108 research outputs found
Collisionless Plasma Shocks in Striated Electron Temperatures
The existence of low frequency waveguide modes of ion acoustic waves is demonstrated in magnetized plasmas for electron temperatures striated along the magnetic field lines. At higher frequencies, in a band between the ion cyclotron and the ion plasma frequency, radiative modes develop and propagate obliquely to the field away from the striation. Arguments for the subsequent formation and propagation of electrostatic shock are presented and demonstrated numerically. For such plasma conditions, the dissipation mechanism is the "leakage'' of the harmonics generated by the wave steepening
Phase space structures generated by an absorbing obstacle in a streaming plasma
The dynamic behavior of an ion flow around an obstacle in a collisionless plasma is investigated. The obstacle consists here of an absorbing cylinder, and a 2 dimensional electrostatic particle-in-cell simulation is used to study the flow characteristics. The formation of irregular filamented density depletions, oblique to the flow, is observed. The dynamics of these structures depend on the physical parameters of the plasma. The structures form at the edges of the wake behind the obstacle, in a region with a strong velocity shear, and are found to be associated with phase-space vortices, observed specially in the velocity direction perpendicular to the flow. The results can be of interest in the interpretation of structures in space plasmas as observed by instrumented space crafts
Weakly nonlinear ion sound waves in gravitational systems
Ion sound waves are studied in a plasma subject to gravitational field. Such
systems are interesting by exhibiting a wave growth that is a result of energy
flux conservation in inhomogeneous systems. The increasing wave amplitude gives
rise to an enhanced interaction between waves and plasma particles that can be
modeled by a modified Korteweg-de Vries equation. Analytical results are
compared with numerical Particle-in-Cell simulations of the problem. Our code
assumes isothermally Boltzmann distributed electrons while the ion component is
treated as a collection of individual particles interacting through collective
electric fields. Deviations from quasi neutrality are allowed for.Comment: 30 pages, 9 figure
Convergence of statistical moments of particle density time series in scrape-off layer plasmas
Particle density fluctuations in the scrape-off layer of magnetically
confined plasmas, as measured by gas-puff imaging or Langmuir probes, are
modeled as the realization of a stochastic process in which a superposition of
pulses with a fixed shape, an exponential distribution of waiting times and
amplitudes represents the radial motion of blob-like structures. With an
analytic formulation of the process at hand, we derive expressions for the
mean-squared error on estimators of sample mean and sample variance as a
function of sample length, sampling frequency, and the parameters of the
stochastic process. % Employing that the probability distribution function of a
particularly relevant shot noise process is given by the gamma distribution, we
derive estimators for sample skewness and kurtosis, and expressions for the
mean-squared error on these estimators.
Numerically generated synthetic time series are used to verify the proposed
estimators, the sample length dependency of their mean-squared errors, and
their performance.
We find that estimators for sample skewness and kurtosis based on the gamma
distribution are more precise and more accurate than common estimators based on
the method of moments.Comment: 31 pages, 10 figure
Weakly nonlinear ion waves in striated electron temperatures
The existence of low-frequency waveguide modes of electrostatic ion acoustic waves is demonstrated in magnetized plasmas for cases where the electron temperature is striated along magnetic field lines. For low frequencies, the temperature striation acts as waveguide that supports a trapped mode. For conditions where the ion cyclotron frequency is below the ion plasma frequency we find a dispersion relation having also a radiative frequency band, where waves can escape from the striation. Arguments for the formation and propagation of an equivalent of electrostatic shocks are presented and demonstrated numerically for these conditions. The shock represents here a balance between an external energy input maintained by ion injection and a dissipation mechanism in the form of energy leakage of the harmonics generated by nonlinear wave steepening. This is a reversible form for energy loss that can replace the time-irreversible losses in a standard Burgers equation
Power Spectra of the Total Occupancy in the Totally Asymmetric Simple Exclusion Process
As a solvable and broadly applicable model system, the totally asymmetric
exclusion process enjoys iconic status in the theory of non-equilibrium phase
transitions. Here, we focus on the time dependence of the total number of
particles on a 1-dimensional open lattice, and its power spectrum. Using both
Monte Carlo simulations and analytic methods, we explore its behavior in
different characteristic regimes. In the maximal current phase and on the
coexistence line (between high/low density phases), the power spectrum displays
algebraic decay, with exponents -1.62 and -2.00, respectively. Deep within the
high/low density phases, we find pronounced \emph{oscillations}, which damp
into power laws. This behavior can be understood in terms of driven biased
diffusion with conserved noise in the bulk.Comment: 4 pages, 4 figure
Scaling Analysis and Evolution Equation of the North Atlantic Oscillation Index Fluctuations
The North Atlantic Oscillation (NAO) monthly index is studied from 1825 till
2002 in order to identify the scaling ranges of its fluctuations upon different
delay times and to find out whether or not it can be regarded as a Markov
process. A Hurst rescaled range analysis and a detrended fluctuation analysis
both indicate the existence of weakly persistent long range time correlations
for the whole scaling range and time span hereby studied. Such correlations are
similar to Brownian fluctuations. The Fokker-Planck equation is derived and
Kramers-Moyal coefficients estimated from the data. They are interpreted in
terms of a drift and a diffusion coefficient as in fluid mechanics. All partial
distribution functions of the NAO monthly index fluctuations have a form close
to a Gaussian, for all time lags, in agreement with the findings of the scaling
analyses. This indicates the lack of predictive power of the present NAO
monthly index. Yet there are some deviations for large (and thus rare) events.
Whence suggestions for other measurements are made if some improved
predictability of the weather/climate in the North Atlantic is of interest. The
subsequent Langevin equation of the NAO signal fluctuations is explicitly
written in terms of the diffusion and drift parameters, and a characteristic
time scale for these is given in appendix.Comment: 6 figures, 54 refs., 16 pages; submitted to Int. J. Mod. Phys. C:
Comput. Phy
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