# Siberian Radioheliograph: First Results by S.V. Lesovoi et al.*

Regular observations of the Sun have started at the first stage of the multiwave Siberian Radioheliograph (SRH). It is a T-shaped interferometer currently consisting of 48 antennas observing in a 4-8 GHz operating frequency range with a 10 MHz instantaneous receiving band. The SRH’s antennas are installed on the central antenna posts of the Siberian Solar Radio Telescope (SSRT: Grechnev et al., 2003). The maximum baseline is currently 107.4 m providing an angular resolution up to 70 arc seconds.

Figure 1. The central part of the SRH antenna array and the northern arm of SSRT.

Imaging Principle

SSRT, which is transformed into SRH, is a cross-shaped interferometer consisting of East-West and South-North equidistant linear arrays of 622.3 m, each with 128 antennas. The images are formed by scanning the Sun, firstly, due to the simultaneous receiving at different frequencies in the 5.67-5.79 GHz band and, secondly, due to Earth’s diurnal rotation. Thus, SSRT can produce images every 2-3 minutes at best.

SRH uses a different imaging principle, a parallel Fourier synthesis (Lesovoi et al., 2014). Both circularly-polarized components are measured. The observing frequencies are set by software. The time to switch from one frequency to another is currently about 2 s, and the accumulation time at each frequency is 0.28 s for each polarized component. The sensitivity to compact sources reaches $10^{-4}$ of total solar flux.

The number of observing frequencies can be changed depending on the observational program. For quasi-stationary objects such as sunspot-associated sources, several hundreds of frequencies can be used to achieve a desired spectral resolution. Observations of flares require high temporal resolution, for which the number of frequencies can be reduced. Since 1 July 2016, observations have been made at five frequencies in a range of 4-8 GHz (see example in Figure 2).

Remaining SSRT antennas continue observations at 5.7 GHz in the original operating mode, providing the images of compact sources with a resolution of up to 21 arc seconds. Daily maps in intensity and circular polarization near local noon are available at http://badary.iszf.irk.ru.

Figure 2. Images of the Sun at five frequencies on 9 June 2016 in the right (RCP) and left (LCP) circularly polarized emissions.

Data

Raw data are available at ftp://badary.iszf.irk.ru/data/srh48. Software has been developed and tested to produce raw solar images, clean them, and calibrate in brightness temperature units. File formats for storage of data and software for their remote access are being developed.

To monitor solar activity and SRH systems, the so-called correlation plots are used. They represent a proxy of radio flux and display temporal variation in the sum of cross-correlations of all antenna pairs. Methods for calculating the correlation plots and their relation with characteristics of solar emission are discussed by Lesovoi and Kobets (2017). Changes in the correlation plots are associated with variations both in the brightness of sources and in their structure. Real-time correlation plots and quick-look images are accessible online at http://badary.iszf.irk.ru/srhCorrPlot.php. The correlation plots in Figure 3 demonstrate an unprecedented observation of three negative bursts during one day. They were caused by screening a microwave source by the surges visible in Figure 4. (For more information about negative bursts see Grechnev et al., 2013 and RHESSI Nugget 206.)

Figure 3. Correlation plots with three negative bursts and a C2.5 flare at 08:55 on 9 August 2016. Observations started at 02:45.

Figure 4. Three surges observed in 304 Angstroms on 9 August 2016 from STEREO-A located $152^{\circ}$ eastward from Earth. The dashed arc denotes the solar limb visible from STEREO-A. The solid arc corresponds to the east limb visible from Earth.

Concluding Remarks

Observations at the first stage of the multiwave Siberian Radioheliograph demonstrate efficiency of design ideas and their accomplishment. The high sensitivity of SRH of about 0.01 sfu combined with a wide dynamic range allows observing the sources of powerful bursts without attenuators. Advantages of SRH are the temporal resolution high enough to study many processes (up to 0.56 s for both polarized components in the single-frequency mode), multi-frequency observations with a tunable frequency set depending on observational program, image synthesis with optimization of required parameters (e.g. spatial resolution or sensitivity), and the absence of geometric distortions from which SSRT images suffered.

SRH allows synthesizing tens of thousands of solar images per day. Expansion of the antenna array up to 96 elements would be the next milestone in upgrading SRH. The spatial resolution of SRH-96 would be as high as 15 arc seconds. Observations with SRH are promising for several studies. In particular, they can shed additional light on the initiation of coronal mass ejections, their propagation up to heights of one to two solar radii, and verification of coronal radio magnetography methods.

*Based on the recent paper:
Lesovoi, S., at al (2017). Siberian Radioheliograph: first results Solar-Terrestrial Physics, 3 (1), 3-16 DOI: 10.12737/24347