bluuebunny/crossref_metadata_2025 huggingface dataset:
Cl and Br atom concentrations during a surface boundary layer ozone depletion event in the Canadian High Arctic
Authors: Boudries Hacene , Bottenheim Jan W.
Published: September 2002

C2‐C5 hydrocarbons were measured in situ at Alert, Nunavut, Canada from April 14 to May 10, 1998 at a minimum frequency of 12 samples per day. During the sampling period, an event occurred where the O3 mixing ratio fell from about 15 ppbv to the instrumental detection limit of less than 1 ppbv in a timespan of about 24 hours. Mixing ratios of alkanes and ethyne decreased concurrently with the O3 decrease. Ancillary data suggested that this was essentially due to chemical reactions only, most probably involving halogen atoms. The hydrocarbon concentrations from the onset of O3 decrease until its virtually complete depletion are used in a first order kinetics analysis to calculate the concentrations of Cl and Br atoms that are required to explain these observations. The Cl atom concentration is calculated to be 7.5 × 104 molec.cm−3. Such a concentration is near the upper limit of earlier estimates, but cannot explain the observed decay of ethyne. Assuming the deficit to be due to bromine atom chemistry, the estimated Br atom concentration is 1.4 × 107 molec.cm−3. The ethyne mixing ratio continued to decrease after virtual total O3 depletion suggesting that Br atoms persisted when O3 was depleted. Hence it is postulated that the depletion of total gaseous mercury (TGM) which terminates upon virtual complete O3 depletion is due to a reaction between gaseous mercury and the BrO molecule.
Can deglaciation trigger earthquakes in N. America?
Authors: Wu Patrick , Johnston Paul
Published: September 2002

A spherical, self‐gravitating viscoelastic earth model is used to calculate the spatial and temporal evolution of the glacially induced lithospheric stress and fault stability in North America. The predicted onset time of a pulse of earthquake activities, the mode of failure and the magnitude of instability are investigated for three sites with different distance from the former ice margin. It is found that glacial unloading is able to trigger paleo‐earthquakes within the ice margin near Charlevoix (47.5°N, 70.1°W) and in Wabash Valley (38.5°N, 87°W) outside the ice margin. However, rebound stress decays away from the former ice margin, thus glacial unloading is unlikely to have triggered the large M8 earthquakes in New Madrid (36.6°N, 89.5°W).
Why is the cloud albedo — Particle size relationship different in optically thick and optically thin clouds?
Authors: Lohmann Ulrike , Tselioudis George , Tyler Chris
Published: September 2002

Recent studies have analyzed satellite data in terms of the relationship of cloud albedo with droplet size for warm clouds. It was found that for optically thick marine clouds (τ > 15) the cloud albedo increases with decreasing cloud droplet effective radius (re). For optically thinner marine clouds (τ < 15) cloud albedo increases with increasing re as to be expected if the liquid water content is adiabatic. Hypotheses for the change in sign in the τ ‐ re relationship are deviations from an adiabatic liquid water content or the presence of single layer versus multi layer clouds. In this study, the ECHAM model, which exhibits this sign change in the τ ‐ re correlation for optically thin and thick marine clouds, is used to test these hypotheses. Probability density functions of τ ‐ re show that the change in sign of the correlation can be attributed to precipitating versus non‐precipitating clouds, but not to the difference in single layer versus multi‐layer clouds.
Seasonal formation of nitrous oxide laminae in the mid and low latitude stratosphere
Authors: Orsolini Yvan J. , Grant William B.
Published: September 2002

The seasonal and global distributions of laminae in nitrous oxide (N2O), a long‐lived tropospheric source gas, are examined in a multi‐year, three‐dimensional model simulation. The frequency of laminae occurrences is described as a function of geographical location and season. Enhanced laminae occurrence is found in the midlatitude surf zones of both hemispheres, but there are zonal asymmetries, especially in the northern hemisphere. In boreal winter and spring, laminae intruding from mid‐latitudes into the deep tropics are found across the Pacific Ocean, on the southern flank of the Aleutian High. A “quiet zone”, free of laminae, lies on the summer side of the equator, and separates the winter from the summer stratosphere. It coincides in latitude with the location of the N2O low‐latitude maximum. High laminae occurrences are again found in the extratropics of the summer hemisphere.
Stratospheric CO at tropical and mid‐latitudes: ATMOS measurements and photochemical steady‐state model calculations
Authors: Rinsland C. P. , Salawitch R. J. , Osterman G. B. , Irion F. W. , Sen B. , Zander R. , Mahieu E. , Gunson M. R.
Published: September 2002

We characterize the spring and fall stratospheric distribution of CO at 49°N‐55°S latitude from ATMOS profiles measured during 4 shuttle flights. Measured mixing ratios increase with potential temperature (θ) from 12 ppbv (10−9 per unit volume) at 525 K, to 30–40 ppbv at 1750 K with only minor variations with latitude and season at a θ level. Evidence for some confinement near 1150 K in the developing November 1994 vortex is indicated from comparison of CO and N2O horizontal gradients. Measured CO mixing ratios at the tropical tropopause are a factor of 10 higher than values calculated with a steady‐state model using standard photochemistry constrained by correlative temperatures and pressures, and ATMOS measurements including CH4 as inputs. Differences decrease with latitude at constant θ and are <20% at 800 K and all latitudes, where the CO photochemical lifetime is 40–50 days.
Rupture history of the 1997 Cariaco, Venezuela, earthquake from teleseismic P waves
Authors: Mendoza C.
Published: September 2002

