bluuebunny/crossref_metadata_2025 huggingface dataset:
Evolution of the Qinling Orogenic Belt
Authors: Huang W. , Wu Z. W.
Published: February 2008

Detailed field mapping, geophysics, and geochemistry show that the Qinling orogenic belt is a southward vergent tectonic wedge consisting of six regional thrust sheets (the Lushan, Luanchun, Qinling, Douling, Wudang, and Zhuqi thrust sheets). These regional thrusts underwent displacement at the end of the mid‐Triassic and the end of the late Jurassic due to the collision between the North and South China blocks and subsequent detachment. The Qinling orogenic belt is thus an Indosinian‐Yanshanian collision‐detachment zone. The so‐called “Caledonian orogeny” in the Qinling Orogenic Belt records a succession of extensional and partial melting events in the continental crust. The basic lavas in the lower Paleozoic were erupted into rifting zones (both in the northern Qinling subzone and southern Qinling subzone). At the same time, local melting of the transitional continental crust occurred and resulted in formation of S‐type granites.
Paleomagnetism and paleogeography of the northern calcareous Alps (Austria)
Authors: Channell J. E. T. , Brandner R. , Spieler A. , Stoner J. S.
Published: February 2008

Prefolding magnetization components have been resolved at 38 sites of Liassic “Adnet” limestones in the central Northern Calcareous Alps (NCA) and at one site in the Drauzug. The site mean directions are northeasterly and down, and are closer to coeval European directions than to Southern Alpine “African” directions. The western end of both the NCA and of the Drauzug yield different site mean directions and negative fold tests indicating remagnetization. The sampling localities have been rotated relative to one another during thrusting, however, comparison of paleomagnetic poles with the European apparent polar wander path implies an average clockwise rotation of about 30° for the NCA relative to Europe. Larger‐scale clockwise rotations in excess of 80°, relative to the Southern Alps, are implied by comparison with the Southern Alpine (African) polar wander path. Facies analysis of Jurassic and Lower Cretaceous basinal sediments in the central part of the NCA suggests that Liassic subsidence was related to presently E‐W sinistral strike‐slip faults, whereas Malm‐Berriasian subsidence was controlled by presently E‐W south‐dipping normal faults. Both paleomagnetic and facies data tend to indicate that the NCA did not undergo the “African” rotation seen in Southern Alpine paleomagnetic data. This implies the existence of a short‐lived tract of Triassic?‐Jurassic ocean floor between the NCA and the Southern Alps, which linked the Pindos Ocean to the Ligurian trough and allowed Adria to rotate independently from the NCA during the Late Jurassic. Eo‐Alpine deformation and metamorphism are associated with the Early Cretaceous closure of this “Hallstatt” trough.
The influence of air and sea exchange on the carbon isotope distribution in the sea
Authors: Broecker Wallace S. , Maier‐Reimer Ernst
Published: February 2008

We explore here the influence of the temperature dependence of isotope fractionation between atmospheric CO2and ocean ΣCO2 on the distribution of carbon isotopes in the ocean. This is accomplished by an analysis of departures from the expected Redfield tie between PO4 and δ13C. We find that for the surface ocean, the temperature influence largely compensates for the biologic influence. In the deep ocean, the temperature influence imprinted at the sites of deepwater formation reduces somewhat the biologically induced difference between the carbon isotope ratios for deep waters produced in the northern Atlantic and in the Antarctic. These same features are reproduced in the Hamburg ocean model. In order to assess the impact of changes in the ratio of ocean mixing rate to wind speed, we have made a model run in which CO2 exchange rates between air and sea were everywhere doubled. As expected, the influence of the thermodynamic effect on the oceanic carbon isotope distribution is magnified.
Methane emission from Minnesota peatlands: Spatial and seasonal variability
Authors: Dise Nancy. B.
Published: February 2008

The variability of methane flux with season, year, and habitat type was investigated in northern Minnesota peatlands from September 1988 through September 1990. Average daily fluxes calculated by integration of annual data for an open poor fen, an open bog, a forested bog hollow, a fen lagg in the forested bog and a forested bog hummock were 180,118, 38, 35, and 10 mg CH4 m−2 d−1, respectively. Fluxes among the five ecosystems were significantly different from one another, although emission from all sites was highest in July and lowest in March. Winter fluxes occurred in all sites but the fen lagg. There was no difference in fluxes measured from the same sites in the spring of 1986, 1989, or 1990, but summer fluxes were significantly higher in the wetter year of 1989 than in 1990, and a summer pulse in methane emission occurred in 1989 that was not seen the next year. Concentrations of methane in pore water, reflecting the seasonal balance of production, oxidation, and release, declined during the month of peak flux, then increased to levels of about 500 μM in December. Consistent spatial and temporal differences in flux could be ascribed to differences in water table, temperature, and peat nutrient status, although additional variability remained. Integration gave an annual average flux of 20 g CH4, m−2 ot; for the three bog ecosystems and 39 g CH4, m−2 for the two fen ecosystems. This gives an estimate of 1–2 Tg CH4, yr−1 from peatlands in the Great Lake states of Minnesota, Wisconsin, and Michigan.
Wind shear rotary spectra in the atmosphere
Authors: Barat Jean , Cot Charles
Published: February 2008

