The presence of continuous cooling and heating processes is a crucial condition that determines the existence of the solar corona. The defining aspects of the corona include the magnetic field, low plasma density, and high temperature of the corona. In this regard, the study of low-contrast structures in optical ranges is limited even with the use of large specialized telescopes. The radio range provides higher sensitivity, which can be used to detect very weak structures of emerging activity. However, the radio astronomical range also faces challenges in observation, both in terms of spatial resolution and the limitations of dynamic range due to the high temperature of the Sun's corona. Observations show that the use of instruments with a large effective area allows us to overcome the main problem associated with the influence of powerful radiation from the quiet Sun, which amplifies equipment noise. The high sensitivity of the RATAN-600 reflector radio telescope to weak signals in radiation flux was used in the decimeter range for the detection of weak microwave bursts at a level of $10^{-3}$ s.f.u. Research and theoretical developments have shown that microwave bursts are caused by magnetic reconnection, which leads to the generation of accelerated particles that excite plasma waves at the second harmonic of the plasma frequency. Due to significant changes in the concept of radio spectroscopy, work has begun on creating a series of broadband spectral complexes covering several octaves. The results of the first series of observations using the complex in the range of 1-3 GHz for searching for and studying quasi-periodic pulsations in the solar corona are presented here. A by-product of these observations was the detection of narrow-band absorption in the frequency range 1560-1665 MHz, near the well-known OH absorption line (1612-1720 MHz).

The existence of continuous cooling and heating processes is an important condition governing the behavior of the solar corona, which is characterized by temperatures of several million Kelvin. These processes can be significantly influenced by small-scale coronal formations, which largely determine the thermal balance of the corona and solar-wind disturbances. High-sensitivity observations of polarized radiation allow us to evaluate the complex structure of magnetic fields that accumulate the energy necessary to excite coronal eruptions, bursts and flares. However, at high altitudes the corona becomes optically thin, and observations of it pose a major challenge, requiring the use of instruments with a large effective area. Many researchers note that the emerging field of coronal magnetometry is hard to develop due to the fact that experimental observations in optical range are limited by the low plasma density in the corona, high temperature, and insufficient sensitivity of the instruments. In contrast, higher sensitivity is achievable in the radio frequency range. In particular, the 1–3 GHz range is optimal for detecting very weak coronal structures of emerging activity, despite limitations in spatial resolution. To carry out radio requency observations of the corona on the RATAN-600 large reflector-type radio telescope, a wide-range spectrometer in the range of 1–3 GHz was created. It has continuous coverage of the entire frequency range with maximum frequency and time resolutions, as well as high sensitivity to radiation flux. The results of the first series of observations of weak coronal structures are presented, and their interpretation in terms of their effect on thermal processes in the corona is discussed.

When observing the Sun with RATAN-600 radio telescope using a spectropolarimetric complex in the range 1–3 GHz, as well as when observing coronal rain, absorption in the radio emission was discovered in the spectral region 1.5–1.65 GHz in active solar regions located under a cold filament or on the limb under a prominence. The observed line structure corresponds to the hyperfine splitting frequencies in the ground state of X2Π3/2 hydroxyl (OH) 1612–1720 MHz. When the observed active region passes through the knife-shaped beam pattern of the telescope antenna, the absorption band shifts in frequency due to a shift in the energy levels of the OH molecule in a magnetic field, which changes along the filament.

Modern studies of solar radio emission are complicated by continuous power amplification and multifrequency external interference, which often completely overlap important frequency ranges. Many topical problems in solar radio astronomy require large effective areas of radio telescopes, high frequency and time resolutions, accurate spatial measurements, and a large dynamic range. It becomes relevant to change the concept of receiving recording equipment. This paper deals with topical problems of the physics of the solar corona in combination with optimal methods of observation with large instruments. The features and difficulties of combining high parameters—dynamic, spatial, temporal, and frequency resolutions—are considered. The proposed solutions of the new-generation observation complex implement the possibilities of intelligent selection of registration conditions in a multioctave mode with multichannel over 8000 channels/GHz with temporary permission up to 8 ms/spectrum. A multiobject observation mode becomes available from powerful flaring objects to faint structures of various nature. High-speed data processing makes it possible to implement an online mode of interference elimination, which is based on a fast statistical analysis of the spectrum with the selection of non-Gaussian (interference) structures. Methods for high-speed analysis of large-volume data (the principal component analysis method) and their presentation to the user are proposed. Examples of the operation of the complex in the range of 1–3 GHz are given. The prospects of a new approach for multiobject radio astronomy observations in the implementation of the RATAN-600 tracking mode are considered: from recombination lines to wide-range spectra, from low-contrast fluctuations to fast changes in flares, etc.

