1,107 research outputs found
Kinetic dissipation and anisotropic heating in a turbulent collisionless plasma
The kinetic evolution of the Orszag-Tang vortex is studied using
collisionless hybrid simulations. In the magnetohydrodynamic regime this vortex
leads rapidly to broadband turbulence. Significant differences from MHD arise
at small scales, where the fluid scale energy dissipates into heat almost
exclusively through the magnetic field because the protons are decoupled from
the magnetic field. Although cyclotron resonance is absent, the protons heat
preferentially in the plane perpendicular to the mean field, as in the corona
and solar wind. Effective transport coefficients are calculated.Comment: 4 pages, 4 figures. Submitted to PR
Transition from ion-coupled to electron-only reconnection: Basic physics and implications for plasma turbulence
Using kinetic particle-in-cell (PIC) simulations, we simulate reconnection
conditions appropriate for the magnetosheath and solar wind, i.e., plasma beta
(ratio of gas pressure to magnetic pressure) greater than 1 and low magnetic
shear (strong guide field). Changing the simulation domain size, we find that
the ion response varies greatly. For reconnecting regions with scales
comparable to the ion Larmor radius, the ions do not respond to the
reconnection dynamics leading to ''electron-only'' reconnection with very large
quasi-steady reconnection rates. The transition to more traditional
''ion-coupled'' reconnection is gradual as the reconnection domain size
increases, with the ions becoming frozen-in in the exhaust when the magnetic
island width in the normal direction reaches many ion inertial lengths. During
this transition, the quasi-steady reconnection rate decreases until the ions
are fully coupled, ultimately reaching an asymptotic value. The scaling of the
ion outflow velocity with exhaust width during this electron-only to
ion-coupled transition is found to be consistent with a theoretical model of a
newly reconnected field line. In order to have a fully frozen-in ion exhaust
with ion flows comparable to the reconnection Alfv\'en speed, an exhaust width
of at least several ion inertial lengths is needed. In turbulent systems with
reconnection occurring between magnetic bubbles associated with fluctuations,
using geometric arguments we estimate that fully ion-coupled reconnection
requires magnetic bubble length scales of at least several tens of ion inertial
lengths
Studies of multiple stellar systems - IV. The triple-lined spectroscopic system Gliese 644
We present a radial-velocity study of the triple-lined system Gliese 644 and
derive spectroscopic elements for the inner and outer orbits with periods of
2.9655 and 627 days. We also utilize old visual data, as well as modern speckle
and adaptive optics observations, to derive a new astrometric solution for the
outer orbit. These two orbits together allow us to derive masses for each of
the three components in the system: M_A = 0.410 +/- 0.028 (6.9%), M_Ba = 0.336
+/- 0.016 (4.7%), and $M_Bb = 0.304 +/- 0.014 (4.7%) M_solar. We suggest that
the relative inclination of the two orbits is very small. Our individual masses
and spectroscopic light ratios for the three M stars in the Gliese 644 system
provide three points for the mass-luminosity relation near the bottom of the
Main Sequence, where the relation is poorly determined. These three points
agree well with theoretical models for solar metallicity and an age of 5 Gyr.
Our radial velocities for Gliese 643 and vB 8, two common-proper-motion
companions of Gliese 644, support the interpretation that all five M stars are
moving together in a physically bound group. We discuss possible scenarios for
the formation and evolution of this configuration, such as the formation of all
five stars in a sequence of fragmentation events leading directly to the
hierarchical configuration now observed, versus formation in a small N cluster
with subsequent dynamical evolution into the present hierarchical
configuration.Comment: 17 pages, 9 figures, Accepted for publication in MNRA
Why Does Steady-State Magnetic Reconnection Have A Maximum Local Rate Of Order 0.1?