In this paper we present the first rotary spectra of the horizontal wind shear measured in the stratosphere with a resolution of 25m. A low frequency gravity wave propagating upward with a vertical wavelength of ≈ 1000 m is clearly identified. This result is quite consistent with many previous observations and may explain why the horizontal velocity spectra may be biased and indicates a power variation law versus vertical wave number steeper than those of both temperature and vertical velocity. Nevertheless, around the small vertical wavelengths (100 m), the asymptotic slope of temperature and horizontal velocity spectra are both close to ‐3 and so in better agreement with the saturated model predictions.
Stability correction functions for the mean wind speed and temperature in the unstable surface layer
Authors: Brutsaert Wilfried
Published: February 2008

In accordance with a theoretical analysis by Kader and Yaglom, interpolation expressions are proposed for the Monin‐Obukhov functions for the wind speed and temperature profiles under unstable conditions in the surface layer of the atmospheric boundary layer. The values of the parameters are adjusted to agree with earlier work under weak instability, as reviewed by Högström. The resulting formulations are valid for considerably larger values of their argument than those that were hitherto available. Their advantage is that they yield concise and closed‐form stability correction functions which can be readily applied in profile analysis and model calculations.
Isothermal plateau fission‐track age and revised distribution of the widespread mid‐Pleistocene rockland tephra in west‐central United States
Authors: Alloway B. V. , Westgate J. A. , Sandhu A. S. , Bright R. C.
Published: February 2008

A tephra bed in southeastern Idaho is correlated to Rockland tephra, a widespread silicic air‐fall unit previously identified in northern California and western Nevada. Correlation of proximal Rockland tephra and its distal correlative is based on glass chemistry determined by electron microprobe and neutron activation analysis. An age estimate of proximal Rockland tephra is precisely determined by application of the isothermal plateau fission‐track (ITPFT) technique to its hydrated glass shards. A weighted mean age of 0.47 ± 0.04 Ma compares well with age estimates based on co‐existing mineral phases using the external detector fission‐track and additive‐dose thermoluminescence techniques. The presence of Rockland tephra 800 km east of its presumed eruptive source will improve the development of detailed chronologies and correlation of Pleistocene sedimentary sequences located in coalescent intermontane basins of west‐central United States.
The semiannual variation of great geomagnetic storms and the postshock Russell‐McPherron effect preceding coronal mass ejecta
Authors: Crooker N. U. , Cliver E. W. , Tsurutani B. T.
Published: February 2008

The occurrence rate of great geomagnetic storms displays a pronounced semiannual variation. Of the forty‐two great storms during the period 1940–1990, none occurred during the solstitial months of June and December, and 40% (17) occurred during the equinoctial months of March and September. This suggests that the semiannual variation found by averaging indices is not the result of some statistical effect superposed on the effects of random storm occurrence but rather is dominated by the storms themselves. Recent results indicate that the intense southward interplanetary magnetic fields (IMFs) responsible for great storms can reside in the postshock plasma preceding the driver gas of coronal mass ejections (CMEs) as well as in the driver gas itself. Here we propose that strong southward fields in the postshock flow result from a major increase in the Russell‐McPherron polarity effect through a systematic pattern of compression and draping within the ecliptic plane. Differential compression at the shock increases the Parker spiral angle and, consequently, the azimuthal field component that projects as a southward component onto Earth's dipole axis. The resulting prediction is that southward fields in the postshock plasma maximize at the spring (fall) equinox in CMEs emerging from toward (away) sectors. This pattern produces a strong semiannual variation in postshock IMF orientation and may account at least in part for the observed semiannual variation of the occurrence of great geomagnetic storms.
Vertical transport of water ice at low latitudes in the Martian atmosphere
Authors: Chassefiére E. , Blamont I. E.
Published: February 2008

Solar occultation measurements made from the Phobos 2 spacecraft show that two condensation levels for H2O are generally present at respectively ≈15 km and ≈50 km altitude at low latitudes. Above ≈15 km, a regular decrease of the water vapor mixing ratio up to ≈35 km is observed (Krasnopolsky et al, 1991) and it is interpreted as the signature of a saturated atmosphere. Around ≈50 km, thin clouds, very likely formed of water ice, are detected (Blamont et al, 1991). This means that above ≈35 km exists a region where the atmosphere is not saturated in H2O, containing enough water vapor to give rise to the formation of clouds. We show that upwelling in the ascending branch of the Hadley cell is able to supply water ice, across the cold saturated region (≈15–35 km), to the non‐saturated layer below the clouds, where ice may sublimate and condense again at cloud level (≈50 km). Different constraints may be introduced relating to the coagulation probability during the upward transport phase, the minimum amount of H2O which must be deposited at the sublimation level and the infrared optical thickness of ice particles. It is shown that these constraints are jointly satisfied for reasonable values of parameters, indicating that a quantity (zonally averaged) of ≈0.01 pr. μm of water ice in the shape of small particles of radius 0.1–0.2 μm might be present in the middle atmosphere (15–35 km) above equatorial regions.
Aerosol optical properties at Mauna Loa Observatory: Long‐range transport from Kuwait?
Authors: Bodhaine B. A. , Harris J. M. , Ogren J. A. , Hofmann D. J.
Published: February 2008

