Bursting activity spreading through asymmetric interactions

People communicate with those who have the same background or share a common interest by using a social networking service (SNS). News or messages propagate through inhomogeneous connections in an SNS by sharing or facilitating additional comments. Such human activity is known to lead to endogenous bursting in the rate of message occurrences. We analyze a multi-dimensional self-exciting process to reveal dependence of the bursting activity on the topology of connections and the distribution of interaction strength on the connections. We determine the critical conditions for the cases where interaction strength is regulated at either the point of input or output for each person. In the input regulation condition, the network may exhibit bursting with infinitesimal interaction strength, if the dispersion of the degrees diverges as in the scale-free networks. In contrast, in the output regulation condition, the critical value of interaction strength, represented by the average number of events added by a single event, is a constant $1-1/\sqrt{2} \approx 0.3$, independent of the degree dispersion. Thus, the stability in human activity crucially depends on not only the topology of connections but also the manner in which interactions are distributed among the connections.Comment: 8 pages, 8 figure

Locally embedded presages of global network bursts

Spontaneous, synchronous bursting of neural population is a widely observed phenomenon in nervous networks, which is considered important for functions and dysfunctions of the brain. However, how the global synchrony across a large number of neurons emerges from an initially non-bursting network state is not fully understood. In this study, we develop a new state-space reconstruction method combined with high-resolution recordings of cultured neurons. This method extracts deterministic signatures of upcoming global bursts in "local" dynamics of individual neurons during non-bursting periods. We find that local information within a single-cell time series can compare with or even outperform the global mean field activity for predicting future global bursts. Moreover, the inter-cell variability in the burst predictability is found to reflect the network structure realized in the non-bursting periods. These findings demonstrate the deterministic mechanisms underlying the locally concentrated early-warnings of the global state transition in self-organized networks

Resonant interaction of fast particles with Alfven waves in spherical tokamaks

The Spherical Tokamak (ST) concept has become one of the main avenues in magneticnuclear fusion research since STs successfully demonstrated plasma operationat [Beta] = 2P[mu]0=B2~1. Next step ST machines aiming at achieving burning plasmaconditions in high [Beta] plasmas are being planned, such as the Spherical TokamakPower Plant (STPP) and the Component Testing Facility (CTF). Instabilities offast particle-driven Alfven eigenmodes are often observed in present-day STs. Suchinstabilities, driven by fusion-born alpha particles as well as by fast ions producedwith auxiliary heating schemes, in the next step STs may pose a major problem asthese instabilities may affect confinement and losses of the fast ions.A theory of compressional Alfven eigenmodes (CAE) with frequencies above thedeuterium cyclotron frequency,[omega] > [omega]cD, is developed for plasma parameters of aSTPP, and modes in the ion-ion hybrid frequency range, [omega]cT < [omega] < [omega]cD, are alsoinvestigated in order to assess the potential of diagnosing the deuterium-tritium(D-T) ratio. For the 1-D character of a STPP equilibrium with [Beta]~1 , a `hollowcylinder toroidal plasma model is employed for studying CAEs with arbitrary valuesof the parallel wave-vector k[||] = k[.]B/|B|. The existence of weakly-damped CAEs,free of mode conversion, is shown to be associated with the `well in the magneticfield profile, B = B (R), that can exist at the magnetic axis.A significant part of this thesis focusses on the experimentally observed effectsof resonant wave-particle interaction between Alfven waves and fast particles in theMega Amp Spherical Tokamak (MAST) device at the Culham Laboratory, UK, andin the LArge Plasma Device (LAPD) in the University of California, Los-Angeles,USA. New robust experimental scenarios for exciting CAEs in the MAST spherical tokamak are developed, and interpretation of the observed CAEs in the frequencyrange [omega]cD/3 < [omega] < [omega]cD is given in the context of the 1-D ST model and the Dopplershifted cyclotron resonance. The e ciency of the Doppler resonance between coand counter directed fast ions and left and right hand polarised Alfven waves isfurther assessed experimentally on the LAPD device, with probe ions injected inthe presence of Alfv en waves launched by an external antenna.The developed theory of CAEs is then applied to a calculation of the linear kineticdrive of CAEs in the MAST experiments. A model representation of the fast iondistribution function, produced by neutral beam injection (NBI), is used by fittingto the TRANSP Monte-Carlo NBI modelling results. The main free energy sourcesassociated with temperature anisotropy and bump-on-tail are estimated analytically,and the CAE stability boundary is qualitatively assessed.In order to explain the experimentally observed difference between steady-stateand pulsating Alfvenic modes, the non-linear theory of fast particle driven modesnear marginal stability is extended to include dynamical friction (drag). For thebump-on-tail problem, the drag is shown to always give an explosive amplitudeevolution in contrast to diffusion in velocity space in the vicinity of the wave-particleresonance. This is then extended to the case of experimentally observed NBI-driventoroidal Alfven eigenmodes (TAEs) in the MAST machine. The experimentallyobserved differences between TAEs driven by fast ions produced with ion cyclotronresonance heating (ICRH) and NBI are then interpreted. The problem of dragdominated collisions for modes excited by fusion-born alpha particles in burningplasmas such as a STPP and ITER is underlined.Imperial Users onl

