211 research outputs found
On the turbulent energy cascade in anisotropic magnetohydrodynamic turbulence
The problem of the occurrence of an energy cascade for Alfv\'enic turbulence
in solar wind plasmas was hystorically addressed by using phenomenological
arguments based to the weakness of nonlinear interactions and the anisotropy of
the cascade in wave vectors space. Here, this paradox is reviewed through the
formal derivation of a Yaglom relation from anisotropic Magnetohydrodynamic
equation. The Yaglom relation involves a third-order moment calculated from
velocity and magnetic fields and involving both Els\"asser vector fields, and
is particularly useful to be used as far as spacecraft observations of
turbulence are concerned
Non-gaussian probability distribution functions in two dimensional Magnetohydrodynamic turbulence
Intermittency in MHD turbulence has been analyzed using high resolution 2D
numerical simulations. We show that the Probability Distribution Functions
(PDFs) of the fluctuations of the Elsasser fields, magnetic field and velocity
field depend on the scale at hand, that is they are self-affine. The departure
of the PDFs from a Gaussian function can be described through the scaling
behavior of a single parameter lambda_r^2 obtained by fitting the PDFs with a
given curve stemming from the analysis of a multiplicative model by Castaing et
al. (1990). The scaling behavior of the parameter lambda_r^2 can be used to
extract informations about the intermittency. A comparison of intermittency
properties in different MHD turbulent flows is also performed.Comment: 7 pages, with 5 figure
Methods for characterising microphysical processes in plasmas
Advanced spectral and statistical data analysis techniques have greatly
contributed to shaping our understanding of microphysical processes in plasmas.
We review some of the main techniques that allow for characterising fluctuation
phenomena in geospace and in laboratory plasma observations. Special emphasis
is given to the commonalities between different disciplines, which have
witnessed the development of similar tools, often with differing terminologies.
The review is phrased in terms of few important concepts: self-similarity,
deviation from self-similarity (i.e. intermittency and coherent structures),
wave-turbulence, and anomalous transport.Comment: Space Science Reviews (2013), in pres
On the probability distribution function of small scale interplanetary magnetic field fluctuations
In spite of a large number of papers dedicated to study MHD turbulence in the
solar wind there are still some simple questions which have never been
sufficiently addressed like: a)do we really know how the magnetic field vector
orientation fluctuates in space? b) what is the statistics followed by the
orientation of the vector itself? c) does the statistics change as the wind
expands into the interplanetary space? A better understanding of these points
can help us to better characterize the nature of interplanetary fluctuations
and can provide useful hints to investigators who try to numerically simulate
MHD turbulence. This work follows a recent paper presented by the same authors.
This work follows a recent paper presented by some of the authors which shows
that these fluctuations might resemble a sort of random walk governed by a
Truncated Leevy Flight statistics. However, the limited statistics used in that
paper did not allow final conclusions but only speculative hypotheses. In this
work we aim to address the same problem using a more robust statistics which on
one hand forces us not to consider velocity fluctuations but, on the other hand
allows us to establish the nature of the governing statistics of magnetic
fluctuations with more confidence. In addition, we show how features similar to
those found in the present statistical analysis for the fast speed streams of
solar wind, are qualitatively recovered in numerical simulations of the
parametric instability. This might offer an alternative viewpoint for
interpreting the questions raised above.Comment: 25pag, 20 jpg small size figures. In press on "ANnales Geophysicae"
(September 2004
Coherent structures and spectral energy transfer in turbulent plasma: A space-filter approach
Plasma turbulence at scales of the order of the ion inertial length is mediated by several mechanisms, including linear wave damping, magnetic reconnection, the formation and dissipation of thin current sheets, and stochastic heating. It is now understood that the presence of localized coherent structures enhances the dissipation channels and the kinetic features of the plasma. However, no formal way of quantifying the relationship between scale-to-scale energy transfer and the presence of spatial structures has been presented so far. In the Letter we quantify such a relationship analyzing the results of a two-dimensional high-resolution Hall magnetohydrodynamic simulation. In particular, we employ the technique of space filtering to derive a spectral energy flux term which defines, in any point of the computational domain, the signed flux of spectral energy across a given wave number. The characterization of coherent structures is performed by means of a traditional two-dimensional wavelet transformation. By studying the correlation between the spectral energy flux and the wavelet amplitude, we demonstrate the strong relationship between scale-to-scale transfer and coherent structures. Furthermore, by conditioning one quantity with respect to the other, we are able for the first time to quantify the inhomogeneity of the turbulence cascade induced by topological structures in the magnetic field. Taking into account the low space-filling factor of coherent structures (i.e., they cover a small portion of space), it emerges that 80% of the spectral energy transfer (both in the direct and inverse cascade directions) is localized in about 50% of space, and 50% of the energy transfer is localized in only 25% of space
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