Observations of the upper frequency cutoffs of the auroral kilometric radiation

Intense auroral kilometric radiation (AKR) is being frequently observed with POLRAD from the Auroral Probe (Interball-2). Observations of the abrupt upper frequency cutoffs (UFCs) in the spectra of AKR are reported. The UFCs can be observed at a frequency range from 300 to 700 kHz, corresponding to AKR generation altitudes from approximately 4800 to 2100 km, and are distributed in magnetic local time (MLT) hours similarly to the AKR events, with a maximum at 1 h MLT. The observed frequency extent of the UFCs is 12 kHz, and is often determined by the instrumental resolution (4 kHz). It is suggested that the UFC may be associated with an abrupt switching on of the generation mechanism, when the electron density becomes sufficiently low inside a plasma depletion at an altitude where the ratio of f_pe/f_ce crosses some threshold value. The steepness of the UFCs can imply a non-linear process of generation. The estimated distance of the e-folding field aligned wave amplification is between 3 and 8 km. The UFCs are sometimes, though very seldom (10%), accompanied by narrow band (less than 4 kHz) ridges of radiation observed at the cutoff frequency. They are smoothly drifting in frequncy for several minutes. The power density of radiation in the ridge can be up to 2 orders of magnitude stronger than in the accompanying wide band emission of AKR. The ridge at UFC can imply either energy concentration at the source bottom, or focusing, if specific conditions for the escape of the radiation are assumed.     

Imaging Offsets in the Moho: Synthetic Tests using Gaussian Beams with Teleseismic Waves

We carry out a sequence of numerical tests to understand conditions under which rapid changes in crustal thickness can be reliably imaged by teleseismic body waves. Using the finite-difference method over a 2-D grid, we compute synthetic seismograms resulting from a planar P-wavefield incident below the grid. We then image the Moho using a migration scheme based on the Gaussian beam representation of the wavefield. The use of Gaussian beams for the downward propagation of the wavefield is particularly advantageous in certain geologically critical cases such as overthrusting of continental lithosphere, resulting in the juxtaposition of high-velocity mantle material over crustal rocks. In contrast to ray-based methods, Gaussian beam migration requires no special treatment to handle such heterogeneities. Our results suggest that with adequate station spacing and signal-to-noise ratios, offsets of the Moho, on the order of 10 km in height, can be reliably imaged beneath thickened crust at depths of about 50 km. Furthermore, even sharp corners and edges are faithfully imaged when precise values of seismic wave speeds are available. Our tests also demonstrate that flexibility in choices of different types of seismic phases is important, because any single phase has trade-offs in issues such as spatial resolution, array aperture, and amplitude of signals.     

Smithsonian Institution’s Global Volcanism Network

Information included in this summary is based on more detailed reports published in the Bulletin of the Global Volcanism Network, vol. 31, no. 3, March 2006 (on the Internet at ). Edited by scientists at the Smithsonian, this bulletin includes reports provided by a worldwide network of correspondents. The reports contain the names and contact information for all sources. Please note that these reports are preliminary and subject to change as events are studied in more detail. The Global Volcanism Program welcomes further reports of current volcanism, seismic unrest, monitoring data, and field observations.     

Some exact solutions for the flow of fluid through tubes with uniformly porous walls

Summary This paper considers an incompressible fluid flowing through a straight, circular tube whose walls are uniformly porous. The flow is steady and one dimensional. The loss of fluid through the wall is proportional to the mean static pressure in the tube. Several formulations of the wall shear stress are considered; these formulations were motivated by the results from Hamel's radial flow problem, boundary layer flows/and boundary layer suction profiles. For each of these formulations exact solutions for the mean axial velocity and the mean static pressure of the fluid are obtained. Sample results are plotted on graphs. For the constant wall shear stress problem, the theoretical solutions compare favorably with some experimental results.Notations A, B, D, E constant parameters - a, b constant parameters - Ai(z), Bi(z) Airy functions - Ai, Bi derivatives of Airy functions - k constant of proportionality betweenV andp - L length of pores - p,p mean static pressure - p ₀ static pressure outside the tube - p ₀ value ofp atx=0 - Q constant exponent - R inside radius of the tube - T wall shear stress - T ₀ shear parameter - t wall thickness - U free stream velocity - ,u mean axial velocity - u ₀ value ofu atx=0 - V,V mean seepage velocity through the wall - v ₀ mean seepage velocity - x,x axial distance along the tube - z transformed axial distance - z ₀ value ofz atx=0 - mean outflow angle through the wall - cos - density of the fluid - wall shear stress - dynamic viscosity of the fluid - over-bar dimensional terms - no bar nondimensional terms The National Center for Atmospheric Research is sponsored by the National Science Foundation     

