bluuebunny/crossref_metadata_2025 huggingface dataset:
Methyl bromide: Ocean sources, ocean sinks, and climate sensitivity
Authors: Anbar A. D. , Yung Y. L. , Chavez F. P.
Published: February 2004

The oceans play an important role in the geochemical cycle of methyl bromide (CH3Br), the major carrier of O3‐destroying bromine to the stratosphere. The quantity of CH3Br produced annually in seawater is comparable to the amount entering the atmosphere each year from natural and anthropogenic sources. The production mechanism is unknown but may be biological. Most of this CH3Br is consumed in situ by hydrolysis or reaction with chloride. The size of the fraction which escapes to the atmosphere is poorly constrained; measurements in seawater and the atmosphere have been used to justify both a large oceanic CH3Br flux to the atmosphere and a small net ocean sink. Since the consumption reactions are extremely temperature‐sensitive, small temperature variations have large effects on the CH3Br concentration in seawater, and therefore on the exchange between the atmosphere and the ocean. The net CH3Br flux is also sensitive to variations in the rate of CH3Br production. We have quantified these effects using a simple steady state mass balance model. When CH3Br production rates are linearly scaled with seawater chlorophyll content, this model reproduces the latitudinal variations in marine CH3Br concentrations observed in the east Pacific Ocean by Singh et al. [1983] and by Lobert et al. [1995]. The apparent correlation of CH3Br production with primary production explains the discrepancies between the two observational studies, strengthening recent suggestions that the open ocean is a small net sink for atmospheric CH3Br, rather than a large net source. The Southern Ocean is implicated as a possible large net source of CH3Br to the atmosphere. Since our model indicates that both the direction and magnitude of CH3Br exchange between the atmosphere and ocean are extremely sensitive to temperature and marine productivity, and since the rate of CH3Br production in the oceans is comparable to the rate at which this compound is introduced to the atmosphere, even small perturbations to temperature or productivity can modify atmospheric CH3Br. Therefore atmospheric CH3Br should be sensitive to climate conditions. Our modeling indicates that climate‐induced CH3Br variations can be larger than those resulting from small (±25%) changes in the anthropogenic source, assuming that this source comprises less than half of all inputs. Future measurements of marine CH3Br, temperature, and primary production should be combined with such models to determine the relationship between marine biological activity and CH3Br production. Better understanding of the biological term is especially important to assess the importance of non anthropogenic sources to stratospheric ozone loss and the sensitivity of these sources to global climate change.
Oceanic primary production: 1. Adaptation of a spectral light‐photosynthesis model in view of application to satellite chlorophyll observations
Authors: Antoine David , Morel André
Published: February 2004

A global equation, designed to estimate the column‐integrated oceanic primary production realized by a given phytoplankton biomass under various environmental conditions, is used to develop a practical method to assess the primary production (P) from the chlorophyll concentration as provided by satellite imagery. This basic equation combines three terms, namely the photosynthetically available radiation impinging at the sea surface, PAR(0+), the column‐integrated chlorophyll content, tot, and the cross section for photosynthesis per unit of chlorophyll, Ψ*. Global monitoring of incident irradiance and near‐surface algal biomass is now achievable from space, and thus the next step toward a monitoring of oceanic primary production would be to dispose in parallel of a “climatological field” of the Ψ* quantity. Actually, Ψ* depends on the two other terms of the equation (PAR(0+) and tot,), and, in addition, on temperature (also detectable from satellite). Therefore such a “climatological field” is variable and complex and it can be conveniently replaced by lookup tables allowing easy interpolation. The entries are date, latitude, cloudiness, temperature, and remotely sensed chlorophyll concentration. This upper layer concentration is extended downward owing to previous results of a statistical analysis of the chlorophyll vertical distribution; accordingly, two parallel tables, corresponding to well‐mixed or stratified upper layers with uniform or non uniform chlorophyll vertical profiles, respectively, are constructed. These tables are produced by systematically using a previously published spectral light‐photosynthesis model. For such extensive computations, the model necessarily relies on, and is operated with, a standard set of ecological and physiological parameters. Therefore sensitivity analyses have been carried out in view of assessing the impact on Ψ*, and on the resulting production of deviations in these parameters or parameterizations, vis‐a‐vis the standard values or formulations which were adopted when building the tables. The effects of the biomass vertical structure, of possible light and temperature adaptation, and of the presence of degraded pigments are among the sensitivity studies which have been performed. The method as proposed can accomodate any improvement and complexity in parameterization to the extent that additional computation time is faced only when generating the lookup tables, not when using them in conjunction with satellite data.
Nitrous oxide in coastal waters
Authors: Bange Hermann W. , Rapsomanikis Spyridon , Andreae Meinrat O.
Published: February 2004

