1,040 research outputs found
Turbulent amplification of magnetic field driven by dynamo effect at rippled shocks
We derive analytically the vorticity generated downstream of a
two-dimensional rippled hydromagnetic shock neglecting fluid viscosity and
resistivity. The growth of the turbulent component of the downstream magnetic
field is driven by the vortical eddies motion. We determine an analytic
time-evolution of the magnetic field amplification at shocks, so far described
only numerically, until saturation occurs due to seed-field reaction to field
lines whirling. The explicit expression of the amplification growth rate and of
the non-linear field back-reaction in terms of the parameters of shock and
interstellar density fluctuations is derived from MHD jump conditions at
rippled shocks. A magnetic field saturation up to the order of milligauss and a
short-time variability in the -ray observations of supernova remnants can be
obtained by using reasonable parameters for the interstellar turbulence.Comment: 9 pages, 4 figures, The Astrophyical Journal in pres
Vortical field amplification and particle acceleration at rippled shocks
Supernova Remnants (SNRs) shocks are believed to accelerate charged particles
and to generate strong turbulence in the post-shock flow. From high-energy
observations in the past decade, a magnetic field at SNR shocks largely
exceeding the shock-compressed interstellar field has been inferred. We outline
how such a field amplification results from a small-scale dynamo process
downstream of the shock, providing an explicit expression for the turbulence
back-reaction to the fluid whirling. The spatial scale of the ray rims and
the short time-variability can be obtained by using reasonable parameters for
the interstellar turbulence. We show that such a vortical field saturation is
faster than the acceleration time of the synchrotron emitting energetic
electrons.Comment: 4 pages, 3 figures; to appear in the proceedings of the RICAP-13,
Roma International Conference on AstroParticle Physic
Early-time velocity autocorrelation for charged particles diffusion and drift in static magnetic turbulence
Using test-particle simulations, we investigate the temporal dependence of
the two-point velocity correlation function for charged particles scattering in
a time-independent spatially fluctuating magnetic field derived from a
three-dimensional isotropic turbulence power spectrum. Such a correlation
function allowed us to compute the spatial coefficients of diffusion both
parallel and perpendicular to the average magnetic field. Our simulations
confirm the dependence of the perpendicular diffusion coefficient on turbulence
energy density and particle energy predicted previously by a model for
early-time charged particle transport. Using the computed diffusion
coefficients, we exploit the particle velocity autocorrelation to investigate
the time-scale over which the particles "decorrelate" from the solution to the
unperturbed equation of motion. Decorrelation time-scales are evaluated for
parallel and perpendicular motions, including the drift of the particles from
the local magnetic field line. The regimes of strong and weak magnetic
turbulence are compared for various values of the ratio of the particle
gyroradius to the correlation length of the magnetic turbulence. Our simulation
parameters can be applied to energetic particles in the interplanetary space,
cosmic rays at the supernova shocks, and cosmic-rays transport in the
intergalactic medium.Comment: 10 pages, 11 figures, The Astrophyical Journal in pres
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