bluuebunny/crossref_metadata_2025 huggingface dataset:
An update on THORPEX-related research in data assimilation and observing strategies
Authors: Rabier F. , Gauthier P. , Cardinali C. , Langland R. , Tsyrulnikov M. , Lorenc A. , Steinle P. , Gelaro R. , Koizumi K.
Published: April 2010

Abstract. The international programme "THORPEX: a World Weather Research Programme" provides a framework in which to tackle the challenge of improving the forecast skill of high-impact weather through international collaboration between academic institutions, operational forecast centres, and users of forecast products. The objectives of the THORPEX Data Assimilation and Observation Strategy Working Group (DAOS-WG) are two-fold. The primary goal is to assess the impact of observations and various targeting methods to provide guidance for observation campaigns and for the configuration of the Global Observing System. The secondary goal is to setup an optimal framework for data assimilation, including aspects such as targeted observations, satellite data, background error covariances and quality control. The Atlantic THORPEX Regional campaign, ATReC, in 2003, has been very successful technically and has provided valuable datasets to test targeting issues. Various data impact experiments have been performed, showing a small but very slightly positive impact of targeted observations. Projects of the DAOS-WG include working on the AMMA field experiment, in the context of IPY and to prepare the future THORPEX-PARC field campaign in the Pacific by comparing sensitivity of the forecasts to observations between several groups.
Continuous dynamic assimilation of the inner region data in hydrodynamics modelling: optimization approach
Authors: Pisnitchenko F. I. , Pisnichenko I. A. , Martínez J. M. , Santos S. A.
Published: April 2010

Abstract. In meteorological and oceanological studies the classical approach for finding the numerical solution of the regional model consists in formulating and solving a Cauchy-Dirichlet problem. The boundary conditions are obtained by linear interpolation of coarse-grid data provided by a global model. Errors in boundary conditions due to interpolation may cause large deviations from the correct regional solution. The methods developed to reduce these errors deal with continuous dynamic assimilation of known global data available inside the regional domain. One of the approaches of this assimilation procedure performs a nudging of large-scale components of regional model solution to large-scale global data components by introducing relaxation forcing terms into the regional model equations. As a result, the obtained solution is not a valid numerical solution to the original regional model. Another approach is the use a four-dimensional variational data assimilation procedure which is free from the above-mentioned shortcoming. In this work we formulate the joint problem of finding the regional model solution and data assimilation as a PDE-constrained optimization problem. Three simple model examples (ODE Burgers equation, Rossby-Oboukhov equation, Korteweg-de Vries equation) are considered in this paper. Numerical experiments indicate that the optimization approach can significantly improve the precision of the regional solution.
Multiscaling of porous soils as heterogeneous complex networks
Authors: Santiago A. , Cárdenas J. P. , Losada J. C. , Benito R. M. , Tarquis A. M. , Borondo F.
Published: April 2010

Abstract. In this paper we present a complex network model based on a heterogeneous preferential attachment scheme to quantify the structure of porous soils. Under this perspective pores are represented by nodes and the space for the flow of fluids between them is represented by links. Pore properties such as position and size are described by fixed states in a metric space, while an affinity function is introduced to bias the attachment probabilities of links according to these properties. We perform an analytical study of the degree distributions in the soil model and show that under reasonable conditions all the model variants yield a multiscaling behavior in the connectivity degrees, leaving a empirically testable signature of heterogeneity in the topology of pore networks. We also show that the power-law scaling in the degree distribution is a robust trait of the soil model and analyze the influence of the parameters on the scaling exponents. We perform a numerical analysis of the soil model for a combination of parameters corresponding to empirical samples with different properties, and show that the simulation results exhibit a good agreement with the analytical predictions.
Structure function analysis and intermittency in the atmospheric boundary layer
Authors: Vindel J. M. , Yagüe C. , Redondo J. M.
Published: April 2010

Abstract. Data from the SABLES98 experimental campaign (Cuxart et al., 2000) have been used in order to study the relationship of the probability distribution of velocity increments (PDFs) to the scale and the degree of stability. This connection is demonstrated by means of the velocity structure functions and the PDFs of the velocity increments. Using the hypothesis of local similarity, so that the third order structure function scaling exponent is one, the inertial range in the Kolmogorov sense has been identified for different conditions, obtaining the velocity structure function scaling exponents for several orders. The degree of intermittency in the energy cascade is measured through these exponents and compared with the forcing intermittency revealed through the evolution of flatness with scale. The role of non-homogeneity in the turbulence structure is further analysed using Extended Self Similarity (ESS). A criterion to identify the inertial range and to show the scale independence of the relative exponents is described. Finally, using least-squares fits, the values of some parameters have been obtained which are able to characterize intermittency according to different models.
Patch behaviour and predictability properties of modelled finite-amplitude sand ridges on the inner shelf
Authors: Vis-Star N. C. , de Swart H. E. , Calvete D.
Published: April 2010

