Orateur
Dr
Elena Amato
(Arcetri observatory)
Description
Streaming instabilities are thought to play a fundamental role in both
the processes of acceleration and propagation of Cosmic Rays in the
Galaxy.
Resonant scattering on self-generated magnetic turbulence has long
been recognized as the most plausible mechanism for the confinemet of
energetic particles. The streaming of these particles along magnetic
field lines at super-alfvenic speeds generates magnetic turbulence at
a wavelength corresponding to the particles' gyroradius: such
turbulence could then provide efficient scattering of the same
particles so as to ensure a small diffusion coefficient and hence
large confinement times in the Galaxy.
Particle streaming is even more important in the acceleration region,
if galactic Cosmic rays up to the knee are indeed accelerated in
Supernova Remnant shocks. Reaching energies in excess of 1 PeV
requires very effective particle scattering, leading to infer a
turbulent magnetic field in the shock region much in excess of the
average field in the ISM. The field amplification is thought to be
provided by the streamining CRs themselves, but sufficient levels of
turbulence, such as to guarantee acceleration up to the knee, are
still difficult to obtain, even after the resonant generation of
Alfven waves is taken into account.
In more recent times, the streaming instability has been reconsidered
to show that in addition to the well known and long studied resonant
modes, there are also non-resonant modes in the dispersion relation,
that might show in some situations very large growth rates, much
larger than for the resonant ones. These non-resonant waves lead to
magnetic field strengths that could ensure, in principle, acceleration
of particles up to the knee. Problem with these modes, in terms of
their efficacy for particle scattering (and hence acceleration) is
that they are born as short wavelength modes and some inverse
cascading is necessary before they can provide effective particle
scattering.
I will give a brief overview of our understanding of streaming
instabilities and their role in Cosmic Ray physics, following the
historical development of studies in this field, from the pioneering
works by Skilling and Wentzel in the '70s to the most recent works,
both theoretical and numerical, on the non-resonant modes.