We determined atmospheric and dissolved nitrous oxide (N2O) in the surface waters of the central North Sea, the German Bight, and the Gironde estuary. The mean saturations were 104 ± 1% (central North Sea, September 1991), 101 ± 2% (German Bight, September 1991), 99 ± 1% (German Bight September 1992), and 132% (Gironde estuary, November 1991). To evaluate the contribution of coastal areas and estuaries to the oceanic emissions we assembled a compilation of literature data. We conclude that the mean saturations in coastal regions (with the exception of estuaries and regions with upwelling phenomena) are only slightly higher than in the open ocean. However, when estuarine and coastal upwelling regions are included, a computation of the global oceanic N2O flux indicates that a considerable portion (approximately 60%) of this flux is from coastal regions, mainly due to high emissions from estuaries. We estimate, using two different parameterizations of the air‐sea exchange process, an annual global sea‐to‐air flux of 11–17 Tg N2O. Our results suggest a serious underestimation of the flux from coastal regions in widely used previous estimates.
Balloon‐borne observations of mid‐latitude hydrofluoric acid
Authors: Sen B. , Toon G. C. , Blavier J.‐F. , Szeto J. T , Fleming E. L. , Jackman C. H.
Published: February 2004

Measurements of stratospheric HF have been made by the JPL MkIV interferometer during high‐altitude balloon flights. Infrared solar absorption spectra were acquired near 35°N at altitudes between local tropopause and 38 km. Volume mixing ratio profiles of HF derived from 4 flights (1990–93), in conjunction with simultaneously observed N2O profiles, indicate an average rate of HF increase of (5.5±0.3)% per year, in agreement with time‐dependent, two‐dimensional model simulations (6% per year) and ATMOS measurements.
Synoptic IPS and Yohkoh soft X‐ray observations
Authors: Hick P. , Jackson B. V. , Rappoport S. , Woan G. , Slater G. , Strong K. , Uchida Y.
Published: February 2004

Interplanetary scintillation measurements of the disturbance factor, g, from October 1991 to October 1992 are used to construct synoptic Carrington maps. These maps, which show the structure of the quiet solar wind, are compared with X‐ray Carrington maps from the Yohkoh SXT instrument. For the period studied the global structure outlined by (weakly) enhanced g‐values apparent in the IPS maps tends to match the active regions (as shown in the X‐ray maps) significantly better than the heliospheric current sheet. Contrary to traditional opinion, which views active regions as magnetically closed structures that do not have any significant impact on the solar wind flow, our results suggest that density fluctuations in the solar wind are significantly enhanced over active regions. These results support the suggestion by Uchida et al. (1992), based on Yohkoh observations of expanding active regions, that active regions play a role in feeding mass into the quiet solar wind.
The effect of anthropogenic sulfate and soot aerosol on the clear sky planetary radiation budget
Authors: Haywood J. M. , Shine K. P.
Published: February 2004

Carbonaceous soot within the troposphere can significantly modify the clear‐sky radiative forcing. Using an extension to a simple radiation calculation and two model‐derived sulfate aerosol data sets, the impact of an assumed soot/sulfate mass ratio of between 0.05 and 0.1 is examined. Fossil fuel derived soot causes a positive global‐mean radiative forcing which for one data set ranges from +0.03 to +0.24Wm−2; the lower estimate is for an external mixture with a soot/sulfate ratio of 0.05 and the upper estimate is for an internal mixture and a soot/sulfate ratio of 0.10. These values compare to a global‐mean radiative forcing of −0.34Wm−2 due to sulfate aerosol. Soot also significantly reduces the interhemispherical difference in the radiative forcing due to sulfate aerosol. The nature and amount of soot must be well established if the climatic role of tropospheric aerosols is to be fully understood.
Changes in long‐term extension rates associated with the Morgan Hill and Loma Prieta earthquakes in California
Authors: Savage J. C. , Lisowski M.
Published: February 2004

Frequent measurements since mid‐1981 of the distances from a geodetic monument located about 100 km south‐southeast of San Francisco to three monuments 30 to 40 km distant provide an unusually complete record of the deformation before and after two nearby earthquakes, the 1984 Morgan Hill (ML = 6.2) and 1989 Loma Prieta (Ms = 7.1) earthquakes. Except possibly for the first few months postseismic, the extension rates indicated by these measurements appear to be steady over the four or five years both preceding and following those earthquakes. However, the preseismic and postseismic rates differ significantly for at least one of the baselines measured for each earthquake. The data over the four to five year postseismic records available are not adequate to demonstrate whether the postseismic rates are relaxing back to the preseismic rates.
Radar internal layers from the Greenland Summit
Authors: Jacobel Robert W. , Hodge Steven M.
Published: February 2004

Ice penetrating radar measurements made over the summit region of Greenland show returns from internal layers which can be used to augment the interpretation of climate information from the two deep cores recently recovered from this area. These reflecting surfaces, believed to represent isochrones, give information about the stress regime near the summit, and may aid in a better calibration of the age depth scale between the two cores — particularly in the lowest 10% of ice thickness where there is currently disagreement. The approximate depth at which internal echoes become discontinuous corresponds with the observations of steep inclinations and overturned folds on the scale of centimeters in the core samples. However the deepest internal layers which can be distinguished in the profiles place constraints on the scale and location of high angle or overturned folds.
Evolution of the Pinatubo volcanic aerosol column above Pasadena, California observed with a mid‐infrared backscatter lidar
Authors: Tratt David M. , Menzies Robert T.
Published: February 2004

