bluuebunny/crossref_metadata_2025 huggingface dataset:
Global climate model high‐obliquity solutions to the ancient climate puzzles of the Faint‐Young Sun Paradox and low‐altitude Proterozoic glaciation
Authors: Jenkins Gregory S.
Published: September 2002

There is a general consensus that warm temperatures throughout Precambrian were caused by very high CO2 or CH4 atmospheric concentrations. Episodes of glaciation in the early and late Proterozoic were caused by a reduction in these greenhouse gases through intensive weathering or large reduction in the methane producing bacteria population. There are suggestions that these episodes were global because the continents occupied low latitudes at or near the time of glaciation. Here it is reported that high‐obliquity throughout the Precambrian serves as a mechanism for producing warm temperatures throughout the Archean and glaciation in the late Proterozoic. In this paper, Global climate model simulations using high‐obliquity values that range from 54° to 70° are undertaken. These simulations use a simple 50‐m slab ocean, a faster rotation rate corresponding to an 18 hour day, an idealized supercontinent located in the tropics which has a north‐south mountain chain and CO2 concentrations which range from 170 to 3400 ppmv. Further, four simulations under global ocean conditions, faster rotation rate and reduced solar constant (15 and 20%) using present‐day and high‐obliquity values have been undertaken. The simulations show warm conditions with high‐obliquity but Snowball Earth conditions for the present‐day obliquity in the global ocean simulations. Near Snowball Earth conditions also occur for the tropical supercontinent simulation for an obliquity of 70° and CO2 concentrations less than 1020 ppmv. Together, these simulations show that for high‐obliquity the ocean remains relatively warm while land areas are cooled because of their lower heat capacity. Large fluxes of latent heat from the ocean balance the longwave energy lost at the top of the atmosphere thereby keeping high latitudes warm during winter seasons. It is suggested from this results that lower and upper limits of 65°–70° for obliquity serve as a solution for the Faint‐Young Sun Paradox and low‐latitude Proterozoic. Finally, the high‐obliquity‐climate relationship sheds some insight on the carbon isotope (δ13C) excursions during the late Proterozoic.
Summertime NOy speciation at the Jungfraujoch, 3580 m above sea level, Switzerland
Authors: Zellweger C. , Ammann M. , Buchmann B. , Hofer P. , Lugauer M. , Rüttimann R. , Streit N. , Weingartner E. , Baltensperger U.
Published: September 2002

During summer 1997, speciated reactive nitrogen (NO, NO2, peroxyacetyl nitrate (PAN), HNO3, and particulate nitrate) was measured in conjunction with total reactive nitrogen (NOy) at the high‐alpine research station Jungfraujoch (JFJ), 3580 m above sea level (asl). The individually measured NOy components averaged to 82% of total NOy. PAN was the most abundant reactive nitrogen compound and composed on average 36% of NOy, followed by NOx, (22%), particulate nitrate (17%), and HNO3 (7%). The NOx/NOy ratio averaged 0.25, but significantly lower values (0.15–0.20) were observed in the presence of high NOy mixing ratios. A classification of the data by synoptic weather conditions indicated that thermally driven vertical transport has a strong impact on the mixing ratios measured at the JFJ during summer. A strong diurnal cycle with maximum mixing ratios in the late afternoon was observed for convective days with north‐westerly advection at 500 hPa. In contrast, during a period of convective days with a wind speed below 7.5 m s−1 at 500 hPa, no obvious diurnal cycle was observed. Under these meteorological conditions the convective boundary layer can be significantly higher over the Alps (i.e., around 4 km asl) than over the surrounding lowlands. Subsequent advection may finally result in the export of reactive nitrogen reservoir compounds to the free troposphere and hence influence global atmospheric chemistry.
Study of water activities of supersaturated aerosols of sodium and ammonium salts
Authors: Ha Zhanyao , Choy Lee , Chan Chak Keung
Published: September 2002

Ammonium and sodium are important cations found in atmospheric aerosols. While the water activities of sodium salts and their mixtures have been studied by many researchers, data for ammonium mixtures are rather limited because of the volatility of ammonium chloride and ammonium nitrate. In this research, water activities of ternary solutions of NH4Cl‐NH4NO3, NH4Cl‐(NH4)2SO4, NaCl‐NH4Cl, NaNO3‐NH4NO3, NaCl‐NH4NO3, and NaNO3‐(NH4)2SO4 of single levitated particles from dilute concentrations to high supersaturations are measured using an electrodynamic balance (EDB). On the basis of a well‐calibrated change of relative humidity (RH) into the EDB, water activity data of a droplet at various concentrations at atmospheric pressure were measured in less than 1 hour. The evaporation loss of NH4Cl and/or NH4NO3 is negligible in this short experimental period at room temperature and atmospheric pressure. The measured results are compared with data from available literature and used to evaluate the performance of four commonly used thermodynamic models: the Zdanovskii‐Stokes‐Robinson (ZSR), Kusik and Meissner (KM), and Pitzer models and aerosol inorganic model (AIM). Predictions made using these models are, in general, consistent with our experimental data. In terms of the total mass fraction of solute (mfs) as a function of water activity (aw), the average standard deviations between the predictions of the models and the measured data are 0.005 for ZSR, 0.011 for KM, 0.016 for Pitzer, and 0.012 for AIM. The ZSR model is recommended for estimating the water activities of Na+−NH4+ salt mixtures because of the high accuracy of its prediction and the wide range of water activities to which it can be applied. Complete crystallization resulting in dry solid particles was observed in the ternary solutions of NH4Cl‐(NH4)2SO4 and NaCl‐NH4Cl. Partial crystallization was observed in the solutions of NaCl‐NH4NO3 and NaNO3‐(NH4)2SO4, in which case the droplets continue to release water as RH decreases after crystallization. A notable exception was the NaNO3‐(NH4)2SO4 of mole ratio 3∶1, where the mfs decreases as RH decreases. The AIM was used to predict the possible phase compositions of these partially crystallized particles. Further research is needed to understand the hygroscopic properties and phase compositions of partially crystallized multicomponent aerosols at low relative humidities.
Atmospheric response time of cosmogenic 14CO to changes in solar activity
Authors: Jöckel Patrick , Brenninkmeijer Carl A. M. , Lawrence Mark G.
Published: September 2002

