The Siberian Solar Radio Telescope: the current state of the instrument,observations, and data

The Siberian Solar Radio Telescope (SSRT) is one of the world's largest solar radio heliographs. It commenced operation in 1983, and since then has undergone several upgrades. The operating frequency of the SSRT is 5.7 GHz. Since 1992 the instrument has had the capability to make one-dimensional scans with a high time resolution of 56 ms and an angular resolution of 15 arc sec. Making one of these scans now takes 14 ms. In 1996 the capability was added to make full, two-dimensional images of the solar disk. The SSRT is now capable of obtaining images with an angular resolution of 21 arc sec every 2 min. In this paper we describe the main features and operation of the instrument, particularly emphasizing issues pertaining to the imaging process and factors limiting data quality. Some of the data processing and analysis techniques are discussed. We present examples of full-disk solar images of the quiet Sun, recorded near solar activity minimum, and images of specific structures: plages, coronal bright points, filaments and prominences, and coronal holes. We also present some observations of dynamic phenomena, such as eruptive prominences and solar flares, which illustrate the high-time-resolution observations that can be done with this instrument. We compare SSRT observations at 5.7 GHz, including computed `light curves', both morphologically and quantatively, with observations made in other spectral domains, such as 17 GHz radio images, Hα filtergrams and magnetograms, extreme-ultraviolet and X-ray observations, and dynamic radio spectra.     

Bursts with temporal fine structure at 5730 MHz

An analysis of solar radio bursts with temporal fine structure (TFS) at 5730 MHz in relation to the magnetic configuration of the corresponding active regions (AR) is presented. We found that the occurrence of TFS bursts increases with increasing complexity of the AR's magnetic configuration. The degree of polarization of TFS bursts varies over a wide range. Most of these fast bursts with a high degree of polarization were observed in active regions with a simple magnetic configuration β. Most of the unpolarized fast bursts were observed in active regions with the most complicated configuration βγδ. Because bursts that are polarized in different modes have different displacements of position with respect to that of associated microwave bursts, we conclude that there are at least two types of TFS bursts at 5730 MHz. We think that fast bursts that are polarized in the ordinary mode are due to microwave type III bursts.     

Coronal mass ejection of April 27, 2003, and evolution of the active region NOAA 10338 in the radio

The results of a study of the coronal mass ejection (CME) of April 27, 2003, which was intrinsically associated with the active region NOAA 10338, are reported. Particular attention is paid to the initial stage of the event, which was accompanied by X-ray bursts of class C9.3 and C6.7, with the aim of determining the origin of CMEs. The energy source of the ejection was in the active region NOAA 10338. This region had a complicated and dynamic magnetic-field topology, and produced a series of CME-type events. The basis for the study was observations at wavelengths of 1.92–17 cm with high spatial resolution, 17″–20″, obtained on the Siberian Solar Radio Telescope (SSRT) and RATAN-600, together with simultaneous data from the Nobeyama Radio Heliograph (NoRH, wavelength 1.76 cm) and 195 Å ultraviolet data from the TRACE spacecraft. The development of the event was followed over three hours, first through observations against the disk at heights of 10,000–100,000 km from the photosphere, then in the post-limb stage to distances of the order of 10⁶ km from the solar center, i.e., in the zone inaccessible to the LASCO coronographs. According to the radio observations, ～10 min before the beginning of the event, the radio structure of the active region NOAA 10338 had an S-shaped (sigmoid) configuration. A rising, gradually expanding dark loop originated at the points where this structure was observed; according to the TRACE data, this loop initiated the event. Subsequently, the structure of the radio image drastically changed, suggesting that coronal plasma was heated and cooled at different sites of the emission region (or was shielded by the cooler material of the ejection). Profiles of the burst that accompanied the ejection are presented for four points in the region. The post-limb part of the event first had a compact (～50″) structure receding from the Sun and visible to distances ～10⁶ km. An asymmetric loop was then formed, with its material falling back onto the Sun at the end of the event. The brightness temperature of the loop was ～15 × 10³ K, and its emission was weakly polarized (P ≈16%). The mean speed of the material was 160 km/s. It is concluded that the observations of the event of April 27, 2003 are most consistent with the model of Amari et al., in which the formation of an eruptive twisted magnetic rope, taken to be responsible for CME-type events, is explained by the emergence of new magnetic flux within an old field of opposite polarity.     

Two-dimensional solar mapping at 5.2 cm with the Siberian Solar Radio Telescope

We present two-dimensional solar maps at 5.2 cm computed from one-dimensinal observations with the Siberian Solar Radio Telescope (SSRT), using Earth rotation aperture synthesis techniques. The resolution attained with the E-W branch of the instrument is 15 by 45 for a solar declination of about 23°. Maps during the period of June 8 to 13, 1988 clearly show the quiet-Sun background, sunspot and plage associated emission as well as compact sources above the neutral line in some active regions. We found that the latter disappear as the gradient of the longitudinal magnetic field decreases. We also detected emission associated with active regions behind the limb, apparently from unresolved loops, extending up to 40. The prospects of the SSRT, as a dedicated solar instrument, are discussed.     

Two-dimensional SSRT observations of the flare-productive active region in July 1996

Two-dimensional images obtained at the Siberian Solar Radio Telescope (SSRT) in the correlation mode are presented. The subject under discussion is the active region NOAA 7978 that produced a flare of X2.6 importance on 9 July 1996. Before the flare a compact, weakly polarized, and reasonably bright long-lived radio source was observed which we have identified as the Neutral Line associated Source (NLS). The correlation of radio and magnetographic images of the active region reveals the NLS brightness center to be localized over the inversion line of the photospheric magnetic field at the place of closest contact of opposite-polarity magnetic hills.