A two‐step finite‐fault waveform inversion scheme is applied to the broadband teleseismic P waves recorded for the strike‐slip, Cariaco, Venezuela, earthquake of 9 July 1997 to recover the distribution of mainshock slip. The earthquake is first analyzed using a long narrow fault with a maximum rise time of 20 sec. This line‐source analysis indicates that slip propagated to the west with a constant rupture velocity and a relatively short rise time. The results are then used to constrain a second inversion of the P waveforms using a 60‐km by 20‐km two‐dimensional fault. The rupture shows a zone of large slip (1.3‐m peak) near the hypocenter and a second, broader source extending updip and to the west at depths shallower than 5 km. The second source has a peak slip of 2.1 meters and accounts for most of the moment of 1.1 × 1026 dyne‐cm (6.6 Mww) estimated from the P waves. The inferred rupture pattern is consistent with macroseismic effects observed in the epicentral area.
Lower stratospheric water vapor measured by UARS MLS
Authors: Pumphrey Hugh C. , Clark Hannah L. , Harwood Robert S.
Published: September 2002

We present zonal mean time series of water vapor in the lower stratosphere as measured by UARS MLS. The data are taken from a development version known as V104; we demonstrate that this version behaves in a physically realistic manner at 100 and 68 hPa, lower altitudes than are useful in previous versions of the data. The 100 hPa data show that the annual cycle in tropical lower stratosphere water vapor is driven by the northern hemisphere. Both low and high mixing ratios spread from the northern hemisphere tropics into the southern hemisphere tropics and into the mid‐latitudes with transport out of the tropics being greater at 100 hPa than at 68 hPa. Transport into mid‐latitudes is greater in the northern hemisphere and greater in spring than in fall. At 46 hPa, the tropical annual cycle is strongly confined to low latitudes.
Model of gamma ray flashes due to fractal lightning
Authors: Milikh G. , Valdivia J. A.
Published: September 2002

A model of γ‐ray flashes is presented. The model relies upon a horizontal fractal lightning discharge, which generates the electromagnetic pulses that produce the stochastic electron runaway discharge in the stratosphere. The model computes the flux and energy spectrum of the γ‐rays generated by the runaway discharge. The results of the computations are compared with the GRO observations after including the γ‐rays attenuation in the atmosphere.
Mechanism of broad spectrum of variabilities in the tropics
Authors: Rameshan K. , Goswami P.
Published: September 2002

The hierarchy of temporal organizations that reveals itself as a broad spectrum of quasi‐periodic oscillations plays important roles in dynamics and predictability in the tropics. We show here that this broad spectrum of oscillations is a response of the tropical circulation to convective internal dynamics modulated by Sea Surface Temperature (SST) and mean conditions. We explore this scenario through a model describing the horizontal dynamics of the first baroclinic mode on an equatorial β‐plane. The highlight of the formulation is inclusion of a convective relaxation time scale that governs the convective heating. The initial fields, SST and the mean wind are obtained from COADS. The model exhibits periodicities at several observed time scales and many of their spatio‐temporal features. The present study can thus help to formulate appropriate convective schemes in large‐scale models.
Radiative forcing by tropospheric ozone: Impact of cloud representation
Authors: Roelofs Geert‐Jan
Published: September 2002

We investigate the effect of clouds on the radiative forcing by tropospheric ozone increases between the pre‐industrial and the present time, using the general circulation model ECHAM4. The results indicate that clouds attenuate the longwave forcing for clear sky conditions by about 20%, whereas it approximately doubles the shortwave forcing. The estimated total clear‐sky and cloudy‐sky forcings are 0.41 and 0.37 W m−2, respectively. Analysis of reported forcing estimates shows that the magnitude of the longwave cloud effect is 20–30% of the clear‐sky longwave forcing. On the other hand, the shortwave cloud effect appears to depend largely on characteristics of the applied cloud distribution and the way clouds are treated in radiative transfer models, and varies between 30–250% of the clear‐sky shortwave forcing. Hence, present estimates of the cloud effect range from zero to as high as 0.1 W m−2.
Observation of Topside Ionospheric MF/HF Radio Emission from Space
Authors: Bale S. D.
Published: September 2002

We present observations of MF/HF ionospheric radio emission observed between 4–7 Re in the terrestrial magnetosphere by the Wind spacecraft. During a perigee pass, and AKR episode, two distinct radio signatures are observed: an intense, sporadic emission near f ≈ 1.8 MHz, and a more steady and weaker emission near f ≈ 4.4 MHz. These emissions show characteristics similar to ground observations of ionospheric ‘auroral roar’, although to propagate to the spacecraft they must have been generated above the ionospheric F‐layer peak. We suggest that the emissions are generated on the topside ionosphere where the condition fuh ≈ nfce is met. These observations represent a new component of the natural terrestrial radio spectrum as viewed from space.
Nitrogen dry deposition at an AmeriFlux site in a hardwood forest in the midwest
Authors: Pryor S. C. , Barthelmie R. J. , Jensen B.
Published: September 2002