Carbon‐14 monoxide is an unique tracer for certain important aspects of atmospheric chemistry and transport. Particularly, it may provide an independent means of testing model‐based OH distributions and seasonality. The cosmogenic source strength of 14CO is inversely correlated with the solar activity. This time dependence demands a rescaling of 14CO measurements or model results to comparable conditions regarding the solar cycle, in order to achieve information on global distributions and trends of OH. For this rescaling, a two‐parameter filter is derived using the three‐dimensional global Model of Atmospheric Transport and Chemistry (MATCH). The filter function defines the atmospheric response time of the 14CO concentration to changes in the total cosmogenic 14CO source strength. The latitude dependence of the response time in addition to the atmospheric lifetime of the tracer provides useful information about the model's transport properties, especially about the role of stratosphere‐troposphere exchange. It may serve as a direct indicator for possible solutions of the unresolved hemispheric asymmetry of the atmospheric 14CO concentration. The general concept can be applied as a diagnostic test for coupled changes in atmospheric transport and chemistry in models. Time series of 14CO measurements can provide this information about the real atmosphere.
A Lagrangian simulation of supersonic and subsonic aircraft exhaust emissions
Authors: Schoeberl M. R. , Morris G. A.
Published: September 2002

To estimate the effect of subsonic and supersonic aircraft exhaust on the stratospheric concentration of NOy, we employ a trajectory model initialized with air parcels based on the standard release scenarios. The supersonic exhaust simulations are in good agreement with two‐dimensional (2‐D) and 3‐D model results and show a perturbation of about 1–2 ppbv of NOy in the stratosphere. The subsonic simulations show that subsonic emissions are almost entirely trapped below the 380 K potential temperature surface. Our subsonic results contradict results from most other models, which show exhaust products penetrating above 380 K, as summarized by Danilin et al. [1998] and Intergovernmental Panel on Climate Change (IPCC) [1999]. The disagreement can likely be attributed to the excessive model diffusion of the strong vertical gradient in NOy that forms at the boundary between the emission zone and the stratosphere above 380 K. Our results suggest that previous assessments of the impact of subsonic exhaust emission on the stratospheric region above 380 K should be considered to be an upper bound.
A parameterization of aerosol activation: 2. Multiple aerosol types
Authors: Abdul‐Razzak Hayder , Ghan Steven J.
Published: September 2002

A parameterization of the activation of a lognormal size distribution of aerosols to form cloud droplets is extended to the case of multiple externally mixed lognormal modes, each composed of a uniform internal mixture of soluble and insoluble material. The Köhler theory is used to relate the aerosol size distribution and composition to the number activated as a function of maximum supersaturation. The supersaturation balance is used to determine the maximum supersaturation, accounting for particle growth both before and after the particles are activated. Comparison of the parameterized activation of two competing aerosol modes with detailed numerical simulations of the activation process yields agreement to within 10% under a wide variety of conditions, including diverse size distributions, number concentrations, compositions, and updraft velocities. The parametization error exceeds 10% only when the mode radius of the two size distributions differs by an order of magnitude. Errors for the mass fraction activated are always much less than errors for the number fraction activated.
Supercooling of type 1 polar stratospheric clouds: The freezing of submicron nitric acid aerosols having HNO3 mol fractions less than 0.5
Authors: Bertram A. K. , Dickens D. B. , Sloan J. J.
Published: September 2002