Aerosol light absorption has been measured continuously at Mauna Loa Observatory, Hawaii (MLO), since April 1990. During the spring of 1991, after oil wells were fired in Kuwait, there was speculation among scientists concerning long‐range transport of smoke particles and its possible effect on global climate. The MLO light absorption record from April 1990 to June 1991 shows low values in the 0.1–1 ×10−7 m−1 range in the summer of 1990, and an increased baseline level of about 2–4 ×10−7 m−1 with numerous superimposed events in the 5–10 ×10−7 m−1 range in the spring of 1991. These levels correspond to black carbon (BC) concentrations of 1–10, 20–40, and 50–100 ng m−3, respectively, under the assumption that BC is the dominant light absorbing species and has a specific absorption of 10 m2 g−1. Large‐scale 500‐hPa trajectories calculated backwards from MLO sometimes show direct transport paths from China and Kuwait to Hawaii that coincide with the black carbon events. These measurements set an upper limit on the possible contribution of Kuwaiti black carbon to the background troposphere near MLO during periods of rapid transport. The aerosol observed at MLO is expected to cause a net cooling of the atmosphere.
A correlation between imf and the limiting primary rigidity for the cosmic ray diurnal anisotropy
Authors: Ahluwalia H. S.
Published: February 2008

A plausible explanation is provided for the existence of the subsidiary maximum in the annual mean cosmic ray diurnal anisotropy, observed during the declining phase [1982–85] of the solar activity cycle 21. For this period, the values of the limiting primary rigidity (Rc), applicable to the steady state diurnal anisotropy, are quite large. Also, the annual mean magnitudes (B) of the interplanetary magnetic field (IMF) are very high. The data for Rc and B are reviewed for 1964–1987 period. A correlation is found to exist between the two parameters.
Role of the BRO + HO2 reaction in the stratospheric chemistry of bromine
Authors: Poulet Gilles , Pirre Michel , Maguin Françoise , Ramaroson Radiela , Le Bras Georges
Published: February 2008

The impact of new laboratory data for the reaction BrO + HO2 → HOBr + O2 has been estimated in a one‐dimensional photochemical modelling of bromine/ozone stratospheric chemistry. The reported 6 fold increase in the measured value for the rate constant of this reaction significantly increases both the HOBr mixing ratio and the global ozone depletion due to bromine compounds (the calculated ozone loss increases from 1.14% to 1.45% for a 20 ppt total bromine content). The higher rate constant makes the bromine partitioning and the ozone depletion very sensitive to the branching ratio of the potential channel forming HBr in the BrO + HO2 reaction. The influence of the existing uncertainty in the photolysis rate of HOBr is also analysed.
The Ulysses Mission: In‐ecliptic phase
Authors: Wenzel K.‐P. , Smith E. J.
Published: February 2008

The Ulysses mission is unique in the history of the exploration of our solar system by spacecraft. The path followed by Ulysses will enable us, for the first time, to explore the heliosphere within a few astronomical units of the Sun over the full range of heliographic latitudes. The mission, a collaboration between ESA and NASA, was launched on 6 October 1990 and employed a Jupiter gravity‐assist in February 1992 to achieve the out‐of‐ecliptic trajectory extending to high solar latitudes (Smith et al. 1991).
Kinetics of the reactions of OH with methyl chloroform and methane: Implications for global tropospheric OH and the methane budget
Authors: Finlayson‐Pitts B. J. , Ezell M. J. , Jayaweera T. M. , Berko H. N. , Lai C. C.
Published: February 2008

The hydroxyl radical plays a central role in the oxidation of methane, larger organics including methyl chloroform, and CO in the troposphere. Hence a knowledge of the kinetics of such reactions is essential in estimating atmospheric lifetimes of these organics as well as global OH concentrations. We report here fast flow discharge studies of the absolute rate constants for both the OH‐CH3CCl3 and OH‐CH4 reactions from 278–378 K in He at 1.05±0.05 Torr pressure. For CH3CCl3,
. The rate constants at 298 K and 277 K, 1.0 and 0.73×10−14 cm3 molecule−1 s−1 respectively, are approximately 15% and 5% below the values now used to estimate global OH concentrations. The use of these lower rate constants in computations of global OH based on CH3CCl3 emissions and ambient measurements will increase the estimated OH concentrations, particularly at higher temperatures found in warmer equatorial regions. For CH4,
. At 298 K and 277 K respectively, this gives values which are ∼23% and 28% below the values which have generally been used in tropospheric models. Our results therefore support the recent suggestion that the methane flux may have been overestimated in the past.
AMPTE CCE observations of Pi 2 pulsations in the inner magnetosphere
Authors: Takahashi Kazue , Ohtani Shin‐ichi , Yumoto Kiyohumi
Published: February 2008

Magnetic field data acquired with the AMPTE Charge Composition Explorer satellite in the inner magnetosphere (L = 2–5) often show Pi 2 pulsations whose waveforms match Pi 2 pulsations simultaneously observed on the ground at Kakioka (L = 1.2). From a study of 25 such events, we find that the magnetic pulsations in the equatorial magnetosphere are dominated by poloidal‐mode oscillations. The relative phase between the compressional component at CCE and the horizontal component at Kakioka is either near zero or near 180°, with the ∼180° lag observed only when the satellite is at L > 3. This observation implies that there is a node of a radial standing wave at L > 3. We argue that the nodal structure arises from reflection of MHD fast‐mode waves at some inner boundary of the magnetosphere and discuss the relevance of the nodal structure to cavity‐mode resonances and oscillations in the inner magnetosphere forced by a source wave external to the inner magnetosphere.
Detachment of part of the downgoing slab and uplift of the New Hebrides (Vanuatu) Islands
Authors: Chatelain Jean‐Luc , Molnar Peter , Prévot Richard , Isacks Bryan
Published: February 2008

