bluuebunny/crossref_metadata_2025 huggingface dataset:
Global and multiscale aspects of magnetospheric dynamics in local‐linear filters
Authors: Ukhorskiy A. Y. , Sitnov M. I. , Sharma A. S. , Papadopoulos K.
Published: December 2002

The magnetospheric dynamics consists of global and multiscale components. The local‐linear filters (LLFs) relating the solar wind input and the magnetospheric output have been used earlier to predict the global dynamical behavior. In this paper, the relative role of global and multiscale processes in the prediction of magnetospheric dynamics is studied. The filters are derived from the reconstructed input–output magnetospheric phase space using time series of VBS as the input and AL index as the output. We show that the conventional formula for the LLF can be broken into two parts corresponding to the global and multiscale constituents. The first part is the zeroth‐order term, which is obtained by the phase space average of the model outputs. This is a feature similar to the mean‐field model in phase transition physics, which yields iterative predictions of the global coherent component. The second part consists of the higher‐order terms of the filter, which are highly irregular and thus cannot be used in dynamical prediction. This irregular behavior represents the departure from the low‐dimensional dynamics underlying earlier studies using LLFs. The earlier prediction studies mixed these two components. However, by separating these two components, the prediction procedure is highly simplified and longer period predictions are achieved. The multiscale nature arises from the perturbations over a wide range of scales and has a power spectrum similar to that of colored noise. When these perturbations are taken into account in the prediction process, the iterative predictions yield a factor of four improvement in the accuracy compared to the mean‐field model. However, the filter technique does not provide a prescription for correctly including the multiscale aspects in a dynamical model and further improvement in forecasting can be achieved by a statistical approach. These results have important implications for space weather forecasting.
Photometric quantities for solar irradiance modeling
Authors: Preminger D. G. , Walton S. R. , Chapman G. A.
Published: December 2002

We analyze photometric quantities for the modeling of the total solar irradiance, S. These quantities are derived from full‐disk solar images taken at the San Fernando Observatory. We introduce a new quantity, the photometric sum, Σ, which is the sum over an entire image of each pixel's contribution to the irradiance in that image. Σ combines both bright and dark features; and because the sum is over the entire image, it will include low contrast features that cannot be identified directly. Specifically, we examine Σr, Σb, and ΣK, the photometric sums over broadband red, broadband blue, and 1‐nm bandpass Ca II K images, respectively. Σr and Σb measure the effects of solar features on the variability in S at two different continuum wavelengths. ΣK measures the variability in spectral lines due to solar features. We find that Σr and Σb have no long‐term trend. ΣK, however, varies in phase with the solar cycle. We carry out several multiple linear regressions on the value of S from cycle 22; the best fit uses Σr and ΣK and reproduces the observed composite S with a multiple regression coefficient R = 0.96. We conclude that the long‐term change in S over the solar cycle can be accounted for by the variability in the spectral lines as measured by ΣK, assuming no change in the quiet Sun; the contribution of the continuum to the variations in S is only on active region timescales.
Ground level muons in coincidence with the solar flare of 15 April 2001
Authors: Poirier J. , D'Andrea C.
Published: December 2002

The counting rate of single muon tracks from the Project GRAND proportional wire chamber array is examined during the Ground Level Event (GLE) of 15 April 2001. The GLE was seen by neutron monitor stations shortly after the time of the solar X‐ray flare. GRAND's single muon data are presented and compared with neutron monitor data from Climax, Newark, and Oulu. The single muon data have mean primary hadron energies higher than those of these neutron monitor stations and so contain information about higher energy hadrons. For the single muon data for Project GRAND the GLE is detected at a statistical significance of 6.1σ.
Effects of HF heater‐produced ionospheric depletions on the ducting of VLF transmissions: A ray tracing study
Authors: Starks M. J.
Published: December 2002

Comprehensive three‐dimensional ray tracing studies suggest that local large‐scale ionospheric density perturbations caused by the HF heater at Arecibo significantly affect the amount of power from upgoing 28.5 kHz VLF transmissions that couples into plasmaspheric ducts on nearby magnetic field lines. Large density depletions can perturb the ray paths of upgoing VLF to enhance or reduce the total power in a local duct. Under certain conditions, ducted power may be enhanced up to 13 dB or attenuated up to 18 dB. The dependence of this effect on the location and cross‐L drift of plasmaspheric ducts and the depth of the heater‐induced depletion may explain the difficulty in obtaining consistent experimental observations. Nongeometrical optics effects from the heating and resultant structures on VLF propagation may also affect the ducted power distribution but are outside the scope of ray tracing analysis.
Tristatic observations of meteors using the 930 MHz European Incoherent Scatter radar system
Authors: Janches Diego , Pellinen‐Wannberg Asta , Wannberg Gudmund , Westman Assar , Häggström Ingemar , Meisel David D.
Published: December 2002

