Time and energy dependence of the cosmic ray gradient in the outer heliosphere

Pioneers 10 and 11, now 35 and 18 AU from the Sun, continue to extend our knowledge of the spatial dependence of cosmic ray intensities in the heliosphere. Radial gradients measured from these spacecraft by UCSD detectors which have integral energy responses above thresholds of 80 and 500 MeV/nucleon are reported. An average gradient of 2%/AU typifies the data set as a whole, but there are time and energy dependences that deviate from this value. With operating lifetimes of 13 and 12 years, respectively, for the two spacecraft, the time dependence was followed for over a solar cycle. The higher energy channel shows less modulation on all time scales. At the start of the present cycle, the gradient is lower than the average value during the last solar cycle

Fine structure in the gamma-ray sky

The EGRET results for gamma-ray intensities in and near the Galactic Plane have been analysed in some detail. Attention has been concentrated on energies above 1 GeV and the individual intensities in a $4^\circ$ longitude bin have been determined and compared with the large scale mean found from a nine-degree polynomial fit. Comparison has been made of the observed standard deviation for the ratio of these intensities with that expected from variants of our model. The basic model adopts cosmic ray origin from supernova remnants, the particles then diffusing through the Galaxy with our usual 'anomalous diffusion'. The variants involve the clustering of SN, a frequency distribution for supernova explosion energies, and 'normal', rather than 'anomalous' diffusion. It is found that for supernovae of unique energy, and our usual anomalous diffusion, clustering is necessary, particularly in the Inner Galaxy. An alternative, and preferred, situation is to adopt the model with a frequency distribution of supernova energies. The results for the Outer Galaxy are such that no clustering is required.Comment: 10 pages, 4 figures, 1 table, accepted for publication in J.Phys.G: Nucl.Part.Phy

Non-thermal Origin of the EUV and Soft X-rays from the Coma Cluster - Cosmic Rays in Equipartition with the Thermal Medium

The role of cosmic rays (CR) in the formation and evolution of clusters of galaxies has been much debated. It may well be related to other fundamental questions, such as the mechanism which heats and virializes the intracluster medium (ICM), and the frequency at which the ICM is shocked. There is now compelling evidence both from the cluster soft excess (CSE) and the `hard-tail' emissions at energies above 10 keV, that many clusters are luminous sources of inverse-Compton (IC) emission. This is the first direct measurement of cluster CR: the technique is free from our uncertainties in the ICM magnetic field, and is not limited to the small subset of clusters which exhibit radio halos. The CSE emitting electrons fall within a crucial decade of energy where they have the least spectral evolution, and where most of the CR pressure resides. However their survival times do not date them back to the relic CR population. By using the CSE data of the Coma cluster, we demonstrate that the CR are energetically as important as the thermal ICM: the two components are in pressure equiparition. Thus, contrary to previous expectations, CR are a dominant component of the ICM, and their origin and effects should be explored. The best-fit CR spectral index is in agreement with the Galactic value.Comment: ApJ accepted; 10 pages LaTeX; 2 figures and 1 table in PostScrip

Galactic Cosmic Rays from Supernova Remnants: II Shock Acceleration of Gas and Dust

This is the second paper (the first was astro-ph/9704267) of a series analysing the Galactic Cosmic Ray (GCR) composition and origin. In this we present a quantitative model of GCR origin and acceleration based on the acceleration of a mixture of interstellar and/or circumstellar gas and dust by supernova remnant blast waves. We present results from a nonlinear shock model which includes (i) the direct acceleration of interstellar gas-phase ions, (ii) a simplified model for the direct acceleration of weakly charged dust grains to energies of order 100keV/amu simultaneously with the gas ions, (iii) frictional energy losses of the grains colliding with the gas, (iv) sputtering of ions of refractory elements from the accelerated grains and (v) the further shock acceleration of the sputtered ions to cosmic ray energies. The calculated GCR composition and spectra are in good agreement with observations.Comment: to appear in ApJ, 51 pages, LaTeX with AAS macros, 9 postscript figures, also available from ftp://wonka.physics.ncsu.edu/pub/elliso

Index

The interest in relativistic beam-plasma instabilities has been greatly rejuvenated over the past two decades by novel concepts in laboratory and space plasmas. Recent advances in this long-standing field are here reviewed from both theoretical and numerical points of view. The primary focus is on the two-dimensional spectrum of unstable electromagnetic waves growing within relativistic, unmagnetized, and uniform electron beam-plasma systems. Although the goal is to provide a unified picture of all instability classes at play, emphasis is put on the potentially dominant waves propagating obliquely to the beam direction, which have received little attention over the years. First, the basic derivation of the general dielectric function of a kinetic relativistic plasma is recalled. Next, an overview of two-dimensional unstable spectra associated with various beam-plasma distribution functions is given. Both cold-fluid and kinetic linear theory results are reported, the latter being based on waterbag and Maxwell–Jüttner model distributions. The main properties of the competing modes (developing parallel, transverse, and oblique to the beam) are given, and their respective region of dominance in the system parameter space is explained. Later sections address particle-in-cell numerical simulations and the nonlinear evolution of multidimensional beam-plasma systems. The elementary structures generated by the various instability classes are first discussed in the case of reduced-geometry systems. Validation of linear theory is then illustrated in detail for large-scale systems, as is the multistaged character of the nonlinear phase. Finally, a collection of closely related beam-plasma problems involving additional physical effects is presented, and worthwhile directions of future research are outlined.Original Publication: Antoine Bret, Laurent Gremillet and Mark Eric Dieckmann, Multidimensional electron beam-plasma instabilities in the relativistic regime, 2010, Physics of Plasmas, (17), 12, 120501-1-120501-36. http://dx.doi.org/10.1063/1.3514586 Copyright: American Institute of Physics http://www.aip.org/</p