We report the freezing temperatures for nitric acid/water aerosol particles having radii about 0.2 μm and concentrations in the range 0.24 < xHNO3 < 0.46 (xHNO3 = nitric acid mol fraction). The aerosols supercool by up to 85 K. The droplets having xHNO3 = 0.33 have the highest freezing point in this concentration range; they supercool by 60 K and produce nitric acid dihydrate (NAD) upon freezing. We describe a simplified principal components analysis that improves the detection of the freezing point and allows the identification of the solid that precipitates upon freezing. This procedure shows that for the temperature range and experimental conditions explored, only NAD precipitates in the concentration range 0.33 < xHNO3 < 0.42 and both NAD and nitric acid trihydrate (NAT) precipitate for the range 0.24 < xHNO3 < 0.33. The temperature dependences of our measured freezing points correlate strongly with NAD saturation ratios, but have no relationship with NAT supersaturation at any concentration. Thus NAD nucleates preferentially from supercooled aerosols in this composition range, probably because of kinetic constraints on the nucleation of NAT. We conclude that NAD nucleation is possible during rapid cooling events in the polar stratosphere, if temperatures lower than 185 K are reached. During the freezing of stratospheric aerosol droplets in this concentration range, it seems likely that NAD nucleates first, thereby providing a surface on which NAT may crystallize.
Measurements of the carbon and hydrogen isotopes of atmospheric methane at Izaña, Tenerife: Seasonal cycles and synoptic‐scale variations
Authors: Bergamaschi Peter , Bräunlich Maya , Marik Thomas , Brenninkmeijer Carl A. M.
Published: September 2002

Atmospheric CH4 and its stable isotope ratios 13C/12C and D/H have been investigated at the Global Atmospheric Watch station Izaña, Tenerife (28°N, 16°W, 2370 m above sea level), since late 1996. Every fortnight both spot samples and integral samples were taken, the latter collected continuously over 2‐week periods. While spot samples show considerable synoptic‐scale variability, the continuous samples clearly define seasonal cycles of CH4, δ13C, and δD, with peak‐to‐peak amplitudes of 30 ppb, 0.2‰ and 3.5‰, respectively. The measurement of the δD seasonality is the first ever reported for atmospheric background CH4 and has been made possible by the development of a tunable diode laser based optical Methane Isotopomer Spectrometer (MISOS). The δD is well in phase with CH4 mixing ratios, and the compact correlation between them allows to derive the average kinetic isotope effect (KIE) of the tropospheric sinks to be 1.23±0.04 (1σ), consistent with recent laboratory measurements of the kinetic isotope effect in the reaction of CH4±OH. In contrast to δD, δ13C is out of phase to CH4 mixing ratios, clearly indicating that δ13C is not only effected by the KIE of the CH4±OH reaction, but also by seasonally varying source mixtures. Considerable short‐term variations are observed in the isotopic composition and mixing ratios of the spot samples. This can be attributed to different origin of air masses arriving at Izaña. Significant CH4 enhancement is observed for air masses originating from the North American continent or Europe, of 12.7±11.7 and 13.4±0.3 ppb, respectively, while air from the African continent or the North Atlantic is depleted (−11.2±2.8 and −7.0 ± 8.2 ppb). Deviations from the mean seasonal CH4 cycle are correlated with significant deviations of δ13C and δD, allowing to estimate the δ13C and δD signatures of major source regions. Furthermore, deviations in CH4 mixing ratio are also clearly correlated with deviations of CO and SF6 mixing ratios. A three‐dimensional inverse model is employed in order to assist with the interpretation of observational data. In general, the model shows excellent agreement with the observed mean seasonal cycles of CH4, δ13C, and δD, including the observed phase behavior.
European Conference on Atmospheric UV Radiation: Overview
Authors: Taalas Petteri , Amanatidis Georgios T. , Heikkilä Anu
Published: September 2002

Interest in atmospheric ultraviolet radiation (UV) research has increased considerably since the discovery of the Antarctic ozone “hole” and after indications of the existence of a similar kind of potential in northern high latitudes. UV measuring and research activities have grown accordingly, including development of new instruments, improvement in quality assurance/quality control methodologies, as well as development of models. Modeling methodologies have been applied for development of spaceborne techniques for estimating the global distribution of solar UV radiation. The European Conference on Atmospheric UV Radiation (ECUV), which was held on June 28 to July 2, 1998 in Helsinki, Finland, brought together 160 scientists from all parts of the world. The Journal of Geophysical Research (JGR) ECUV special issue summarizes 32 papers presented at the conference. New UV trend analyses based on spectral and broadband measurements and modeled UV data for the most recent decades indicate increases of UV‐B radiation in various parts of Europe. Additional observational and modeled information on the impact of cloudiness, total ozone, ozone profiles, aerosols, and albedo on ground level UV irradiance has been gathered. The regional UV albedo has been studied over snow‐covered surfaces, vegetation, and sea areas by aircraft‐based, ground‐based, and modeled methods. Further instrument development, improved QA/QC practices, and the application of data correction methods have already taken place but are also clearly needed in the future. Different methods for estimating UV irradiance using spaceborne information have been developed. Additional efforts to improve our understanding of the impact of cloudiness, albedo, and aerosol on UV are expected to improve spaceborne UV retrievals in the future. In Europe the European Commission and national funding agencies have been supporting research projects aimed at understanding the UV climatology in Europe through UV instrument development and intercomparisons, UV database establishment, and UV process, trend, and scenario studies.
Comparison of observed and simulated NO2 photodissociation frequencies in a cloudless atmosphere and in continental boundary layer clouds
Authors: Früh Barbara , Trautmann Thomas , Wendisch Manfred , Keil Andreas
Published: September 2002