We report results from the first tristatic measurements of radar meteors obtained during 17 November 1997 and 1998, using the UHF (930 MHz) European Incoherent Scatter (EISCAT) radar system. The observing technique utilized for these observations was first reported by Pellinen‐Wannberg et al. [1998a]. This system consists of three 32‐m parabolic antennae located in northern Scandinavia. Since EISCAT observes mostly meteor head echoes, a general characteristic of high‐power/large‐aperture radars, direct Doppler velocity (±1 km/s) determinations are possible. In addition, using the technique reported here, absolute geocentric meteor velocity and good radiant information (±5°) are deduced for those meteors that are detected simultaneously by all three receivers. An overview of the methodology and a summary of the results obtained so far are reported in this work. We compare the results obtained using this method with those reported by previous large‐aperture meteor radar work at lower frequencies and find general agreement. EISCAT detects mainly sporadic particles extending the fast daily sporadic micrometeor storms first suggested by Janches et al. [2000b] and Mathews et al. [2001] to submillimeter particles. To the best of our knowledge, these observations represent the first of their kind and prove EISCAT to be a crucial instrument for the study of extraterrestrial particles entering the Earth's atmosphere, in particular at very high geocentric latitudes and high geocentric speeds.
Reconsidering the effectiveness of quasi‐static thunderstorm electric fields for whistler duct formation
Authors: McCormick Robert J. , Rodger Craig J. , Thomson Neil R.
Published: December 2002

It has been suggested that E × B mixing of magnetospheric plasma can lead to geomagnetic field‐aligned ionization enhancements termed whistler ducts. DC electric fields from thunderstorms have been put forward as the source of the required radial electrostatic field. Recent experimental observations have indicated that quasi‐static radial thunderstorm electric fields are not responsible for whistler duct formation. This evidence appears to be in contradiction to the current theoretical calculations. In this paper we reconsider whistler duct formation through quasi‐static thunderstorm electric fields. Both the charge distributions and ionospheric profiles previously used are based on rather dated assumptions. We find that more realistic thunderstorm charge distributions in conjunction with the earlier ionospheric profiles produce high‐altitude electric fields that are of insufficient strength, given average thunderstorm effective charge, to create ducting within a reasonable time period. This is, however, not confirmed when the same charge distributions are examined using a more modern ionospheric profile based on international standard models for the ionosphere and neutral atmosphere. In this case some charge distributions will lead to realistic whistler duct creation. Our modeling suggests that quasi‐electrostatic radial fields from thunderstorms could drive whistler duct formation in some situations.
On the transport of pick‐up ions in the heliosphere: A compendium of new analytical solutions
Authors: Stawicki Olaf , Lerche Ian , Fichtner Horst
Published: December 2002

We present new solutions of the pitch‐angle‐averaged transport equation of pick‐up ions in the heliosphere. These solutions supplement those available so far for the case of a vanishing momentum diffusion. A general solution is derived for nonvanishing momentum diffusion. This solution, which is valid for a simple spatial power law behavior of the solar wind plasma and for arbitrary source functions, is applied to the stochastic acceleration of pick‐up ions and generalizes earlier solutions. Based on this, we present several asymptotic phase space integrals for the outer heliosphere and solve them analytically for a standard pick‐up ion source function. Numerical calculations for interstellar neutral hydrogen yield phase space distributions depending sensitively on the underlying turbulence and its dependence on heliocentric distance.
Electron and proton excitation of the FUV aurora: Simultaneous IMAGE and NOAA observations
Authors: Coumans V. , Gérard J.‐C. , Hubert B. , Evans D. S.
Published: December 2002

The Far Ultraviolet (FUV) imaging system on board the IMAGE satellite provides a global view of the north auroral region in different spectral channels. The Wideband Imaging Camera (WIC) is sensitive to the N2 LBH emission and NI emissions produced by both electron and proton precipitations. The SI12 camera images the Lyman‐α emission due to incident protons only. We compare WIC and SI12 observations with model predictions based on particle measurements from the TED and the MEPED detectors on board NOAA‐TIROS spacecraft. Models of the interaction of auroral particles with the atmosphere are used together with the in situ proton and electron flux and characteristic energy data to calculate the auroral brightness at the magnetic footprint of the NOAA‐15 and NOAA‐16 orbital tracks. The MEPED experiment measures the precipitating particles with energy higher than 30 keV, so that these comparisons include all auroral energies, in contrast to previous comparisons. A satisfactory agreement in morphology and in magnitude is obtained for most satellite overflights. The observed FUV‐WIC signal is well modeled if the different spatial resolution of the two sensors is considered and the in situ measurements properly smoothed. The calculated count rate includes contributions from LBH emission, the NI 149.3 nm line, and the OI 135.6 nm line excited by electrons and protons. The proton contribution in WIC can locally dominate the electrons. The comparisons indicate that protons can significantly contribute to the FUV aurora at specific times and places and cannot be systematically neglected. The results confirm the shift of the proton auroral oval equatorward of the electron oval in the dusk sector. We also show that in some regions, especially in the dusk sector, high‐energy protons dominate the proton energy flux and account for a large fraction of the Lyman‐α and other FUV emissions.
Implications of diverse fault orientations imaged in relocated aftershocks of the Mount Lewis, ML 5.7, California, earthquake
Authors: Kilb D. , Rubin A. M.
Published: December 2002

