Narrow-band Oscillations Research Articles

Peculiarities of the Mode Spectrum in Free-Electron Masers Based on Oversized Bragg Resonators with a Corrugation Phase Step

We study the operating mode splitting caused by interaction of the neighboring Bragg scattering zones in an oversized Bragg resonator with a corrugation phase step, which is operated at the coupled forward and backward waveguide modes with different transverse structures. This effect is described within the framework of the coupled-wave approach using an advanced four-wave model. It is shown that this effect deteriorates the selective properties of the resonator and, finally, restricts the output power and reduces stability of the narrow-band operating regime in the free-electron masers (FEMs) based on such resonators. The results of the theoretical analysis were corroborated by 3D simulations and “cold” electrodynamic tests. Experimental studies of 30-GHz FEMs with the Bragg resonators having different corrugation depths demonstrated the onset of both narrow-band single-mode and multifrequency multimode oscillation regimes in such resonators. The possibility of power enhancement by using passive compression of the FEM output pulse in a double-frequency oscillation regime is discussed.

Pattern-based computing via sequential phase transitions in hierarchical mean field neuropercolation

In this work, we describe operational principles of a pattern-based computing paradigm based on the neuropercolation model, which can be used as associative memory supporting sensory processing and pattern recognition. Neuropercolation extends the concept of phase transitions to interactive populations exhibiting frequent transients in their spatio-temporal dynamics, which can be viewed as manifestations of an asynchronous computer working with a sequence of meta-stable spatial patterns, in a bid to unravel the limitations of Turing computing principles. The model is motivated by the structural and dynamical properties of large-scale neural populations in the cerebral cortex and it implements basic building blocks of neurodynamics following the hierarchy of Freeman K-sets.The introduced mean-field approximation allows rigorous mathematical analysis of the emergent dynamics, which is the major novel contribution of this work. Specifically, we derive exact conditions for the onset of non-zero background activity, for the transition from steady state to narrow-band (limit cycle) oscillations, and for the transition from narrow-band to broad-band (chaotic) dynamics. We describe an array of connected oscillators, which exhibits transient synchronization episodes manifesting meta-stable collective states. The corresponding meta-stable spatial amplitude patterns are destabilized by inputs or spontaneously and jump to another pattern, yielding a sequence of transient patterns. These patterns are shaped by the connections between the nodes modifiable through learning. The sequence of patterns manifest the steps of the computation, which embody the meaning of the input data in the context of the system past experiences.

A statistical study of fundamental toroidal mode standing Alfvén waves using THEMIS ion bulk velocity data

AbstractWe have studied the statistical properties of toroidal mode standing Alfvén waves with a fundamental eigenmode structure along the field line, denoted T1 waves, in the L range 7–12, using THEMIS‐D data for 2008–2013. T1 wave events were identified in hourly data segments using an automated procedure that detects narrowband oscillations in the azimuthal component of the ion bulk velocity. For each event we determined the frequency, amplitude, ellipticity, and the orientation angle of the polarization ellipse, and we examined the L and magnetic local time dependence of the detection rate and physical properties of the T1 waves. Confirming previous observations in space and on the ground, we found a pronounced dawn‐dusk asymmetry in the wave detection rate and amplitude. The detection rate in the dawn sector is approximately twice as high as that in the dusk sector, and the amplitude in the dawn sector is larger by ∼50%. The same asymmetry is also evident in the velocity amplitude averaged in the fixed Pc5 band (1.7–6.7 mHz) and in the amplitude of the electric field and field line displacement that are derived from the velocity amplitude. The ellipticity and the orientation angle of the polarization ellipse are organized by local time, in accordance with the theoretical prediction of toroidal mode waves excited by field line resonance with tailward propagating waves in the magnetosphere, which are in turn driven by external sources. Although infrequently, T1 waves are also detected in the midnight sector, suggesting magnetotail sources for this subset of events.

