bluuebunny/crossref_metadata_2025 huggingface dataset:
Crater features diagnostic of oblique impacts: The size and position of the central peak
Authors: Ekholm Andreas G. , Melosh H. Jay
Published: February 2004

Using Magellan data, we investigated two crater characteristics that have been cited as diagnostic of oblique impacts: an uprange offset of the central peak in complex craters, and an increasing central peak diameter relative to crater diameter with decreasing impact angle. We find that the offset distribution is random and very similar to that for high‐angle impacts, and that there is no correlation between central peak diameter and impact angle. Accordingly, these two crater characteristics cannot be used to infer the impact angle or direction.
Freshwater fluxes through the Western Fram Strait
Authors: Meredith Michael , Heywood Karen , Dennis Paul , Goldson Laura , White Rowan , Fahrbach Eberhard , Schauer Ursula , Østerhus Svein
Published: February 2004

Two hydrographic and δ18O transects across Fram Strait (Aug–Sept 1997, 1998) are used to examine freshwater contributions to the East Greenland Current (EGC). The EGC featured up to ∼16% meteoric water in both years, but was made comparatively more saline through the formation of up to ∼11 m of sea ice. We derive meteoric water fluxes of ∼3680 km³yr−1 in Aug‐Sept 1997, and ∼2000 km³yr−1 in Aug–Sept 1998. The 1997 and 1998 data show a long‐term mean sea ice flux through Fram Strait around half the long‐term mean meteoric water flux. A 1991 δ18O section [Bauch et al., 1995] yielded a very similar ratio. Our 1998 section reveals fresh, low‐δ18O water on the East Greenland shelf whose comparatively large volume constitutes a potentially significant contribution to the total freshwater flux through Fram Strait. Such fluxes are important to the regional and global thermohaline circulation; we suggest that efforts towards monitoring both the EGC and East Greenland shelf waters are thus required.
Long‐term changes in the solar photosphere associated with changes in the coronal source flux
Authors: Foster S. , Lockwood M.
Published: February 2004

Using sunspot observations from Greenwich and Mount Wilson, we show that the latitudinal spread of sunspot groups has increased since 1874, in a manner that closely mirrors the long‐term (∼100 year) changes in the coronal source flux, Fs, as inferred from geomagnetic activity. This latitude spread is shown to be well correlated with the flux emergence rate required by the model of the coronal source flux variation by Solanki et al. [2000]. The time constant for the decay of this open flux is found to be 3.6±0.8 years. Using this value, and quantifying the photospheric flux emergence rate using the latitudinal spread of sunspot groups, the model reproduces the observed coronal source flux variation. The ratio of the 100‐year drift to the solar cycle amplitude for the flux emergence rate is found to be half of the same ratio for Fs.
Role of climate feedback on methane and ozone studied with a Coupled Ocean‐Atmosphere‐Chemistry Model
Authors: Johnson C. E. , Stevenson D. S. , Collins W. J. , Derwent R. G.
Published: February 2004

We present results from two experiments carried out with a coupled ocean‐atmosphere‐tropospheric chemistry model run continously over the period 1990 to 2100. In the control experiment, climate is unforced, but emissions of trace gases to the chemical model increase in line with an illustrative scenario for future trace gas emissions with medium high growth. In the climate change experiment trace gas emissions are identical to the control, but climate is also forced using greenhouse gas concentrations and SO2 emissions from the same scenario. Global average methane in the climate change experiment increased from 1670 ppbv in 1990 to 3230 ppbv by 2100, compared to 3650 ppbv by 2100 in the control. The methane increase in the control experiment is therefore 27% more than in the control. This difference is due to both temperature and OH changes which increase the rate of methane oxidation and act in the opposite direction to the negative feedback of methane on itself through OH. Mid‐latitude northern hemisphere ozone concentrations in July for the mid‐troposphere rose from 39 ppbv in 1990s to 64 ppbv in the 2090s in the control experiment and to 49 ppbv in the climate change experiment. The direct role of climate change is therefore predicted to be a negative feedback on the radiative forcing from the change to tropospheric ozone and methane concentrations.
Satellite observations of upwelling on the continental shelf south of Madagascar
Authors: DiMarco Steven F. , Chapman Piers , Nowlin Worth D.
Published: February 2004

We report on upwelling seen in satellite AVHRR sea surface temperature imagery over the continental slope and shelf of southern Madagascar during February and March 2000. The upwelling is concurrent with anomalously high pseudo wind‐stress over the region during this period. However, the western boundary East Madagascar Current, which is seen over the continental slope region, may contribute to the upwelling effect. The upwelling covers an area of 2° longitude by 1° latitude and at its peak is about 3–5°C cooler than the local ambient sea surface temperature. The paucity of in situ wind and current data in the region, however, prohibit a quantitative assessment of the relative forcing.
Positive feedback between future climate change and the carbon cycle
Authors: Friedlingstein Pierre , Bopp Laurent , Ciais Philippe , Dufresne Jean‐Louis , Fairhead Laurent , LeTreut Hervé , Monfray Patrick , Orr James
Published: February 2004