This paper presents observations of ammonia, nitric acid and size segregated chemically speciated aerosols above the canopy of a secondary successional deciduous forest in Indiana during spring. Average nitric acid concentrations range between 0.1–1 µg m−3 and vertical gradients from 1 to 9 ng m−3 / m. The ammonia probability distribution indicates an average concentration of 0.8 µg m−3, although concentrations in excess of 2 µg m−3 are observed. Aerosol measurements indicate ammonium is largely confined to the accumulation mode (0.1 < Dp < 1.0 µm) with a mean concentration of 1 µg m−3. Nitrate exhibits a bi‐modal distribution (with mass in both the accumulation and coarse (Dp > 1.0 µm) modes), and is present at mean concentrations of 1.2 µg m−3. Average dry deposition fluxes calculated from these measurements are: Nitric acid = 0.38 mg‐N m−2 dy−1, ammonia = 1.07 mg‐N m−2 dy−1, aerosol‐nitrate = 0.27 mg‐N m−2 dy−1, aerosol‐ammonium = 0.25 mg‐N m−2 dy−1. The total dry deposition nitrogen flux of over 1.9 mg‐N m−2 dy−1 is approximately equal to the flux in precipitation as measured at NADP sites in the region.
An estimate of the uptake of atmospheric methyl bromide by agricultural soils
Authors: Varner Ruth K. , Crill Patrick M. , Talbot Robert W. , Shorter Joanne H.
Published: September 2002

Published estimates of removal of atmospheric methyl bromide (CH3Br) by agricultural soils are 2.7 Gg yr−1 (Gg = 109 g) [Shorter et al., 1995] and 65.8 Gg yr−1 [Serça et al., 1998]. The Serça et al. estimate, if correct, would suggest that the current value for total removal of atmospheric CH3Br by all sinks of 206 Gg yr−1 (based on Shorter et al., 1995) would be 30% too low. We have calculated a new rate of global agricultural soil uptake of atmospheric CH3Br from a larger sampling of cultivated soils collected from 40 sites located in the United States, Costa Rica, and Germany. First order reaction rates were measured during static laboratory incubations. These data were combined with uptake measurements we reported earlier based on field and laboratory experiments [Shorter et al. 1995]. Tropical (10.2°–10.4°N) and northern (45°–61°N) soils averaged lower reaction rate constants than temperate soils probably due to differing physical and chemical characteristics as well as microbial populations. Our revised global estimate for the uptake of ambient CH3Br by cultivated soils is 7.47±0.63 Gg yr−1, almost three times the value that we reported in 1995.
Propagation and mechanism of decadal upper‐ocean variability in the North Pacific
Authors: Liu Zhengyu , Zhang Rong‐Hua
Published: September 2002

Two types of anomaly pathways associated with decadal thermocline variability in the North Pacific Ocean are identified on constant density surfaces (isopycnals): one westward and the other southwestward around the subtropical gyre. A theory, based on two buroclinic planetary wave modes in the thermocline, is proposed to account for the observed variability pattern. The variability along the westward wave path is ascribed to the non‐Doppler‐shift 1st mode and seems to be forced mainly by wind stress forcing; the anomaly along the southwestward subduction path is associated with the advective nature of the 2nd mode and could be excited mainly by surface buoyancy anomalies. This study may provide a dynamically unified, three‐dimensional picture of decadal thermocline variability in the North Pacific Ocean.
Spring 1996 and 1997 ozonesonde measurements over McMurdo Station, Antarctica
Authors: Nardi Bruno , Bellon Willard , Oolman Larry D. , Deshler Terry
Published: September 2002

Ozone and temperature profiles were measured with balloon‐borne instruments from McMurdo Station, Antarctica (77.85°S, 166.67°E) from late August to late October, on 38 occasions in 1996 and on 26 occasions in 1997. Minimum column ozone values of 147 DU and 141 DU were measured in 1996 and 1997, reduced from initial measurements in late August of 277 DU and 313 DU, respectively. These ozone minima are slightly higher than the lowest seasonal minimum detected in 1993, and are similar to observations of 1994, 1995. The observed recoveries during 1996, and especially 1997, as indicated by increasing temperatures and ozone above 20 km as the polar vortex became unstable, were among the weakest measured since 1986 with low ozone extending into late October. We continue to observe in both 1996 and 1997 almost complete ozone depletion over a layer more than 4 km thick and centered near 17 km, similar to the first such observation over McMurdo in 1995. The maximum observed thickness of this layer has remained relatively constant since 1995.
Unified structure in Quaternary climate
Authors: Gauthier John H.
Published: September 2002

The Quaternary climate record exhibits a structure of superimposed, aperiodic oscillations starting at the 11‐yr sunspot cycle and spaced by powers of 2 in period through the major 90,000‐yr glacial cycle. Climate cycles that do not fall in this structure typically correspond to harmonics of the structure oscillations. The inclusion of the known solar cycles and the presence of increased abundances of cosmogenic radionuclides at many structure periods suggest that the structure is related to long‐period solar variability.
Auroral‐plasma sheet electron anisotropy
Authors: Kletzing C. A. , Scudder J. D.
Published: September 2002

Observations from 90 passes through the high‐latitude extension of the Earth's plasma sheet made by the Hydra particle spectrometer on Polar show clear, repeatable structure in the electron pitch angle anisotropy. Two distinct regions are seen. The first region begins at the poleward edge of the plasma sheet and consists of alternating regions of field‐aligned electron beams moving away from the ionosphere and loss‐cone distributions indicative of electron acceleration and precipitation below the spacecraft. The second region begins equatorward of the first region and is characterized by a higher temperature loss‐cone distribution superimposed on lower temperature, field aligned population. With decreasing latitude, the boundary in energy between these two populations in the second region decreases until only the trapped, loss‐cone population remains. The two regions are identified as connecting to the discrete and diffuse auroral regions, respectively, and it is shown that the observed pitch angle anisotropies can be explained as arising from auroral processes below the spacecraft.
Hydrogen‐enhanced electrical conductivity of diopside crystals
Authors: Wang Zichao , Ji Shaocheng , Dresen Georg
Published: September 2002

