2. Origin and nature of the emissions 2.1 The radio waves with which the radioastronomy service is concerned are generated in extra-terrestrial sources by four distinct mechanisms: — thermal emission from hot ionized gas and from solid bodies; - non-thermal processes, mainly synchrotron emission from electrons spiralling in a magnetic field, but including also emission from plasmas (as in the solar atmosphere); and the pulses emitted by pulsars; — “line” emission resulting from transitions within individual atoms and molecules; — primordial radiation fields. These combine to produce: 2.1.1 A continuum of radiation, which extends relatively smoothly over the whole frequency range accessible to observation; upper and lower limits of observation are imposed by the Earth’s atmosphere at roughly 300 GHz and 1.5 MHz respectively. The continuum is composed of a background together with numerous small “bright” regions, the discrete radio sources. The background shows a general distribution over the whole sky with a broad maximum in the direction of the galactic centre, together with a ridge of intense emission around the galactic equator (the Milky Way), showing a marked maximum in the direction of the centre. The discrete sources, often referred to as radio stars, are, with a few exceptions, (notably the Sun, and special types of nearby stars), not stars but radio “nebulae”. They are of two kinds, those of extra-galactic origin and those originating within our galaxy. The extra-galactic sources are, in general, distributed randomly over the sky while the galactic sources are for the most part confined to within a few degrees of the galactic equator. 2.1.2 “Line” emission which, though occurring at the source at one or more precise frequencies determined by the transitions involved, is observable over a band of frequencies, the result of Doppler shifts due to relative motions in the line-of-sight. Spectral lines are also observable in absorption when a strong source of continuum emission is viewed through an intervening gaseous medium. 2.1.3 Intermittent emission (“bursts”) of durations which may vary from seconds to hours. They are most intense in the HF and VHF bands, and those from disturbances in the solar atmosphere may vary progressively in frequency, from high to low, during their lifetime. Those so far detected originate in localized areas on the Sun, some types of stars, the planet Jupiter and (at a lower level) X-ray sources. 2.1.4 Pulsating emission (pulsars) was discovered in 1967 [Hewish et al., 1968] and is believed to be radiation from stars composed only of neutrons and thus to be matter in its most highly condensed state. The neutron star rotates and the interaction of its magnetic field with the surrounding plasma generates the pulses of radio waves. The rate of emission of these pulses varies in different pulsars from about 30 per second to about one pulse every 4 seconds. Some pulsars are slowing down; cases have occurred where the pulse recurrence frequency has changed suddenly. A general description of pulsars has been published [Radhakrishnan, 1969]. Pulsars are not only important astrophysical objects but they often serve for the investigation of the interstellar and interplanetary media. Furthermore, observation of a pulsar member of a binary star system offers a possible new technique for investigating the general theory of relativity. 2.2 Continuum radiation and discrete sources The discovery of the largest class of radio sources and the bulk of current knowledge about their nature and distribution, and the processes responsible for the radio emission from them, has come about through observations of the continuum radiation, made at a limited number of frequencies at the lower end of the band transmitted by the ionosphere. Observations of intensity need to be made at a number of frequencies to determine the characteristic “spectra” of sources, but, because the distribution of continuum radiation with frequency is relatively smooth, observations of this kind do not need to be made at specific or closely adjacent frequencies. For many types of observation, bands spaced at intervals of an octave are satisfactory. However, there are some unusual sources, for example those showing self-absorption, newly-created sources such as novae and pulsars for which observations at intervals of less than half an octave are desirable. In addition the study of polarization often requires observations at closely spaced frequencies. The detailed structure of many radio sources is an important feature which can lead to a better understanding of the ways in which radio energy is generated. To observe this structure, high angular resolutions are needed. Antenna systems (arrays) capable of producing details with a resolution of a few seconds of arc have very large dimensions (100 000 wavelengths or more). Much finer detail can be observed by using very long base line interferometers with antennae separated by thousands of kilometres or by observing when the Moon occults the source. World-wide protection of the continuum bands is needed to ensure that no transmitter in the protected bands will illuminate the Moon or be able to interfere via any antenna of an interferometer system which often extends over different countries or different continents.
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