Several seismological observations suggest that there is a gap within the downgoing slab of Australian lithosphere plunging beneath the New Hebrides, and ages of elevated coral terraces on the New Hebrides Islands suggest that the islands are rising rapidly. We suggest that the creation of the gap in the slab, within the last 1 M.y., occurred by part of the slab detaching from the rest and sinking rapidly into the underlying asthenosphere. This lowered the downward force on the slab at shallower depths and therefore on the overriding plate, and the continued decrease of this force, as the deeper slab sinks freely, has allowed the islands above it to rise. Although this suggestion cannot account for all aspects of the uplift of the islands, it provides a simple mechanism for explaining the fairly young uplift and its distribution along the New Hebrides arc.
238U and 232Th series nuclides in groundwater from the J‐13 well at the Nevada Test Site: Implications for ion retardation
Authors: Copenhaver Sally A. , Krishnaswami S. , Turekian Karl K. , Shaw Henry
Published: February 2008

The activities of members of the 238U and 232Th decay chains have been determined in water from well J‐13 at the Nevada Test Site near Yucca Mountain. Using 222Rn as a measure of recoil flux, adsorption (k1) and desorption (k2) rate constants for Ra and Pb have been determined. The ratio (k1/k2), defined as the dimensionless distribution coefficient, K, is 1.3 × 104 for Ra and 2.7 × 104 for Pb and is estimated to be about 104 for Th.
Postglacial rebound with a non‐Newtonian upper mantle and a Newtonian lower mantle rheology
Authors: Gasperini Paolo , Yuen David A. , Sabadini Roberto
Published: February 2008

We have employed a composite rheology consisting of both linear and nonlinear creep mechanisms which are connected by a ‘transition’ stress. Background stress due to geodynamical processes is included. For models with a non‐Newtonian upper‐mantle overlying a Newtonian lower‐mantle the temporal responses of the displacements can reproduce those of Newtonian models. The average effective viscosity profile under the ice‐load at the end of deglaciation turns out to be the crucial factor governing mantle relaxation. This can explain why simple Newtonian rheology has been successful in fitting the uplift data over formerly glaciated regions.
O+ and He+ restricted and extended (BI‐modal) ion conic distributions
Authors: Peterson W. K. , Collin H. L. , Doherty M. F. , Bjorklund C. M.
Published: February 2008

We have developed an automated procedure using standard image processing techniques that finds and characterizes energetic ion conic events in the data acquired by the Energetic Ion Composition Spectrometer on DE‐1 in the altitude range 8,000 to 24,000 km. The algorithm discriminates between the two types of ion conic distribution, those restricted to a narrow angular range and those extended in angle. Extended (bi‐modal) ion conic distributions also have a significant flux of field‐aligned energetic ions. Extended ion conies constitute more than one third of the ion conics found. The two types of ion conic distribution have different altitude dependences. The average properties of energetic conic distributions suggest that conic formation by localized, explosive, transverse energization is not the dominant mechanism responsible for producing energetic conic distributions above 8,000 km.
Chemical dieferentiation of a convecting planetary interior: Consequences for a one plate planet such as Venus
Authors: Parmentier E. M. , Hess P. C.
Published: February 2008

Partial melting to generate the crust of a planet creates compostionally buoyant residual mantle. In the absence of mantle flow associated with plate tectonics, this buoyant, refractory layer may collect at the top of the mantle with important implications for the evolution of the interior and surface. In this study models of the thermal and chemical evolution of a planetary interior demonstrate the possible consequences of a chemically buoyant depleted mantle layer. As the depleted layer thickens the melting temperature at the top of the underlying convecting mantle also increases and the degree of partial melting of mantle added to the depleted layer decreases. As less depleted mantle with less positive compositional buoyancy is added, negative thermal buoyancy of the layer eventually exceeds its positive compositional buoyancy. The depleted layer then sinks into and mixes with the convecting interior. The top of the convecting mantle then moves to a shallower depth, larger degrees of melting resume, and a new depleted layer accumulates. This accumulation and instability of the depleted layer occurs repeatedly over a substantial portion of the planet's evolution with a period of 300–500 Myr. On Venus the population of impacts craters is indistinguishable from a random distribution over the surface and give a surface age of about 500 Myr. We speculate that the mechanism described above may explain this episodic global resurfacing of Venus.
Black carbon concentration in a Greenland Dye‐3 Ice Core
Authors: Chylek Petr , Johnson Bruce , Wu Hong
Published: February 2008

We have determined the black carbon concentration in Greenland Dye‐3 ice core samples covering the time period from about 3380 to 100 years before present. The average concentration found is 1.53 μg of black carbon per 1 kg of ice. We have found significantly lower black carbon concentrations during the time period from 750 to 100 years before present connected possibly with the Little Ice Age and corresponding changes in atmospheric general circulation patterns and black carbon sources.
Impacts on regional climate of Amazon deforestation
Authors: Dickinson Robert E. , Kennedy Patrick
Published: February 2008