Dehydration and electrical conductivity (σ) experiments were performed on diopsides from India (In), Sri Lanka (Sri) and Russia (Ru) to study the effect of hydrogen (H) on σ. Dehydration showed that hydrogen concentrations (CH) of In‐ and Sri‐diopsides remained constant (∼50 and ∼900 H/106 Si), and CH in Ru‐diopside decreased continuously from 1000 to 100 H/106 Si within 72 h. σ's were measured at T = 1193–1473 K, P = 0.1 MPa and fO2 = 10−9 MPa. Different changes in σ with time were observed: the σ of Ru‐diopside was reduced by one order of magnitude within 72 h as crystals dehydrated; σs of In‐ and Sri‐diopsides remained time‐independent. Sri‐diopside shows the highest σ corresponding to its highest CH among the three crystals. Our results suggest an important effect of H on the σ of diopsides. We interpret this as a result from the fast diffusion of H incorporated into diopside. Also the concentration and mobility of intrinsic charged defects may increase due to the presence of H.
Paleomagnetic constraints on the tectonic relationship between the Alashan/Hexi Corridor Terrane and the North China Block
Authors: Huang B. C. , Otofuji Y. , Yang Z. Y.
Published: September 2002

We report paleomagnetic results for Early‐Middle Ordovician limestone rocks from the eastern part of the Hexi Corridor, northwest China. The characteristic high temperature component B isolated from 74 out of 114 samples exhibited dual polarity and passed the fold and reversal tests at the 95% confidence level. The corresponding paleopole, at 326.5°E, 31.8°N with A95=9.5°, is in agreement with the coeval poles from the stable North China Block (NCB). It implies that the Alashan/Hexi Corridor (AHC) terrane was part of the NCB since the Early‐Middle Ordovician, which is consistent with geological evidence. The Silurian and Devonian paleomagnetic results from the AHC terrane may therefore be used to discuss the middle Paleozoic tectonic displacement and paleogeographic reconstruction of the NCB.
Sources and transport of the Deep Western Boundary Current East of the Kerguelen Plateau
Authors: Donohue Kathleen A. , Hufford Gwyneth E. , McCartney Michael S.
Published: September 2002

East of the Kerguelen Plateau, a deep western boundary current in the Australian‐Antarctic Basin brings cold dense waters north from the margins of Antarctica. Geostrophic velocities referenced to acoustic Doppler current profiler data, both shipboard and lowered, suggest the flow is unidirectional throughout the water column with estimated northwestward transport below potential temperature 1°C of 28 ± 7 × 106 m³s−1 and 49 ± 9 × 106 m³s−1 respectively. Hydrographie and acoustic Doppler current profiler data show that the deep boundary current is supplied by northward flow emerging from the confluence of westward flow along the Antarctic continental slope and eastward flow of Weddell Basin waters through the Princess Elizabeth Trough to the south of the Kerguelen Plateau.
Validation of ILAS Version 3.10 ozone with ozonesonde measurements
Authors: Sasano Y. , Nakajima H. , Kanzawa H. , Suzuki M. , Yokota T. , Nakane H. , Gernandt H. , Schmidt A. , Herber A. , Yushkov V. , Dorokhov V. , Deshler T.
Published: September 2002

Ozone (O3) measurements made with the Improved Limb Atmospheric Spectrometer (ILAS) onboard the Advanced Earth Observing Satellite (ADEOS) were validated with correlative ozonesonde measurements conducted at five stations, Andoya, Kiruna and Yakutsk in the Northern Hemisphere, and Neumayer and Syowa in the Southern Hemisphere. The ILAS Version 3.10 O3 vertical profiles were compared with 79 correlative ozonesonde measurements that were made within 500 km and 3 hours in distance and time differences, respectively. The comparisons indicate that ILAS O3 typically has an accuracy within 20% between 12 and 35 km. The precision of the ILAS O3 is estimated to be ±10–25% between 12 and 20 km, ±5–7% between 20 and 30 km, and ±5% between 30 and 40 km.
Seasonal and IMF By effect on the polar cap convection orientation
Authors: Jayachandran P. T. , MacDougall J. W.
Published: September 2002

Seasonal and IMF By effect on the polar cap convection orientation is studied using the Canadian Advanced Digital Ionosondes (CADI) situated at Eureka (CGM lat. 88.67°N) and Resolute Bay (CGM lat. 83.55°N), two polar cap stations. In this study, we determined the daily convection orientation (MLT towards which the convection is directed) for each day of 1995. The orientation shows clear seasonal variation, being latest in winter and earliest in summer (strong northward IMF conditions are excluded from the study). The convection pattern is rotated clockwise by ∼ 4hrs from winter to summer. This includes rotation due to the curvature of the flow, ∼2 hours during summer months. The convection pattern also shows an IMF By dependence. The orientation is earlier by about 2 ½ hours when IMF By is positive relative to when it is negative. The observed seasonal changes in the convection orientation can be broadly explained by the changes in the ratio of the Region 1 and Region 2 field aligned currents.
Impact of nitric acid on ice evaporation rates
Authors: Warshawsky Matthew S. , Zondlo Mark A. , Tolbert Margaret A.
Published: September 2002

