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Detailed calculations of the interaction of electrons and photons with the relict radiation (inverse Compton effect and pair production process, respectively) have been made. The attenuation length for energy and intensity of the photons and electrons has thereby been established. Si sono eseguiti calcoli dettagliati dell’interazione degli elettroni e dei protoni con la radiazione di relitto (effetto Compton inverso e processo di produzione di coppie, rispettivamente). Sono state conseguentemente determinate la lunghezza di attenuazione per l’energia e l’intensità dei fotoni e degli elettroni.

Golden et al.1have demonstrated that the measured ratio of the flux of antiprotons to protons (\[pmacr]/p ratio) is significantly greater than expected from the commonly adopted ‘leaky box’ model of cosmic ray propagation, a model in which the \[pmacr]'s are secondaries generated by the interaction of cosmic rays with the interstellar medium (ISM) in the Galaxy. At the energy at which measurements were made (kinetic energy≃8 GeV), disturbing effects due to interplanetary modulation are negligible and there is now general agreement2–5that the excess is a factor of 3 at least (an earlier calculation6of the expected \[pmacr]/p which gave a result near to observation has been shown to be incorrect2–5). Bogolomov7found a similar but statistically less compelling enhancement at somewhat lower energies. A prominent characteristic of the \[pmacr]/p ratio for galactic secondaries is a rapid fall with decreasing energy, below≃5 GeV, due to kinematic factors. It was therefore remarkable that Buffington et al.8recently reported a ratio only a factor of 2.5 smaller at a very low kinetic energy (average≃0.2 GeV). This result, if correct, necessitates serious consideration of more exotic explanations. An important feature at these very low energies is the modulation of the flux by the interplanetary field and we have now examined this facet. We describe an improved calculation of the energy spectrum of \[pmacr] expected for the standard leaky box model and show that even the enhanced secondary \[pmacr] production in the ‘closed Galaxy’ model of Peters and Westergaard9fails by a wide margin to reproduce the high observed flux at low energies. A more radical departure from conventional thinking is to regard the observed \[pmacr]s as primaries; a universal baryon-symmetric model and a black hole evaporation model are considered.

There has long been controversy over the question of mass composition and interaction characteristics of cosmic rays with energies above ∼1012eV since in this region direct determination of particle mass has not been possible and accelerator data on interaction mechanisms have been scarce. Accelerator results in the 1970s led to the suggestion that particle interactions could be represented by the so-called scaling of the type enunciated by Feynman1. In this model2an asymptotic condition is reached by 1011eV such that most interaction energy is carried off by a small, constant number of secondaries. Studies of cosmic rays showed that by 1015eV if scaling remained valid then the primary mass composition must have changed considerably. An alternative model3assumes that the mass composition remains unchanged, in which case the interaction characteristics would have to be very different4. The advent of the p\[pmacr] collider results, corresponding to an energy (with respect to a stationary target) of ∼2×1014eV, now allows us to make a distinction between the two models. Here we present evidence supporting the view that the mass composition of high-energy cosmic rays is not much different from that of lower energy rays, but the scaling model is invalid.

A NUMBER of suggestions have been made1–3that encounters of the solar system with extra-solar phenomena in earlier epochs led to climatic and biological catastrophes. Most recently Whitten et al.4and Clark et al.5have drawn attention to the role of nearby supernovae (within ≃ 10 pc) from which catastrophic effects are expected (primarily the depletion of the atmospheric ozone layer and consequent transmission of damaging solar ultraviolet.) Whitten et al. estimate the probability of an encounter within a period of 108yr to be 1–5% and Clark et al. calculate 50% for the same quantity. In this letter, we argue that it is likely that rare and very big solar flares, and their attendant cosmic-ray fluxes, will give even more dramatic effects. This idea is not new, in that Reid et al.6have drawn attention to the effect of solar flares on the evolution of life, but here we endeavour to estimate the likely frequency of solar flares of sufficient strength to have significant effects. Furthermore, we draw attention to the ‘explosion’ at the galactic centre postulated by some7to have occurred about 107yr ago; if this actually took place then its effect too would have been important and exceeded likely supernova phenomena.