Future climate change due to increased atmospheric CO2 may affect land and ocean efficiency to absorb atmospheric CO2. Here, using climate and carbon three‐dimensional models forced by a 1% per year increase in atmospheric CO2, we show that there is a positive feedback between the climate system and the carbon cycle. Climate change reduces land and ocean uptake of CO2, respectively by 54% and 35% at 4 × CO2. This negative impact implies that for prescribed anthropogenic CO2 emissions, the atmospheric CO2 would be higher than the level reached if climate change does not affect the carbon cycle. We estimate the gain of this climate‐carbon cycle feedback to be 10% at 2 × CO2 and 20% at 4 × CO2. This translates into a 15% higher mean temperature increase.
Inversion studies of magnetic cloud structure at 0.7 AU: Solar cycle variation
Authors: Mulligan T. , Russell C. T. , Elliott D. , Gosling J. T. , Luhmann J. G.
Published: February 2004

Pioneer Venus observations from 1979–1988 have revealed the solar cycle evolution of magnetic clouds in the inner heliosphere. Non‐force‐free magnetic rope models, fit to 44 magnetic clouds, allow us to determine the structures of these clouds and compare their properties with potential field maps of their solar source regions. The leading magnetic polarity in these clouds is correlated with the polarity of the sun's global field. The axis of symmetry of the clouds varies with the inclination of the magnetic streamer belt. At 0.72 AU, the median radius of a magnetic flux rope is 0.10 AU. The median rope contains 11 TWb and carries 0.5 GAmps of parallel electric current. The occurrence frequency of different handedness of the ropes is equally left and right throughout the solar cycle and shows no correlation with other modeled parameters.
Tropospheric and stratospheric aspects of the Arctic oscillation
Authors: Kodera Kunihiko , Kuroda Yuhji
Published: February 2004

The role of the stratospheric variability in the Arctic Oscillation (AO) is investigated by selecting 18 large month‐to‐month variation events. It is shown that although tropospheric features are very similar, the vertical structure of the AO differs considerably depending on whether it is initiated by a downward propagation of zonal‐mean zonal wind anomalies from the stratosphere (type S) or it is produced within the troposphere (type T). Type S events preferentially occur during February and March, while type T events are seen during November and December. The results of the analysis suggest that diverse aspects of the AO‐related variability can more easily be understood as coupling and uncoupling of stratospheric and tropospheric modes of variability.
Seismic anisotropy beneath the Lau back‐arc basin inferred from sScS‐ScS splitting data
Authors: Iidaka Takashi , Niu Fenglin
Published: February 2004

We determine upper mantle anisotropy just beneath active basins by comparing ScS and sScS waveform splitting. We have measured sScS‐ScS shear‐wave splitting with data from the SPANET and GSN seismic networks. The observed sScS‐ScS shear‐wave splitting suggests that there is a large lateral variation of anisotropy beneath the Lau back‐arc basin. ScS‐ and sScS‐waves which sample the southern part of the Lau basin show small splitting. In contrast, the northern part of the Lau basin, which is characterized to be tectonically active, is observed to have large seismic anisotropy with a WNW‐ESE direction. The cause of anisotropy may be related to the mantle dynamics of the active basin.
Electromagnetic imaging of a transpressional tectonics in SW Iberia
Authors: Almeida Eugénio , Pous Jaume , Santos F. Monteiro , Fonseca Paulo , Marcuello Alex , Queralt Pilar , Nolasco Rita , Mendes‐Victor Luís
Published: February 2004

Forty‐one magnetotelluric soundings were carried out along a 200 km‐long profile (approximately NNE‐SSW) across the three major geotectonic units in SW Iberia. A model obtained from two‐dimensional inversion of the magnetotelluric data set reveals high conductivity zones in the middle‐lower crust (10–30 km). Two of these zones correspond to the transition between the main geotectonic units: one between the South Portuguese Zone and the Ossa Morena Zone, interpreted as having been caused by metasediments, and the other between the Ossa Morena Zone and the Central Iberia Zone associated with a shear zone and metasediments. Another high conductivity anomaly related to black shales with major graphite impregnation was detected within the Ossa Morena Zone. The resistive features, located preferentially in the upper crust (1–10 km), coincide with gabbroic and granitic complexes.
A linkage between polar patches and plasmaspheric drainage plumes
Authors: Su Yi‐Jiun , Thomsen Michelle F. , Borovsky Joseph E. , Foster John C.
Published: February 2004

Two distinct features are shown here to follow the same convection pattern: polar ionization patches and plasmaspheric drainage plumes. In this study, the enhanced ionospheric electron concentration, observed from Millstone Hill incoherent scatter radar, is mapped to the equatorial plane. The resulting trajectory there is found to coincide with the location of the draining plasmasphere as observed from a geosynchronous satellite. The result indicates that both the F‐region electron concentration and the plasmaspheric drainage plume follow the newest separatrix (the boundary between open and closed drift trajectories) convecting sunward. Observations have shown that the polar patches are transported over the polar cap into the night side. Hence, we conclude that the draining plasmasphere should follow a similar trajectory transporting plasmaspheric material over the polar cap into the tail plasma sheet.
Multipoint observations of substorm intensifications: The High‐latitude aurora and electron injections in the inner equatorial plasma sheet
Authors: Sandholt P. E. , Farrugia C. J.
Published: February 2004