Abstract. The long-term evolution of shoreface-connected sand ridges is investigated with a nonlinear spectral model which governs the dynamics of waves, currents, sediment transport and the bed level on the inner shelf. Wave variables are calculated with a shoaling-refraction model instead of using a parameterisation. The spectral model describes the time evolution of amplitudes of known eigenmodes of the linearised system. Bottom pattern formation occurs if the transverse bottom slope of the inner shelf, β, exceeds a critical value βc. For fixed model parameters the sensitivity of the properties of modelled sand ridges to changes in the number (N−1) of resolved subharmonics (of the initially fastest growing mode) is investigated. For any N the model shows the growth and subsequent saturation of the height of the sand ridges. The saturation time scale is several thousands of years, which suggests that observed sand ridges have not reached their saturated stage yet. The migration speed of the ridges and the average longshore spacing between successive crests in the saturated state differ from those in the initial state. Analysis of the potential energy balance of the ridges reveals that bed slope-induced sediment transport is crucial for the saturation process. In the transient stage the shoreface-connected ridges occur in patches. The overall characteristics of the bedforms (saturation time, final maximum height, average longshore spacing, migration speed) hardly vary with N. However, individual time series of modal amplitudes and bottom patterns strongly depend on N, thereby implying that the detailed evolution of sand ridges can only be predicted over a limited time interval. Additional experiments show that the critical bed slope βc increases with larger offshore angles of wave incidence, larger offshore wave heights and longer wave periods, and that the corresponding maximum height of the ridges decreases whilst the saturation time increases.
Modeling nonlinear compressional waves in marine sediments
Authors: McDonald B. E.
Published: April 2010

Abstract. A computational model is presented which will help guide and interpret an upcoming series of experiments on nonlinear compressional waves in marine sediments. The model includes propagation physics of nonlinear acoustics augmented with granular Hertzian stress of order 3/2 in the strain rate. The model is a variant of the time domain NPE (McDonald and Kuperman, 1987) supplemented with a causal algorithm for frequency-linear attenuation. When attenuation is absent, the model equations are used to construct analytic solutions for nonlinear plane waves. The results imply that Hertzian stress causes a unique nonlinear behavior near zero stress. A fluid, in contrast, exhibits nonlinear behavior under high stress. A numerical experiment with nominal values for attenuation coefficient implies that in a water saturated Hertzian chain, the nonlinearity near zero stress may be experimentally observable.
Characterizing water fingering phenomena in soils using magnetic resonance imaging and multifractal theory
Authors: Posadas A. , Quiroz R. , Tannús A. , Crestana S. , Vaz C. M.
Published: April 2010

Abstract. The study of water movement in soils is of fundamental importance in hydrologic science. It is generally accepted that in most soils, water and solutes flow through unsaturated zones via preferential paths or fingers. This paper combines magnetic resonance imaging (MRI) with both fractal and multifractal theory to characterize preferential flow in three dimensions. A cubic double-layer column filled with fine and coarse textured sand was placed into a 500 gauss MRI system. Water infiltration through the column (0.15×0.15×0.15 m3) was recorded in steady state conditions. Twelve sections with a voxel volume of 0.1×0.1×10 mm3 each were obtained and characterized using fractal and multifractal theory. The MRI system provided a detailed description of the preferential flow under steady state conditions and was also useful in understanding the dynamics of the formation of the fingers. The f(α) multifractal spectrum was very sensitive to the variation encountered at each horizontally-oriented slice of the column and provided a suitable characterization of the dynamics of the process identifying four spatial domains. In conclusion, MRI and fractal and multifractal analysis were able to characterize and describe the preferential flow process in soils. Used together, the two methods provide a good alternative to study flow transport phenomena in soils and in porous media.
Conjugate flows and amplitude bounds for internal solitary waves
Authors: Makarenko N. I. , Maltseva J. L. , Kazakov A. Yu.
Published: April 2010

Abstract. Amplitude bounds imposed by the conservation of mass, momentum and energy for strongly nonlinear waves in stratified fluid are considered. We discuss the theoretical scheme which allows to determine broadening limits for solitary waves in the terms of a given upstream density profile. Attention is focused on the continuously stratified flows having multiple broadening limits. The role of the mean density profile and the influence of fine-scale stratification are analyzed.
Spectral properties of electromagnetic turbulence in plasmas
Authors: Shaikh D. , Shukla P. K.
Published: April 2010

