1,194 research outputs found
Small-Scale Turbulence in a Closed-Field-Line Geometry
Plasma turbulence due to small-scale entropy modes is studied with gyrokinetic simulations in a simple closed-field-line geometry, the Z pinch, in low-β parameter regimes that are stable to ideal interchange modes. We find an enormous variation in the nonlinear dynamics and particle transport as a function of two main parameters, the density gradient and the plasma collisionality. This variation is explained in part by the damping and stability properties of spontaneously formed zonal flows in the system. As in toroidal systems, the zonal flows can lead to a strong nonlinear suppression of transport below a critical gradient that is determined by the stability of the zonal flows
Correlated and zonal errors of global astrometric missions: a spherical harmonic solution
We propose a computer-efficient and accurate method of estimation of
spatially correlated errors in astrometric positions, parallaxes and proper
motions obtained by space and ground-based astrometry missions. In our method,
the simulated observational equations are set up and solved for the
coefficients of scalar and vector spherical harmonics representing the output
errors, rather than for individual objects in the output catalog. Both
accidental and systematic correlated errors of astrometric parameters can be
accurately estimated. The method is demonstrated on the example of the JMAPS
mission, but can be used for other projects of space astrometry, such as SIM or
JASMINE.Comment: Accepted by AJ, to be published in 201
Collisional damping of zonal flows due to finite Larmor radius effects
The collisional damping of seeded E X B zonal flows on the ion Larmor radius scale is studied using a gyrokinetic model. The focus is on flow damping due to finite Larmor radius effects, which cause a nu(parallel to)/nu anisotropy of the ion distribution function that is damped by ion-ion collisions. The gyrokinetic equations are solved in a slab geometry with no gradients or curvature, and a gyroaveraged Lorentz collision operator that conserves particle number, momentum, and energy is used. The solution of the gyrokinetic equations explores the dependence of the damping rate on the wavelength of the flows and the impact of the collisions on the ion distribution function. These numerical results can be used as a benchmark test during the implementation of finite Larmor radius effects in the collision operator of gyrokinetic codes. (C) 2010 American Institute of Physics. [doi:10.1063/1.3447875
The Fundamental Reference AGN Monitoring Experiment (FRAMEx)
The U.S. Naval Observatory (USNO), in collaboration with Paris Observatory
(OP), is conducting the Fundamental Reference AGN Monitoring Experiment, or
FRAMEx. FRAMEx will use USNO's and OP's in-house observing assets in the radio,
infrared (IR) and visible, as well as other ground- and space-based telescopes
(e.g., in the X-ray) that we can access for these purposes, to observe and
monitor current and candidate Reference Frame Objects (RFOs) -- consisting of
Active Galactic Nuclei (AGN) -- as well as representative AGN, in order to
better understand astrometric and photometric variability at multiple
timescales. FRAMEx will improve the selection of RFOs as well as provide
significant new data to the AGN research community. This paper describes the
FRAMEx objectives, specific areas of investigation, and the initial data
collection campaigns.Comment: 8 pages, 2 figures. To be published in the Proceedings of 2019
Journ\'ees meetin
X-point collapse and saturation in the nonlinear tearing mode reconnection
We study the nonlinear evolution of the resistive tearing mode in slab
geometry in two dimensions. We show that, in the strongly driven regime (large
Delta'), a collapse of the X-point occurs once the island width exceeds a
certain critical value ~1/Delta'. A current sheet is formed and the
reconnection is exponential in time with a growth rate ~eta^1/2, where eta is
the resistivity. If the aspect ratio of the current sheet is sufficiently
large, the sheet can itself become tearing-mode unstable, giving rise to
secondary islands, which then coalesce with the original island. The saturated
state depends on the value of Delta'. For small Delta', the saturation
amplitude is ~Delta' and quantitatively agrees with the theoretical prediction.
If Delta' is large enough for the X-point collapse to have occured, the
saturation amplitude increases noticeably and becomes independent of Delta'.Comment: revtex4, 4 pages, 18 figure
Astrophysical Gyrokinetics: Basic Equations and Linear Theory
Magnetohydrodynamic (MHD) turbulence is encountered in a wide variety of
astrophysical plasmas, including accretion disks, the solar wind, and the
interstellar and intracluster medium. On small scales, this turbulence is often
expected to consist of highly anisotropic fluctuations with frequencies small
compared to the ion cyclotron frequency. For a number of applications, the
small scales are also collisionless, so a kinetic treatment of the turbulence
is necessary. We show that this anisotropic turbulence is well described by a
low frequency expansion of the kinetic theory called gyrokinetics. This paper
is the first in a series to examine turbulent astrophysical plasmas in the
gyrokinetic limit. We derive and explain the nonlinear gyrokinetic equations
and explore the linear properties of gyrokinetics as a prelude to nonlinear
simulations. The linear dispersion relation for gyrokinetics is obtained and
its solutions are compared to those of hot-plasma kinetic theory. These results
are used to validate the performance of the gyrokinetic simulation code {\tt
GS2} in the parameter regimes relevant for astrophysical plasmas. New results
on global energy conservation in gyrokinetics are also derived. We briefly
outline several of the problems to be addressed by future nonlinear
simulations, including particle heating by turbulence in hot accretion flows
and in the solar wind, the magnetic and electric field power spectra in the
solar wind, and the origin of small-scale density fluctuations in the
interstellar medium.Comment: emulateapj, 24 pages, 10 figures, revised submission to ApJ:
references added, typos corrected, reorganized and streamline
Gyrokinetic simulations of the tearing instability
Linear gyrokinetic simulations covering the collisional -- collisionless
transitional regime of the tearing instability are performed. It is shown that
the growth rate scaling with collisionality agrees well with that predicted by
a two-fluid theory for a low plasma beta case in which ion kinetic dynamics are
negligible. Electron wave-particle interactions (Landau damping), finite Larmor
radius, and other kinetic effects invalidate the fluid theory in the
collisionless regime, in which a general non-polytropic equation of state for
pressure (temperature) perturbations should be considered. We also vary the
ratio of the background ion to electron temperatures, and show that the
scalings expected from existing calculations can be recovered, but only in the
limit of very low beta.Comment: 7 pages, 10 figures, submitted to Po
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