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Analysis of coronal emissions observed at meter wavelengths
Received 18 February 1999 / Accepted 26 August 1999
We present an extensive analysis of large and medium scale radio sources observed with the Nançay Radioheliograph at 169 MHz, over more than four solar rotations in 1984 (during the declining phase of the solar cycle). The large scale emission is dominated by the coronal plateau, the radio counterpart of the coronal plasma sheet, which had an oscillatory structure around the equator. Medium scale local sources include both faint noise storm continua and thermal sources. Noise storm continua have greater brightness (8 105 to 3.8 106 K) and smaller diameter than thermal sources. We present distributions of brightness temperatures, sizes as well as longitude and latitude of the thermal sources and we study their location with respect to faculae and neutral lines. Based on their rotation rate, we estimate their altitude at 70 000 km. We find that most thermal sources at 169 MHz are located between faculae and neutral lines and we present a geometrical model to account for this observation, according to which these sources are located in the upper leg of large scale loops. A small number of thermal sources is closer to neutral lines; these might be loops at the base of isolated coronal streamers.
Key words: Sun: corona Sun: radio radiation
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Online publication: November 2, 1999