Quantifying submarine eruptive flux from interpretation of hydroacoustic signals, West Mata Volcano, Lau Basin

West Mata is a submarine volcano in the northeast Lau Basin. Hydroacoustic data from a 5-month period provide insight into the nature of eruptive behavior at this volcano. Previous studies have used acoustic data to estimate the eruption velocity at subaerial volcanoes (Woulff and McGetchin, 1976; Vergniolle et al., 2004; Vergniolle and Caplan- Auerbach, 2004, 2006; Caplan-Auerbach et al., 2010). In this study, the hydroacoustic data from West Mata are used to calculate eruption velocities and volumes (both lava and gas) for explosions during the 5-month hydrophone deployment. The method used in this study, developed by Vergniolle and Caplan-Auerbach (2006), was found to be most successful when events were hand-picked for a portion of the data during processing, and then extrapolated for explosions from the entire data set. However, there are two factors that contribute to an underestimation of the total eruptive volume. Only one signal type (explosions, representing discrete gas bubble bursts at the volcanic vent) could be used for estimating the eruptive velocities and volumes at West Mata, due to interference resulting from signal reflections. Another issue for accurately estimating cumulative eruptive volume resulted from raypaths reflecting on the sea surface, and arriving at the hydrophones 20 – 80 ms after the direct wave. This also caused hydroacoustic signals to overlap, limiting the calculations to the first 20 ms instead of the entire waveform. Eruptive velocities for vent explosions recorded at West Mata’s four hydrophones were found to range from 2.4 – 24.8 m/s. Eruptive flux ranged from 1.9 – 9.7 m3/s for each event. The volumes erupted per explosion ranged from 0.03 – 0.15 m3. Yearly eruption rates were calculated to be 5.7 x 103 – 9.1 x 104 m3/yr for both gas and lava. However, if only lava is used (assumed to contain 25% vesicularity), then the yearly eruptive rate at West Mata is estimated to be 1.9 x 103 – 5.6 x 104 m3/yr. These results were compared to estimates determined from video imagery at West Mata and long-term bathymetric surveys at West Mata and another submarine volcano, NW Rota-1. Velocities estimated from video imagery compare well with velocities estimated from hydroacoustic signals in this study, ranging from 0.2 – 5.2 m/s, compared to a range of 1.0 – 31 m/s using hydroacoustic signals. Volumes estimated from hydroacoustic date are up to three orders of magnitude lower than those derived from bathymetric surveys at West Mata. Given that the results from this study are expected to be an underestimate of eruptive volume, these estimates are considered to be reasonable, suggesting that the hydroacoustic method of estimating eruptive flux is also viable for the submarine environment. It is recommended that future studies deploy hydrophone stations in the near-field (\u3c 5 km) to reduce the effects of sea surface reflections, which would ensure the most accurate results

Many Attractors, Long Chaotic Transients, and Failure in Small-World Networks of Excitable Neurons

We study the dynamical states that emerge in a small-world network of recurrently coupled excitable neurons through both numerical and analytical methods. These dynamics depend in large part on the fraction of long-range connections or `short-cuts' and the delay in the neuronal interactions. Persistent activity arises for a small fraction of `short-cuts', while a transition to failure occurs at a critical value of the `short-cut' density. The persistent activity consists of multi-stable periodic attractors, the number of which is at least on the order of the number of neurons in the network. For long enough delays, network activity at high `short-cut' densities is shown to exhibit exceedingly long chaotic transients whose failure-times averaged over many network configurations follow a stretched exponential. We show how this functional form arises in the ensemble-averaged activity if each network realization has a characteristic failure-time which is exponentially distributed.Comment: 14 pages 23 figure

Gradual Translocation of Spatial Correlates of Neuronal Firing in the Hippocampus toward Prospective Reward Locations

SummaryIn a continuous T-maze alternation task, CA1 complex-spike neurons in the hippocampus differentially fire as the rat traverses overlapping segments of the maze (i.e., the stem) repeatedly via alternate routes. The temporal dynamics of this phenomenon were further investigated in the current study. Rats learned the alternation task from the first day of acquisition and the differential firing pattern in the stem was observed accordingly. More importantly, we report a phenomenon in which spatial correlates of CA1 neuronal ensembles gradually changed from their original firing locations, shifting toward prospective goal locations in the continuous T-maze alternation task. The relative locations of simultaneously recorded firing fields, however, were preserved within the ensemble spatial representation during this shifting. The within-session shifts in preferred firing locations in the absence of any changes in the environment suggest that certain cognitive factors can significantly alter the location-bound coding scheme of hippocampal neurons