We use seismic waveform cross correlation to determine the relative positions of 2747 microearthquakes near Mount Lewis, California, that have waveforms recorded from 1984 to 1999. These earthquakes include the aftershock sequence of the 1986 ML5.7 Mount Lewis earthquake. Approximately 90% of these aftershocks are located beyond the tips of the approximately north striking main shock, defining an hourglass with the long axis aligned approximately with the main shock. Surprisingly, our relocation demonstrates that many of these aftershocks illuminate a series of near‐vertical east‐west faults that are ∼0.5–1 km long and separated by as little as ∼200 m. We propose that these structures result from the growth of a relatively young fault in which displacement across a right‐lateral approximately north striking fault zone is accommodated by slip on secondary left‐lateral approximately east striking faults. We derive the main shock‐induced static Coulomb failure function (Δσf) on the dominant fault orientation in our study area using a three‐dimensional (3‐D) boundary element program. To bound viable friction coefficients, we measure the correlation between the rank ordering of relative amplitudes of Δσf and seismicity rate change. We find that likely friction coefficients are 0.2–0.6 and that the assumed main shock geometry introduces the largest uncertainties in the favored friction values. We obtain similar results from a visual correlation of calculated Δσf contours with the distribution of aftershocks. Viable rate‐and‐state constitutive parameters bound the observed relationship between magnitude of Δσf and seismicity rate change, and for our favored main shock model a maximum correlation is achieved when Δσf is computed with friction coefficients of 0.3–0.6. These values are below those previously cited for young faults.
Seismic velocity and attenuation structures in the top of the Earth's inner core
Authors: Wen Lianxing , Niu Fenglin
Published: December 2002

We collect a global data set of PKIKP and PKiKP phases recorded by the Global Seismic Network and many regional seismic arrays to study seismic structure in the top of the Earth's inner core. The PKIKP and PKiKP observations show different characteristics between those sampling the “eastern” hemisphere (40°E–180°E) of the inner core and those sampling the “western” hemisphere (180°W–40°E). PKIKP phases (1) arrive about 0.4 s earlier than the theoretical arrivals based on Preliminary Reference Earth Model (PREM) for those sampling the eastern hemisphere of the inner core and about 0.3 s later for those sampling the western hemisphere (131°–141°); (2) bifurcate at smaller epicentral distances for those sampling the eastern hemisphere, compared to those sampling the western hemisphere; and (3) have smaller amplitudes for those sampling the eastern hemisphere. Waveform modeling of these observations suggests two different types of models for the two “hemispheres” of the top of the inner core, with a model in the eastern hemisphere having a P velocity increase of 0.765 km/s across the inner core boundary, a small radial velocity gradient of 0.000055 (km/s)/km, and an average Q value of 250, and a model in the western hemisphere with a P velocity increase of 0.633 km/s across the inner core boundary, a radial velocity gradient of 0.000533 (km/s)/km and an average Q value of 600. The hemispherical difference of seismic structures may be explained by different geometric inclusions of melt and/or different alignments of iron crystals with anisotropic properties in both velocity and attenuation. We speculate that this large‐scale pattern of seismic heterogeneities may be caused by a large‐scale heat flow anomaly at the bottom of the outer core and/or different vigorousness of convection in the top of the inner core between the two hemispheres.
A gelation analogy of crustal formation derived from fractal conductive structures
Authors: Bahr Karsten , Smirnov Maxim , Steveling Erich ,   BEAR Working Group
Published: December 2002

The statistical evaluation of numerical random media models and of electromagnetic array data provides evidence of fractal conductive structures in the middle and lower crust. During the search for an electrical conduction mechanism that is compatible with the geophysical anomalies in the middle and lower crust, random resistor networks were developed. These networks contain two types of resistors, representing the rock matrix and the conductive phase. Here we demonstrate that random resistor network models can also explain the statistical properties of the electric field distortion measured in large‐scale electromagnetic array experiments. Correspondence between measured and modeled distortion statistics is obtained if the structures formed by the two resistor types in the networks have a fractal geometry. This can indicate that the natural conductive networks also have a fractal geometry and stay close to a percolation threshold. A simple scale model of crust formation mechanics is considered in order to find an analog medium and an analog process that creates such a geometry. We suggest gelation as a rheological analogy that allows for a coexistence of ductile and brittle behavior of material in the lower crust. This process forms fractal structures in the host matrix and fractal conductive networks, provided that conductive material is available at that time of crustal evolution. The statistical evaluation of the field data provides evidence for a fractal structure with an upper bound of the size of Fennoscandia.
Thermohydraulic explosions in phreatomagmatic eruptions as evidenced by the comparison between pyroclasts and products from Molten Fuel Coolant Interaction experiments
Authors: Büttner R. , Dellino P. , La Volpe L. , Lorenz V. , Zimanowski B.
Published: December 2002

