bluuebunny/crossref_metadata_2025 huggingface dataset:
Phase space vortices in collisionless plasmas
Authors: Guio P. , Børve S. , Daldorff L. K. S. , Lynov J. P. , Michelsen P. , Pécseli H. L. , Juul Rasmussen J. , Saeki K. , Trulsen J.
Published: April 2010

Abstract. Results on the formation and propagation of electron phase space vortices from laboratory experiments are summarized. The electron phase space vortices were excited in a strongly magnetized Q-machine plasma by applying a pulse to a segment of a waveguide surrounding the plasma. Depending on the temporal variation of the applied pulse, one or more phase space vortices can be excited, and their interaction can be followed in space and time. We were able to demonstrate, for instance, an irreversible coalescence of two such vortices. These results are extended by numerical simulations, showing how electron phase space vortices can also be formed by beam instabilities. Furthermore, a study of ion phase space vortices is performed by numerical simulations. Both codes allow for an externally applied magnetic field in three spatial dimensions. Ion phase space vortices are formed by the nonlinear saturation of the ion-ion two-stream instability, excited by injecting an ion beam at the plasma boundary. By following the evolution of the ion distribution of the velocity perpendicular to the direction of propagation of the injected ion beam, we find a significant ion heating in the direction perpendicular to the magnetic field associated with the ion phase space vortices being formed. The results are relevant, for instance, for the interpretation of observations by instrumented spacecraft in the Earth's ionosphere and magnetosphere.
Solitary waves observed in the auroral zone: the Cluster multi-spacecraft perspective
Authors: Pickett J. S. , Kahler S. W. , Chen L.-J. , Huff R. L. , Santolík O. , Khotyaintsev Y. , Décréau P. M. E. , Winningham D. , Frahm R. , Goldstein M. L. , Lakhina G. S. , Tsurutani B. T. , Lavraud B. , Gurnett D. A. , André M. , Fazakerley A. , Balogh A. , Rème H.
Published: April 2010

Abstract. We report on recent measurements of solitary waves made by the Wideband Plasma Wave Receiver located on each of the four Cluster spacecraft at 4.5-6.5RE (well above the auroral acceleration region) as they cross field lines that map to the auroral zones. These solitary waves are observed in the Wideband data as isolated bipolar and tripolar waveforms. Examples of the two types of pulses are provided. The time durations of the majority of both types of solitary waves observed in this region range from about 0.3 up to 5ms. Their peak-to-peak amplitudes range from about 0.05 up to 20mV/m, with a few reaching up to almost 70mV/m. There is essentially no potential change across the bipolar pulses. There appears to be a small, measurable potential change, up to 0.5V, across the tripolar pulses, which is consistent with weak or hybrid double layers. A limited cross-spacecraft correlation study was carried out in order to identify the same solitary wave on more than one spacecraft. We found no convincing correlations of the bipolar solitary waves. In the two cases of possible correlation of the tripolar pulses, we found that the solitary waves are propagating at several hundred to a few thousand km/s and that they are possibly evolving (growing, decaying) as they propagate from one spacecraft to the next. Further, they have a perpendicular (to the magnetic field) width of 50km or greater and a parallel width of about 2-5km. We conclude, in general, however, that the Cluster spacecraft at separations along and perpendicular to the local magnetic field direction of tens of km and greater are too large to obtain positive correlations in this region. Looking at the macroscale of the auroral zone at 4.5-6.5RE, we find that the onsets of the broadband electrostatic noise associated with the solitary waves observed in the spectrograms of the WBD data are generally consistent with propagation of the solitary waves up the field lines (away from Earth), or with particles or waves propagating up the field line, which leads to local generation of the solitary waves all along the field lines. A discussion of the importance of these solitary waves in magnetospheric processes and their possible generation mechanisms, through electron beam instabilities and turbulence, is provided.
Association of Alfvén waves and proton cyclotron waves with electrostatic bipolar pulses: magnetic hole events observed by Polar
Authors: Lakhina G. S. , Tsurutani B. T. , Pickett J.
Published: April 2010

Abstract. Two magnetic hole events observed by Polar on 20 May 1996 when it was in the polar cap/polar cusp boundary layer are studied. Low-frequency waves, consisting of nonlinear Alfvén waves and large amplitude (±14nT peak-to-peak) obliquely propagating proton cyclotron waves (with frequency f~0.6 to 0.7 fcp), accompanied by electric bipolar pulses (electron holes) and electron heating have been observed located within magnetic holes. It is shown that low-frequency waves can provide free energy to drive some high frequency instabilities which saturate by trapping electrons, thus, leading to the generation of electron holes.
Electron acoustic solitons in the Earth's magnetotail
Authors: Tagare S. G. , Singh S. V. , Reddy R. V. , Lakhina G. S.
Published: April 2010

