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1.
Retrieval of the quasi-optimal signal activating the excitable systems using preceding noise samples
We propose a method for obtaining a signal leading to the activation of an excitable dynamic system for a signal energy close
to minimal. The efficiency of this technique, which is based on recording and processing of noise samples preceding the activation
was tested using the FitzHugh–Nagumo, Hodgkin–Huxley, and Luo–Rudy models as examples. It is shown that the proposed procedure
gives good results when the noise intensity is smaller than or close to the system activation energy. The criteria of “low”
and “high” intensities of fluctuations are proposed. The method of increasing the stability of the excitable system with respect
to “low-intensity” noise by filtering or another way of suppression of the spectral components that make the main contribution
to the energetically optimal activation signal is justified. The relation between eigenvalues of the linearized system of
the Hamiltonian equations, which describe the optimal trajectories and the activation signal, and eigenvalues of the excitable
system linearized near the initial equilibroum state is found. 相似文献
2.
Some distinctive features in the behavior of small-scale artificial ionospheric irregularities at mid-and high latitudes 总被引:1,自引:0,他引:1
N. F. Blagoveshchenskaya T. D. Borisova V. A. Kornienko V. L. Frolov M. T. Rietveld A. Brekke 《Radiophysics and Quantum Electronics》2007,50(8):619-632
We present the results of experimental studies of some features in the behavior of small-scale artificial irregularities (SSAIs)
at mid-and high latitudes based on the “Sura” and EISCAT/HEATING HF facilities. Observations were performed by the method
of aspect scattering using a network of diagnostic paths having a common reception point located near St. Petersburg. We found
that an extremely long duration of the second (slow) stage of SSAI relaxation of up to 5 min occurs in the evening hours when
the ionosphere above the “Sura” facility is illuminated by the Sun, but the solar terminator travels through the magnetically
conjugated ionosphere. The conjecture is made that the processes initiated by the terminator are mostly responsible for secondary
ionospheric turbulence maintaining the irregularities above “Sura.” A drastic increase in the Doppler spectra width of the
scattered signals is revealed when the magnetically conjugate point of the ionosphere is located on the shade side of the
terminator, but the ionosphere above the “Sura” facility is still lighted. It is assumed that the “ run away” of photoelectrons
from the day to the night side could reduce the threshold of excitation of artificial irregularities, leading to an increase
in their intensity. The presence of fairly intense scattered signals was detected from the “Sura” and EISCAT/HEATING experimental
results both under conditions of pulsed HF heating after continuous heater-on periods and cycled HF heating by short pulses.
In the case of pulsed heating by short pulses with duration τp < 100 ms and average radiated power Pa below the threshold power Pthr of the SSAI generation cutoff the irregularities can be maintained due only to striction parametric instabilities. The excitation
of irregularites under the cycled HF pumping with the pulse duration τp = 384 ms for Pa comparable with Pthr was detected. The aspect-angle dependence, or the so-called magnetic zenith effect, was found in the SSAI intensity. The
residual turbulence aftereffects played a significant role in the SSAI development.
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Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 8, pp. 678–694, August 2007. 相似文献
3.
We report the results of theoretical investigations of tunneling current noise spectra in a wide range of applied bias voltage.
Localized states of individual impurity atoms play an important role in tunneling current noise formation. It was found that
switching “on” and “off” of Coulomb interaction of conduction electrons with two charged localized states results in power
law singularity of low-frequency tunneling current noise spectrum (1/f
α) and also results on high frequency component of tunneling current spectra (singular peaks appear).
The article is published in the original. 相似文献
4.
Yu. V. Tokarev M. L. Kaiser P. Rodrigues V. A. Alimov Yu. I. Belov G. N. Boiko G. P. Komrakov N. V. Muravyova A. V. Rakhlin 《Radiophysics and Quantum Electronics》1999,42(8):705-714
We present experimental results on reception of VHF signals of the transmitting facility “Sura” of the Radiophysical Research
Institute by the NASA spacecraft WIND. The experiments were performed during daytime during the summer seasons 1997–1998.
The dependence of power and spectral characteristics of the VHF radio waves on the power of the sounding radiation is analyzed.
We find that, in a wide range of effective powers of the VHF radiation from about 40 kW to 160 MW, the phenomena observed
in the radiation received onboard WIND (such as focusing, scintillations, or frequency deviations of signals) does not usually
depend on the operation mode of the “Sura” facility. At the same time, broadening of the radiation pattern of the facility
and decrease of the mean level of the received signal by about 6 dB toward the direction of maximum of the radiation pattern
were observed at the peak radiation power of the facility. The experimental results are compared to present concepts on effects
of self-focusing instability and nonlinear defocusing of VHF radio waves in the ionospheric plasma.
Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 8, pp. 799–809, August 1999. 相似文献
5.
M. Ozer L. J. Graham 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,61(4):499-503
In a Hodgkin-Huxley neuron model driven just above threshold, external noise can increase both jitter and latency of the first
spike, an effect called “noise delayed decay” (NDD). This phenomenon is important when considering how neuronal information
is represented, thus by the precise timing of spikes or by their rate. We examine how NDD can be affected by network activity
by varying the model's membrane time constant, τm. We show that NDD is significant for small τm or high network activity, and decreases for large τm, or low network activity. Our results suggest that for inputs just above threshold, the activity of the network constrains
the neuronal coding strategy due to, at least in part, the NDD effect. 相似文献
6.
Modes of the waveguide are selected from signals of individual receivers by time reversing them with the use of the frequency
response of a plane wave. The experimental distribution of the modes on the “arrival time — mode number” plane corresponds
to the model of an ideal waveguide, differing in that higher modes advance lower ones in the experiment. A modification of
the frequency response, which eliminates that contradiction, is proposed, the modification accounting for the dependence of
the effective thickness of the waveguide on the frequency. As a result, a method of determining the distance between the sound
source and receiver is proposed and tested, and the interpretation of the noise immunity of signal reception on the basis
of the time reversal is presented. The experimental data are obtained in the Barents Sea, at depths of about 100 m and distances
of 7, 10.5, and 12 km, with the signals in a band of 100–300 Hz. 相似文献
7.
Vladimir Kalitvianski 《Central European Journal of Physics》2009,7(1):1-11
We show that the electrostatic potential of an atomic nucleus “seen” by a fast charged projectile at short distances is quantum
mechanically smeared due to nucleus motion around the atomic center of inertia. For example, the size of the “positive charge
cloud” in the Hydrogen ground state is much larger than the proper proton size. For target atoms in excited initial states,
the effect is even larger. The elastic scattering at large angles is generally weaker than the Rutherford scattering since
the effective potential at short distances is softer than the Colombian one due to a natural “cutoff”. In addition, the large-angle
scattering leads to target atom excitations due to pushing the nucleus (⇒ inelastic processes). The Rutherford cross section
is in fact inclusive rather than elastic. These results are analogous to those from QED. Non-relativistic atomic calculations
are presented. The difference and the value of these calculations arise from nonperturbatively (exact) nucleus “dressing”
that immediately leads to correct physical results and to significant technical simplifications. In these respects a nucleus
bound in an atom is a simple but rather realistic model of a “dressed” charge in the QFT. This idea is briefly demonstrated
on a real electron model (electronium) which is free from infinities.
相似文献
8.
Rafael D. Sorkin 《International Journal of Theoretical Physics》1997,36(12):2759-2781
In seeking to arrive at a theory of “quantum gravity,” one faces several choices among alternative approaches. I list some
of these “forks in the road” and offer reasons for taking one alternative over the other. In particular, I advocate the following:
the sum-over-histories framework for quantum dynamics over the “observable and state-vector” framework; relative probabilities
over absolute ones; spacetime over space as the gravitational “substance” (4 over 3+1); a Lorentzian metric over a Riemannian
(“Euclidean”) one; a dynamical topology over an absolute one; degenerate metrics over closed timelike curves to mediate topology
change; “unimodular gravity” over the unrestricted functional integral; and taking a discrete underlying structure (the causal
set) rather than the differentiable manifold as the basis of the theory. In connection with these choices, I also mention
some results from unimodular quantum cosmology, sketch an account of the origin of black hole entropy, summarize an argument
that the quantum mechanical measurement scheme breaks down for quantum field theory, and offer a reason why the cosmological
constant of the present epoch might have a magnitude of around 10−120 in natural units.
This paper is the text of a talk given at the symposium on Directions in General Relativity held at the University of Maryland,
College Park, Maryland, in May 1993 in honor of Dieter Brill and Charles Minser. 相似文献
9.
10.
T. V. Kulikova A. B. Shubin V. A. Bykov K. Yu. Shunyaev 《Bulletin of the Russian Academy of Sciences: Physics》2010,74(8):1158-1159
The vapor pressure of the gas phase components over Al-Sc melts was studied. The temperatures and the variation of the enthalpy
upon “melt-gas” phase transformations were determined. The simulation was performed in an initial medium of argon at a total
pressure of 105 Pa and temperatures T = 1873−4000 K. 相似文献
11.