In recent years, focus in substorm research has been directed to high speed jets of plasma traveling earthward in the magnetotail, called bursty bulk flows (BBFs) and a search for their ionospheric signature(s). One candidate ionospheric signature consists of episodic brightenings of the poleward branch of the aurora and associated forms propagating from the higher to the lower branch. This study extends previous efforts by employing ground‐based, optical observations of the high‐latitude (70°–80° magnetic latitude) aurora at high spatial and temporal resolution, and supplementing these by equatorial, near‐Earth (6 RE) particle measurements during a strong substorm. We describe the brightening and equatorward motion of the high latitude aurora and relate it in time and space to electron injections in the 5–20 keV range earthward of geostationary heights (6 events in 20 min). These observations are placed in the context of the observed 3‐phase poleward expansion of the aurora, and the corresponding magnetotail dynamics during the strong substorm under study.
Can the direct and semi‐direct aerosol effect compete with the indirect effect on a global scale?
Authors: Lohmann Ulrike , Feichter Johann
Published: February 2004

During the Indian Ocean Experiment (INDOEX) Ackerman et al. [2000] found that black carbon (BC) aerosols can reduce cloud cover and liquid water path (“semi‐direct” aerosol effect). Absorption of solar radiation by BC leads to a heating of the air which can result in an evaporation of cloud droplets. This warming can partially offset the cooling due to the indirect aerosol effect. We use the ECHAM4 general circulation model to investigate whether this effect is discernible on a global scale. The results of this sensitivity study, where the absorption of BC only takes place in clear air and interstitially, indicate that the zonal mean reduction in cloud cover and liquid water path in highly polluted regions of the Northern Hemisphere can amount to 4% and 10 g m−2, respectively. However, the indirect aerosol effects of increasing cloud lifetime and cloud albedo dominate.
Accumulation map for the Greenland Ice Sheet: 1971–1990
Authors: Bales Roger C. , McConnell Joseph R. , Mosley‐Thompson Ellen , Lamorey Gregg
Published: February 2004

An updated accumulation map for the Greenland Ice Sheet for the period 1971–1990, based on kriging of accurate point estimates developed during the past decade at over 40 points plus over 200 historical points, shows an average water accumulation of 30 g cm−2yr−1. Average uncertainty (standard deviation) in estimates of point accumulation for the interior part of the ice sheet, where the kriging drift surface is well constrained, is approximately 7 g cm−2yr−1, or 24% of mean accumulation. Because there are multiple cores in most regions, the regional uncertainty in accumulation should be considerably lower than the 7 g cm−2yr−1 average uncertainty at a point. There are still many areas on the ice sheet, including northwest and southern Greenland, where annual accumulation remains poorly constrained.
Impact of ocean dynamics on the simulation of the neoproterozoic “snowball Earth”
Authors: Poulsen Christopher J. , Pierrehumbert Raymond T. , Jacob Robert L.
Published: February 2004

A fully coupled ocean‐atmosphere general circulation model (the Fast Ocean‐Atmosphere Model) is used to simulate the Neoproterozoic climate with a reduced solar luminosity (95% of present‐day), low atmospheric CO2 (140 ppmv), and an idealized tropical supercontinent. Two coupled simulations were completed with present‐day and cold initial ocean temperatures. These experiments are compared with uncoupled (i.e., mixed‐layer) model experiments to determine the impact of a dynamical ocean on the Neoproterozoic simulations. In contrast to global sea‐ice coverage in the uncoupled experiments, the sea‐ice margin seasonally advances to 46 and 55° latitude in the coupled experiments. The coupled simulations demonstrate that dynamic ocean processes can prevent a snowball solution and suggest that a reduced solar luminosity and low atmospheric CO2 are not by themselves sufficient conditions for a snowball solution. Heat exchange through vertical mixing in the mid‐latitudes, caused by static instability, is identified as the primary process halting the advance of the sea‐ice margin.
Anomalies of mantle‐derived helium during the 1997–1998 seismic swarm of Umbria‐Marche, Italy
Authors: Italiano F. , Martinelli G. , Nuccio P. M.
Published: February 2004

More than 100 ³He/4He isotope ratios were measured in CO2‐dominated gas samples collected during the 1997–1998 seismic crisis that shook the Umbria‐Marche, Central Apennines, Italy. Although the gas emissions were located in a typical “crustal” environment the measured values delineate variable mixing proportions between mantle‐derived and crustal helium. During the monitoring period anomalous ³He/4He ratios were observed, together with a significant increase of fluid outflows. The observed ³He/4He anomalies appear to be driven by stress‐induced crustal deformations affecting micro‐fracturing and bulk permeability and, in turn, by a seismogenic‐induced modification of the crustal permeability of rocks. Since crustal deformations can also occur aseismically, the geochemical anomalies must be considered indicators of processes occurring below the surface, which could eventually help to estimate the probability increase of an impending earthquake, instead of pure empirical precursors.
Numerical simulation of asymmetric corona with multi‐streamer structures
Authors: Jingqun Li , Fengsi Wei , Xueshang Feng
Published: February 2004

A new procedure is proposed to generate planar asymmetric coronas with multi‐streamer structures, as shown in SOHO/LASCO observations. The initial coronal magnetic field is separated into potential part and non‐potential part. We first fit the potential part by using the sum of magnetic multi‐poles, whereas the non‐potential part is approximated by the magnetic field induced by some properly fitted current densities. Then the total field is numerically modified in terms of Maxwell's equations. Based on the initial magnetic fields obtained in such a way, we can further compute various complex asymmetric corona structures by solving MHD equations. In order to verify this procedure, we compute the 2‐D coronal structures prior to the December, 1996 CME and August, 1999 CME events. The numerical result is on the whole in agreement with observations, and hence set proper backgrounds for further studying the propagation of CME in various coronal structures using numerical computations.
Deceleration of the high‐latitude thermospheric wind by polar cap gravity waves
Authors: Innis J. L.
Published: February 2004