To validate radiative transfer models for the actinic flux and photodissociation frequencies, it is essential to compare model results with observed data. For this purpose, we used in situ vertical profile measurements of thermodynamic, aerosol particle, and cloud drop properties in order to compare simulated photodissociation frequencies of NO2, J(NO2), with respective measurements. Two case studies in cloudless and four cases in cloudy conditions were investigated. We found that the deviations between the simulated J(NO2) profiles and the measured data are less than 10% in all cases which is in the range given by the measurement uncertainties. For quantifying the sensitivity of the simulated J(NO2) to the different input parameters the impact of experimental uncertainties on the measurement of aerosol and drop microphysical parameters was investigated. In cloudless conditions the largest effect on the modeled J(NO2) is due to the uncertainty of the aerosol volume (about 2% at ground level). In cloudy atmosphere the uncertainty of the observed liquid water content (about 20% at ground level) is of largest importance. Furthermore, the influence of different quantum yield and absorption cross section spectra of NO2 was examined: The different spectra result in changes of ±5% for the computed J(NO2). Additionally, we investigated the effect of different solution methods for the radiative transfer equation on J(NO2). The delta‐four‐stream method was shown to be a very fast and accurate technique with errors of less than 2%. On the other hand, the delta‐two‐stream method may cause relative errors of up to 10 to 20%.
Erosion rates on Mars and implications for climate change: Constraints from the Pathfinder landing site
Authors: Golombek M. P. , Bridges N. T.
Published: September 2002

The observation that the Mars Pathfinder landing site looks very similar to its appearance after it was deposited by catastrophic floods around 1.8–3.5 Ga allows quantitative constraints to be placed on the rate of change of the site since that time. The abundance of erosional features such as an exhumed former soil horizon, sculpted wind tails, ripplelike and other lag deposits, and ventifacts (fluted and grooved rocks) all suggest the site has undergone net deflation or loss of 3–7 cm of material. The presence of barchan dunes and ventifacts argues for erosion by saltating crystalline sand‐size particles entrained in the wind. Most ventifacts probably formed soon after the catastrophic flood, which likely introduced a large, fresh supply of sand‐size particles distributed across the rocky plain. The strongest winds blew toward the northwest during this time, resulting in the sculpting of ventifacts, deflation of the surface, collections of dunes within Big Crater and other lows, and possibly preferentially eroding small crater rims. The predominant wind direction changed to blow toward the southwest, similar to today. These winds further deflated the surface, completed the deposition of sand‐size material in dunes and ultimately trapped these dunes in lows. The erosional features observed by Pathfinder indicate extremely low longterm deflation rates of 0.01–0.04 nm/yr since the end of the Hesperian (1.8–3.5 Ga) similar to less precise rates of <1 nm/yr based on the preservation of craters at the Viking 1 and Pathfinder landing sites. Short‐term redistribution rates (deposition and removal) of atmospheric dust at the Pathfinder landing site and preexisting dust and sand at other locations on Mars are up to 105nm/yr. Estimates of erosion rates on Mars show a rapid decrease by 3–6 orders of magnitude from 102–104nm/yr in Noachian terrains (characterized by rimless, flat‐floored craters and valley networks) to the exceedingly slow rates (10−2–10−1nm/yr) operating during the Hesperian and Amazonian. Noachian erosion rates comparable to low continental denudation rates on Earth are consistent with erosion by running water and perhaps a more clement climate. The rapid decrease in erosion rates is consistent with a major climatic change during the Noachian, at the tail end of heavy bombardment, and a cold, dry, desiccating climate similar to today's since that time.
Mars secular obliquity change due to water ice caps
Authors: Parry Rubincam David
Published: September 2002

Mars may have substantially changed its average axial tilt over geologic time due to the waxing and waning of water ice caps through the phenomenon of climate friction (also called obliquity‐oblateness feedback). Depending upon Mars' climate and internal structure, water caps of the order of 1017–1018kg cycling with the obliquity oscillations could have either increased or decreased the average obliquity by possibly tens of degrees. This is in contrast to previous results, which indicated that 1017kg carbon dioxide caps only increased the axial tilt. Since the south polar cap appears to be mostly uncompensated, Mars may be largely rigid on the obliquity timescale. Further, Mars may be a water‐rich planet so that there is a large phase angle between insolation forcing and the size of the obliquity‐driven water caps. A stiff, water‐rich planet indicates the obliquity may have decreased over the eons. Such a decrease might account for the apparent youthfulness of the polar layered terrain, the idea being that fewer volatiles were available to be cycled into and out of the terrain at high obliquity because of more even insolation between equator and pole, so that the movement of volatiles produced thin layers or perhaps no layers at all. As the obliquity decreased, the insolation contrast between high and low latitiudes increased, and more volatiles might have shuttled into and out of the polar regions, forming the observed thick layers. In another but perhaps less likely scenario, Mars' average obliquity may have either increased or decreased until it became “stuck” at its present value of ∼24°. In this case the idea is that Mars' climate dynamics altered as the average tilt changed. Once the rate of increase in tilt caused by the deformation of the solid planet equaled the rate of decrease caused by the caps, the obliquity evolution ceased, leaving Mars at its present tilt.
A spectroscopic search for molecular oxygen in the Venus middle atmosphere
Authors: Mills Franklin P.
Published: September 2002

