386 research outputs found
One-point statistics and intermittency of induced electric field in the solar wind
The interplanetary induced electric field e=vxb is studied, using solar wind
time series. The probability distribution functions (PDFs) of the electric
field components are measured from the data and their non-gaussianity is
discussed. Moreover, for the first time we show that the electric field
turbulence is characterized by intermittency. This point is addressed by
studying, as usual, the scaling of the PDFs of field increments, which allows a
quantitative characterization of intermittency.Comment: Accepted for publication on Europhysics Letters, April 22th, 200
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
Turbulence, intermittency and cross-scale energy transfer in an interplanetary coronal mass ejection
Solar wind measurements carried out by NASA's Wind spacecraft before, during
and after the passing of an interplanetary coronal mass ejection (ICME)
detected on 12-14 September 2014 have been used in order to examine several
properties of magnetohydrodynamic (MHD) turbulence. Spectral indices and
flatness scaling exponents of magnetic field, velocity and proton density
measurements were obtained, and provided a standard description of the
characteristics of turbulence within different sub-regions of the ICME and its
surroundings. This analysis was followed by the validation of the third-order
moment scaling law for isotropic, incompressible MHD turbulence in the same
sub-regions, which confirmed the fully developed nature of turbulence in the
ICME plasma. The energy transfer rate was also estimated in each ICME
sub-region and in the surrounding solar wind. An exceptionally high value was
found within the ICME sheath, accompanied by enhanced intermittency, possibly
related to the powerful energy injection associated with the arrival of the
ICME
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
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