The evolution of the volcanic debris plume originating from the June 1991 eruption of Mt. Pinatubo has been monitored since its genesis using a ground‐based backscatter lidar facility sited at the Jet Propulsion Laboratory (JPL). Both absolute and relative pre‐ and post‐Pinatubo backscatter observations are in accord with Mie scattering projections based on measured aerosol particle size distributions reported in the literature. The post‐Pinatubo column‐integrated backscatter coefficient peaked approximately 400 days after the eruption, and the observed upper boundary of the aerosol column subsided at a rate of ∼200 m mon−1.
Cloud‐radiative effects on implied oceanic energy transports as simulated by Atmospheric General Circulation Models
Authors: Gleckler P. J. , Randall D. A. , Boer G. , Colman R. , Dix M. , Galin V. , Helfand M. , Kiehl J. , Kitoh A. , Lau W. , Liang X.‐Y. , Lykossov V. , McAvaney B. , Miyakoda K. , Planton S. , Stern W.
Published: February 2004

This paper summarizes the ocean surface net energy flux simulated by fifteen atmospheric general circulation models constrained by realistically‐varying sea surface temperatures and sea ice as part of the Atmospheric Model Intercomparison Project. In general, the simulated energy fluxes are within the very large observational uncertainties. However, the annual mean oceanic meridional heat transport that would be required to balance the simulated surface fluxes is shown to be critically sensitive to the radiative effects of clouds, to the extent that even the sign of the Southern Hemisphere ocean heat transport can be affected by the errors in simulated cloud‐radiation interactions. It is suggested that improved treatment of cloud radiative effects should help in the development of coupled atmosphere‐ocean general circulation models.
Relation of Na+ concentration and δ18O in winter precipitation with weather conditions
Authors: Suzuki Keisuke , Endo Yasoichi
Published: February 2004

A chemical study of the winter precipitation was conducted in Tohkamachi, central Japan. The ratios of Cl−/Na+ and Mg2+/Na+ found in the winter precipitation samples are nearly equal to those of sea water. This result suggests that Na+, Mg2+, and Cl− found in the winter precipitation are originated from sea water. A relationship was determined between the Na+ concentration in the winter precipitation sample and the height of the convective mixing layer. Cloud droplets containing sea salt particles are believed to be captured by snow crystals through the rain‐out process. The results indicate that the higher the convective mixing layer heights, the larger the amount of sea salt particles captured by the snow crystals. A relationship between the δ18O values in the winter precipitation and the air temperature at the top of convective mixing layer was determined for several synoptic weather conditions that produced winter precipitation. The δ18O values in samples collected under conditions of winter monsoon are larger than those under other weather conditions at the same temperature. A good relationship is observed between the temperatures at the top of convective mixing layer and the δ18O of samples of winter precipitation under conditions of the winter monsoon.
On the downward bias in vertical velocity measurements by VHF radars
Authors: Hoppe U.‐P. , Fritts D. C.
Published: February 2004

EISCAT VHF (224 MHz) radar observations of the vertical velocity under Polar Mesosphere Summer Echo (PMSE) conditions were performed with 2 s time and 300 m height resolution and velocity uncertainties better than 0.1 ms−1. We show that upward velocities that persist for ∼ 10 min or more lead to greater variance in the motion field and tend to make the PMSE weaker. As smaller radar cross section also correlates with larger velocity uncertainty, any vertical mean velocity composed of thresholded radar data tends to have a downward (negative) bias. We argue that this phenomenon can contribute to the discrepancy between the mean upwelling accounting for the summer mesopause thermal structure and radar observations of the mean vertical velocity.
Stratospheric OClO measurements as a poor quantitative indicator of chlorine activation
Authors: Sessler J. , Chipperfield M. P. , Pyle J. A. , Toumi R.
Published: February 2004

In the past measurements of OClO have been used to determine the degree of chlorine activation in the Arctic and Antarctic lower stratospheres. In this study we have carried out box model calculations to investigate the behaviour of OClO concentrations at 50hPa for cold denoxified conditions, typical of the polar winter. The dependence of the OClO abundance on ClOx (ClO+2Cl2O2) and BrOx (BrO+BrCl), as well as the sensitivity to nitrogen species, latitude and temperature is investigated. The net production of OClO from the reaction ClO + BrO → OClO + Br, the major source of OClO, is found to be first order in BrO for BrOx values from 0 to 22 pptv, whereas it becomes pseudo zero order in ClO for ClOx greater than ∼400 pptv. This behaviour is due to the rate at which BrOx is tied up as BrCl at sunset. The insensitivity of the OClO concentration to high levels of ClO makes OClO a poor indicator for distinguishing between medium and high levels of ClO. OClO is only a qualitatively good indicator of chlorine activation but is a good quantitative indicator of BrO.
On the variability of aftershock ground motions in the San Fernando Valley
Authors: Hough S. E. , Dietel C. , Glassmoyer G. , Sembera E.
Published: February 2004

