Cosmic ray propagation

Figure 16.23. Two examples of neutralino models that provide a good fit to the excess of cosmic ray positrons observed by the HEAT collaboration. The two sets of data points (open and filled squares) are derived from two different instruments flown in 1994-95 and 2000. The lines represent (i) the best expectation we have from models of cosmic ray propagation in the galaxy ( bkg. only fit ), which underestimate the data points above 7 GeV (ii) the effect of adding positrons from neutralino annihilations (lines SUSY component , SUSY+bkg. fit , and bkg. component , the latter being the resulting background component when the data are fitted to the sum of background and neutralino contributions). (Figures from Baltz, Edsjo, Freese, Gondolo(2002)).

The thickness of the disk of the galaxy is of order 300 pc = 1000 light years, which is much shorter than the characteristic propagation time of 10 million years. The explanation is that the charged particles are trapped in the turbulent magnetized plasma of the interstellar medium and only diffuse slowly away from the disk, which is assumed to be where the sources are located. Measurements of the ratio of unstable to stable secondary nuclei (especially 10Be/9 lie) are used to determine resc independently of the product np resc and hence to constrain further the models of cosmic-ray propagation. 

Antiprotons and positrons. Antiprotons and positrons are of special interest because an excess over what is expected from production by protons during propagation could reflect an exotic process such as evaporation of primordial black holes or decay of exotic relic particles (Bottino et al., 1998). At a more practical level, they are important because they are secondaries of the dominant proton component of the cosmic radiation. As a consequence their spectra and abundances provide an independent constraint on models of cosmic-ray propagation (Moskalenko et al., 1998). 

Secondary antiprotons have a kinematic feature analogous to that in 7r°-decay gamma rays but at a higher energy related to the nucleon mass. In this case the feature is related to the high threshold for production of a nucleon-antinucleon pair in a proton-proton collision. This kinematic feature is observed in the data (Orito et al., 2000), and suggests that an exotic component of antiprotons is not required. Antiproton fluxes are consistent with the basic model of cosmic-ray propagation described in the Introduction. 

It is worth noting that the knee observed in the cosmic ray spectrum at 4 x 1015 eV may arise not in the sources but in the process of cosmic ray propagation in the Galaxy, e.g. as a result of interplay between ordinary and Hall diffusion in galactic magnetic fields [54], [Roulet 2004], Of course, this explanation requires the existence of a power-law source spectrum which extends without essential breaks up to about 1018 eV or even further. 

The primary cosmic rays propagate through the interstellar medium (ISM) until they either escape into extragalactic space, or are removed by interaction or energy losses in the ISM. Their interstellar equilibrium intensity may be recorded with a detector which is usually carried above the earth s atmosphere on spacecraft or balloon. Secondary cosmic rays are those that are generated as products from interactions of the primaries in the ISM positrons and antiprotons mostly come from interactions of primary protons, while the secondary nuclei such as Li, Be, B, and the elements just below iron, which cannot be produced by primary nucleosynthesis, are the products of spallation reactions of heavier primaries in the ISM. The overall arriving cosmic-ray intensity represents a mix of primary and secondary particles. 

Electromagnetic radiation (Section 13.1) Various forms of radiation propagated at the speed of light. Electromagnetic radiation includes (among others) visible light infrared, ultraviolet, and microwave radiation and radio waves, cosmic rays, and X-rays. 

Radiation The emission and propagation of energy through space or through a material medium in the form of waves. The term also includes subatomic particles, such as a, P, and cosmic rays and electromagnetic radiation. 

Fig. 9.2. Schematic view of the life history of a cosmic ray from acceleration in the source through propagation in the Galaxy to observation above the Earth s atmosphere. Adapted from Rolfs and Rodney (1988).

C. Tanford and R. N. Pease, J. Cham, Phys.y 16, 861 (1947), have developed a model in which propagation occurs by diffusion of radicals. Since wall initiation of chains is not possible in most flames, it seems quite reasonable to expect that initiation by diffusion of chain carriers may be a necessary condition for propagation. It is difficult, however, to see this as a limiting or controlling condition even in fast flames, particularly for branching chain reactions in which, because of the rapid multiplication of centers, even cosmic rays can act as initiations once ignition temperatures have been reached. 

In this picture, the amount of secondary nuclei is a measure of the characteristic time for propagation of cosmic rays before they escape from the galaxy into inter-galactic space. A simplified version of the diffusion equation that relates the observed abundances and spectra to initial values is... 

To a first approximation, the all-particle spectrum of cosmic rays can be described by a power law on more than 11 decades on particle energy, so that the dependence of cosmic ray intensity on particle energy is close to E 2-7 at energy more than about 10 GeV. Closer examination reveals some structure in the galactic cosmic ray spectrum that includes the knee at 4 x 1015 eV, the second knee at about 1018 eV, and the ankle at 1019 eV. The steady-state spectrum is shaped by two principle processes - the acceleration at the sources and the subsequent propagation in the Galaxy. 

Ptuskin, V.S. (2001). Propagation, confinement models, and large-scale dynamical effects of galactic cosmic rays , Space. Sci. Rev. 99, 281... 

ELEMENTAL COMPOSITION AND PROPAGATION OF COSMIC RAYS AT HIGH ENERGIES... 

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