The results from a new attempt to detect ground state molecular oxygen in the Venus middle atmosphere are presented. The upper limit inferred from the January 1995 observations using the Ultra High Resolution Facility (UHRF) spectrometer at the Anglo‐Australian Observatory (AAO) is equivalent to a uniform volume mixing ratio of 3×10−6 at and above the 300 mbar level. This is comparable to the upper limit inferred from observations in the early 1970s and is a factor of 10 larger than the upper limit inferred from observations in 1982. These observations indicate the observed secular decrease in the abundance of SO2 and SO from the early 1980s to 1995 did not result in an O2 abundance that could be detected within the sensitivity of the present observations. Radiative transfer calculations show the sensitivity of observations to the vertical profile of O2. These calculations show that all three spectroscopic observations (1973, 1982, and 1995) would have detected the presence of O2 if its mixing ratio near 50 km altitude had been 10−5 as was reported by in situ measurements in 1980.
Diaspores and related hydroxides: Spectral‐compositional properties and implications for Mars
Authors: Cloutis Edward A. , Bell James F.
Published: September 2002

Differences in spectral reflectance properties (0.3–26 μm) of a suite of metal hydroxides (gibbsite, böhmite, diaspore, goethite, and manganite) have been found to be a function of both structural and compositional differences between these minerals. The properties of the O‐H stretching fundamental bands in particular can be used to identify the presence of these minerals and to discriminate isostructural and heteromorphous species. This is due to the fact that the O‐H stretching fundamental bands are very intense in these minerals and occur at longer wavelengths than those of other minerals, and also due to the fact that these metal hydroxides lack strong absorption bands at shorter wavelengths. In the case of dimorphous minerals such as böhmite (γ‐AlO(OH)) and diaspore (α‐AlO(OH)), differences in the wavelength positions of O‐H stretching fundamental bands can be of the order of 0.1–0.3 μm. Reflectance spectra of metal hydroxide‐bearing “ores” (bauxites) and impure samples indicate that accessory phases, in particular iron‐bearing minerals, have a more pronounced effect on spectral reflectance toward shorter wavelengths. Intimate mixtures of diaspore + orthopyroxene indicate that diaspore abundances as low as 5 wt% (or less) can be detected by the appearance of characteristic absorption bands at 3.33 and 3.41 μm. This finding has particular relevance to the study of near‐infrared spectra of Mars, in that the conditions which favor diaspore formation on the Earth may have also been prevalent early in Mars' history. If diaspore did form on Mars in the past, it is likely to have persisted to the present.
Variation of lunar sodium during passage of the Moon through the Earth's magnetotail
Authors: Potter Andrew E. , Killen Rosemary M. , Morgan Thomas H.
Published: September 2002

We measured sodium emission above the lunar equator over a range of lunar altitudes from 100 to 4000 km. The measurements were repeated approximately every 24 hours from June 7 to 16, 1998, covering the period during which the Moon passed through the Earth's magnetotail. Sodium temperatures derived from the altitude dependence of emission intensity ranged from 1200 to 2900 K. This result supports the view that photodesorption is a primary source of sodium in the exosphere since the most probable temperature of sodium from this source is in this range. Passage of the Moon through the Earth's magnetotail (where solar wind is essentially absent) affected the sodium density, such that it was higher before the Moon entered the Earth's magnetotail than after the Moon left it. This suggests that the solar wind plays a role in production of lunar sodium. We propose that its function is to mobilize sodium and bring it to the surface, where photodesorption can eject it into the exosphere. A two‐step process such as this could help to explain the latitude dependence of sodium density, which varies as the second or higher power of cosine latitude.
Influence of early plate tectonics on the thermal evolution and magnetic field of Mars
Authors: Nimmo F. , Stevenson D. J.
Published: September 2002

Recent magnetic studies of Mars suggest that (1) it possessed a periodically reversing magnetic field for the first ∼500 Myr of its existence and (2) plate tectonics may have been operating during this time. On Earth the geodynamo is thought to occur because of convection in the outer core. This paper estimates the amount of heat the Martian core can conduct in the absence of convection. It uses parameterized, variable‐viscosity thermal evolution models to show that the core heat flux increases if the planet's surface heat flux is increased above the value required to eliminate instantaneous radiogenic heat production. Conversely, a sudden reduction in surface heat flux causes the mantle to heat up and the core heat flux to become negative. Thus, if plate tectonics, or some other process causing high surface heat flux, was occurring on early Mars, it is likely to have caused convection in the core and hence generated a magnetic field. Conversely, a reduction in surface heat flux would probably have caused the core to stop convecting and shut off the magnetic field. There is thus an important link between surface processes and core magnetism, which may also be relevant to planets such as Earth and Venus.
Small‐scale models of multiring basins
Authors: Allemand Pascal , Thomas Pierre
Published: September 2002