Abstract. Small amplitude electron - acoustic solitons are studied in a magnetized plasma consisting of two types of electrons, namely cold electron beam and background plasma electrons and two temperature ion plasma. The analysis predicts rarefactive solitons. The model may provide a possible explanation for the perpendicular polarization of the low-frequency component of the broadband electrostatic noise observed in the Earth's magnetotail.
Generation of zonal flows by Rossby waves in the atmosphere
Authors: Onishchenko O. G. , Pokhotelov O. A. , Sagdeev R. Z. , Shukla P. K. , Stenflo L.
Published: April 2010

Abstract. A novel mechanism for the short-scale Rossby waves interacting with long-scale zonal flows in the Earth's atmosphere is studied. The model is based on the parametric excitation of convective cells by finite amplitude Rossby waves. We use a set of coupled equations describing the nonlinear interaction of Rossby waves and zonal flows which admits the excitation of zonal flows. The generation of such flows is due to the Reynolds stresses of the finite amplitude Rossby waves. It is found that the wave vector of the fastest growing mode is perpendicular to that of the pump Rossby wave. We calculate the maximum instability growth rate and deduce the optimal spatial dimensions of the zonal flows as well as their azimuthal propagation speed. A comparison with previous results is made. The present theory can be used for the interpretation of existing observations of Rossby type waves in the Earth's atmosphere.
Null modes effect in Rossby wave model
Authors: Goncharov V. , Pavlov V.
Published: April 2010

Abstract. The problem of the null-modes existence and some particularities of their interaction with nonlinear vortex-wave-like structures is discussed. We show that the null-modes are fundamental elements of nonlinear wave fields. The conditions under which null-modes can manifest themselves are elucidated. The Rossby-Hasegawa-Mima (RHM) model is used for the illustration of features of null-modes-waves interactions.
Flow partitioning and unstable divergence in fluviokarst evolution in central Kentucky
Authors: Phillips J. D. , Walls M. D.
Published: April 2010

Abstract. In fluviokarst landscapes flow may be partitioned into its surface and subsurface components as well as into diffuse and concentrated flow. The competition among these is hypothesized to be responsible for the divergence of the landscape in a fluviokarst region of central Kentucky into depression-rich, unchannelled karst-rich and channel-poor (KRCP) and strongly fluvially dissected channel-rich karst-poor (CRKP) zones. The interrelationships between diffuse surface runoff, channelized surface flow, diffuse recharge, and point recharge are dynamically unstable and chaotic, implying that small changes in the partitioning are likely to have disproportionately large and long-lived impacts, reflected in geomorphology. In the Kentucky River gorge, rapid Quaternary incision has resulted in local slope changes which should induce instability in the flow partitioning system. A GIS-based landscape classification scheme showed that there is a relationship between slope gradients and the degree of karstification or fluvial dissection. Geomorphic interpretation of landforms in the river gorge area indicates that CRKP and KRCP zones are growing at the expense of other landscape classes. This results in an increase in Kolmogorov entropy, a characteristic of a dynamically, unstable, chaotic system. Results support the hypothesis that divergent landscape evolution is linked to the complex nonlinear dynamics of low partitioning.
A non-linear and stochastic response surface method for Bayesian estimation of uncertainty in soil moisture simulation from a land surface model
Authors: Hossain F. , Anagnostou E. N. , Lee K.-H.
Published: April 2010

Abstract. This study presents a simple and efficient scheme for Bayesian estimation of uncertainty in soil moisture simulation by a Land Surface Model (LSM). The scheme is assessed within a Monte Carlo (MC) simulation framework based on the Generalized Likelihood Uncertainty Estimation (GLUE) methodology. A primary limitation of using the GLUE method is the prohibitive computational burden imposed by uniform random sampling of the model's parameter distributions. Sampling is improved in the proposed scheme by stochastic modeling of the parameters' response surface that recognizes the non-linear deterministic behavior between soil moisture and land surface parameters. Uncertainty in soil moisture simulation (model output) is approximated through a Hermite polynomial chaos expansion of normal random variables that represent the model's parameter (model input) uncertainty. The unknown coefficients of the polynomial are calculated using limited number of model simulation runs. The calibrated polynomial is then used as a fast-running proxy to the slower-running LSM to predict the degree of representativeness of a randomly sampled model parameter set. An evaluation of the scheme's efficiency in sampling is made through comparison with the fully random MC sampling (the norm for GLUE) and the nearest-neighborhood sampling technique. The scheme was able to reduce computational burden of random MC sampling for GLUE in the ranges of 10%-70%. The scheme was also found to be about 10% more efficient than the nearest-neighborhood sampling method in predicting a sampled parameter set's degree of representativeness. The GLUE based on the proposed sampling scheme did not alter the essential features of the uncertainty structure in soil moisture simulation. The scheme can potentially make GLUE uncertainty estimation for any LSM more efficient as it does not impose any additional structural or distributional assumptions.
Detecting nonlinearity in time series driven by non-Gaussian noise: the case of river flows
Authors: Laio F. , Porporato A. , Ridolfi L. , Tamea S.
Published: April 2010

