1,973 research outputs found
Turbulence in collisionless plasmas : statistical analysis from numerical simulations with pressure anisotropy
In recent years, we have experienced increasing interest in the understanding of the physical properties of collisionless plasmas, mostly because of the large number of astrophysical environments (e. g. the intracluster medium (ICM)) containing magnetic fields that are strong enough to be coupled with the ionized gas and characterized by densities sufficiently low to prevent the pressure isotropization with respect to the magnetic line direction. Under these conditions, a new class of kinetic instabilities arises, such as firehose and mirror instabilities, which have been studied extensively in the literature. Their role in the turbulence evolution and cascade process in the presence of pressure anisotropy, however, is still unclear. In this work, we present the first statistical analysis of turbulence in collisionless plasmas using three-dimensional numerical simulations and solving double-isothermal magnetohydrodynamic equations with the Chew-Goldberger-Low laws closure (CGL-MHD). We study models with different initial conditions to account for the firehose and mirror instabilities and to obtain different turbulent regimes. We found that the CGL-MHD subsonic and supersonic turbulences show small differences compared to the MHD models in most cases. However, in the regimes of strong kinetic instabilities, the statistics, i.e. the probability distribution functions (PDFs) of density and velocity, are very different. In subsonic models, the instabilities cause an increase in the dispersion of density, while the dispersion of velocity is increased by a large factor in some cases. Moreover, the spectra of density and velocity show increased power at small scales explained by the high growth rate of the instabilities. Finally, we calculated the structure functions of velocity and density fluctuations in the local reference frame defined by the direction of magnetic lines. The results indicate that in some cases the instabilities significantly increase the anisotropy of fluctuations. These results, even though preliminary and restricted to very specific conditions, show that the physical properties of turbulence in collisionless plasmas, as those found in the ICM, may be very different from what has been largely believed. Implications can range from interchange of energies to cosmic ray acceleration.Publisher PDFPeer reviewe
Scale dependent superconductor-insulator transition
We study the disorder driven superconductor to insulator transition in
amorphous films of high carrier-concentration indium-oxide. Using thin films
with various sizes and aspect ratios we show that the `critical'
sheet-resistance depends systematically on sample
geometry; superconductivity disappears when exceeds
k in large samples. On the other hand, wide and sufficiently
short samples of the same batch exhibit superconductivity (judged by
conductivity versus temperature) up to which is
considerably larger. These results support the inhomogeneous scenario for the
superconductor-insulator transition.Comment: Contribution to the proceedings of "Fluctuations and phase
transitions in superconductors", Nazareth Ilit, Israel, June 10-14, 200
The role of pressure anisotropy in the turbulent intracluster medium
In low-density plasma environments, such as the intracluster medium (ICM),
the Larmour frequency is much larger than the ion-ion collision frequency. In
such a case, the thermal pressure becomes anisotropic with respect to the
magnetic field orientation and the evolution of the turbulent gas is more
correctly described by a kinetic approach. A possible description of these
collisionless scenarios is given by the so-called kinetic magnetohydrodynamic
(KMHD) formalism, in which particles freely stream along the field lines, while
moving with the field lines in the perpendicular direction. In this way a
fluid-like behavior in the perpendicular plane is restored. In this work, we
study fast growing magnetic fluctuations in the smallest scales which operate
in the collisionless plasma that fills the ICM. In particular, we focus on the
impact of a particular evolution of the pressure anisotropy and its
implications for the turbulent dynamics of observables under the conditions
prevailing in the ICM. We present results from numerical simulations and
compare the results which those obtained using an MHD formalism.Comment: 7 pages, 14 figures, Journal of Physics: Conference Serie
Reversible Superconductivity in Electrochromic Indium-Tin Oxide Films
Transparent conductive indium tin oxide (ITO) thin films, electrochemically
intercalated with sodium or other cations, show tunable superconducting
transitions with a maximum at 5 K. The transition temperature and the
density of states, (extracted from the measured Pauli susceptibility
exhibit the same dome shaped behavior as a function of electron
density. Optimally intercalated samples have an upper critical field T and . Accompanying the development of
superconductivity, the films show a reversible electrochromic change from
transparent to colored and are partially transparent (orange) at the peak of
the superconducting dome. This reversible intercalation of alkali and alkali
earth ions into thin ITO films opens diverse opportunities for tunable,
optically transparent superconductors
Particle Acceleration in Turbulence and Weakly Stochastic Reconnection
Fast particles are accelerated in astrophysical environments by a variety of
processes. Acceleration in reconnection sites has attracted the attention of
researchers recently. In this letter we analyze the energy distribution
evolution of test particles injected in three dimensional (3D)
