Morphology of the quiet Sun between 150 and 450 MHz as observed with the Nancay radioheliograph

Context. Since its last upgrading in 2004, the Nancay radioheliograph (NRH) is able to produce reliable images of the quiet Sun in the 150-450 MHz range, which corresponds to the low and medium corona. These images are better than those previously obtained with the NRH or other instruments in this range and are suitable for quantitative and systematic exploitation. Aims. We aim to study the radio aspects of the solar atmosphere. We aim to focuss on the description of the morphology and the comparisons with observations in EUV and X-ray ranges and with magnetic structures. Methods. We used the rotational aperture synthesis technique (suitable for non time-varying objects) along with an original self-calibration procedure and a specific deconvolution algorithm (a variant of CLEAN that includes a scale analysis). Results. We present results from radio imaging of the quiet Sun with the NRH. The analysis was carried over about 160 days during the summers (June-August) from 2004 to 2011. We confirms and extend our first results, which were published from a much smaller data sample. We emphasize new aspects of the corona observed in this frequency range, in particular the existence of coronal holes darker than previously reported and dark channels at high frequencies. We give examples that illustrate the complex morphology of coronal structures as revealed by radio imaging, the center-to-limb effects (radio occultation) and the variation with the phase of the solar cycle. We compare our images with large-scale coronal magnetic structures. We show that dark coronal holes and channels seen at high frequencies and bright ribbons seen at low frequencies seem to be associated with particular types of magnetic structures. Conclusions. Detailed radio images in this frequency range are a new tool with high potential for investigating the low and middle corona, since these images can emphasize structures that are not (or much less) visible at other wavelengths. In the near future, there is much to learn from observations with NRH during the ascending part of the cycle (never observed at m/dcm wavelengths) and from composite images of combined NRH and GMRT data, which will have a much better resolution. In a second step, it will be interesting to obtain circularly polarized images (giving a diagnostic of the coronal magnetic field) and images with new-generation instruments, which will yield detailed images with shorter synthesis or even snapshot images, from which processes evolving on shorter time scales can be studied.