Microwave View on Particle Acceleration in Flares

The thermal-to-nonthermal partition was found to vary greatly from one flare to another resulting in a broad variety of cases from 'heating without acceleration' to 'acceleration without heating'. Recent analysis of microwave data of these differing cases suggests that a similar acceleration mechanism, forming a power-law nonthermal tail up to a few MeV or even higher, operates in all the cases. However, the level of this nonthermal spectrum compared to the original thermal distribution differs significantly from one case to another, implying a highly different thermal-to-nonthermal energy partition in various cases. This further requires a specific mechanism capable of extracting the charged particles from the thermal pool and supplying them to a bulk acceleration process to operate in flares \textit{in addition} to the bulk acceleration process itself, which, in contrast, efficiently accelerates the seed particles, while cannot accelerate the thermal particles. Within this 'microwave' view on the flare energy partition and particle acceleration I present a few contrasting examples of acceleration regions detected with microwave data and compare them with the most popular acceleration mechanisms---in DC fields, in collapsing traps, and stochastic acceleration by a turbulence spectrum - to identify the key elements needed to conform with observations. In particular, I point out that the turbulence needed to drive the particle acceleration is generated in nonpotential magnetic structures, which results in nonzero helicity of the turbulence. This helicity, in its turn, produces a nonzero mean DC electric field on top of stochastic turbulent fields driving the main stochastic acceleration; thus, acceleration by helical turbulence combines properties of the standard stochastic acceleration with some features of acceleration in DC electric fields, exactly what is demanded by observation.