520 research outputs found
A numerical study of diffusive shock acceleration of cosmic rays in supernova shocks
The evolution of the energy spectrum of cosmic rays accelerated by the first order Fermi mechanism, by a supernova remnant shock wave, including adiabatic deceleration effects behind the front, is carried out by means of a time-dependent numerical code. The calculations apply to the adiabatic stage (or Sedov stage) of the supernova explosion, and the energetic particle spectrum is calculated in the test particle limit (i.e., the back reaction of the cosmic rays on the flow is not included). The particles are injected mono-energetically at the shock. The radial distribution, The radial distribution, and the spectrum of the accelerated and decelerated particles is shown
Cosmic ray drift, shock wave acceleration and the anomalous component of cosmic rays
A model of the anomalous component of the quiet-time cosmic ray flux is presented in which ex-interstellar neutral particles are accelerated continuously in the polar regions of the solar-wind termination shock, and then drift into the equatorial regions of the inner heliosphere. The observed solar-cycle variations, radial gradient, and apparent latitude gradient of the anomalous component are a natural consequence of this model
Pick-up ion dynamics at the structured quasi-perpendicular shock
We study the pickup ion dynamics and mechanism of multiple reflection and
acceleration at the structured quasi-perpendicular supercritical shock. The
motion of the pickup ions in the shock is studied analytically and numerically
using the test particle analysis in the model shock front. The analysis shows
that slow pickup ions may be accelerated at the shock ramp to high energies.
The maximum ion energy is determined by the fine structure of the
electro-magnetic field at the shock ramp and decreases when the angle between
magnetic field and shock normal decreases. Evolution of pickup ion distribution
across the nearly-perpendicular shock and pickup ion spectrum is also studied
by direct numerical analysis.Comment: LaTeX (elsart.cls), packages: times,amsmath,amssymb; 15 pages + 13
figures (GIF). To appear in Planetary and Space Science
Particle Transport in intense small scale magnetic turbulence with a mean field
Various astrophysical studies have motivated the investigation of the
transport of high energy particles in magnetic turbulence, either in the source
or en route to the observation sites. For strong turbulence and large rigidity,
the pitch-angle scattering rate is governed by a simple law involving a mean
free path that increases proportionally to the square of the particle energy.
In this paper, we show that perpendicular diffusion deviates from this behavior
in the presence of a mean field. We propose an exact theoretical derivation of
the diffusion coefficients and show that a mean field significantly changes the
transverse diffusion even in the presence of a stronger turbulent field. In
particular, the transverse diffusion coefficient is shown to reach a finite
value at large rigidity instead of increasing proportionally to the square of
the particle energy. Our theoretical derivation is corroborated by a dedicated
Monte Carlo simulation. We briefly discuss several possible applications in
astrophysics.Comment: (9 pages, 6 figures, revised version with minor changes
Emergence and prevalence of/Mactamase producing Haemophilus influenzae in Finland and susceptibility of 102 respiratory isolates to eight antibiotics
A survey of 102 consecutive clinical isolates of Haemophilus influenzae mainly from otolaryngological patients revealed 13 ampicillin resistant ones, while 2 years earlier none were found. All the 13 strains which were resistant according to the broth dilution minimal inhibitory concentration (MIC) could be shown to produce /Mactamase using the chromogenic cephalosporin 87/312. Routine disc diffusion susceptibility testing had under-estimated the resistance and five of the /Mactamase producing strains had been reported as ampicillin susceptible. Amoxycillin and azidocillin were equally active and slightly less active than ampicillin against /Mactamase negative strains. The 13 ampicillin resistant strains were also resistant to amoxycillin and azidocillin, and also the MICs of cephalothin and cephalexin were significantly increased by /Mactamase production, while cefuroxime, erythromycin and chloramphenicol were not affected. A marked inoculum effect was demonstrated with the /Mactam antibiotics, and only a narrow (about tenfold) range of inoculum concentrations distinguished reliably between susceptible and resistant strains. The MICs of the penicillins against /Mactamase positive H. influenzae increased markedly with incubation time
Cosmic ray acceleration at supergalactic accretion shocks: a new upper energy limit due to a finite shock extension
Accretion flows onto supergalactic-scale structures are accompanied with
large spatial scale shock waves. These shocks were postulated as possible