A simulation of the climate response to Amazon deforestation has been carried out. Precipitation is decreased on the average by 25% or 1.4 mm/day, with ET and runoff both decreasing by 0.7 mm/day. Modifications of surface energy balance through change of albedo and roughness are complicated by cloud feedbacks. The initial decrease of the absorption of solar radiation by higher surface albedos is largely cancelled by a reduction in cloud cover, but consequent reduction in downward longwave has a substantial impact on surface energy balance. Smoke aerosols might have an effect comparable to deforestation during burning season.
Seismic imaging of shallow reflectors in the Eastern Kapuskasing Structural Zone, with correction of crossdip attitudes
Authors: Kim Jisoo , Moon Wooil M. , Percival John A. , West F. Gordon
Published: February 2008

Cascaded processes of crossdip correction and residual statics are tested and applied in the reprocessing of regional data from LITHOPROBE Kapuskasing Transect line 2. The objective was to improve seismic imaging of shallow, gently dipping reflectors in the eastern Kapuskasing structural zone (KSZ), a thrusted slice of Archean middle to lower crust. This focusing strategy proved to be very effective in improving the image of the reflected energy and in identifying a set of conformally dipping reflectors whose true crossdip is estimated to be approximately 17° NW. The estimated crossdips for a reflective, compositionally layered zone and for the basal thrust, the Ivanhoe Lake Fault zone (ILFZ), support the previously estimated average dip of 15°–20°.
Hybrid simulations of intermediate shocks: Coplanar and noncoplanar solutions
Authors: Karimabadi H. , Omidi N.
Published: February 2008

The kinetic structure and stability of subfast intermediate shocks (IS) are investigated using a hybrid code. The shocks are formed dynamically by the interaction between a flowing plasma and a stationary piston. For θBN = 60° and plasma β = 0.46, the strong IS is found to be stable with a width in the range of 10 to 20 ion inertial lengths (λi). The rotation of the transverse component of the magnetic field is in the ion sense. The weak IS has a more complex structure and consists of both Alfvén and slow waves. The leading edge of the shock is dominated by the Alfvén mode and is associated with a S‐shape electron sense field rotation with a small decrease (increase) in the magnetic field (density) across it. Some of the ion dissipation occurs within this layer, which is relatively thin (∼ 17 – 20λi). However, the transition to the downstream density and magnetic field occurs in the much wider (∼ 150λi) trailing slow wave. The main heating associated with this trailing edge occurs in the direction parallel to the magnetic field. This slow wave has a phase velocity larger than the Alfvén speed due to kinetic corrections to linear wave properties. As a result, the slow wave stays attached to the leading edge of the shock which remains time‐stationary. The classification of IS's, based on phase velocity of MHD modes, becomes ambiguous in the kinetic limit. When the magnetic field is noncoplanar, the strong IS becomes time‐dependent and expands self‐similarly in time, whereas the weak IS disintegrates into an RD.
The moho as a magnetic boundary revisited
Authors: Wasilewski P. J. , Mayhew M. A.
Published: February 2008

We have re‐examined the question of whether it is generally true that in continental regions a relatively magnetic crust overlies a relatively non‐magnetic mantle. Whereas the original conclusion was based on limited data, the present study is based on about 400 globally‐distributed xenolith samples. While it is now clear that the “Moho” represents a maximum velocity gradient in a complex transition from crust to mantle, and while crustal magnetization is highly variable, the original concept of a “magnetic crust” overlying a non‐magnetic mantle is strongly supported by the new data. A large upper mantle xenolith suite of ultramafic composition is characterized by non‐magnetic chrome spinels and magnesian ilmenites. Such rocks are magnetic only where demonstrably altered. The most significantly magnetic rocks within the crust are mafic types prograded to granulite grade or mafic melts of basaltic composition which crystallized deep in the crust at high pressure and temperature. Prograded rocks have characteristic magnetite Curie points, and are strongly magnetic, especially the more mafic compositions, while the mafic rocks formed by crystallization at depth have Curie points between 400°–570°C and are strongly magnetic. Induced magnetizations are commonly several A/m, and can readily account for long‐wavelength magnetic anomalies measured by satellite and aircraft; thus, there is no need to search for exotic sources for “missing” crustal magnetization. No metal or other peculiar magnetic species have been observed by us in any of our (unaltered) samples. Mineral assemblages (including magnetic components) are consistent with oxidizing conditions close to the FMQ buffer.
ion acceleration at the current sheet of the Martian magnetosphere
Authors: Ip W.‐H.
Published: February 2008

Plasma measurements of the Martian magnetosphere by the Phobos 2 spacecraft have revealed many interesting features connected with the mass and energy transport processes in the solar wind interaction. For example, the ASPERA instrument and the TAUS experiment showed that there is a very significant ionospheric outflow in the tail region which could lead to a depletion of the oxygen content of the present CO2 ‐ atmosphere of Mars in just a few hundred million years. At the flank of the ionosphere where scavenging of the heavy exospheric ions is most effective, viscous effects and other mechanisms might play a role in accelerating the ionospheric plasma. However, a different process has to be invoked to explain the presence of field‐aligned ion (FAI) beams detected by the ASPERA experiment near the central tail. In the present work, we consider the acceleration effect of a thin current sheet. Furthermore, we wish to explore the possibility that the observed FAI outflow might be the signature of formation of neutral lines in the near‐tail region of Mars. Besides the tearing mode instability of the current sheet induced by the draping of the interplanetary magnetic field, magnetic neutral lines could also form as the result of a hybrid magnetosphere with a component of the intrinsic magnetic field interconnected with the interplanetary magnetic field.
Sedimentary records of reversal transitions ‐ Magnetization smoothing artefact or geomagnetic field behaviour?
Authors: Weeks R. , Fuller M. , Laj C. , Mazaud A. , Herrero‐Bervera E.
Published: February 2008