Role of tides on the formation of the Antarctic Slope Front at the Weddell‐Scotia Confluence

AbstractThe structure of the Antarctic Slope Front (ASF) and the associated Antarctic Slope Current (ASC) on the Scotia Sea side of the Weddell‐Scotia Confluence (WSC) is described using data from a hydrographic survey and three 1 year long moorings across the continental slope. The ASC in this region flows westward along isobaths with an annual mean speed of ∼0.2 m s−1, with time variability dominated by the K1 and O1 tidal diurnal constituents, a narrowband oscillation with ∼2‐week period attributable to the spring/neap tidal cycle, and seasonal variability. Realistic and idealized high‐resolution numerical simulations are used to determine the contribution of tides to the structure of the ASF and the speed of the ASC. Two simulations forced by realistic atmospheric forcing and boundary conditions integrated with and without tidal forcing show that tidal forcing is essential to reproduce the measured ASF/ASC cross‐slope structure, the time variability at our moorings, and the reduced stratification within the WSC. Two idealized simulations run with tide‐only forcing, one with a homogeneous ocean and the other with initial vertical stratification that is laterally homogeneous, show that tides can generate the ASC and ASF through volume flux convergence along the slope initiated by effects including the Lagrangian component of tidal rectification and mixing at the seabed and in the stratified ocean interior. Climate models that exclude the effects of tides will not correctly represent the ASF and ASC or their influence on the injection of intermediate and dense waters from the WSC to the deep ocean.

Extended lunar precursor regions: Electron‐wave interaction

AbstractWe present Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) observations of electron‐wave interactions which extend to quite large distances upstream from the Moon. We first study electron velocity distributions and wave spectra on an event basis. In both the solar wind and terrestrial plasma sheet, we observe strong whistler wave activity on the magnetic field lines connected to the dayside lunar surface. These whistlers are most likely driven by the anisotropy of upward electrons caused by surface absorption. The whistler growth rates computed from the measured electron distributions successfully reproduced the spectral characteristics of the observed ∼100 Hz narrowband oscillations and those reaching lower frequencies. Meanwhile, the incoming solar wind strahl beam is occasionally isotropized near the Moon, and broadband electrostatic waves are observed simultaneously, suggesting streaming instabilities between the incoming and outgoing beams. Based on the case study, we statistically survey the spatial variations of the characteristic quantities of the upstream electron‐wave interactions. Both the electron anisotropy and electromagnetic wave intensity decay with increasing field line distances but remain higher than the ambient level at 6 lunar radii (∼10,000 km) or more. The strahl electron isotropization and electrostatic waves are found mainly at lower altitudes below 1 lunar radius. The electron anisotropy and whistler intensity exhibit clear anticorrelation with crustal magnetic fields, indicating that the magnetic anomalies suppress the whistler wave growth. The ARTEMIS observations convincingly illustrate that the lunar influence on electrons reaches out to 6 lunar radii or more upstream from the Moon.

The effect of linear mixing in the EEG on Hurst exponent estimation

Although the long-range temporal correlation (LRTC) of the amplitude fluctuations of neuronal EEG/MEG oscillations is widely acknowledged, the majority of studies to date have been performed in sensor space, disregarding the mixing effects implied by volume conduction and confounding noise. While the effect of mixing on the evaluation of evoked responses and connectivity measures has been extensively studied, there are, to date, no studies reporting on the differences in the values of the estimated Hurst exponents when moving between sensor and source space representations of the multivariate data or on the effect of noise. Such differences, if not duly acknowledged, may lead to erroneous data interpretations. We show in simulations and in theory that measuring Hurst exponents in sensor space may lead to an incomplete picture of the LRTC properties of the underlying data and that noise may significantly bias the estimate of the Hurst exponent of the underlying signal. Moreover, these predictions are confirmed in real data, where we analyze the amplitude dynamics of neuronal oscillations in the resting state from EEG data. By moving either to an independent components representation or to a source representation which maximizes the signal to noise ratio in the alpha frequency range, we observe greater variance, skewness and kurtosis over measured Hurst exponents than in sensor space. We confirm the suitability of conventional source separation methodology by introducing a novel algorithm HeMax which obtains a source maximizing the Hurst exponent in the amplitude dynamics of narrow band oscillations. Our findings imply that the long-range correlative properties of the EEG should be studied in source space, in such a way that the SNR is maximized, or at least with spatial decomposition techniques approximating source activities, rather than in sensor space.

