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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present the results of forecasting flare activity based on the data from microwave spectropolarimetric observations of active regions (ARs) obtained with the RATAN-600 radio telescope and the X-ray data from the GOES satellite as well as monitoring data. The method is designed for short-term (1-3-days) flare forecasts. Proton events are considered as part of the general flare problem. Obtaining a reliable forecast is a difficult process in view of the multi-parameter and multi-dimensional system of plasma parameter variations and multiple non-linear interconnections.We used a modified Tanaka-Enome criterion, as well a database of observational material collected over many years. The forecasting efficiency was analyzed depending on the threshold values of the criterion. We show that the quality of the radio astronomical forecast is determined by the level of sensitivity of the detector at short centimeterwavelengths and by the solar activity level. [ABSTRACT FROM AUTHOR]

We present the results of a study of the subsecond pulses (SSPs) registered in the microwaves during the C2.4 solar flare. The event occurred on August 10, 2011, in the 11236 active region near the western limb and reached a maximum at 0935 UT. Subsecond pulses were registered by RATAN-600 for the first time when this flare reached its maximum. The fact of detection of SSPs was confirmed by simultaneous observations carried out with a spectropolarimeter at the Radio Astrophysical Observatory, Institute of Solar-Terrestrial Physics. A former analysis of the emission nature of the main flare source revealed the presence of a high-temperature ( T > 30 MK) source that determined both the microwave and X-ray emission parameters of this event. We have compared the temporal variations of the SSPs microwave emission with the variations of the plasma parameters (temperature and emission measure) taken from observations with the FERMI space telescope with a subsecond time resolution. The obtained results suggest that the nature of SSP generation, as well as of the flare itself, was determined by the high-temperature source. [ABSTRACT FROM AUTHOR]

The physical conditions in the low corona and chromosphere of solar active regions are studied. A diagnostics technique based on multiwave observations in the centimeter range, photospheric magnetic field extrapolation, and radioemission calculations has been applied. The calculated spatial and spectral structure of the radioemission has been compared with RATAN-600 spectral-polarization observations with a high spatial resolution. The effect of the plasma physical parameters on the emission structure character in a complex magnetic field topology in active regions is analyzed. Modeling of the spectral singularities at a quasi-periodic propagation of the radioemission is presented. [ABSTRACT FROM AUTHOR]

A CME/flare event occurred at the eastern limb on 25 January, 2007. Seven successive multi-wavelength scans in the range 1.8 cm – 5.0 cm were obtained with the RATAN-600 radio telescope starting just at the beginning of the post-eruptive arcade formation (30 min after a C6.3 flare peak) and lasting for 3.5 hours. The conditions were favorable to study the off-limb microwave radio source associated with the post-eruptive arcade in different phases of its formation. Microwave radio emission of the arcade was rather intense initially and then considerably decreased; its maximum was co-spatial with the 195 Å Fe xii loop tops. The RATAN-600 total flux spectra of the off-limb radio source were practically flat during the first two hours indicating a predominant contribution of thermal emission. The X-ray spectrum was thermal (according to RHESSI data) at that time. Data available in the meter wavelength range during this phase were indicative of weak non-thermal processes likely due to accelerated particles. However, free – free emission of an isothermal source dominated in microwaves. This is indicative of the presence of a large amount of plasma in the region of arcade formation at the initial stage of the event. The weak microwave emission during the decay phase might be interpreted as the thermal cyclotron emission of the loops in the arcade. [ABSTRACT FROM AUTHOR]

A method is developed for estimation of the vertical structure of the magnetic field in active regions using multi-wave spectral-polarization measurements of radio waves which gives not only the dependence of magnetic field strength on height but also determines two-dimensional form of a magnetic flux tube, emitted in the microwave range of wavelengths. [ABSTRACT FROM PUBLISHER]

High-altitude measurements and magnetic field distribution in active regions are always an important problem to verify existing models at heights from units to hundreds of Mm. Optical methods of analysis of the magnetic structure work well only at the photosphere level. At the same time, the progress of radio astronomy methods of analyzing the solar radio emission [1] and of the theory of solar plasma radiation [2] facilitates introduction of new methods for measuring the magnetic field strength at various altitudes in the solar atmosphere. In this paper we use multi-wave observations of polarization of the radio emission of active regions in the microwave range together with precise magnetosphere data in order to develop a method of projection for measuring the height distribution of the magnetic field. [ABSTRACT FROM AUTHOR]