Sedimentary records of reversals for the past 12 million years exhibit preferred longitudes of their VGP paths. This observation may be related to long term aspects of core‐mantle dynamics. However, it has been suggested that sedimentary VGP paths could be an artefact due to smoothing which mixes non antipodal pre‐ and post‐transitional directions. We show here that smoothing over unrealistically long time scales is required to generate intermediate directions of magnetization which are purely mixtures of pre‐ and post‐transitionals directions in sediments whose remanence is primarily depositional (DRM) or post‐depositional remanence (PDRM). Sedimentary records of transitions showing VGP paths confined to the planes of immediately pre‐ and post‐transitional VGPs have however been observed. These can be explained by non‐axisymmetric bias in the non‐reversing field [Gubbins and Bloxham, 1987; Constable, 1992], which plays a role in reversals. Therefore, provided that magnetization directions are properly cleaned to give single component directions, the preferential longitudes of reversal VGP paths will be due to field behaviour and not to an artefact of the magnetization process. If, however, mixtures of primary magnetization acquired at the time of the formation of the rock and secondary magnetization acquired later are measured, then artefacts will be generated.
Tropospheric heterogeneous chemistry of haloacetyl and carbonyl halides
Authors: De Bruyn W. J. , Duan S. X. , Shi X. Q. , Davidovits P. , Worsnop D. R. , Zahniser M. S. , Kolb C. E.
Published: February 2008

A series of uptake studies have been completed for the gas phase carbonyl halides CCl2O and CF2O and the haloacetyl halides CCl3CClO, CF3CFO and CF3CClO which are oxidation intermediates of volatile fluorocarbon, chlorocarbon and hydrofluorocarbon (HCFC) species in the atmosphere. The experimental apparatus in these studies employs a fast moving monodispersed train of droplets (50 to 200 μm in diameter) passed through a low pressure flow reactor. Experiments were done as a function of droplet pH and gas/droplet interaction time. The measured uptake coefficients (γ) for all these molecules are less than our experimental detection limit of 5 × 10−4 for pH 7 droplets at 300 K. From the results of the measurements and other available data, the uptake coefficients for these species are estimated to be 10−6 < γ < 10−4. The lower limit is sufficiently large so that the tropospheric removal rate of the species is not expected to affect the ozone depletion potential values of the HCFC parent species for CF2O, CF3CFO and CF3CClO. However with such uncertainty it is not possible to determine the partitioning of the ocean/land deposition of the species.
Acoustic sounder measurements of the vertical velocity of volcanic jets at Stromboli Volcano
Authors: Weill Alain , Brandeis Geneviève , Vergniolle Sylvie , Baudin François , Bilbille Jacques , Fèvre Jean‐François , Piron Brigitte , Hill Xavier
Published: February 2008

We have used a Sodar (acoustic doppler sounder) to characterize the behavior of volcanoes, and in particular, to measure the vertical velocities of volcanic jets. We have analyzed more than 100 explosions on Stromboli volcano (Eolian islands). First results show vertical speeds close to the vent ranging from 20 m/s to 80 m/s. The uncertainty in the measurements is close to 10 %, due mainly to the inaccurate knowledge of the sound speed in volcanic jets. This is small, compared to other volcanological methods. This method is well suited for monitoring temporal variations during an eruption and should be particularly interesting for the study of the eruptive dynamics of lava fountains. The Doppler spectrum in explosions appears to be related to different types of echoes and contains information on particle sizes in the jet.
A reappraisal of the geomagnetic polarity time scale to 4 MA using data from the Turkana Basin, East Africa
Authors: McDougall I. , Brown F. H. , Cerling T. E. , Hillhouse J. W.
Published: February 2008

Recalibration of the Pliocene and early Pleistocene geomagnetic time scale using the K‐Ar dated fluvial sequence of the Turkana Basin in East Africa agrees with calibrations based on astronomical calculations. Ages estimated here are: Olduvai Subchron, 1.78–1.96 Ma; Réunion Subchrons, 2.11–2.15 Ma and 2.19–2.27 Ma; Matuyama‐Gauss boundary, 2.60 Ma; Kaena Subchron 3.02–3.09 Ma; Mammoth Subchron, 3.21–3.29 Ma; Gauss‐Gilbert boundary, 3.57 Ma.
Solar cycle variations in H+ and d+ densities in the Venus ionosphere: Implications for escape
Authors: Donahue Thomas M. , Hartle Richard E.
Published: February 2008

