Beginning at dusk, an intense airglow emission layer descends from a mean altitude of 95 km, reaching 89 km by midnight after which the emission rapidly decays. Shortly after midnight it reappears weakly at a higher altitude and remains at this level as the emission rate gradually increases towards dawn. This strong local time dependence leads us to conclude that the effect is tidally driven. Comparison with the Forbes [1982a,b] model suggests that total density perturbations and changes in the atomic oxygen mixing ratio may the cause of the changes in emission rate distribution between dusk and midnight. The reappearance of the emission after midnight may be caused by downward winds bringing oxygen‐rich air from above.
Axisymmetry of mantle aspherical structures
Authors: Matyska C.
Published: February 2004

Recent seismic tomographic models have revealed broad, low velocity anomalies in the lower mantle beneath Africa and the central Pacific that correspond to the poles of the surface geotectonic reference network. We test the hypothesis that the tomographic models are axisymmetrical with respect to axes passing near the geotectonic poles. The symmetry is apparent at the bottom of the lower mantle as well as in the transition zone in the upper mantle. This supports the idea of massive flushing of cold upper mantle material into the bottom part of the lower mantle and points to a stabilizing role of the lower mantle megaplumes on whole mantle processes.
Is there truly a “hard” rock site in the Valley of Mexico?
Authors: Singh S. K. , Quaas R. , Ordaz M. , Mooser F. , Almora D. , Torres M. , Vásquez R.
Published: February 2004

To understand the cause of observed amplification of seismic waves even at hill‐zone sites in the Valley of Mexico, digital accelerographs have been installed at three especially chosen sites. Two of these sites, MADI and TEXC, located on hard Pleistocene lava (a few meters in thickness) overlying Oligocene andesites, were expected to be free of site effects. Analysis of the data recorded by these and other accelerographs during three moderate, shallow subduction zone events, however, shows significant amplification at MADI and TEXC between 0.2 and 0.6 Hz with respect to established attenuation relations. The cause of the amplification at hill‐zone sites in the Valley of Mexico, including MADI and TEXC, may be pervasive low S‐wave velocity in, and complex structure of, the upper layers of the volcanic rocks. If so, then there may not be a truly “hard” rock site in the valley.
A possible mechanism for combined chlorine and bromine catalyzed destruction of tropospheric ozone in the Arctic
Authors: Le Bras G. , Platt U.
Published: February 2004

Recent field measurements of BrO radical concentrations and estimates of ClO radical concentrations from field measurements of hydrocarbons have been used to calculate the ozone depletion rate in the Arctic boundary layer which occurs during springtime. Considering the ozone cycles involving the ClO/BrO reaction and the BrO self‐reaction the calculated ozone depletion rates, as high as 6 ppb/day, are comparable to the observations.
Static compression of ε‐FeSi and an evaluation of reduced silicon as a deep Earth constituent
Authors: Knittle Elise , Williams Quentin
Published: February 2004

The volume of ε‐FeSi has been measured to pressures of 50 GPa. No high pressure transformations are observed in this system, including after laser‐heating at high pressures. The bulk modulus of this phase is 209 (±6) GPa, with a pressure derivative of 3.5 (±0.4). This high bulk modulus will slightly elevate the thermochemically‐inferred pressure conditions at which chemical reactions between iron and mantle silicates should commence at mid‐mantle depths. Moreover, because of the relatively low pressure derivatives of the bulk moduli of FeSi alloys manifested both in our data and in previous shock results, silicon as the sole light alloying component of the outer core is unlikely to produce a sufficiently large change in the bulk sound speed of the outer core relative to pure iron liquid. Therefore, silicon is not likely to be the primary alloying component in the outer core, unless its effect on the elasticity of the outer core is fortuitously offset by other light alloying constituents: its role is probably that of a subsidiary constituent. Thus, silicon is unique (to date) among proposed major outer core alloying constituents in being potentially precludable on purely geophysical grounds.
Quantitative fault studies on the East Pacific Rise: A comparison of sonar imaging techniques
Authors: Cowie Patience A. , Malinverno Alberto , Ryan William B. F. , Edwards Margo H.
Published: February 2004

A synthesis is presented of published statistics on the lengths, scarp heights, and spacing of normal faults on the flanks of the East Pacific Rise (EPR) between 19°S and 21°N. Nine data sets are included which were collected at different times employing one of four different sonar mapping systems (the Scripps deep‐tow instrument, SeaMARC I, SeaMARC II, and GLORIA) with widely ranging data collection capabilities. The EPR in this region is morphologically quite similar, and in some cases the data sets were obtained from the same area of the seafloor. It is assumed, therefore, that data collection procedure and system resolution have had a greater influence on the fault statistics observed than varying tectonic processes. We try to account for these sampling effects such that quantitative comparisons of fault development at midocean ridges may then be made. We discuss the problem of identifying faults in sonar images and the errors associated with measurements of the relevant parameters. We also investigate the statistical properties of the fault populations and derive expressions for correcting observed values of the population means at finite resolution to obtain the corresponding values at infinite resolution. The theoretical predictions compare well with the results of a decimation procedure applied to real data sets to simulate diminishing resolution. We also discuss the effects that surveying geometry can have on the interpretation of fault lengths. The data synthesis is augmented by an analysis of the correlation between scarp height and fault length for areas near 3.5°S and 12°N on the EPR. The correlation, which we find to be linear, is used to estimate (1) the minimum operational fault target size for each of the four mapping systems, and (2) the topography due to faulting in an area where only fault lengths are known.