And the Interstellar Magnetic Field

Cosmic Rays. Elementary particles and atomic nuclei reaching the Earth from space are called cosmic rays. They occur throughout interstellar space with an energy density of the same order of magnitude as that of the radiation from stars. Cosmic rays are therefore important for the ionization and heating of interstellar gas.

Since cosmic rays are charged, their direction of propagation in space is constantly changed by the magnetic field. Their direction of arrival therefore gives no information about their place of origin. The most important properties of cosmic rays that can be observed from the Earth are their particle composition and energy distribution. As noted in Sect. 3.6, these observations have to be made in the upper atmosphere or from satellites, since cosmic ray particles are destroyed in the atmosphere.

The main constituent of the cosmic rays (about 90%) is hydrogen nuclei or protons. The second most important constituent (about 9%) is helium nuclei or a particles. The rest of the particles are electrons and nuclei more massive than helium.

Most cosmic rays have an energy smaller than 109 eV. The number of more energetic particles drops rapidly with increasing energy. The most energetic protons have an energy of 1020 eV, but such particles are very rare - the energy of one such proton could lift this book about one centimetre. (The largest particle accelerators reach "only" energies of 1012 eV.)

The distribution of low-energy (less than 108 eV) cosmic rays cannot be reliably determined from the Earth, since solar "cosmic rays", high-energy protons and electrons formed in solar flares fill the solar system and strongly affect the motion of low-energy cosmic rays.

The distribution of cosmic rays in the Milky Way can be directly inferred from gamma-ray and radio observations. The collisions of cosmic ray protons with interstellar hydrogen atoms gives rise to pions which then decay to form a gamma-ray background. The radio background is formed by cosmic ray electrons which emit synchrotron radiation in the interstellar magnetic field.

Both radio and gamma-ray emission are strongly concentrated in the galactic plane. From this it has been concluded that the sources of cosmic rays must also be located in the galactic plane. In addition there are individual peaks in the backgrounds around known supernova remnants. In the gamma-ray region such peaks are observed at e. g. the Crab nebula and the Vela pulsar; in the radio region the North Polar Spur is a large, nearby ring-like region of enhanced emission.

Apparently a large fraction of cosmic rays have their origin in supernovae. An actual supernova explosion will give rise to energetic particles. If a pulsar is formed, observations show that it will accelerate particles in its surroundings. Finally the shock waves formed in the expanding supernova remnant will also give rise to relativistic particles.

On the basis of the relative abundances of various cosmic ray nuclei, it can be calculated how far they have travelled before reaching the Earth. It has been found that typical cosmic ray protons have travelled for a period of a few million years (and hence also a distance of a few million light-years) from their point of origin. Since the diameter of the Milky Way is about 100,000 light-years, the protons have crossed the Milky Way tens of times in the galactic field.

The Interstellar Magnetic Field. The strength and direction of the interstellar magnetic field are difficult to determine reliably. Direct measurements are impossible, since the magnetic fields of the Earth and the Sun are much stronger. However, using various sources it has been possible to deduce the existence and strength of the field.

We have already seen that interstellar grains give rise to interstellar polarization. In order to polarize light, the dust grains have to be similarly oriented; this can only

Fig. 15.27. The polarization of starlight. The dashes give the direction and degree of the polarization. The thinner dashes correspond to stars with polarization smaller than 0.6%; the thicker dashes to stars with larger polarization.

The scale is shown in the upper left-hand corner. Stars with polarization smaller than 0.08% are indicated by a small circle. (Mathewson, D.S., Ford, V.L. (1970): Mem. R.A.S. 74, 139)

Fig. 15.27. The polarization of starlight. The dashes give the direction and degree of the polarization. The thinner dashes correspond to stars with polarization smaller than 0.6%; the thicker dashes to stars with larger polarization.

The scale is shown in the upper left-hand corner. Stars with polarization smaller than 0.08% are indicated by a small circle. (Mathewson, D.S., Ford, V.L. (1970): Mem. R.A.S. 74, 139)

be achieved by a general magnetic field. Figure 15.27 shows the distribution of interstellar polarization over the sky. Stars near each other generally have the same polarization. At low galactic latitudes the polarization is almost parallel to the galactic plane, except where one is looking along a spiral arm.

More precise estimates of the strength of the magnetic field can be obtained from the rotation of the plane of polarization of the radio radiation from distant sources. This Faraday rotation is proportional to the strength of the magnetic field and to the electron density. Another method is to measure the Zeeman splitting of the 21 cm radio line. These measurements have fairly consistently given a value of 10-10-10-9 T for the strength of the interstellar magnetic field. This is about one millionth of the interplanetary field in the solar system.

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