We analyze aftershocks of the 1/17/94 Mw6.7 Northridge earthquake recorded at a 3‐element small‐aperture array within the town of Northridge, above the mainshock rupture plane. Many of the M4‐5 aftershocks are observed to have a prolonged shaking duration, up to ˜8 seconds, with conspicuous longer period (≈1 s) arrivals in the latter part of the wave train. Recordings of a M4.0 aftershock that occurred at 23:49 GMT on 1/17 show the origin of these waves. A slant‐stack cross‐correlation method on each of the three components shows that the late arrivals are characterized by low apparent velocities and a back‐azimuth that is approximately 10 degrees off that of the direct arrivals. Based on the inferred apparent velocities and consideration of studies in other sedimentary basins, we conclude that these later arrivals consist of surface waves generated within the San Fernando Valley. Similar results are obtained for a M3.4 event recorded across the array. The surface waves are not, however, a ubiquitous feature of the aftershock recordings. We show that other M˜4 events recorded at the same site are characterized by simple displacement pulses and durations that are typical for their magnitude, suggesting that 3‐dimensional site response may be difficult to predict in cases where the sources are close to a valley or basin and/or the basin structure is complex.
Excitation of left‐hand polarized nonlinear MHD waves at comet Grigg‐Skjellerup
Authors: Shevchenko V. I. , Galinsky V. L. , Ride S. K. , Baine M.
Published: February 2004

We present a self‐consistent theory and the results of numerical simulations describing the excitation and nonlinear evolution of left‐hand polarized Alfven waves in the solar wind near comet P/Grigg‐Skjellerup. During the Giotto spacecraft encounter with this comet, the interplanetary magnetic field was almost perpendicular to the solar wind flow, so the newly created (by photoionization) cometary ions excited left‐hand polarized waves through normal Doppler resonance. In our calculations we take into account two types of nonlinearity (1) the nonlinearity of the cometary ions' motion due to their resonant interaction with excited waves (i.e., trapping and bounce oscillations), and (2) the nonlinearity of the solar wind particle motion that leads to the steepening of the wave profile. Results of the study are in reasonable agreement with observations. This analysis may also be relevant to the related problem of nonlinear wave excitation by reflected protons near planetary bow‐shocks.
Filamentation and layering of an idealized tracer by observed winds in the lower stratosphere
Authors: Orsolini Y. , Simon P. , Cariolle D.
Published: February 2004

The isentropic transport of a passive tracer on synoptic time scales in the winter lower stratosphere is modelled with the use of a high‐resolution transport model, which is forced by winds derived from global meteorological analyses. The study has focused on a meteorological situation which occured in late January 1992. Repeated poleward intrusions of mid‐latitude air are shown to lead to the filamentation of a tracer distribution, which was initially compact and located inside the polar vortex. The effect of wind resolution on the filamentation process is examined. By performing isentropic advection on many closely spaced independent levels, the vertical structure of these filaments can be studied.
Macroscale modeling and mesoscale observations of plasma density structures in the polar cap
Authors: Basu Santimay , Basu Sunanda , Sojka J. J. , Schunk R. W. , MacKenzie E.
Published: February 2004

The seasonal and UT variation of mesoscale structures (10 km – 100 m) in the central polar cap has been obtained from an analysis of 250‐MHz intensity scintillation observations made at Thule, Greenland. It has been established earlier [Su. Basu et al., 1990] that mesoscale structures causing scintillations of satellite signals may develop at the edges of macroscale structures (several hundred km) such as discrete polar cap plasma density enhancements or patches through the gradient drift instability process. As such, we examined the seasonal and UT variation of polar cap patches simulated by using the USU Time Dependent Ionospheric Model (TDIM) under conditions of southward Bz. A fairly remarkable similarity is found between the scintillation observations and the model predictions of patch occurrence. For instance, both the patch and scintillation occurrences are minimized during the winter solstice (northern hemisphere) between 0800–1200 UT while also having their largest seasonal intensity between 2000–2400 UT. Little UT dependence of patches and scintillations is seen at equinox with high intensity being observed throughout the day, while during local summer the intensity of macroscale patches and mesoscale irregularities are found to be a minimum at all UT. These results indicate that macroscale features in the polar cap are routinely associated with plasma instabilities giving rise to smaller scale structures and that the specific patch formation mechanism assumed in the simulation is consistent with the observations. This ability to bridge between macroscale modeling and mesoscale observations provides a natural framework for the modeling of mesoscale structures themselves. This mesoscale modeling, in turn, can be utilized in a variety of radar and communication systems applications in the polar region.
Scaling nature of crustal susceptibilities
Authors: Pilkington Mark , Todoeschuck John P.
Published: February 2004

Scale invariant or self‐similar properties have been demonstrated for a wide range of geophysical processes and rock properties. Evidence from well logs and inferences from aeromagnetic field power spectra suggest that crustal susceptibility also shows this behaviour with a power spectrum proportional to the −4 power of the spatial frequency. We examine two large susceptibility measurement data sets and find that they both support the scaling hypothesis.
First results from POAM II: The dissipation of the 1993 Antarctic Ozone Hole
Authors: Bevilacqua R. M. , Hoppel K. W. , Hornstein J. S. , Lucke R. L. , Shettle E. P. , Ainsworth T. L. , Debrestian D. , Fromm M. D. , Krigman S. S. , Lumpe J. , Glaccum W. , Olivero J. J. , Clancy R. T. , Randall C. E. , Rusch D. W. , Chassefière E. , Dalaudier F. , Deniel C. , Brogniez C. , Lenoble J.
Published: February 2004

