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.
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