Abstract. Several methods exist for the detection of nonlinearity in univariate time series. In the present work we consider riverflow time series to infer the dynamical characteristics of the rainfall-runoff transformation. It is shown that the non-Gaussian nature of the driving force (rainfall) can distort the results of such methods, in particular when surrogate data techniques are used. Deterministic versus stochastic (DVS) plots, conditionally applied to the decay phases of the time series, are instead proved to be a suitable tool to detect nonlinearity in processes driven by non-Gaussian (Poissonian) noise. An application to daily discharges from three Italian rivers provides important clues to the presence of nonlinearity in the rainfall-runoff transformation.
Direct numerical simulations of helical dynamo action: MHD and beyond
Authors: Gómez D. O. , Mininni P. D.
Published: April 2010

Abstract. Magnetohydrodynamic dynamo action is often invoked to explain the existence of magnetic fields in several astronomical objects. In this work, we present direct numerical simulations of MHD helical dynamos, to study the exponential growth and saturation of magnetic fields. Simulations are made within the framework of incompressible flows and using periodic boundary conditions. The statistical properties of the flow are studied, and it is found that its helicity displays strong spatial fluctuations. Regions with large kinetic helicity are also strongly concentrated in space, forming elongated structures. In dynamo simulations using these flows, we found that the growth rate and the saturation level of magnetic energy and magnetic helicity reach an asymptotic value as the Reynolds number is increased. Finally, extensions of the MHD theory to include kinetic effects relevant in astrophysical environments are discussed.
Influence of a discontinuity on the spectral and fractal analysis of one-dimensional data
Authors: Berton R. P. H.
Published: April 2010

Abstract. The analysis of a data area or segment containing steep transitions between regions with different textures (for example a cloud and its background) leads to addressing the problem of discontinuities and their impact on texture analysis. In that purpose, an original one-dimensional analytical model of spectrum and roughness function has been worked out, with a discontinuity between two fractal regions, each one specified by its average µ, standard deviation σ, spectral index β and Hurst exponent H. This has the advantage of not needing the generation of a fractal structure with a particular algorithm or random functions and clearly puts into evidence the role played by the average in generating spectral poles and side lobes. After validation of the model calibration, a parametric study is carried out in order to understand the influence of this discontinuity on the estimation of the spectral index β and the Hurst parameter H. It shows that for a pure µ-gap, H is well estimated everywhere, though overestimated, and β is overestimated in the anti-correlation range and saturates in the correlation range. For a pure σ-gap the retrieval of H is excellent everywhere and the behaviour of β is better than for a µ-gap, leading to less overestimation in the anti-correlation range. For a pure β-gap, saturation degrades measurements in the case of raw data and the medium with smaller spectral index is predominant in the case of trend-corrected data. For a pure H-gap, there is also dominance of the medium with smaller fractal exponent.
A mixture theory for geophysical fluids
Authors: Eringen A. C.
Published: April 2010

Abstract. A continuum theory is developed for a geophysical fluid consisting of two species. Balance laws are given for the individual components of the mixture, modeled as micropolar viscous fluids. The continua allow independent rotational degrees of freedom, so that the fluids can exhibit couple stresses and a non-symmetric stress tensor. The second law of thermodynamics is used to develop constitutive equations. Linear constitutive equations are constituted for a heat conducting mixture, each species possessing separate viscosities. Field equations are obtained and boundary and initial conditions are stated. This theory is relevant to an atmospheric mixture consisting of any two species from rain, snow and/or sand. Also, this is a continuum theory for oceanic mixtures, such as water and silt, or water and oil spills, etc.
Nonlinear multidimensional scaling and visualization of earthquake clusters over space, time and feature space
Authors: Dzwinel W. , Yuen D. A. , Boryczko K. , Ben-Zion Y. , Yoshioka S. , Ito T.
Published: April 2010

Abstract. We present a novel technique based on a multi-resolutional clustering and nonlinear multi-dimensional scaling of earthquake patterns to investigate observed and synthetic seismic catalogs. The observed data represent seismic activities around the Japanese islands during 1997-2003. The synthetic data were generated by numerical simulations for various cases of a heterogeneous fault governed by 3-D elastic dislocation and power-law creep. At the highest resolution, we analyze the local cluster structures in the data space of seismic events for the two types of catalogs by using an agglomerative clustering algorithm. We demonstrate that small magnitude events produce local spatio-temporal patches delineating neighboring large events. Seismic events, quantized in space and time, generate the multi-dimensional feature space characterized by the earthquake parameters. Using a non-hierarchical clustering algorithm and nonlinear multi-dimensional scaling, we explore the multitudinous earthquakes by real-time 3-D visualization and inspection of the multivariate clusters. At the spatial resolutions characteristic of the earthquake parameters, all of the ongoing seismicity both before and after the largest events accumulates to a global structure consisting of a few separate clusters in the feature space. We show that by combining the results of clustering in both low and high resolution spaces, we can recognize precursory events more precisely and unravel vital information that cannot be discerned at a single resolution.
Nonlinear dynamics of turbulent waves in fluids and plasmas
Authors: He K. , Chian A. C.-L.
Published: April 2010

