FINE-STRUCTURES IN SOLAR RADIO-EMISSION AT DECAMETER WAVELENGTHS

Millisecond-duration structures in solar radio emission have been stud ied at decametre wavelengths using the Acousto-Optic Spectrograph at t he Nancay Radio Astronomy Station of the Paris-Meudon Observatory. Wit h this instrument, dynamic spectra can be obtained in left- and right- hand polarization over a 21 MHz band between 20 and 80MHz with a frequ ency resolution of 30kHz and a time resolution down to 42 ms for a sin gle polarization. Observations during 1989 to 1991 have revealed a num ber of interesting fine structures within the frequency range 26 to 55 MHz. Of particular interest are successions of fine structures a few MHz wide, having durations down to the limit of resolution, which may be super-imposed upon some larger outburst. These bursts have negative drift rates of some 10 MHz/s or faster. The envelope of a group of th ese short-duration bursts may show an overall slow negative drift in f requency. These structures could not be identified with any previously known decametric fine structure and it is suggested that they be deca metre-wave spike bursts, as they bear some resemblance to the spike bu rsts observed at centimetre- and metre-wavelengths. Average values of about 200 and 600 ms are estimated, respectively, for the e-folding de cay time and the exciter duration of a burst between 38 and 50 MHz. Th ese values are consistent with extrapolations from observations at hig her frequencies which have been interpreted as plasma oscillations. Al ternatively, these fine structures can be interpreted as the signature of an electron cyclotron masing source operating in a stationary loss -cone. With a background electron temperature of 10(6) K, we derive an estimation of the energy of hot electrons (approximately 1 or 2 keV) and of source sizes (less-than-or-equal-to 450 km). These emissions se em to be unusual at decametric frequencies although they are quite oft en observed at higher frequencies. The proposed mechanism is interesti ng as a possible source of energy exchange and heating in the solar co rona. A few solar S-bursts, showing a characteristic negative frequenc y drift of about 2 MHz/s, were also observed although this type of act ivity was not prolific.