Spike Emission Research Articles

Sputtering of indium usingAumprojectiles: Transition from linear cascade to spike regime

We have investigated the yields and emission velocity distributions of neutral In atoms and In2 dimers sputtered from a pure indium surface under bombardment with Aum − m=1,2 ,3 projectile ions. It is shown that 5-keV Au1 bombardment leads to results in full compliance with linear cascade sputtering theory. All polyatomic projectiles are found to generate an additional low-energy contribution to the sputtered flux, which increases with increasing projectile nuclearity and energy and completely dominates the spectra under 10-keV Au3 bombardment. Analysis shows that this contribution cannot be explained in terms of thermal spike sputtering models. Instead, the results indicate a spike emission mechanism, which closely resembles a free expansion of a supercritically heated subsurface volume.

Millisecond radio spikes in the decimetre band and their related active solar phenomena

We present here a brief description of thirteen events of the narrowband solar millisecond radio spike emissions observed between February 2000 and December 2001. The total observing time was 1990.4 h, collected during the 357 ob- serving days. The data were collected with the 15-m radio telescope and fast radiospectrograph of Toru´ n Observatory, Poland. The dynamic spectra of the spikes were recorded in the 1352−1490 MHz frequency band split into 46 frequency channels with temporal resolution equals to 12 500 measurements per second per channel. The presented observations have probably the highest time resolution ever published. Using X-ray, UV and ground based observations we have analysed the main properties of the active phenomena correlated in time with the observed spikes. We found that probably all spikes are emitted as a result of some processes related to the solar flares. The spikes observed during the solar flares were emitted from events of various morphologies, various scenarios of evolution and various configurations of the interacting magnetic fields. All other emissions of the spikes, not correlated in time with solar flares, were observed during the slow increases of the X-ray flux leading to the flares. On the basis of the GOES database we have estimated that merely 2% of the solar flares are associated with the radio spikes.

A New Catalogue of Fine Structures Superimposed on Solar Microwave Bursts

The 2.6–3.8 GHz, 4.5–7.5 GHz, 5.2–7.6 GHz and 0.7–1.5 GHz component spectrometers of Solar Broadband Radio Spectrometer (SBRS) started routine observations, respectively, in late August 1996, August 1999, August 1999, and June 2000. They just managed to catch the coming 23rd solar active maximum. Consequently, a large amount of microwave burst data with high temporal and high spectral resolution and high sensitivity were obtained. A variety of fine structures (FS) superimposed on microwave bursts have been found. Some of them are known, such as microwave type III bursts, microwave spike emission, but these were observed with more detail; some are new. Reported for the first time here are microwave type U bursts with similar spectral morphology to those in decimetric and metric wavelengths, and with outstanding characteristics such as very short durations (tens to hundreds ms), narrow bandwidths, higher frequency drift rates and higher degrees of polarization. Type N and type M bursts were also observed. Detailed zebra pattern and fiber bursts at the high frequency were found. Drifting pulsation structure (DPS) phenomena closely associated with CME are considered to manifest the initial phase of the CME, and quasi-periodic pulsation with periods of tens ms have been recorded. Microwave ``patches", unlike those reported previously, were observed with very short durations (about 300 ms), very high flux densities (up to 1000 sfu), very high polarization (about 100% RCP), extremely narrow bandwidths (about 5%), and very high spectral indexes. These cannot be interpreted with the gyrosynchrotron process. A superfine structure in the form of microwave FS (ZPS, type U), consisting of microwave millisecond spike emission (MMS), was also found.

Decimetric Spike Bursts versus Microwave Continuum

We analyze properties of decimetric spike bursts occurring simultaneously with microwave gyrosynchrotron continuum bursts. We found that all of the accompanying microwave bursts were highly polarized in the optically thin range. The sense of polarization of the spike clusters is typically the same as that of the optically thin gyrosynchrotron emission, implying preferential extraordinary wave-mode spike polarization. Optically thick spectral indices of the continuum in spike-producing events were not observed to be larger than 2.5, suggesting low or absent Razin suppression. This implies that the plasma frequency-to-gyrofrequency ratio is systematically lower in the spike-producing bursts than in other bursts. The spike cluster flux density is found to be tightly correlated with the high-frequency spectral index of the microwave continuum for each event, while the flux-to-flux correlation may not be present. We discovered strong evidence that the trapped fast electrons producing the microwave gyrosynchrotron continuum have an anisotropic pitch-angle distribution of the loss cone type in the spike-producing bursts. The spike clusters are mainly generated when the trapped electrons have the hardest and the most anisotropic distributions. The new properties are discussed against the currently available ideas about emission processes and models for spike generation. We conclude that the findings strongly support the electron cyclotron maser mechanism of spike emission, with characteristics agreeing with expectations from the local-trap model.

