Although the solar flare phenomenon is widely accepted to be a consequence of release of excessive magnetic energy stored in the coronal currents (stated another way — in nonpotential magnetic fields), many essential details of this energy release remain poorly understood. Initially, the released flare energy is somehow divided between thermal and nonthermal components through plasma heating and particle acceleration, respectively, although this proportion can then change in the course of the flare due, e.g., to fast particle Coulomb losses leading to additional plasma heating and/or chromospheric evaporation. So far, 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" (Battaglia et al. 2009) to "acceleration without heating" (Fleishman et al. 2011). 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 in addition to the bulk acceleration process, which, in contrast, efficiently accelerates the seed particles, while cannot accelerate the thermal particles. 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.

Page created on September 24, 2012.