Abstract. In a model drift wave system that is interesting both in fluids and plasmas, we find that an embedded moving saddle point plays an important role at the onset of turbulence. Here the saddle point is actually a saddle steady wave, in its moving frame the wave system can be transformed into a set of coupled oscillators whose motion is affected by the saddle steady wave as if it is a potential. It is found that a collision with the saddle point triggers a crisis, following the collision another dynamic event occurs which involves a transition in the phase state of the master oscillator. Only after the latter event the spatial regularity is destroyed. The phase dynamics before and after the transition is further investigated. It is found that in a spatially coherent state before the transition the oscillators reach a functional phase synchronization collectively with or without phase slips, after the transition in the turbulent state an on-off imperfect synchronization is established among the oscillators with long wavelengths. When the synchronization is on, their amplitudes grow up simultaneously, giving rise to a burst in the total wave energy. A power law behavior is observed in the correlation function between phases of the oscillators. Potential application of our results in prediction of energy bursts in turbulence is discussed.
Prebifurcation noise amplification and noise-dependent hysteresis as indicators of bifurcations in nonlinear geophysical systems
Authors: Surovyatkina E.
Published: April 2010

Abstract. The phenomena of prebifurcation noise amplification and noise-dependent hysteresis are studied as prospective indicators of bifurcations ("noisy precursor") in nonlinear Geophysical systems. The phenomenon of prebifurcation noise amplification arises due to decreasing of damping coefficients just before bifurcation. A simple method for the estimation of the forced fluctuation variance is suggested which is based on results of linear theory up to the boundary of its validity. The upper level for the fluctuation variance before the onset of the bifurcation is estimated from the condition that the contribution of the non-linear term becomes comparable (in the sense of mean squares) with that of the linear term. The method has proved to be efficient for two simple bifurcation models (period doubling bifurcation and pitchfork bifurcation) and might be helpful in application to geophysics problems. The transition of a nonlinear system through the bifurcation point offers a new opportunity for estimating the internal noise using the magnitude of the noise-dependent hysteretic loop, which occurs when the control parameter is changed in the forward and backward direction.
Exact theory for localized envelope modulated electrostatic wavepackets in space and dusty plasmas
Authors: Kourakis I. , Shukla P. K.
Published: April 2010

Abstract. Abundant evidence for the occurrence of modulated envelope plasma wave packets is provided by recent satellite missions. These excitations are characterized by a slowly varying localized envelope structure, embedding the fast carrier wave, which appears to be the result of strong modulation of the wave amplitude. This modulation may be due to parametric interactions between different modes or, simply, to the nonlinear (self-)interaction of the carrier wave. A generic exact theory is presented in this study, for the nonlinear self-modulation of known electrostatic plasma modes, by employing a collisionless fluid model. Both cold (zero-temperature) and warm fluid descriptions are discussed and the results are compared. The (moderately) nonlinear oscillation regime is investigated by applying a multiple scale technique. The calculation leads to a Nonlinear Schrödinger-type Equation (NLSE), which describes the evolution of the slowly varying wave amplitude in time and space. The NLSE admits localized envelope (solitary wave) solutions of bright-(pulses) or dark- (holes, voids) type, whose characteristics (maximum amplitude, width) depend on intrinsic plasma parameters. Effects like amplitude perturbation obliqueness (with respect to the propagation direction), finite temperature and defect (dust) concentration are explicitly considered. Relevance with similar highly localized modulated wave structures observed during recent satellite missions is discussed.
Relativistic whistler oscillitons - do they exist?
Authors: McKenzie J. F. , Dubinin E. M. , Sauer K.
Published: April 2010

Abstract. We examine the effects of relaxing the assumption of quasi-charge neutrality on the propagation of stationary whistler waves. This necessitates that in the wave frame the equations of motion must be made fully relativistic and when this is done, the correct form of the relativistic dispersive effects is recovered. Our numerical solutions indicate that fully relativistic effects can prevent the formation of oscillitons by virtue of the weakening of dispersive effects required to counterbalance those of nonlinear steepening.
Long-term predictability of mean daily temperature data
Authors: von Bloh W. , Romano M. C. , Thiel M.
Published: April 2010

