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 $f(p) \propto p^{-(3+\alpha)}$ exist, if the mean scattering time $\tau_c \propto p^{\alpha}$ 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 ($E\simeq3\cdot 10^{15}$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 $k_\perp S_{3\rm d}(k_\parallel,k_\perp)\propto k_\parallel^{-1}k_\perp^{-\alpha}$ (with $k_\parallel$ and $k_\perp$ 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 $k_\parallel \propto k_\perp^{2/3}$, 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