This letter explores the hypothesis that gravity wave drag may account for the observed deceleration of the thermospheric anti‐sunward wind near the nightside auroral oval, which at times has been seen to reduce to near zero in a horizontal distance of a few hundred kilometres. Other high‐latitude observations have revealed thermospheric gravity waves that appear to propagate across the polar cap from the nightside to the dayside, and hence are moving counter to the wind near magnetic midnight. Approximate constancy with altitude of the vertical velocity amplitudes of these waves suggests a moderate level of energy dissipation is present. Calculations indicate a wave‐mean flow interaction could lead to a deceleration of the wind of magnitude comparable to that observed.
The preferred structure of variability of the northern hemisphere atmospheric circulation
Authors: Monahan A. H. , Pandolfo L. , Fyfe J. C.
Published: February 2004

A nonlinear generalisation of Principal Component Analysis (PCA) is applied to the 500mb geopotential height field of the Northern Hemisphere extratropical atmosphere. It is found that the low‐frequency variability of the mid‐troposphere is characterised by three distinct quasi‐stationary states. The states are described and compared to those obtained from applications of cluster analyses and linear PCA to the height field. Evidence is provided that modes obtained through PCA (notably the Arctic Oscillation (AO)) are not independent dynamical modes of variability of the Northern Hemisphere extratropics. Rather they arise as the optimal linear compromise between the preferred quasi‐stationary states of the circulation.
A measurement of the quiet network contribution to solar irradiance variation
Authors: Foukal Peter , Milano Leo
Published: February 2004

A large increase in quiet network area since the 17th century Maunder Minimum has been suggested as a mechanism for increasing solar irradiance sufficiently to drive global warming. We show that this mechanism requires essentially complete disappearance of network proceeding back in time to the beginning of the 20th century. This disappearance is ruled out by the many Ca K spectroheliograms taken since the discovery of the network in the early 1890's. Furthermore, network area measurements we have carried out on Ca K spectroheliograms digitized from the Mt. Wilson and NSO/Sacramento Peak archives, for the nine solar activity minima between 1914 and 1996, show no evidence of network area variations large enough to produce a significant long‐term component of total irradiance variation. A network brightness variation of sufficient magnitude is also unlikely, given the linear dependence of solar microwave flux on area of bright structures.

More generally, recent analyses of cycle 21,22 pyrheliometry, and of broadband stellar photometry, provide little support for any long‐term irradiance component These results do not rule out a secular irradiance increase. But they suggest that high climate sensitivity to the relatively small changes in solar total and UV irradiance that have been observed, provides a more likely explanation of the global temperature‐solar activity correlation.
Thermal conductivity of sea ice; dependence on temperature and depth
Authors: Trodahl H. J. , Wilkinson S. O. F. , McGuinness M. J. , Haskell T. G.
Published: February 2004

We report an in situ experimental determination of the thermal conductivity of natural sea ice, a parameter required by models of climate and sea ice growth. The results obtained in the experiment differ in detail from the theoretically predicted values used by those models. In particular, we find that experimental results are lower than predicted, due to disorder in the crystalline structure of the ice. The reduction is nearly a factor of two near the top surface of the ice. Deeper in the ice, in contrast, heat flow is enhanced by a contribution from brine convection.
Coseismic and postseismic displacements related with the 1997 Earthquake Sequence in Umbria‐Marche (Central Italy)
Authors: Basili Roberto , Meghraoui Mustapha
Published: February 2004

We study the coseismic and postseismic displacements related with the 1997 Umbria‐Marche earthquake sequence by means of leveling lines along a deformed aqueduct located in the epicentral area. Comparing the 1960 and 10/1997 measurements we obtain 0.49±0.10 m of coseismic displacement distributed along 3 km across the normal fault zone. Modeling of the coseismic surface dislocation is obtained from a combination of low angle (38°) faults at depth and high angle (80°) upper fault branches. The best fit model indicates that the upper branches stop at 0.4 km below the ground surface and have 60% of slip with respect to the lower faults. The postseismic displacement measured during 1998 is 0.18 m and represents 36% of the apparent coseismic deformation. Moderate earthquakes in the Apennines and related surface deformation may thus result from curved faults that reflect the brittle‐elastic properties of the uppermost crustal structures.
Application of a three‐dimensional numerical model to Lake Vostok: An Antarctic subglacial lake
Authors: Williams Michael J. M.
Published: February 2004

The circulation in two water column thickness scenarios for Lake Vostok, a subglacial lake in central East Antarctica, is investigated using a three‐dimensional numerical model. In a scenario with a constant water column thickness (the distance between the sloping ice sheet which caps the lake and the lake bed) the circulation is barotropic, but with a more realistic water column thickness the flow is baroclinic. The different circulations result from the geothermal heat flux warming shallow regions of the realistic lake more efficiently, thus forming a different horizontal density structure. The differences and uncertainties between the two scenarios highlight the importance that the largely unknown water column thickness and geothermal heat flux have in determining the circulation in the lake.
The Moon and El Niño
Authors: Cerveny Randall S. , Shaffer John A.
Published: February 2004