Thomas J. Mueller 《显形杂志》2005,8(2):187-191
The 11th International Symposium on Flow Visualization (ISFV) was held at Notre Dame, IN, USA, August 8–12, 2004. The Symposium attracted
236 participants from around the world. The 52 Technical Sessions and two Poster Sessions covered a wide range of topics as
indicated in the Keywords. Of the 182 submitted papers, 162 were presented. The presented papers included 8 invited lectures.
Each morning and afternoon began with an invited lecture by an outstanding, recognized leader in the field. Toshio Kobayashi
received the Leonardo da Vinci Award, an engraved plate, and presented the Leonardo da Vinci Memorial Lecture on “High-performance
Computing and Visualization of Unsteady Turbulent Flows.” Toshio Kobayashi is very well known for his outstanding contributions
in computational science and flow visualization as well as his leadership in organizing conferences, workshops, and symposia
on flow visualization. Ronald J. Adrian discussed “Visualization in Extreme Environments,” Rolf H. Engler described “Pressure-sensitive
Paints and Temperature-sensitive Paints in Quantitative Wind Tunnel Studies,” William K. Blake explained “Cavitation as Flow
Visualization Seeding,” Giovanni M. Carlomagno discussed “The Use of Colors in Thermo-fluid Dynamic Studies,” Ajit Yoganathan
presented “A Gallery of Cardiovascular Fluid Flow Fields: From Heart Valves to Congenital Heart Disease,” Richard B. Miles
described “Flow Visualization by Filtered Molecular and Particular Scattering,” and Thomas C. Gruber Jr., displayed a technique
for “Visualization of Foreign Gases in Atmospheric Air.” At the end of the last day, Jurgen Kompenhaus from DLR discussed
the 12th ISFV to be held in Germany in 2006. After this presentation there was a tour of the Hessert Laboratory for Aerospace
Research. 相似文献
12.
V. O. Rapoport V. L. Frolov G. P. Komrakov G. A. Markov A. S. Belov M. Parrot J. L. Rauch 《Radiophysics and Quantum Electronics》2007,50(8):645-656
We present the results of measuring the characteristics of electromagnetic and plasma disturbances at altitudes of about 700
km, obtained by using the onboard equipment of the French microsatellite DEMETER during its passage through the magnetic tube
resting upon the region of intense generation of artificial ionospheric turbulence created due to modification of the ionospheric
F2 region by high-power radio emission from the “Sura” facility. It is shown that an artificial density duct emerging from the
disturbed region and extending to the Earth’s magnetosphere can be formed.
__________
Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 8, pp. 709–721, August 2007. 相似文献
13.
O. P. Shustova N. N. Kalmykov A. V. Uryson 《Bulletin of the Russian Academy of Sciences: Physics》2011,75(3):313-315
The composition of cosmic rays as they propagate from their source is noticeably distorted due to the photodisintegration
of nuclei against the background cosmic radiation. We analyze the propagation of different nuclei groups of ultrahigh-energy
cosmic rays on the way from their source to an earth-based facility. Two types of sources are considered: “nearby” (within
an area having a radius of ≈40 Mpc) and “remote” (at distances of hundreds of Mpc) active galactic nuclei. We show that within
the accepted model, the composition of the particles incident on the facility depends on the source type. 相似文献
14.
Reconstruction of the Fermi surface of high-temperature superconducting cuprates in the pseudogap state is analyzed within
a nearly exactly solvable model of the pseudogap state, induced by short-range order fluctuations of the antiferromagnetic
(AFM), spin-density wave (SDW), or a similar charge-density wave (CDW) order parameter, competing with the superconductivity.
We explicitly demonstrate the evolution from “Fermi arcs” (on the “large” Fermi surface) observed in the ARPES experiments
at relatively high temperatures (when both the amplitude and phase of the density waves fluctuate randomly) towards the formation
of typical “small” electron and hole “pockets,” which are apparently observed in the de Haas-van Alphen and Hall resistance
oscillation experiments at low temperatures (when only the phase of the density waves fluctuate and the correlation length
of the short-range order is large enough). A qualitative criterion for the quantum oscillations in high magnetic fields to
be observable in the pseudogap state is formulated in terms of the cyclotron frequency, the correlation length of fluctuations,
and the Fermi velocity.
The text was submitted by the authors in English. 相似文献
15.
