1,660 research outputs found
Dissipative Dynamics of Collisionless Nonlinear Alfven Wave Trains
The nonlinear dynamics of collisionless Alfven trains, including resonant
particle effects is studied using the kinetic nonlinear Schroedinger (KNLS)
equation model. Numerical solutions of the KNLS reveal the dynamics of Alfven
waves to be sensitive to the sense of polarization as well as the angle of
propagation with respect to the ambient magnetic field. The combined effects of
both wave nonlinearity and Landau damping result in the evolutionary formation
of stationaryOA S- and arc-polarized directional and rotational
discontinuities. These waveforms are freqently observed in the interplanetary
plasma.Comment: REVTeX, 6 pages (including 5 figures). This and other papers may be
found at http://sdphpd.ucsd.edu/~medvedev/papers.htm
Noncommutative Field Theories and (Super)String Field Theories
In this lecture notes we explain and discuss some ideas concerning
noncommutative geometry in general, as well as noncommutative field theories
and string field theories. We consider noncommutative quantum field theories
emphasizing an issue of their renormalizability and the UV/IR mixing. Sen's
conjectures on open string tachyon condensation and their application to the
D-brane physics have led to wide investigations of the covariant string field
theory proposed by Witten about 15 years ago. We review main ingredients of
cubic (super)string field theories using various formulations: functional,
operator, conformal and the half string formalisms. The main technical tools
that are used to study conjectured D-brane decay into closed string vacuum
through the tachyon condensation are presented. We describe also methods which
are used to study the cubic open string field theory around the tachyon vacuum:
construction of the sliver state, ``comma'' and matrix representations of
vertices.Comment: 160 pages, LaTeX, 29 EPS figures. Lectures given by I.Ya.Aref'eva at
the Swieca Summer School, Brazil, January 2001; Summer School in Modern
Mathematical Physics, Sokobanja, Yugoslavia, August 2001; Max Born Symposium,
Karpacz, Poland, September, 2001; Workshop "Noncommutative Geometry, Strings
and Renormalization", Leipzig, Germany, September 2001. Typos corrected,
references adde
Weibel instability and associated strong fields in a fully 3D simulation of a relativistic shock
Plasma instabilities (e.g., Buneman, Weibel and other two-stream
instabilities) excited in collisionless shocks are responsible for particle
(electron, positron, and ion) acceleration. Using a new 3-D relativistic
particle-in-cell code, we have investigated the particle acceleration and shock
structure associated with an unmagnetized relativistic electron-positron jet
propagating into an unmagnetized electron-positron plasma. The simulation has
been performed using a long simulation system in order to study the nonlinear
stages of the Weibel instability, the particle acceleration mechanism, and the
shock structure. Cold jet electrons are thermalized and slowed while the
ambient electrons are swept up to create a partially developed hydrodynamic
(HD) like shock structure. In the leading shock, electron density increases by
a factor of 3.5 in the simulation frame. Strong electromagnetic fields are
generated in the trailing shock and provide an emission site. We discuss the
possible implication of our simulation results within the AGN and GRB context.Comment: 4 pages, 3 figures, ApJ Letters, in pres
The geometry of spontaneous spiking in neuronal networks
The mathematical theory of pattern formation in electrically coupled networks
of excitable neurons forced by small noise is presented in this work. Using the
Freidlin-Wentzell large deviation theory for randomly perturbed dynamical
systems and the elements of the algebraic graph theory, we identify and analyze
the main regimes in the network dynamics in terms of the key control
parameters: excitability, coupling strength, and network topology. The analysis
reveals the geometry of spontaneous dynamics in electrically coupled network.
Specifically, we show that the location of the minima of a certain continuous
function on the surface of the unit n-cube encodes the most likely activity
patterns generated by the network. By studying how the minima of this function
evolve under the variation of the coupling strength, we describe the principal
transformations in the network dynamics. The minimization problem is also used
for the quantitative description of the main dynamical regimes and transitions
between them. In particular, for the weak and strong coupling regimes, we
present asymptotic formulas for the network activity rate as a function of the
coupling strength and the degree of the network. The variational analysis is
complemented by the stability analysis of the synchronous state in the strong
coupling regime. The stability estimates reveal the contribution of the network
connectivity and the properties of the cycle subspace associated with the graph
of the network to its synchronization properties. This work is motivated by the
experimental and modeling studies of the ensemble of neurons in the Locus
Coeruleus, a nucleus in the brainstem involved in the regulation of cognitive
performance and behavior
Towards a Simple Model of Compressible Alfvenic Turbulence
A simple model collisionless, dissipative, compressible MHD (Alfvenic)
turbulence in a magnetized system is investigated. In contrast to more familiar
paradigms of turbulence, dissipation arises from Landau damping, enters via
nonlinearity, and is distributed over all scales. The theory predicts that two
different regimes or phases of turbulence are possible, depending on the ratio
of steepening to damping coefficient (m_1/m_2). For strong damping
(|m_1/m_2|<1), a regime of smooth, hydrodynamic turbulence is predicted. For
|m_1/m_2|>1, steady state turbulence does not exist in the hydrodynamic limit.