An information theoretic approach to the contributions of the firing rates and the correlations between the firing of neurons.

To analyze the extent to which populations of neurons encode information in the numbers of spikes each neuron emits or in the relative time of firing of the different neurons that might reflect synchronization, we developed and analyzed the performance of an information theoretic approach. The formula quantifies the corrections to the instantaneous information rate that result from correlations in spike emission between pairs of neurons. We showed how these cross-cell terms can be separated from the correlations that occur between the spikes emitted by each neuron, the auto-cell terms in the information rate expansion. We also described a method to test whether the estimate of the amount of information contributed by stimulus-dependent synchronization is significant. With simulated data, we show that the approach can separate information arising from the number of spikes emitted by each neuron from the redundancy that can arise if neurons have common inputs and from the synergy that can arise if cells have stimulus-dependent synchronization. The usefulness of the approach is also demonstrated by showing how it helps to interpret the encoding shown by neurons in the primate inferior temporal visual cortex. When applied to a sample dataset of simultaneously recorded inferior temporal cortex neurons, the algorithm showed that most of the information is available in the number of spikes emitted by each cell; that there is typically just a small degree (approximately 12%) of redundancy between simultaneously recorded inferior temporal cortex (IT) neurons; and that there is very little gain of information that arises from stimulus-dependent synchronization effects in these neurons.

A type IV radio burst associated with type III burst, pulsation and spike emissions

A type IV radio burst accompanied by several normal- and reverse-drifting type III bursts, multiple long-term quasi-periodic pulsations and spikes was observed with the radio spectrometers (1.0-2.0 and 2.6-3.8 GHz) at National Astronomical Observatories of China (NAOC) on 23 September 1998. In combination with the images of Siberian Solar Radio Telescope (SSRT) of Russia, the complex and multiple magnetic structures inferred from the radio bursts reveal the existence of both large-scale and small-scale magnetic structures. This event suggests that the geometries of coronal magnetic fields contain multiple discrete electron acceleration/injection sites at different heights, and extended open and closed magnetic field lines. It can be shown that the energetic electrons gain access to open, diverging and closed field lines thus producing different types of radio bursts. From the characteristics of position, polarization, dispersion and displacement of the sources, the model of the type IV event is supported, which involves synchrotron emission from the electrons confined by the rapid scattering through the interaction of hydromagnetic wave with particles.

From subthreshold to firing-rate resonance.

Many types of neurons exhibit subthreshold resonance. However, little is known about whether this frequency preference influences spike emission. Here, the link between subthreshold resonance and firing rate is examined in the framework of conductance-based models. A classification of the subthreshold properties of a general class of neurons is first provided. In particular, a class of neurons is identified in which the input impedance exhibits a suppression at a nonzero low frequency as well as a peak at higher frequency. The analysis is then extended to the effect of subthreshold resonance on the dynamics of the firing rate. The considered input current comprises a background noise term, mimicking the massive synaptic bombardment in vivo. Of interest is the modulatory effect an additional weak oscillating current has on the instantaneous firing rate. When the noise is weak and firing regular, the frequency most preferentially modulated is the firing rate itself. Conversely, when the noise is strong and firing irregular, the modulation is strongest at the subthreshold resonance frequency. These results are demonstrated for two specific conductance-based models and for a generalization of the integrate-and-fire model that captures subthreshold resonance. They suggest that resonant neurons are able to communicate their frequency preference to postsynaptic targets when the level of noise is comparable to that prevailing in vivo.

Population dynamics of interacting spiking neurons.

