8,622 research outputs found
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
Nonlinear evolution of the parametric instability: numerical predictions versus observations in the heliosphere
International audienceLow-frequency turbulence in the solar wind is characterized by a high degree of Alfvénicity close to the Sun. Cross-helicity, which is a measure of Alfvénic correlation, tends to decrease with increasing distance from the Sun at high latitudes as well as in slow-speed streams at low latitudes. In the latter case, large scale inhomogeneities (velocity shears, the heliospheric current sheet) are present, which are sources of decorrelation; yet at high latitudes, the wind is much more homogeneous, and a possible evolution mechanism is represented by the parametric instability. The parametric decay of an circularly polarized broadband Alfvén wave is then investigated, as a source of decorrelation. The time evolution is followed by numerically integrating the full set of nonlinear MHD equations, up to instability saturation. We find that, for ~ 1, the final cross-helicity is ~ 0.5, corresponding to a partial depletion of the initial correlation. Compressive fluctuations at a moderate level are also present. Most of the spectrum is dominated by forward propagating Alfvénic fluctuations, while backscattered fluctuations dominate large scales. With increasing time, the spectra of ElsÀsser variables tend to approach each other. Some results concerning quantities measured in the high-latitude wind are reviewed, and a qualitative agreement with the results of the numerical model is found
Dissipation of Alfven waves in compressible inhomogeneous media
In weakly dissipative media governed by the magnetohydrodynamics (MHD) equations, any efficient mechanism of energy dissipation requires the formation
of small scales. Using numerical simulations, we study the properties of AlfvÂŽen waves propagating in a compressible inhomegeneous medium, with an inhomogeneity transverse to the direction of wave propagation. Two dynamical effects, energy pinching and phase mixing, are responsible for the small-scales formation, similarly to the incompressible case. Moreover, compressive perturbations, slow waves and a static entropy wave are generated; the former are subject to steepening and form shock waves, which efficiently dissipate their energy, regardless of the Reynolds number. Rough estimates show that the dissipation times are consistent with those required to dissipate AlfvÂŽen waves of photospheric origin inside the solar corona
Eco-friendly gas mixtures for Resistive Plate Chambers based on Tetrafluoropropene and Helium
Due to the recent restrictions deriving from the application of the Kyoto
protocol, the main components of the gas mixtures presently used in the
Resistive Plate Chambers systems of the LHC experiments will be most probably
phased out of production in the coming years. Identifying possible replacements
with the adequate characteristics requires an intense R&D, which was recently
started, also in collaborations across the various experiments. Possible
candidates have been proposed and are thoroughly investigated. Some tests on
one of the most promising candidate - HFO-1234ze, an allotropic form of
tetrafluoropropane- have already been reported. Here an innovative approach,
based on the use of Helium, to solve the problems related to the too elevate
operating voltage of HFO-1234ze based gas mixtures, is discussed and the
relative first results are shown.Comment: 9 pages, 6 figures, 1 tabl
Proper orthogonal decomposition of solar photospheric motions
The spatio-temporal dynamics of the solar photosphere is studied by
performing a Proper Orthogonal Decomposition (POD) of line of sight velocity
fields computed from high resolution data coming from the MDI/SOHO instrument.
Using this technique, we are able to identify and characterize the different
dynamical regimes acting in the system. Low frequency oscillations, with
frequencies in the range 20-130 microHz, dominate the most energetic POD modes
(excluding solar rotation), and are characterized by spatial patterns with
typical scales of about 3 Mm. Patterns with larger typical scales of 10 Mm, are
associated to p-modes oscillations at frequencies of about 3000 microHz.Comment: 8 figures in jpg in press on PR
Growth characteristics of American Ginseng (Panax quinquefolius L.) woods and field - cultivated at Northern Europe
Received: February 19th, 2022 ; Accepted: May 15th, 2022 ; Published: May 16th, 2022 ; Correspondence: [email protected] Latvia, Northern Europe, American ginseng was grown in three forest types with
different dominant species, as well as in agricultural field conditions - cultivated under artificial
shade with three different types of mulches. Field cultivation yielded higher yields, root length,
and root weight than wood cultivation under dominant species Corylus avellana, Betula pendula,
and Picea abies. Mulching had a positive impact on ginseng growth in the field. Mulching with
straw and buckwheat hulls resulted in longer and heavier roots.
In American ginseng roots, the contents of six ginsenosides were determined: Rg1, Re, Rb1, Rc,
Rb2, and Rd. Re was the most abundant ginsenoside, followed by Rb1 > Rd > Rg1 = Rb2 > Rd.
The total content of ginsenosides in our study did not reach the 4 percent threshold set by US
Pharmacopeia.
These findings show that Panax quinquefolium can be grown in Northern Europe at 57°N, but it
takes more than four years to achieve adequate yields and ginsenoside content
Characterization of the water diffusion in GEM foil material
Systematic studies on the GEM foil material are performed to measure the moisture diffusion rate and saturation level.These studies are important because the presence of this compound inside the detectorâs foil can possibly change its mechanical and electrical properties,and in such a way,the detector performance can be affected.To understand this phenomenon,a model is developed with COMSOL Multiphysicsv.4.3 which described the adsorption and diffusion within the geometry of GEM foil,the concentration profiles and the time required to saturate the foil.The COMSOL model is verified by experimental observations on a GEM foil sample.This note will describe the model and its experimental verification results
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