Abstract. We report on the nonlinear turbulent processes associated with electromagnetic waves in plasmas. We focus on low-frequency (in comparison with the electron gyrofrequency) nonlinearly interacting electron whistlers and nonlinearly interacting Hall-magnetohydrodynamic (H-MHD) fluctuations in a magnetized plasma. Nonlinear whistler mode turbulence study in a magnetized plasma involves incompressible electrons and immobile ions. Two-dimensional turbulent interactions and subsequent energy cascades are critically influenced by the electron whisters that behave distinctly for scales smaller and larger than the electron skin depth. It is found that in whistler mode turbulence there results a dual cascade primarily due to the forward spectral migration of energy that coexists with a backward spectral transfer of mean squared magnetic potential. Finally, inclusion of the ion dynamics, resulting from a two fluid description of the H-MHD plasma, leads to several interesting results that are typically observed in the solar wind plasma. Particularly in the solar wind, the high-time-resolution databases identify a spectral break at the end of the MHD inertial range spectrum that corresponds to a high-frequency regime. In the latter, turbulent cascades cannot be explained by the usual MHD model and a finite frequency effect (in comparison with the ion gyrofrequency) arising from the ion inertia is essentially included to discern the dynamics of the smaller length scales (in comparison with the ion skin depth). This leads to a nonlinear H-MHD model, which is presented in this paper. With the help of our 3-D H-MHD code, we find that the characteristic turbulent interactions in the high-frequency regime evolve typically on kinetic-Alfvén time-scales. The turbulent fluctuation associated with kinetic-Alfvén interactions are compressive and anisotropic and possess equipartition of the kinetic and magnetic energies.
Coupling between reconnection and Kelvin-Helmholtz instabilities in collisionless plasmas
Authors: Grasso D. , Borgogno D. , Pegoraro F. , Tassi E.
Published: April 2010

Abstract. In a collisionless plasma, when reconnection instability takes place, strong shear flows may develop. Under appropriate conditions these shear flows become unstable to the Kelvin-Helmholtz instability. Here, we investigate the coupling between these instabilities in the framework of a four-field model. Firstly, we recover the known results in the low β limit, β being the ratio between the plasma and the magnetic pressure. We concentrate our attention on the dynamical evolution of the current density and vorticity sheets which evolve coupled together according to a laminar or a turbulent regime. A three-dimensional extension in this limit is also discussed. Secondly, we consider finite values of the β parameter, allowing for compression of the magnetic and velocity fields along the ignorable direction. We find that the current density and vorticity sheets now evolve separately. The Kelvin-Helmholtz instability involves only the vorticity field, which ends up in a turbulent regime, while the current density maintains a laminar structure.
Finite Larmor radius influence on MHD solitary waves
Authors: Mjølhus E.
Published: April 2010

Abstract. MHD solitons are studied in a model where the usual Hall-MHD model is extended to include the finite Larmor radius (FLR) corrections to the pressure tensor. The resulting 4-dimensional set of differential equations is treated numerically. In this extended model, the point at infinity can be of several types. Necessary for the existence of localized solutions is that it is either a saddle-saddle, a saddle-center, or, possibly, a focus-focus. In cases of saddle-center, numerical solutions for localized travelling structures have been obtained, and compared with corresponding results from the Hall-MHD model.
Alfvén wave filamentation and dispersive phase mixing in a high-density channel: Landau fluid and hybrid simulations
Authors: Borgogno D. , Hellinger P. , Passot T. , Sulem P. L. , Trávníček P. M.
Published: April 2010

Abstract. The propagation of dispersive Alfvén waves in a low-beta collisionless plasma with a high-density channel aligned with the ambient magnetic field, is studied in three space dimensions. A fluid model retaining linear Landau damping and finite Larmor radius corrections is used, together with a hybrid particle-in-cell simulation aimed to validate the predictions of this Landau-fluid model. It is shown that when the density enhancement is moderate (depending on the pump wavelength and the plasma parameters), the wave energy concentrates into a filament whose transverse size is prescribed by the dimension of the channel. In contrast, in the case of a stronger density perturbation, the early formation of a magnetic filament is followed by the onset of thin helical ribbons and the development of strong gradients. This "dispersive phase mixing" provides a mechanism permitting dissipation processes (not included in the present model) to act and heat the plasma.
How much does inclusion of non-linearity and multi-point pattern recognition improve the spatial mapping of complex patterns of groundwater contamination?
Authors: Chowdhury M. , Alouani A. , Hossain F.
Published: April 2010

Abstract. In this brief communication, we discuss the implication of the hypothesis that "non-linearity and multi-point pattern recognition can improve the spatial mapping of complex patterns of groundwater contamination". The discussion is based on our recently published work in Stochastic Environmental Research and Risk Assessment. Therein we have found that the use of a highly non-linear pattern learning technique in the form of an artificial neural network (ANN) can yield significantly superior results under the same set of constraints when compared to the more linear two-point ordinary kriging method.
Characterization of peak flow events with local singularity method
Authors: Cheng Q. , Li L. , Wang L.
Published: April 2010