A dynamical equation is derived for the spike emission rate nu(t) of a homogeneous network of integrate-and-fire (IF) neurons in a mean-field theoretical framework, where the activity of the single cell depends both on the mean afferent current (the "field") and on its fluctuations. Finite-size effects are taken into account, by a stochastic extension of the dynamical equation for the nu; their effect on the collective activity is studied in detail. Conditions for the local stability of the collective activity are shown to be naturally and simply expressed in terms of (the slope of) the single neuron, static, current-to-rate transfer function. In the framework of the local analysis, we studied the spectral properties of the time-dependent collective activity of the finite network in an asynchronous state; finite-size fluctuations act as an ongoing self-stimulation, which probes the spectral structure of the system on a wide frequency range. The power spectrum of nu exhibits modes ranging from very high frequency (depending on spike transmission delays), which are responsible for instability, to oscillations at a few Hz, direct expression of the diffusion process describing the population dynamics. The latter "diffusion" slow modes do not contribute to the stability conditions. Their characteristic times govern the transient response of the network; these reaction times also exhibit a simple dependence on the slope of the neuron transfer function. We speculate on the possible relevance of our results for the change in the characteristic response time of a neural population during the learning process which shapes the synaptic couplings, thereby affecting the slope of the transfer function. There is remarkable agreement of the theoretical predictions with simulations of a network of IF neurons with a constant leakage term for the membrane potential.

A pair of solar spike emissions

Using the 2.6-3.8 GHz solar radio spectrometer of the National Astronomical Observatories of China (NAOC), a pair of microwave millisecond spike (MMS) emissions were observed, and their frequency drift rate was measured. The separatrix frequency of the MMS pair was at 2900 MHz. Its emission layer was about 2x10(4) km above the photosphere. The polarization degree was wave-like variation with an average value of about 25% in LCP. An MMS pair differs greatly from the type III bursts pair. For the latter, in a certain frequency range, there is no emission around separatrix frequency. This phenomenon may help better understand the mechanism of MMS.

Location of narrowband spikes in solar flares

Narrowband spikes of the decimeter type have been identified in dynamic spectrograms of Phoenix-2 of ETH Zurich and located in position with the Nancay Radioheliograph at the same frequency. The spike positions have been compared with the location of hard X-ray emission and the thermal flare plasma in soft X-rays and EUV lines. The decimetric spikes are found to be single sources located some 20´´ to 400´´ away from the flare site in hard or soft X-rays. In most cases there is no bright footpoint nearby. In at least two cases the spikes are near loop tops. These observations do not confirm the widely held view that the spike emission is produced by some loss-cone instability masering near the footpoints of flare loops. On the other hand, the large distance to the flare sites and the fact that these spikes are all observed in the flare decay phase make the analyzed spike sources questionable sites for the main flare electron acceleration. They possibly indicate coronal post-flare acceleration sites.

The Giant Flare of 1998 August 27 from SGR 1900+14. II. Radiative Mechanism and Physical Constraints on the Source

The extraordinary 1998 August 27 giant flare places strong constraints on the physical properties of its source, SGR 1900+14. We make detailed comparisons of the published data with the magnetar model, which identifies the soft gamma repeaters as neutron stars endowed with ~1015 G magnetic fields. The giant flare evolved through three stages, whose radiative mechanisms we address in turn. The extreme peak luminosity L > 106LEdd, hard spectrum, and rapid variability of the initial ~0.5 s spike emission all point to an expanding pair fireball with very low baryon contamination. We argue that this energy must have been deposited directly through shearing and reconnection of a magnetar-strength external magnetic field. Low-order torsional oscillations of the star fail to transmit energy rapidly enough to the exterior, if the surface field is much weaker. A triggering mechanism is proposed, whereby a helical distortion of the core magnetic field induces large-scale fracturing in the crust and a twisting deformation of the crust and exterior magnetic field. After the initial spike (whose ~0.4 s duration can be related to the Alfven crossing time of the core), very hot (T 1 MeV) plasma rich in electron-positron pairs remains confined close to the star on closed magnetic field lines. The envelope of the August 27 flare can be accurately fitted, after ~40 s, by the contracting surface of such a fireball. The form of this fit gives evidence that the temperature of the trapped pair plasma decreases outward from its center. We quantify the effects of direct neutrino pair emission on the X-ray light curve, thereby deducing a lower bound μmin ~ 1 × 1032 G cm3 to the magnetic moment of the confining field, comparable to the strongest fields measured in radio pulsars. The radiative flux during the intermediate ~40 s of the burst appears to exceed the trapped fireball fit. The lack of strong rotational modulation and intermediate hardness of this smooth tail are consistent with the emission from an extended pair corona, in which O-mode photons are heated by Compton scattering. This feature could represent residual seismic activity within the star and accounts for ~10% of the total flare fluence. We consider in detail the critical luminosity, below which a stable balance can be maintained between heating and radiative cooling in a confined, magnetized pair plasma, but above which the confined plasma runs away to a trapped fireball in LTE. The emergence of large-amplitude pulsations at ~40 s probably represents a transition to a pair-depleted photosphere whose main source of opacity is electrons (and ions) ablated from the heated neutron star surface. The best-fit temperature of the blackbody component of the spectrum equilibrates at a value that agrees well with the regulating effect of photon splitting. The remarkable four-peaked substructure within each 5.16 s pulse, as well as the corresponding collimation of the X-ray flux, has a simple explanation based on the strong inequality between the scattering cross sections of the two photon polarization modes. The width of each X-ray jet is directly related to the amount of matter advected outward by the high cross section ordinary mode.