POAM II is a space‐borne instrument which uses the solar occultation technique to measure the vertical distribution of ozone, aerosols and polar stratospheric clouds, and other properties of the stratosphere and mesosphere. POAM II was launched aboard the SPOT 3 satellite in time to observe the dissipation of the 1993 Antarctic ozone hole. POAM data indicates that the Antarctic ozone hole dissipates from the top downward. It also supports the hypothesis that the Antarctic vortex is an effective containment vessel. However the strength of the containment appears to decrease markedly at altitudes below 18 km
The topographic torque on a bounding surface of a rotating gravitating fluid and the excitation by core motions of decadal fluctuations in the Earth's rotation
Authors: Hide Raymond
Published: February 2004

General expressions (with potential applications in several areas of geophysical fluid dynamics) are derived for all three components of the contribution made by the geostrophic part of the pressure field associated with flow in a rotating gravitating fluid to the topographic torque exerted by the fluid on a rigid impermeable bounding surface of any shape. When applied to the Earth's liquid metallic core, which is bounded by nearly spherical surfaces and can be divided into two main regions, the “torosphere” and “polosphere,” the expressions reduce to formulae given previously by the author, thereby providing further support for his work and that of others on the rôle of topographic coupling at the core‐mantle boundary in the excitation by core motions of Earth rotation fluctuations on decadal time scales. They also show that recent criticisms of that work are vitiated by mathematical and physical errors. Contrary to these criticisms, the author's scheme for exploiting Earth rotation and other geophysical data (either real or simulated in computer models) in quantitative studies of the topography of the core‐mantle boundary (CMB) by intercomparing various models of (a) motions in the core based on geomagnetic secular variation data and (b) CMB topography based on seismological and gravity data has a sound theoretical basis. The practical scope of the scheme is of course limited by the accuracy of real data, but this is a matter for investigation, not a priori assessment.
Brunhes‐Matuyama Boundary: 790 k.y. date consistent with ODP Leg 130 oxygen isotope records based on fit to Milankovitch Template
Authors: Berger Wolfgang H. , Bickert T. , Wefer G. , Yasuda M. K.
Published: February 2004

The age of the Brunhes‐Matuyama boundary, until recently taken as 730 k.y. in standard time scales for the Quaternary (e.g., Imbrie et al., 1984; Prell et al., 1986) and in Ocean Drilling Program results (Kroenke et al., 1991; Shackleton et al., 1992) has been revised to between 780 k.y. and 790 k.y., based on both orbital tuning of oxygen isotope data (e.g., Shackleton et al., 1990) and on new 40Ar‐39Ar dates on volcanic rocks (e.g., Baksi et al., 1992). We apply the revised chronology to an oxygen isotope record for Ontong Java Plateau (ODP Leg 130, western equatorial Pacific). Using this time scale, we find excellent agreement between the record and a template based on orbital forcing. The equation for the template is where the change in ice‐mass is given as the difference between constant ice growth (IGR) and variable amounts of melting. Melting depends on summer insolation at 65°N (INS), the current ice‐mass present (ICE), and the average ice‐mass (MEM) over a given time interval (L). The memory base L and the exponents are adjusted for best fit. The fit to the relevant portion of the record yields an age of 790±6 k.y. for the Brunhes‐Matuyama boundary. The age assignment is robust relative to substantial changes in parameter settings.
Motion of the termination shock in response to an 11 Year variation in the solar wind
Authors: Karmesin S. Roy , Liewer P. C. , Brackbill J. U.
Published: February 2004

A two‐dimensional hydrodynamic numerical model has been used to study the motion of the termination shock in response to an 11 year variation in the solar wind ram pressure. We find that for a total variation in the ram pressure by a factor of 2, a termination shock at 89 AU moves inward and outward about ±8% of its distance with a typical velocity of 12 km/sec. This movement may be understood in terms of the various time scales associated with the response of the termination shock and heliopause to variations in the solar wind ram pressure.
Carbonyl sulfide: No remedy for global warming
Authors: Taubman Steven J. , Kasting James F.
Published: February 2004

The enhancement of the stratospheric aerosol layer caused by the eruption of Mt. Pinatubo (June 15, 1991), and the subsequent cooling of the earth's lower atmosphere [Dutton and Christy, 1992; Minnis et al., 1993] shows that stratospheric aerosols can have a strong effect on the earth's climate. This supports the notion that the intentional enhancement of the stratospheric aerosol layer through increased carbonyl sulfide (OCS) emissions might be an effective means for counteracting global warming. Through the use of a one‐dimensional photochemical model, we investigate what effect such a program might have on global average stratospheric ozone. In addition, we consider the impact of enhanced OCS emissions on rainwater acidity and on the overall health of both plants and animals. We find that while the warming produced by a single CO2 doubling (1 to 4°C) might be offset with ozone losses of less than 5%, any attempt to use carbonyl sulfide as a permanent solution to global warming could result in depletion of global average ozone by 30% or more. We estimate that in order to achieve cooling of 4°C rainwater pH would fall to between 3.5 and 3.8. Finally, a 4°C cooling at the surface will require that ambient near ground OCS levels rise to above 10 ppmv which is probably greater than the safe exposure limit for humans. Thus, enhanced OCS emissions do not provide an environmentally acceptable solution to the problem of global warming.
Structural instability of the Rikitake Disk Dynamo
Authors: Hide Raymond
Published: February 2004

