Source characteristics of main and post-burst-increase phases of solar bursts at 17 GHz

Twenty four solar bursts of peak fluxes above 50 sfu are analyzed which were observed with the 17 GHz interferometer at Nobeyama during the period from 1978 September to 1979 December. Source characteristics and their temporal evolutions are investigated on a statistical basis with high time resolutions up to 0.8 s. Use of a model-fitting technique recently developed by Kosugi is made to derive both the position of centroid and size (proportionalFWHM) of burst source with an uncertainty of a few arc sec. The results of this study are the following: (i) Two different phases in the burst, that is to say, the main phase and the post-burst-increase (PBI) phase, are distinguished clearly not only by the morphological difference of flux time profile, but also by the differences of brightness temperature (107-> or approx.109 K vs 105-107 K), circular polarization degree (0-50% vs 0-10%), and size (< or approx.5-25'' vs 10-70''). There is no definite correlation between the peak fluxes in the two phases. (ii) The majority of the selected bursts show in the main phase source characteristics of the impulsive burst. The total flux varies rapidly (characteristic time scale defined by FWHM < or approx.100 s), often associated with the rapid shift of position and the rapid change of polarization degree. The source height of the impulsive source is lower than that of the PBI source. On the other hand, the type IVsub(μ) source, seen in three events, shows a gradual variation and the source ascends to a height of proportional40 000 km above the photosphere. (iii) In the PBI phase, the expansion and ascension of the source occur in general. The velocities of both the movements are of the order of 5 km s-1. (orig./WB)