Rather, spikey, small scale structure is predicted.Comment: 6 pages, one figure, REVTeX; this version to be published in PRE. For
related papers, see http://sdphpd.ucsd.edu/~medvedev/papers.htm
Shaping bursting by electrical coupling and noise
Gap-junctional coupling is an important way of communication between neurons
and other excitable cells. Strong electrical coupling synchronizes activity
across cell ensembles. Surprisingly, in the presence of noise synchronous
oscillations generated by an electrically coupled network may differ
qualitatively from the oscillations produced by uncoupled individual cells
forming the network. A prominent example of such behavior is the synchronized
bursting in islets of Langerhans formed by pancreatic \beta-cells, which in
isolation are known to exhibit irregular spiking. At the heart of this
intriguing phenomenon lies denoising, a remarkable ability of electrical
coupling to diminish the effects of noise acting on individual cells.
In this paper, we derive quantitative estimates characterizing denoising in
electrically coupled networks of conductance-based models of square wave
bursting cells. Our analysis reveals the interplay of the intrinsic properties
of the individual cells and network topology and their respective contributions
to this important effect. In particular, we show that networks on graphs with
large algebraic connectivity or small total effective resistance are better
equipped for implementing denoising. As a by-product of the analysis of
denoising, we analytically estimate the rate with which trajectories converge
to the synchronization subspace and the stability of the latter to random
perturbations. These estimates reveal the role of the network topology in
synchronization. The analysis is complemented by numerical simulations of
electrically coupled conductance-based networks. Taken together, these results
explain the mechanisms underlying synchronization and denoising in an important
class of biological models
Magnetic Field Generation in Core-Sheath Jets via the Kinetic Kelvin-Helmholtz Instability
We have investigated magnetic field generation in velocity shears via the
kinetic Kelvin-Helmholtz instability (kKHI) using a relativistic plasma jet
core and stationary plasma sheath. Our three-dimensional particle-in-cell
simulations consider plasma jet cores with Lorentz factors of 1.5, 5, and 15
for both electron-proton and electron-positron plasmas. For electron-proton
plasmas we find generation of strong large-scale DC currents and magnetic
fields which extend over the entire shear-surface and reach thicknesses of a
few tens of electron skin depths. For electron-positron plasmas we find
generation of alternating currents and magnetic fields. Jet and sheath plasmas
are accelerated across the shear surface in the strong magnetic fields
generated by the kKHI. The mixing of jet and sheath plasmas generates
transverse structure similar to that produced by the Weibel instability.Comment: 28 pages, 12 figures, in press, ApJ, September 10, 201
Radiation from relativistic jets
Nonthermal radiation observed from astrophysical systems containing
relativistic jets and shocks, e.g., gamma-ray bursts (GRBs), active galactic
nuclei (AGNs), and Galactic microquasar systems usually have power-law emission
spectra. Recent PIC simulations of relativistic electron-ion
(electron-positron) jets injected into a stationary medium show that particle
acceleration occurs within the downstream jet. In the presence of relativistic
jets, instabilities such as the Buneman instability, other two-streaming
instability, and the Weibel (filamentation) instability create collisionless
shocks, which are responsible for particle (electron, positron, and ion)
acceleration. The simulation results show that the Weibel instability is
responsible for generating and amplifying highly nonuniform, small-scale
magnetic fields. These magnetic fields contribute to the electron's transverse
deflection behind the jet head. The ``jitter'' radiation from deflected
electrons in small-scale magnetic fields has different properties than
synchrotron radiation which is calculated in a uniform magnetic field. This
jitter radiation, a case of diffusive synchrotron radiation, may be important
to understand the complex time evolution and/or spectral structure in gamma-ray
bursts, relativistic jets, and supernova remnants.Comment: 8 pages,3 figures, accepted for the Proceedings of Science of the
Workshop on Blazar Variability across the Electromagnetic Spectrum, April 22
to 25, 200
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