Thermohydraulic explosions were produced by Molten Fuel Coolant Interaction (MFCI) experiments using remelted shoshonitic rocks from Vulcano (Italy). The fragmentation history and energy release were recorded. The resulting products were recovered and analyzed with the scanning electron microscope. Fine particles from experiments show shape and surface features that result from melt fragmentation in brittle mode. These clasts relate to the thermohydraulic phase of the MFCI, where most of the mechanical energy is released; they are here called “active” particles. The total surface area of such particles is proportional to the energy of the respective explosions. Other particles from experiments show shape and surface features that result from melt fragmentation in a ductile regime. These fragments, called “passive” particles, form after the thermohydraulic phase, during the expansion phase of the MFCI. In order to verify thermohydraulic explosions in volcanic eruptions, we compared experimental products with samples from phreatomagmatic base‐surge deposits of Vulcano. Ash particles from the experiments show features similar to those from the deposits, suggesting that the experiments reproduced the same fragmentation dynamics. To achieve discrimination between active and passive particles, we calculated shape parameters from image analysis. The mass of active particles in base‐surge deposits was calculated. As the material properties for the natural samples are identical to the experimental ones, the energy measurements and calculations of the experiments can be applied. For a single phreatomagmatic eruption at Vulcano, a maximum mechanical energy release of 2.75 × 1013 J was calculated, representing a TNT analogue of 6.5 kt.
The propagation of a buoyant liquid‐filled fissure from a source under constant pressure: An experimental approach
Authors: Menand Thierry , Tait Stephen R.
Published: December 2002

We study the propagation of a buoyant liquid‐filled fissure from a reservoir under constant pressure within the framework of linear elastic fracture mechanics. We conducted laboratory experiments by injecting aqueous solutions into gelatin solid: an analogue for elastic and brittle crustal rocks. Fissure velocity and injection rate of liquid were measured rather than being imposed. Our experimental results allow evaluation of how the different driving and resistive pressures evolved during fissure propagation and highlight the influence of the fracture resistance of the host solid. In an initial transient propagation regime, elastic pressure generated by the fissure is balanced by the fracture pressure; the fissure propagates radially with decreasing velocity and increasing injection rate, controlled by the source conditions. Subsequently, buoyancy overcomes the source pressure as the driving force, and vertical steady state propagation is established. The fissure develops a bulbous head and propagation is controlled by the balance in this head, between buoyancy pressure and fracture pressure. Even after this transition, the constant values of velocity, flux, and strain energy release rate reflect the source conditions. Our model suggests that greater horizontal dyke cross section reflects larger source pressure and that mafic dykes propagating from shallow magma chambers are unlikely to attain steady state. Moreover, our experiments place constraints on the mechanics of time‐dependent failure of the solid as a process that resists fissure propagation: propagation velocity scales with the square of the height of the fissure head, and fracture toughness of rocks would be length scale dependent rather than a material property.
Slip‐size‐dependent renewal processes and Bayesian inferences for uncertainties
Authors: Ogata Yosihiko
Published: December 2002

This paper is initially concerned with uncertainty of hazard estimates using renewal process models where the parameters are poorly constrained because of the scarcity of large earthquakes occurring along the same fault segment. A Bayesian inference is adopted, taking account of the whole likelihood function of the parameters. Also, new statistical models are considered which make use of knowledge on the slip associated with earthquake ruptures, may help to improve the hazard estimate in a positive way. Including these models, the predictive efficiency is compared by using the Akaike's Bayesian information criterion (ABIC). Three data sets are analyzed for the illustrations. The Brownian Passage Time (BPT) model is selected to fit the first data set, consisting of 10 historical great earthquakes from Nanaki trough in Japan. However, its predictive hazard function shows a large uncertainty (>100 years) of likely occurrence time around 2070. The second data set consists of the last three events of the first data set but associated with record of slip sizes, for which the ABIC selected the extended lognormal renewal process model where the time intervals between successive events are normalized by the corresponding slip sizes of the starting events of the intervals. The estimated predictive hazard function implies that the next event is likely to occur around 2040 ± 10. The last data set, consisting of 4 events with estimated occurrence times and slip sizes from a submarine fault. The ABIC selected the slip‐size‐dependent BPT model for this data. This model indicates that the likelihood of occurrence time of the next event is decreasing from now, and the period of its half decay is more than several hundred years. For the data sets of the last two examples, it was also shown that the slip size records are useful for better prediction of the next event.
Decoupled thermal and mantle helium anomalies: Implications for the transport regime in continental rift zones
Authors: Clauser Christoph , Griesshaber Erika , Neugebauer Horst J.
Published: December 2002

