62 research outputs found
Stochastic Acceleration by Turbulence
The subject of this paper is stochastic acceleration by plasma turbulence, a
process akin to the original model proposed by Fermi. We review the relative
merits of different acceleration models, in particular the so called first
order Fermi acceleration by shocks and second order Fermi by stochastic
processes, and point out that plasma waves or turbulence play an important role
in all mechanisms of acceleration. Thus, stochastic acceleration by turbulence
is active in most situations. We also show that it is the most efficient
mechanism of acceleration of relatively cool non relativistic thermal
background plasma particles. In addition, it can preferentially accelerate
electrons relative to protons as is needed in many astrophysical radiating
sources, where usually there are no indications of presence of shocks. We also
point out that a hybrid acceleration mechanism consisting of initial
acceleration by turbulence of background particles followed by a second stage
acceleration by a shock has many attractive features. It is demonstrated that
the above scenarios can account for many signatures of the accelerated
electrons, protons and other ions, in particular He and He, seen
directly as Solar Energetic Particles and through the radiation they produce in
solar flares.Comment: 29 pages 7 figures for proceedings of ISSI-Bern workshop on Particle
Acceleration 201
Hydrodynamical simulations of Galactic fountains - I. Evolution of single fountains
The ejection of the gas out of the disc in late-type galaxies is related to star formation and
is due mainly to Type II supernovae. In this paper, we studied in detail the development of
the Galactic fountains in order to understand their dynamical evolution and their influence
on the redistribution of the freshly delivered metals over the disc. To this aim, we performed
a number of 3D hydrodynamical radiative cooling simulations of the gas in the Milky Way
where the whole Galaxy structure, the Galactic differential rotation and the supernova explosions
generated by a single OB association are considered. A typical fountain powered by
100 Type II supernovae may eject material up to ∼2 kpc which than collapses back mostly in
the form of dense, cold clouds and filaments. The majority of the gas lifted up by the fountains
falls back on the disc remaining within a radial distance R = 0.5 kpc from the place where
the fountain originated. This localized circulation of disc gas does not influence the radial
chemical gradients on large scale, as required by the chemical models of the Milky Way which
reproduce the metallicity distribution without invoking large fluxes of metals. Simulations of
multiple fountains fuelled by Type II supernovae of different OB associations will be presented
in a companion paper
Magnetic field amplification and evolution in turbulent collisionless magnetohydrodynamics:an application to the intracluster medium
The amplification of magnetic fields (MFs) in the intracluster medium (ICM) is attributed to turbulent dynamo (TD) action, which is generally derived in the collisional-MHD framework. However, this assumption is poorly justified a priori, since in the ICM the ion mean free path between collisions is of the order of the dynamical scales, thus requiring a collisionless MHD description. The present study uses an anisotropic plasma pressure that brings the plasma within a parametric space where collisionlessinstabilities take place. In this model, a relaxation term of the pressure anisotropy simulates the feedback of the mirror and firehose instabilities, in consistency with empirical studies. Our three-dimensional numerical simulations of forced transonic turbulence, aiming the modeling of the turbulent ICM, were performed for different initial values of the MF intensity and different relaxation rates of the pressure anisotropy. We found that in the high-β plasma regime corresponding to the ICM conditions, a fast anisotropy relaxation rate gives results that are similar to the collisional-MHD model, as far as the statistical properties of the turbulence are concerned. Also, the TD amplification of seed MFs was found to be similar to the collisional-MHD model. The simulations that do not employ the anisotropy relaxation deviate significantly from the collisional-MHD results and show more power at the small-scale fluctuations of both density and velocity as a result of the action of the instabilities. For these simulations, the large-scale fluctuations in the MF are mostly suppressed and the TD fails in amplifying seed MFs.</p
Numerical models for the 19th century outbursts of η carinae
[No abstract available
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