P. H. Chavanis 《The European Physical Journal B - Condensed Matter and Complex Systems》2011,80(3):275-306
We study the thermodynamics of the Hamiltonian mean field
(HMF) model with an external potential playing the role of a
“magnetic field”. If we consider only fully stable states,
the caloric curve does not present any phase
transition. However, if we take into account metastable states (for
a restricted class of perturbations), we find a very rich
phenomenology. In particular, the caloric curve
displays a region of negative specific heat in the microcanonical
ensemble in which the temperature decreases as the energy
increases. This leads to ensembles inequivalence and to zeroth
order phase transitions similar to the “gravothermal catastrophe”
and to the “isothermal collapse” of self-gravitating systems. In
the present case, they correspond to the reorganization of the
system from an “anti-aligned” phase (magnetization pointing in
the direction opposite to the magnetic field) to an “aligned”
phase (magnetization pointing in the same direction as the magnetic
field). We also find that the magnetic susceptibility can be
negative in the microcanonical ensemble so that the magnetization
decreases as the magnetic field increases. The magnetic curves can
take various shapes depending on the values of energy or temperature. We
describe first order phase
transitions and hysteretic cycles involving positive or negative
susceptibilities. We also show that this model exhibits gaps in the
magnetization at fixed energy, resulting in ergodicity breaking. 相似文献
16.
An investigation of the behaviour of amplitude and frequency noise of radiation of an active interferometer in the regime
of amplified optical bistability is presented. The phenomenon of non-amplification of external signal noise at sharp amplification
of the external signal is established. The impact of strong amplitude fluctuations is studied. Our results have shown that
the active interferometer allows an effective separation of the valid signal from the amplitude noise. The spectral density
of a fluctuation of the field frequency in the active interferometer is shown to fall sharply in comparison with the “quantum
limit”. The linewidth of radiation is determined by the spectral density at zero frequency and is equal to the natural linewidth
of the external signal. The influence of quantum noise of the amplifying and absorbing media of the active interferometer
is discussed.
Received: 26 January 1996 相似文献
17.
V. N. Buravtsev A. V. Nikolaev A. V. Ukrainets 《Moscow University Physics Bulletin》2009,64(4):442-445
The impact of platelet collisions on their distribution in a viscous liquid flow has been analyzed. It has been shown that
platelet distribution in the flow perpendicular to current lines resulting from their collisions can be described as “shear”
diffusion. In the wide physiological range of shear velocities specific for blood, the coefficient of “shear” diffusion is
much larger than the coefficient of “Brownian” diffusion. For a parabolic fluid velocity profile (Poiseuille flow) in cylindrical
vessel “shear” diffusion causes uneven radial distribution of platelets over the vessel radius. “Shear” diffusion causes platelet
concentration to grow from the wall toward the center (vessel axis). This effect appears to be a consequence of the specific
distribution of platelet collision frequency reaching its maximum at the vessel wall. 相似文献
18.
Susumu Kuwamura 《Optical Review》2006,13(3):129-137
We have developed a wavelet denoising (thresholding) method for a tomographically reconstructed image to which the conventional
wavelet methods are not necessarily applicable because of their limitation of applicable noise models. The basic idea of our
new method is that noise variance is, in general, spatially varying and the threshold must be adapted to it. Specifically,
our algorithm includes two key steps: The first is to estimate local variances in image space to produce a “σ-map”. The second
is to calculate the standard deviations of individual wavelet coefficients from the σ-map by a formula of “covariance propagation”.
Spatially adaptive thresholds are then given as those proportional to the standard deviations. Our method is applicable to
a wider range of noise models, and numerical experiments have shown that it can yield a denoised image with 10% less residual
error than that in the boxcar smoothing or the median filtering. 相似文献
19.
Yu. V. Tokarev 《Radiophysics and Quantum Electronics》2009,52(1):1-12
We derived analytical expressions for the Stokes parameters of the “Sura” facility radiation. Results of the first experiments
on studying the linearly polarized components of the facility radiation on oblique paths by the method of reception of signals
on remote spacecraft are presented. The dominant contribution of the plasmasphere into the rotation measure of the radio-wave
polarization plane in near-Earth space is noted.
Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 52, No. 1, pp. 1–13, January 2009. 相似文献
20.
L. A. Gribov 《Journal of Applied Spectroscopy》2006,73(3):323-327
The concept of the adiabatic approximation is introduced from a few new standpoints, and the conditions are refined under
which we can assume that the total energy of an electronic-vibrational (vibronic) state is the sum of the energies of the
“electronic” and the “nuclear” problems and the wave function is represented as the product of the corresponding functions.
An expression exactly corresponding to such an approximation is considered for the optical transition matrix element, and
its individual terms are analyzed for any change in the geometric structure of the molecule upon optical excitation.
__________
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 3, pp. 290–293, May–June, 2006. 相似文献