Regulating Cortical Oscillations in an Inhibition-Stabilized Network.

Understanding the anatomical and functional architecture of the brain is essential for designing neurally inspired intelligent systems. Theoretical and empirical studies suggest a role for narrowband oscillations in shaping the functional architecture of the brain through their role in coding and communication of information. Such oscillations are ubiquitous signals in the electrical activity recorded from the brain. In the cortex, oscillations detected in the gamma range (30-80 Hz) are modulated by behavioral states and sensory features in complex ways. How is this regulation achieved? Although several underlying principles for the genesis of these oscillations have been proposed, a unifying account for their regulation has remained elusive. In a network of excitatory and inhibitory neurons operating in an inhibition-stabilized regime, we show that strongly superlinear responses of inhibitory neurons facilitate bidirectional regulation of oscillation frequency and power. In such a network, the balance of drives to the excitatory and inhibitory populations determines how the power and frequency of oscillations are modulated. The model accounts for the puzzling increase in their frequency with the salience of visual stimuli, and a decrease with their size. Oscillations in our model grow stronger as the mean firing level is reduced, accounting for the size dependence of visually evoked gamma rhythms, and suggesting a role for oscillations in improving the signal-to-noise ratio (SNR) of signals in the brain. Empirically testing such predictions is still challenging, and implementing the proposed coding and communication strategies in neuromorphic systems could assist in our understanding of the biological system.

Multispacecraft observations of fundamental poloidal waves without ground magnetic signatures

AbstractPoloidal standing Alfvén waves observed by spacecraft usually have a second harmonic standing wave structure. On very rare occasions, fundamental poloidal waves have been observed in association with giant pulsations observed on the ground. In this paper, we report multisatellite observations of fundamental poloidal waves that did not produce any clearly associated magnetic pulsations on the ground. The waves were observed on 10 November 2008, at ~1830 Universal Time (UT) at THEMIS‐A and THEMIS‐D and at ~2010 UT at THEMIS‐E as these spacecraft passed L ~ 11 and magnetic local time (MLT) ~ 0900. The GOES‐11 geostationary satellite (L ~ 7) also observed poloidal waves at ~1730 UT when it was at ~0900 MLT. The poloidal waves at THEMIS were characterized by narrow‐band oscillations (frequency ~4 mHz) of the ion bulk velocity and magnetic field in the radial direction. We identify the waves at THEMIS to be the fundamental mode on the basis of the wave properties observed slightly south of the magnetic equator: large velocity amplitude, small magnetic field amplitude, and ~90° phase delay of the magnetic field relative to the velocity. The azimuthal wave number is found to be ~70 (if we assume westward propagation) or ~200 (if we assume eastward propagation) from the phase delay between THEMIS‐A and THEMIS‐D. This wave number explains why there were no corresponding magnetic field oscillations on the ground. These observations imply that only a subset of fundamental poloidal waves excited in the magnetosphere is observed on the ground as giant pulsations.

On the scenario of transition to the broadband oscillation regime in the prototype of a low-voltage vircator

This study is devoted to experimental investigation of the scenario for a transition from the narrow-band (single-frequency) oscillation regime to the broadband noise-like oscillation in a prototype of a low-voltage oscillator operating with a virtual cathode. The experimental setup and the control instrumentation are considered. Detailed experimental analysis of the dynamics of variation of temporal realizations of the signals generated by the laboratory prototype upon the variation of the chosen control parameters is performed. Analysis of the power spectra, temporal realizations of output signals, phase portraits, and the bifurcation diagram constructed upon a change of the chosen control parameters shows that a transition from the single-frequency oscillation regime to broadband oscillations occurs via a cascade of period-doubling bifurcations (classical Feugenbaum scenario is implemented).