Simulations suggest collisionless steady-state magnetic reconnection of Harris-type current sheets proceeds with a rate of order 0.1, independent of dissipation mechanism. We argue this long-standing puzzle is a result of constraints at the magnetohydrodynamic (MHD) scale. We perform a scaling analysis of the reconnection rate as a function of the opening angle made by the upstream magnetic fields, finding a maximum reconnection rate close to 0.2. The predictions compare favorably to particle-in-cell simulations of relativistic electron-positron and non-relativistic electron-proton reconnection. The fact that simulated reconnection rates are close to the predicted maximum suggests reconnection proceeds near the most efficient state allowed at the MHD-scale. The rate near the maximum is relatively insensitive to the opening angle, potentially explaining why reconnection has a similar fast rate in differing models
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Model for evaluating nuclear strategies with proliferation resistance
A model was developed at HEDL to specifically analyze proliferation resistant strategies. The model was not designed to predict the future, but rather to provide a method for estimating the consequences of decisions affecting proliferation resistance in a rational and plausible manner. The characteristics of the model are described
Magnetic Reconnection As An Element Of Turbulence
In this work, recent advances on the study of reconnection in turbulence are reviewed. Using direct numerical simulations of decaying incompressible two-dimensional magnetohydrodynamics (MHD), it was found that in fully developed turbulence complex processes of reconnection locally occur (Servidio et al., 2009, 2010a). In this complex scenario, reconnection is spontaneous but locally driven by the fields, with the boundary conditions provided by the turbulence. Matching classical turbulence analysis with a generalized Sweet-Parker theory, the statistical features of these multiple-reconnection events have been identified. A discussion on the accuracy of our algorithms is provided, highlighting the necessity of adequate spatial resolution. Applications to the study of solar wind discontinuities are reviewed, comparing simulations to spacecraft observations. New results are shown, studying the time evolution of these local reconnection events. A preliminary study on the comparison between MHD and Hall MHD is reported. Our new approach to the study of reconnection as an element of turbulence has broad applications to space plasmas, shedding a new light on the study of magnetic reconnection in nature
Overview on numerical studies of reconnection and dissipation in the solar wind
In this work, recent advances in numerical studies of local reconnection events in the turbulent plasmas are reviewed. Recently [1], the nonlinear dynamics of magnetic reconnection in turbulence has been investigated through high resolution numerical simulations. Both fluid (MHD and Hall MHD) and kinetic (HybridVlasov) 2D simulations reveal the presence of a large number of X-type neutral points, where magnetic reconnection locally occurs. The associated reconnection rates are distributed over a wide range of values and they depend on the local geometry of the diffusion region. This new approach to the study of magnetic reconnection has broad applications to the turbulent solar wind (SW). Strong magnetic SW discontinuities are in fact strongly related to these intermittent processes of reconnection [2, 3]. Methods employed to identify sets of possible reconnection events along a one-dimensional path through the turbulent field (emulating experimental sampling by a single detector in a highspeed flow) are here reviewed. These local reconnection/discontinuity events may be the main sites of heating and particle acceleration processes [4]. Results from hybrid-Vlasov kinetic simulations support these observations [5, 6]. In the turbulent regime, in fact, kinetic effects manifest through a deformation of the ion distribution function. These patterns of non-Maxwellian features are concentrated in space nearby regions of strong magnetic activity. These results open a new path on the study of kinetic processes such as heating, particle acceleration, and temperature anisotropy, commonly observed in astrophysics.Fil: Donato, S.. UniversitĂ della Calabria; ItaliaFil: Servidio, S.. UniversitĂ della Calabria; ItaliaFil: Dmitruk, Pablo Ariel. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de FĂsica; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Ciudad Universitaria. Instituto de FĂsica de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de FĂsica de Buenos Aires; ArgentinaFil: Valentini, F.. UniversitĂ della Calabria; ItaliaFil: Greco, A.. UniversitĂ della Calabria; ItaliaFil: Veltri, P.. UniversitĂ della Calabria; ItaliaFil: Wan, M.. University of Delaware. Department of Physics and Astronomy; Estados UnidosFil: Shay, M. A.. University of Delaware. Department of Physics and Astronomy; Estados UnidosFil: Cassak, P. A.. West Virginia University. Department of Physics; Estados UnidosFil: Matthaeus, W. H.. University of Delaware. Department of Physics and Astronomy; Estados Unido
Establishment of clonal myogenic cell lines from severely affected dystrophic muscles - CDK4 maintains the myogenic population
<p>Abstract</p> <p>Background</p> <p>A hallmark of muscular dystrophies is the replacement of muscle by connective tissue. Muscle biopsies from patients severely affected with facioscapulohumeral muscular dystrophy (FSHD) may contain few myogenic cells. Because the chromosomal contraction at 4q35 linked to FSHD is thought to cause a defect within myogenic cells, it is important to study this particular cell type, rather than the fibroblasts and adipocytes of the endomysial fibrosis, to understand the mechanism leading to myopathy.</p> <p>Results</p> <p>We present a protocol to establish clonal myogenic cell lines from even severely dystrophic muscle that has been replaced mostly by fat, using overexpression of CDK4 and the catalytic component of telomerase (human telomerase reverse transcriptase; hTERT), and a subsequent cloning step. hTERT is necessary to compensate for telomere loss during <it>in vitro </it>cultivation, while CDK4 prevents a telomere-independent growth arrest affecting CD56+ myogenic cells, but not their CD56- counterpart, <it>in vitro</it>.</p> <p>Conclusions</p> <p>These immortal cell lines are valuable tools to reproducibly study the effect of the FSHD mutation within myoblasts isolated from muscles that have been severely affected by the disease, without the confounding influence of variable amounts of contaminating connective-tissue cells.</p
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