sources of ultra-high energy cosmic rays. The highest particle energies were
expected for perpendicular shock configuration in the so-called "Jokipii
diffusion limit", involving weakly turbulent conditions in the large-scale
magnetic field imbedded in the accreting plasma. For such configuration we
discuss the process limiting the highest energy that particles can obtain in
the first-order Fermi acceleration process due to finite shock extensions to
the sides, along and across the mean magnetic field. Cosmic ray outflow along
the shock structure can substantially lower (below ~10^18 eV for protons) the
upper particle energy limit for conditions considered for supergalactic shocks.Comment: A&A, accepte
The mixing of interplanetary magnetic field lines: A significant transport effect in studies of the energy spectra of impulsive flares
Using instrumentation on board the ACE spacecraft we describe short-time scale (~3 hour) variations observed in the arrival profiles of ~20 keV nucleon^(–1) to ~2 MeV nucleon^(–1) ions from impulsive solar flares. These variations occurred simultaneously across all energies and were generally not in coincidence with any local magnetic field or plasma signature. These features appear to be caused by the convection of magnetic flux tubes past the observer that are alternately filled and devoid of flare ions even though they had a common flare source at the Sun. In these particle events we therefore have a means to observe and measure the mixing of the interplanetary magnetic field due to random walk. In a survey of 25 impulsive flares observed at ACE between 1997 November and 1999 July these features had an average time scale of 3.2 hours, corresponding to a length of ~0.03 AU. The changing magnetic connection to the flare site sometimes lead to an incomplete observation of a flare at 1 AU; thus the field-line mixing is an important effect in studies of impulsive flare energy spectra
Particle acceleration in rotating and shearing jets from AGN
We model the acceleration of energetic particles due to shear and centrifugal
effects in rotating astrophysical jets. The appropriate equation describing the
diffusive transport of energetic particles in a collisionless, rotating
background flow is derived and analytical steady state solutions are discussed.
In particular, by considering velocity profiles from rigid, over flat to
Keplerian rotation, the effects of centrifugal and shear acceleration of
particles scattered by magnetic inhomogeneities are distinguished. In the case
where shear acceleration dominates, it is confirmed that power law particle
momentum solutions exist, if the mean scattering
time is an increasing function of momentum. We show
that for a more complex interplay between shear and centrifugal acceleration,
the recovered power law momentum spectra might be significantly steeper but
flatten with increasing azimuthal velocity due to the increasing centrifugal
effects. The possible relevance of shear and centrifugal acceleration for the
observed extended emission in AGN is demonstrated for the case of the jet in
the quasar 3C273.Comment: 15 pages (including 8 pages Appendix), 4 figures; accepted for
publication in A&
Turbulence and particle acceleration in collisionless supernovae remnant shocks: II- Cosmic-ray transport
Supernovae remnant shock waves could be at the origin of cosmic rays up to
energies in excess of the knee (eV) if the magnetic
field is efficiently amplified by the streaming of accelerated particles in the
shock precursor. This paper follows up on a previous paper \citep{pell05} which
derived the properties of the MHD turbulence so generated, in particular its
anisotropic character, its amplitude and its spectrum. In the present paper, we
calculate the diffusion coefficients, also accounting for compression through
the shock, and show that the predicted three-dimensional turbulence spectrum
(with and the
wavenumber components along and perpendicular to the shock normal) generally
leads to Bohm diffusion in the parallel direction. However, if the anisotropy
is constrained by a relation of the form ,
which arises when the turbulent energy cascade occurs at a constant rate
independent of scale, then the diffusion coefficient loses its Bohm scaling and
scales as in isotropic Kolmogorov turbulence. We show that these diffusion
coefficients allow to account for X-ray observations of supernova remnants.
This paper also calculates the modification of the Fermi cycle due to the
energy lost by cosmic rays in generating upstream turbulence and the
concomittant steepening of the energy spectrum. Finally we confirm that cosmic
rays can produced an amplified turbulence in young SNr during their free
expansion phase such that the maximal energy is close to the knee and the
spectral index is close to 2.3 in the warm phase of the interstellar mediumComment: 13 pages, 4 figures, accepted for publication in Astronomy &
Astrophysics main journa
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