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Astron. Astrophys. 364, 793-798 (2000)

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5. Conclusion

Type IV solar radio bursts exhibit a wealth of well documented fine structures between 100 MHz and 1 GHz. The BAO radio spectrograph aims at carrying observations with high time and frequency resolutions at the higher frequencies where fine structures are still poorly known. In this paper we have described an observation by this instrument of a type IV-DCIM burst lasting more than one hour, between 3 and 4 GHz.

This event exhibits a remarkable fine structure which has a M appearance in dynamic spectra, we call it M-burst after we interpret it as an emission produced by a same electron beam, which is originally formed from the plasmoid-the source of this type IV-DCIM, moving along closed magnetic arch lines and mirrored. So the M burst is a new sub-class of type III bursts, however it is a fine structure of a type IV-DCIM burst.

In 1959, within a year of Boischot's classification of type IV bursts on the basis of their long durations, wide spectra and moving sources, Wild et al. (1959) classified type V bursts on the basis of their wide spectra, moderately long durations and association with type III bursts. On dynamic spectrograms they appear as diffuse continua following certain type III bursts or bursts groups. Since type V bursts are characterised by continuum emissions sometimes following type III bursts; Weiss & Stewart (1965) suggested that type V bursts could be interpreted by several consecutive reflections on magnetic mirrors near the footpoints of magnetic arches. Hence it is natural that such a structure as a M emission would have been detected in a type V burst. Hovever, as mentioned above, this M burst is a fine structure of a type IV-DCIM burst. The type IV bursts are generally thought to be different from type V bursts in meter wavelengths. A major question that has to be clarified in the future is whether the type IV and type V bursts have some similar aspects. It would be important and interesting if this kind of structure is also detected in a type V burst.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001