5,093 research outputs found
Why we need to see the dark matter to understand the dark energy
The cosmological concordance model contains two separate constituents which
interact only gravitationally with themselves and everything else, the dark
matter and the dark energy. In the standard dark energy models, the dark matter
makes up some 20% of the total energy budget today, while the dark energy is
responsible for about 75%. Here we show that these numbers are only robust for
specific dark energy models and that in general we cannot measure the abundance
of the dark constituents separately without making strong assumptions.Comment: 4 pages, to be published in the Journal of Physics: Conference Series
as a contribution to the 2007 Europhysics Conference on High Energy Physic
Magnetorotational Turbulence and Dynamo in a Collisionless Plasma
We present results from the first 3D kinetic numerical simulation of
magnetorotational turbulence and dynamo, using the local shearing-box model of
a collisionless accretion disc. The kinetic magnetorotational instability grows
from a subthermal magnetic field having zero net flux over the computational
domain to generate self-sustained turbulence and outward angular-momentum
transport. Significant Maxwell and Reynolds stresses are accompanied by
comparable viscous stresses produced by field-aligned ion pressure anisotropy,
which is regulated primarily by the mirror and ion-cyclotron instabilities
through particle trapping and pitch-angle scattering. The latter endow the
plasma with an effective viscosity that is biased with respect to the
magnetic-field direction and spatio-temporally variable. Energy spectra suggest
an Alfv\'en-wave cascade at large scales and a kinetic-Alfv\'en-wave cascade at
small scales, with strong small-scale density fluctuations and weak
non-axisymmetric density waves. Ions undergo non-thermal particle acceleration,
their distribution accurately described by a kappa distribution. These results
have implications for the properties of low-collisionality accretion flows,
such as that near the black hole at the Galactic center.Comment: 6 pages, 6 figures, accepted for publication in Physical Review
Letter
Firehose and Mirror Instabilities in a Collisionless Shearing Plasma
Hybrid-kinetic numerical simulations of firehose and mirror instabilities in
a collisionless plasma are performed in which pressure anisotropy is driven as
the magnetic field is changed by a persistent linear shear . For a
decreasing field, it is found that mostly oblique firehose fluctuations grow at
ion Larmor scales and saturate with energies ; the pressure
anisotropy is pinned at the stability threshold by particle scattering off
microscale fluctuations. In contrast, nonlinear mirror fluctuations are large
compared to the ion Larmor scale and grow secularly in time; marginality is
maintained by an increasing population of resonant particles trapped in
magnetic mirrors. After one shear time, saturated order-unity magnetic mirrors
are formed and particles scatter off their sharp edges. Both instabilities
drive sub-ion-Larmor--scale fluctuations, which appear to be
kinetic-Alfv\'{e}n-wave turbulence. Our results impact theories of momentum and
heat transport in astrophysical and space plasmas, in which the stretching of a
magnetic field by shear is a generic process.Comment: 5 pages, 8 figures, accepted for publication in Physical Review
Letter
Pegasus: A New Hybrid-Kinetic Particle-in-Cell Code for Astrophysical Plasma Dynamics
We describe Pegasus, a new hybrid-kinetic particle-in-cell code tailored for
the study of astrophysical plasma dynamics. The code incorporates an
energy-conserving particle integrator into a stable, second-order--accurate,
three-stage predictor-predictor-corrector integration algorithm. The
constrained transport method is used to enforce the divergence-free constraint
on the magnetic field. A delta-f scheme is included to facilitate a
reduced-noise study of systems in which only small departures from an initial
distribution function are anticipated. The effects of rotation and shear are
implemented through the shearing-sheet formalism with orbital advection. These
algorithms are embedded within an architecture similar to that used in the
popular astrophysical magnetohydrodynamics code Athena, one that is modular,
well-documented, easy to use, and efficiently parallelized for use on thousands
of processors. We present a series of tests in one, two, and three spatial
dimensions that demonstrate the fidelity and versatility of the code.Comment: 27 pages, 12 figures, accepted for publication in Journal of
Computational Physic
Ar-40 to Ar-39 dating of pseudotachylites from the Witwatersrand basin, South Africa, with implications for the formation of the Vredefort Dome
The formation of the Vredefort Dome, a structure in excess of 100 km in diameter and located in the approximate center of the Witwatersrand basin, is still the subject of lively geological controversy. It is widely accepted that its formation seems to have taken place in a single sudden event, herein referred to as the Vredefort event, accompanied by the release of gigantic amounts of energy. It is debated, however, whether this central event was an internal one, i.e., a cryptoexplosion triggered by volcanic or tectonic processes, or the impact of an extraterrestrial body. The results of this debate are presented
Dual phase-space cascades in 3D hybrid-Vlasov-Maxwell turbulence
To explain energy dissipation via turbulence in collisionless, magnetized
plasmas, the existence of a dual real- and velocity-space cascade of
ion-entropy fluctuations below the ion gyroradius has been proposed. Such a
dual cascade, predicted by the gyrokinetic theory, has previously been observed
in gyrokinetic simulations of two-dimensional, electrostatic turbulence. For
the first time we show evidence for a dual phase-space cascade of ion-entropy
fluctuations in a three-dimensional simulation of hybrid-kinetic,
electromagnetic turbulence. Some of the scalings observed in the energy spectra
are consistent with a generalized theory for the cascade that accounts for the
spectral anisotropy of critically balanced, intermittent, sub-ion-Larmor-scale
fluctuations. The observed velocity-space cascade is also anisotropic with
respect to the magnetic-field direction, with linear phase mixing along
magnetic-field lines proceeding mainly at spatial scales above the ion
gyroradius and nonlinear phase mixing across magnetic-field lines proceeding at
perpendicular scales below the ion gyroradius. Such phase-space anisotropy
could be sought in heliospheric and magnetospheric data of solar-wind
turbulence and has far-reaching implications for the dissipation of turbulence
in weakly collisional astrophysical plasmas.Comment: version accepted in ApJ
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