Abstract. We quantify the long-term predictability of global mean daily temperature data by means of the Rényi entropy of second order K2. We are interested in the yearly amplitude fluctuations of the temperature. Hence, the data are low-pass filtered. The obtained oscillatory signal has a more or less constant frequency, depending on the geographical coordinates, but its amplitude fluctuates irregularly. Our estimate of K2 quantifies the complexity of these amplitude fluctuations. We compare the results obtained for the CRU data set (interpolated measured temperature in the years 1901-2003 with 0.5° resolution, Mitchell et al., 2005)with the ones obtained for the temperature data from a coupled ocean-atmosphere global circulation model (AOGCM, calculated at DKRZ). Furthermore, we compare the results obtained by means of K2 with the linear variance of the temperature data.
Ensemble Kalman filter assimilation of temperature and altimeter data with bias correction and application to seasonal prediction
Authors: Keppenne C. L. , Rienecker M. M. , Kurkowski N. P. , Adamec D. A.
Published: April 2010

Abstract. To compensate for a poorly known geoid, satellite altimeter data is usually analyzed in terms of anomalies from the time mean record. When such anomalies are assimilated into an ocean model, the bias between the climatologies of the model and data is problematic. An ensemble Kalman filter (EnKF) is modified to account for the presence of a forecast-model bias and applied to the assimilation of TOPEX/Poseidon (T/P) altimeter data. The online bias correction (OBC) algorithm uses the same ensemble of model state vectors to estimate biased-error and unbiased-error covariance matrices. Covariance localization is used but the bias covariances have different localization scales from the unbiased-error covariances, thereby accounting for the fact that the bias in a global ocean model could have much larger spatial scales than the random error.The method is applied to a 27-layer version of the Poseidon global ocean general circulation model with about 30-million state variables. Experiments in which T/P altimeter anomalies are assimilated show that the OBC reduces the RMS observation minus forecast difference for sea-surface height (SSH) over a similar EnKF run in which OBC is not used. Independent in situ temperature observations show that the temperature field is also improved. When the T/P data and in situ temperature data are assimilated in the same run and the configuration of the ensemble at the end of the run is used to initialize the ocean component of the GMAO coupled forecast model, seasonal SSH hindcasts made with the coupled model are generally better than those initialized with optimal interpolation of temperature observations without altimeter data. The analysis of the corresponding sea-surface temperature hindcasts is not as conclusive.
Nonlinear data-assimilation using implicit models
Authors: Terwisscha van Scheltinga A. D. , Dijkstra H. A.
Published: April 2010

Abstract. We show how the traditional 4D-Var method can be adapted for implicit time-integration and extended for multi-parameter estimation. We present the algorithm for this new method, which we call I4D-Var, and demonstrate its performance using a fully-implicit barotropic quasi-geostrophic model of the wind-driven double-gyre ocean circulation. For the latter model, the different regimes of flow behavior and the regime boundaries (i.e. bifurcation points) are well known and hence the parameter estimation problem can be systematically studied. It turns out that I4D-Var is able to correctly estimate parameter values, even when background flow and "observations" are in different dynamical regimes.
Aggregation and sampling in deterministic chaos: implications for chaos identification in hydrological processes
Authors: Salas J. D. , Kim H. S. , Eykholt R. , Burlando P. , Green T. R.
Published: April 2010

Abstract. A review of the literature reveals conflicting results regarding the existence and inherent nature of chaos in hydrological processes such as precipitation and streamflow, i.e. whether they are low dimensional chaotic or stochastic. This issue is examined further in this paper, particularly the effect that certain types of transformations, such as aggregation and sampling, may have on the identification of the dynamics of the underlying system. First, we investigate the dynamics of daily streamflows for two rivers in Florida, one with strong surface and groundwater storage contributions and the other with a lesser basin storage contribution. Based on estimates of the delay time, the delay time window, and the correlation integral, our results suggest that the river with the stronger basin storage contribution departs significantly from the behavior of a chaotic system, while the departure is less significant for the river with the smaller basin storage contribution. We pose the hypothesis that the chaotic behavior depicted on continuous precipitation fields or small time-step precipitation series becomes less identifiable as the aggregation (or sampling) time step increases. Similarly, because streamflows result from a complex transformation of precipitation that involves accumulating and routing excess rainfall throughout the basin and adding surface and groundwater flows, the end result may be that streamflows at the outlet of the basin depart from low dimensional chaotic behavior. We also investigate the effect of aggregation and sampling using series derived from the Lorenz equations and show that, as the aggregation and sampling scales increase, the chaotic behavior deteriorates and eventually ceases to show evidence of low dimensional determinism.
On the nonexistence of large amplitude stationary solitary waves in symmetric unmagnetized pair plasmas
Authors: Verheest F.
Published: April 2010

Abstract. Waves in pair plasmas have a fundamentally different dispersion due to the equal charge-to-mass ratios between negative and positive charges. In view of possible applications e.g. to electron-positron and fullerene pair plasmas, it is shown that there are no stationary large amplitude nonlinear structures in symmetric unmagnetized pair plasmas.
Bifurcation analysis of a paradigmatic model of monsoon prediction
Authors: Kumar Mittal A. , Dwivedi S. , Chandra Pandey A.
Published: April 2010