Recent studies have suggested that nitric acid uptake by ice clouds may decrease ice evaporation rates and thereby prolong the cloud lifetimes. To test this suggestion, ice desorption rates were studied as a function of HNO3 partial pressure (10−6–10−5 Torr), relative humidity (28–92%), and temperature (192–204 K) using optical interference of a helium neon laser. Ice evaporation rates in the presence of 1 × 10−6 Torr HNO3 were indistinguishable from those of pure ice. In contrast, ice evaporation in the presence of 8 × 10−6 Torr HNO3 resulted in lower evaporation rates by 33% relative to pure ice. Higher partial pressures of HNO3 result in a supercooled H2O/HNO3 liquid layer over ice, which may freeze to form a sealed NAT coating. This causes a lowering of the ice evaporation rate and prolongs the lifetime of ice. Ice exposed to lower partial pressures of HNO3 will not form a liquid layer and will thus evaporate at the same rate as pure ice.
ELF sub‐ionospheric pulse in time domain
Authors: Nickolaenko Alexander P. , Hayakawa Masashi , Kudintseva Irina G. , Myand Stanislav V. , Rabinowicz Leonid M.
Published: September 2002

We present an analytical time domain solution for ELF electromagnetic waves propagating in the Earth‐ionosphere cavity. The workability of the formula obtained is demonstrated by computations of the pulse waveforms originating from a point vertical electric source.
The vertical distribution of ClO at Ny‐Ålesund during March 1997
Authors: Ruhnke R. , Kouker W. , Reddmann Th. , Berg H. , Hochschild G. , Kopp G. , Krupa R. , Kuntz M.
Published: September 2002

Results of the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA) are compared with vertical ClO profiles measured by the ground‐based Millimeter Wave Radiometer MIRA2 inside the vortex during March 1997 at Ny‐Ålesund. The influence of the OH + ClO and HO2 + ClO reaction branching ratio and of the absorption cross section of Cl2O2 on the calculated mixing ratios of ClO and ozone has been investigated. In the upper stratosphere the ClO mixing ratio is reduced by 90% by using a minor channel of the OH + ClO reaction with a branching ratio of 0.07. A temperature dependent minor channel of the HO2 + ClO reaction reduces the upper stratospheric ClO mixing ratio by 22%. Different absorption spectra of Cl2O2 alter the ClO mixing ratios up to 12% at noon at 20 km. This causes differences of 15% in the ozone loss during winter.
INTERBALL tail observations of dayside magnetopause oscillations and simultaneous POLAR cusp measurements
Authors: Dempsey D. L. , Avanov L. A. , Waite J. H. , Vaisberg O. L. , Burch J. L. , Fuselier S. A. , Smirnov V. N. , Skalsky A. A.
Published: September 2002

On April 13, 1996, while Russia's Interball Tail spacecraft was travelling through the dayside magnetosheath, NASA's Polar spacecraft traversed the northern cusp. Observed transients seen in the SCA‐1 Interball Tail data are shown to be multiple crossings of the magnetopause. Analysis of the moments data from the SCA‐1 instrument and magnetic field data from the MIF instrument on Interball Tail show quasi‐periodic motion of the magnetopause. Simultaneous observations of changes in the He2+/H+ density ratio in the cusp by the Polar/TIMAS instrument show variations on the same time scale. We conclude that the variations in cusp data observed by Polar are the result of changes in the reconnection rate at the magnetopause and that these changes are associated with the magnetopause oscillations observed by Interball.
Heterogeneity and the earthquake magnitude‐frequency distribution
Authors: Steacy Sandra J. , McCloskey John
Published: September 2002

Regional earthquake populations are widely observed to have power‐law magnitude‐frequency statistics. Whether earthquake distributions on individual faults are similarly power‐law is presently the subject of considerable research effort. Recent careful observations suggest that event statistics on rough faults are power‐law whereas earthquake distributions on smooth faults are better described by a characteristic model in which there are more large earthquakes than would be expected by extrapolation from the distribution of small events. Here the relation between heterogeneity and the magnitude‐frequency distribution is investigated in a cellular automaton that includes new nearest‐neighbor stress transfer rules which produce realistic stress concentrations. The results agree with the suggestion that smooth faults have more characteristic earthquake distributions and support the view that an earthquake's size is controlled by rupture arrest.
The September 26, 1997 Colfiorito, Italy, earthquakes: Modeled coseismic surface displacement from SAR interferometry and GPS
Authors: Stramondo S. , Tesauro M. , Briole P. , Sansosti E. , Salvi S. , Lanari R. , Anzidei M. , Baldi P. , Fornaro G. , Avallone A. , Buongiorno M. F. , Franceschetti G. , Boschi E.
Published: September 2002

The largest events of the 1997 Umbria‐Marche seismic sequence were the two September 26 earthquakes of Mw = 5.7 (00:33 GMT) and Mw = 6.0 (09:40 GMT), which caused severe damage and ground cracks in a wide area around the epicenters. We created an ERS‐SAR differential interferogram, where nine fringes are visible in and around the Colfiorito basin, corresponding to 25 cm of coseismic surface displacement. GPS data show a maximum horizontal displacement of (14±1.8) cm and a maximum subsidence of (24±3) cm. We used these geodetic data and the seismological parameters to estimate geometry and slip distribution on the fault planes. Modeled fault depths and maximum slip amplitudes are 6.5 km and 47 cm for the first event and 7 km and 72 cm for the second one, in good agreement with those derived from the seismological data.
Constraints on the shallow seismic structure at Ocean Drilling Program Site 1107, Ninetyeast Ridge, from implosive bottom sources and airgun shots
Authors: Grevemeyer Ingo , Flueh Ernst R. , Herber Rolf , Villinger Heinrich
Published: September 2002