Abstract. Three methods, return period, power-law frequency plot (concentration-area) and local singularity index, are introduced in the paper for characterizing peak flow events from river flow data for the past 100 years from 1900 to 2000 recorded at 25 selected gauging stations on rivers in the Oak Ridges Moraine (ORM) area, Canada. First a traditional method, return period, was applied to the maximum annual river flow data. Whereas the Pearson III distribution generally fits the values, a power-law frequency plot (C-A) on the basis of self-similarity principle provides an effective mean for distinguishing "extremely" large flow events from the regular flow events. While the latter show a power-law distribution, about 10 large flow events manifest departure from the power-law distribution and these flow events can be classified into a separate group most of which are related to flood events. It is shown that the relation between the average water releases over a time period after flow peak and the time duration may follow a power-law distribution. The exponent of the power-law or singularity index estimated from this power-law relation may be used to characterize non-linearity of peak flow recessions. Viewing large peak flow events or floods as singular processes can anticipate the application of power-law models not only for characterizing the frequency distribution of peak flow events, for example, power-law relation between the number and size of floods, but also for describing local singularity of processes such as power-law relation between the amount of water released versus releasing time. With the introduction and validation of singularity of peak flow events, alternative power-law models can be used to depict the recession property as well as other types of non-linear properties.
Synchronization in a coupled two-layer quasigeostrophic model of baroclinic instability – Part 1: Master-slave configuration
Authors: Castrejón-Pita A. A. , Read P. L.
Published: April 2010

Abstract. Synchronization is studied using a pair of diffusively-coupled, two-layer quasi-geostrophic systems each comprising a single baroclinic wave and a zonal flow. In particular, the coupling between the systems is in the well-known master-slave or one-way configuration. Nonlinear time series analysis, phase dynamics, and bifurcation diagrams are used to study the dynamics of the coupled system. Phase synchronization, imperfect synchronization (phase slips), or complete synchronization are found, depending upon the strength of coupling, when the systems are either in a periodic or a chaotic regime. The results of investigations when the dynamics of each system are in different regimes are also presented. These results also show evidence of phase synchronization and signs of chaos control.
Improved moment scaling estimation for multifractal signals
Authors: Veneziano D. , Furcolo P.
Published: April 2010

Abstract. A fundamental problem in the analysis of multifractal processes is to estimate the scaling exponent K(q) of moments of different order q from data. Conventional estimators use the empirical moments μ^rq=⟨ | εr(τ)|q⟩ of wavelet coefficients εr(τ), where τ is location and r is resolution. For stationary measures one usually considers "wavelets of order 0" (averages), whereas for functions with multifractal increments one must use wavelets of order at least 1. One obtains K^(q) as the slope of log( μ^rq) against log(r) over a range of r. Negative moments are sensitive to measurement noise and quantization. For them, one typically uses only the local maxima of | εr(τ)| (modulus maxima methods). For the positive moments, we modify the standard estimator K^(q) to significantly reduce its variance at the expense of a modest increase in the bias. This is done by separately estimating K(q) from sub-records and averaging the results. For the negative moments, we show that the standard modulus maxima estimator is biased and, in the case of additive noise or quantization, is not applicable with wavelets of order 1 or higher. For these cases we propose alternative estimators. We also consider the fitting of parametric models of K(q) and show how, by splitting the record into sub-records as indicated above, the accuracy of standard methods can be significantly improved.
Seasonal variability of the subpolar gyres in the Southern Ocean: a numerical investigation based on transfer operators
Authors: Dellnitz M. , Froyland G. , Horenkamp C. , Padberg-Gehle K. , Sen Gupta A.
Published: April 2010

Abstract. The detection of regions in the ocean that are coherent over an extended period of time is a fundamental problem in many oceanic applications. For instance such regions are important for studying the transport of marine species and for the distribution of nutrients. In this study we demonstrate the efficacy of transfer operators in detecting and analysing such structures. We focus first on the detection of the Weddell and Ross Gyre for the four seasons spanning December 2003–November 2004 within the 3-D oceanic domain south of 30° S, and show distinct seasonal differences in both the three-dimensional structure and the persistence of the gyres. Further, we demonstrate a new technique based on the discretised transfer operators to calculate the mean residence time of water within parts of the gyres and determine pathways of water leaving and entering the gyres.
Earthquake simulations with time-dependent nucleation and long-range interactions
Authors: Dieterich J. H.
Published: April 2010

Abstract. A model for rapid simulation of earthquake sequences is introduced which incorporates long-range elastic interactions among fault elements and time-dependent earthquake nucleation inferred from experimentally derived rate- and state-dependent fault constitutive properties. The model consists of a planar two-dimensional fault surface which is periodic in both the x- and y-directions. Elastic interactions among fault elements are represented by an array of elastic dislocations. Approximate solutions for earthquake nucleation and dynamics of earthquake slip are introduced which permit computations to proceed in steps that are determined by the transitions from one sliding state to the next. The transition-driven time stepping and avoidance of systems of simultaneous equations permit rapid simulation of large sequences of earthquake events on computers of modest capacity, while preserving characteristics of the nucleation and rupture propagation processes evident in more detailed models. Earthquakes simulated with this model reproduce many of the observed spatial and temporal characteristics of clustering phenomena including foreshock and aftershock sequences. Clustering arises because the time dependence of the nucleation process is highly sensitive to stress perturbations caused by nearby earthquakes. Rate of earthquake activity following a prior earthquake decays according to Omori's aftershock decay law and falls off with distance.
Conjectured chaotic nature of the ionosphere-magnetosphere coupling in a reconfigurating magnetosphere
Authors: Wodnicka E. B.
Published: April 2010