The development of methods for the radio frequency interference mitigation becomes a fundamental challenge for any radio astronomy research. In this paper, we report on the development and creation of a new spectrometer designed for solar observations at the RATAN-600 radio telescope, capable of excising radio interference using a statistical algorithm based on the spectral kurtosis estimation. The implementation of the external analog interface and the operation of the digital signal processing system based on the FPGA are described. The maximum spectral resolution reached is 122 kHz in the frequency band of 1.0–3.0 GHz. The output spectrum has from 64 to 8192 frequency bins, depending on the requirements of the observer, the output rate is 120 spectra per second. The test results and the first observations of the Sun show that the research method can successfully detect and suppress interference from most of the local radio interference sources.

To implement the tracking modes of the RATAN-600 radio telescope, it is necessary to introduce new automation tools that provide a new quality of monitoring and control of the antenna system geometry [1, 2].In this work, the ACS is considered by the movement of the type 3 feed and the carriage, its structure is shown, new capabilities of the complex, previously unavailable on the RATAN-600 radio telescope, are considered.

The study involves the obtainment of regular tracers of the solar cycle in the radio range by activity manifestations in the chromosphere and corona. Our research method consists of the systematic processing of the RATAN-600 radio data at frequencies 3–18 GHz obtained during more than one cycle of solar activity and their comparison with other databases of time series characterizing the magnetic activity of the Sun at the photospheric level. In the first phase of this work, we identified an array of approximately 600 active regions that evenly covers all phases of solar cycle 24 in approximately 9 years, for which we reliably identified the local radio sources based on RATAN-600 data, as well as data from other databases, such as spacecraft of the Space Weather Prediction Center of the National Oceanic and Atmospheric Administration and the Solar Dynamics Observatory. The sample is statistically homogeneous and evenly distributed over solar latitudes in both hemispheres. The statistical relationships between the sunspot areas and the parameters of intensity and polarization of local radio sources are obtained. It is shown how the spectral flux density of local radio sources in the chromosphere and corona at different frequencies increases with the increase in the sunspot area (and, consequently, the magnetic flux). The estimates of exponents of this dependence are obtained.

There is a need today to recreate the Havana observational solar station because of the importance of obtaining a regular forecast of solar activity in a wide range of time intervals. The concept of the created observation network and the infrastructure of the monitoring station as a network element are described in detail. The functions of this network will provide continuous observational material for the Russian Sun and Space Weather Services and will be independent of space observations, but capable of using them for quality control. The physical foundations of complex observations for a wide range of heliogeophysical phenomena are considered.

The results of long-term mesoscale observations in the microwave range are reviewed. The rationale for the resumption of observations at the Havana Radio Astronomy Station is justified. Domestic observations at a wavelength of λ5 cm have a long observation interval and can effectively complement the global series of observations. A comparative analysis of the results of solar observation in the radio range and satellite observations in the UV and X-ray range is performed. The value of continuous observations for the assessment of space weather factors is shown.

Abstract—We present a method of determination of numbers of gyroresonance layers most effectively radiating in the transition region of the active solar atmosphere. It is based on determination of frequency in the spectrum of an extraordinary wave of an active region, at which the gradient of this spectrum abruptly rises. Observations of 29 active regions carried out at the RATAN-600 radio telescope have been analyzed. The magnetic field in the transition region was determined from the frequency of bending point in the spectrum of antenna temperatures in an extraordinary mode under condition of emission in the third harmonic of the gyrofrequency. The relation between the photospheric magnetic field strength and the magnetic field strength in the transition region (1.52–2.28) is substantially greater than previously obtained, namely, the field isdecreasedonlyby10–20% in the transition region. The heights in the transition region were determined from the reconstructed magnetic field in the nonlinear force-free approximation, and are in the range of 1.00–3.57 Mm.