Small‐scale sand‐silicone simulations of multiring impact structures have been undertaken in order to understand the effects of the rheology of the lithosphere on the variability of natural multiring structures. For low sand‐silicone thickness ratio (1:3), brittle strain is accommodated by spiral strike‐slip faults. For higher sand‐silicone ratios (1:1 or 2:1), an inner concentric ring affected by strike‐slip faults is relayed by an external ring affected by concentric normal faults. The diameter of the inner ring decreases with the increase of the sand‐silicone thickness ratio. It is suggested that the flexure of the brittle layer due to the silicone flow is responsible for the brittle strain field which is enhanced by the channel flow of the lower crust. The characteristic geometry of the intersection of conjugated strike‐slip faults can be observed around large multiring basins on silicate crust such as Orientale on the Moon and on icy crust, such as Valhalla on Callisto and Gilgamesh on Ganymede. The strain field around these large craters is discussed in terms of mechanical properties of the lithospheres. On the Moon, large craters without relaxation faults, such as Imbrium are located on thin crust regions. The crust was too thin to have a ductile lower layer at the time of impact. Gilgamesh on Ganymede is surrounded mainly by strike‐slip faults. Asgard on Callisto has the same diameter as Gilgamesh but is surrounded by concentric normal faults. The brittle‐ductile thickness ratio is thus higher on Callisto than on Ganymede.
Accumulation mechanisms and the weathering of Antarctic equilibrated ordinary chondrites
Authors: Benoit P. H. , Sears D. W. G.
Published: September 2002

Induced thermoluminescence (TL) is used to quantitatively evaluate the degree of weathering of meteorites found in Antarctica. We find a weak correlation between TL sensitivity and descriptions of weathering in hand specimens, the highly weathered meteorites having lower TL sensitivity than unweathered meteorites. Analysis of samples taken throughout large meteorites shows that the heterogeneity in TL sensitivity within meteorite finds is not large relative to the range exhibited by different weathered meteorites. The TL sensitivity values can be restored by minimal acid washing, suggesting the lower TL sensitivities of weathered meteorites reflects thin weathering rims on mineral grains or coating of these grains by iron oxides produced by hydration and oxidation of metal and sulfides. Small meteorites may tend to be more highly weathered than large meteorites at the Allan Hills ice fields. We find that meteorite fragments >150 g may take up to 300,000 years to reach the highest degrees of weathering, while meteorites <150 g require <40,000 years. However, at other fields, local environmental conditions and variability in terrestrial history are more important in determining weathering than size alone. Weathering correlates poorly with surface exposure duration, presumably because weathering occurs primarily during interglacial periods. The Allan Hills locality has served as a fairly stable surface over the last 100,000 years or so and has efficiently preserved both small and large meteorites. Meteorites from Lewis Cliff, however, have experienced extensive weathering, probably because of increased surface melt water from nearby outcrops. Meteorites from the Elephant Moraine locality tend to exhibit only minor degrees of weathering, but small meteorites are less weathered than large meteorites, which we suggest is due to the loss of small meteorites by aeolian transport.
Midinfrared optical constants of calcite and their relationship to particle size effects in thermal emission spectra of granular calcite
Authors: Lane Melissa D.
Published: September 2002

Application of dispersion theory and the Fresnel reflection equations in this midinfrared (2000–400 cm−1) emission study of calcite enables 11 oscillators and their band parameters to be described. Of the 11, two have not been defined previously in the literature: one for the ordinary ray (at 891 cm−1) and one for the extraordinary ray (at 848 cm−1). Updated optical constants (n and k) of calcite are presented as well. Emissivity spectra of a suite of calcite particulate samples are also shown. The variations in the spectral shape of the fundamental and transparency bands that are influenced by particle size are related to the new optical constants. Type I, II, and III band behaviors are discussed as they relate to the optical and physical properties of the calcite particle size fractions. It is shown here that for Type II band behavior, the spectral bands that invert with decreased particle size shift to the position of maximum k for that band rather than to “lower frequencies,” as has been concluded in previous studies.
Incorporation and preservation of Mg in Globigerinoides sacculifer: implications for reconstructing the temperature and 18O/16O of seawater
Authors: Rosenthal Y. , Lohmann G. P. , Lohmann K. C , Sherrell R. M.
Published: September 2002

Using bathymetric transects of surface sediments underlying similar sea surface temperatures but exposed to increasing dissolution, we examined the processes which affect the relationship between foraminiferal Mg/Ca and δ18O. We found that Globigerinoides saccculifer calcifies over a relatively large range of water depth and that this is apparent in their Mg content. On the seafloor, foraminiferal Mg/Ca is substantially altered by dissolution with the degree of alteration increasing with water depth. Selective dissolution of the chamber calcite, formed in surface waters, shifts the shell's bulk Mg/Ca and δ18O toward the chemistries of the secondary crust acquired in colder thermocline waters. The magnitude of this shift depends on both the range of temperatures over which the shell calcified and the degree to which it is subsequently dissolved. In spite of this shift the initial relationship between Mg/Ca and δ18O, determined by their temperature dependence, is maintained. We conclude that paired measurements of δ18O and Mg/Ca can be used for reconstructing δ18Owater, though care must be taken to determine where in the water column the reconstruction applies.
A reinterpretation of Mid‐Cretaceous shallow marine temperatures through model‐data comparison
Authors: Poulsen Chris J. , Barron Eric J. , Peterson William H. , Wilson Paul A.
Published: September 2002

