Radio Emission of the Quiet Sun and Active Regions (Invited Review)

Shibasaki, Κ.; Alissandrakis, C. E.; Pohjolainen, S.

doi:10.1007/s11207-011-9788-4

Cited by 97 publications

(86 citation statements)

References 108 publications

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“…At all the NRH frequencies, the corona exhibits a variety of scales, from more than 1 R s (long dark channels, large holes at HF, elongated bright areas at LF) down to the resolution limit (small holes, fine structure in large holes, transverse dimension of dark channels, bright spots in particular near cycle minimum). It was already known from a few observations that the aspect of the corona was not the same at 408 and 169 MHz (Shibasaki et al 2011). We showed that this aspect progressively changes from high to low frequencies, with a loss of contrast (in particular for coronal holes) from 450 down to 230 MHz and that a marked transition occurs below 230 MHz.…”

Section: Discussionsupporting

confidence: 46%

“…We had already shown that the structure of the CV in the uv plane is complex and has smallscale features (see Fig. 6 in Shibasaki et al 2011) and requires a dense uv coverage to be properly described. It follows that radio imaging is difficult but, when performed simultaneously at several frequencies, is a promising tool for investigating the 3D structure of the corona.…”

Section: Discussionmentioning

confidence: 99%

“…It was defined as a quasi-uniform area, brighter (ΔT b = 50−100 kK) than the average QS, surrounding all local emission sources and running "around the Sun in a continuous manner", and it was considered the dominant feature of the large-scale coronal emission. The plateau had a close association with the zone of enhanced K-corona emission and was identified "with the diffuse emission of a high-altitude loop system which overrides the principal inversion line of the general magnetic field at the base of the heliosheet" (Shibasaki et al 2011).…”

Section: Introductionmentioning

confidence: 96%

“…The QS emission has been reviewed in the dcm/m range by Lantos (1980Lantos ( , 1998, Alissandrakis (1994), and in the mm/dam range by Shibasaki et al (2011). Decades of observations have shown that the radio Sun is a non-uniform and variable source, traditionally described as consisting of three components: 1) a wide background, the QS, involving only large scales on the order of 1 R s and slowly varying with the solar cycle, 2) a slowly varying component (SVC), classically described as the emission of a few of localized regions with sizes less than a few arc minutes, superimposed on the QS and varying over time scales from one day to several weeks, 3) non-thermal continua and bursts, generally much brighter.…”

Section: Introductionmentioning

confidence: 99%

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

Chambe¹

2012

A&A

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Context. Since its last upgrading in 2004, the Nançay 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 selfcalibration 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.

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Section: Discussionsupporting

confidence: 46%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 96%

Section: Introductionmentioning

confidence: 99%

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

Chambe¹

2012

A&A

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“…It consists of free-free emission from quiet sun coronal plasma and gyromagnetic emission from sunspots in active regions (e.g. Shibasaki et al 2011, and references therein).…”

Section: Indices From Observations Of the Transition Region And Solarmentioning

confidence: 99%

Solar Cycle Indices from the Photosphere to the Corona: Measurements and Underlying Physics

Ermolli

Shibasaki

Tlatov

et al. 2015

Space Sciences Series of ISSI

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A variety of indices have been proposed in order to represent the many different observables modulated by the solar cycle. Most of these indices are highly correlated with each other owing to their intrinsic link with the solar magnetism and the dominant eleven year cycle, but their variations may differ in fine details, as well as on short-and long-term trends. In this paper we present an overview of the indices that are often employed to describe the many features of the solar cycle, moving from the ones referring to direct observations of the inner solar atmosphere, the photosphere and chromosphere, to those deriving from measurements of the transition region and solar corona. For each index, we summarize existing measurements and typical use, and for those that quantify physical observables, we describe the underlying physics.

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