magnetohydrodynamic (MHD) simulations of different magnetic reconnection
configurations. When considering a single Sweet-Parker topology, the particles
accelerate predominantly through a first-order Fermi process, as predicted in
previous work (de Gouveia Dal Pino & Lazarian, 2005) and demonstrated
numerically in Kowal, de Gouveia Dal Pino & Lazarian (2011). When turbulence is
included within the current sheet, the acceleration rate, which depends on the
reconnection rate, is highly enhanced. This is because reconnection in the
presence of turbulence becomes fast and independent of resistivity (Lazarian &
Vishniac, 1999; Kowal et al., 2009) and allows the formation of a thick volume
filled with multiple simultaneously reconnecting magnetic fluxes. Charged
particles trapped within this volume suffer several head-on scatterings with
the contracting magnetic fluctuations, which significantly increase the
acceleration rate and results in a first-order Fermi process. For comparison,
we also tested acceleration in MHD turbulence, where particles suffer
collisions with approaching and receding magnetic irregularities, resulting in
a reduced acceleration rate. We argue that the dominant acceleration mechanism
approaches a second order Fermi process in this case.Comment: 6 pages, 1 figur
Dynamo in the Intra-Cluster Medium: Simulation of CGL-MHD Turbulent Dynamo
The standard magnetohydrodynamic (MHD) description of the plasma in the hot,
magnetized gas of the intra-cluster (ICM) medium is not adequate because it is
weakly collisional. In such collisionless magnetized gas, the microscopic
velocity distribution of the particles is not isotropic, giving rise to kinetic
effects on the dynamical scales. These kinetic effects could be important in
understanding the turbulence, as so as the amplification and maintenance of the
magnetic fields in the ICM. It is possible to formulate fluid models for
collisonless or weakly collisional gas by introducing modifications in the MHD
equations. These models are often referred as kinetic MHD (KMHD). Using a KMHD
model based on the CGL-closure, which allows the adiabatic evolution of the two
components of the pressure tensor (the parallel and perpendicular components
with respect to the local magnetic field), we performed 3D numerical
simulations of forced turbulence in order to study the amplification of an
initially weak seed magnetic field. We found that the growth rate of the
magnetic energy is comparable to that of the ordinary MHD turbulent dynamo, but
the magnetic energy saturates in a level smaller than of the MHD case. We also
found that a necessary condition for the dynamo works is to impose limits to
the anisotropy of the pressure.Comment: 3 pages, 1 figure, 274 IAU Symposium: Advances in Plasma Astrophysic
Cosmic-ray driven dynamo in galaxies
We present recent developments of global galactic-scale numerical models of
the Cosmic Ray (CR) driven dynamo, which was originally proposed by Parker
(1992). We conduct a series of direct CR+MHD numerical simulations of the
dynamics of the interstellar medium (ISM), composed of gas, magnetic fields and
CR components. We take into account CRs accelerated in randomly distributed
supernova (SN) remnants, and assume that SNe deposit small-scale, randomly
oriented, dipolar magnetic fields into the ISM. The amplification timescale of
the large-scale magnetic field resulting from the CR-driven dynamo is
comparable to the galactic rotation period. The process efficiently converts
small-scale magnetic fields of SN-remnants into galactic-scale magnetic fields.
The resulting magnetic field structure resembles the X-shaped magnetic fields
observed in edge-on galaxies.Comment: 6 pages, 4 figures, to appear in Proceedings of IAU Symp. 274,
Advances in Plasma Astrophysics, ed. A. Bonanno, E. de Gouveia dal Pino and
A. Kosoviche
Features of collisionless turbulence in the intracluster medium from simulated Faraday Rotation maps
Observations of the intracluster medium (ICM) in galaxy clusters suggest for
the presence of turbulence and the magnetic fields existence has been proved
through observations of Faraday Rotation and synchrotron emission. The ICM is
also known to be filled by a rarefied weakly collisional plasma. In this work
we study the possible signatures left on Faraday Rotation maps by collisionless
instabilities. For this purpose we use a numerical approach to investigate the
dynamics of the turbulence in collisionless plasmas based on an
magnetohydrodynamical (MHD) formalism taking into account different levels of
pressure anisotropy. We consider models covering the sub/super-Alfv\'enic and
trans/supersonic regimes, one of them representing the fiducial conditions
corresponding to the ICM. From the simulated models we compute Faraday Rotation
maps and analyze several statistical indicators in order to characterize the
magnetic field structure and compare the results obtained with the
collisionless model to those obtained using standard collisional MHD framework.
We find that important imprints of the pressure anisotropy prevails in the
magnetic field and also manifest in the associated Faraday Rotation maps which
evidence smaller correlation lengths in the collisionless MHD case. These
points are remarkably noticeable for the case mimicking the conditions
prevailing in ICM. Nevertheless, in this study we have neglected the decrease
of pressure anisotropy due to the feedback of the instabilities that naturally
arise in collisionless plasmas at small scales. This decrease may not affect
the statistical imprint differences described above, but should be examined
elsewhere.Comment: 24 pages, 15 figures, MNRAS accepte
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