Trapping Of Cosmic Rays Research Articles

Cosmic-ray propagation in the bi-stable interstellar medium

Context. Cosmic rays propagate through the galactic scales down to the smaller scales at which stars form. Cosmic rays are close to energy equipartition with the other components of the interstellar medium and can provide a support against gravity if pressure gradients develop. Aims. We study the propagation of cosmic rays within the turbulent and magnetised bi-stable interstellar gas. The conditions necessary for cosmic-ray trapping and cosmic-ray pressure gradient development are investigated. Methods. We derived an analytical value of the critical diffusion coefficient for cosmic-ray trapping within a turbulent medium, which follows the observed scaling relations. We then presented a numerical study using 3D simulations of the evolution of a mixture of interstellar gas and cosmic rays, in which turbulence is driven at varying scales by stochastic forcing within a box of 40 pc. We explored a large parameter space in which the cosmic-ray diffusion coefficient, the magnetisation, the driving scale, and the amplitude of the turbulence forcing, as well as the initial cosmic-ray energy density, vary. Results. We identify a clear transition in the interstellar dynamics for cosmic-ray diffusion coefficients below a critical value deduced from observed scaling relations. This critical diffusion depends on the characteristic length scale L of Dcrit ≃ 3.1 × 1023 cm2 s−1(L/1 pc)q+1, where the exponent q relates the turbulent velocity dispersion σ to the length scale as σ ~ Lq. Hence, in our simulations this transition occurs around Dcrit ≃ 1024–1025 cm2 s−1. The transition is recovered in all cases of our parameter study and is in very good agreement with our simple analytical estimate. In the trapped cosmic-ray regime, the induced cosmic-ray pressure gradients can modify the gas flow and provide a support against the thermal instability development. We discuss possible mechanisms that can significantly reduce the cosmic-ray diffusion coefficients within the interstellar medium. Conclusions. Cosmic-ray pressure gradients can develop and modify the evolution of thermally bi-stable gas for diffusion coefficients D0 ≤ 1025 cm2 s−1 or in regions where the cosmic-ray pressure exceeds the thermal one by more than a factor of ten. This study provides the basis for further works including more realistic cosmic-ray diffusion coefficients, as well as local cosmic-ray sources.

He and Ne Ages of Large Presolar Silicon Carbide Grains: Solving the Recoil Problem

AbstractKnowledge about the age of presolar grains provides important insights into Galactic chemical evolution and the dynamics of grain formation and destruction processes in the Galaxy. Determination from the abundance of cosmic ray interaction products is straightforward, but in the past has suffered from uncertainties in correcting for recoil losses of spallation products. The problem is less serious in a class of large (tens of μm) grains. We describe the correction procedure and summarise results for He and Ne ages of presolar SiC ‘Jumbo’ grains that range from close to zero to ∼850 Myr, with the majority being less than 200 Myr. We also discuss the possibility of extending our approach to the majority of smaller SiC grains and explore possible contributions from trapping of cosmic rays.

Galactic cosmic ray modulation and interplanetary medium perturbations due to a long‐living active region during October 1989

During October 1989, three very energetic flares were ejected by the same active region at longitudes 9° E, 32° W, and 57° W, respectively. The shape of the galactic cosmic ray variations suggests the presence of large magnetic cloud structures (Nagashima et al., 1990) following the shock‐associated perturbations. In spite of long data gaps the interplanetary observations at IMP 8 (near the Earth) and ICE (∼1 AU, ∼65° W) confirm this possibility for the event related to the 9° E flare; the principal axes analysis shows that the interplanetary magnetic field variations at both spacecraft locations are mainly confined on a meridian plane. This result suggests that the western longitudinal extension of this cloud is indeed very large (≥75°). The nonnegligible depression in the cosmic ray intensity observed inside the possible cloud related to the 57° W flare indicates that also the eastern extension could be very wide. The analysis of neutron monitor data shows clearly the cosmic ray trapping effect of magnetic clouds; this mechanism seems to be responsible for the enhanced diurnal effect often observed during the recovery phase of Forbush decreases. We give an interpretation for the anisotropic cosmic ray peak occurring in the third event, and, related to that, we suggest that the Forbush decrease modulated region at the Earth's orbit could be somewhat wider than the magnetic cloud, as already anticipated by Nagashima et al. (1990). By this analysis, based mainly on cosmic ray data, we show that it is possible to do reasonable inferences on the large‐scale structure of flare‐related interplanetary perturbations when interplanetary medium data are not completely present.

Confinement of cosmic rays in molecular clouds

We explore the trapping of cosmic rays produced by a supernova in a dense molecular cloud and the implications for COS B ..gamma..-ray sources. After an extensive study of the Alfven wave streaming instability of cosmic rays in such an environment, we conclude that unless the magnetic field in the cloud is as large as 10/sup -3/ gauss, the cosmic rays are unable to escape from the cloud before losing their energy through nuclear collisions. Energy losses will always occur for a sufficiently large cloud, regardless of the self-trapping effect. Buried supernovae could therefore be detectable sources of 100 MeV ..pi../sup 0/-decay ..gamma..-rays. We discuss possible escape mechanisms involving stellar wind cavities and their relationship to recent studies of apparently excessive antiproton fluxes in cosmic rays.

Hydromagnetic waves in high Beta plasmas

We have determined the wave propagation and damping properties in a collisionless thermal plasma for which ..beta.., the ratio of the plasma pressure to the magnetic pressure, is much greater than unity. We achieve this by solving the full collisionless dispersion relation as an expansion in 1/..beta... To do this we develop a set of iteration methods which converge to this solution. We illustrate these results with two applications, the trapping of cosmic rays in supernovae remnants and the collisionless damping of hydromagnetic turbulence.

The cosmic-ray flare effect: 3. Deductions regarding the interplanetary magnetic field

The observations of cosmic-ray flare effects made at sea level are used to investigate the magnetic fields within the inner solar system. Considering the temporal dependence of intensity, the degree of anisotropy, and the direction in space from which the fluxes were greatest it is concluded that, when a large, active sunspot group is on the far western portion of the solar disk, magnetic lines of force essentially connect the sunspot group to the earth. It is shown that small-scale irregularities exist in these lines of force. The over-all configuration of the magnetic field regime is investigated; the trapping of cosmic rays injected by solar flares and the exclusion of galactic cosmic rays are discussed. The relative merits of two contemporary theoretical models of the interplanetary fields established by solar activity are discussed.