1,178 research outputs found
Turbulence and Mixing in the Intracluster Medium
The intracluster medium (ICM) is stably stratified in the hydrodynamic sense
with the entropy increasing outwards. However, thermal conduction along
magnetic field lines fundamentally changes the stability of the ICM, leading to
the "heat-flux buoyancy instability" when and the "magnetothermal
instability" when . The ICM is thus buoyantly unstable regardless of
the signs of and . On the other hand, these
temperature-gradient-driven instabilities saturate by reorienting the magnetic
field (perpendicular to when and parallel to when ), without generating sustained convection. We show that
after an anisotropically conducting plasma reaches this nonlinearly stable
magnetic configuration, it experiences a buoyant restoring force that resists
further distortions of the magnetic field. This restoring force is analogous to
the buoyant restoring force experienced by a stably stratified adiabatic
plasma. We argue that in order for a driving mechanism (e.g, galaxy motions or
cosmic-ray buoyancy) to overcome this restoring force and generate turbulence
in the ICM, the strength of the driving must exceed a threshold, corresponding
to turbulent velocities . For weaker driving, the ICM
remains in its nonlinearly stable magnetic configuration, and turbulent mixing
is effectively absent. We discuss the implications of these findings for the
turbulent diffusion of metals and heat in the ICM.Comment: 8 pages, 2 figs., submitted to the conference proceedings of "The
Monster's Fiery Breath;" a follow up of arXiv:0901.4786 focusing on the
general mixing properties of the IC
Radio-wave propagation through a medium containing electron-density fluctuations described by an anisotropic Goldreich-Sridhar spectrum
We study the propagation of radio waves through a medium possessing density
fluctuations that are elongated along the ambient magnetic field and described
by an anisotropic Goldreich-Sridhar power spectrum. We derive general formulas
for the wave phase structure function, visibility, angular broadening,
diffraction-pattern length scales, and scintillation time scale for arbitrary
distributions of turbulence along the line of sight, and specialize these
formulas to idealized cases.Comment: 25 pages, 3 figures, submitted to Ap
Scale dependent alignment between velocity and magnetic field fluctuations in the solar wind and comparisons to Boldyrev's phenomenological theory
(Abridged abstract) A theory of incompressible MHD turbulence recently
developed by Boldyrev predicts the existence of a scale dependent angle of
alignment between velocity and magnetic field fluctuations that is proportional
to the lengthscale of the fluctuations to the power 1/4. In this study, plasma
and magnetic field data from the Wind spacecraft are used to investigate the
angle between velocity and magnetic field fluctuations in the solar wind as a
function of the timescale of the fluctuations and to look for the power law
scaling predicted by Boldyrev.Comment: Particle Acceleration and Transport in the Heliosphere and Beyond,
7th Annual International Astrophysics Conference, Kauai, Hawaii, G. Li, Q.
Hu, O. Verkhoglyadova, G. P. Zank, R. P. Lin, J. Luhmann (eds), AIP
Conference Proceedings 1039, 81-8
Secondary peak on asymmetric magnetization loop of type-II superconductors
Asymmetric magnetization loops with a second peak effect were parameterized
by the extended critical state model. The magnetic field distribution in a
sample is considered. Expression is suggested for a peak of the critical
current density and corresponding depression on field dependence of the depth
of surface layer with equilibrium magnetization. These functions determine the
width and the asymmetry of a magnetization loop. Asymmetry of the secondary
peak height on magnetization branches for increasing and decreasing field is
reproduced on the computed magnetization curves.Comment: 6 pages, 2 figures, Equation 6 is modified to be f=0 at B=
- …