Mid‐Cretaceous δ18O paleotemperatures are compared with temperatures predicted by an ocean general circulation model. The δ18O paleotemperatures, calculated assuming a δw of −1.0‰ (SMOW), are 5.6°C lower at low latitudes and 2.4°C higher at middle and high latitudes than the model‐predicted, mixed‐layer temperatures. The model‐data comparison is improved after accounting for the spatial variability of δw and after considering the paleohabitats of the marine organisms from which δ18O values were measured. Paleotemperatures adjusted using the Broecker [1989] δw‐S relationship and the model‐predicted salinity are 3.4°C higher at low‐latitude sites and 1.9°C lower at higher‐latitude sites. Moreover, seven of nine low‐latitude paleotemperatures intersect the model‐predicted temperature within the upper 100 m of the water column, a depth consistent with the interpreted paleohabitat of the analyzed organisms. However, adjusted paleotemperatures for three of five Southern Hemisphere high‐latitude locations are at least 9°C higher than the model‐predicted temperatures for these locations. The model‐data comparison suggests that mid‐Cretaceous δ18O values are compatible with sea surface temperatures as high or higher than those simulated by an ocean general circulation model of the mid‐Cretaceous with 4 × present‐day atmospheric pCO2.
The terrestrial ring current: Origin, formation, and decay
Authors: Daglis Ioannis A. , Thorne Richard M. , Baumjohann Wolfgang , Orsini Stefano
Published: September 2002

The terrestrial ring current is an electric current flowing toroidally around the Earth, centered at the equatorial plane and at altitudes of ∼10,000–60,000 km. Changes in this current are responsible for global decreases in the Earth's surface magnetic field, which are known as geomagnetic storms. Intense geomagnetic storms have severe effects on technological systems, such as disturbances or even permanent damage to telecommunication and navigation satellites, telecommunication cables, and power grids. The main carriers of the storm ring current are positive ions, with energies from ∼1 keV to a few hundred keV, which are trapped by the geomagnetic field and undergo an azimuthal drift. The ring current is formed by the injection of ions originating in the solar wind and the terrestrial ionosphere. The injection process involves electric fields, associated with enhanced magnetospheric convection and/or magnetospheric substorms. The quiescent ring current is carried mainly by protons of predominantly solar wind origin, while geospace activity tends to increase the abundance (both absolute and relative) of O+ ions, which are of ionospheric origin. During intense magnetic storms, the O+ abundance increases dramatically, resulting in a rapid intensification of the ring current and an O+ dominance around storm maximum. This compositional change affects, among other processes, the decay of the ring current through the species‐ and energy‐dependent charge exchange and wave‐particle scattering loss. Energetic neutral atoms, products of charge exchange, enable global imaging of the ring current and are the most promising diagnostic tool of ring current evolution. This review will cover the origin of ring current particles, their transport and acceleration, the effects of compositional variations in the ring current, the effects of substorms on ring current growth, and the dynamics of ring current decay with an emphasis on the process of charge exchange and the potential for wave scattering loss.
A review of discontinuities and Alfvén waves in interplanetary space: Ulysses results
Authors: Tsurutani Bruce T. , Ho Christian M.
Published: September 2002

The Ulysses mission is the first to explore our heliosphere at all latitudes up to ±80° and therefore is an ideal mission to study potential gradients in heliolatitude (and radial distance) of discontinuity occurrence rates and types. Directional discontinuities (DDs) are shown to be dependent on the type of solar wind streams that they are embedded in. The occurrence rate of DDs is 5–10 times higher in high‐speed streams than in slow streams. The explanation is that nonlinear Alfvén waves dominate the high‐speed streams and rotational discontinuities are the phase‐steepened edges of the Alfvén waves. Dissipation at these phase‐steepened Alfvén waves have been sought but not found. An e−(R−1)/5 decrease in discontinuity rate with increasing radial distance (R in units of AU) is partially an artifact of the selection criteria (discontinuity thickening), but dissipation at a relatively slow rate cannot be ruled out at this time. There is no obvious latitudinal gradients in discontinuity types or occurrence rates. Somewhat surprisingly, tangential discontinuities are detected at high latitudes. These have been associated with the edges of local small‐scale magnetic decreases. A pair of slow shocks were detected at 5.3 AU. The speeds are similar to fast mode shock speeds. When Alfvén waves in high‐speed streams impinge upon the Earth's magnetosphere, near‐continuous substorms (called HILDCAAs) occur, leading to the pumping of an extraordinary amount of energy into the nightside ionosphere. Current discontinuity and Alfvén wave research problems are discussed.
Remote sensing of solar magnetic fields
Authors: Lites Bruce W.
Published: September 2002

New techniques for remote sensing of solar magnetic fields now provide measures of the magnetic field vector within the solar atmosphere with high angular resolution and high precision. These measurements have enabled a much improved physical understanding of magnetic processes and phenomena in the solar atmosphere, processes that drive the variability of the Sun's radiative and particulate output. The new techniques are reviewed here in the context of the scientific advances they have fostered. Emphasis is given to techniques for inferring the field vector. The quantitative nature of the information needed to explore the solar phenomena sharply constrains the needed precision and angular resolution of the observations. These requirements are reviewed here, along with an assessment of how future improvements in observing capabilities will address these requirements. One may also attribute much of the recent advance in our understanding of solar magnetic fields to ongoing progress in techniques for analysis of the polarization measurements that underlie solar magnetometry. The status and prospects of analysis techniques are also reviewed.
Tropospheric measurements of turbulence and characteristics of Bragg scatterers using the Jicamarca VHF radar
Authors: Chau Jorge L. , Doviak Richard J. , Muschinski Andreas , Holloway Christopher L.
Published: September 2002