Abstract. During substorms the magnetic field configuration changes in time; stretching of the magnetosphere during growth phase is followed by its collapse after the onset of the expansion phase. In this study the ionospheric origin oxygen ion dynamics in a time-dependent magnetosphere is analyzed. An induction electric field of several mV/m due to the reconfigurating magnetic field determines the details of the ion extraction from the auroral topside ionosphere. Two regimes of motion are discernible; a regime in regions far from the equatorial plane where the magnetic moment is conserved and a regime near the equatorial plane, in which the motion produces magnetic moment jumps and oscillations. The time spent by the ion in the regions is determined by the initial characteristics of the ion and by the field transition features. Lyapunov characteristic exponents are calculated to estimate the sensitivity of the system to initial conditions. Their values are higher for orbits with chaotic segments compared with the orbits in the static magnetic field and depend on the amplitude of the induced electric field. It follows from the study that the region of chaos usually localized far beyond 10 RE in the plasma sheet is expected to approach closer to the Earth ( r = 6 - 7 RE) during substorm associated reconfigurations of the magnetosphere, due to the auroral ionospheric ions.
Geophysical turbulence data and turbulence theory
Authors: van Atta C. W.
Published: April 2010

Abstract. Many vital insights into the nature of turbulence in fluids have originated from experimental data obtained in geophysical flows. Geophysical data have often helped to stimulate the creation of new turbulence theory, while theory has in many cases motivated the experimental efforts. The present brief review discusses several key examples of this interaction between experiment and theory, citing mainly work which is of particular interest to the author. No attempt is made to provide a complete listing of the extensive and currently rapidly developing literature for some of the problems discussed.
Microstructure induced nonlinearity of unconsolidated rocks as related to seismic diagnostic problems
Authors: Belyaeva I. Yu. , Ostrovsky L. A. , Zaitsev V. Yu.
Published: April 2010

Abstract. Manifestations of the so-called structure induced nonlinearity are considered for the case of a granular medium, the latter being a generally accepted model of nonconsolidated rocks in seismics. The consideration is carried out using the medium model in the form of the "ideal" random packing of spherical elastic granules in which the interparticle space can be filled with a fluid. A physical equation of such a medium is derived; the dependencies of nonlinear parameters on the grain material elastic moduli, the fluid compressibility and the initial medium strain are analyzed. The influence of defects in nonideal grain packings (that is, the presence of a fraction of unloaded intergranular contacts) upon the nonlinear properties of the medium is investigated. It is shown that the packing nonideality has the stronger effect on higher-order nonlinear properties. It is demonstrated that the nonlinear parameters may be used in exploration seismology as a much more sensitive and informative characteristic compared with conventionally used linear moduli.
Lagrangian transport and chaos in the near wake of the flow around an obstacle: a numerical implementation of lobe dynamics
Authors: Duan J. , Wiggins S.
Published: April 2010

Abstract. In this paper we study Lagrangian transport in the near wake of the flow around an obstacle, which we take to be a cylinder. In this case, for the range of Reynolds numbers investigated, the flow is two-dimensional and time periodic. We use ideas and methods from transport theory in dynamical systems to describe and quantify transport in the near wake. We numerically solve the Navier-Stokes equations for the velocity field and apply these methods to the resulting numerical representation of the velocity field. We show that the method of lobe dynamics can be used in conjunction with computational fluid dynamics methods to give very detailed and quantitative information about Lagrangian transport. In particular, we show how the stable and unstable manifolds of certain saddle-type stagnation points on the cylinder, and one in the wake, can be used to divide the flow into three distinct regions, an upper wake, a lower wake, and a wake cavity. The significance of the division using stable and unstable manifolds lies in the fact that these invariant manifolds form a template on which the transport occurs. Using this, we compute fluxes from the upper and lower wakes into the wake cavity using the associated turnstile lobes. We also compute escape time distributions as well as compare transport properties for two different Reynolds numbers.
Factors affecting multiscaling analysis of rainfall time series
Authors: Harris D. , Seed A. , Menabde M. , Austin G.
Published: April 2010