Advances on Brain Inspired Computing

This special issue of Cognitive Computation includes 11 original articles, which have been selected among the highest quality submissions to the 2012 Brain Inspired Cognitive Systems (BICS 2012) Conference. BICS 2012 provided a high-level international forum for scientists, engineers, and educators to present the state-of-the-art research on brain inspired computing and cognitive systems, with applications in multi-disciplinary fields. The conference featured plenary speeches given by worldwide renowned scholars, regular sessions with broad coverage, and some special sessions focusing on interesting topics for the related scientific community. Based on the recommendation of symposium organizers and reviewers, a number of authors were invited to resubmit an extended version of their contributions, originally submitted as conference papers, for this special issue of Cognitive Computation. All these journal articles went through the same rigorous review procedure by at least three independent experts before being accepted for publication. This special issue focuses on recent advancements in the field of brain inspired computing. The selected 11 articles can be divided into two main groups. The first group consists of four papers, and it is more oriented to new architectures and algorithms, whereas the second group contains the remaining seven papers, dealing with challenging applications in diverse areas. The special issue starts with the first group of papers and with the contribution by Kozma and Puljic, titled ‘‘Learning Effects in Coupled Arrays of Cellular Neural Oscillators,’’ where the authors analyze the spatio-temporal dynamics of coupled neural oscillatory arrays. In particular, a neuropercolation model encompassing the Freeman principles of neurodynamics is used, and the modulation effects due to distributed input biases in interconnected excitatory-inhibitory oscillators spanning the lattice graph are investigated. It is shown that the proposed neuropercolation model is able to generate large-scale synchronized, narrow-band oscillations in response to learned stimuli, as observed in EEG and ECoG experiments. The second contribution by Xunan Zhang et al., addresses the Bayesian classification problem in the presence of incomplete data. The usual approach adopted in the literature consists in simply ignoring the samples with missing values or imputing missing values before classification. This is not really effective when the amount of missing values is considerable and/or the data acquisition phase is expensive. The authors thus propose an innovative expectation–maximization-based learning algorithm, applying it to a multivariate Gaussian mixture model and a multiple kernel density estimator. Such a technique is able to avoid list-wise deletion or mean imputation in solving classification tasks with incomplete data. Experimental tests on several benchmark problems and also on practical classification tasks on the lithology identification of hydrothermal minerals and license plate character recognition have shown that the proposed approach shows remarkable classification S. Squartini (&) Department of Information Engineering, UniversitaPolitecnicadelle Marche, Via BrecceBianche 1, 60131 Ancona, Italy e-mail: s.squartini@univpm.it

Learning Effects in Coupled Arrays of Cellular Neural Oscillators

We analyze spatio-temporal dynamics of coupled neural oscillatory arrays. The interconnected oscillators can produce a wide range of dynamics, including quasi-periodic limit cycles, chaotic waveforms, and intermittent chaotic oscillations. We study the role of distributed input bias and develop methods for learning the input patterns. After learning, the coupled oscillators produce large-scale synchronized, narrow-band oscillations in response to the learned patterns. We study patterns of amplitude modulations that span the whole lattice graph. The presented results correspond to Freeman’s 6th building block of neurodynamics.

Does rhythmic entrainment represent a generalized mechanism for organizing computation in the brain?

Does rhythmic entrainment represent a generalized mechanism for organizing computation in the brain?

Frequency of Subthreshold Oscillations at Different Membrane Potential Voltages in Neurons at Different Anatomical Positions on the Dorsoventral Axis in the Rat Medial Entorhinal Cortex

Neurons from layer II of the medial entorhinal cortex show subthreshold membrane potential oscillations (SMPOs) which could contribute to theta-rhythm generation in the entorhinal cortex and to generation of grid cell firing patterns. However, it is unclear whether single neurons have a fixed unique oscillation frequency or whether their frequency varies depending on the mean membrane potential in a cell. We therefore examined the frequency of SMPOs at different membrane potentials in layer II stellate-like cells of the rat medial entorhinal cortex in vitro. Using whole-cell patch recordings, we found that the fluctuations in membrane potential show a broad band of low power frequencies near resting potential that transition to more narrowband oscillation frequencies with depolarization. The transition from broadband to narrowband frequencies depends on the location of the neuron along the dorsoventral axis in the entorhinal cortex, with dorsal neurons transitioning to higher-frequency oscillations relative to ventral neurons transitioning to lower-frequency oscillations. Once SMPOs showed a narrowband frequency, systematic frequency changes were not observed with further depolarization. Using a Hodgkin-Huxley-style model of membrane currents, we show that differences in the influence of depolarization on the frequency of SMPOs at different dorsal to ventral positions could arise from differences in the properties of the h current. The properties of frequency changes in this data are important for evaluating models of the generation of grid cell firing fields with different spacings along the dorsal-to-ventral axis of medial entorhinal cortex.