In the present work a statistical analysis of long-lived microbursts (MBs) in the decimetric wavelength range was performed for the first time. Long-lived microbursts at decimetric wavelengths were observed with one-dimensional scans on the RATAN-600 radio telescope in intensity and circular polarization with a sensitivity of about 5 – 10 Jy. MBs have fluxes in the range of 0.001 – 0.1 s.f.u. and polarization degrees of 10 – 100%, and the duration of individual bursts is about 1 – 2 s. Microbursts and background sources exist for several days and appear at the sites of prolonged energy release. In this work MBs were compared with noise storms (NSs) in the metric wavelength range. Our analysis shows with high confidence that MBs are manifestation of NSs in the decimetric wavelength range. The reason for the significant difference in flux between MBs and NSs could be because MBs (unlike NSs) are related to incoherent generation of Langmuir waves. The nature of the MB emission is similar to the smoothly varying (background) emission of the NSs, butthe MB emission is impulsive because of the high rate of pitch-angle diffusion. [ABSTRACT FROM AUTHOR]

Application of the mode of azimuthal observations on RATAN-600 has made it possible to realize multiple daily observations [1]. The results of multiwavelength observations of the Sun in this mode point to the existence of sharp spectral irregularities in the polarized radiation of flare-productive active regions (FPARs) producing especially powerful flares [2]. These phenomena occur in a broad range of radio-emission fluxes (0.05–200 s.f.u.), in a rather narrow wavelength band (2–5 cm), and they are recorded during the time interval from several hours to several days before a powerful flare, as well as during the flare. Frequently, such events indicate an eruption of powerful coronal mass ejections and high-energy particle (proton) fluxes, which, when reaching the Earth’s magnetosphere, trigger its disturbances. Here, we analyze events in the microwave band connected with the strongest flares which took place on October 28 and November 4, 2003. [ABSTRACT FROM AUTHOR]

New observations of solar microbursts are reported. The spectra of microbursts can contain narrow features (Δf/f ≤ 0.03). Possible mechanisms for the generation of the microburst radio emission are analyzed, focusing on mechanisms that preferentially generate ordinary waves. Together with the well-known generation of radio emission at the fundamental plasma frequency, mechanisms associated with upper-hybrid waves and resonant-transition radiation are considered. The radio emission at the upper-hybrid frequency always corresponds to ordinary waves. Ordinary waves can also dominate in resonant-transition radiation, but the presence of well-developed small-scale turbulence in the emission region is required. Possible mechanisms for the generation of this turbulence include thermal fluctuations, Langmuir turbulence, quasi-stationary structures created by plasma waves, and ion-acoustic waves. Probable origins of the quasi-periodicity of microbursts are analyzed.© 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]

The observations of the solar radio emission on September 11, 2001, with the RATAN-600 radio telescope (southern sector) at four centimeter wavelengths (1.92, 2.24, 2.74, and 3.21 cm) revealed synchronous brightenings in solar radio sources. These were identified on the solar photosphere with active regions that were spaced up to ∼106 km apart (AR 9608 and AR 9616). We discuss manifestations of the possible mechanisms of synchronous brightenings in solar sources in a narrow microwave spectral band. The significant linear correlation (ρc = 0.84–0.92) between the relative fluxes of AR 9610 and AR 9608 at 1.92 and 2.24 cm and the significant linear correlation (ρc = 0.65–0.84) between the relative fluxes of AR 9606 and AR 9608 at 3.21 cm in a two-hour interval of observations are indicative of the interconnection between these active regions not only during flares and bursts, but also in the periods of their absence. This confirms the existence of a large-scale temporal component in the dynamics of the radio flux variations for these active regions. We found a difference between the temporal variations of the radio emission from the halo and the solar radio sources under consideration. The times of increase in the total solar soft X-ray (0.5–4.0 Å, 1.0–8.0 Å; GOES 8, GOES 10) flux are shown to coincide with the times of increase in the fluxes from the solar radio sources at short centimeter wavelengths. © 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

A series of coronal magnetograms, related to the active region NOAA 9068 is derived using the effects of the quasi-transverse (QT-) propagation at microwaves. In the assumption that the product of the electron density $N$ and the scale of magnetic divergence $L_d$ is constant and equal to $10^{18}$ cm$^{-2}$, the sensitivity of the measurements peaks at $B(G) \approx 180 \, \lambda^{-4/3} (cm)$, where $\lambda$ is the observational wavelength. The radio maps of the Sun taken with the Solar Siberian Radio Telescope (SSRT) at $\lambda = 5.2$ cm were sampled and converted into the 2D coronal magnetograms supplying absolute values of the total magnetic vector $\vec B$ in the range 10-30 G. Each coronal magnetogram covers the portion of the microwave source in the active region affected by the QT-propagation conditions in the corona.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html [ABSTRACT FROM PUBLISHER]

