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In some flares, the thermal component appears much earlier than the nonthermal component in the X-ray range. Using sensitive microwave observations, we revisit this finding made by Battaglia et al. based on a thorough analysis of RHESSI data. We have found that nonthermal microwave emission produced by accelerated electrons with energy of at least several hundred keV appears as early as the thermal soft X-ray emission, indicating that the electron acceleration takes place at the very early flare phase. The non-detection of the hard X-rays at that early stage of the flares is thus an artifact of a limited RHESSI sensitivity. In all of the considered events, the microwave emission intensity increases at the early flare phase. We found that either thermal or nonthermal gyrosynchrotron emission can dominate the low-frequency (optically thick) part of the microwave spectrum below the spectral peak occurring at 3-10 GHz. In contrast, the high-frequency optically thin part of the spectrum is always formed by the nonthermal, accelerated electron component, whose power-law energy spectrum can extend up to a few MeV at this early flare stage. This means that even though the total number of accelerated electrons is small at this stage, their nonthermal spectrum is fully developed. This implies that an acceleration process of available seed particles is fully operational. While creation of this seed population (the process commonly called "injection" of the particles from the thermal pool into the acceleration process) has a rather low efficiency at this stage, the plasma heating efficiency is high. This imbalance between the heating and acceleration (in favor of the heating) is difficult to reconcile within most of available flare energization models. Being reminiscent of the trade off between the Joule heating and runaway electron acceleration, it puts additional constraints on the electron injection into the acceleration process. As a byproduct of this study, we demonstrate that for those cases when the optically thick part of the radio spectrum is dominated by the thermal contribution, the microwave spectral data yield reliable estimates of the magnetic field and source area at the early flare phase.

Here we briefly present some design approaches for a multifrequency 96-antenna radioheliograph. The array antenna configuration, transmission lines and digital receivers are the main focus of this work. The radioheliograph is a T-shaped centrally-condensed radiointerferometer operating at the frequency range 4-8 GHz. The justification for the choice of such a configuration is discussed. The antenna signals are transmitted to a workroom by analog optical links. The dynamic range and phase errors of the microwave-over-optical signal are considered. The signals after downconverting are processed by the digital receivers for delay tracking and fringe stopping. The required delay tracking step and data rates are considered. Two 3-bit data streams (I and Q) are transmitted to a correlator with the transceivers embedded in FPGA (Field Programmed Gate Array) chips and with PCI Express cables.

The first 48-antenna stage of the Siberian Radioheliograph (SRH) started single-frequency test observations early in 2016, and since August 2016 it routinely observes the Sun at several frequencies in the 4-8 GHz range with an angular resolution of 1-2 arc minutes and an imaging interval of about 12 seconds. With limited opportunities of the incomplete antenna configuration, a high sensitivity of about 100 Jy allows the SRH to contribute to the studies of eruptive phenomena along three lines. First, some eruptions are directly visible in SRH images. Second, some small eruptions are detectable even without a detailed imaging information from microwave depressions caused by screening the background emission by cool erupted plasma. Third, SRH observations reveal new aspects of some events to be studied with different instruments. We focus on an eruptive C2.2 flare on 16 March 2016 around 06:40, one of the first flares observed by the SRH. Proceeding from SRH observations, we analyze this event using extreme-ultraviolet, hard X-ray, white-light, and metric radio data. An eruptive prominence expanded, brightened, and twisted, which indicates a time-extended process of the flux-rope formation together with the development of a large coronal mass ejection (CME). The observations rule out a passive role of the prominence in the CME formation. The abrupt prominence eruption impulsively excited a blast-wave-like shock, which appeared during the microwave burst and was manifested in an "EUV wave" and Type II radio burst. The shock wave decayed and did not transform into a bow shock because of the low speed of the CME. Nevertheless, this event produced a clear proton enhancement near Earth. Comparison with our previous studies of several events confirms that the impulsive-piston shock-excitation scenario is typical of various events.