Spectral polarization observations of radio sources above sunspots are regularly carried out with the RATAN-600 radio telescope (RATAN is a Russian acronym for the Radio Astronomical Telescope of the Academy of Sciences). The in-depth studies of the spectra reveal new effects. In this paper, we analyze the manifestation of radio emission of the fourth gyrofrequency harmonic in microwave spectra obtained with 1-percent frequency resolution in a range of 3–18 GHz. Registration of the extraordinary mode in the short-wavelength range of the spectrum is compared to the model calculations of the second-to-fifth gyrofrequency harmonics against a background of the thermal bremsstrahlung emission of flocculi, surrounding the spot structure of an active region. The brightening of the extraordinary mode in the short-wavelength spectral range and the kinks in the intensity spectra of emission are analyzed. The interpretation of the RATAN-600 observational data with probable diagnostics of the emission of the fourth gyrofrequency harmonic is considered as examples.

We develope a method to compute the temperature and density structure along the line of sight by inversion of the differential emission measure (DEM), under the assumptions of stratification and hydrostatic equilibrium. We apply this method to the DEM obtained from the Atmospheric Imaging Assembly (AIA) observations and use the results, together with potential extrapolations of the photospheric magnetic field, to compute the microwave emission of three sunspots, which we compare with observations from the Academy of Sciences Radio Telescope - 600 (RATAN-600) radio telescope and the Nobeyama Radioheliograph (NoRH). Our DEM-based models reproduce very well the observations of the moderate-size spot on October 2011 and within 25% the data of a similar sized spot on March 2016, but predict too low values for the big spot of 14 April 2016. The latter is better fitted by a constant conductive flux atmospheric model which, however, cannot reproduce the peak brightness temperature of $4.7\times 10^{6}~\mbox{K}$ and the shape of the source at the NoRH frequency. We propose that these deviations are due to the low intensity non-thermal emission associated to a moving pore and to an opposite polarity light bridge. We also find that the double structure of the big spot at high RATAN-600 frequencies can be interpreted in terms of the variation of the angle between the magnetic field and the line of sight along the sunspot.

The paper covers characteristics of big interferometers and reflecting radio telescopes intended for the studies of the solar atmosphere radiation. Solar radio interferometers (radioheliographs) have proven to be more cost-effective than single dish radio telescopes, since using the aperture synthesis allows for fast imaging of the Sun and its active regions with higher spatial resolution, thus, studying solar eruptive processes more efficiently. From the other hand, the studies of many fundamental aspects of the solar atmosphere physics requiring studies of low-contrast structures before and after flares gained no proper development. The efficiency of radio reflectors was enhanced in last decades mainly due to the development of matrix methods of reception, focal arrays, and also due to spectroscopic analysis in the wide wavelength range with high flux density sensitivity. Meanwhile, the diversity of issues related to the physics of the solar atmosphere and active processes requires an optimal combination of capabilities of various microwave interferometric and reflecting systems for joint studies of the Sun. In view of the started implementation of solar observation programs with a new instrument, ALMA, and obtaining newly observed data in the millimeter and submillimeter ranges, the areas of development of solar studies with the RATAN-600 radio telescope in the wide wavelength range are discussed.

The distributions of temperature and density in the sunspot atmosphere are crucial for an understanding of the process of energy transfer from the photosphere upward to the corona in strong magnetic fields. The joint analysis of millimeter observations at ALMA (Atacama Large Millimeter/Submillimeter Array) and observations performed in the microwave range (centimeter wavelengths) with the RATAN-600 radio telescope of the Russian Academy of Sciences provides new data on the temperature distribution and physical processes at different altitudes above a spot. The NOAA 12470 active region was observed and mapped at 1.3 and 3 mm (ALMA) and 2–10 cm (RATAN-600) in December 2015. These observations are analyzed, and the results are compared to models of sunspot atmospheres. The fundamental problems arising in the study of atmospheres of active regions in the millimeter and centimeter ranges are determined, and the importance of the 3–18 mm range in the physics of the generation and transfer of energy for corona heating is demonstrated.

The RATAN-600 archive of regular solar observations contains data from the beginning of 1997 (for 23 and 24 solar cycles), which are data of the left and right circular polarizations and cover the range of 3-18 GHz with high frequency and spatial resolutions.The paper presents the proposed data processing technique, which includes: rejecting defects, calibrating, finding the background in the form of a quiet Sun, searching for cyclotron radiation sources in active regions, and also using auxiliary data on active regions (ARs) Solar Region Summary (SRS) provided by SWPC NOAA.The ultimate goal is to obtain solar activity parameters such as flux density in ARs in the range of 3-18 GHz, magnetic field values and search for other parameters to study the cyclic activity of the Sun, as well as the dynamics of coronal jets.