The continuum radiation from most discrete sources and from parts of the background is partially plane polarized. A study of this polarization over a range of frequencies can be used to deduce the Faraday rotation, and hence the magnetic field conditions in the radiation source, the ionosphere and the interstellar regions of the Milky Way. For this work, observations at more closely spaced frequency intervals than the normally adequate octave separation are desirable at frequencies below about 2 GHz. The bands made available to the radioastronomy service, in accordance with the Final Acts of the World Administrative Radio Conference for Space Telecommunications, Geneva, 1971, represent a significant improvement over the international allocations made to the service in 1959 and 1963 and are a partial fulfilment of the requirements of the service. However, many of the allocated bands have insufficient bandwidths; they are in most cases shared with other services; many apply to limited areas of the world; and there are large intervals between some of the allocated bands. 2.3 Line radiation The first line radio radiation discovered in 1951 was that due to neutral atomic hydrogen at a rest frequency of 1420.4 MHz [Ewen and Purcell, 1951]. In 1963, the hydroxyl molecule OH was found [Weinreb et al., 1963]; it has a group of 4 lines at about 1612, 1665, 1667 and 1721 MHz and other transitions including isotopes of the molecule, are known, which give frequencies near 4.6, 6.0 and 8.1 GHz. More complete descriptions of specific molecular lines in emission and absorption can be found in Report 223-4. Since that time, many other radio lines have been found. Hydrogen, helium and carbon in their excited states give rise to a whole series of lines throughout the spectrum. These have been observed at frequencies as low as 400 MHz and as high as 85.69 GHz [Lilley and Palmer, 1968], Lines from more complex molecules and their isotopes have recently been discovered at a remarkable rate. Water, ammonia, carbon monoxide, cyanogen, hydrogen cynanide, formaldehyde, formic acid, methyl alcohol and cyano-acetylene are all now known to exist in interstellar space. More than thirty-five different molecules have now been identified in interstellar space by means of radio observations of spectral lines. Isotopic forms of many of these molecules have also been identified. When it is considered that each of these molecules and their isotopic forms may have a number of spectral lines, each associated with different energy level transitions, then it is understandable that the radio spectrum is found to be heavily populated with line signals. More than two hundred and fifty radio spectral lines have already been detected in interstellar space and are listed in Report 223-4. Not all are of equal astronomical interest, and the most important are listed in Annex I of this present Report. The value to astronomy of line observations is now very great. By using the neutral hydrogen line, the distribution and motion of hydrogen is mapped, both within our own galaxy and in many of our neighbouring galaxies. Studies of the Zeeman eifect on radio waves absorbed in clouds of hydrogen allowed the very small magnetic fields in interstellar space to be measured [Verschuur, 1969], The recombination lines allow measurements to be made of temperature as well as of the relative abundance of hydrogen and helium in the ionized nebulae in our galaxy. The OH lines, the water lines and the ammonia lines lead to complex theories of maser action in space to explain their behaviour. The formaldehyde line was the precursor to other, more complex, organic compound lines, with all the implications that their discovery might have on our knowledge of how life can be generated in our Universe. The study of radio frequency lines is one of the most difficult fields in radioastronomy. The best antennae and receivers and the most highly developed electronic processing equipment is used. Integration times of many hours are common. For all these reasons, freedom from harmful interference is necessary. It is necessary over a band of frequencies for each line, a band wide enough to include broadening and shifting of the original emission due to Doppler effects, together with a band of comparable width for comparison or reference purposes, adjacent to that containing the line. 2.4 Bursts, pulsars and variable sources The Sun is an outstanding source of short-period bursts of radio energy of many types which give important knowledge of processes of solar and plasma physics [Wild et al., 1963]. Some stars seem, like the Sun, to show large increases in their output in the form of flares of optical and radio waves together, and these short duration flares can be detected by radio astronomers. Jupiter is a source of large bursts of radio energy, observed sporadically at frequencies below about 30 MHz [Roberts, 1963]. Pulsars are sources which emit pulses of remarkably regular periodicities, in the approximate range from 30 pulses per second to one pulse every four seconds. The emissions can be observed in the frequency range between 30 MHz and 15 GHz, and observation at several frequencies in this range are needed. Only for strong pulsars is the detection of single pulses possible. For weak sources pulse averaging techniques with integration times of up to some hours are used to detect the mean pulse profile. Pulse arrival time measurements, extending over some years give not only information about proper motions of the pulsars and their positions, with an accuracy of 0.01 arc second, but also about the long-term stability of the pulsar period. Some radio sources, particularly the quasars, show variability of their radio emission over a time scale of a few weeks, and, recently, novae and X-ray sources have been found to emit a changing level of their radio noise as their optical brightness changes.
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