Almost all our knowledge of the Universe comes from the study of photons, whether they be of X-ray, visible, infrared or radio wave¬length. There is, however, an increasingly important branch of astronomy that is concerned with quite different particles, COSMIC RAYS. The term cosmic ray is applied to elementary particles, usually protons, electrons or the nuclei of atoms, which travel through space with a velocity close to that of light. Since most of these particles are absorbed by the Earth’s atmosphere they must be studied from the tops of high mountains, or from rockets, balloons, or satellites.

Most photons can travel across the Galaxy from their point of origin to the Earth with almost no deflection from a straight line. This is not true for cosmic rays. Protons, electrons and nuclei all possess electric charge and are therefore continuously deflected in different directions as they encounter the constantly-changing magnetic fields of interstellar and interplanetary space. Except possibly for the very highest energy particles, we cannot tell where a cosmic ray came from by looking at the direction it was travelling when it reached the Earth. The observational problems faced by cosmic ray astronomers are somewhat analogous to those of optical astronomers if the latter were forced always to observe the skies through a frosted glass window which randomly scattered all star¬light as it entered the telescope!

The main properties of cosmic rays that can be measured are their energies and the numbers of the different types of particles reaching the Earth. It is found that some 90 per cent of the particles are protons, the nuclei of hydrogen atoms, with most of the rest being a-particles, the nuclei of helium atoms. The bulk of the particles detected have energies around 108-1O9 eV, but many particles with much greater energies are also seen. The number of particles of a given energy depends on the energy. Above 109 eV the particle flux drops steadily with increasing energy all the way to 1020eV. We know very little about cosmic rays with energy greater than 1019eV because so few of them arrive on the Earth each day. There is, nevertheless, very great interest in such particles, as they have energies many orders of magnitude greater than can be generated by particle accelerators in terrestrial laboratories. The energy contained in a single 1020eV cosmic-ray proton is about 16 Joules – enough to lift this book several centimeters off the table.

High-energy cosmic rays possess so much momentum that they are essentially undeflected as they pass through the Solar System. Cosmic rays with energies less than 109ey, however, are much more susceptible to the influence of the Sun, in particular of the solar wind. As a result, comparatively few cosmic rays with energies below 108eV reach the Earth. This effect, known as SOLAR MODULATION, prevents us from measuring the true flux of low-energy cosmic rays in interstellar space.

The only place in the Galaxy where we can measure cosmic rays directly is the vicinity of our Sun. Several lines of indirect evidence, however, indicate that the flux of cosmic rays we detect on Earth is typical of many regions of the Galaxy and that cosmic rays are a universal constituent of the interstellar medium. In the rest of this section we will describe some of the secondary effects of cosmic rays such as radio and gamma-ray emission, and also discuss what id known about their origin and propagation.

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