In continental rift zones, thermal anomalies and enhanced occurrence of mantle‐derived fluids in groundwaters often coincide on a continental but rarely on the local scale. For areas of recent intraplate volcanism this suggests a time shift between the transport of mantle‐derived volatiles and heat to the Earth's surface. As a particularly illustrative example, we studied the central European Rhine graben, which is distinguished by extensional tectonics, high seismicity, young, Upper Tertiary volcanism, abundant mineral water springs, and a heat flow anomaly. A combination of thermal, helium isotope, and major element data of mineral waters with numerical simulations of flow and heat transport leads to a profound understanding of crustal‐scale transport processes and allows a clear distinction between diffusion and flow dominated regimes. The simulations show that for a crustal‐scale system the transition between these two regimes occurs within one decade of permeability (10−16–10−17 m2). Flow is likely to be strongly controlled by fault systems both in the lower and upper crust. While thermal anomalies highlight fluid circulation within the upper brittle crust, mantle‐derived volatiles provide information on transport phenomena from lower crustal sections. The thermal and hydro‐chemical anomalies in the Upper Rhine graben are directly related to mass flow rates. Therefore lack of thermal anomalies around the Kaiserstuhl volcano suggests that mass flow rates must be very small, while positive thermal anomalies in the central part of the graben can directly be related to advection of fluids into basin sediments and to redistribution of heat within these sediments.
Neotectonic modeling of the Ibero‐Maghrebian region
Authors: Negredo Ana M. , Bird Peter , Sanz de Galdeano Carlos , Buforn Elisa
Published: December 2002

Thin‐shell finite element techniques have been applied to model the neotectonics of the Ibero‐Maghrebian region, in the westernmost Mediterranean. This region is characterized by a complex seismotectonic pattern and moderate seismic activity associated with the convergence between Africa and Eurasia. We compare two end‐member models using different fault networks. Model predictions, including anelastic strain rates, vertically integrated stresses and velocity fields, are compared to the seismicity map and to data on directions of maximum horizontal compression. Best results are obtained assuming a low fault friction coefficient (0.05) and when the Betics and Rif are modeled as parts of the same arc‐shaped chain. The highest predicted fault slip rates are in the Tell mountains. Farther to the west, fault slip is more homogeneously distributed over the Betic‐Rif chain, Gulf of Cadiz, and Alboran Sea, indicating a diffuse geometry of the plate boundary in this area. The areas of highest predicted strain rates coincide with the most seismically active regions, located in northern Algeria and northeastern Morocco. Our best model also reproduces a major change of the stress regime, from thrusting in the east (Tell mountains) to predominantly strike‐slip and normal faulting in the west (Betic‐Rif chain and Alboran Sea). The Alboran basin is shown to be undergoing significant internal transpression and therefore cannot be considered as a rigid microplate. The western part of the Alboran Sea and surrounding areas are being extruded to the WNW with respect to Iberia.
Lateral heterogeneity in the upper mantle beneath the Tibetan plateau and its surroundings from SS‐S travel time residuals
Authors: Dricker I. G. , Roecker S. W.
Published: December 2002

We use SS‐S differential travel times to map the upper mantle structure of the Tibetan plateau and its surroundings. New observations from analog stations in the former Soviet Union help to fill in gaps in the spatial coverage of previous studies, providing new data for eastern Tibet, the Kunlun range, and the Tarim basin. The effects of crustal thickness variations, anisotropy, and global heterogeneity are evaluated and explicitly accounted for by incorporating results of previous studies and by ray tracing through a global 3D model. The resulting residuals suggest that, with the exception of the eastern part of the plateau, Tibet is underlain almost entirely by lithosphere of normal to greater than normal thickness. In general, there appear to be strong lateral gradients in wave speed between eastern and western Tibet. We find little evidence for low wave speeds under northern Tibet to the west of about 87°E. Instead, the average shear wave speeds in the upper 300 km of the mantle beneath this part of Tibet are about 0.5 km/s faster than those directly to the north beneath the Tarim basin. These results support models of Tibetan dynamics in which the lithosphere underlying northern Tibet underplates the crust as opposed to those in which large portions of this lithosphere disappear by convection, subduction, or detachment. At the same time, the dominant E‐W gradient in residuals implies that 2D models inadequately explain the dynamics of all of Tibet.
Using fractal analysis to quantitatively characterize the shapes of volcanic particles
Authors: Maria Anton , Carey Steven
Published: December 2002

The shapes of volcanic particles reflect numerous eruptive parameters (e.g., magma viscosity, volatile content, and degree of interaction with water) and are useful for understanding fragmentation and transport processes associated with volcanic eruptions. However, quantitative analysis of volcanic particle shapes has proven difficult because of their morphological complexity and variability. Here a newly developed procedure for shape analysis based on fractal geometry is described and tested. Although volcanic particle shapes are not truly fractal, their complexity can be effectively characterized in terms of fractal values (pseudofractal dimensions) reflecting morphological invariance over discrete ranges of scale. Using fractal data produced by dilation of a particle's two‐dimensional boundary, a spectrum of fractal dimensions is calculated for each particle by taking the first derivative of the dilation data. Compared with fractal methods that express shape in terms of only one or two fractal dimensions, typically derived from the slope of data on a Richardson plot, this technique results in better discrimination between samples. In addition, use of multiple fractal values allows incorporation of multivariate statistical analysis, further strengthening the differentiating power of the technique. This fractal spectrum technique yields promising results for samples of sideromelane shards from Iceland and is likely to be effective at characterizing other kinds of volcanic particle shapes.
Preeruptive inflation and surface interferometric coherence characteristics revealed by satellite radar interferometry at Makushin Volcano, Alaska: 1993–2000
Authors: Lu Zhong , Power John A. , McConnell Vicki S. , Wicks Charles , Dzurisin Daniel
Published: December 2002