Narrowband Oscillations in the Upper Equatorial Ocean. Part I: Interpretation as Shear Instabilities

Abstract Extended measurements of temperature fluctuations that include the turbulence wavenumber band have now been made using rapidly sampled fast thermistors at multiple depths above the core of the Equatorial Undercurrent on the Tropical Atmosphere Ocean (TAO) mooring at 0°, 140°W. These measurements include the signature of narrowband oscillations as well as turbulence, from which the temperature variance dissipation rate χT and the turbulence energy dissipation rate εχ are estimated. The narrowband oscillations are characterized by the following:groupiness—packets consist of O(10) oscillations;spectral peaks of up to two orders of magnitude above background;a clear day–night cycle with more intensive activity at night;enhanced mixing rates;frequencies of 1–2 × 10−3 Hz, close to both the local buoyancy and shear frequencies, N/2π and S/2π, which vary slowly in time;high vertical coherence over at least 30-m scales; andabrupt vertical phase change (π/2 over <20 m).The abrupt vertical phase change is consistent with instabilities formed in stratified shear flows. Linear stability analysis applied to measured velocity and density profiles leads to predicted frequencies that match those of the observed oscillations. This correspondence suggests that the observed oscillation frequencies are set by the phase speed and wavelength of instabilities as opposed to the Doppler shifting of internal gravity waves with intrinsic frequency set by the local stratification N.

Narrowband Oscillations in the Upper Equatorial Ocean. Part II: Properties of Shear Instabilities

Abstract Narrowband oscillations observed in the upper equatorial Pacific are interpreted in terms of a random ensemble of shear instability events. Linear perturbation analysis is applied to hourly averaged profiles of velocity and density over a 54-day interval, yielding a total of 337 unstable modes. Composite profiles of mean states and eigenfunctions surrounding the critical levels suggest that the standard hyperbolic tangent model of Kelvin–Helmholtz (KH) instability is a reasonable approximation, but the symmetry of the composite perturbation is broken by the stratification and vorticity gradient of the underlying equatorial undercurrent. Unstable modes are found to occupy a range of frequencies with a peak near 1.4 mHz, consistent with the frequency content of the observed oscillations. A probabilistic theory of random instabilities predicts this peak frequency closely. An order of magnitude estimate suggests that the peak frequency is of order N, in accord with the observations. This results not from gravity wave physics but from the balance of shear and stratification that governs shear instability in geophysical flows. More generally, it is concluded that oscillatory signals with frequency bounded by N can result from a process that has nothing to do with gravity waves.

MHD nature of ionospheric wave packets generated by the solar terminator

We obtained the first experimental evidence for the magnetohydrodynamic (MHD) nature of ionospheric medium-scale travelling wave packets (MSTWP). We used data on total electron content (TEC) measurements obtained at the dense Japanese network GPS/GEONET (1220 stations) in 2008-2009. We found that the diurnal, seasonal and spectral MSTWP characteristics are specified by the solar terminator (ST) dynamics. MSTWPs are the chains of narrow-band TEC oscillations with single packet's duration of about 1-2 hours and oscillation periods of 10-20 minutes. Their total duration is about 4--6 hours. The MSTWP spatial structure is characterized by a high degree of anisotropy and coherence at the distance of more than 10 wavelengths. The MSTWP direction of travelling is characterized by a high directivity regardless of seasons. Occurrence rate of daytime MSTWPs is high in winter and during equinoxes. Occurrence rate of nighttime MSTIDs has its peak in summer. These features are consistent with previous MS travelling ionosphere disturbance (TID) statistics obtained from 630-nm airglow imaging observations in Japan. In winter, MSTWPs in the northern hemisphere are observed 3-4 hours after the morning ST passage. In summer, MSTWPs are detected 1.5-2 hours before the evening ST occurrence at the point of observations, at the moment of the evening ST passage in the magneto-conjugate point. Both the high Q-factor of oscillatory system and synchronization of MSTWP occurrence with the solar terminator passage at the point of observations and in the magneto-conjugate area testify the MHD nature of ST-excited MSTWP generation. The obtained results are the first experimental evidence for the hypothesis of the ST-generated ion sound waves.