On Ninetyeast Ridge the seismological community drilled Ocean Drilling Program (ODP) Site 1107, coincident with ODP Site 757, for the future host of a long‐term seismological observatory in the Central Indian Ocean. To study the shallow seismic structure at the drill site, we developed a unusual on‐bottom seismic tool. Five implosive sources were placed along a 2‐km line. Four ocean bottom sensors recorded seismic waves over epicentral distances of 400 m to 2200 m, revealing seismic velocities of the immediately subjacent sediments. Velocities within this portion of crust are generally not detectable by traditional marine seismic techniques. Airgun shots, however, can be used to determine velocities of basement rocks. One‐dimensional travel‐time inversion of both bottom sources and water path corrected surface shots yields a velocity structure which is in excellent agreement with ODP drilling results. Three lithological units are imaged. A 150 m thick sediment section with velocities of 1600–1720 m/s. Underneath, a layer of 230 m thickness with velocities ranging from 2080 to 2250 m/s, corresponding to volcanic ashes and tuffs. The volcanic basement occured at 380 m below seafloor. Velocities rapidly increase from 3500 m/s to values of more than 4500 m/s within a few hundred metres.
Micromagnetic calculation of hysteresis as a function of temperature in pseudo‐single domain magnetite
Authors: Muxworthy A. R. , Williams W.
Published: September 2002

Using an unconstrained three‐dimensional micromagnetic model, magnetic hysteresis was simulated in magnetite in the grain size range 0.08–0.3 µm as a function of temperature between 100 K and the Curie temperature (851 K). For all temperatures in the cubic phase above the Verwey transition (Tυ ≈ 120 K), 0.08 µm grains switched by coherent rotation, and 0.3 µm grains switched with an intermediate vortex phase. The mode of switching for 0.1 µm grains was found to be dependent on both field direction and temperature. For all grain sizes in the monoclinic phase below Tυ, switching was by coherent rotation, with the domain state rotating in the monoclinic magnetocrystalline anisotropy “easy” a‐plane. Calculations for coercive force (Hc) as a function of temperature display similar trends to experimental data. Below Tυ, the model correctly predicts the large increase in Hc. However, the absolute values of Hc are higher than the experimental data.
Monthly and fortnightly tidal variations of the Earth's rotation rate predicted by a TOPEX/POSEIDON Empirical Ocean Tide Model
Authors: Desai Shailen D. , Wahr John M.
Published: September 2002

Empirical models of the two largest constituents of the long‐period ocean tides, the monthly and the fortnightly constituents, are estimated from repeat cycles 10 to 210 of the TOPEX/POSEIDON (T/P) mission. These models are first validated with tide gauge observations which show residual variances with the T/P‐derived models that are of the order of 2–3 mm² for both the monthly and fortnightly constituents, and which are smaller than residual variances from respective equilibrium models of these long‐period ocean tides. These empirical models are used to predict the contribution that they each have on tidal variations of the Earth's rotation rate. For tidal variations in UT1 these contributions are predicted to be of the order of 0.1 ms with a scatter of the order of 8 and 3 µs for the monthly and fortnightly ocean tides, respectively.
A 2D microphysical model of the polar stratospheric CN layer
Authors: Mills Michael J. , Toon Owen B. , Solomon Susan
Published: September 2002

Each spring a layer of small particles forms between 20 and 30 km altitude in the polar regions. We present the first self‐consistent explanation of the observed “CN layer” from a 2D microphysical model of sulfate aerosol. Our theory relies on photolysis of H2SO4 and SO3, consistent with recent laboratory measurements, to produce SO2 in the upper stratosphere and mesosphere. An additional source of SO2 may be required. Nucleation throughout the polar winter extends the top of the aerosol layer to higher altitudes, despite strong downward transport of ambient air. This may affect heterogeneous chemistry at the top of the aerosol layer in polar winter and spring.
Intercomparison of the influence of tropospheric clouds on UV‐visible absorptions Detected during the NDSC Intercomparison Campaign at OHP in June 1996
Authors: Pfeilsticker K. , Arlander D. W. , Burrows J. P. , Erle F. , Gil M. , Goutail F. , Hermans C. , Lambert J.‐C. , Platt U. , Pommereau J.‐P. , Richter A. , Sarkissian A. , Van Roozendael M. , Wagner T. , Winterrath T.
Published: September 2002

The influence of tropospheric clouds on zenith sky light (or brief ZSL‐DOAS) measurements of stratospheric gases is investigated. From a large set of intercomparison studies including six simultaneously operated UV/visible spectrometers, the zenith sky absorptions of O3, O4, NO2, and H2O are found to increase considerably under the investigated Cumulonimbus (Cb) cloud. The accuracy of the inferred visible O3 absorptions, however, are affected by interfering H2O absorptions. The increased cloudy sky absorptions are attributed to increased pathlengths due to multiple Mie scattering and hence interstitial gaseous absorptions inside the cloud. The absorptions detected for chemically inert gases like O4 (and H2O) are found to be inconsistent with those detected for NO2 and O3. This finding indicates that O3 and NO2 are modified by cloud related transport or chemical processes.
Upper‐mantle flow in eastern Europe
Authors: Dricker Ilya , Vinnik Lev , Roecker Steven , Makeyeva Larissa
Published: September 2002