The hydrogen ion concentrations recently observed on Venus, near solar minimum, by the Ion Mass Spectrometer on the Pioneer Venus Orbiter in the anti‐solar sector (22:00–02:00 LST) of the ionosphere are more than an order of magnitude less than those previously observed at solar maximum. This strong solar cycle variation has a profound effect on the escape of hydrogen (and deuterium) from Venus; almost all escape occurs during solar maximum. After adjustment for solar cycle variation, a planet‐averaged hydrogen escape flux of 0.6–1.4 × 107 cm−2 s−1 is obtained along with a large deuterium fractionation factor of 0.1–0.14. These results suggest at least two plausible scenarios for the evolution of water on Venus: (1) Water vapor on Venus may be approaching a steady state if the escape flux is balanced by endogenous or exogenous sources of water. The source of water must be highly fractionated, with a D/H ratio differing by less than an order of magnitude from the present ratio of 2.4 × 10−2, thus precluding low D/H water from comets, asteroids or a mantle reservoir. (2) The present day D/H ratio of 2.4 × 10−2 could be established by Rayleigh fractionation of an early low D/H water reservoir if the escape flux was sufficiently large in earlier times. An early water endowment at least 340 times today's abundance, equivalent to 4.2 to 14 m of liquid water on the surface, would be needed.
Model calculation of the N2+ first negative intensity and vibrational enhancement from energetic incident O+ energy spectra
Authors: Ishimoto M. , Romick G. J. , Meng C. ‐I.
Published: February 2008

The N2+ first‐negative band system with high vibrational and rotational enhancement appears in the midlatitude region during large geomagnetic storms and its (0–0) band emission intensity ranges up to 1000 Rayleighs (R). The observed ratios of the (1–1) to (0–0) bands are high (8%–22%) compared to the value of 4.5% for electron auroras excited by keV electrons. The ratio of the (2–2) to (0–0) bands is more than an order of magnitude larger than that in the case of electron aurora. Analysis of the observed ground based spectra has led to the conclusion that energetic O+ precipitation causes the midlatitude emissions. We investigated whether energetic O+ could produce the N2+ 1N (0–0), (1–1) and (2–2) band emissions with similar intensities and ratios to those observed at midlatitude. An O+ transport model with measured N2+ 1N cross sections calculated the emissions with variation in several parameters: the incident O+ spectra, scattering cross sections, and the model atmosphere. The results indicate that 1) the majority of the O+ must have energies below 10 keV to produce the observed vibrational enhancement, and 2) the total energy flux must be on the order of 10 erg cm−2 s−1. Such energy flux would also cause peak atmospheric heating a factor of three larger than that due to the noon solar extreme ultraviolet energy deposition.
Simulation of the critical ionization velocity: Effect of using physically correct mass ratios
Authors: Biasca Rodger , Hastings Daniel , Cooke David
Published: February 2008

The concept of the “critical ionization velocity” (CIV) of a neutral gas has received much attention since it was proposed by Alfvén. In the last several years, numerical simulations have been a useful method of examining the physical mechanism behind CIV. Since these simulations have been run using explicit particle‐in‐cell (PIC) codes, artificial mass ratios (e.g. mi/me = 100) have been employed to reduce the time scales in the simulation and the computational power required for the problem. However, the effect of the use of unphysical mass ratios on the simulation results has not been well discussed. In this study, simulations using artificial and realistic mass ratios are compared. The study employs an implicit PIC code to allow the realistic mass ratio simulations to be performed efficiently. Several numerical aspects of using implicit codes, such as the undesirable damping of upper hybrid heating, are discussed. The results indicate that when scaled appropriately, simulations using physical and unphysical mass ratios provide nearly identical results when the anomalous ionization rate is either much lower than the ion cyclotron frequency (νion/Ωi ≪ 1) or greater than the ion cyclotron frequency (νion/Ωi > 1). In the case of an anomalous ionization rate near the cyclotron frequency, the unphysical mass ratio results cannot be scaled easily to regain the results from the physical mass ratio simulation.
Further evolution of velocity shell distribution of cometary and interstellar pickup ions and excitation of oblique Alfvén waves
Authors: Yoon Peter H.
Published: February 2008

This paper shows how an initially isotropic spherical shell velocity distribution of cometary or interstellar pickup ions can become thermalized. The basic instability mechanism responsible for the thermalization is based on papers by Wu and Yoon (1990) and Yoon (1990). The unstable mode is in the Alfvén mode branch and propagates in an oblique direction, with the mode stable for exactly parallel and perpendicular propagations. The present paper discusses the nonlinear evolution of the ion distribution as well as the unstable waves. The method is based on the self‐consistent quasi‐linear equations. It is found that the thermalization can take place in 6.4×10³ to 1.6×104 ion gyroperiods for the parameters used in the present analysis, while the unstable waves can grow to an appreciable level of the initial fluctuation.
Modifications of magnetohydrodynamics as applied to the solar wind
Authors: Montgomery David
Published: February 2008

The effect of including the Braginskii viscous stress tensor in magnetohydrodynamics is remarked upon. It is shown that semiquantitative agreement with a recently observed anisotropy in the turbulent solar wind spectrum can be achieved in this way. The modifications of the dynamical equations are simple enough to permit their inclusion in numerical codes. The effects of large “ion parallel viscosities” also may be significant for plasmas in quite different regimes than the solar wind.
Drifting holes in the energetic electron flux at geosynchronous orbit following substorm onset
Authors: Sergeev V. A. , Bösinger T. , Belian R. D. , Reeves G. D. , Cayton T. E.
Published: February 2008