Pilot reports in January 1995 and geologic field observations from the summer of 1996 indicate that a relatively small explosive eruption of Makushin, one of the more frequently active volcanoes in the Aleutian arc of Alaska, occurred on 30 January 1995. Several independent radar interferograms that each span the time period from October 1993 to September 1995 show evidence of ∼7 cm of uplift centered on the volcano's east flank, which we interpret as preeruptive inflation of a ∼7‐km‐deep magma source (ΔV = 0.022 km3). Subsequent interferograms for 1995–2000, a period that included no reported eruptive activity, show no evidence of additional ground deformation. Interferometric coherence at C band is found to persist for 3 years or more on lava flow and other rocky surfaces covered with short grass and sparsely distributed tall grass and for at least 1 year on most pyroclastic deposits. On lava flow and rocky surfaces with dense tall grass and on alluvium, coherence lasts for a few months. Snow and ice surfaces lose coherence within a few days. This extended timeframe of coherence over a variety of surface materials makes C band radar interferometry an effective tool for studying volcano deformation in Alaska and other similar high‐latitude regions.
Modeling the stress variations induced by great earthquakes on the largest volcanic eruptions of the 20th century
Authors: Marzocchi Warner , Casarotti Emanuele , Piersanti Antonio
Published: December 2002

One of the most recent novelties in volcanology is the finding of a statistically significant influence of remote strong earthquakes on the largest explosive eruptions of the last century. Here, we model such interaction in terms of the coseismic and postseismic stress diffusion. The stress variation consists of the elastic response of the lithosphere, and the viscoelastic relaxation of the asthenosphere and mantle, computed through a spherical, stratified, self‐gravitating, viscoelastic Earth model. Our results show that the tectonic earthquakes induce on the volcanoes significant stress variations, from a few tenths to tens of bars, both in dilatation/compression and in shear. Remarkably, the model explains the main features resulting from the empirical study: the time lag (from several months to a few decades), the spatial distances (hundreds of kilometers), and the different intensities of the coupling. This indicates that the coseismic and postseismic stress diffusion might remotely induce eruptions, particularly for volcanoes close to a critical state. However, the results show only weak evidence of a common preeruptive stress evolution, with an overall compression and shear pattern at a few kilometers depth, pointing out the complexity of the mechanism responsible for the reactivation of a volcano. These findings confirm previous suggestions, depicting the volcanic areas to be open systems governed by a high number of degrees of freedom, where the remote seismic activity might represent one of the most relevant. The modeling of such interactions might shed a new light on evaluating the probability of occurrence of a large explosive eruption.
Locations of mid‐oceanic earthquakes constrained by seafloor bathymetry
Authors: Pan Jianfeng , Antolik Michael , Dziewonski Adam M.
Published: December 2002

Earthquakes associated with the creation of new oceanic crust are difficult to locate precisely because they normally occur far away from seismic stations. We assume that epicenters of these events can be constrained by seafloor topography and use the Harvard centroid moment tensor (CMT) focal mechanism of an interplate earthquake to relate the event to either a transform fault (a parallel in a coordinate system defined by the pole of rotation between two plates) or a mid‐oceanic ridge (a meridian). Using arrival times from the International Seismological Center (ISC) Bulletin, we have relocated about 1500 globally distributed events for the time period 1976–1998. The ISC locations can be as much as 70 km away from appropriate topographic features. Our new locations support that large interplate earthquakes occur on or very close to the primary bathymetric features. For earthquakes with unknown mechanism or magnitude below the CMT threshold or predate the CMT catalog, we use a simultaneous Joint Hypocenter Determination procedure with respect to the already relocated Master Events (JHDME). We invert for earthquakes first for the Romanche Fracture Zone (RFZ) and then for a region extending from 30°N to 30°S. Seismicity agrees well with the main active ridge or transform. However, a small number of events might be related to the secondary bathymetry features. The improved locations for mid‐oceanic earthquakes based on bathymetry could help improve our understanding of the oceanic crustal and deep Earth structure, seismic slip and budget constraint along plate boundaries, and dynamics of plate tectonics.
Magnetic anisotropy and paleomagnetic inclination shallowing in red beds: Evidence from the Mississippian Mauch Chunk Formation, Pennsylvania
Authors: Tan Xiaodong , Kodama K. P.
Published: December 2002