Abstract. Local and global bifurcation structure of the forced Lorenz model in the r-F plane is investigated. The forced Lorenz model is a conceptual model for understanding the influence of the slowly varying boundary forcing like Sea Surface Temperature (SST) on the Indian summer monsoon rainfall variability. Shift in the probability density function between the two branches of the Lorenz attractor as a function of SST forcing is calculated. It is found that the one-dimensional return map (cusp map) splits into two cusps on introduction of forcing.
Hysteresis-controlled instability waves in a scale-free driven current sheet model
Authors: Uritsky V. M. , Klimas A. J.
Published: April 2010

Abstract. Magnetospheric dynamics is a complex multiscale process whose statistical features can be successfully reproduced using high-dimensional numerical transport models exhibiting the phenomenon of self-organized criticality (SOC). Along this line of research, a 2-dimensional driven current sheet (DCS) model has recently been developed that incorporates an idealized current-driven instability with a resistive MHD plasma system (Klimas et al., 2004a, b). The dynamics of the DCS model is dominated by the scale-free diffusive energy transport characterized by a set of broadband power-law distribution functions similar to those governing the evolution of multiscale precipitation regions of energetic particles in the nighttime sector of aurora (Uritsky et al., 2002b). The scale-free DCS behavior is supported by localized current-driven instabilities that can communicate in an avalanche fashion over arbitrarily long distances thus producing current sheet waves (CSW). In this paper, we derive the analytical expression for CSW speed as a function of plasma parameters controlling local anomalous resistivity dynamics. The obtained relation indicates that the CSW propagation requires sufficiently high initial current densities, and predicts a deceleration of CSWs moving from inner plasma sheet regions toward its northern and southern boundaries. We also show that the shape of time-averaged current density profile in the DCS model is in agreement with steady-state spatial configuration of critical avalanching models as described by the singular diffusion theory of the SOC. Over shorter time scales, SOC dynamics is associated with rather complex spatial patterns and, in particular, can produce bifurcated current sheets often seen in multi-satellite observations.
On the separation of timescales in spring-block earthquake models
Authors: Hergarten S. , Jansen F.
Published: April 2010

Abstract. One of the most widespread spring-block earthquake models, the Olami-Feder-Christensen model, is investigated without making the assumption that the duration of individual earthquakes is negligible. While the Gutenberg-Richter law for the size distribution of earthquakes is preserved qualitatively for earthquakes of finite duration, the b-value decreases with increasing earthquake duration. The effect decreases with increasing lattice size, although it is not clear whether it completely vanishes in the limit of infinite grid size. For realistic values of earthquake duration, the effect is negligible, so that the original model with zero earthquake duration is appropriate for most applications.
On the predictability of ice avalanches
Authors: Pralong A. , Birrer C. , Stahel W. A. , Funk M.
Published: April 2010

Abstract. The velocity of unstable large ice masses from hanging glaciers increases as a power-law function of time prior to failure. This characteristic acceleration presents a finite-time singularity at the theoretical time of failure and can be used to forecast the time of glacier collapse. However, the non-linearity of the power-law function makes the prediction difficult. The effects of the non-linearity on the predictability of a failure are analyzed using a non-linear regression method. Predictability strongly depends on the time window when the measurements are performed. Log-periodic oscillations have been observed to be superimposed on the motion of large unstable ice masses. The value of their amplitude, frequency and phase are observed to be spatially homogeneous over the whole unstable ice mass. Inclusion of a respective term in the function describing the acceleration of unstable ice masses greatly increases the accuracy of the prediction.
Numerical modelling of disintegration of basin-scale internal waves in a tank filled with stratified water
Authors: Stashchuk N. , Vlasenko V. , Hutter K.
Published: April 2010

Abstract. We present the results of numerical experiments performed with the use of a fully non-linear non-hydrostatic numerical model to study the baroclinic response of a long narrow tank filled with stratified water to an initially tilted interface. Upon release, the system starts to oscillate with an eigen frequency corresponding to basin-scale baroclinic gravitational seiches. Field observations suggest that the disintegration of basin-scale internal waves into packets of solitary waves, shear instabilities, billows and spots of mixed water are important mechanisms for the transfer of energy within stratified lakes. Laboratory experiments performed by D. A. Horn, J. Imberger and G. N. Ivey (JFM, 2001) reproduced several regimes, which include damped linear waves and solitary waves. The generation of billows and shear instabilities induced by the basin-scale wave was, however, not sufficiently studied. The developed numerical model computes a variety of flows, which were not observed with the experimental set-up. In particular, the model results showed that under conditions of low dissipation, the regimes of billows and supercritical flows may transform into a solitary wave regime. The obtained results can help in the interpretation of numerous observations of mixing processes in real lakes.
Earthquake forecasting and its verification
Authors: Holliday J. R. , Nanjo K. Z. , Tiampo K. F. , Rundle J. B. , Turcotte D. L.
Published: April 2010

