This study is devoted to the investigation of pre-flare processes preceding the eruptive X3. 2-class solar flare that occurred on May 14, 2013. This event was selected because of its favorable position near the solar limb, the presence of a well-pronounced pre-flare phase, and the availability of high-quality observed data from the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA), Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), Nobeyama Radio Heliograph (NoRH), and Siberian Solar Radio Telescope (SSRT). The main objective of this work is to identify possible eruption triggers and to perform a detailed multiwavelength analysis of the pre-flare energy release properties. In this study, we consider a pre-flare time interval of about one and a half hours. From the point of view of temporal evolution, the pre-flare phase of the selected event consists of two stages. During the first stage, a quasi-stationary compact X-ray source is observed in the 5–25 keV energy range. The radio sources are also relatively stable, and their centroids coincide with the X-ray brightness center. This is followed by a sharp burst (nonthermal emission up to 100 keV is detected) and a subsequent increase in emission intensity over a broad spectral range. The observed sources become nonstationary. The second stage, following the burst, lasts for about one hour. During this period, the sources expand, and the growth of a coronal loop system is observed. Afterward, the eruption and the main flare take place. It is noteworthy that the trigger burst (TB) between the first and second pre-flare stages was associated with a very compact X-ray source and a strong brightening in all available ultraviolet (EUV and UV) channels. To determine the magnetic field structure at the photospheric level, vector magnetograms obtained by the Helioseismic and Magnetic Imager/Solar Dynamics Observatory (HMI/SDO) were used. These data showed that the pre-flare energy release and the TB were localized near the magnetic neutral line. Estimates of the thermodynamic parameters of the flare plasma, the energy of accelerated electrons, and the thermal energy of the pre-flare plasma were obtained based on the analysis of microwave and X-ray spectra. The observed microwave spectra are well explained by a gyrosynchrotron model consisting of an extended source associated with high coronal loops and the plasma temperature T 5-7 MK, a compact source in lower loops (T 10-20 MK), and bremsstrahlung emission at frequencies of 17–34 GHz. In general, the X-ray data from the compact source are in good agreement with the observed radio emission.
Шарыкин et al. (Sun,) studied this question.