New Class II Methanol Masers in W3(OH)

We report interferometric observations of nine class II methanol maser candidate lines toward W3(OH). Narrow maser emission spikes at vLSR = -43.1 km s-1 are present in three of the lines: 31-40 A+, 72-63 A+, and 72-63 A-. For all three lines the maser position is near the northern edge of the W3(OH) ultracompact H II region (maser emission is also seen near the southern edge in the 31-40 A+ line). For the remaining six lines there is no obvious counterpart to the narrow maser spike at -43.1 km s-1. Additional spatially extended emission is present in all nine lines over the range from -41 to -48 km s-1. By comparing our observed flux densities with an extensive set of model calculations, we infer physical characteristics of the maser region. In these calculations the methanol is excited by infrared radiation from warm dust, and this excited gas amplifies the free-free background emission from the ultracompact H II region. The gas forming the narrow maser spikes appears to have both high kinetic temperature, Tkin ≥ 110 K, and high density, n ≈ 107 cm-3. Low-temperature solutions are ruled out by the observed line ratios and low-density solutions by the unphysically large path length that would be required. The gas is rich in methanol (2NM = NA + NE 10-6N), and the methanol column density in the tangential direction for each symmetry species (divided by line width) is NM/ΔV ≈ 1012 cm-3 s. Somewhat lower values of n and NM/ΔV are also acceptable. The size of the region emitting the maser spike is of order 100 × 1000 AU. In most of the lines the broad emission from -41 to -48 km s-1 can also be attributed to weak maser action, produced in gas with similar physical conditions (high density and temperature). It differs from the narrow spike emission mainly through a beaming factor that can be interpreted as an elongation factor for clumps of maser gas. The combination of narrow and broad emission can arise naturally from an ensemble of clumps of different elongations and orientations. In this unified picture the best fit to the data is provided by n ≈ 2 × 106 cm-3 and NM/ΔV ≈ 4 × 1011 cm-3 s, somewhat lower than the values obtained for just the spike component. The methanol maser clumps may be present in an expanding shell surrounding the H II region, similar to the material producing OH maser emission in this source.

Neural code for sound localization at low frequencies

Several nuclei in the auditory pathway perform sound localization. At lower sound frequencies, they compute the interaural time difference. This difference is transduced by the dedicated neuronal circuit into a labeled line difference. The neurons of the circuit fire only when synaptic inputs from both ears arrive within a short time window. We have shown previously the minimum timing difference detectable in the linear model and in the Hodgkin–Huxley-type equations. To better describe the spike code for the sound localization we define a generalized vector strength for point processes. Based on calculations in single cell models, we show how the neural code is transformed depending on the main sound frequency. We conclude that the performance limits of the low-frequency system are set by the frequency of the sound and therefore by both the vector strength and the probability of spike emission in the single unit. This shows the necessity for the existence of two different mechanisms for sound localization (1) one based on the interaural time delay, operating at low sound frequencies, and (2) another based on the interaural intensity difference, operating at high frequencies. The two systems then converge in higher-order neural relay stations.

