SYNOPTIC STUDY OF CORONAL STRUCTURES OBSERVED AT METER WAVELENGTHS DURING THE DECLINING PHASE OF THE SOLAR-CYCLE

We have produced synoptic maps from daily images of the solar corona o n the disk obtained with the Nancay Radioheliograph at 169 MHz during June-August 1984. We discuss briefly the structures seen on the synopt ic maps and we compare the location of radio sources with photospheric neutral lines and Her filaments derived from synoptic charts from Meu don, Boulder, and Stanford. We measured the distance of radio sources from these optical features and compared their distribution with compu ted random distributions. A criterion of confidence is proposed which, when applied to our data, shows that the metric radio sources of ther mal origin are best associated with the large-scale neutral lines show n on the low resolution Stanford magnetograms. This association indica tes that the emission comes from arcades of moderately dense loops spa nning the neutral line. However, the radio sources are not usually loc ated directly above or very close to neutral lines, but their distribu tion peaks at distances between 4 and 6 degrees. Our results show no s tatistically significant association with Meudon filaments.