Abstract. No proven method is currently available for the reliable short time prediction of earthquakes (minutes to months). However, it is possible to make probabilistic hazard assessments for earthquake risk. In this paper we discuss a new approach to earthquake forecasting based on a pattern informatics (PI) method which quantifies temporal variations in seismicity. The output, which is based on an association of small earthquakes with future large earthquakes, is a map of areas in a seismogenic region ("hotspots'') where earthquakes are forecast to occur in a future 10-year time span. This approach has been successfully applied to California, to Japan, and on a worldwide basis. Because a sharp decision threshold is used, these forecasts are binary--an earthquake is forecast either to occur or to not occur. The standard approach to the evaluation of a binary forecast is the use of the relative (or receiver) operating characteristic (ROC) diagram, which is a more restrictive test and less subject to bias than maximum likelihood tests. To test our PI method, we made two types of retrospective forecasts for California. The first is the PI method and the second is a relative intensity (RI) forecast based on the hypothesis that future large earthquakes will occur where most smaller earthquakes have occurred in the recent past. While both retrospective forecasts are for the ten year period 1 January 2000 to 31 December 2009, we performed an interim analysis 5 years into the forecast. The PI method out performs the RI method under most circumstances.
Modelling the outer radiation belt as a complex system in a self-organised critical state
Authors: Crosby N. B. , Meredith N. P. , Coates A. J. , Iles R. H. A.
Published: April 2010

Abstract. The dynamic behaviour of the outer electron radiation belt makes this area of geo-space a candidate for the concept of self-organized criticality. It is shown here that frequency distributions of measured outer electron radiation belt data are well-represented by power-laws over two decades. Applying the concept of self-organized criticality to interpret the shape of the distributions suggests another approach to complement existing methods in the interpretation of how this complicated environment works. Furthermore sub-grouping the radiation belt count rate data as a function of spatial location or temporal interval (e.g. L-shell, magnetic local time, solar cycle, ...) shows systematic trends in the value of the slope of the power-laws. It is shown that the inner part of the outer radiation belt is influenced in a similar manner to the outer part, but in a less profound way. Our results suggest that the entire outer radiation belt appears to be affected as the sum of its individual parts. This type of study also gives the probability of exceeding a given threshold value over a given time; limiting the size of "an event". The average values could then be compared with models used in spacecraft design.
The influence of convective current generator on the global current
Authors: Morozov V. N.
Published: April 2010

Abstract. The mathematical generalization of classical model of the global circuit with taking into account the convective current generator, working in the planetary boundary layer was considered. Convective current generator may be interpreted as generator, in which the electromotive force is generated by processes, of the turbulent transport of electrical charge. It is shown that the average potential of ionosphere is defined not only by the thunderstorm current generators, working at the present moment, but by the convective current generator also. The influence of the convective processes in the boundary layer on the electrical parameters of the atmosphere is not only local, but has global character as well. The numerical estimations, made for the case of the convective-unstable boundary layer demonstrate that the increase of the average potential of ionosphere may be of the order of 10% to 40%.
A theoretical model of a wake of a body towed in a stratified fluid at large Reynolds and Froude numbers
Authors: Troitskaya Y. I. , Druzhinin O. A. , Sergeev D. A. , Papko V. V. , Balandina G. N.
Published: April 2010

Abstract. The objective of the present paper is to develop a theoretical model describing the evolution of a turbulent wake behind a towed sphere in a stably stratified fluid at large Froude and Reynolds numbers. The wake flow is considered as a quasi two-dimensional (2-D) turbulent jet flow whose dynamics is governed by the momentum transfer from the mean flow to a quasi-2-D sinuous mode growing due to hydrodynamic instability. The model employs a quasi-linear approximation to describe this momentum transfer. The model scaling coefficients are defined with the use of available experimental data, and the performance of the model is verified by comparison with the results of a direct numerical simulation of a 2-D turbulent jet flow. The model prediction for the temporal development of the wake axis mean velocity is found to be in good agreement with the experimental data obtained by Spedding (1997).
An attempt to find precursors in the ULF geomagnetic data by means of flicker noise spectroscopy
Authors: Hayakawa M. , Timashev S. F.
Published: April 2010

Abstract. The flicker noise spectroscopy which is a new phenomenological method for the retrieval of information contained in chaotic time signals, is based on the analysis of recognizable irregularities (pulse, jumps, and discontinuities of derivatives of various order). This method is applied to the ULF (ultra-low-frequency) data observed at Guam in 1992–1994, in order to study the temporal nonlinear behavior of the lithospheric activity prior to the large 1993 Guam earthquake (8 August 1993). We have found that the lithosphere must have exhibited the step-like discontinuous behaviors in the lithosphere 101, 78, 54, 31 and 8 days before the main shock. This kind of nonlinear temporal behavior can be tracked by means of our flicker noise spectroscopy.
A conceptual ENSO model under realistic noise forcing
Authors: Saynisch J. , Kurths J. , Maraun D.
Published: April 2010