Abstract. Simulations based on random multiplicative cascade models are used to investigate the uncertainty in estimates of parameters characterizing the multiscaling nature of rainfall time series. The principal parameters used and discussed are the spectral exponent, β, and the K(q) function which characterizes the scaling of the moments. By simulating a large number of series, the sampling variability of parameter estimates in relation to the length of the time series is assessed and found to be in excess of 10%-20% for fields less than ~104 points in length. The issue of long time series which may consist of physically distinct processes with different statistics is addressed and it is shown that highly variable data mixed with an equal amount of less variable data of similar strength is dominated entirely by the statistics of the highly variable data. The effects on the estimates of β and K(q) with the addition of white noise or the tipping bucket effect (quantization) can also be significant, particularly following gradient transformations. Some high resolution rainfall data are also analyzed to illustrate how a single instrumental glitch can strongly bias results and how mixing physically different processes together can lead to incorrect conclusions.
Width dependent collisionless electron dynamics in the static fields of the shock ramp, 2, Phase space portrait
Authors: Gedalin M. , Griv U. , Balikhin M. A.
Published: April 2010

Abstract. We study numerically in detail the behaviour of electrons in the strongly inhomogeneous static magnetic and electric fields, which are typical for thin quasiperpendicular collisionless shocks. We pay particular attention to the dependence of the final electron velocities on their initial velocities, for different shock widths. Electrons are completely magnetized when the shock is wide, but become demagnetized, and the energies that they acquire rapidly increase with the steepening of the field structure. One of the clear manifestations of the electron demagnetization is the loss of even approximate one-to-one correspondence of the downstream perpendicular velocity to the upstream perpendicular velocity. Electron reflection occurs despite the large cross-shock potential which accelerates electrons along the magnetic field (the regime of complete magnetization) or across the shock (strong demagnetization). The reflected ion fraction is sensitive to the potential, magnetic field jump, and ramp width.
Closure of multi-fluid and kinetic equations for cyclotron-resonant interactions of solar wind ions with Alfvén waves
Authors: Marsch E.
Published: April 2010

Abstract. Based on quasilinear theory, a closure scheme for anisotropic multi-component fluid equations is developed for the wave-particle interactions of ions with electromagnetic Alfvén and ion-cyclotron waves propagating along the mean magnetic field. Acceleration and heating rates are calculated. They may be used in the multi-fluid momentum and energy equations as anomalous transport terms. The corresponding evolution equation for the average wave spectrum is established, and the effective growth/damping rate for the spectrum is calculated. Given a simple power-law spectrum, an anomalous collision frequency can be derived which depends on the slope and average intensity of the spectrum, and on the gyrofrequency and the differential motion (with respect to the wave frame) of the actual ion species considered. The wave-particle interaction terms attain simple forms resembling the ones for collisional friction and temperature anisotropy relaxation (due to pitch angle scattering) with collision rates that are proportional to the gyrofrequency but diminished substantially by the relative wave energy or the fluctuation level with respect the background field. In addition, a set of quasilinear diffusion equations is derived for the reduced (with respect to the perpendicular velocity component) velocity distribution functions (VDFs), as they occur in the wave dispersion equation and the related dielectric function for parallel propagation. These reduced VDFs allow one to describe adequately the most prominent observed features, such as an ion beam and temperature anisotropy, in association with the resonant interactions of the particles with the waves on a kinetic level, yet have the advantage of being only dependent upon the parallel velocity component.
Observation of equatorial Kelvin solitary waves in a slowly varying thermocline
Authors: Zheng Q. , Susanto R. D. , Yan X.-H. , Liu W. T. , Ho C.-R.
Published: April 2010

Abstract. TOPEX/POSEIDON (T/P) sea level deviation (SLD) time series from 3 October 1992 to 15 May 1997 combined with upper ocean thermal structures are used to observe the characteristics and analyze the dynamics of equatorial waves in the Pacific Ocean. The evolution of the Kelvin wave propagating along an eastward shoaling thermocline in the equatorial Pacific is investigated. The behaviour of this wave as it propagates eastward can be approximately described with the solutions of the perturbed Korteweg-de Vries (PKDV) equation and modified Green's Law. Assuming that the nonlinear term and dispersive term of this equation are balanced, the amplitude increases as the thermocline decreases to the power -3/8. Approaching the eastern Pacific, the nonlinearity increases and the relation changes to the power -9/8. The dispersion relation, and mass and energy conservations are investigated. The results indicate that under a varying thermocline, the nonlinear Kelvin solitary waves indeed exist in the real ocean.
Breakdown coefficients and scaling properties of rain fields
Authors: Harris D. , Menabde M. , Seed A. , Austin G.
Published: April 2010

Abstract. The theory of scale similarity and breakdown coefficients is applied here to intermittent rainfall data consisting of time series and spatial rain fields. The probability distributions (pdf) of the logarithm of the breakdown coefficients are the principal descriptor used. Rain fields are distinguished as being either multiscaling or multiaffine depending on whether the pdfs of breakdown coefficients are scale similar or scale dependent, respectively. Parameter  estimation techniques are developed which are applicable to both multiscaling and multiaffine fields. The scale parameter (width), σ, of the pdfs of the log-breakdown coefficients is a measure of the intermittency of a field. For multiaffine fields, this scale parameter is found to increase with scale in a power-law fashion consistent with a bounded-cascade picture of rainfall modelling. The resulting power-law exponent, H, is indicative of the smoothness of the field. Some details of breakdown coefficient analysis are addressed and a theoretical link between this analysis and moment scaling analysis is also presented. Breakdown coefficient properties of cascades are also investigated in the context of parameter estimation for modelling purposes.
Dissipation and wave-ion interaction in the solar wind: Links between fluid and kinetic theory
Authors: Marsch E.
Published: April 2010

