Fine structure in solar radio spectra

Observations of the solar radio spectrum have been made with high time and frequency resolution. Spectra were recorded over six 3 MHz bands between 30 and 82 MHz. The receivers used were capable of time and frequency resolutions of 1 ms and 2 kHz respectively. A large number of radio bursts exhibiting a variety of fine spectral structure were recorded. The most interesting bursts recorded, referred to as solar S bursts, were identified with radio emission previously described by Ellis (Aust. J. Phys., 22, 177, 1969) who called them fast drift storm bursts. Improved time and frequency resolution has allowed a much closer study than did the original observations. The bursts were observed throughout the 30-82 MHz frequency range but were most numerous in the 33-44 MHz band and were very rare at 80 MHz. On a dynamic spectrum the bursts appeared as narrow sloping lines with the centre frequency of each burst decreasing with time. The rate of frequency drift was about 1/3 that of type III bursts. The variation of frequency drift rate with wave frequency was consistent with their being emitted from plasma radiation sources which were travelling out of the solar corona with a constant velocity. Most bursts were observed over only a limited frequency range (< 5 MHz) but some drifted for more than 10 MHz. The durations measured at a single frequency and the instantaneous bandwidths of S bursts was small; ˜ívÆt = 49 ¬¨¬± 34 ms and ˜ívÆf = 123 ¬¨¬± 56 kHz for bursts observed near 40 MHz. A significant number had ˜ívÆt‚Äöv߬©20 ms. Flux densities of S burst sources were estimated to fall in the range 10 -23 -5 x 10- nW M-2 Hz -1 . A small proportion (1-2 %) Of bursts showed a fine structure in which the burst source apparently only emitted at discrete, regularly spaced frequencies causing the spectrogram to exhibit a series of bands or fringes. The fringe spacing increased with wave frequency and was (˜í¬•f = 90 kHz for fringes near 40 MHz. The bandwidths of fringes was narrow, often less than 30 kHz and some cases down to 10-15 kHz. The following conclusions are reached. Firstly, the bursts are due to plasma radiation, probably at the fundamental. Secondly, the fringed bursts may be indicative of a wave propagating in the corona whose wavelength is ˜í¬™ = 10 3 km. This idea is strongly supported by the observed correspondence between fringes in bursts which occur close together in time. Plasma radiation mechanisms are discussed in terms of their ability to produce the observed intensities and bandwidths and the small proportion of fringed bursts. A plasma radiation mechanism necessarily includes a conversion process for transferring energy from longitudinal plasma (Langmuir) waves to escaping electromagnetic radiation. Four conversion processes are considered as possibilities for the radiation of solar S bursts. Two of these, the scattering of Langmuir waves by thermal ions and the coalescence of two Langmuir waves, are rejected. Mode coupling across the plasma level and the coalescence of Langmuir waves with low frequency waves are both retained as possible conversion processes.