228 research outputs found
Nonextensive entropy approach to space plasma fluctuations and turbulence
Spatial intermittency in fully developed turbulence is an established feature
of astrophysical plasma fluctuations and in particular apparent in the
interplanetary medium by in situ observations. In this situation the classical
Boltzmann-Gibbs extensive thermo-statistics, applicable when microscopic
interactions and memory are short ranged, fails. Upon generalization of the
entropy function to nonextensivity, accounting for long-range interactions and
thus for correlations in the system, it is demonstrated that the corresponding
probability distributions (PDFs) are members of a family of specific power-law
distributions. In particular, the resulting theoretical bi-kappa functional
reproduces accurately the observed global leptokurtic, non-Gaussian shape of
the increment PDFs of characteristic solar wind variables on all scales.
Gradual decoupling is obtained by enhancing the spatial separation scale
corresponding to increasing kappa-values in case of slow solar wind conditions
where a Gaussian is approached in the limit of large scales. Contrary, the
scaling properties in the high speed solar wind are predominantly governed by
the mean energy or variance of the distribution. The PDFs of solar wind scalar
field differences are computed from WIND and ACE data for different time-lags
and bulk speeds and analyzed within the nonextensive theory. Consequently,
nonlocality in fluctuations, related to both, turbulence and its large scale
driving, should be related to long-range interactions in the context of
nonextensive entropy generalization, providing fundamentally the physical
background of the observed scale dependence of fluctuations in intermittent
space plasmas.Comment: 21 pages, 8 figures, accepted for publication, to appear in Advances
in Geosciences 2, chapter 04, 2006 (with minor corrections
Dark matter density profiles: A comparison of nonextensive theory with N-body simulations
Density profiles of simulated galaxy cluster-sized dark matter haloes are
analysed in the context of a recently introduced nonextensive theory of dark
matter and gas density distributions. Nonextensive statistics accounts for
long-range interactions in gravitationally coupled systems and is derived from
the fundamental concept of entropy generalisation. The simulated profiles are
determined down to radii of ~1% of R_200. The general trend of the relaxed,
spherically averaged profiles is accurately reproduced by the theory. For the
main free parameter kappa, measuring the degree of coupling within the system,
and linked to physical quantities as the heat capacity and the polytropic index
of the self-gravitating ensembles, we find a value of -15. The significant
advantage over empirical fitting functions is provided by the physical content
of the nonextensive approach.Comment: 6 pages, 3 figures, accepted for publication in A&
Is current disruption associated with an inverse cascade?
Current disruption (CD) and the related kinetic instabilities in the
near-Earth magnetosphere represent physical mechanisms which can trigger
multi-scale substorm activity including global reorganizations of the
magnetosphere. Lui et al. (2008) proposed a CD scenario in which the kinetic
scale linear modes grow and reach the typical dipolarization scales through an
inverse cascade. The experimental verification of the inverse nonlinear cascade
is based on wavelet analysis. In this paper the Hilbert-Huang transform is used
which is suitable for nonlinear systems and allows to reconstruct the
time-frequency representation of empirical decomposed modes in an adaptive
manner. It was found that, in the Lui et al. (2008) event, the modes evolve
globally from high-frequencies to low-frequencies. However, there are also
local frequency evolution trends oriented towards high-frequencies, indicating
that the underlying processes involve multi-scale physics and non-stationary
fluctuations for which the simple inverse cascade scenario is not correct.Comment: 6 pages, 4 figure
A universal mirror wave-mode threshold condition for non-thermal space plasma environments
Magnetic fluctuations are recognized in a large variety of space plasmas by increasingly high resolution, in situ observations as mirror wave mode structures. A typical requirement for the excitation of mirror modes is a dominant perpendicular pressure in a high-beta plasma environment. Contrary, we demonstrate from a realistic kinetic analysis how details of the velocity space distributions are of considerable significance for the instability threshold. Introducing the most common characteristics of observed ion and electron distributions by a mixed suprathermal-loss-cone, we derive a universal mirror instability criterion from an energy principle for collisionless plasmas. As a result, the transition from two temperature Maxwellians to realistic non-thermal features provides a strong source for the generation of mirror wave mode activity, reducing drastically the instability threshold. In particular, a number of space-related examples illuminate how the specific structure of the velocity space distribution dominates as a regulating excitation mechanism over the effects related to changes in the plasma parameters
A nonextensive entropy path to probability distributions in solar wind turbulence
International audienceThe observed scale dependence of the probability distributions of the differences of characteristic solar wind variables is analyzed. Intermittency of the turbulent fluctuations at small-scale spatial separations is accompanied by strongly non-Gaussian distributions that turn into a normal distribution for large-scale separation. Conventional theoretical models are subject to insufficient physical justification since nonlocality in turbulence should be based on long-range interactions, provided recently by the bi-kappa distribution in the context of nonextensive thermo-statistics. Observed WIND and ACE probability distributions are accurately reproduced for different time lags by the one-parameter bi-kappa functional, a core-halo convolution, where kappa measures the degree of nonlocality or nonextensivity in the system. Gradual decoupling is obtained by enhancing the spatial separation scale corresponding to increasing kappa values, where a Gaussian is approached for infinite kappa. Consequently, long-range interactions introduced on the fundamental level of entropy generalization, are able to provide physically the source of the observed scale dependence of the turbulent fluctuations in the intermittent interplanetary medium
High-resolution kinetic energy release distributions and dissociation energies for fullerene ions C(n)(+), 42 \u3c= n \u3c= 90
We have measured the kinetic energy released in the unimolecular dissociation of fullerene ions, C(n)(+)--\u3eC(n-2)(+)+C(2), for sizes 42less than or equal tonless than or equal to90. A three-sector-field mass spectrometer equipped with two electric sectors has been used in order to ensure that contributions from isotopomers of different masses do not distort the experimental kinetic energy release distributions. We apply the concept of microcanonical temperature to derive from these data the dissociation energies of fullerene cations. They are converted to dissociation energies of neutral fullerenes with help of published adiabatic ionization energies. The results are compared with literature values. (C) 2004 American Institute of Physics
- …