Abstract. We investigated the influence of atmospheric noise on the generation of interannual El Niño variability. Therefore, we perturbed a conceptual ENSO delay model with surrogate windstress data generated from tropical windspeed measurements. The effect of the additional stochastic forcing was studied for various parameter sets including periodic and chaotic regimes. The evaluation was based on a spectrum and amplitude-period relation comparison between model and measured sea surface temperature data. The additional forcing turned out to increase the variability of the model output in general. The noise-free model was unable to reproduce the observed spectral bandwidth for any choice of parameters. On the contrary, the stochastically forced model is capable of producing a realistic spectrum. The weakly nonlinear regimes of the model exhibit a proportional relation between amplitude and period matching the relation derived from measurement data. The chaotic regime, however, shows an inversely proportional relation. A stability analysis of the different regimes revealed that the spectra of the weakly nonlinear regimes are robust against slight parameter changes representing disregarded physical mechanisms, whereas the chaotic regime exhibits a very unstable realistic spectrum. We conclude that the model including stochastic forcing in a parameter range of moderate nonlinearity best matches the real conditions. This suggests that atmospheric noise plays an important role in the coupled tropical pacific ocean-atmosphere system.
Statistical analysis of Stromboli VLP tremor in the band [0.1–0.5] Hz: some consequences for vibrating structures
Authors: De Lauro E. , De Martino S. , Falanga M. , Palo M.
Published: April 2010

Abstract. We analyze time series of Strombolian volcanic tremor, focusing our attention on the frequency band [0.1–0.5] Hz (very long period (VLP) tremor). Although this frequency band is largely affected by noise, we evidence two significant components by using Independent Component Analysis with the frequencies, respectively, of ~0.2 and ~0.4 Hz. We show that these components display wavefield features similar to those of the high frequency Strombolian signals (>0.5 Hz). In fact, they are radially polarised and located within the crater area. This characterization is lost when an enhancement of energy appears. In this case, the presence of microseismic noise becomes relevant. Investigating the entire large data set available, we determine how microseismic noise influences the signals. We ascribe the microseismic noise source to Scirocco wind. Moreover, our analysis allows one to evidence that the Strombolian conduit vibrates like the asymmetric cavity associated with musical instruments generating self-sustained tones.
Evolution of localized vortices in the presence of stochastic perturbations
Authors: Bécu E. , Pavlov V.
Published: April 2010

Abstract. We consider the evolution of a distribution of N identical point vortices when stochastic perturbations in the Hamiltonian are present. It is shown that different initial configurations of vorticity with identical integral invariants may exist. Using the Runge-Kutta scheme of order 4, it is also demonstrated that different initial configurations with the same invariants may evolve without having any tendency to approach to a unique final, axially symmetric, distribution. In the presence of stochastic perturbations, if the initial distribution of vortices is not axially symmetric, vortices can be trapped in certain domains whose location is correlated with the configuration of the initial vortex distribution.
Multifractal earth topography
Authors: Gagnon J.-S. , Lovejoy S. , Schertzer D.
Published: April 2010

Abstract. This paper shows how modern ideas of scaling can be used to model topography with various morphologies and also to accurately characterize topography over wide ranges of scales. Our argument is divided in two parts. We first survey the main topographic models and show that they are based on convolutions of basic structures (singularities) with noises. Focusing on models with large numbers of degrees of freedom (fractional Brownian motion (fBm), fractional Levy motion (fLm), multifractal fractionally integrated flux (FIF) model), we show that they are distinguished by the type of underlying noise. In addition, realistic models require anisotropic singularities; we show how to generalize the basic isotropic (self-similar) models to anisotropic ones. Using numerical simulations, we display the subtle interplay between statistics, singularity structure and resulting topographic morphology. We show how the existence of anisotropic singularities with highly variable statistics can lead to unwarranted conclusions about scale breaking. We then analyze topographic transects from four Digital Elevation Models (DEMs) which collectively span scales from planetary down to 50 cm (4 orders of magnitude larger than in previous studies) and contain more than 2×108 pixels (a hundred times more data than in previous studies). We use power spectra and multiscaling analysis tools to study the global properties of topography. We show that the isotropic scaling for moments of order ≤2 holds to within ±45% down to scales ≈40 m. We also show that the multifractal FIF is easily compatible with the data, while the monofractal fBm and fLm are not. We estimate the universal parameters (α, C1) characterizing the underlying FIF noise to be (1.79, 0.12), where α is the degree of multifractality (0≤α≤2, 0 means monofractal) and C1 is the degree of sparseness of the surface (0≤C1, 0 means space filling). In the same way, we investigate the variation of multifractal parameters between continents, oceans and continental margins. Our analyses show that no significant variation is found for (α, C1) and that the third parameter H, which is a degree of smoothing (higher H means smoother), is variable: our estimates are H=0.46, 0.66, 0.77 for bathymetry, continents and continental margins. An application we developped here is to use (α, C1) values to correct standard spectra of DEMs for multifractal resolution effects.