A new type of substorm‐related particle flux change was discovered at geosynchronous orbit. It is manifested as a decrease in energetic electron flux (of initial duration about 1–2 min) starting near local magnetic midnight at a time of high‐energy particle injection to geosynchronous distance. Unlike “dropouts,” which are observed only in the midnight local time sector, this flux signature drifted eastward showing the typical energy and LT dependent time patterns of electron drift. This phenomenon was seen only in electron fluxes above a certain threshold energy and was characterized by a moderate amplitude of flux variation (within a factor of 3). Below the threshold energy the normal substorm‐associated increase in electron flux (drifting cloud of accelerated particles) was detected. The phenomenon in question, called a drifting electron hole, is explained as being a result of adiabatic redistribution of an already existing high‐energy electron component recently described by Cayton et al. (1989). It is argued that in the case of a strong (inward directed) radial gradient of electron flux and of a hard energy spectrum the electron fluxes which are brought to a lower drift shell by the inward injection may still be smaller than the previously existing fluxes at this drift shell. This leads to an apparent loss of electrons in some localized portion of the drift shells (e.g., at geosynchronous orbit). Another explanation of the drifting electron hole based on rapid particle losses due to precipitation seems less probable.
Relativistic electron enhancements observed over a range of L shells trapped at high altitudes and precipitating at low altitudes into the atmosphere
Authors: Imhof W. L. , Nightingale R. W.
Published: February 2008

The fluxes of relativistic electrons at high altitudes were measured at the SCATHA satellite over the L shell range of 5.3–8.7 during both enhancements and depletions. Simultaneous observations of the precipitating fluxes of electrons (>1 MeV) were performed at low altitudes from the P78‐1 satellite. On a given pass of this satellite the fluxes of directly precipitating electrons in the bounce loss cone displayed bursts of enhancement, generally with observing times of less than 10 s. On the average these fluxes had a maximum value at L = 4.5–5.0. In the drift loss cone the quasi‐trapped electrons, all of which were about to precipitate into the atmosphere within a few minutes, were less intermittent and generally more intense, thereby providing a greater sensitivity for detecting precipitation. The ratio of the average fluxes of the directly precipitating electrons to the quasi‐trapped electrons increased with increasing L value. The coordinated data presented here have permitted for the first time a comparison to be made as a function of L shell between the precipitating and the high‐altitude trapped electron fluxes during relativistic electron enhancement events, some of which included electrons with energies extending to at least 5 MeV. The low‐ and high‐altitude fluxes of >1‐MeV electrons were found to track each other during the period studied (April–June 1979), with respect to both time and L shell variations. The average precipitating electron fluxes measured at low altitudes in the drift loss cone were lower than those at high altitude by a factor of about 3×10². For L = 5.3–7 the average fluxes in the bounce loss cone at low altitudes were estimated to be lower than the trapped fluxes at high altitude by a factor of about 3×104.
High‐latitude F region ionospheric interchange modes in the presence of powerful radio waves
Authors: Chaturvedi P. K. , Keskinen M. J. , Ossakow S. L.
Published: February 2008

We examine the possibility of influencing medium scale size (0.1 ∼ 10 km) interchange modes in the high‐latitude F region ionosphere by a high‐power, high‐frequency (HF) electromagnetic (“pump”) wave launched from the ground. The parametric coupling of interchange modes with the pump wave will occur near the critical layer where the pump frequency matches the local plasma frequency with the nonlinear Joule heating providing the dominant coupling mechanism. We find that the parametric excitation (or suppression) of the interchange process at high latitudes is possible, in the local approximation, for short wavelengths (k⊥LN ≫ 1, where k⊥ and LN are the transverse wavenumber of the interchange mode and density gradient scalelength, respectively) using HF power densities (≳10−6 W/m²) that are attainable by current ionospheric heater facilities. These power density estimates for longer wavelength interchange modes, i.e., for k⊥LN ≲ 1, are increased. The results are discussed in light of previous and possible future ionospheric heating experiments.
A class of high‐m pulsations and its auroral radar signature
Authors: Grant I. F. , McDiarmid D. R. , McNamara A. G.
Published: February 2008

Bistatic Auroral Radar System radar events showing a repetitive equatorward (or radarward) traveling band signature in the range‐time‐intensity plots have been analyzed. The associated pulsations have many features in common with previously reported high‐m storm time Pc 5 pulsations. The periods, azimuthal wave numbers (m), Dst dependence, and polarization ellipses are similar. One difference is also seen. The pulsation phase velocity of our events often has a significant equatorward component in the ionosphere. However, of the three cases for which the phase velocity direction in the magnetospheric equatorial plane was determined, one had phase velocities close to azimuthal at both locations, one had an azimuthal phase velocity in the magnetosphere but an equatorward component of the phase velocity in the ionosphere, and the final event had equatorward or inward components of phase velocity at both locations. These events are very similar to the equatorward traveling bands events observed by the Sweden and Britain Auroral Radar Experiment system (Tian et al., 1991; Yeoman et al., 1992) except that our m values are distinctly larger. If coupling to energetic protons is involved in exciting these pulsations, this difference implies that the energy of the protons involved decreases with increasing L of the interaction location. Finally, our data do not lend themselves to the plasmapause field line resonance interpretation of Tian et al. (1991).