Anti-matter in the cosmic radiation incident on the Earth can be of two types: antiparticles from distant antigalaxies and secondary particles generated by collisions of cosmic rays with nuclei of the interstellar medium (ISM) either in our Galaxy or in other galaxies. Insofar as the 2.7 K relict radiation inhibits the former for positrons (with energy above some tens of MeV) and the expected ratio of extragalactic particles to galactic particles is very small for protons (and thus antiprotons) it is conventional to assume that collisions in the ISM are the source of the e+and \[pmacr] detected. We consider here the manner in which cosmic rays are propagated in the Galaxy by studying the fluxes of these antiparticles.

The origin of the predominant particle component of cosmic rays is still undecided although at ‘low’ energies (E∼109–1010eV) γ-ray data provide circumstantial evidence in favour of an origin in galactic sources1. The galactic magnetic field deflects trajectories, at least until about 1018eV, and almost completely destroys any relationship between the directions from which cosmic rays arrive and the directions to their sources, making the elucidation of origins a problem. Above 1018eV, if the primary cosmic rays are mainly protons, as would be expected and as is claimed3, then the absence of large anisotropies associated with the galactic plane strongly suggests that the particles are from extragalactic sources. If they are extragalactic and their sources are distributed widely in the Universe then, because of interactions between the protons and the photons of the 2.7 K relict radiation, the observed spectrum should steepen considerably above several times 1019eV. However, the opposite appears to be happening: the measured spectrum flattens above about 1019eV (refs 4–6) (see Fig. 2). Several suggestions have been made to account for this paradox, in each case the idea being to generate in extragalactic space a production spectrum which is so ‘flat’ that even after energy losses are taken into account, the resulting spectrum at the Earth has the characteristic observed flattening (for example ref. 7). Here we put forward an alternative model where the observed spectral shape arises in a rather natural way, being due to propagation characteristics in the local supercluster, the bulk of the very energetic particles having originated in its central region.

Recent experiments using extensive air shower techniques1–5have given evidence for the presence of significant fluxes of cosmic γ rays, in the energy range 1015–1016eV, from specific cosmic sources. We argue here that the flux from these sources, and others as yet unresolved, is probably sufficient to allow the explanation of a number of previously puzzling features of the cosmic radiation, in particular the way in which the amplitude and phase of the anisotropy of cosmic rays varies with energy. It is possible that the effects of cosmic γ rays will be detectable at energies as high as 1017eV.

THE controversy over the question of the origin of energetic cosmic rays is well known. There are those who believe that the bulk of the radiation comes from Galactic sources, such as supernovae1and others2who find rather good reasons for supposing that the radiation comes primarily from extragalactic sources. In an intermediate group there are those3who think that perhaps only the most energetic particles (E>1018eV) are derived from extra galactic sources.

AT energies above ∼ 1017eV it becomes increasingly difficult to maintain the observed near isotropy of ultra high energy cosmic rays if they are of Galactic origin, unless there is an extensive halo magnetic field, and the primaries are heavy nuclei1; instead, extragalactic origin is often assumed. The models proposed can be divided into three main groups: (1) ‘universal’, in which there is a class of galaxies in the Universe (of which our own is not a member) which are roughly uniformly distributed, and which generate the ultra high energy particles; (2) ‘universal plus supercluster enhancement’2, in which the existence of a supercluster is assumed—the enhanced density of galaxies in the SC allows a larger fraction, than is its share by volume, to have come from this region; and (3) ‘Supercluster trapping’3, where energetic phenomena in the SC generate the particles which are then trapped for periods of perhaps 109–1010yr.

THE problem of the origin of cosmic rays is well known; but in spite of considerable advances in knowledge concerning energetic extra-terrestrial objects in recent years, no satisfactory solution has appeared. It is not clear, even, whether the important sources are within the Galaxy or outside it1,2.