As early as 1900. a British physicist, Sir Oliver Lodge, attempted to detect radio waves from the Sun. His failure to do HO can be attributed mainly to the primitive equipment then available. Marconi, in 191(5, and various radio enthusiasts thereafter, surmised that the static disturbances might be of solar or cosmic origin. Military investigations at radar stations in Britain during 1942 left no doubt that the Sun is a radio source, which occasionally radiates bursts of amazing intensity. The behavior of the Sun at radio frequencies is quite different to its features in the optical domain: the radio disc is larger, and the fluctuations in intensity are more marked since the total solar radio flux can increase by one to 100 000 times during active storms.

Radio emission from the quiet Sun is best mapped at periods of sunspot minimum, since solar activity is then least likely to modify the radio picture. As we saw when discussing the sharpness of the optical disc, most radiation at a given frequency comes from regions where the atmosphere is just becoming transparent at that frequency. .The properties of the radio Sun are mainly governed by the electron density in the chromospheres ‘and corona, since free electrons are the major source of opacity at radio wavelengths. The electron density decreases with distance above the photosphere, but in the lower parts of the atmosphere it is sufficiently great to absorb long wavelengths. Consequently only radio waves shorter than a few millimeters propagate in, and are detectable from, the lower chromospheres . Wavelengths below 10 cm travel in the upper chromospheres unimpeded and metre-wavelength radiation is only encountered in the corona.

By mapping the solar radio emission at a series of wavelengths it is possible to investigate the physical conditions at different heights in the solar atmosphere. At longer radio wavelengths the Sun sub¬tends a larger angular size and the emission comes from regions with an electron temperature of 106K, since the corona is the major source. Radio observations show limb brightening in the corona at wavelengths longer than 10cm, giving added confirmation that temperature is higher in the corona. The variation of brightness temperature, as a function of distance from the centre of the Sun’s disc.

Radio observations of the solar corona are also made indirectly by observing the transmission through the corona of signals from very remote radio sources, such as quasars, as they are occulted by the outer solar atmosphere. In this way scattering and refraction by the corona and solar wind have been detected out to 1.5 x 10s km. The Sun is studied daily at many radio observatories hi order to keep track of the various manifestations of solar activity. At Culgoora in Australia, there is a 96-element radio heliograph. This interferometer is able to map solar radio waves from the entire disc every few minutes. At centimeter wavelengths the total radio power varies gradually on a day-to-day basis. This S-COMPONENT of the solar emission correlates almost exactly with the surface area of the disc that is covered by sunspots; so the S – component is presumably associated with active regions. High-resolution obser¬vations have shown that the radio waves are emitted from regions in the corona which are larger in size than the sunspots . Because the regions correspond to the plages mentioned earlier they are called RADIO PLAGES. They have a brightness temperature of 106K, and are due to higher-density regions in the corona overlying the sunspots; thermal emission from dense regions in the corona probably accounts for the radio plages.

It was an intense NOISE STORM on the Sun which led to the discovery of solar radio emission in 1942. The subject of radio bursts from the active Sun is a complex and fascinating part of solar studies, which has made important contributions to plasma physics. Several types of radio burst have been differentiated; those identified at metre wavelengths are designated as types I-V.

TYPE i BURSTS are a prominent feature of solar activity at meter wavelengths. They are the only type of burst which is not specifically associated with solar flares. When their intensity is plotted against time they appear as myriads of spikes of radio emission, lasting 0.1-10 seconds, imposed on a greatly increased background. They are associated with sunspots, and the high brightness temperature of 1011K signifies that the radiation is not thermal in origin. Noise storms have durations of a few hours to several days.

The larger solar flares are frequently accompanied by outbursts at meter wavelengths lasting up to 30 minutes as well as strong X ray bursts. These are termed TYPE n BURSTS. One of their interesting properties is that the burst is detectable at a later time at lower frequencies: the emission drifts from higher to lower frequencies at about 1 MHz/sec during the burst. Consequently special radio receivers had to be developed for studying the type II bursts. Research conducted by J. Paul Wild and his colleagues in Australia suggests that the frequency drift is a consequence of the stream of radiating particles’ motion from the Sun which excites lower frequency waves as it gets higher up in the atmosphere. High-resolution radio maps, made during the course of solar bursts, have essentially confirmed this model.

After a flare begins, TYPE III BURSTS may be detectable as short duration spikes which drift down in frequency by about 20 MHz/sec. These also appear to be caused by clouds of electrons being expelled from the Sun, possibly at velocities 0.25 that of light.

Sometimes a type II burst is followed by emission covering all wavelengths, from microwaves to tens of meters, and lasting an hour or so. These are TYPE iv BURSTS. They are caused by synchrotron radiation from electrons that they have been trapped and accelerated to relativistic velocities by the magnetic fields in active regions. Solar cosmic rays are frequently ejected during type II bursts.

TYPE v is a broadband emission lasting a few minutes which displays some frequency drift.

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