Spatial analysis of solar type III events associated with narrow band spikes at metric wavelengths

The spatial association of narrow band metric radio spikes with type III bursts is analyzed. The analysis addresses the question of a possible causal relation between the spike emission and the acceleration of the energetic electrons causing the type III burst. The spikes are identified by the Phoenix-2 spectrometer (ETH Zurich) from survey solar observations in the frequency range from 220 MHz to 530 MHz. Simultaneous spatial information was provided by the Nancay Radioheliograph (NRH) at several frequencies. Five events were selected showing spikes at one or two and type III bursts at two or more Nancay frequencies. The 3-dimensional geometry of the single events has been reconstructed by applying different coronal density models. As a working hypothesis it is assumed that emission at the plasma frequency or its harmonic is the responsible radiation process for the spikes as well as for the type III bursts. It has been found that the spike source location is consistent with the backward extrapolation of the trajectory of the type III bursts, tracing a magnetic field line. In one of the analyzed events, type III bursts with two different trajectories originating from the same spike source could be identified. These findings support the hypothesis that narrow band metric spikes are closely related to the acceleration region.

Oscillations and irregular emission in networks of linear spiking neurons.

The dynamics of a network of randomly connected inhibitory linear integrate and fire (LIF) neurons (with a floor for the depolarization), in the presence of stochastic external afferent input, is considered in various parameter regimes of the neurons and of the network. Applying a technique recently introduced by Brunel and Hakim, we classify the regimes in which such a network has stable stationary states and in which spike emission rates oscillate. In the vicinity of the bifurcation line, the oscillation frequency and its amplitude are computed and compared with simulations. As for leaky IF neurons, the space of parameters can be compacted into two. Yet despite significant technical differences between the two models, related to both the different dynamics of the depolarization as well as to the different boundary conditions, the qualitative behavior is rather similar. The significance of LIF neurons and of the differences with leaky IF neurons is discussed.

Ion-sound model of microwave spikes with fast shocks in the reconnection region

A new model for solar spike bursts is considered based on the interaction of Langmuir waves with ion-sound waves: l+s→t. Such a mechanism can operate in shock fronts, propagating from a magnetic reconnection region. New observations of microwave millisecond spikes are discussed. They have been observed in two events: 4 November 1997 between 05:52–06:10 UT and 28 November 1997 between 05:00–05:10 UT using the multichannel spectrograph in the range 2.6–3.8 GHz of Beijing AO. Yohkoh/SXT images in the AR and SOHO EIT images testify to a reconstruction of bright loops after the escape of a CME. A fast shock front might be manifested as a very bright line in T e SXT maps (up to 20 MK) above dense structures in emission measure (EM) maps. Moreover one can see at the moment of spike emission (for the 28 November 1997 event) an additional maximum at the loop top on the HXR map in the AR as principal evidence of fast shock propagation. The model gives the ordinary mode of spike emission. Sometimes we observed a different polarization of microwave spikes that might be connected with the depolarization of the emission in the transverse magnetic field and rather in the vanishing magnetic field in the middle of the QT region. Duration and frequency band of isolated spikes are connected with parameters of fast particle beams and shock front. Millisecond microwave spikes are probably a unique manifestation of flare fast shocks in the radio emission.

Polarization observations of solar microwave spike emission at high temporal and spectral resolutions

11 microwave spike events were observed with the 2.6–3.8 GHz solar radio spectrometer of Beijing Astronomical Observatory. The spikes have short durations (tens of ms) and weak flux densities (<500 sfu), similar to the previous reports. Their degrees of polarization span over a large range, some spikes have frequency drifts as high as several GHz/s. Time delays of 8 ms or more are found between the two polarizations of some spikes, and drastic variations in thedegree of polarization with the frequency, even reversal of the polarizations, are found.