The Oligocene Lund Tuff, Great Basin, USA: a very large volume monotonous intermediate

Unusual monotonous intermediate ignimbrites consist of phenocryst-rich dacite that occurs as very large volume (>1000 km³) deposits that lack systematic compositional zonation, comagmatic rhyolite precursors, and underlying plinian beds. They are distinct from countless, usually smaller volume, zoned rhyolite–dacite–andesite deposits that are conventionally believed to have erupted from magma chambers in which thermal and compositional gradients were established because of sidewall crystallization and associated convective fractionation. Despite their great volume, or because of it, monotonous intermediates have received little attention. Documentation of the stratigraphy, composition, and geologic setting of the Lund Tuff – one of four monotonous intermediate tuffs in the middle-Tertiary Great Basin ignimbrite province – provides insight into its unusual origin and, by implication, the origin of other similar monotonous intermediates.The Lund Tuff is a single cooling unit with normal magnetic polarity whose volume likely exceeded 3000 km³. It was emplaced 29.02±0.04 Ma in and around the coeval White Rock caldera which has an unextended north–south diameter of about 50 km. The tuff is monotonous in that its phenocryst assemblage is virtually uniform throughout the deposit: plagioclase>quartz≈hornblende>biotite>Fe–Ti oxides≈sanidine>titanite, zircon, and apatite. However, ratios of phenocrysts vary by as much as an order of magnitude in a manner consistent with progressive crystallization in the pre-eruption chamber. A significant range in whole-rock chemical composition (e.g., 63–71 wt% SiO₂) is poorly correlated with phenocryst abundance.These compositional attributes cannot have been caused wholly by winnowing of glass from phenocrysts during eruption, as has been suggested for the monotonous intermediate Fish Canyon Tuff. Pumice fragments are also crystal-rich, and chemically and mineralogically indistinguishable from bulk tuff. We postulate that convective mixing in a sill-like magma chamber precluded development of a zoned chamber with a rhyolitic top or of a zoned pyroclastic deposit. Chemical variations in the Lund Tuff are consistent with equilibrium crystallization of a parental dacitic magma followed by eruptive mixing of compositionally diverse crystals and high-silica rhyolite vitroclasts during evacuation and emplacement. This model contrasts with the more systematic withdrawal from a bottle-shaped chamber in which sidewall crystallization creates a marked vertical compositional gradient and a substantial volume of capping-evolved rhyolite magma. Eruption at exceptionally high discharge rates precluded development of an underlying plinian deposit.The generation of the monotonous intermediate Lund magma and others like it in the middle Tertiary of the western USA reflects an unusually high flux of mantle-derived mafic magma into unusually thick and warm crust above a subducting slab of oceanic lithosphere.     

An extrusion energy absorber suitable for the protection of structures during an earthquake

A structure moving under the influence of an earthquake is normally required to absorb its own energy of motion. However, in many cases it is possible to attach to the structure energy absorbing devices which absorb most of its energy of motion. One such device is an energy absorber which works by extruding lead back and forth through an orifice. On being extruded the deformed lead recrystallizes immediately, thereby recovering its original mechanical properties before the next extrusion or stroke. Accordingly, the amount of energy absorbed is not limited by work hardening or fatigue of the lead, but rather by the heat capacity of the device, the melting point of lead being the upper limit to the operating temperature. Furthermore, the device is able to absorb energy during a large number of earthquakes. A number of 20 kN × 2 cm stroke to 200 kN × 26 cm stroke extrusion energy absorbers have been tested at rates of 10^?6 to 3.6 × 10^?6 cm/min. They behaved as ‘plastic solids’ or ‘coulomb dampers’ with nearly rectangular hysteresis loops and little rate dependence.     