The Rikitake‐type disk dynamo, consisting of two identical single Faraday‐disk dynamos of the Bullard type coupled together, has been analyzed intensively for more than thirty years by: (a) mathematicians interested in its ability to behave chaotically, and (b) geophysicists needing low‐dimensional electromechanical analogues of the geodynamo in their efforts to elucidate the complex time series of geomagnetic polarity reversals over geological time. Implicit in the theory is what now appears to be an unwarranted assumption, namely that mechanical friction is unimportant both qualitatively and quantitatively as a dissipative agency in comparison with ohmic effects, and can therefore safely be neglected. The assumption is inconsistent with the findings of the present study, which shows that mechanical friction can render the Rikitake dynamo “structurally unstable” and consequently incapable of producing chaotic oscillations. Research on low‐dimensional analogues of the geodynamo should concentrate attention on structurally stable systems, with properties less dependent on the starting conditions.
Physical properties of the aerosol debris generated by the impact of fragment H of comet P/Shoemaker‐Levy 9 on Jupiter
Authors: Moreno F. , Muñoz O. , Molina A. , López‐Moreno J. J. , Ortiz J. L. , Rodríguez J. , López‐Jiménez A. , Girela F. , Larson S. M. , Campins H.
Published: February 2004

The physical properties of the aerosol debris coming from the impact of fragment H of comet P/Shoemaker‐Levy on Jupiter are analyzed. Based on a high spatial resolution image set obtained at the 4.2m William Herschel Telescope at La Palma (Spain), we derived detailed limb darkening curves at three wavelengths (3600, 8920 and 9480 Å), which we used to retrieve the aerosol physical properties and spatial distribution in the central core region of the impact site. We derived these properties combining the observations at two dates, that corresponding to the night of July 18th, 1994, just after the impact time, and that of July 21st, 1994. Radiative transfer models in combination with a Mie scattering program were used to retrieve the possible values of aerosol properties and spatial distribution compatible with these observations. The main results of the model computations are that the particles are rather dark, with imaginary refractive indexes varying between 0.02 at 3600 Å and 0.006 in the 9000 Å region, have a mean radius of about 0.15 µm, and are distributed between 1 and 450 mbar with a total column concentration of 8.2 × 109 cm−2, giving a total optical thickness of 3.6 at 9480 Å. A lower limit to the volume of the impact debris aerosol is estimated to be that corresponding to a sphere of about 230 m in diameter.
Evolution of plasma bubbles in the equatorial F region with different seeding conditions
Authors: Sekar R. , Suhasini R. , Raghavarao R.
Published: February 2004

As there are a significant number of observed onset conditions for the post‐sunset equatorial spread F that are not evidently associated with the required (5%) seed perturbation as assumed by the earlier simulation studies, an investigation is carried out with different seeding conditions for the evolution of plasma bubbles in the equatorial F region by means of a nonlinear numerical simulation model. The present investigation reveals that even an order of magnitude less perturbation (0.5%) than the earlier belief, grows into a bubble provided the nonlinear effects of other agencies such as the vertical wind and the electric field, in addition to gravity are taken into account. It is suggested that the above amplitude of the initial perturbation in plasma density can be generated through electrodynamical effects by the observed fluctuations in the electric fields.
Evolution of the Pinatubo volcanic cloud over Hampton, Virginia
Authors: Osborn Mary T. , DeCoursey Robert J. , Trepte Charles R. , Winker David M. , Woods David C.
Published: February 2004

A ground‐based lidar system at NASA Langley Research Center in Hampton, Virginia, has monitored the stratospheric aerosol vertical distribution and loading since 1974. The eruption of Mt. Pinatubo in June 1991 produced the largest enhancement of stratospheric aerosol loading ever observed by lidar over this mid‐latitude location. Low altitude layers (<20 km) were the first to arrive over Hampton in early August, the result of transport associated with a tropospheric anticyclonic cell over North America. The maximum peak scattering ratio, 34 at 22.4 km, and the maximum stratospheric integrated backscatter of 0.0053 sr−1, both at 694 nm, observed since the eruption were measured on February 20, 1992. After decreasing during the spring and summer of 1992, the aerosol burden increased significantly during the winter of 1992–3, evidence of poleward winter transport from the equatorial reservoir. Over the period from February 1992 to February 1994, the stratospheric aerosol loading decreased with an average 1/e decay time of 10.1 months. The vertical distribution, intensity, and transport of Pinatubo aerosols over Hampton, Virginia, are described in detail and compared with similar measurements after El Chichon.
Superparamagnetism and reduction diagenesis in pelagic sediments: Enhancement or depletion?
Authors: Tarduno John A.
Published: February 2004

Superparamagnetic grains, ultra‐fine (d < 0.05µm) particles unable to retain a remanence at room temperature, have come under increased scrutiny as indicators of the diagenetic and authigenic history of sedimentary rocks. In marine sediments, fine‐grained magnetite with large surface area to volume is thought to be preferentially removed during magnetic mineral reduction. On the basis of new magnetic hysteresis, low‐temperature, and geochemical studies of pelagic sediments from the western equatorial Pacific Ocean, it is proposed that reduction processes increase rather than deplete the ultra‐fine magnetic grain population. At the modern Fe‐redox boundary enhanced superparamagnetism coincides with a coarsening of remanence‐carrying grains. Superparamagnetism also tracks proposed temporal changes in magnetite reduction caused by climatically‐driven fluctuations in organic carbon (Corg) supply. Together with hysteresis characteristics, changes in superparamagnetism may help the identification of paleointensity artifacts resulting from non steady‐state magnetic mineral reduction.
The earthquake sequence of November 1987 and March 1988 in the Gulf of Alaska: A new insight
Authors: Quintanar Luis , Madariaga Raúl , Deschamps Anne
Published: February 2004