A New Measurement of Cosmic Ray Composition at the Knee

The Dual Imaging Cerenkov Experiment (DICE) was designed and operated for making elemental composition measurements of cosmic rays near the knee of the spectrum at several PeV. Here we present the first results using this experiment from the measurement of the average location of the depth of shower maximum, , in the atmosphere as a function of particle energy. The value of near the instrument threshold of ~0.1 PeV is consistent with expectations from previous direct measurements. At higher energies there is little change in composition up to ~5 PeV. Above this energy is deeper than expected for a constant elemental composition implying the overall elemental composition is becoming lighter above the knee region. These results disagree with the idea that cosmic rays should become on average heavier above the knee. Instead they suggest a transition to a qualitatively different population of particles above 5 PeV.Comment: 7 pages, LaTeX, two eps figures, aas2pp4.sty and epsf.sty included, accepted by Ap.J. Let

Plasma Wakefield Acceleration for Ultrahigh Energy Cosmic Rays

A cosmic acceleration mechanism is introduced which is based on the wakefields excited by the Alfven shocks in a relativistically flowing plasma, where the energy gain per distance of a test particle is Lorentz invariant. We show that there exists a threshold condition for transparency below which the accelerating particle is collision-free and suffers little energy loss in the plasma medium. The stochastic encounters of the random accelerating-decelerating phases results in a power-law energy spectrum: f(e) 1/e^2. The environment suitable for such plasma wakefield acceleration can be cosmically abundant. As an example, we discuss the possible production of super-GZK ultra high energy cosmic rays (UHECR) through this mechanism in the atmosphere of gamma ray bursts. We show that the acceleration gradient can be as high as G ~ 10^16 eV/cm. The estimated event rate in our model agrees with that from UHECR observations.Comment: 11 pages, 1 figure, submitted to Phys. Rev. Let

Diffusive Shock Acceleration with Magnetic Amplification by Non-resonant Streaming Instability in SNRs

We investigate the diffusive shock acceleration in the presence of the non-resonant streaming instability introduced by Bell (2004). The numerical MHD simulations of the magnetic field amplification combined with the analytical treatment of cosmic ray acceleration permit us to calculate the maximum energy of particles accelerated by high-velocity supernova shocks. The estimates for Cas A, Kepler, SN1006, and Tycho historical supernova remnants are given. We also found that the amplified magnetic field is preferentially oriented perpendicular to the shock front downstream of the fast shock. This explains the origin of the radial magnetic fields observed in young supernova remnants.Comment: 18 pages, 9 figures, accepted to Ap

Technology requirements of exploration beyond Neptune by solar sail propulsion

This paper provides a set of requirements for the technology development of a solar sail propelled Interstellar Heliopause Probe mission. The mission is placed in the context of other outer solar systems missions, ranging from a Kuiper Belt mission through to an Oort cloud mission. Mission requirements are defined and a detailed parametric trajectory analysis and launch date scan performed. Through analysis of the complete mission trade space a set of critical technology development requirements are identified which include an advanced lightweight composite High-Gain Antenna, a high-efficiency Ka-band travelling-wave tube amplifier and a radioisotope thermoelectric generator with power density of approximately 12 W/kg. It is also shown that the Interstellar Heliopause Probe mission necessitates the use of a spinning sail, limiting the direct application of current hardware development activities. A Kuiper Belt mission is then considered as a pre-curser to the Interstellar Heliopause Probe, while it is also shown through study of an Oort cloud mission that the Interstellar Heliopause Probe mission is the likely end-goal of any future solar sail technology development program. As such, the technology requirements identified to enable the Interstellar Heliopause Probe must be enabled through all prior missions, with each mission acting as an enabling facilitator towards the next

Nonthermal Emission from a Supernova Remnant in a Molecular Cloud

In evolved supernova remnants (SNRs) interacting with molecular clouds, such as IC 443, W44, and 3C391, a highly inhomogeneous structure consisting of a forward shock of moderate Mach number, a cooling layer, a dense radiative shell and an interior region filled with hot tenuous plasma is expected. We present a kinetic model of nonthermal electron injection, acceleration and propagation in that environment and find that these SNRs are efficient electron accelerators and sources of hard X- and gamma-ray emission. The energy spectrum of the nonthermal electrons is shaped by the joint action of first and second order Fermi acceleration in a turbulent plasma with substantial Coulomb losses. Bremsstrahlung, synchrotron, and inverse Compton radiation of the nonthermal electrons produce multiwavelength photon spectra in quantitative agreement with the radio and the hard emission observed by ASCA and EGRET from IC 443. We distinguish interclump shock wave emission from molecular clump shock wave emission accounting for a complex structure of molecular cloud. Spatially resolved X- and gamma- ray spectra from the supernova remnants IC 443, W44, and 3C391 as might be observed with BeppoSAX, Chandra XRO, XMM, INTEGRAL and GLAST would distinguish the contribution of the energetic lepton component to the gamma-rays observed by EGRET.Comment: 14 pages, 4 figure, Astrophysical Journal, v.538, 2000 (in press