Regional climates around the world display cycles corresponding to the 18.61‐year maximum lunar declination (MLD) periodicity. We suggest that these cycles are created by a relationship between MLD and El Niño / Southern Oscillation (ENSO). Both equatorial Pacific sea‐surface temperature and South Pacific atmospheric pressure significantly correlate with maximum lunar declination. Low MLDs are associated with warmer equatorial Pacific sea‐surface temperatures and negative values of the Southern Oscillation Index. A lunar‐influenced change in the Pacific gyre circulation presents a viable physical mechanism for explaining these relationships. We suggest that the gyre is enhanced by tidal forces under high MLDs, inducing cold‐water advection into the equatorial region but is restricted by the weak tidal forcing of low MLDs thereby favoring El Niño episodes. An astronomical model utilizing this relationship produces a forecast of increased non‐El Niño (either La Niña or neutral) activity for the early part of this decade.
Depth‐dependent attenuation in the uppermost inner core from the Taiwan short period seismic array PKP data
Authors: Tseng Tai‐Lin , Huang Bor‐Shouh , Chin Byau‐Heng
Published: February 2004

Inner core attenuation properties from two spots were examined in detail using PKIKP and PKP(BC) phases with a distance range of between 147° and 154° as recorded by both the short‐period regional seismic array in Taiwan and the broadband stations of the German Regional Seismic Network. The ray paths from the six South American events to Taiwan sampled the inner core region beneath the eastern part of the northern Pacific Ocean, while those from Fiji to Europe reflected the inner core boundary under the western part of the northern Pacific Ocean. Results of this study show the depth‐dependent attenuation of Qp beneath the eastern part of the northern Pacific Ocean from 241±21.8 (at depths <210 km) to 337±46.5 (at depths >270 km) with a weighted mean of 305±25.4 within the frequency band 0.5∼2.0Hz. The average Qp under the western part of the northern Pacific Ocean is 160±33.5 (0.5 ∼ 1.5Hz) which is consistent with that previously reported. It is found that Qp under the western part of the northern Pacific is clearly lower than that under the eastern part. This discrepancy might be related to regional variations in the uppermost inner core.
Measurements of an anomalous global methane increase during 1998
Authors: Dlugokencky E. J. , Walter B. P. , Masarie K. A. , Lang P. M. , Kasischke E. S.
Published: February 2004

Measurements of atmospheric methane from a globally distributed network of air sampling sites indicate that the globally averaged CH4 growth rate increased from an average of 3.9 ppb yr−1 during 1995–1997 to 12.7±0.6 ppb in 1998. The global growth rate then decreased to 2.6±0.6 ppb during 1999, indicating that the large increase in 1998 was not a return to the larger growth rates observed during the late‐1970s and early‐1980s. The increased growth rate during 1998 corresponds to an increase in the imbalance between CH4 sources and sinks equal to ∼24 Tg CH4, the largest perturbation observed in 16 years of measurements. We suggest that wetland and boreal biomass burning sources may have contributed to the anomaly. An adaptation of a global process‐based model, which included soil‐temperature and precipitation anomalies, was used to calculate emission anomalies of 11.6 Tg CH4 from wetlands north of 30°N and 13 Tg CH4 for tropical wetlands during 1998 compared to average emissions calculated for 1982–1993. For 1999, calculated wetland emission anomalies were negative for high northern latitudes and the tropics, contributing to the low growth rate observed in 1999. Also 1998 was a severe fire year in boreal regions where ∼1.3 × 105 km² of forest and peat land burned releasing an estimated 5.7 Tg CH4.
Crustal displacements due to continental water loading
Authors: van Dam T. , Wahr J. , Milly P. C. D. , Shmakin A. B. , Blewitt G. , Lavallée D. , Larson K. M.
Published: February 2004

The effects of long‐wavelength (>100 km), seasonal variability in continental water storage on vertical crustal motions are assessed. The modeled vertical displacements (ΔrM) have root‐mean‐square (RMS) values for 1994–1998 as large as 8 mm, with ranges up to 30 mm, and are predominantly annual in character. Regional strains are on the order of 20 nanostrain for tilt and 5 nanostrain for horizontal deformation. We compare ΔrM with observed Global Positioning System (GPS) heights (ΔrO) (which include adjustments to remove estimated effects of atmospheric pressure and annual tidal and non‐tidal ocean loading) for 147 globally distributed sites. When the ΔrO time series are adjusted by ΔrM, their variances are reduced, on average, by an amount equal to the variance of the ΔrM. Of the ΔrO time series exhibiting a strong annual signal, more than half are found to have an annual harmonic that is in phase and of comparable amplitude with the annual harmonic in the ΔrM. The ΔrM time series exhibit long‐period variations that could be mistaken for secular tectonic trends or postglacial rebound when observed over a time span of a few years.
Energetic oxygen ions of ionospheric origin observed in the cusp
Authors: Chen Jiasheng , Fritz Theodore A.
Published: February 2004

On 8/27/96 from 8 to 10:36 UT the POLAR satellite crossed the dayside cusp and observed very energetic ionospheric O ≤ +2 ions which had a magnetic moment spectrum, M(E) that followed a power law. Later at 14:45 to 15:30 UT the satellite passed through the ring current at 15–16 MLT where the O ≤ +2 ions again had a M(E) power law form but with a spectral amplitude that was two orders of magnitude lower. During this period from 8 to 16 UT there were no magnetic storms as Dst had an average value of ∼ −5 nT. The comparison of the fluxes at constant M means that the energetic ions in the cusp cannot have a source in the ring current. Arguments are given that the comparison of these sequential measurements of M(E) are valid and that a new mechanism is needed for producing these energetic ionospheric ions observed in the cusp.
Observations of rapid photochemical destruction of ozone in snowpack interstitial air
Authors: Peterson M. C. , Honrath R. E.
Published: February 2004