New information has been obtained by reanalyzing the rupture process of three large strike‐slip earthquakes that occurred in the Gulf of Alaska on November 17 and 30, 1987 and March 6, 1988. We used spectral analysis of P‐waves complemented with a simple technique for inverting body waves using a fixed source mechanism. Our results show that the Ms=7.0 event of November 17, which can be considered to be a foreshock to the November 30 event, consisted of two pulses of large seismic moment release: the first one with a duration of ∼15 sec coincides with those obtained by other studies; the second, which occurs 20 sec after the first arrival is clearly visible on the records, but was not previously reported. The Ms=7.4 earthquake of November 30 is a complex event that ruptured on a fault plane trending north‐south. The Ms=7.7 shock of March 6, 1988 was a simple event along the same north‐south trend south of the November events. P‐wave far field spectra exhibit an ω−1 intermediate trend between the usual ω0 and ω−2 slopes. This trend is due to rupture along a long, narrow fault, so the corner frequency associated with this intermediate trend is related to the reciprocal of rupture propagation time. We suggest that these events were due to a tear in the Pacific plate.
Four micron infrared observations of the comet Shoemaker‐Levy 9 collision with Jupiter at the Zelenchuk Observatory: Spectral evidence for a stratospheric haze and determination of its physical properties
Authors: Rosenqvist J. , Biraud Y. G. , Cuisenier M. , Marten A. , Hidayat T. , Chountonov G. , Moreau D. , Muller C. , Maslov I. , Ackerman M. , Balega Y. , Korablev O.
Published: February 2004

We present preliminary results derived from low spectral resolution infrared spectra of the comet impact sites onto Jupiter. Fragments W/K were tracked several days after the collision at the SAO 6m‐telescope on July 22 and 27. A systematic increase of the spectral radiance is observed for fragment W/K spectra as compared to undisturbed region spectra. This change is interpreted as the evidence for a stratospheric haze produced by the collision. From the spectral shape of this relative increase, the albedo of the haze can reach values as low as 0.06 but is constrained between 0.13 and 0.15 if the spot size is 3′. No stringent constraint is obtained for the altitude of this haze even if a 5–50 mbar level seems to be possibly the best estimate. The particles in the haze should play a significant role in the thermal balance over impact sites. The fragments W/K spectra between July 22 and July 27 show a high stability of the haze between both dates.
Models of the SL‐9 collision‐generated hazes
Authors: Ortiz J. L. , Muñoz O. , Moreno F. , Molina A. , Herbst T. M. , Birkle K. , Böhnhardt H. , Hamilton D. P.
Published: February 2004

From subarcsec‐seeing infrared images and spectra in the K and H bands, we present reflectivities and limb‐darkening results for two of the hazy regions generated during the SL‐9 collision. These highly reflective impact areas at infrared methane wavelengths have been studied in terms of multiple scattering radiative models. The results for both the H and D/G impact remnants show that a considerable amount of particles have been injected in Jovian stratospheric levels from ∼1 mbar to the upper level of the assumed Jovian tropospheric NH3 ice haze (at ∼350 mbar). Our models suggest a concentration of small particles, with mean radius of 0.15 µm for a real refractive index of 1.7. The imaginary refractive indices are in the range 1 × 10−4 to 1 × 10−3. For the H impact region, absorption at 2.0 µm is enhanced compared to unperturbed regions at the same scattering geometry. This enhancement is not observed in the older D/G complex.
Formation of a very thin current sheet in the near‐Earth magnetotail and the explosive growth phase of substorms
Authors: Lee L. C. , Zhang L. , Choe G. S. , Cai H. J.
Published: February 2004

A magnetofrictional method is used to construct two‐dimensional MHD equilibria of the Earth's magnetosphere for a given distribution of entropy function (S = pVγ), where p is the plasma pressure and V is the tube volume per unit magnetic flux. It is found that a very thin current sheet with Bz < 0.5nT and thickness < 1000km can be formed in the near‐earth magnetotail (x ∼ −8 to −20 Re) during the growth phase of substorm. The tail current sheets are found to become thinner as the entropy or the entropy gradient increases. We suggest that the new “entropy anti‐diffusion instability” associated with plasma transport across field lines leads to magnetic field dipolarization and accelerates the formation of thin current sheet, which may explain the observed explosive growth phase of substorms.
Seismic‐geodynamic determination of the origin of excess ellipticity of the core‐mantle boundary
Authors: Forte Alessandro M. , Mitrovica Jerry X. , Woodward Robert L.
Published: February 2004

We demonstrate that the excess dynamic ellipticity of the core‐mantle‐boundary (CMB) inferred from the period of the Earth's retrograde free core nutation (RFCN) may be explained in terms of viscous stresses generated by mantle convective flow. The 3D elastic structure and radial viscosity profile which define the model used in the viscous flow calculations are constrained to simultaneously reconcile a large set of seismic and geodynamic data.
Aftershock source mechanisms from the June 9, 1994, Deep Bolivian Earthquake
Authors: Tinker Mark Andrew , Wallace Terry C. , Beck Susan L. , Silver Paul G. , Zandt George
Published: February 2004