Remote sensing retrieval of surface suspended sediment concentration in the Yellow River Estuary

Accurate assessment of surface suspended sediment concentration(SSSC) in estuary is essential to address several important issues: erosion, water pollution, human health risks, etc. In this study, an empirical cubic retrieval model was developed for the retrieval of SSSC from Yellow River Estuary. Based on sediments and seawater collected from the Yellow River and southeastern Laizhou Bay, SSSC conditions were reproduced in the laboratory at increasing concentrations within a range common to field observations. Continuous spectrum measurements of the various SSSCs ranging from 1 to 5700 mg/l were carried out using an Ava Field-3 spectrometer. The results indicated the good correlation between water SSSC and spectral reflectance(Rrs) was obtained in the spectral range of 726–900 nm. At SSSC greater than 2700 mg/L, the 740–900 nm spectral range was less susceptible to the effects of spectral reflectance saturation and more suitable for retrieval of high sediment concentrations. The best correlations were obtained for the reflectance ratio of 820 nm to 490 nm. Informed by the correlation between Rrs and SSSC, a retrieval model was developed(R2 = 0.992). The novel cubic model, which used the ratio of a near-infrared(NIR) band(740–900 nm) to a visible band(400–600 nm) as factors, provided robust quantification of high SSSC water samples. Two high SSSC centers, with an order of 103 mg/l, were found in the inversion results around the abandoned Diaokou River mouth, the present Yellow River mouth to the abandoned Qingshuigou River mouth. There was little sediment exchange between the two high SSSC centers due to the directions of the residual currents and vertical mixing.     

Partial melting in the Inzie Head gneisses: the role of water and a petrogenetic grid in KFMASH applicable to anatectic pelitic migmatites

A sequence of prograde isograds is recognized within the Dalradian Inzie Head gneisses where pelitic compositions have undergone variable degrees of partial melting via incongruent melting reactions consuming biotite. Three leucosome types are identified. At the lowest grades, granitic leucosomes containing porphyroblasts of cordierite (CRD‐melt) are abundant. At intermediate grades, CRD‐melt mingles with garnetiferous leucosomes (GT‐melt). At the highest grades, CRD‐melt coexists with orthopyroxene‐bearing leucosomes (OPX‐melt), while garnet is conspicuously absent. The prograde metamorphic field gradient is constrained to pressures of 2–3 kbar below the CRD‐melt isograd, and no greater than 4.5 kbar at the highest grade around Inzie Head. A petrogenetic grid, calculated using thermocalc , is presented for the K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (KFMASH) system for the phases orthopyroxene, garnet, cordierite, biotite, sillimanite, H₂O and melt with quartz and K‐feldspar in excess. For the implied field gradient, the reaction sequence predicted by the grid is consistent with the successive prograde development of each leucosome type. Compatibility diagrams suggest that, as anatexis proceeded, bulk compositions may have been displaced towards higher MgO content by the removal of (relatively) ferroan granitic leucosome. An isobaric (P = 4 kbar) T–a_H2O diagram shows that premigmatization fluids must have been water‐rich (a_H2O > 0.85) and suggests that, following the formation of small volumes of CRD‐melt, the system became fluid‐absent and melting reactions buffered a_H2O to lower values as temperatures rose. GT‐ and OPX‐melt formed by fluid‐absent melting reactions, but a maximum of 7–11% CRD‐melt fraction can be generated under fluid‐absent conditions, much less than the large volumes observed in the field. There is strong evidence that the CRD‐melt leucosomes could not have been derived by buoyantly aided upwards migration from levels beneath the migmatites. Their formation therefore required a significant influx of H₂O‐rich fluid, but in a quantity insufficient to have exhausted the buffering capacity of the solid assemblage plus melt. Fluid : rock ratios cannot have exceeded 1 : 30. The fluid was channelled through a regionally extensive shear zone network following melt‐induced failure. Such an influx of fluid at such depths has obvious consequences for localized crustal magma production and possibly for cordierite‐bearing granitoids in general.     

Background of ground penetrating radar measurements

Characterization of the shallow subsurface (0.25 to 10 m) is of growing importance for engineering activities, solutions of environmental problems, and archaeological investigations. Ground-penetrating radar (GPR) is an appropriate technique considering the depth range of interest, the strength of electric and magnetic contrasts between different subsurface layers and buried objects, and the required resolution. GPR surveys can detect subsurface structures by recording electromagnetic reflections from discontinuities. The detectability of objects and the delineation of subsurface structures increases with increasing wave velocity and conductivity differences between the object and its surroundings or between adjacent layers. However, unwanted reflections from objects above the surface influence the images. Shielded antennas can be used to avoid strong reflections from these objects. The data thus obtained are, however, more difficult to interpret. The fundamentals of GPR and two different acquisition setups for a GPR system are discussed. Basic interpretation tools for travel-time and velocity estimation are described, and finally, case studies are presented, followed by conclusions.