The Generating Region of Bidirectional Electron Beams in the Corona

Metric and decimetric type III bursts and microwave spike emissions with negative and positive frequency drift rates which were observed with radio spectrometers at Yunnan and Beijing Observatories are presented. The frequencies and heights at which the bidirectional electron beams originated are estimated. Three events reveal a separatrix frequency (at 250, 1300, and 2900 MHz) between normal- and reverse-drifting radio bursts, indicating a compact acceleration source where electron beams are injected in both upward and downward directions. These cases may indicate that the changeover frequencies of bidirectional electron beams are within a large band from 250 to 2900 MHz and the frequency bands of separatrices are in very small (4 to 100 MHz) and different bands. These type III bursts appear to be a plasma emission phenomenon from a beam of electrons which seem to have widely separated acceleration regions from the high to the low corona. These cases suggest that current sheets that separate open and closed magnetic fluxes in the low corona, and oppositely directed open field lines in the high corona are possible sites for bidirectional electron acceleration. The regions of magnetic topology from closed to open magnetic field structures should be very large (from about 20 000 to 107 000 km above the photosphere).

Collision-induced intersystem crossing from NH(a 1Δ,b 1Σ+) to NH(A 3Π): Gateway-mediated and direct mechanisms

Metastable NH* radicals in a molecular beam, generated in a discharge, were allowed to collide with target particles (He through Xe rare gas atoms, and H2, CO, N2, NO, O2) in a cell or a crossed jet. Optical emission was observed issuing from the collision zone (and in the case of the jet also from different points along the primary beam). Spectral analysis (∼0.13 nm FWHM resolution) revealed two components; (a) a pair of sharp P, R lines (“spikes,” originating from the (perturbed) level NH(A 3Π, v=2, J=5, F3, Λ-component “e”; (b) broad NH(A 3Π→X 3∑−) emission in the (0, 0), (1, 1), and (2, 2) bands. Component (a) was shown to be due to a gateway coupling with the (perturbed) level NH(b 1∑+, v=5, J=5). From the collision gas pressure dependence of the “spike” intensity, relative cross sections were derived. They varied by less than a factor of 3 between He and NO. Weak spike emission was also observed issuing from the NH* beam without collisions. From the exponential decay of this “afterglow” intensity along 20 cm of the beam, the lifetime of the long-lived gateway emission component was found to be 52 μs, with a beam speed of ∼1220 m/s (measured using a chopper wheel and a particle multiplier detector). There is also a fast gateway component, having a (calculated) lifetime of ∼0.21 μs. It is too close (∼1 cm−1) to the slow component to be spectrally resolved and is, moreover, much weaker. The calculated branching ratio of the fast and the slow component is 1:247. Experimentally an upper limit of 1:20 was derived from simulations of the observed emission intensity profile downstream from the beam/jet crossing point. It is pointed out that only the weak, fast component of the “spike” intensity should properly be termed “gateway” emission, while the dominant, slow component is better described as being due to an “emission window” at a particular level of the otherwise dark NH(b) state. The broadband component (b) of the NH(A–X) emission is due to direct spin-changing energy transfer from (mainly) NH(a 1Δ) to NH(A 3Π). Surprisingly all target gases except He were effective, although the relative cross sections varied here by a factor of 120 between Ne and NO. NH(a) was identified as the dominant reactant species from the different beam attenuation in the target cell, compared to that of NH(b) (as measured using the spike attenuation). The contours of the intense NH(A–X) bands observed with Xe, O2, and NO were computer-simulated, yielding high rotational “temperatures” and, with O2, a striking excess population of the “f” Λ component (e:f=1:5).

Correlations and the encoding of information in the nervous system.

Is the information transmitted by an ensemble of neurons determined solely by the number of spikes fired by each cell, or do correlations in the emission of action potentials also play a significant role? We derive a simple formula which enables this question to be answered rigorously for short time-scales. The formula quantifies the corrections to the instantaneous information rate which result from correlations in spike emission between pairs of neurons. The mutual information that the ensemble of neurons conveys about external stimuli can thus be broken down into firing rate and correlation components. This analysis provides fundamental constraints upon the nature of information coding, showing that over short time-scales correlations cannot dominate information representation, that stimulus-independent correlations may lead to synergy (where the neurons together convey more information than they would if they were considered independently), but that only certain combinations of the different sources of correlation result in significant synergy rather than in redundancy or in negligible effects. This analysis leads to a new quantification procedure which is directly applicable to simultaneous multiple neuron recordings.