Coronal structures observed at metric wavelengths with the Nan�ay radioheliograph

Abstract

We present and discuss two-dimensional maps of the Sun at 169 MHz, obtained with the Nancay radioheliograph used as an Earth rotation aperture synthesis instrument. The maps have been computed on the basis of about 6 hr of one-dimensional observations by the east-west and the north-south arrays of the radioheliograph and have a resolution of 1.5′ by 4.2′ for a solar declination near 23°. In addition to a broad background component, the maps show several features both brighter and darker than the background. Some of the bright features are sources of noise storm continua, as evidenced by their positions relative to active regions and by the occurrence of type I bursts. Weaker emission regions are apparently associated with neutral lines of the photospheric magnetic field. We found no sources associated with extended quiescent filaments. Some of the depressions on the maps correspond to coronal holes both in the equatorial region and near the poles, while the more shallow ones may be arch regions with low electron temperature and/or emission measure. The distribution of brightness temperature at a height of 0.15 solar radii above the photospheric limb shows a gross similarity with coronal green line observations. The present results indicate that the notion of the slowly varying component at metric wavelengths may have to be reexamined, since sources of different nature may have been grouped in this component in the past.