Abstract. In this paper we establish links between turbulence dissipation and wave-particle interactions in the solar corona and wind. Based on quasilinear theory, a set of anisotropic, multi-component fluid equations is derived, which describe the wave-particle interactions of ions with Alfvén waves and ion-cyclotron waves or magnetosonic waves propagating along the mean magnetic field. The associated equations for the wave spectrum and the heating and acceleration of the ions are derived. In fast solar wind streams heavy ions have about equal thermal speeds as the protons and flow faster than them. In order to explain the observed relations, Tj / Tp ≈ mj /mp and Uj Up ≈ VA, a numerical fluid-type model is developed, which takes into account the relevant wave-particle interactions. It is shown that left- and right-handed polarized waves propagating away from the Sun parallel to the interplanetary magnetic field can resonantly heat and accelerate minor ions preferentially with respect to the protons in close agreement with the measured characteristics of ion velocity distributions. Finally, some results from a simple analytical model are discussed.
Remarks on the parallel propagation of small-amplitude dispersive Alfvénic waves
Authors: Champeaux S. , Laveder D. , Passot T. , Sulem P. L.
Published: April 2010

Abstract. The envelope formalism for the description of a small-amplitude parallel-propagating Alfvén wave train is tested against direct numerical simulations of the Hall-MHD equations in one space dimension where kinetic effects are neglected. It turns out that the magnetosonic-wave dynamics departs from the adiabatic approximation not only near the resonance between the speed of sound and the Alfvén wave group velocity, but also when the speed of sound lies between the group and phase velocities of the Alfvén wave. The modulational instability then does not anymore affect asymptotically large scales and strong nonlinear effects can develop even in the absence of the decay instability. When the Hall-MHD equations are considered in the long-wavelength limit, the weakly nonlinear dynamics is accurately reproduced by the derivative nonlinear Schrödinger equation on the expected time scale, provided no decay instabilities are present. The stronger nonlinear regime which develops at later time is captured by including the coupling to the nonlinear dynamics of the magnetosonic waves.
Properties of fast solitary structures
Authors: Ergun R. E. , Carlson C. W. , Muschietti L. , Roth I. , McFadden J. P.
Published: April 2010

Abstract. We present detailed observations of electromagnetic waves and particle distributions from the Fast Auroral SnapshoT (FAST) satellite which reveal many important properties of large-amplitude, spatially-coherent plasma structures known as "fast solitary structures" or "electron phase space holes". Similar structures have been observed in several regions of the magnetosphere including the auroral zone, plasma sheet boundary layer, and bow shock. There has been rapid theoretical progress in understanding these structures. Solitary structures can develop from bidirectional electron beams. Once developed, the one-dimensional properties parallel to the magnetic field can be adequately described by analytical treatment as BGK structures. There remains, however, several unanswered questions. The origin of the bidirectional electron beams, the development of two-or three-dimensional structures, and the observed association with the ion cyclotron frequency are not well understood.
Collisionless mirror mode trapping
Authors: Treumann R. A. , Baumjohann W.
Published: April 2010

Abstract. We investigate trapping of mirror modes in a magnetic slab. This model is a simplification of a real situation in front of the magnetopause where mirror waves may become trapped in a region close to the magnetopause for tangential discontinuity conditions and an unidentified (hypothetical) boundary deeper in the sheath which we, for simplicity, assume to be another tangential discontinuity. Such magnetic slabs may trap mirror modes selecting a particular perpendicular wave lengths which follows from a quantization condition on the perpendicular wavenumber.
Functional background of the Tsallis entropy: "coarse-grained" systems and "kappa" distribution functions
Authors: Milovanov A. V. , Zelenyi L. M.
Published: April 2010

Abstract. The concept of the generalized entropy is analyzed, with the particular attention to the definition postulated by Tsallis [J. Stat. Phys. 52, 479 (1988)]. We show that the Tsallis entropy can be rigorously obtained as the solution of a nonlinear functional equation; this equation represents the entropy of a complex system via the partial entropies of the subsystems involved, and includes two principal parts. The first part is linear (additive) and leads to the conventional, Boltzmann, definition of entropy as the logarithm of the statistical weight of the system. The second part is multiplicative and contains all sorts of multilinear products of the partial entropies; inclusion of the multiplicative terms is shown to reproduce the generalized entropy exactly in the Tsallis sense. We speculate that the physical background for considering the multiplicative terms is the role of the long-range correlations supporting the "macroscopic" ordering phenomena (e.g., formation of the "coarse-grained" correlated patterns). We prove that the canonical distribution corresponding to the Tsallis definition of entropy, coincides with the so-called "kappa" redistribution which appears in many physical realizations. This has led us to associate the origin of the "kappa" distributions with the "macroscopic" ordering ("coarse-graining") of the system. Our results indicate that an application of the formalism based on the Tsallis notion of entropy might actually have sense only for the systems whose statistical weights, Ω, are relatively small. (For the "coarse-grained" systems, the weight \\omega could be interpreted as the number of the "grains".) For large Ω (i.e., Ω -> ∞), the standard statistical mechanical formalism is advocated, which implies the conventional, Boltzmann definition of entropy as ln Ω.
Permanent bedforms in a theoretical model of wave-sea-bed interactions
Authors: Becker J. M. , Bercovici D.
Published: April 2010