We present new observational data on the phenomena of extremely high activity on the Sun and in the heliosphere that took place in October-November 2003. A large variety of solar and heliospheric parameters give evidence that the interval under consideration is unique over the entire observation time. Based on these data, comparing them with similar situations in the past and using available theoretical concepts, we discuss possible cause-and-effect connections between the processes observed. The paper includes the first results and conclusions derived by the collaboration "Solar Extreme Events-2003" organized in Russia for detailed investigations of these events. As a result of our consideration, it is beyond question that the physical causes of solar and heliospheric phenomena in October-November 2003 are not exclusively local and do not belong only to the active regions and solar atmosphere above them. The energy reservoirs and driving forces of these processes have a more global nature. In general, they are hidden from an observer, since ultimately their sources lie in the subphotospheric layers of the Sun, where changes that are fast and difficult to predict can sometimes take place (and indeed they do). Solar flares can serve as sufficiently good tracers of these sudden changes and reconstructions on the Sun, although one can still find other diagnostic indicators among the parameters of magnetic fields, motions of matter, and emission characteristics. [ABSTRACT FROM AUTHOR]

This paper is devoted to the development of capabilities of the RATAN-600 radio telescope by putting the regime of multi-azimuth observations at the southern sector with periscope reflector into regular operation. This method makes it possible to study the dynamics of variable radio sources on temporal scales down to a few minutes and increase by one order of magnitude the number of variable sources observed during sky surveys. To date, over ten series of observations of the Sun were performed in the new regime using the multi-wavelength receiver complex, and new astrophysical results were obtained. The software for multi-wavelength mapping of the Sun and wavelet analysis of time series is developed. The following characteristics of the regime are achieved: a temporal resolution of 4 min during a time interval of up to 4.5 h for observations at azimuth angles in the interval from 30° to -30°. [ABSTRACT FROM AUTHOR]

RATAN-600 multiwavelength observations of the Sun reveal sharp spectral inhomogeneities in the polarized radiation from active regions that produce intense flares. These events occur in a wide range of radio fluxes (0.05-10 s.f.u.) in a relatively narrow wavelength range (2-5 cm). They are detected on time scales from several hours to several days before and during an intense flare. We analyze the detected events and their relationship to the preliminary phase of intense flares. Significant statistical material was obtained in 2001. The new flare-plasma properties can be used to test existing solar-flare models and to develop new criteria of flaring activity. [ABSTRACT FROM AUTHOR]

We report the detection of long-lived sources of radio bursts accompanied by polarized background emission in solar active regions. Both types of radio sources were detected at several decimeter wavelengths in observations on the RATAN-600 radio telescope in one-dimensional scans in intensity and circular polarization with a sensitivity of about 5–10 Jy. The degree of polarization is from 70 to 100%. The microburst and background sources exist for several days and appear at sites of prolonged energy release. The typical duration of an individual microburst is about 1–2 s, and the time interval between microbursts is about 3–5 s. A negative microburst frequency drift of about 100 MHz/s or more is also observed. Some interpretations of the microburst and background sources are discussed. The most probable microburst model involves the generation of radio emission via the plasma mechanism, with the upward propagation of fast electrons above an active region. In this case, the required energy of the Langmuir waves is 2×10[sup –8] of the heat energy of the background plasma. Microbursts appear in different places in an active region. New methods for determining the magnetic-field intensity in the regions of generation of the decimeter-wavelength emission are proposed. © 2001 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

Observations of the active region AR 7962 obtained at 2–32 cm on the RATAN-600 radio telescope on May 10–12, 1996, are presented. The high-resolution measurements detected a narrow feature near 8.5 cm against the background of the smooth spectrum of the local source associated with sunspots. This narrow-band emission is identified with a bright, pointlike, high-frequency source at 1.7 cm recorded on maps made using the Nobeyama radio telescope. The characteristics of the observed line (lifetime 3 days, brightness temperature of the order of several million Kelvin, relative width of about 10%) suggest that it can be explained as thermal cyclotron radiation at the third harmonic of the electron gyrofrequency from a compact source containing a dense, hot plasma; the corresponding higher frequency emission could be due to thermal Bremsstrahlung. Analysis of the RATAN-600 and Nobeyama data can be used to probe the magnetic field, kinetic temperature, and electron density in the radiation source in the corona. © 2000 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

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 4.7×106K 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. [ABSTRACT FROM AUTHOR]

This work is devoted to the new phenomena appearing in radio emission in preflare active regions that were found with the use of radio telescope RATAN-600. The first is the complicated spectral-polarization behavior in narrow frequency range and, in particularly, the double changing of the polarization emission sign in active region at the time interval of several days before big flare. The second, the detection of regular decreasing of radio brightness radio darkening of active regions, which is also observed in a few days prior to flare [Tokhchukova & Bogod (2003)].To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html [ABSTRACT FROM PUBLISHER]