We present and discuss estimates of shear‐wave splitting in the upper mantle of eastern Europe, based on SKS techniques. Estimates of the direction of polarization of the fast split wave α and the delay of the slow wave δt are obtained at seven stations: UZH, KOV and LVV, within and near the Carpathians, SIM, SEV and YAL in the Crimean peninsula, and KIEV in the Russian plate. Fast directions at all stations are around 130°, close to the values previously found in central Europe. In the Carpathians, similar values of δt are observed within the Pannonian depression, where the subcrustal lithosphere, if present, is very thin, and in the foredeep with a thick subcrustal lithosphere. This similarity suggests that the observed anisotropy is not inherited from earlier orogenies and preserved in the lithosphere. In the Crimea, the fast direction of anisotropy in the mantle is perpendicular to the strike of the Crimean mountains of late Jurassic‐Cretaceous age, contrary to the expected alignment of the fast direction with the strike of a collisional belt. In general, the data are consistent with a present‐day or recent large‐scale mantle flow in central and eastern Europe, parallel to the Alpine belt.
The structure of iron under the conditions of the Earth's inner core
Authors: Vočadlo Lidunka , Brodholt John , Alfè Dario , Price Geoffrey D. , Gillan Michael J.
Published: September 2002

The inferred density of the solid inner core indicates that it is predominantly made of iron. In order to interpret the observed seismic anisotropy and understand the high pressure and temperature behaviour of the core, it is essential to establish the crystal structure of iron under core conditions. On the basis of extrapolated experimental data, a number of candidate structures for the high P/T iron phase have been proposed, namely, body‐centred cubic (bcc), body‐centred tetragonal (bct), hexagonal close‐packed (hcp), double‐hexagonal close‐packed (dhcp) and an orthorhombically distorted hcp polymorph (Matsui, 1993; Stixrude and Cohen, 1995; Boehler, 1993; Saxena et al., 1996; Andrault et al., 1997). Here we present the results of the first fully ab initio free energy calculations for all of these polymorphs of iron at core pressures and temperatures. Our results show that hcp‐Fe is the most stable polymorph of iron under the conditions of the Earth's inner core.
Seasonal variation of middle latitudes wind fields of the mesopause region – A comparison between observation and model calculation
Authors: Kremp Christine , Berger Uwe , Hoffmann Peter , Keuer Dieter , Sonnemann Gerd R.
Published: September 2002

Mean MF‐radar prevailing mesospheric wind data over Juliusruh (54.6°N, 13.4°E) for 8 years (1990–1997) have been compared with daytime averaged winds obtained from calculations using the Cologne global three‐dimensional dynamic model of the middle atmosphere COMMA. The altitude‐seasonal cross section shows that the model reflects the seasonal zonal wind pattern fairly well. The mesospheric summer and especially the winter winds are somewhat weaker in the observations. The model results are better reflected by measurements over Saskatoon (52°N, 107°W). The height of the observed maximum of the summer jet occurs more than 5 km above that one inferred from the model calculations. The long‐term averages of the zonal winds will not be significantly changed if the periods with stratospheric warming events, which can lead to short term wind reversals at mesospheric heights during the winter months, are eliminated from the observational basis. The daytime averaged meridional wind component matches the observations over Juliusruh partly and agrees also with the published results obtained from measurements over Saskatoon.
Anthropogenic perturbations of tropospheric ion composition
Authors: Beig G. , Brasseur G.
Published: September 2002

This paper presents a simple model of the vertical distribution of the most abundant positive ions in the troposphere. The model is based on a simple ion chemical scheme, measured rate constants, and observed or calculated atmospheric densities of parent neutral compounds. It suggests that in many cases the most abundant gaseous ions are heavy protonated water cluster ions of pyridine, picoline and lutidine with ammonia molecules attached (pyridinated cluster ions) in the lower troposphere (below 7 km altitude), acetone cluster ions in the upper troposphere (between 7 and 15 km) and methyl cyanide cluster ions above 15 km. The relative abundance of heavy clustered aerosol ions is predicted to be highest near the surface. The model suggests that ion composition could be perturbed as a result of human activities (i.e., increasing emissions of parent neutral species including alkaline compounds), and that in particular, the relative atmospheric abundance of pyridinated water cluster ions is increasing dramatically and should further increase in the future.
Prediction of seasonal mean United States precipitation based on El Niño sea surface temperatures
Authors: Wang Hui , Ting Mingfang , Ji Ming
Published: September 2002

A method for seasonal predictions of U.S. precipitation based on tropical Pacific sea surface temperature (SST) anomalies is developed in this letter with the singular value decomposition (SVD). This method was applied to the 1997/98 El Niño. The 1997 summer and 1997/98 winter precipitation over the United States were predicted using forecast SST from the National Centers for Environmental Prediction (NCEP) coupled ocean‐atmosphere model. A cross validation, based on the percentages of hit between the hindcast and observed precipitation for the past El Niño and La Niña events, indicates a certain degree of predictability in both winter and summer seasons. During El Niño, significant predictability of summer precipitation is found over the Northern Plains and Atlantic States, while during La Niña it is in the Midwest. Above normal precipitation in the Northern Plains and the Midwest and below normal precipitation in the Atlantic States is associated with a warm phase of the El Niño‐Southern Oscillation (ENSO) in summer. For winter precipitation, significant predictability is detected over the Gulf Coast States, the Southern Plains and California. Over these regions, wetter conditions are generally associated with the warm phase of ENSO.
Measurements of the solar soft X‐ray irradiance from the Student Nitric Oxide Explorer
Authors: Bailey Scott M. , Woods Thomas N. , Barth Charles A. , Solomon Stanley C.
Published: September 2002

