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Abstract: The paper presents a study of the evolution of the photospheric magnetic field of active regions (ARs) of the Sun in which flares (larger than the M9 class in the Geostationary Operational Environmental Satellite (GOES) X-ray classification) occurred in 2010-2017. The purpose of this paper is to detect the precursors of flares. Thirteen ARs at a distance of not more than 45 degrees from the central meridian are selected for analysis out of 31 ARs in which flares larger than M9.0 were detected in the specified period. The magnetographic characteristics of the selected ARs are studied based on the Solar Dynamics Observatory (SDO) data. A flare index is proposed. It is calculated from the data of the Helioseismic and Magnetic Imager of SDO (SDO/HMI) and reflects the distance between regions of opposite magnetic polarity computed between the field boundaries by the threshold value. The analysis showed that a sharp increase in the flare index is detected in all the studied ARs 2-3 days before large flares. Flares larger than the M9.0 class occurred 5-20 h after the global or local maximum of the flare index. It is shown using the example of individual events that regions of the opposite polarity first converged and then relatively quickly separated from each other before large flares. The revealed features of evolution of ARs before large flares can be used to develop methods for predicting them. [ABSTRACT FROM AUTHOR]

This paper continues the cycle of authors’ works on the detection of precursors of large flares (M5 and higher classes) in active regions (ARs) of the Sun by their microwave radiation and magnetographic characteristics. Generalization of the detected precursors of strong flares can be used to develop methods for their prediction. This paper presents an analysis of the development of NOAA AR 12242, in which an X1.8 flare occurred on December 20, 2014. The analysis is based on regular multiazimuth and multiwavelength observations with the RATAN-600 radio telescope in the range 1.65-10 cm with intensity and circular polarization analysis and data from the Solar Dynamics Observatory (SDO). It was found that a new component appeared in the AR microwave radiation two days before the X-flare. It became dominant in the AR the day before the flare and significantly decreased after the flare. The use of multiazimuth observations from RATAN-600 and observations at 1.76 cm from the Nobeyama Radioheliograph made it possible to identify the radio source that appeared before the X-flare with the site of the closest convergence of opposite polarity fields near the neutral line in the AR. It was established that the X-flare occurred 20 h after the total gradient of the magnetic field of the entire region calculated from SDO/HMI data reached its maximum value. Analysis of the evolution of the microwave source that appeared before the X-flare in AR 12242 and comparison of its parameters with the parameters of other components of the AR microwave radiation showed that the new source can be classified as neutral line associated sources (NLSs), which were repeatedly detected by the RATAN-600 and other radio telescopes 1-3 days before the large flares. [ABSTRACT FROM AUTHOR]

The evolution of the microwave radiation from four active regions, where strong X-ray flares (X-class, GOES) occurred in 2011, has been studied. Daily multiwavelength RATAN-600 radio observations of the Sun in the 1.6–8.0 cm range have been used. It has been indicated that the radiosource above the photospheric magnetic field neutral line (above the region with the maximal convergence of the fields opposite in sign) becomes predominant in the structure of the active region microwave radiation one to two days before a powerful flare as in the eruptive events previously studied with RATAN-600. The appearance of such a radiosource possibly reflects the current sheet formation in the corona above the active region. The energy necessary for a flare is stored in the magnetic field of active region, which can be considered as a factor for predicting a powerful flare. [ABSTRACT FROM AUTHOR]