Red, hematite‐bearing sedimentary rocks are an important source of paleomagnetic data, particularly for continental apparent polar wander paths during the Paleozoic. This study presents magnetic anisotropy data from the Mississippian Mauch Chunk Formation of eastern Pennsylvania, indicating that these red beds have suffered from a significant amount of paleomagnetic inclination shallowing. Fourteen oriented block samples were collected from normal and reversed polarity strata identified in a previous study. Thermal demagnetization isolated the characteristic remanent magnetization at seven normal polarity horizons at unblocking temperatures greater than 670°C (mean direction, D = 354.0°, I = −18.4°, α95 = 10.2°). Anisotropy of remanence measurements (anisotropy of isothermal remanent magnetization and thermal remanent magnetization) indicate a bedding parallel, foliated magnetic fabric with foliations ranging from 1.1 to 1.35. Thermal demagnetization at 670°C of the isothermal remanent magnetization isolates the magnetic fabric of the characteristic remanence‐carrying grains and indicates stronger foliations (1.15–1.48). Anisotropy of magnetic susceptibility (AMS) also indicates bedding parallel magnetic foliations of 1.01–1.04, typical of red beds. Chemical leaching isolates the AMS of the characteristic remanence‐carrying grains to range from 1.02 to 1.07. This AMS fabric was used to correct the characteristic remanence inclination isolated by thermal demagnetization. The critical parameter needed for an accurate inclination correction, the individual particle anisotropy aχ, was determined from the correlation between the normalized principal axes of remanence and susceptibility anisotropy to be 1.06. The corrected Mauch Chunk direction, D = 354.5°, I = −56.4° results in a corrected Mauch Chunk Formation paleopole (12°N, 108°E) that is consistent with a European igneous paleopole.
On a modified arrested topographic wave in Fram Strait
Authors: Schlichtholz P.
Published: December 2002

The bottom geostrophic flow in Fram Strait is explained in terms of a modified arrested topographic wave model. The model is based on a combination of the vorticity balance between the topographic vortex stretching and the bottom Ekman pumping due to liner friction with a relation between the bottom flow and the bottom pressure gradient over a sloping bottom. Such a combination emphasizes a dynamical relevance of along‐isobath density variations at the bottom. Using an inverse solution based on the hydrographic data from the MIZEX 84 experiment, it is shown that the along‐isobath bottom density variations should be dynamically important, for example, in the East Greenland Current. A relatively high correlation between an along‐isobath distribution of the bottom geostrophic flow and the bottom density is reported for that current. The density variations identified in the data should be able to drive a bottom geostrophic flow with a magnitude consistent with the inverse solution and observations.
Short‐wave directional properties in the vicinity of atmospheric and oceanic fronts
Authors: Hisaki Yukiharu
Published: December 2002

The author estimated and investigated mean wave directions and directional spread parameters of 0.505 Hz ocean waves using data from 24.5 MHz‐HF ocean radar. The author described temporal and spatial variabilities of the short‐wave field. The short‐wave direction is a good indication of wind direction. The spread parameter was related to the variability of wind direction. The author examined the case that an atmospheric front or an oceanic front exists in the HF ocean radar observation area. In the case of an atmospheric front the short‐wave direction changed with the passage of the atmospheric front. Furthermore, the complicated frontal structure, which cannot be verified from a weather chart, was revealed. Narrow directional spread was observed in the vicinity of the convergent front, which was reproduced using a simplified model based on the wave action conservation law. The result shows that wave refraction associated with a frontal convergent zone is important in determining the spatial variability of short‐wave directional properties.
Air‐sea interactions during the passage of a winter storm over the Gulf Stream: A three‐dimensional coupled atmosphere‐ocean model study
Authors: Li Yongping , Xue Huijie , Bane John M.
Published: December 2002

A three‐dimensional, regional coupled atmosphere‐ocean model with full physics is developed to study air‐sea interactions during winter storms off the U. S. east coast. Because of the scarcity of open ocean observations, models such as this offer valuable opportunities to investigate how oceanic forcing drives atmospheric circulation and vice versa. The study presented here considers conditions of strong atmospheric forcing (high wind speeds) and strong oceanic forcing (significant sea surface temperature (SST) gradients). A simulated atmospheric cyclone evolves in a manner consistent with Eta reanalysis, and the simulated air‐sea heat and momentum exchanges strongly affect the circulations in both the atmosphere and the ocean. For the simulated cyclone of 19–20 January 1998, maximum ocean‐to‐atmosphere heat fluxes first appear over the Gulf Stream in the South Atlantic Bight, and this results in rapid deepening of the cyclone off the Carolina coast. As the cyclone moves eastward, the heat flux maximum shifts into the region near Cape Hatteras and later northeast of Hatteras, where it enhances the wind locally. The oceanic response to the atmospheric forcing is closely related to the wind direction. Southerly and southwesterly winds tend to strengthen surface currents in the Gulf Stream, whereas northeasterly winds weaken the surface currents in the Gulf Stream and generate southwestward flows on the shelf. The oceanic feedback to the atmosphere moderates the cyclone strength. Compared with a simulation in which the oceanic model always passes the initial SST to the atmospheric model, the coupled simulation in which the oceanic model passes the evolving SST to the atmospheric model produces higher ocean‐to‐atmosphere heat flux near Gulf Stream meander troughs. This is due to wind‐driven lateral shifts of the stream, which in turn enhance the local northeasterly winds. Away from the Gulf Stream the coupled simulation produces surface winds that are 5 ∼ 10% weaker. Differences in the surface ocean currents between these two experiments are significant on the shelf and in the open ocean.
Interannual variability in the tropical Indian Ocean
Authors: Huang Bohua , Kinter James L.
Published: December 2002