The Mw 8.3 Bolivia earthquake occurred on June 9, 1994, at a depth of 636 km. This is the largest deep event in recorded history and ruptured a portion of the down‐going Nazca slab unknown to have ruptured previously. We recorded the main shock and aftershocks on the BANJO and SEDA portable, broadband seismic arrays deployed in Bolivia during this event. Myers et al. (this issue) identified and located 36 aftershocks (M>2) for the 10‐day period following the main shock. We use a grid search technique to determine focal mechanisms for 12 of these aftershocks ranging in magnitude from 2.7 to 5.3. We compare the observed P to SV and SH ratios to a series of synthetics that represent different fault plane orientations. We find consistent focal mechanisms with the T‐axis roughly horizontal and oriented approximately east‐west, and the P‐axis predominantly vertical. The aftershock focal mechanisms indicate a rotation of the P‐axis within the slab from down‐dip compression prior to the main shock to a near‐vertical direction afterwards. This observation is consistent with the release of shear stress on the near‐horizontal rupture plane and the subsequent rotation of the maximum compressive stress to a fault ‐normal orientation.
Laboratory and seismological observations of lower mantle isotropy
Authors: Meade Charles , Silver Paul G. , Kaneshima Satoshi
Published: February 2004

We have carried out seismological and mineral physics investigations to identify the extent and origin of elastic anisotropy in the lower mantle. Based on observations of shear wave splitting, we conclude that the lower mantle is effectively isotropic. This result is surprising since the lower mantle is composed of elastically anisotropic minerals (silicate perovskite, MgO, SiO2) that have been extensively strained by geologic processes. To reconcile the seismic observations, we have measured texture development during deformation and high temperature recrystallization of mantle silicates. We show that these experiments provide a direct explanation for the shear wave splitting measurements: the lower mantle appears isotropic because silicate perovskite maintains an isotropic texture during deformation and recrystallization.
Noachian and Hesperian modification of the original chryse impact basin topography
Authors: Stockman S. A. , Frey H.
Published: February 2004

We propose a new center and ring assignment for the original Chryse impact basin based upon photogeologic mapping of Noachian outcrops and re‐examination of the published geology using orthographic projections. While others have centered the Chryse impact on the topographic low associated with Hesperian volcanic and fluvial deposits, we suggest that the center of the Noachian‐age excavation cavity was located approximately 800 km to the north, and that the basin topography was modified significantly from the Noachian into the Hesperian. Evolution of the topographic low included structural modification by a later impact centered in Acidalia, restricted volcanic deposition and loading, localized subsidence, and restricted deposition from the circum‐Chryse outflow channels.
The latitudinal distribution (50°N–50°S) of NO2 and O3 in October/November 1990
Authors: Kreher K. , Fiedler M. , Gomer T. , Stutz J. , Platt U.
Published: February 2004

The present paper reports on shipboard DOAS (Differential Optical Absorption Spectroscopy) measurements of stratospheric NO2 and O3 vertical column abundances. The data were obtained between 50°N and 50°S during October and November 1990, before the eruption of Mt. Pinatubo. The daily measurements were made during morning and evening twilight. The retrieved column abundances were averaged over a solar zenith angle (SZA) range of 87° to 91°, for both NO2 measured in the visible wavelength region and for O3 measured in the UV. The instrumentation and the data evaluation technique are briefly discussed.

In the Northern Hemisphere (autumn) the NO2 vertical column abundances show a steady decrease from 50°N to the equator, while in the Southern Hemisphere (spring) the corresponding increase with latitude is both more pronounced and more variable. Depending on latitude, the measured mean am/pm ratios are between 0.55 and 0.63, generally consistent with the pre‐Pinatubo overnight conversion of NO2 to N2O5.

Within the estimated error of our DOAS measurements, the retrieved O3 vertical column amounts are in good agreement with the TOMS data. Both data sets show a slightly different longitudinal distribution than expected from the O3 climatology.
Changes in brightness of Jupiter's radiation belts at 13 and 22 cm during and after impacts of comet SL‐9
Authors: Leblanc Y. , Dulk G. A.
Published: February 2004

The Australia Telescope provided 2‐D images of the changes in Jupiter radiation belts caused by comet Shoemaker‐Levy 9 with a resolution of 4″ × 18″ at 13 cm and 7″ × 30″ at 22 cm. Before the impacts, at 22 cm, the images and 1‐D scans along the magnetic equator were very similar to those observed at 20 cm by the VLA 12 years earlier at the same longitudes. During and after the SL‐9 impacts, the belts were brighter than before, and remained bright at least 12 days after the first impact. The increase in brightness was not the same at all longitudes; it was almost entirely confined to one hemisphere, between 100° and 240° longitude, where the magnetic field is the stronger. The separation between the peaks of intensity decreased from 2.92 RJ to 2.82 RJ over the 12 days. The changes in the belts at 13 cm were similar to those observed at 22 cm. The new asymmetry implies the existence of a new or a newly‐accelerated population of electrons confined to a portion of the magnetic field of the planet.