Measurements in central Greenland demonstrate that ozone is rapidly destroyed through a photochemical process in the surface snowpack. The rate of destruction is too high to result from reaction with NOx or HOx, but could result from catalytic destruction by bromine if its release from snowpack bromide is highly efficient. The pristine nature of the Greenland snowpack implies that ozone destruction may be widespread in illuminated snowpacks and thus influence the budget of boundary‐layer ozone. Similar destruction on tropospheric cloud ice crystals may explain observations of very low ozone levels associated with cirrus clouds.
Comparison of measured and modeled stratospheric UV/Visible actinic fluxes at large solar zenith angles
Authors: Bösch H. , Camy‐Peyret C. , Chipperfield M. , Fitzenberger R. , Harder H. , Schiller C. , Schneider M. , Trautmann T. , Pfeilsticker K.
Published: February 2004

Measured and modeled stratospheric filter sensitivity weighted ultraviolet/visible (UV/vis) actinic fluxes—approximating the NO2 photolysis rate coefficients (jNO2)— are compared. The measurements were performed with two calibrated 2π‐actinometers assembled on the azimuth angle‐controlled LPMA/DOAS (Laboratoire de Physique Moléculaire et Applications/Differential Optical Absorption Spectroscopy) gondola during a series of balloon flights. Since the actinometer's spectral sensitivity curve did not exactly match the actinic spectrum of NO2 and the skylight's spectrum shape changes with atmospheric height and solar illumination, only proxies (proxy‐)jNO2 rather than true jNO2 values were monitored during balloon ascents (0–30 km) for solar zenith angle (SZA) 75° < SZA < 86°, and at balloon float altitude during solar occultation (86° < SZA < 95°). The measured direct and diffuse total proxy‐jNO2 values compare excellently with radiative transfer (RT) modeling. That finding allows us to rule out uncertainties in computing UV/vis actinic fluxes as a significant factor in the still insufficient modeling of stratospheric NO2 at large SZAs.
Pioneer Venus Orbiter observations of a solar wind quasi‐invariant
Authors: Fainberg J. , Osherovich Vladimir A. , Stone R. G.
Published: February 2004

The recently suggested solar wind quasi‐invariant [QI≡( B²/8π)/(ρυ²/2)] is a combination of magnetic field strength B, plasma density ρ and solar wind speed υ. Measured in the solar wind near the Earth, the median value of QI was shown to be a reliable index of solar activity for the 28 year period holding a linear relation with sunspot numbers (SSN) with a high correlation coefficient (cc) of 0.98 [Osherovich, Fainberg and Stone, 1999]. Here, using hourly data from the Pioneer Venus Orbiter for the period from 1978–1988, we demonstrate that QI near Venus also closely follow SSN with a correlation coefficient of 0.94 for the yearly mean values. We found that the calibration coefficient between QIVenus and QIEarth is slightly larger than unity, namely, QIVenus=a* · QIEarth where a*=1.134±0.001. We suggest that, for any point in the heliosphere, QI can be viewed as a useful measure of solar activity.
Geomagnetic quiet time (Sq) variations at high latitudes
Authors: Detrick D. L. , Lanzerotti L. J.
Published: February 2004

The availability of geomagnetic data from two Antarctic Geophysical Observatories (AGOs P5 and P6; invariant geomagnetic latitudes 87° and 85°S) in the deep southern hemisphere polar cap allows for the first time the study of the geomagnetic “quiet time” variations (Sq) at the highest latitudes possible. We present analyses of the Sq variations at these two AGO sites for data obtained in 1997, and compare them with model predictions. We find strong evidence that the Sq variations near 90°S geomagnetic latitude are considerably larger (factor of two or more) than would be expected from the model, independent of season. Comparison of the measurements of the vertical components to the model suggests that there may be large differences in the Earth's deep mantle conductivity under the Antarctic ice cap at the two locations. These observations highlight the need for incorporating high latitude data into Sq models.
Comparison of the monthly precipitation derived from the TRMM satellite
Authors: Shin Dong‐Bin , Chiu Long S. , Kafatos Menas
Published: February 2004

A comparison of monthly rainfall derived from the version 5 of TRMM Microwave Imager (TMI), Precipitation Radar (PR), TRMM Combined algorithm (TCA) and TMI‐emission algorithm (TMIE) using two years (1998 to 1999) of TRMM data was made. The global (TRMM domain, 40°N∼ 40°S) average rain rates are 3.29, 2.62 and 2.93 mm/day over land and 3.02, 2.47 and 2.54 mm/day over oceans for TMI, PR, and TCA respectively. The TMIE oceanic average is 2.90 mm/day. For both the global and zonal means, the TMI rain rates are the largest and the PR estimates lowest. Regression analyses show the offsets of algorithms are close to zero. According to a paired t‐test, significant differences exist between TMI and PR and between TMI and TCA, especially in oceanic dry regions. However, the difference between PR and TCA was judged to be insignificant. Comparison of PR and TMIE shows that a statistically significant difference is evident in the oceanic dry regions.
Laboratory observation of Alfvén resonance
Authors: Mitchell C. , Vincena S. , Maggs J. , Gekelman W.
Published: February 2004