Beginning on March 11, 1998, the Student Nitric Oxide Explorer (SNOE) satellite has made daily observations of the solar soft x‐ray irradiance. These measurements are carried out by a multi‐channel photometer system utilizing x‐ray sensitive photodiodes with individual thin film filters. Current from the photodiodes is analyzed in terms of a solar reference spectrum in order to determine solar irradiances. The irradiances from 2 to 10 nm vary between 0.15 and 0.40 ergs cm−2 s−1 from March 11 to July 19, 1998 while the solar 10.7 cm fluxes varied between 85 and 140 × 10−22 W m−2 Hz−1. A 27‐day periodicity is observed with variations in the soft x‐ray irradiance on the order of a factor of two. The measured irradiances are shown to be correlated with the solar 10.7 cm flux but only mildly so with the GOES‐8 0.1 to 0.8 nm solar fluxes.
A comparison of the new filter ozonometer MICROTOPS II with Dobson and Brewer spectrometers at Hohenpeissenberg
Authors: Köhler U.
Published: September 2002

The recently developed filter ozonometer MICROTOPS II No. 3128 (MTops 3128) was purchased in October 1996. Almost two years of intensive comparative measurements with the regular spectrometers Dobson No. 104 (D104) and Brewer No. 10 (BR10) and intercomparison with the standard Dobsons No. 065 (D065) and 064 (D064) on selected days confirmed that the handy MTops 3128 is able to yield good total ozone measurements. The differences between the various instruments are mostly less than ±2%, increasing at air masses higher than 3.5 or at hazy sky conditions. Both instrumental types contribute to this increase with opposite effects. The handling during observation and data processing turned out to be easy with only few minor shortcomings. Replacement of the quartz cover didn't change the original MTops calibration significantly. Adverse effects of the filter aging or a calibration drift could not be detected over this 21‐months period.
Twilight observations suggest unknown sources of HOx
Authors: Wennberg P. O. , Salawitch R. J. , Donaldson D. J. , Hanisco T. F. , Lanzendorf E. J. , Perkins K. K. , Lloyd S. A. , Vaida V. , Gao R. S. , Hintsa E. J. , Cohen R. C. , Swartz W. H. , Kusterer T. L. , Anderson D. E.
Published: September 2002

Measurements of the concentrations of OH and HO2 (HOx) in the high‐latitude lower stratosphere imply the existence of unknown photolytic sources of HOx. The strength of the additional HOx source required to match the observations depends only weakly on solar zenith angle (SZA) for 80° < SZA < 93°. The wavelengths responsible for producing this HOx must be longer than 650 nm because the flux at shorter wavelengths is significantly attenuated at high SZA by scattering and absorption. Provided that the sources involve only a single photon, the strength of the bonds being broken must be < 45 kcal mole−1. We speculate that peroxynitric acid (HNO4) dissociates after excitation to an unknown excited state with an integrated band cross section of 2‐3 × 10−20 cm² molecule−1 nm (650 < λ < 1250 nm).
Melt distribution in mantle rocks deformed in shear
Authors: Zimmerman Mark E. , Zhang Shuqing , Kohlstedt David L. , Karato Shun‐ichiro
Published: September 2002

Shear experiments on olivine‐basalt aggregates provide compelling evidence that the dynamic distribution of melt is controlled by the magnitude and orientation of the differential stress. Our results suggest that deformed, partially molten upper mantle rocks will have highly anisotropic physical properties including seismic wave velocities and melt permeability. In addition, our results provide a basis for interpreting geophysical observations, such as shear‐wave splitting and for modeling melt migration processes beneath mid‐ocean ridges, specifically focused flow of melt toward the ridge axis.
Testing plausible upper‐mantle compositions using fine‐scale models of the 410‐km discontinuity
Authors: Gaherty James B. , Wang Yanbin , Jordan Thomas H. , Weidner Donald J.
Published: September 2002

We constructed models of the 410‐km discontinuity, in which the shape and width of the velocity and density increase were constrained by mineral physics data on the α‐β transition in (Mg,Fe)2SiO4. The transition was represented as cubic functions of depth, and its width was estimated to range between 8–24 km. Reflection coefficients were calculated for these models for competing estimates of the percentage of olivine in the mantle, using synthetic seismograms that include the finite‐frequency effects of the distributed transition. Comparing the synthetic reflectivities with the average reflectivity observed in previous analyses of ScS reverberations, we conclude that the mantle composition is close to that of pyrolite (55 vol. % olivine). Furthermore, much of the range of reflectivity can be explained by temperature variation in a pyrolite mantle. An olivine‐poor composition (35 vol. % olivine) marginally satisfies the seismic data only if the transition thickness averages less than 10 km.
Magnetospheric electric fields from ion data
Authors: Whipple E. C. , Starr D. L. , Halekas J. S. , Scudder J. D. , Holdaway R. D. , Faden J. B. , Puhl‐Quinn P. , Maynard N. C. , Russell C. T.
Published: September 2002

We have obtained the magnetospheric electric potential distribution along the path of Polar using ion data from Hydra and magnetic field data from MFE. We used the technique described by Whipple et al. [1998] to identify ion drift paths that intersected the spacecraft path at two different locations and times. The difference in energy of the ions at the two times, under certain restrictive assumptions, indicates the potential difference between the two locations. We did this for five minute intervals over 1.5 hours and summed these differences to obtain the potential variation along the spacecraft path. The results agree well with that obtained by the Polar Electric Field Instrument, and are consistent with a co‐rotation potential plus a small tailward field of magnitude 0.10 mV/m. This technique can complement and aid in the interpretation of other electric field data, or provide electric field data where such instruments are not available.