320 research outputs found
The transport of cosmic rays in self-excited magnetic turbulence
The process of diffusive shock acceleration relies on the efficacy with which
hydromagnetic waves can scatter charged particles in the precursor of a shock.
The growth of self-generated waves is driven by both resonant and non-resonant
processes. We perform high-resolution magnetohydrodynamic simulations of the
non-resonant cosmic-ray driven instability, in which the unstable waves are
excited beyond the linear regime. In a snapshot of the resultant field,
particle transport simulations are carried out. The use of a static snapshot of
the field is reasonable given that the Larmor period for particles is typically
very short relative to the instability growth time. The diffusion rate is found
to be close to, or below, the Bohm limit for a range of energies. This provides
the first explicit demonstration that self-excited turbulence reduces the
diffusion coefficient and has important implications for cosmic ray transport
and acceleration in supernova remnants.Comment: 8 pages, 8 figures, accepted for publication in MNRA
A cosmic ray current driven instability in partially ionised media
We investigate the growth of hydromagnetic waves driven by streaming cosmic
rays in the precursor environment of a supernova remnant shock. It is known
that transverse waves propagating parallel to the mean magnetic field are
unstable to anisotropies in the cosmic ray distribution, and may provide a
mechanism to substantially amplify the ambient magnetic field. We quantify the
extent to which temperature and ionisation fractions modify this picture. Using
a kinetic description of the plasma we derive the dispersion relation for a
collisionless thermal plasma with a streaming cosmic ray current. Fluid
equations are then used to discuss the effects of neutral-ion collisions. We
calculate the extent to which the environment into which the cosmic rays
propagate influences the growth of the magnetic field, and determines the range
of possible growth rates. If the cosmic ray acceleration is efficient, we find
that very large neutral fractions are required to stabilise the growth of the
non-resonant mode. For typical supernova parameters in our galaxy, thermal
effects do not significantly alter the growth rates. For weakly driven modes,
ion-neutral damping can dominate over the instability at more modest ionisation
fractions. In the case of a supernova shock interacting with a molecular
clouds, such as in RX J1713.7-3946, with high density and low ionisation, the
modes can be rapidly damped.Comment: 5 pages, 2 figures, accepted to A&A. Corrections made. Applications
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Environmental limits on the non-resonant cosmic-ray current-driven instability
We investigate the so-called non-resonant cosmic-ray streaming instability,
first discussed by Bell (2004). The extent to which thermal damping and
ion-neutral collisions reduce the growth of this instability is calculated.
Limits on the growth of the non-resonant mode in SN1006 and RX J1713.7-3946 are
presented.Comment: 4 pages, 1 figure, contributed talk at the workshop: High Energy
Phenomena in Relativistic Outflows (HEPRO), Dublin, 24-28 September 200
Modeling magnetized star-planet interactions: boundary conditions effects
Conference ProceedingCopyright © International Astronomical Union 2013We model the magnetized interaction between a star and a close-in planet (SPMIs), using global, magnetohydrodynamic numerical simulations. In this proceedings, we study the effects of the numerical boundary conditions at the stellar surface, where the stellar wind is driven, and in the planetary interior. We show that is it possible to design boundary conditions that are adequate to obtain physically realistic, steady-state solutions for cases with both magnetized and unmagnetized planets. This encourages further development of numerical studies, in order to better constrain and undersand SPMIs, as well as their effects on the star-planet rotational evolution. © 2013 International Astronomical Union
A current driven instability in parallel, relativistic shocks
Recently, Bell has reanalysed the problem of wave excitation by cosmic rays
propagating in the pre-cursor region of a supernova remnant shock front. He
pointed out a strong, non-resonant, current-driven instability that had been
overlooked in the kinetic treatments, and suggested that it is responsible for
substantial amplification of the ambient magnetic field. Magnetic field
amplification is also an important issue in the problem of the formation and
structure of relativistic shock fronts, particularly in relation to models of
gamma-ray bursts. We have therefore generalised the linear analysis to apply to
this case, assuming a relativistic background plasma and a monoenergetic,
unidirectional incoming proton beam. We find essentially the same non-resonant
instability noticed by Bell, and show that also under GRB conditions, it grows
much faster than the resonant waves. We quantify the extent to which thermal
effects in the background plasma limit the maximum growth rate.Comment: 8 pages, 1 figur
Probing Nearby CR Accelerators and ISM Turbulence with Milagro Hot Spots
Both the acceleration of cosmic rays (CR) in supernova remnant shocks and
their subsequent propagation through the random magnetic field of the Galaxy
deem to result in an almost isotropic CR spectrum. Yet the MILAGRO TeV
observatory discovered a sharp ( arrival anisotropy of CR
nuclei. We suggest a mechanism for producing a weak and narrow CR beam which
operates en route to the observer. The key assumption is that CRs are scattered
by a strongly anisotropic Alfven wave spectrum formed by the turbulent cascade
across the local field direction. The strongest pitch-angle scattering occurs
for particles moving almost precisely along the field line. Partly because this
direction is also the direction of minimum of the large scale CR angular
distribution, the enhanced scattering results in a weak but narrow particle
excess. The width, the fractional excess and the maximum momentum of the beam
are calculated from a systematic transport theory depending on a single scale
which can be associated with the longest Alfven wave, efficiently
scattering the beam. The best match to all the three characteristics of the
beam is achieved at pc. The distance to a possible source of the beam
is estimated to be within a few 100pc. Possible approaches to determination of
the scale from the characteristics of the source are discussed. Alternative
scenarios of drawing the beam from the galactic CR background are considered.
The beam related large scale anisotropic CR component is found to be energy
independent which is also consistent with the observations.Comment: 2 figures, ApJ accepted version2 minor changes and correction
A kinetic approach to cosmic ray induced streaming instability at supernova shocks
We show that a purely kinetic approach to the excitation of waves by cosmic
rays in the vicinity of a shock front leads to predict the appearance of a
non-alfv\'enic fastly growing mode which has the same dispersion relation as
that previously found by \cite{bell04} by treating the plasma in the MHD
approximation. The kinetic approach allows us to investigate the dependence of
the dispersion relation of these waves on the microphysics of the current which
compensates the cosmic ray flow. We also show that a resonant and a
non-resonant mode may appear at the same time and one of the two may become
dominant on the other depending on the conditions in the acceleration region.
We discuss the role of the unstable modes for magnetic field amplification and
particle acceleration in supernova remnants at different stages of the remnant
evolution.Comment: Accepted for publication in MNRA
Time evolution and asymmetry of a laser produced blast wave
Studies of a blast wave produced from carbon rods and plastic spheres in an argon background gas have been conducted using the Vulcan laser at the Rutherford Appleton Laboratory. A laser of 1500 J was focused onto these targets, and rear-side observations of an emission front were recorded using a fast-framing camera. The emission front is asymmetrical in shape and tends to a more symmetrical shape as it progresses due to the production of a second shock wave later in time, which pushes out the front of the blast wave. Plastic spheres produce faster blast waves, and the breakthrough of the second shock is visible before the shock stalls. The results are presented to demonstrate this trend, and similar evolution dynamics of experimental and simulation data from the FLASH radiation-hydrodynamics code are observed
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