The interstellar medium is a very strong source of radio waves. Besides individual bright objects such as the supernova remnant Cassiopeia A and the H+ region Orion A, there is diffuse emission visible from all directions in the sky. This radio emission is the result of the interaction between cosmic-ray electrons and the Galactic magnetic field.

A map of this diffuse radio emission there is a strong concentration towards the plane of the Galaxy, but there are other prominent features as well. The largest of these, the North Polar Spur, rises from the plane near galactic longitude 30°, and may possibly be the result of a supernova explosion near the Earth some 105 years ago. The diffuse radio emission is seen at frequencies from about 1 MHz to 1 GHz, and, in common with many other celestial objects such as radio galaxies and supernova remnants, is strongest at long wavelengths, declining towards higher frequencies. Such a spectrum is characteristic of SYNCHROTRON RADIATION, which arises when high-speed electrons spiral around magnetic field lines. The realization around 1950 that the diffuse galactic radio emission was due to the synchrotron process was the first evidence for the existence of cosmic-ray electrons, as opposed to protons, in the interstellar medium. At most energies, cosmic-ray electrons are outnumbered 100 to 1 by protons and it was only in the H)(50s that cosmic-ray detection techniques became sensitive enough to detect cosmic-ray electrons at the top of the Earth’s atmosphere directly.

The radio frequency at which a synchrotron electron radiates depends on its energy and on the strength of the magnetic field about which it is spiralling. For example, a 3-GeV electron in a magnetic field of 3 x 10-10T(3 ? G) radiates predominantly at about 300MHz, a typical radio-astronomy frequency. In the same way that gamma rays can tell us about distant cosmic-ray protons, therefore, radio astronomy gives us a picture of the cosmic-ray electrons throughout the Galaxy. Neither case is straightforward, however, since the gamma rays are dependent also on the hydrogen density distribution, and the radio flux depends on the galactic magnetic field.

We do not know very much about the interstellar magnetic field. We cannot measure it directly because it is so much weaker inside the Solar System than that of the Sun or the Earth. We must there¬fore use indirect means. One method is to measure the Faraday rotation of the plane of polarization of cosmic radio waves, as they pass through the interstellar medium. Another is to look for the Zeeman splitting of the 21-cm hydrogen line as it is absorbed in an interstellar cloud containing a magnetic field. Rough estimates of the interstellar field strength can also be made on the basis of the optical polarization of starlight and from a comparison of the measured spectrum of the diffuse galactic radio emission with that predicted from the cosmic-ray electrons as observed near the earth. All these methods suggest that magnetic fields in the range 10-10 to 10-9T are fairly typical in our Galaxy, although stronger fields must exist in the denser regions of the interstellar medium. The direction of the magnetic field appears to be in the plane of the Galaxy in ‘ places and, near the Sun, probably runs along the local spiral arm.

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