Quiet Sun and slowly varying component at meter and decameter wavelengths

Abstract

Comparison of maps of the Sun obtained over the period June 29 to July 8, 1982 at 169 MHz with the Nan~ay Radioheliograph and at 73.8, 50, and 30.9 MHz with the Clark Lake Radioheliograph shows that the slowly varying component at meter and decameter wavelengths is not always thermal emission. During the period under study weak noise storm continua were the most frequent sources of slowly varying component at 169 and 73.8 MHz. Most filaments show no radio counterpart on the disk. A streamer has been detected on the disk from 169 to 30.9 MHz with an optimum observability at 50 MHz. The brightest source of the slowly varying component from 73.8 to 30.9 MHz for most of the period was located above an extended coronal hole in a region where a depression was observed at 169 MHz. In favorable cases, electron densities can be derived from the positions of noise storms and radio streamers; these are in agreement with previous K-corona eclipse observations.

Synoptic study of coronal structures observed at meter wavelengths during the declining phase of the solar cycle

“…These results provide a quantitative verification of the qualitative assessment of Alissandrakis, Lantos, and Nicolaidis (1985) and Lantos et al (1987). A similar relation also exists between metric radio sources and Boulder filaments.…”

“…These results were largely confirmed by subsequent observations (Lantos et al, 1987;Lantos, Alissandrakis, and Rigaud, 1992), however no systematic study of possible associations was ever made.…”

“…On the other hand, using two-dimensional images at the same wavelength during the solar cycle maximum, Alissandrakis, Lantos, and Nicolaidis (1985) found 'no obvious association between the emission sources and Ha filaments'. The only rather systematic association of radio sources with underlying structures found by these authors and by Lantos et al (1987) was with magnetic field neutral lines. This was however based on a qualitative approach only.…”

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

“…Holes can be either darker or brighter than the mean disk at 150 MHz, and the question of their aspect at lower frequencies remains open. Do they systematically become brighter than the disk at low frequencies, as seen by Lantos et al (1987) in one case observed at 37 and 75 MHz with the CLRO radioheliograph? It would also be interesting to investigate whether the bright elongated structures we observe at 150 MHz present a different aspect at still lower frequencies.…”

“…They show more contrast at 408 MHz (about 50% of the QS) than at lower frequencies. Most of them are still visible at 169 MHz as shallow depressions (10-20%), more rarely at 80 MHz and at lower frequencies, where they can occasionally appear as faint brightness enhancements (Lantos et al 1987). Because of the limited resolution and dynamic range in images, the shapes in radio and EUV could not be precisely compared.…”