Resonant interaction of fast particles with Alfvén waves in spherical tokamaks

The Spherical Tokamak (ST) concept has become one of the main avenues in magnetic nuclear fusion research since STs successfully demonstrated plasma operation at [Beta] = 2P[mu]0=B2~1. Next step ST machines aiming at achieving burning plasma conditions in high [Beta] plasmas are being planned, such as the Spherical Tokamak Power Plant (STPP) and the Component Testing Facility (CTF). Instabilities of fast particle-driven Alfven eigenmodes are often observed in present-day STs. Such instabilities, driven by fusion-born alpha particles as well as by fast ions produced with auxiliary heating schemes, in the next step STs may pose a major problem as these instabilities may affect confinement and losses of the fast ions. A theory of compressional Alfven eigenmodes (CAE) with frequencies above the deuterium cyclotron frequency,[omega] > [omega]cD, is developed for plasma parameters of a STPP, and modes in the ion-ion hybrid frequency range, [omega]cT < [omega] < [omega]cD, are also investigated in order to assess the potential of diagnosing the deuterium-tritium (D-T) ratio. For the 1-D character of a STPP equilibrium with [Beta]~1 , a `hollow cylinder' toroidal plasma model is employed for studying CAEs with arbitrary values of the parallel wave-vector k[||] = k[.]B/|B|. The existence of weakly-damped CAEs, free of mode conversion, is shown to be associated with the `well' in the magnetic field profile, B = B (R), that can exist at the magnetic axis. A significant part of this thesis focusses on the experimentally observed effects of resonant wave-particle interaction between Alfven waves and fast particles in the Mega Amp Spherical Tokamak (MAST) device at the Culham Laboratory, UK, and in the LArge Plasma Device (LAPD) in the University of California, Los-Angeles, USA. New robust experimental scenarios for exciting CAEs in the MAST spherical tokamak are developed, and interpretation of the observed CAEs in the frequency range [omega]cD/3 < [omega] < [omega]cD is given in the context of the 1-D ST model and the Doppler shifted cyclotron resonance. The e ciency of the Doppler resonance between co and counter directed fast ions and left and right hand polarised Alfven waves is further assessed experimentally on the LAPD device, with probe ions injected in the presence of Alfv en waves launched by an external antenna. The developed theory of CAEs is then applied to a calculation of the linear kinetic drive of CAEs in the MAST experiments. A model representation of the fast ion distribution function, produced by neutral beam injection (NBI), is used by fitting to the TRANSP Monte-Carlo NBI modelling results. The main free energy sources associated with temperature anisotropy and bump-on-tail are estimated analytically, and the CAE stability boundary is qualitatively assessed. In order to explain the experimentally observed difference between steady-state and pulsating Alfvenic modes, the non-linear theory of fast particle driven modes near marginal stability is extended to include dynamical friction (drag). For the bump-on-tail problem, the drag is shown to always give an explosive amplitude evolution in contrast to diffusion in velocity space in the vicinity of the wave-particle resonance. This is then extended to the case of experimentally observed NBI-driven toroidal Alfven eigenmodes (TAEs) in the MAST machine. The experimentally observed differences between TAEs driven by fast ions produced with ion cyclotron resonance heating (ICRH) and NBI are then interpreted. The problem of drag dominated collisions for modes excited by fusion-born alpha particles in burning plasmas such as a STPP and ITER is underlined

Alterations in the surface properties of sea spray aerosols introduced by the presence of sterols

SSCI-VIDE+CARE+CGOInternational audiencehe mixed stearic acid (SA)/sterol systems were used as sea spray aerosol mimics to get more insights into the al-terations in surface properties of aerosols induced by sterols. By means of surface pressure (π)–area(A) isotherms and polarization modulation-infrared reflection absorption spectroscopy (PM-IRRAS), the effect ofcholesterol (chol), stigmasterol (stig) and ergosterol (erg) on the lateral packing and chain conformation of SAmonolayer was explored. The fact that the excess areas of mixing of the mixed monolayers exhibit significant devi-ations from ideally mixedfilm proves that, the sterols are miscible withSA throughout all the monolayer composi-tions and surface pressures examined. The lift-off areas inπ–A isotherms were found to increase with increasingmole fraction of sterols, indicating that expulsive interactions exist between SA and sterols, which are more pro-nounced when the mole fraction of sterols is 0.7. In addition, the peak intensities ofνa(CH2)andνs(CH2)inIRRASspectra decrease with increasing sterols levels, which is consistent with ourfindings in theπ–A isotherms, thatthe addition of sterols leads to a looser chain packing in SA monolayer. The proportion ofgauchedefects in SA mono-layer induced by the sterols follows the order cholesterol stigmast ergosterol at a certain sterol level, as reflected by the decreasing peak intensities fνa(CH2)andνs(CH2). Consequently, the sterols generally give riseto considerable expanding effects on SA monolayer, which are particularly pronounced for stigmasterol and ergos-terol, suggesting that the additional alkyl side chains and double bonds of thesterols play a role on disordering SAmonolayer. The present study is likely to shed light on many boundary processes take place at the interface ofSSAs, in particular, transport processes of water and trace gases across the interfac