We present laboratory observations of shear Alfvén wave resonances in a cylindrical plasma column. Fourier data are presented which show the existence of many standing eigenmodes in the shear Alfvén frequency range. Harmonic frequency dependence on ambient
is presented for three resonances. A wavelet analysis is used to show the lifetimes of the identified harmonic modes. Experimental Q values for the first five eigenfrequencies are measured and compared to theoretical values.
Enhanced chlorophyll associated with tropical instability waves in the equatorial Pacific
Authors: Strutton Peter G. , Ryan John P. , Chavez Francisco P.
Published: February 2004

High resolution mooring time series are used to quantify significant chlorophyll anomalies associated with tropical instability waves (TIWs) in the equatorial Pacific. Distinct peaks characterized by very high chlorophyll (up to 3.5 mg m−3) are observed in association with TIW cold cusps. These high‐chlorophyll peaks appear to differ with respect to scale and intensity from those previously observed at subductive fronts. The physical processes responsible for the observed chlorophyll distributions are not mutually exclusive, and include advection, horizontal mixing, enhanced upwelling and concentration of biomass at fronts. Given the potentially large spatial extent of these high chlorophyll bands, their importance as regions of increased productivity and CO2 uptake is discussed.
Dust transport to Dome C, Antarctica, at the Last Glacial Maximum and present day
Authors: Lunt Daniel J. , Valdes Paul J.
Published: February 2004

The Antarctic polar ice‐core records show large changes in the concentration of dust over glacial‐interglacial timescales. This paper explores how much of this variation is due to changes in the transport of dust from arid regions to the ice cores. Back trajectories, initialised from the site of Dome C ice‐core, Antarctica, are calculated using an offline trajectory code forced by modelled winds, output from the UKMO Unified Model running under present day and LGM boundary conditions. As well as comparing the present day and LGM back trajectories, their seasonal and interannual variability is also explored. The results suggest that the Patagonian provenance of the Dome C dust can be understood in terms of the atmospheric transport whereas the total change in dust concentration requires changes to the sources or sinks.
Soil moisture in a regional climate model: Simulation and projection
Authors: Pan Zaitao , Arritt Raymond W. , Gutowski William J. , Takle Eugene S.
Published: February 2004

Regional climate simulations driven by three sets of initial and lateral boundary conditions—analyzed observations, GCM control climate, and GCM enhanced greenhouse‐gas scenario climate—are used to assess model accuracy in predicting soil moisture and to examine changes in soil moisture in the scenario climate. Simulated soil moisture does not show noticeable drift during the 10‐year simulations. Observed and simulated soil moisture for Illinois and Iowa correspond reasonably well for the top 10 cm soil layer but a consistent low bias is present in the top 1 m. Growing season depletion of soil water is simulated well but recharge after growing season is slower than observed, at least in part due to underprediction of precipitation in autumn. This suggests that improvements in simulating soil moisture depend greatly on improvements in simulating precipitation. The climate change scenario produces drier soil in the top 10 cm during winter but wetter top soil in warm seasons because of greater precipitation, while top l m soil is wetter in all seasons.
Observations of time‐dependent meniscus behavior with implications for seismic attenuation in three‐phase systems
Authors: Brunner W. , Spetzler H. A.
Published: February 2004

Meniscus dynamics causing strain energy dissipation in three‐phase systems, e.g., partially saturated rocks, is being studied as a method of detecting changes in the chemical composition of these systems. New observations of dynamic menisci and thin films on contaminated and clean surfaces have been made. Thin films on contaminated surfaces were observed to exhibit time‐dependent relaxation. We propose that this relaxation is responsible for a poorly understood attenuation peak previously observed in partially saturated, contaminated systems.
Joint observations of sodium enhancements and field‐aligned ionospheric irregularities
Authors: Kane T. , Grime B. , Franke S. , Kudeki E. , Urbina J. , Kelley M. , Collins S.
Published: February 2004

Resonance Lidar observations of neutral sodium and VHF coherent scatter radar observations of field‐aligned 3‐meter irregularities were obtained during the Coqui II rocket campaign in Puerto Rico. The Lidar, a facility instrument at the Arecibo Observatory (18.3°N, 66.8°W), and the University of Illinois Radar, located near Salinas on the south of the island, both monitored volumes near where the uplegs of the nominal rocket trajectories intersected the E‐region. The Observatory's Incoherent Scatter Radar was also used to characterize the plasma layers. Preliminary investigation of the data sets has shown a potential correspondence between VHF backscatter from plasma layers and a new class of characteristic enhancements in the neutral sodium.
Alfvén wave generated electron time dispersion
Authors: Kletzing C. A. , Hu S.
Published: February 2004

The results from a model of kinetic Alfvén waves which includes varying magnetic field and density show that time‐dispersed bursts of auroral electrons can be accelerated by Alfvén wave pulses propagating from the magnetosphere to the ionosphere. The modeled electron signatures have similar energy range and temporal structure to those observed on sounding rockets and satellites suggesting that electron time dispersion is generated by Alfvén waves.
Slab‐like high velocity anomaly in the uppermost mantle beneath the Dabie‐Sulu Orogen
Authors: Xu Peifen , Liu Futian , Wang Qingchen , Cong Bolin , Chen Hui
Published: February 2004