Travelling wave packets generated by the solar terminator in the upper atmosphere

For the first time, the morphology of a new type of midlatitude, medium scale, travelling ionospheric disturbances—travelling wave packets (TWPs)—is presented with the use of total electron content (TEC) measurements from the data of a global net of GPS receivers over a long time period from 1998 to 2007. The total amount of registered TWPs exceeds 275 000. In the context of time, TWPs are narrow-band TEC oscillations of an hour length with a period of about 10–20 min. TWPs are predominantly observed within 3 h after passing the morning solar terminator, when the TEC derivatives reach their maximum.

Broadband Shifts in Local Field Potential Power Spectra Are Correlated with Single-Neuron Spiking in Humans

A fundamental question in neuroscience concerns the relation between the spiking of individual neurons and the aggregate electrical activity of neuronal ensembles as seen in local field potentials (LFPs). Because LFPs reflect both spiking activity and subthreshold events, this question is not simply one of data aggregation. Recording from 20 neurosurgical patients, we directly examined the relation between LFPs and neuronal spiking. Examining 2030 neurons in widespread brain regions, we found that firing rates were positively correlated with broadband (2-150 Hz) shifts in the LFP power spectrum. In contrast, narrowband oscillations correlated both positively and negatively with firing rates at different recording sites. Broadband power shifts were a more reliable predictor of neuronal spiking than narrowband power shifts. These findings suggest that broadband LFP power provides valuable information concerning neuronal activity beyond that contained in narrowband oscillations.

Layer-specific network oscillation and spatiotemporal receptive field in the visual cortex

A quintessential feature of the neocortex is its laminar organization, and characterizing the activity patterns in different layers is an important step in understanding cortical processing. Using in vivo whole-cell recordings in rat visual cortex, we show that the temporal patterns of ongoing synaptic inputs to pyramidal neurons exhibit clear laminar specificity. Although low-frequency (approximately 2 Hz) activity is widely observed in layer 2/3 (L2/3), a narrow-band fast oscillation (10-15 Hz) is prominent in layer 5 (L5). This fast oscillation is carried exclusively by excitatory inputs. Moreover, the frequency of ongoing activity is strongly correlated with the spatiotemporal window of visual integration: Neurons with fast-oscillating spontaneous inputs exhibit transient visual responses and small receptive fields (RFs), whereas those with slow inputs show prolonged responses and large RFs. These findings suggest that the neural representation of visual information within each layer is strongly influenced by the temporal dynamics of the local network manifest in spontaneous activity.

Spatio-temporal structure of the wave packets generated by the solar terminator

Using long-term (1998--2009) total electron content (TEC) measurements from the GPS global network including dense network of GPS sites in USA and Japan, we have obtained the first data regarding the spatio-temporal structure and the statistics of medium-scale traveling wave packets (MS TWPs) excited by the solar terminator (ST). Total amount of the detected TWPs exceeds 565,000. There is no correlation between TWPs occurrence and geomagnetic and solar activity. We found that the diurnal, seasonal and spectral MS TWPs characteristics are specified by the solar terminator (ST) dynamics. MS TWPs are the chains of narrow-band TEC oscillations with single packet’s duration of about 1–2 h and oscillation periods of 10–20 min. The total duration of chain is about 4–6 h. The MS TWPs spatial structure is characterized by a high degree of anisotropy and coherence at the distance of more than 10 wavelengths. Occurrence rate of daytime MS TWPs is high in winter and during equinoxes. Occurrence rate of nighttime MS TWPs has its peak in summer. These features are consistent with previous MS travelling ionosphere disturbance (TID) statistics obtained from 630-nm airglow imaging observations in Japan. In winter, MS TWPs in the northern hemisphere are observed 3–4 h after the morning ST passage. In summer, MS TWPs are detected 1.5–2 h before the evening ST appearance at the point of observations, but at the moment of the evening ST passage in the magneto-conjugate point. The obtained results are the first experimental evidence for the hypothesis of the ST-generated ion sound waves.