Abstract. The interaction between sea waves and a deformable sea-bed is studied with a simple two-layer model in which the upper-layer fluid is inviscid and the lower-layer fluid is bi-viscous to account for non-Newtonian behaviour of sand and sediments. The nonlinear response of the system to periodic forcing by an external surface pressure is determined. It is shown that a simple bi-viscous rheology allows small wavelength morphology in the lower layer to be generated from large wavelength surface waves in the upper inviscid layer, although the morphology is not permanent. For a bi-viscous rheology with a pressure-dependent yield stress (which accounts for the fact that sand yields less readily under loading than unloading), however, small wavelength and permanent features are formed in the seabed.
A Monte Carlo simulation of magnetic field line tracing in the solar wind
Authors: Pommois P. , Zimbardo G. , Veltri P.
Published: April 2010

Abstract. It is well known that the structure of magnetic field lines in solar wind can be influenced by the presence of the magnetohydrodynamic turbulence. We have developed a Monte Carlo simulation which traces the magnetic field lines in the heliosphere, including the effects of magnetic turbulence. These effects are modelled by random operators which are proportional to the square root of the magnetic field line diffusion coefficient. The modelling of the random terms is explained, in detail, in the case of numerical integration by discrete steps. Furthermore, a proper evaluation of the diffusion coefficient is obtained by a numerical simulation of transport in anisotropic magnetic turbulence. The scaling of the fluctuation level and of the correlation lengths with the distance from the Sun are also taken into account. As a consequence, plasma transport across the average magnetic field direction is obtained. An application to the propagation of energetic particles from corotating interacting regions to high heliographic latitudes is considered.
Nonlinear saturation of Rayleigh-Taylor instability and generation of shear flow in equatorial spread-F plasma
Authors: Chakrabarti N. , Lakhina G. S.
Published: April 2010

Abstract. An analysis of low order mode coupling equations is used to describe the nonlinear behaviour of the Rayleigh-Taylor (RT) instability in the equatorial ionosphere. The nonlinear evolution of RT instability leads to the development of shear flow. It is found that there is an interplay between the nonlinearity and the shear flow which compete with each other and saturate the RT mode, both in the collisionless and collisional regime. However, the nonlinearly saturated state, normally known as vortices or bubbles, may not be stable. Under certain condition these bubbles are shown to be unstable to short scale secondary instabilities that are driven by the large gradients which develop within these structures. Some understanding of the role of collisional nonlinearity in the  shear flow generations is also discussed.
Self-organized criticality: Does it have anything to do with criticality and is it useful?
Authors: Turcotte D. L.
Published: April 2010

Abstract. Three aspects of complexity are fractals, chaos, and self-organized criticality. There are many examples of the applicability of fractals in solid-earth geophysics, such as earthquakes and landforms. Chaos is widely accepted as being applicable to a variety of geophysical phenomena, for instance, tectonics and mantle convection. Several simple cellular-automata models have been said to exhibit self-organized criticality. Examples include the sandpile, forest fire and slider-blocks models. It is believed that these are directly applicable to landslides, actual forest fires, and earthquakes, respectively. The slider-block model has been shown to clearly exhibit deterministic chaos and fractal behaviour. The concept of self-similar cascades can explain self-organized critical behaviour. This approach also illustrates the similarities and differences with critical phenomena through association with the site-percolation and diffusion-limited aggregation models.
Sensitivity of the attractor of the barotropic ocean model to external influences: approach by unstable periodic orbits
Authors: Kazantsev E.
Published: April 2010

Abstract. A description of a deterministic chaotic system in terms of unstable periodic orbits (UPO) is used to develop a method of an a priori estimate of the sensitivity of statistical averages of the solution to small external influences. This method allows us to determine the forcing perturbation which maximizes the norm of the perturbation of a statistical moment of the solution on the attractor. The method was applied to the barotropic ocean model in order to determine the perturbation of the wind field which provides the greatest perturbation of the model's climate. The estimates of perturbations of the model's time mean solution and its mean variance were compared with directly calculated values. The comparison shows that some 20 UPOs is sufficient to realize this approach and to obtain a good accuracy.