Seismic tomography is used to determine three‐dimensional velocity structure of the Dabie‐Sulu orogen in eastern China. The high resolution image of the P‐waves velocity structure is obtained for the uppermost mantle to a depth of at least 150 km, along the latitudes of 30°–38°N and longitudes of 114°–122°E. Results of this inversion reveal a slab‐like high velocity anomaly under this orogen in the depth from Moho discontinuity to at least 110 km. It may represent a remnant of the subducted Yangtze block during the period of Triassic continent‐continent collision.
Inferring volcanic degassing processes from vesicle size distributions
Authors: Blower J. D. , Keating J. P. , Mader H. M. , Phillips J. C.
Published: February 2004

Both power law and exponential vesicle size distributions (VSDs) have been observed in many different types of volcanic rocks. We present results of computer simulations and laboratory analogue experiments which reproduce these findings and show that the distributions can be interpreted as the product of continuous bubble nucleation resulting from non‐equilibrium degassing. This ongoing nucleation causes the bubbles to evolve through an exponential size distribution into a power law size distribution as nucleation and growth progress. These findings may explain the apparent contradiction between present models of bubble growth in magmas, which predict that degassing in explosive eruptions is a non‐equilibrium process, and models of conduit flow, which assume perfect equilibrium degassing. The process of continuous nucleation is the mechanism whereby the volcanic system maintains near‐equilibrium in the case of rapid depressurization and slow volatile diffusion.
Retrieval of mesospheric and lower thermospheric kinetic temperature from measurements of CO2 15 µm Earth Limb Emission under non‐LTE conditions
Authors: Mertens Christopher J. , Mlynczak Martin G. , López‐Puertas Manuel , Wintersteiner Peter P. , Picard R. H. , Winick Jeremy R. , Gordley Larry L. , Russell James M.
Published: February 2004

We present a new algorithm for the retrieval of kinetic temperature in the terrestrial mesosphere and lower thermosphere from measurements of CO2 15 µm earth limb emission. Non‐local‐thermodynamic‐equilibrium (non‐LTE) processes are rigorously included in the new algorithm, necessitated by the prospect of satellite‐based limb radiance measurements to be made from the TIMED/SABER platform in the near future between 15 km and 120 km tangent altitude. The algorithm requires 20 seconds to retrieve temperature to better than 3 K accuracy on a desktop computer, easily enabling its use in operational processing of satellite data. We conclude this letter with a study of the sensitivity of the retrieved temperatures to parameters used in the non‐LTE models, including sensitivity to the rate constant for physical quenching of CO2 bending mode vibrations by atomic oxygen.
Upper mantle discontinuity structure in the region of the Tonga Subduction Zone
Authors: Gilbert Hersh J. , Sheehan Anne F. , Wiens Douglas A. , Dueker Kenneth G. , Dorman LeRoy M. , Hildebrand John , Webb Spahr
Published: February 2004

We study the mantle structure below the southwest Pacific in order to examine the geometry of the Tonga slab at depth and its interaction with the 410‐ and 660‐km discontinuities (hereafter called the 410 and the 660). We utilize data from stations of both the Lau Basin Ocean Bottom Seismogram experiment and island stations of the Southwest Pacific Seismic Experiment. The tectonic complexity of this region, containing both the Tonga subduction zone and the associated Lau back arc spreading center make it an ideal area to investigate the upper mantle discontinuities using a high resolution technique such as common conversion point stacking of receiver functions. We produce a high‐resolution image of the upper mantle near the Tonga subduction zone to show the interaction between the discontinuities and the subducting slab. Our results show the 410 uplifted by 30 km near the Tonga slab and the 660 depressed by 20 to 30 km as expected for thermally controlled olivine phase transitions.
Gaseous mercury emissions from a fire in the Cape Peninsula, South Africa, during January 2000
Authors: Brunke Ernst‐G. , Labuschagne Casper , Slemr F.
Published: February 2004

During mid‐January 2000 the plume from a fire, which destroyed 9000 ha of mixed vegetation in the southern part of the Cape Peninsula, passed over the Cape Point Global Atmosphere Watch station (34°S, 18°E). The smoke plume was characterized by a CO/CO2 emission ratio (ER) of 0.0548±0.0018 mol/mol, typical for biomass burning. Measurements of total gaseous mercury (TGM) made during this episode provided Hg/CO and Hg/CO2 emission ratios of (2.10±0.21) * 10−7 and (1.19±0.30) * 10−8 mol/mol, respectively. Based on the presently accepted CO source estimate for biomass burning of 621 * 10³ kt yr−1 (range 400‐700 * 10³ kt yr−1), the ER(Hg/CO) suggests that the global mercury emission from biomass burning amounts to approximately 0.93 kt annually (range 0.51–1.14 kt yr−1). Similarly, based on a CO2 emission from biomass burning of 3460 * 10³ ktC yr−1 (range 3000–6200 ktC yr−1), the ER(Hg/CO2) suggests that the global mercury emission from biomass burning amounts to approximately 0.59 kt annually (range 0.38–1.33 kt yr−1), in good agreement with the emission estimated from the ER(Hg/CO). If supported by measurements from fires in other regions, mercury emissions from biomass burning of the order of 1 kt yr−1 could represent one of the major, hitherto neglected, sources of atmospheric mercury.