We apply the Holloway et al. [1997a] spaced antenna (SA) method to data collected with the Jicamarca VHF radar. Measurements are presented of an indicator of turbulent intensity (i.e., the standard deviation of the velocity fluctuation) and the parameters (orientation and correlation lengths) of horizontally anisotropic Bragg scatterers at tropospheric heights. We compare SA indicators of turbulence intensity to concurrent measurements obtained with a very narrow vertical beam (∼0.8°). This comparison results in a very good agreement. We also show that the Jicamarca observations are due to a Fresnel scattering mechanism rather than a reflecting one. Finally, we find an L‐shaped regression between the correlation lengths of Bragg scatterers and the indicators of turbulent intensity. When turbulence is weak (σt < 0.1 m s−1), the horizontal correlation lengths have a large spread of relatively large values (i.e., large aspect sensitivity), but when turbulence is strong, the horizontal correlation lengths are much smaller.
A case study of the auroral E region neutral wind on a quiet summer day: Comparison of the European Incoherent Scatter UHF radar for deriving the E region wind
Authors: Nozawa S. , Brekke A.
Published: September 2002

Comparison study of three methods of the European Incoherent Scatter (EISCAT) radar for furnishing E region neutral wind data is performed on the basis of observational data. A Special Program by using the EISCAT UHF radar located in northern Scandinavia was conducted for 24 hours starting at 1000 UT on July 14, 1997. Two modes of EISCAT Common Program 1 and 2 are combined in this Special Program, so that the capability of the tristatic method and monostatic (or beamswinging) methods are compared on the basis of observations at four heights between 99 and 108 km under geomagnetic quiet conditions. The meridional neutral wind is derived by the field‐aligned method as well. Derived wind velocities by the three methods are compared, and fairly good agreement is found in terms of a relatively longer temporal variation (∼2–3 hours). The comparison of the derived velocities shows that the tristatic method (i.e., Common Program 1) is superior to the monostatic method (i.e., Common Program 2) in terms of an instantaneous measurement, while the monostatic method has an advantage on a tidal study. Regarding the meridional wind component, the filed‐aligned method has significant advantages compared to the other methods concerning temporal data coverage, error, and height coverage. The derived neutral wind in this Special Program is compared with that from a statistical study by Nozawa and Brekke [1999a]. Generally good agreement for mean and semidiurnal winds is found with the statistical result, but relatively strong amplitude (∼35 m s−1) of 8‐hour meridional component is found in this case study. Comparison of the zonal mean wind and mean temperature is made with the CIRA (1986) model as well. Good agreement is obtained for the temperature, while poor agreement is found for the zonal mean wind.
Explicit full identification of a transient dipole source in the atmosphere from measurement of the electromagnetic fields at several points at ground level
Authors: He Sailing , Popov Mikhail , Romanov Vladimir
Published: September 2002

Identification of a transient dipole source over a perfectly conducting ground by measuring electromagnetic fields at several ground points is considered. It is shown that the full information concerning the dipole source (including the location, the orientation, and the transient moment of the dipole) can be determined uniquely by measuring the electromagnetic fields at two ground points if the dipole orientation does not lie on the plane formed by the dipole and the two measurement points. Explicit identification formulas are given to reconstruct all the parameters which describe the transient dipole source. Under any circumstances all the parameters can be determined by measurements at any three ground points which are not along a line. The reconstruction algorithm is tested with clean and noisy data.
Scattering layer thickness and position estimated by radar frequency domain interferometry: 1. Effects of the limited horizontal extent and advection of the scattering layers.
Authors: Luce H. , Röttger J. , Yamamoto M. , Fukao S.
Published: September 2002

Frequency domain interferometry (FDI) is a mesosphere‐stratosphere‐troposphere radar technique used for probing thin‐layered structures of the atmosphere. The position and thickness of a scattering layer embedded within the illuminated volume can be deduced from the complex coherence of received signals at two closely spaced frequencies. This technique has permitted us to identify layered structures (called “FDI layers”) with thicknesses estimated to be around 50–200 m in the troposphere‐stratosphere. However, its application needs very restrictive hypotheses. For example, the layers are assumed to have a large extent in the horizontal plane with respect to the horizontal extent defined by the ‐6‐dB angular width of the two‐way antenna pattern. This hypothesis seems not always to be verified in light of observations with other radar techniques. In this paper, theoretical calculations are performed in order to estimate the consequences of the limited extent of the scattering structure in the horizontal plane on the FDI parameters, i.e. the vertical extent and position in height of the single layer. The first analysis concerns the effects of the limited extent by itself, and the second one deals with the consequences of the advection of the scattering structure by the wind. It is shown that in some limiting cases, substantial biases in the FDI parameters can occur. Finally, this work stresses the necessity to confront the FDI data with other technique measurement results or to complete it with space domain interferometry data. A future paper will be devoted to the study of the effects of the tilt of a scattering layer on the thickness and position estimations.