The interannual variability in the tropical Indian Ocean is examined using 41‐year (1958–1998) seasonal anomalies of the upper‐ocean heat content (HCA), sea surface temperature (SSTA), and surface wind stress. Precipitation anomalies from a shorter period (1979–1998) have also been analyzed. This analysis demonstrates that a coupled ocean–atmosphere interannual oscillation with a period ranging from 2 to 5 years is the major variability in the tropical Indian Ocean. At the peak phase, anomalous equatorial zonal winds over the central and the eastern ocean and anomalous trade winds to the south induce zonal SSTA and HCA gradients near the equator and an east–west shift of the convection. This interannual oscillation is the dominant signal from the boreal autumn to the next spring. The westward propagating HCA causes a phase delay between the peaks of the surface cooling near the eastern coast and the warming near the western coast near the equator. During its propagation, the southern HCA branch is strengthened by the anomalous wind curl of the equatorial and southeast trade wind anomalies over the southern ocean. As a result, the southern HCA is maintained near the western coast for a much longer period. This Indian Ocean oscillation is significantly correlated with the El Niño/Southern Oscillation (ENSO) variability in the Pacific Ocean.
Atmospheric contributions to the surface mass balance of Greenland in the HadAM3 atmospheric model
Authors: Murphy Bradley F. , Marsiat Isabelle , Valdes Paul
Published: December 2002

Atmospheric climate models now present powerful tools for studying atmospheric parameters relevant to the mass balance of the Greenland ice sheet, a significant uncertainty in global sea level. This study evaluates the simulations of the UK Hadley Centre atmospheric model (HadAM3) against observational data and reanalyses. HadAM3 reproduces the main precipitation patterns, but a positive bias exists in autumn over southern Greenland. The modeled North Atlantic storm track is well placed, but it and the polar vortex and hemispheric‐scale wave activity are weaker than observed, which may impact on moisture flux over Greenland. Surface temperatures are 2°–3°C too warm in summer and up to 5°C too cold in winter, probably owing to the snow albedo varying too little throughout the year, and excess summer downward short‐wave radiation flux and winter long‐wave loss, related to insufficient cloud cover. The summer warm bias and the too coarsely represented ice sheet area lead to Greenland‐mean melting being overestimated, and a negative surface mass balance is simulated. A higher‐resolution ice sheet mask improves these quantities significantly. Increasing resolution is expected to remove most errors when orography and the ice sheet, particularly around the coastal margins where gradients are large, are better defined, as orographic dynamical deficiencies are minimized. A high‐resolution version of the model is seen to improve the simulation of precipitation. Minor ice and snow physics improvements are needed, but the large‐scale simulation produces good‐quality inputs with no major bias for higher resolution limited area or downscaling models.
Simulation of the ozone formation in the northern part of the Po Valley
Authors: Martilli A. , Neftel A. , Favaro G. , Kirchner F. , Sillman S. , Clappier A.
Published: December 2002

The formation of photooxidants and aerosols in the Milan area (Po Valley, Italy) has been simulated for 13 May 1998 with an Eulerian model. The high‐pressure conditions and the weak synoptic circulation favor the formation of strongly enhanced photooxidants and aerosol levels downwind of the metropolitan area of Milan. The polluted air masses are transported northward where the highest ozone (O3) concentrations of up to 200 ppb are reached 30 to 40 km away from the city center of Milan. The core of the Milan plume has an extension of only 10 km in length and 15 km in width at 1600 LT. The focus of the simulation was the sensitivity of the O3 formation to the reduction of nitrogen oxides (NOx) and volatile organic compounds (VOC) emissions. The model results are compared to the measurements in order to evaluate the quality of the simulation. The spatial and temporal variations of the concentrations of O3 and other trace compounds are reasonably well reproduced. Several reasons for systematic deviations have been analyzed in order to understand their possible influence on the sensitivity analysis. Moreover, the simulation showed that indicator species such as the H2O2/HNO3 or O3/NOy ratio could be applied to characterize the VOC/NOx regime of the O3 production in the Milan area, but that it is necessary to modify the indicator ratios to include the aerosol nitrate. Highest O3 values appear at the transition from VOC to NOx sensitivity approximately 30 km downwind of the highest emissions.